# Degradation of antibiotics by homogeneous and heterogeneous Fenton processes: A review

Hasna Befenzi, Amine Ezzariai, Tahar Mechichi, Lamfeddal Kouisni, Mohamed Hafidi, Eric Record, Loubna El Fels

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Hasna Befenzi, Amine Ezzariai, Tahar Mechichi, Lamfeddal Kouisni, Mohamed Hafidi, et al., Degradation of antibiotics by homogeneous and heterogeneous Fenton processes: A review. Journal of Hazardous Materials Advances, 2025, 17, pp.100522. 10.1016/j.hazadv.2024.100522. hal-05088719

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https://hal.inrae.fr/hal-05088719v1

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# Degradation of antibiotics by homogeneous and heterogeneous Fenton processes: A review

Hasna Befenzi<sup>a,b</sup>, Amine Ezzariai<sup>c</sup>, Tahar Mechichi<sup>d</sup>, Lamfeddal Louisni<sup>c</sup>, Mohamed Hafidi<sup>a,c</sup>, Eric Record<sup>b,*</sup>, Loubna EL Fels<sup>a,*</sup>

## ARTICLE INFO

### Keywords:

Antibiotics

Advanced oxidation processes

Fenton

Photo-Fenton

Electro-Fenton

Heterogeneous catalysts

Degradation products

## ABSTRACT

Global antibiotic consumption has been rising uncontrollably. Antibiotics ultimately get flushed into the environment, where they can pose risks to ecosystems and human health. Conventional wastewater treatment processes are not effective at removing these antibiotics. However, the application of Fenton processes in water treatment has attracted attention due to their fast reaction speeds and effective performances. Here we review recent research related into Fenton processes for antibiotic degradation, including homogeneous and heterogeneous Fenton, photo-Fenton and electro-Fenton reactions. We look at the impact of several key parameters such as target antibiotic, hydrogen peroxide, ferrous ion concentrations, pH and temperature on the efficiency of antibiotic degradation. We also provide an in-depth analysis of commonly used catalysts, such as natural heterogeneous catalysts and synthetic iron-based catalysts, and go on to propose typical mechanisms for antibiotic degradation by homogeneous and heterogeneous Fenton processes based on products identified in the literature.

## 1. Introduction

Antibiotics have been widely employed in both human and veterinary medicine to combat infections, stimulate animal growth, and safeguard human health (Dai et al., 2020). Daily global consumption of antibiotics has increased worldwide by 65% (from 21.1 billion to 34.8 billion daily doses) (Zhu et al., 2021). Unfortunately, a large proportion of antibiotic compounds does not get fully metabolized by humans and animals, and is thus excreted via urine and excrement, ending up in sewerage and wastewater treatment plants where they get discharged into the environment. The net result is pollution of soil and water, where transfers of resistant genes drive the development of antibiotic-resistant bacteria (Chen et al., 2021; Zhu et al., 2021).

Several methods have been developed to remove antibiotics from wastewater, including adsorption, biological and chemical treatments (Phoon et al., 2020). However, adsorption is limited to simply separating antibiotics from wastewater without degrading the compound,

whereas advanced oxidation processes that can generate OH<sup>•</sup> radicals (HR-AOPs) or SO<sub>4</sub><sup>2-</sup> sulfate radicals (SR-AOPs) that exhibit highly efficient antibiotic degradation (Chen et al., 2021; Khan et al., 2020). Note, however, that AOPs do have crucial limitations, as the byproducts generated during the process may carry unknown toxicity may promote antimicrobial resistance gene (ARG) microorganisms, especially pathogenic bacteria (Li et al., 2023; Yin et al., 2018) demonstrated that the transformation products of phenicol antibiotics (PABs), particularly florfenicol (FLO) and trimphenicol (THA), after UV/H<sub>2</sub>O<sub>2</sub> degradation showed high acute toxicity in *Vibrio fischeri*, potentially due to the production of hydroxylated by-product during the oxidation of FLO and THA.

AOPs remain the preferred methods for water treatment and pollutant degradation, due to their reaction speed and their ability to degrade and completely remove emergent pollutants. They break down antibiotics into various oxidation intermediates and lower-toxicity products (Hasani et al., 2020) than the original molecules.

<footnote><sup>a</sup> Laboratory of Microbial Biotechnologies, Agrosciences and Environment (BioMAgE), Labeled Research Unit CNRST No.4, Faculty of Sciences Semlalia, Cadi Ayyad University, Marrakesh, Morocco</footnote>

<footnote><sup>b</sup> INRAE, Aix Marseille Univ, BBF, Biodiversité et Biotechnologie Fongiques, Marseille, France</footnote>

<footnote><sup>c</sup> African Sustainable Agriculture Research Institute (ASARI), College of Agriculture and Environmental Sciences, Mohammed VI Polytechnic University (UM6P), Laayoune, Morocco</footnote>

<footnote><sup>d</sup> National Engineering School of Sfax-Laboratory of Lipase Biochemistry and Enzyme Engineering, University of Sfax, Sfax 3038, Tunisia</footnote>

<footnote>* Corresponding authors at: Laboratory of Microbial Biotechnologies, Agrosciences and Environment (BioMAgE), Labeled Research Unit CNRST N°4, Faculty of Science Semlalia, Cadi Ayyad University, Morocco. - INRAE, Aix Marseille Univ, BBF, Biodiversité et Biotechnologie Fongiques, Marseille, France.

E-mail addresses: eric record@inrae.fr (E. Record), loubna.elfels@uca.ac.ma (L. EL Fels).</footnote>
<table><caption>**Table 1**
Summary of the results obtained by homogeneous and heterogeneous Fenton.</caption><thead><tr><th></th><th>Antibiotic</th><th>Experimental conditions</th><th>Removal efficiency</th><th>Refs.</th></tr></thead><tbody><tr><td>Heterogeneous Fenton</td><td>Tetracycline<br/>C<sub>22</sub>H<sub>24</sub>N<sub>2</sub>O<sub>8</sub><br/>10 mg/L</td><td>pH: 3, catalyst concentration: 0.4 g/L, H<sub>2</sub>O<sub>2</sub>: 9 mmol.g<sup>-1</sup><br/>Catalyst: iron-loaded granular activated<br/>carbon (Fe-GAC)</td><td>92.6% degradation in 60 min</td><td>(He et al., 2022)</td></tr><tr><td>Heterogeneous Fenton</td><td>Ciprofloxacin<br/>C<sub>17</sub>H<sub>18</sub>FN<sub>3</sub>O<sub>3</sub><br/>20 mg/L</td><td>pH: 4, catalyst concentration: 1 g/L, H<sub>2</sub>O<sub>2</sub>: 2 mM<br/>Catalyst: hexapod-like pyrite nanosheet mineral clusters</td><td>Complete degradation in 10 min</td><td>(Nie et al., 2022)</td></tr><tr><td>Heterogeneous Fenton</td><td>Ciprofloxacin<br/>C<sub>17</sub>H<sub>18</sub>FN<sub>3</sub>O<sub>3</sub><br/>30 mg/L</td><td>pH: 3, catalyst concentration: 1.5 g/L, H<sub>2</sub>O<sub>2</sub>: 5 mM<br/>Catalyst: FeS<sub>2</sub></td><td>83.2% degradation in 20 min</td><td>(Sang et al., 2022)</td></tr><tr><td>Heterogeneous Fenton</td><td>Sulfamethoxazole<br/>C<sub>10</sub>H<sub>11</sub>N<sub>3</sub>O<sub>3</sub>S<br/>1.27 mg/L</td><td>pH: 7, catalyst concentration: 0.8 g/L, H<sub>2</sub>O<sub>2</sub>: 560 µM<br/>Catalyst: Fe<sup>2+</sup>-zeolite</td><td>10% degradation in 50 min</td><td>(Wang et al., 2020)</td></tr><tr><td>Heterogeneous Fenton</td><td>Ofloxacin<br/>C<sub>18</sub>H<sub>20</sub>FN<sub>3</sub>O<sub>4</sub><br/>50 mg/L</td><td>pH: 7, catalyst concentration: 0.75 g/L, H<sub>2</sub>O<sub>2</sub>: 5 mL/L<br/>Catalyst: Fe<sub>3</sub>O<sub>4</sub>-montmorillonite</td><td>81% degradation in 120 min</td><td>(A. R. D. Ahmad et al., 2020)</td></tr><tr><td>Heterogeneous Fenton</td><td>Tetracycline<br/>C<sub>22</sub>H<sub>24</sub>N<sub>2</sub>O<sub>8</sub><br/>100 mg/L</td><td>pH: 3, catalyst concentration: 0.1 g/L, H<sub>2</sub>O<sub>2</sub>: 100 mM, temperature: 26 °C<br/>Catalyst: nZVI/CeO<sub>2</sub></td><td>93% removal in 60 min</td><td>(Zhang et al., 2019)</td></tr><tr><td>Homogeneous Fenton</td><td>Amoxicillin<br/>C<sub>16</sub>H<sub>19</sub>N<sub>3</sub>O<sub>5</sub>S<br/>10 mg/L</td><td>pH: 3, catalyst concentration: 30 mg/L, H<sub>2</sub>O<sub>2</sub>: 375 mg/L<br/>Catalyst: FeSO<sub>4</sub>-7H<sub>2</sub>O</td><td>Complete degradation in 12 min</td><td>(Verma and Haritash, 2019)</td></tr><tr><td>Heterogeneous Fenton</td><td>Ciprofloxacin<br/>C<sub>17</sub>H<sub>18</sub>FN<sub>3</sub>O<sub>3</sub><br/>10 mg/L</td><td>pH: 3, catalyst concentration: 1.75 g/L, H<sub>2</sub>O<sub>2</sub>: 12 mM, room temperature: 25 °C<br/>Catalyst: magnetite Fe<sub>3</sub>O<sub>4</sub></td><td>88.92% removal in 120 min</td><td>(Hassani et al., 2018)</td></tr><tr><td>Homogeneous Fenton</td><td>Trimethoprim<br/>C<sub>14</sub>H<sub>18</sub>N<sub>4</sub>O<sub>3</sub><br/>14.5 mg/L</td><td>pH: 3, catalyst concentration: 0.05 mM, H<sub>2</sub>O<sub>2</sub>: 1 mM<br/>Catalyst: FeSO<sub>4</sub></td><td>Complete degradation in 240 min</td><td>(Wang and Wang, 2018)</td></tr><tr><td>Homogeneous Fenton</td><td>Ceftriaxone<br/>C<sub>18</sub>H<sub>18</sub>N<sub>8</sub>O<sub>7</sub>S<sub>3</sub><br/>500 mg/L</td><td>pH: 2.6, catalyst concentration: 0.4 M<br/>Catalyst: FeSO<sub>4</sub>-7H<sub>2</sub>O</td><td>84.6% degradation in 30 min</td><td>(Puddoo et al., 2017)</td></tr><tr><td>Heterogeneous Fenton</td><td>Sulfamethazine<br/>C<sub>12</sub>H<sub>14</sub>N<sub>4</sub>O<sub>2</sub>S<br/>20 mg/L</td><td>pH: 7, catalyst concentration: 0.5 g/L, H<sub>2</sub>O<sub>2</sub>: 10 µL, temperature: 25 °C<br/>Catalyst: Ce<sup>0</sup>-Fe<sup>0</sup>-reduced graphene oxide<br/>Ce<sup>0</sup>/Fe<sup>0</sup>-RGO</td><td>Complete degradation in 5 min</td><td>(Wan and Wang, 2016)</td></tr></tbody></table>

<table><caption>**Table 2**
Summary of the results obtained by homogeneous photo-Fenton processes.</caption><thead><tr><th>Antibiotic</th><th>Experimental conditions</th><th>Light source</th><th>Removal efficiency</th><th>Refs.</th></tr></thead><tbody><tr><td>Isoniazid<br/>C<sub>6</sub>H<sub>7</sub>N<sub>3</sub>O<br/>25 mg/L</td><td>pH: 3, catalyst concentration: 10 mg/L, H<sub>2</sub>O<sub>2</sub>: 125 mg/L<br/>Catalyst: FeSO<sub>4</sub>-7H<sub>2</sub>O</td><td>125 W high-pressure mercury-vapor lamp</td><td>99% removal in 10 min</td><td>(Rodrigues-Silva et al., 2022)</td></tr><tr><td>Cephalexin<br/>C<sub>16</sub>H<sub>17</sub>N<sub>3</sub>O<sub>4</sub>S<br/>50 mg/L</td><td>pH: 3.5, catalyst concentration: 0.2016 mg/L, H<sub>2</sub>O<sub>2</sub>: 1.2096 mg/L<br/>Catalyst: FeSO<sub>4</sub>-7H<sub>2</sub>O</td><td>Ultraviolet irradiation</td><td>53.37% removal in 180 min</td><td>(Gou et al., 2021b)</td></tr><tr><td>Tetracycline<br/>C<sub>22</sub>H<sub>24</sub>N<sub>2</sub>O<sub>8</sub><br/>100 mg/L</td><td>Catalyst concentration: 5 mg/L, H<sub>2</sub>O<sub>2</sub>: 20 mg/L<br/>Catalyst: FeCl<sub>2</sub>-4H<sub>2</sub>O</td><td>4 W low-pressure Hg UV lamp</td><td>91% tetracycline removal in 30 min<br/>92.4% oxytetracycline hydrochloride removal in 30 min</td><td>(Han et al., 2020a)</td></tr><tr><td>Oxytetracycline hydrochloride<br/>C<sub>22</sub>H<sub>24</sub>N<sub>2</sub>O<sub>9</sub>.ClH<br/>100 mg/L</td><td></td><td></td><td></td><td></td></tr><tr><td>Ciprofloxacin<br/>C<sub>17</sub>H<sub>18</sub>FN<sub>3</sub>O<sub>3</sub><br/>50 mg/L</td><td>pH: 3.5, catalyst concentration: 1.25 mM, H<sub>2</sub>O<sub>2</sub>: 10 mM, temperature: 25 °C<br/>catalyst:(FeNH<sub>4</sub>)<sub>2</sub>(SO<sub>4</sub>).6H<sub>2</sub>O</td><td>UV lamp 12 W/m<sup>2</sup></td><td>93.1% removal in 45 min</td><td>(Giri and Golder, 2019a)</td></tr><tr><td>Ampicillin<br/>C<sub>16</sub>H<sub>19</sub>N<sub>3</sub>O<sub>4</sub>S<br/>0.1 mg/L</td><td>pH: 8, catalyst concentration: 5 mg/L, H<sub>2</sub>O<sub>2</sub>: 50 mg/L<br/>Catalyst: FeSO<sub>4</sub>-7H<sub>2</sub>O</td><td>1 kW lamp (Xe-OP)</td><td>99.9% degradation in 5 min</td><td>(Ioannou-Ttofa et al., 2019)</td></tr><tr><td>Sulfamethoxazole<br/>C<sub>10</sub>H<sub>11</sub>N<sub>3</sub>O<sub>3</sub>S<br/>40 mg/L</td><td>Solar radiation<br/>pH: 3, catalyst concentration: 268.8 µM, H<sub>2</sub>O<sub>2</sub>: 30 µM, temperature: 25 °C</td><td>1500 W Xenon lamp (solar radiation)<br/>Low-pressure Hg lamp TNN 15/32 (UV radiation)</td><td>Complete sulfamethoxazole degradation in 50 min (solar and UV radiation)<br/>56% trimethoprim removal in 50 min (solar radiation)<br/>75% trimethoprim removal in 50 min (UV radiation)</td><td>(Martínez-Costa et al., 2018)</td></tr><tr><td>Trimethoprim<br/>C<sub>14</sub>H<sub>18</sub>N<sub>4</sub>O<sub>3</sub><br/>40 mg/L</td><td>pH: 3, catalyst concentration: 89.6 µM, H<sub>2</sub>O<sub>2</sub>: 10 µM, temperature: 25 °C<br/>Catalyst: Fe<sup>3+</sup>(catalyst)</td><td></td><td></td><td></td></tr><tr><td>Tetracycline<br/>C<sub>22</sub>H<sub>24</sub>N<sub>2</sub>O<sub>8</sub><br/>25 mg/L</td><td>Catalyst concentration: 166.7 mg/L, H<sub>2</sub>O<sub>2</sub>: 112 mg/L<br/>Catalyst: FeSO<sub>4</sub>-7H<sub>2</sub>O</td><td>Sunlight</td><td>88.7% removal in 120 min</td><td>(Cahino et al., 2018)</td></tr><tr><td>Ciprofloxacin<br/>C<sub>17</sub>H<sub>18</sub>FN<sub>3</sub>O<sub>3</sub><br/>17 mg/L</td><td>pH: 3, catalyst concentration: 0.09 mM, H<sub>2</sub>O<sub>2</sub>: 4.2 mM<br/>Catalyst: FeSO<sub>4</sub>-7H<sub>2</sub>O</td><td>15-W lamp</td><td>100% removal in 1 min</td><td>(Villegas-Guzman et al., 2017)</td></tr><tr><td>Sulfamethoxazole<br/>C<sub>10</sub>H<sub>11</sub>N<sub>3</sub>O<sub>3</sub>S<br/>10 mg/L</td><td>pH: 3, catalyst concentration: 11.3 mg/L, H<sub>2</sub>O<sub>2</sub>: 20 mg/L<br/>Catalyst: Fe<sup>2+</sup></td><td>Black light UV-A(365 nm, 8 W) lamp</td><td>Complete degradation in 5 min</td><td>(Cruz et al., 2017)</td></tr></tbody></table>
<table><caption>**Table 3**
Summary of the results obtained by heterogeneous photo-Fenton processes.</caption><thead><tr><th>Antibiotic</th><th>Experimental conditions</th><th>Light source</th><th>Removal efficiency</th><th>Refs.</th></tr></thead><tbody><tr><td>Oxytetracycline<br/>C<sub>22</sub>H<sub>24</sub>N<sub>2</sub>O<sub>9</sub><br/>10 mg/L</td><td>pH: 4-9, catalyst concentration: 10 mg (80 mL),<br/>H<sub>2</sub>O<sub>2</sub>: 1 mol/L<br/>Catalyst: FePc/g-C<sub>3</sub>N<sub>4</sub> (CNFP 60)</td><td>Xe lamp (300 W, 420 nm cut-off<br/>filter) served as visible light<br/>source</td><td>88.48% removal in 60 min</td><td>(Xu et al., 2023)</td></tr><tr><td>Tetracycline<br/>C<sub>22</sub>H<sub>24</sub>N<sub>2</sub>O<sub>8</sub><br/>20 mg/L</td><td>pH: 3-11, catalyst concentration: 0.4 g/L, H<sub>2</sub>O<sub>2</sub>: 1.5<br/>mM<br/>Catalyst: Fe-POM/BMO-3</td><td>5 W-LED<br/>λ > 400 nm</td><td>99.79% removal in 18 min</td><td>(Yang et al., 2023)</td></tr><tr><td>Tetracycline<br/>C<sub>22</sub>H<sub>24</sub>N<sub>2</sub>O<sub>8</sub><br/>10 mg/L</td><td>pH: 3-11, catalyst concentration: 0.15 g/L, H<sub>2</sub>O<sub>2</sub>: 0.1<br/>M<br/>Catalyst: FeP/Fe<sub>1</sub>-GO</td><td>300 W xenon light source</td><td>Complete degradation in 30 min<br/>and 79.2% removal in 30 min</td><td>(X. Li et al., 2023b)</td></tr><tr><td>Tetracycline<br/>hydrochloride<br/>C<sub>22</sub>H<sub>25</sub>ClN<sub>2</sub>O<sub>8</sub><br/>20 mg/L</td><td>pH: 3.5, catalyst concentration: 0.1 g/L, H<sub>2</sub>O<sub>2</sub>: 0.23<br/>mol/L, room temperature<br/>Catalyst: Fe<sub>2</sub>O<sub>3</sub>-on-ZrO<sub>2</sub>-2</td><td>Xenon lamp (300 W)</td><td>95% removal in 25 min</td><td>(Zhang et al., 2023)</td></tr><tr><td>Oxytetracycline<br/>C<sub>22</sub>H<sub>24</sub>N<sub>2</sub>O<sub>9</sub><br/>20 mg/L</td><td>pH: 5, catalyst concentration: 0.5 g/L, H<sub>2</sub>O<sub>2</sub>: 50 mM<br/>Catalyst: DyFeO<sub>3</sub>/g-C<sub>3</sub>N<sub>4</sub>-2</td><td>40 W white LED</td><td>98.1% removal in 180 min</td><td>(Feng et al., 2023)</td></tr><tr><td>Ciprofloxacin<br/>C<sub>17</sub>H<sub>18</sub>FN<sub>3</sub>O<sub>3</sub><br/>20 mg/L</td><td>pH: 5.6-5.8, catalyst concentration: 0.5 g/L, H<sub>2</sub>O<sub>2</sub>:<br/>1.6 mM, Catalyst: FeSA/WO<sub>3</sub>/BiOBr (FWB)</td><td>Xenon lamp</td><td>95.8% of removal in 30 min and<br/>100% removal in 60 min</td><td>(Yu et al., 2023)</td></tr><tr><td>Tetracycline<br/>C<sub>22</sub>H<sub>24</sub>N<sub>2</sub>O<sub>8</sub><br/>100 mg/L</td><td>pH: 6, catalyst concentration: 0.7 g/L, H<sub>2</sub>O<sub>2</sub>: 8 mM,<br/>Power: 120 W, temperature: 25<br/>°C ± 2<br/>Catalyst: S-ZnFe<sub>2</sub>O<sub>4</sub></td><td>30 W LED<br/>Lamp</td><td>95.41% removal in 70 min</td><td>(Usman et al., 2022)</td></tr><tr><td>Oxytetracycline<br/>C<sub>22</sub>H<sub>24</sub>N<sub>2</sub>O<sub>9</sub><br/>30 mg/L</td><td>Catalyst concentration: 10 mg (40 mL), H<sub>2</sub>O<sub>2</sub>: 0.5 mL<br/>(30%)<br/>Catalyst: 30% MnFe<sub>2</sub>O<sub>4</sub>/g-C<sub>3</sub>N<sub>4</sub></td><td>300 W xenon lamp</td><td>80.5% removal in 10 min</td><td>(Sun et al., 2022)</td></tr><tr><td>Tetracycline<br/>C<sub>22</sub>H<sub>24</sub>N<sub>2</sub>O<sub>8</sub><br/>20 mg/L</td><td>pH: 7, catalyst concentration: 0.5 g/L, H<sub>2</sub>O<sub>2</sub>: 80 mM,<br/>temperature: 20 °C<br/>Catalyst: 15Fe/SCN<sup>-</sup></td><td>300 W xenon lamp</td><td>90.3% removal in<br/>80 min and 61.2% TOC removal<br/>in<br/>80 min</td><td>(Li et al., 2022a)</td></tr><tr><td>Sulfamethoxazole<br/>C<sub>10</sub>H<sub>11</sub>N<sub>3</sub>O<sub>3</sub>S<br/>5 mg/L</td><td>pH: 5, catalyst concentration: 0.3 g/L, H<sub>2</sub>O<sub>2</sub>: 200 µL/<br/>L, temperature: 25 ± 0.3 °C<br/>Catalyst: Fe<sub>3</sub>S<sub>4</sub></td><td>Visible light irradiation (350<br/>mW low power LED light)</td><td>100% removal in 10 min</td><td>(Hang et al., 2022)</td></tr><tr><td>Sulfamethoxazole<br/>C<sub>10</sub>H<sub>11</sub>N<sub>3</sub>O<sub>3</sub>S<br/>0.2 mg/L</td><td>pH: 6, catalyst concentration: 3 mmol/L, H<sub>2</sub>O<sub>2</sub>: 2<br/>mmol/L, room temperature<br/>Catalyst: MIL-53(Fe)</td><td>Solar irradiance</td><td>96% removal in 120 min</td><td>(Ortega-Moreno et al.,<br/>2022)</td></tr><tr><td>Sulfamethoxazole<br/>C<sub>10</sub>H<sub>11</sub>N<sub>3</sub>O<sub>3</sub>S<br/>5 mg/L</td><td>pH: 5.8, catalyst concentration: 0.3 g/L,<br/>H<sub>2</sub>O<sub>2</sub>: 2.94 mM, room temperature<br/>Catalyst: MgFe<sub>2</sub>O<sub>4</sub>/MIL-88A (MF140M60)</td><td>Visible light (350 mW low-<br/>power LED light)</td><td>100% removal in 40 min</td><td>(Shi et al., 2022)</td></tr><tr><td>Ciprofloxacin<br/>C<sub>17</sub>H<sub>18</sub>FN<sub>3</sub>O<sub>3</sub><br/>16.56 mg/L</td><td>pH: 7, catalyst concentration: 10 µg/mL, H<sub>2</sub>O<sub>2</sub>: 0.5<br/>mmol/L<br/>Catalyst: Fe-TCPP-3</td><td>300 W xenon lamp equipped<br/>with a UV cut-off filter.</td><td>73% removal in 60 min</td><td>(Yang et al., 2022)</td></tr><tr><td>Tetracycline<br/>hydrochloride<br/>C<sub>22</sub>H<sub>25</sub>ClN<sub>2</sub>O<sub>8</sub><br/>50 mg/L</td><td>pH: 4, catalyst concentration: 0.5 g/L, H<sub>2</sub>O<sub>2</sub>: 10 mM,<br/>temperature: 25 °C<br/>Catalyst: 35%-LaFeO<sub>3</sub>/BiOCl</td><td>500 W xenon lamp</td><td>91% removal in 20 min</td><td>(Cao et al., 2022)</td></tr><tr><td>Tetracycline<br/>C<sub>22</sub>H<sub>24</sub>N<sub>2</sub>O<sub>8</sub><br/>25 mg/L</td><td>pH: 7, catalyst concentration: 0.2 g/L, H<sub>2</sub>O<sub>2</sub>: 20 mM<br/>Catalyst: Fe<sub>3</sub>O<sub>4</sub>@FeS<sub>2</sub>@C@MoS<sub>2</sub></td><td>300 W xenon lamp with a 420<br/>nm cut-off filter</td><td>81.5% removal in 40 min</td><td>(Deng et al., 2022)</td></tr><tr><td>Tetracycline<br/>C<sub>22</sub>H<sub>24</sub>N<sub>2</sub>O<sub>8</sub><br/>20 mg/L</td><td>Catalyst concentration: 0.2 g/L, H<sub>2</sub>O<sub>2</sub>: 1 mM, room<br/>temperature, light intensity: 398.73 mW/cm<sup>2</sup><br/>Catalyst: 1FMW</td><td>300 W xenon lamp</td><td>89.9% removal in 60 min</td><td>(Wei et al., 2022)</td></tr><tr><td>Tetracycline<br/>C<sub>22</sub>H<sub>24</sub>N<sub>2</sub>O<sub>8</sub><br/>20 mg/L</td><td>pH: 3, catalyst concentration: 0.75 g/L, H<sub>2</sub>O<sub>2</sub>: 17.4<br/>mM<br/>Catalyst: 11Fe-BOI</td><td>500 W xenon lamp with a 420<br/>nm cut-off filter</td><td>94.0% removal in 80 min</td><td>(Zhu et al., 2022)</td></tr><tr><td>Tetracycline<br/>C<sub>22</sub>H<sub>24</sub>N<sub>2</sub>O<sub>8</sub><br/>10 mg/L</td><td>pH: 6.5, catalyst concentration: 0.4 g/L<br/>H<sub>2</sub>O<sub>2</sub>: 1 mM, Catalyst:<br/>2 Fe-g-C<sub>3</sub>N<sub>4</sub>/Bi<sub>2</sub>WO<sub>6</sub></td><td>500 W xenon lamp</td><td>93.90% removal in 120 min</td><td>(Liu et al., 2022)</td></tr><tr><td>Tetracycline<br/>hydrochloride<br/>C<sub>22</sub>H<sub>25</sub>ClN<sub>2</sub>O<sub>8</sub><br/>50 mg/L</td><td>pH: 4-10, catalyst concentration: 0.8 g/L,<br/>H<sub>2</sub>O<sub>2</sub>: 0.1 mM (50 µL 30%)<br/>Catalyst: FeWO<sub>4</sub>/BiOCl (1:1)</td><td>300 W xenon lamp</td><td>97% removal in 80 min</td><td>(Shi et al., 2022)</td></tr><tr><td>Ciprofloxacin<br/>C<sub>17</sub>H<sub>18</sub>FN<sub>3</sub>O<sub>3</sub><br/>20 mg/L</td><td>pH: 7, catalyst concentration: 0.1 g/L, H<sub>2</sub>O<sub>2</sub>: 5 mM,<br/>temperature: 25 °C<br/>Catalyst: t-MIL-53(Fe,Mn)</td><td>300 W xenon lamp equipped<br/>with a<br/>UV cut-off filter (λ≥420 nm)</td><td>89% removal in 30 min</td><td>(Wu et al., 2021)</td></tr><tr><td>Tetracycline<br/>hydrochloride<br/>C<sub>22</sub>H<sub>25</sub>ClN<sub>2</sub>O<sub>8</sub><br/>10 mg/L</td><td>pH: 5, catalyst concentration: 0.5 g/L, H<sub>2</sub>O<sub>2</sub>: 10 mM,<br/>temperature: 25 °C<br/>Catalyst: Fe<sub>3</sub>O<sub>4</sub>/α-FeOOH-1</td><td>300 W xenon lamp</td><td>96% removal in 90 min and<br/>100% in 90 min at pH 7</td><td>(Huang et al., 2021)</td></tr><tr><td>Roxarsone<br/>C<sub>6</sub>H<sub>6</sub>As<sub>2</sub>NO<sub>6</sub><br/>10 mg/L</td><td>pH: 3, catalyst concentration: 1 g/L, H<sub>2</sub>O<sub>2</sub>: 8.8 mM<br/>Catalyst: MoS<sub>2</sub>@ schwertmannite</td><td>Hg lamp (365 nm, 100 W)</td><td>85% removal in 40 min</td><td>(X. Wang et al., 2021)</td></tr></tbody></table>

(continued on next page)
<table><caption>Table 3 (continued)</caption><thead><tr><th>Antibiotic</th><th>Experimental conditions</th><th>Light source</th><th>Removal efficiency</th><th>Refs.</th></tr></thead><tbody><tr><td>Levofloxacin<br>C<sub>18</sub>H<sub>20</sub>FN<sub>3</sub>O<sub>4</sub><br>200 mg/L</td><td>pH: 6.81, catalyst concentration: 0.33 g/L, H<sub>2</sub>O<sub>2</sub>: 5.27 g/L<br>Catalyst: Fe<sub>3</sub>O<sub>4</sub>@MIL-100(Fe) (1:4)</td><td>Xenon lamp</td><td>94.3% removal in 180 min</td><td>(He et al., 2021)</td></tr><tr><td>Tetracycline<br>hydrochloride<br>C<sub>22</sub>H<sub>25</sub>ClN<sub>2</sub>O<sub>8</sub><br>25 mg/L</td><td>pH: 3, catalyst concentration: 1 g/L, H<sub>2</sub>O<sub>2</sub>: 5 mM, temperature: 20 ± 2 °C<br>Catalyst: 7% Fe<sub>3</sub>O<sub>4</sub>/g-C<sub>3</sub>N<sub>4</sub></td><td>500 W xenon lamp with a 420 nm cut-off filter</td><td>99.8% removal in 100 min</td><td>(Cui et al., 2022)</td></tr><tr><td>Tetracycline<br>hydrochloride<br>C<sub>22</sub>H<sub>25</sub>ClN<sub>2</sub>O<sub>8</sub><br>80 mg/L</td><td>pH: 7, catalyst concentration: 0.1 g/L, H<sub>2</sub>O<sub>2</sub>: 50 mM<br>Catalyst: MnFe<sub>2</sub>O<sub>4</sub>/diatomite composite</td><td>300 W xenon lamp with a 420 nm UV-cutoff filter</td><td>91.8% removal in 60 min<br>96.7% removal in 60 min at pH 4</td><td>(Sun et al., 2021)</td></tr><tr><td>Tetracycline<br>C<sub>22</sub>H<sub>24</sub>N<sub>2</sub>O<sub>8</sub><br>20 mg/L</td><td>pH: 6.4, catalyst concentration: 220 mg/L, H<sub>2</sub>O<sub>2</sub>: 22 mM<br>Catalyst: CuFeO<sub>2</sub>/biochar-1.0</td><td>300 W xenon lamp with a 420 nm cut-off filter</td><td>97.6% removal in 120 min</td><td>(Xin et al., 2021)</td></tr><tr><td>Tetracycline<br>hydrochloride<br>C<sub>22</sub>H<sub>25</sub>ClN<sub>2</sub>O<sub>8</sub><br>50 mg/L</td><td>pH: 4, catalyst concentration: 0.2 g/L, H<sub>2</sub>O<sub>2</sub>: 20 mL/L, irradiation intensity: 200 mW/cm<sup>2</sup><br>Catalyst: CUS-MIL-250</td><td>300 W Xe lamp</td><td>95.6% removal in 80 min</td><td>(J. Guo et al., 2021)</td></tr><tr><td>Tetracycline<br>C<sub>22</sub>H<sub>24</sub>N<sub>2</sub>O<sub>8</sub><br>20 mg/L</td><td>pH: 7, catalyst concentration: 0.5 g/L, H<sub>2</sub>O<sub>2</sub>: 80 mM, temperature: 20 °C<br>Catalyst: 0.8MLD/g-C<sub>3</sub>N<sub>4</sub>/Fe<sub>3</sub>O<sub>4</sub></td><td>300 W xenon lamp with a &gt; 420 nm cut-off filter</td><td>95.8% removal in 80 min<br>Mineralization rate was 55.7% within 80 min</td><td>(Zhang et al., 2021)</td></tr><tr><td>Cefuroxime<br>C<sub>16</sub>H<sub>16</sub>N<sub>4</sub>O<sub>8</sub>S<br>20 mg/L</td><td>pH: 3, catalyst concentration: 1 g/L, H<sub>2</sub>O<sub>2</sub>: 5.3 mmol/L, light intensity: 30 W/m<sup>2</sup><br>Catalyst: (1.0% wt.) α-Fe<sub>2</sub>O<sub>3</sub>-TiO<sub>2</sub></td><td>Atlas Suntest XLS + solar simulator<br>(390-800 nm)</td><td>Complete degradation in 150 min<br>73% mineralization</td><td>(Hernández-Coronado et al., 2021)</td></tr><tr><td>Tetracycline<br>C<sub>22</sub>H<sub>24</sub>N<sub>2</sub>O<sub>8</sub><br>20 mg/L</td><td>pH: 4.5, catalyst concentration: 0.4 g/L, H<sub>2</sub>O<sub>2</sub>: 20 mM<br>Catalyst: Fe<sub>3</sub>(hexaiminotriphenylene)<sub>2</sub></td><td>300 W xenon arc lamp with a 420 nm cut-off filter</td><td>96.7% removal in 30 min</td><td>(Xing et al., 2021)</td></tr><tr><td>Sulfamethoxazole<br>C<sub>10</sub>H<sub>11</sub>N<sub>3</sub>O<sub>3</sub>S<br>30 mg/L</td><td>pH: 3, catalyst concentration: 0.5 g/L, H<sub>2</sub>O<sub>2</sub>: 10 mM<br>Catalyst: CUCS-MIL-88B-Fe/Ti<sub>3</sub>C<sub>2</sub></td><td>LSH-500 W Xenon lamp</td><td>99% removal in 120 min</td><td>(M. Ahmad et al., 2020)</td></tr><tr><td>Tetracycline<br>C<sub>22</sub>H<sub>24</sub>N<sub>2</sub>O<sub>8</sub><br>10 mg/L</td><td>Catalyst concentration: 0.2 g/L, H<sub>2</sub>O<sub>2</sub>: 20 mM<br>Catalyst: Ti<sub>3</sub>C<sub>2</sub>Tx/MIL-53(Fe)</td><td>500 W xenon lamp with a UV-cut-off filter (420 nm &lt; λ &lt; 760 nm)</td><td>90.3% removal in 80 min</td><td>(N. Liu et al., 2020)</td></tr><tr><td>Tetracycline<br>hydrochloride<br>C<sub>22</sub>H<sub>25</sub>ClN<sub>2</sub>O<sub>8</sub><br>20 mg/L</td><td>pH: 8, catalyst concentration: 1 g/L, H<sub>2</sub>O<sub>2</sub>: 20 mM, temperature: 10 °C<br>Catalyst: CuFeO<sub>2</sub></td><td>Light illumination</td><td>90% removal in 120 min</td><td>(Liu et al., 2020c)</td></tr><tr><td>Ciprofloxacin<br>C<sub>17</sub>H<sub>18</sub>FN<sub>3</sub>O<sub>3</sub><br>3.31 mg/L</td><td>pH: 7.4, catalyst concentration: 0.16 g/L, H<sub>2</sub>O<sub>2</sub>: 0.1 mL (35% v/v), temperature: 25 °C<br>Catalyst: α-Fe<sub>2</sub>O<sub>3</sub> NPs</td><td>UV lamp (11 W, λ=395 nm)</td><td>58.82% removal in 60 min</td><td>(Hakimi and Alikhani, 2020)</td></tr><tr><td>Tetracycline<br>C<sub>22</sub>H<sub>24</sub>N<sub>2</sub>O<sub>8</sub><br>10 mg/L</td><td>pH: 5.7, catalyst concentration: 0.2 g/L, H<sub>2</sub>O<sub>2</sub>: 10 mM<br>Catalyst: LFO-HP21</td><td>Metal halide lamp (λ: 400-800 nm)</td><td>90% removal in 180 min</td><td>(Alpay et al., 2020)</td></tr><tr><td>Ofloxacin<br>C<sub>18</sub>H<sub>20</sub>FN<sub>3</sub>O<sub>4</sub><br>30 mg/L</td><td>pH: 4.7, catalyst concentration: 5 mg/L, H<sub>2</sub>O<sub>2</sub>: 0.14 mg/L<br>Catalyst: Biogenic Fe-Mn oxides</td><td>Mercury lamp (500 W, wavelength &lt; 420 nm)</td><td>98.1% removal in 120 min</td><td>(Du et al., 2020)</td></tr><tr><td>Sulfamethoxazole<br>C<sub>10</sub>H<sub>11</sub>N<sub>3</sub>O<sub>3</sub>S<br>5 mg/L</td><td>pH: 9, catalyst concentration: 0.025 g/L, H<sub>2</sub>O<sub>2</sub>: 1000 mg/L<br>Catalyst: NiFe-CNT</td><td>Mercury lamp (100 W)</td><td>99% removal in 120 min</td><td>(Nawaz et al., 2020)</td></tr><tr><td>Ciprofloxacin<br>C<sub>17</sub>H<sub>18</sub>FN<sub>3</sub>O<sub>3</sub><br>50 mg/L</td><td>pH: 3.5, catalyst concentration: 1.25 mmol/L (Fe<sup>2+</sup>)/1.25 g/L (TiO<sub>2</sub>), H<sub>2</sub>O<sub>2</sub>: 10 mmol/L, temperature: 25 °C<br>Catalyst: Fe<sup>2+</sup>/TiO<sub>2</sub></td><td>UV lamp (12 W/m<sup>2</sup>)</td><td>93.1% removal in 45 min and 47.3% mineralization in 45min</td><td>(Giri and Golder, 2019b)</td></tr><tr><td>Tetracycline<br>C<sub>22</sub>H<sub>24</sub>N<sub>2</sub>O<sub>8</sub><br>50 mg/L</td><td>pH: 6, catalyst concentration: 0.4 g/L, H<sub>2</sub>O<sub>2</sub>: 10 mM<br>Catalyst: g-C<sub>3</sub>N<sub>4</sub>/PDI@NH<sub>2</sub>-MIL-53(Fe)</td><td>LED white light lamp (380-800 nm, 5 W)</td><td>90% removal in 60 min</td><td>(Li et al., 2019a)</td></tr><tr><td>Tetracycline<br>C<sub>22</sub>H<sub>24</sub>N<sub>2</sub>O<sub>8</sub><br>40 mg/L</td><td>pH: 5.5, catalyst concentration: 500 mg/L, H<sub>2</sub>O<sub>2</sub>: 100 mM<br>Catalyst: MnFe<sub>2</sub>O<sub>4</sub>/biochar (1:2)</td><td>300 W xenon lamp with a 420 nm cut-off filter</td><td>95% removal in 120 min</td><td>(Lai et al., 2019)</td></tr><tr><td>Tetracycline<br>hydrochloride<br>C<sub>22</sub>H<sub>25</sub>ClN<sub>2</sub>O<sub>8</sub><br>20 mg/L</td><td>pH: 6.5, catalyst concentration: 1 g/L, H<sub>2</sub>O<sub>2</sub>: 10 mM<br>Catalyst: 10% α-Fe<sub>2</sub>O<sub>3</sub>/BiOI</td><td>Xenon lamp with a cut-off filter (k &lt; 420 nm)</td><td>82.1% removal in 30 min</td><td>(Jiang et al., 2019)</td></tr><tr><td>Sulfamethazine<br>C<sub>12</sub>H<sub>14</sub>N<sub>4</sub>O<sub>2</sub>S<br>500 mg/L</td><td>pH: 6, catalyst concentration: 0.5 g/L, H<sub>2</sub>O<sub>2</sub>: 60 mM (30% w/w), temperature: 20 °C<br>Catalyst: CuFeO</td><td>300 W xenon lamp (λ &gt; 420 nm)</td><td>95.42% removal in 30 min</td><td>(Cheng et al., 2019)</td></tr><tr><td>Amoxicillin<br>C<sub>16</sub>H<sub>19</sub>N<sub>3</sub>O<sub>5</sub>S<br>50 mg/L</td><td>pH: 7.1, catalyst concentration: 1 g/L, H<sub>2</sub>O<sub>2</sub>: 10 mM, temperature: 25 °C<br>Catalyst: Fly ash</td><td>150 W TQ150-Z0 lamp</td><td>36.1% removal in 240 min</td><td>(Ramírez-Franco et al., 2019)</td></tr><tr><td>Ciprofloxacin<br>C<sub>17</sub>H<sub>18</sub>FN<sub>3</sub>O<sub>3</sub><br>3.3 mg/L</td><td>pH: 2.5, catalyst concentration: 0.56 mg/L, H<sub>2</sub>O<sub>2</sub>: 30 mg/L<br>Catalyst: Fe<sup>3+</sup>-citrate (1:1) or Fe<sup>3+</sup>-oxalate (1:3)</td><td>10 W dark light lamp</td><td>Complete removal in 20 min (FeCit and FeOx)</td><td>(Görmez et al., 2019)</td></tr></tbody></table>

(continued on next page)
<table><caption>**Table 3 (continued)**</caption><thead><tr><th>Antibiotic</th><th>Experimental conditions</th><th>Light source</th><th>Removal efficiency</th><th>Refs.</th></tr></thead><tbody><tr><td>Metronidazole<br>C<sub>6</sub>H<sub>9</sub>N<sub>3</sub>O<sub>3</sub><br>6 mg/L</td><td>pH: 7, catalyst concentration: 1 g/L, H<sub>2</sub>O<sub>2</sub>: 10 mM<br>Catalyst: TiO<sub>2</sub>-codoped Fe/Si</td><td>Xenon lamp (220 W) with a 400 nm long-pass filter</td><td>93% removal in 50 min</td><td>(Du et al., 2018)</td></tr><tr><td>Tetracycline<br>hydrochloride<br>C<sub>22</sub>H<sub>25</sub>ClN<sub>2</sub>O<sub>8</sub><br>60 mg/L</td><td>pH: 3, catalyst concentration: 0.2 g/L, H<sub>2</sub>O<sub>2</sub>: 0.55 mM<br>Catalyst: Fe<sub>3</sub>O<sub>4</sub>@ carbon shell</td><td>300 W xenon lamp</td><td>100% removal in 10 min</td><td>(Yan et al., 2018)</td></tr><tr><td>Sulfonamide<br>C<sub>6</sub>H<sub>8</sub>N<sub>2</sub>O<sub>2</sub>S<br>10 mg/L</td><td>pH: 6.73, catalyst concentration: 0.2 g/L, H<sub>2</sub>O<sub>2</sub>: 10 mM<br>Catalyst: mesoporous CuFe<sub>2</sub>O<sub>4</sub></td><td>300 W xenon lamp</td><td>96% removal in 120 min<br>31.42% SMZ mineralization</td><td>(Gao et al., 2018)</td></tr><tr><td>Tetracycline<br>hydrochloride<br>C<sub>22</sub>H<sub>25</sub>ClN<sub>2</sub>O<sub>8</sub><br>10 mg/L</td><td>pH: 3–10, catalyst concentration: 0.5 g/L, H<sub>2</sub>O<sub>2</sub>: 10 mM<br>Catalyst: α-FeOOH/γ-Fe<sub>2</sub>O<sub>3</sub></td><td>300 W xenon lamp</td><td>93% removal in 60 min</td><td>(Ma et al., 2018)</td></tr><tr><td>Tetracycline<br>hydrochloride<br>C<sub>22</sub>H<sub>25</sub>ClN<sub>2</sub>O<sub>8</sub><br>10 mg/L</td><td>Catalyst concentration: 200 mg/L, H<sub>2</sub>O<sub>2</sub>: 10 mM<br>Catalyst: MIL-100(Fe)@Fe<sub>3</sub>O<sub>4</sub>/CA</td><td>150 W xenon arc lamp equipped with a 400 nm long-pass filter</td><td>85% removal in 210 min</td><td>(Rasheed et al., 2018)</td></tr><tr><td>Tetracycline<br>C<sub>22</sub>H<sub>24</sub>N<sub>2</sub>O<sub>8</sub><br>40 mg/L</td><td>pH: 3, catalyst concentration: 0.25 g/L, H<sub>2</sub>O<sub>2</sub>: 0.377 M, room temperature<br>Catalyst: Fe<sub>3</sub>O<sub>4</sub>@void@TiO<sub>2</sub> NPs</td><td>300 W Xenon lamp</td><td>100% removal in 6 min</td><td>(Du et al., 2017)</td></tr><tr><td>Tetracycline<br>C<sub>22</sub>H<sub>24</sub>N<sub>2</sub>O<sub>8</sub><br>43 mg/L</td><td>pH: 3, catalyst concentration: 0.11 g/L, H<sub>2</sub>O<sub>2</sub>: 1.97 mM<br>Catalyst: Fe<sub>3</sub>O<sub>4</sub>@C</td><td>4 W UV-C lamp (Philips)</td><td>79.25% removal in 44 min</td><td>(Kakavandi et al., 2016)</td></tr></tbody></table>

**FePc/g-C<sub>3</sub>N<sub>4</sub>:** Graphite carbon nitride coupled with high-dispersed iron (II) phthalocyanine; **Fe-POM/BMO:** Fe-polyoxometalate (Fe-POM)-modified Bi<sub>2</sub>MoO<sub>6</sub> with dominant {010} facets; **FeP/Fe<sub>1</sub>-GO:** FeP/Fe single-atom/graphene oxide; **FeSA:** Fe single-atom; **S-ZnFe<sub>2</sub>O<sub>4</sub>:** non-metal (sulphur)-doped zinc ferrite; **Fe/SCN:** Fe-doped porous carbon nitride; **Fe<sub>3</sub>S<sub>4</sub>:** Greigite; **FMW:** flower-like MnFe<sub>2</sub>O<sub>4</sub>–MoS<sub>2</sub>; **Fe-BOI:** Fe-doped Bi<sub>4</sub>O<sub>5</sub>I<sub>2</sub>; **MIL:** Lavoisier Institute Material; **MLD:** malonamide; **CUCS-MIL-88B-Fe/Ti<sub>3</sub>C<sub>2</sub>:** MIL-88B-Fe with mixed-valence coordinatively unsaturated iron centers on ultrathin Ti<sub>3</sub>C<sub>2</sub>; **NiFe-CNT:** multi-walled carbon nanotube-NiFe<sub>2</sub>O<sub>4</sub>, **CA:** carbon aerogel.

The most common antibiotics degraded by AOPs are tetracycline and ciprofloxacin (see Tables 1, 2, 3, 4, 5). Analysis by Brillas (2022a) found that ciprofloxacin (18.2% of studies) and tetracycline (14.5%) are the most widely studied antibiotics.

This review paper presents a critical synthesis of the most recent findings reported on AOPs frequently used for the treatment of antibiotics in wastewater. First we give a comprehensive explanation on the basic characteristics of different families of antibiotics. We then detail the fundamental concepts of AOPs in order to understand the key reactions involved in the generation of oxidative agents and the criteria used to assess their efficacy in each process.

## 2. Antibiotics

Antibiotics are antimicrobial substances used to inhibit the growth of microorganisms, specifically bacteria and fungi. Antibiotics are of microbial origin but they can also be semi-synthetic or fully synthetic. Antibiotics were discovered in 1928 and quickly considered as ‘miracle drugs’ due to their ability to fight infectious (bacterial) diseases. They have revolutionized medicine—without them, major surgeries, organ transplants, and cancer chemotherapy would all be impossible (Chopra and Roberts, 2001).

Antibiotics divide into several classes: β-lactams, macrolides, quinolones, tetracyclines, sulfonamides, arsphenamines, polypeptides, aminoglycosides, amphenicols, lipopeptides, oxazolidinones, streptogramins, glycopeptides, streptogramins, ansamycins, and lincomamides (Lewis, 2020).

β-lactams are a broad class of antibiotics frequently used in the treatment of severe infections. They inhibit the activities of transpeptidases involved in the synthesis of peptidoglycan, a component of the bacterial cell wall (Hernández-Alomía et al., 2023; Tooke et al., 2019).

The tetracyclines class encompasses chlortetracycline, tetracycline, limecycline, oxytetracycline, clomocycline, methacycline, demethyl-chlortetracycline, rolitetracycline, doxycycline, minocycline and tertiary-butylglycylamidominocycline (Chopra and Roberts, 2001). They block bacterial protein synthesis by binding to the ribosome (site

A) and preventing aminoacyl-tRNA from binding to it (Nguyen et al., 2014).

The macrolides class, which is made up of roxithromycin, clarithromycin, azithromycin, and erythromycin, inhibit bacterial protein synthesis and are eliminated in the bile (Monahan et al., 2021).

The sulfonamides class consists mainly of sulfadiazine, sulfamethazine, sulfachlopyridazine, sulfamethizole, sulfamethoxazole, sulfamethoxine, sulfamerazine, sulfamoxole, sulfapyridine, sulfathiazole, and sulfisoxazole. The sulfonamides possesses a structure similar to para-aminobenzoic acid (PARA) that enables them to act as false substrates competing with PARA for the enzyme dihydrofolate synthase, leading to the inhibition of dihydrofolic acid synthesis (DHFA), which inhibits bacterial growth (Paumelle et al., 2021).

The presence of antibiotics in environmental matrices, drinking water, surface water, groundwater, sediments, and soil, is mainly due to human consumption. Antibiotics are excreted through urine in an unmodified form or metabolized after ingestion, and ultimately collected in wastewater treatment plants. Most wastewater treatment plants have not been designed to treat these chemical compounds, especially in trace amounts (Darmey et al., 2023).

The antibiotics used in veterinary medicine for both livestock and pets are another source of environmental contamination. Their elimination by fecal or urinary route has a negative impact on the environment, either directly when animals graze or indirectly by reaching the groundwater via manure and sludge without passing through a sewage treatment plant (Broens and Van Geijlswijk, 2018). Furthermore, antibiotic manufacturing industries could directly contribute to the increase of antibiotic concentrations in wastewater treatment plants, as manufacturing residues can leach into the aquatic environment (Fig. 1) (Mompelat et al., 2009).

The fate of antibiotics in the environment is what drives the production of bacteria that are resistant and/or multi-resistant to antibiotics through the appearance of ARGs that spread by horizontal gene transfer (HGT) in microorganisms (Caneschi et al., 2023). This makes it crucial and urgent to develop highly efficient methods to eliminate these compounds in wastewater treatment plants. Here we summarize some of the methods that have been developed so far.
<table><caption>**Table 4**
Summary of the results obtained by homogeneous electro-Fenton processes.</caption><thead><tr><th>Antibiotic</th><th>Cathode material</th><th>Anode material</th><th>Experimental conditions</th><th>Removal efficiency</th><th>Refs.</th></tr></thead><tbody><tr><td>Ciprofloxacin<br/>C<sub>17</sub>H<sub>18</sub>FN<sub>3</sub>O<sub>3</sub><br/>50 mg/L</td><td>Graphene<br/>aerogel</td><td>Platinum</td><td>pH: 3, catalyst concentration: 0.8 mM, Na<sub>2</sub>SO<sub>4</sub>: 0.05 M, current density: 30 mA/cm<sup>2</sup><br/>Catalyst: FeSO<sub>4</sub></td><td>Nearly 100% degradation in 90 min<br/>91% TOC removal in 120 min<br/>96.97% degradation of AMX</td><td>(Wang et al., 2021c)<br/>(Zhang et al., 2021)</td></tr><tr><td>Amoxicillin<br/>C<sub>16</sub>H<sub>19</sub>N<sub>3</sub>O<sub>5</sub>S<br/>10 mg/L</td><td>Stainless steel</td><td>Rod-like DSA (Ti/<br/>IrO<sub>2</sub>-RuO<sub>2</sub>)</td><td>pH: 2.29, catalyst concentration: 26.17 mg/L, Na<sub>2</sub>SO<sub>4</sub>: 0.05 M, current: 366.08 mA<br/>Catalyst: FeSO<sub>4</sub>·7H<sub>2</sub>O</td><td>Complete degradation in 120 min<br/>38% of TOC removal in 360 min<br/>98.83% removal efficiency after 120 min</td><td>(Wang et al., 2021a)<br/>(Liu et al., 2020b)</td></tr><tr><td>Cefoperazone<br/>C<sub>25</sub>H<sub>27</sub>N<sub>9</sub>O<sub>8</sub>S<sub>2</sub><br/>200 mg/L</td><td>Activated<br/>carbon fiber felt</td><td>RuO<sub>2</sub>/Ti</td><td>pH: 3, catalyst concentration: 1 mM, Na<sub>2</sub>SO<sub>4</sub>: 0.05 M, current density: 6.67 mA·cm<sup>-2</sup>, O<sub>2</sub> flow rate: 100 mL/min<br/>Catalyst: FeSO<sub>4</sub>·7H<sub>2</sub>O</td><td>Total degradation in 10 min<br/>(CS), 40 min (CF), 60 min (SS).<br/>TOC degradation after 480 min at 300 mA:<br/>63% (CS), 56% (CF), 37% (SS)</td><td>(Sopaj et al., 2016b)</td></tr><tr><td>Ciprofloxacin<br/>C<sub>17</sub>H<sub>18</sub>FN<sub>3</sub>O<sub>3</sub><br/>50 mg/L</td><td>ZnO-CeO<sub>2</sub>/CF</td><td>Platinum</td><td>pH: 3, catalyst concentration: 0.1 mmol·L<sup>-1</sup>, Na<sub>2</sub>SO<sub>4</sub>: 0.05 mol·L<sup>-1</sup>, E<sub>cell</sub>: 4 V<br/>Catalyst: FeSO<sub>4</sub></td><td>Complete degradation in 1 min</td><td>(Lai et al., 2020)</td></tr><tr><td>Sulfamethazine<br/>C<sub>12</sub>H<sub>14</sub>N<sub>4</sub>O<sub>2</sub>S<br/>55.6 mg/L</td><td>Carbon sponge<br/>Carbon felt<br/>Stainless steel</td><td>Platinum</td><td>pH: 3, catalyst concentration: 0.2 mM, Na<sub>2</sub>SO<sub>4</sub>: 0.05 M, current: 400 mA<br/>Catalyst: FeSO<sub>4</sub>·7H<sub>2</sub>O</td><td></td><td></td></tr><tr><td>Oxytetracycline<br/>C<sub>22</sub>H<sub>24</sub>N<sub>2</sub>O<sub>9</sub><br/>20.25 mg/L</td><td>NaOH-GF<br/>NaOH activated<br/>graphite felt</td><td>Platinum</td><td>pH: 3, catalyst concentration: 0.20 mM, Na<sub>2</sub>SO<sub>4</sub>: 50 mM, current density: 5.17 mA·cm<sup>-2</sup>, O<sub>2</sub> flow rate: 0.6 L/min, temperature: 25 °C<br/>Catalyst: FeSO<sub>4</sub>·7H<sub>2</sub>O</td><td>95.1% removal in 20 min and<br/>57.4% COD removal in 20 min</td><td>(Shoorangiz<br/>et al., 2019b)</td></tr><tr><td>Ciprofloxacin<br/>C<sub>17</sub>H<sub>18</sub>FN<sub>3</sub>O<sub>3</sub><br/>45 mg/L</td><td>Graphite felt</td><td>Stainless steel</td><td>pH: 5, Na<sub>2</sub>SO<sub>4</sub>: 0.05 M, current: 75 mA, air flow rate: 2 L/min<br/>Catalyst: Fe<sup>2+</sup><br/>Optimized by response surface methodology (RSM)</td><td>93% degradation at 90 min.<br/>82% TOC removal at 480 min.</td><td>(Chen et al., 2019b)</td></tr><tr><td>Tetracycline<br/>C<sub>22</sub>H<sub>24</sub>N<sub>2</sub>O<sub>8</sub><br/>80 mg/L</td><td>Carbon felt</td><td>Ti/RuO<sub>2</sub>-IrO<sub>2</sub></td><td>pH: 3, catalyst concentration: 0.1 mM, Na<sub>2</sub>SO<sub>4</sub>: 0.05 M, current: 60 mA, room temperature<br/>Catalyst: FeSO<sub>4</sub>·7H<sub>2</sub>O</td><td>Complete degradation in 11min</td><td>(Yahya et al., 2017)</td></tr><tr><td>Moxifloxacin<br/>C<sub>21</sub>H<sub>24</sub>FN<sub>3</sub>O<sub>4</sub><br/>0.15 mM<br/>60.21 mg/L</td><td>Carbon felt</td><td>Platinum</td><td>pH: 3, catalyst concentration: 0.5 mM, Na<sub>2</sub>SO<sub>4</sub>: 0.05 M, current: 400 mA, room temperature<br/>Catalyst: FeSO<sub>4</sub>·7H<sub>2</sub>O</td><td></td><td></td></tr><tr><td>Amoxicillin<br/>C<sub>16</sub>H<sub>19</sub>N<sub>3</sub>O<sub>5</sub>S<br/>36.5 mg/L</td><td>Carbon-felt</td><td>Ti<sub>4</sub>O<sub>7</sub></td><td>pH: 3, catalyst concentration: 0.1 mM, Na<sub>2</sub>SO<sub>4</sub>: 0.05 M, current: 120 mA, O<sub>2</sub> flow rate: 1 L/min, room temperature: 23±2 °C<br/>Catalyst: FeSO<sub>4</sub>·7H<sub>2</sub>O</td><td>Total removal in 5 min<br/>89% TOC degradation in 120 min</td><td>(Oturan et al., 2017)</td></tr><tr><td>Norfloxacin<br/>C<sub>16</sub>H<sub>18</sub>FN<sub>3</sub>O<sub>3</sub><br/>80 mg/L</td><td>Boron-doped<br/>diamond</td><td>Carbon felt</td><td>pH: 3, catalyst concentration: 0.1 mM, Na<sub>2</sub>SO<sub>4</sub>: 0.05 M, current: 60 mA, temperature: 25 °C<br/>Catalyst: Fe<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>·5H<sub>2</sub>O<br/>pH: 3, catalyst concentration: 0.1 mM, Na<sub>2</sub>SO<sub>4</sub>: 0.05 M, current: 300 mA<br/>Catalyst: Fe<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>·5H<sub>2</sub>O</td><td>Complete degradation in 30 min<br/>97.7% TOC removal in 300 min (5h)</td><td>(Özcan et al., 2016b)</td></tr><tr><td>Sulfamethazine<br/>C<sub>12</sub>H<sub>14</sub>N<sub>4</sub>O<sub>2</sub>S<br/>28.8 mg/L</td><td>Graphite felt</td><td>Boron-doped<br/>diamond</td><td>pH: 3, catalyst concentration: 0.2 mM, Na<sub>2</sub>SO<sub>4</sub>: 50 mM, current density: 20.83 mA/cm<sup>2</sup>, room temperature<br/>Catalyst: Fe<sup>2+</sup></td><td>Complete degradation in 20 min<br/>96.9% TOC degradation in 360 min</td><td>(Sopaj et al., 2016a)</td></tr><tr><td>Levofloxacin<br/>C<sub>18</sub>H<sub>20</sub>FN<sub>3</sub>O<sub>4</sub><br/>200 mg/L</td><td>Activated<br/>carbon fiber felt</td><td>RuO<sub>2</sub>/Ti</td><td>pH: 3, catalyst concentration: 1 mM, Na<sub>2</sub>SO<sub>4</sub>: 0.05 M, current density: 6.67 mA·cm<sup>-2</sup>, O<sub>2</sub> flow rate: 100 mL·min<sup>-1</sup><br/>Catalyst: Fe<sup>2+</sup></td><td>Complete removal in 120 min<br/>and 61% TOC removal in 360 min</td><td>(Gong et al., 2016)</td></tr><tr><td>Enoxacin<br/>C<sub>15</sub>H<sub>17</sub>FN<sub>4</sub>O<sub>3</sub><br/>50 mg/L</td><td>Vitreous carbon<br/>disk</td><td>Platinum</td><td>pH: 3, catalyst concentration: 0.2 mmol/L, Na<sub>2</sub>SO<sub>4</sub>: 50 mmol/L, current: 300 mA<br/>Catalyst: FeSO<sub>4</sub>·7H<sub>2</sub>O</td><td>98% degradation in 60 min<br/>43% TOC removal in 120 min</td><td>(Annabi et al., 2016)</td></tr><tr><td>Levofloxacin<br/>C<sub>18</sub>H<sub>20</sub>FN<sub>3</sub>O<sub>4</sub><br/>54 mg/L</td><td>Carbon felt</td><td>Platinum</td><td>pH: 3, catalyst concentration: 0.1 mM, Na<sub>2</sub>SO<sub>4</sub>: 0.05 mM, current: 400 mA, room temperature<br/>Catalyst: FeSO<sub>4</sub>·7H<sub>2</sub>O</td><td>Complete removal in 10 min</td><td>(Yahya et al., 2016)</td></tr></tbody></table>

### 3. Advanced oxidation processes

AOPs are among the most popular and effective technologies developed to eliminate the organic pollutants present in different water sources. They work by degrading and mineralizing organic compounds via the in situ generation of strong and powerful oxidant radicals. The hydroxyl radical, for instance, is characterized by an oxidation potential E° = 2.80 V and can react with a wide range of contaminants. AOPs essentially include photocatalysis, ozonation, the Fenton process, the photo-Fenton process, and the electro-Fenton process (Cuerda-Correa et al., 2019).

Photocatalysis involves semiconductor materials that, when exposed to light, produce reactive species, enabling the breakdown of contaminants. Titanium dioxide (TiO<sub>2</sub>) is the most commonly used photocatalyst, generating electron-hole pairs under UV light and facilitating oxidation and reduction reactions (Kujawska et al., 2022). Advances

such as modified TiO<sub>2</sub> and Z-scheme photocatalysts are being developed to improve efficiency under visible light, enhancing photocatalytic activity under solar irradiation (Hassani et al., 2021). In addition to photocatalysis, ozonation has gained attention as an effective method for degrading organic and inorganic pollutants (Saravanan et al., 2022a). The ozone molecule (O<sub>3</sub>) exhibits impressive oxidizing properties, with a high oxidation potential (E° = 2.07 V), allowing for the breakdown of a wide range of organic compounds. Moreover, ozonation facilitates direct oxidation and produces reactive species, such as hydroxyl radicals OH*, which significantly enhance the overall oxidation process. However, ozonation has its limitations, including high operational costs and a relatively short half-life, which can affect its overall efficacy (Germán et al., 2017). This review will specifically address the Fenton process, photo-Fenton process, and electro-Fenton process, focusing on their effectiveness in degrading antibiotics.

Here the peer-reviewed literature on oxidation processes applied for
<table><caption>**Table 5**
Summary of the results obtained by heterogeneous electro-Fenton processes.</caption><thead><tr><th>Antibiotic</th><th>Cathode material</th><th>Anode material</th><th>Experimental conditions</th><th>Removal efficiency</th><th>Refs.</th></tr></thead><tbody><tr><td>Tetracycline<br>hydrochloride<br>C<sub>22</sub>H<sub>25</sub>ClN<sub>2</sub>O<sub>8</sub><br>20 mg/L</td><td>Stainless-steel</td><td>Ti/IrO<sub>2</sub>-RuO<sub>2</sub><br>-TiO<sub>2</sub></td><td>pH: 3, catalyst concentration: 0.5 g/L, Na<sub>2</sub>SO<sub>4</sub>: 25 mmol/L, current density: 15 mA/cm<sup>2</sup>, peroxydisulfate: 2.5 mmol/L<br>Catalyst: Fe-GAC</td><td>86.38% removal in 60 min</td><td>(Wu et al., 2022)</td></tr><tr><td>Ofloxacin<br>C<sub>18</sub>H<sub>20</sub>FN<sub>3</sub>O<sub>4</sub><br>100 mg/L</td><td>CB-Bio-FeMnOx</td><td>Ti/PbO<sub>2</sub></td><td>pH: 6.5, NaCl: 70 mM, current density: 50 mA/cm<sup>2</sup>, air flow rate: 1 L/min<br>pH: 6.5, Na<sub>2</sub>SO<sub>4</sub>: 50 mM, current density: 50 mA/cm<sup>2</sup>, air flow rate: 1 L/min<br>Catalyst: Fe-GAC</td><td>Complete degradation in 20 min<br>85% removal in 120 min</td><td>(Xu et al., 2023a)</td></tr><tr><td>Ceftazidime<br>C<sub>22</sub>H<sub>22</sub>N<sub>6</sub>O<sub>7</sub>S<sub>2</sub><br>5 mg/L</td><td>Fe/Cu-CTA</td><td>Pt</td><td>pH: 6, Na<sub>2</sub>SO<sub>4</sub>: 0.1 mol/L<br>current: 25 mA</td><td>99.5 ± 0.4% removal within 120 min</td><td>(Wang et al., 2023)</td></tr><tr><td>Levofloxacin<br>C<sub>18</sub>H<sub>20</sub>FN<sub>3</sub>O<sub>4</sub><br>20 mg/L<br>Sulfamethoxazole<br>C<sub>10</sub>H<sub>11</sub>N<sub>3</sub>O<sub>3</sub>S<br>20 mg/L<br>Meropenem<br>C<sub>17</sub>H<sub>25</sub>N<sub>3</sub>O<sub>5</sub>S<br>20 mg/L<br>Tetracycline<br>C<sub>22</sub>H<sub>24</sub>N<sub>2</sub>O<sub>8</sub><br>20 mg/L</td><td>CF/Fe-N-C</td><td>Raw carbon felt</td><td>pH: 3, catalyst concentration: 1 mM, Na<sub>2</sub>SO<sub>4</sub>: 40 mM, current density: 100 mA/cm<sup>2</sup><br>Catalyst: Fe<sup>2+</sup></td><td>96.1% levofloxacin degradation<br>99.7% sulfamethoxazole removal<br>98.8% of meropenem removal<br>95.5% tetracycline removal in 40 min</td><td>(Qian et al., 2023)</td></tr><tr><td>Norfloxacin<br>C<sub>16</sub>H<sub>18</sub>FN<sub>3</sub>O<sub>3</sub><br>50 mg/L</td><td>Modified Graphite felts EH-GF</td><td>Pt</td><td>pH: 3, catalyst concentration: 0.3 mM, O<sub>2</sub> flow rate:<br>200 mL/min, potential: -0.6 V<br>Catalyst: Fe<sup>2+</sup></td><td>93% degradation in 120 min<br>63% TOC removal in 120 min</td><td>(Guo et al., 2023)</td></tr><tr><td>Ciprofloxacin<br>C<sub>17</sub>H<sub>18</sub>FN<sub>3</sub>O<sub>3</sub><br>20 mg/L</td><td>Reticulated vitreous carbon</td><td>Ti/RuO<sub>2</sub>-IrO<sub>2</sub></td><td>pH: 3, catalyst concentration: 0.5 g/L, Na<sub>2</sub>SO<sub>4</sub>: 2 g/L, O<sub>2</sub> flow rate: 400 mL/min, temperature: 25 ± 1 °C<br>Catalyst: AFRB (antibiotic fermentation residue biochar)</td><td>Complete degradation in 10 min</td><td>(Jin et al., 2023)</td></tr><tr><td>Doxycycline<br>C<sub>22</sub>H<sub>24</sub>N<sub>2</sub>O<sub>8</sub><br>20 mg/L</td><td>CoFeCe-LDH@CF</td><td>Pt</td><td>pH: 3, Na<sub>2</sub>SO<sub>4</sub>: 50 mM, current: 20 mA, air flow rate: 500 mL/min</td><td>95.2% removal in 180 min</td><td>(Xu et al., 2023c)</td></tr><tr><td>Tetracycline<br>C<sub>22</sub>H<sub>24</sub>N<sub>2</sub>O<sub>8</sub><br>50 mg/L</td><td>ACF</td><td>Pt</td><td>pH: 3.85, catalyst concentration: 0.08 g/L, Na<sub>2</sub>SO<sub>4</sub>: 0.05 M, current 50 mA, air flow rate: 0.1 L/min<br>Catalyst: Fe@Fe<sub>2</sub>O<sub>3</sub>-CeO<sub>2</sub></td><td>90.7% removal within 1 h TOC removal efficiency was 86.9% within 6 h</td><td>(Zhang et al., 2022)</td></tr><tr><td>Sulfamethoxazole<br>C<sub>10</sub>H<sub>11</sub>N<sub>3</sub>O<sub>3</sub>S<br>10 mg/L</td><td>CoFe-NC</td><td>Pt</td><td>pH: 1, Na<sub>2</sub>SO<sub>4</sub>: 0.05 M, potential: -0.7 V</td><td>91% removal in 300 min</td><td>(Wang et al., 2022a)</td></tr><tr><td>Sulfamethoxazole<br>C<sub>10</sub>H<sub>11</sub>N<sub>3</sub>O<sub>3</sub>S<br>20 mg/L</td><td>Cu-doped Fe<sub>2</sub>O<sub>3</sub> nanoparticles/ etched graphite felt (Cu-Fe<sub>2</sub>O<sub>3</sub>/ EGF)</td><td>BDD</td><td>pH: 6, Na<sub>2</sub>SO<sub>4</sub>: 0.05 M, current density: 6 mA/cm<sup>-2</sup>, air flow rate: 0.6 L/min, conductivity: 0.17 mS/cm</td><td>100% degradation in 90 min</td><td>(Qi et al., 2022)</td></tr><tr><td>Amoxicillin<br>C<sub>16</sub>H<sub>19</sub>N<sub>3</sub>O<sub>5</sub>S<br>100 mg/L</td><td>Multi-walled carbon nanotubes<br>Carbon black/Graphite felt tubes<br>Carbon black/Graphite felt (MWCNT-CB/GF)</td><td>Ti/RuO<sub>2</sub>-IrO<sub>2</sub></td><td>pH: 4, catalyst concentration: 2 g/L, Na<sub>2</sub>SO<sub>4</sub>: 0.05 M, current: 120 mA, conductivity: 0.1 µS/cm<br>Catalyst: Cu-Fe-Fe<sub>3</sub>C@NDB</td><td>100% removal In 35 min</td><td>(Qi et al., 2022a)</td></tr><tr><td>Enrofloxacin<br>C<sub>19</sub>H<sub>22</sub>FN<sub>3</sub>O<sub>3</sub><br>10 mg/L</td><td>Fe/Co/Zn@C-NCNFs-800</td><td>Pt</td><td>pH: 3, Na<sub>2</sub>SO<sub>4</sub>: 0.10 mol/L, current: 40 mA</td><td>47.1% TOC removal after 150 min<br>99.33% degradation after 30 min</td><td>(Xian Li et al., 2022b)</td></tr><tr><td>Ciprofloxacin<br>C<sub>17</sub>H<sub>18</sub>FN<sub>3</sub>O<sub>3</sub><br>20 mg/L</td><td>Mn<sub>3</sub>Co<sub>3.3</sub>@C-GF</td><td>Pt</td><td>pH: 3, catalyst concentration: 0.02 mM, Na<sub>2</sub>SO<sub>4</sub>: 0.05 M, air flow rate: 350 mL.min<sup>-1</sup><br>Catalyst: FeSO<sub>4</sub>.7H<sub>2</sub>O</td><td>99.8% removal in 60 min</td><td>(Huang et al., 2022)</td></tr><tr><td>Sulfamethoxazole<br>C<sub>10</sub>H<sub>11</sub>N<sub>3</sub>O<sub>3</sub>S<br>10 mg/L</td><td>Cu<sub>0.33</sub>Fe<sub>0.67</sub>NBDC-300/GF</td><td>Pt</td><td>pH: 5.6, Na<sub>2</sub>SO<sub>4</sub>: 0.05 mol/L, current density: 7.5 mA/cm<sup>2</sup>, O<sub>2</sub> flow rate: 0.6 L/min, temperature: 25 °C</td><td>100% removal within 45 min</td><td>(Fu et al., 2022)</td></tr><tr><td>Penicillin G<br>C<sub>16</sub>H<sub>18</sub>N<sub>2</sub>O<sub>4</sub>S<br>50 mg/L</td><td>Carbon felt</td><td>MMO-Ti/RuO<sub>2</sub>IrO</td><td>pH: 3, catalyst concentration: 0.5 mM, current density: 120 mA.cm<sup>-2</sup>, temperature: 25 °C<br>Catalyst: FeSO<sub>4</sub>.7H<sub>2</sub>O</td><td>98% removal in 480 min</td><td>(Gonzaga et al., 2021b)</td></tr><tr><td>Sulfanilamide<br>C<sub>6</sub>H<sub>8</sub>N<sub>2</sub>O<sub>2</sub>S<br>86.1 mg/L</td><td>Carbon foam</td><td>Platinum gauze</td><td>pH: 3, catalyst concentration: 0.5 mM, Na<sub>2</sub>SO<sub>4</sub>: 0.05 M, current: 60 mA, conductivity: 0.1 µS/cm<br>Catalyst: Fe<sup>2+</sup></td><td>92% COD removal in 240 min</td><td>(Ganiyu et al., 2021)</td></tr><tr><td>Tetracycline<br>C<sub>22</sub>H<sub>24</sub>N<sub>2</sub>O<sub>8</sub><br>50 mg/L</td><td>3.6-Cu/CuFe<sub>2</sub>O<sub>4</sub>/CB@GF</td><td>Pt</td><td>pH: 3, catalyst concentration: 0.84 ± 0.11 mg/L, Na<sub>2</sub>SO<sub>4</sub>: 0.05 M, current:</td><td>80% removal in 120 min</td><td>(Cui et al., 2021)</td></tr></tbody></table>

(continued on next page)
<table><caption>Table 5 (continued)</caption><thead><tr><th>Antibiotic</th><th>Cathode material</th><th>Anode material</th><th>Experimental conditions</th><th>Removal efficiency</th><th>Refs.</th></tr></thead><tbody><tr><td>Amoxicillin<br/>C<sub>16</sub>H<sub>19</sub>N<sub>3</sub>O<sub>5</sub>S<br/>30 mg/L</td><td>Graphite recovered from used batteries</td><td>Stainless steel</td><td>30 mA, conductivity: 0.1 µS/cm<br/>Catalyst: Cu/CuFe<sub>2</sub>O<sub>4</sub></td><td>95% removal in 180 min</td><td>(Kadji et al., 2021)</td></tr><tr><td>Tetracycline<br/>C<sub>22</sub>H<sub>24</sub>N<sub>2</sub>O<sub>8</sub><br/>50 mg/L</td><td>BDD</td><td>CFP@MnO<sub>2</sub>-Fe<sub>3</sub>O<sub>4</sub>-C</td><td>pH: 3, Na<sub>2</sub>SO<sub>4</sub>: 0.1 mol/L, current density: 5 mA/cm<sup>2</sup></td><td>100% removal in 120 min</td><td>(Dong et al., 2021)</td></tr><tr><td>Tetracycline<br/>C<sub>22</sub>H<sub>24</sub>N<sub>2</sub>O<sub>8</sub><br/>20 mg/L</td><td>ACF</td><td>Pt</td><td>pH: 7, catalyst concentration: 0.3 g/L, Na<sub>2</sub>SO<sub>4</sub>: 1.42 g (200 mL), current: 100 mA, room temperature<br/>Catalyst: Fe<sub>2</sub>Co<sub>1</sub>/NPC</td><td>91% removal in 60 min<br/>90.3% TOC removal in 240 min.</td><td>(Hu et al., 2021)</td></tr><tr><td>Tetracycline<br/>C<sub>22</sub>H<sub>24</sub>N<sub>2</sub>O<sub>8</sub><br/>17.7 mg/L</td><td>Ti sheet</td><td>Fe<sub>2</sub>O<sub>3</sub>-in-CNT</td><td>pH: 6.8, Na<sub>2</sub>SiO<sub>3</sub>: 10 mmol/L, air flow rate: 1.5 mL/min</td><td>97.5% ± 3% removal in 3 h</td><td>(Guo et al., 2021a)</td></tr><tr><td>Sulfamethazine<br/>C<sub>12</sub>H<sub>14</sub>N<sub>4</sub>O<sub>2</sub>S<br/>10 mg/L</td><td>Carbon felt modified with carbon black</td><td>DSA (RuO<sub>2</sub>) or Pt</td><td>pH: 4, catalyst concentration: 25 mg/L, Na<sub>2</sub>SO<sub>4</sub>: 50 mM, current: 25 mA<br/>Catalyst: FeCu@PC-800/6</td><td>94.7% removal in 30 min<br/>(DSA (RuO<sub>2</sub>) as anode)<br/>91.5% SMT removal in 30 min (Pt as anode)</td><td>(Du et al., 2021)</td></tr><tr><td>Levofloxacin<br/>C<sub>18</sub>H<sub>20</sub>FN<sub>3</sub>O<sub>4</sub><br/>80 mg/L</td><td>Hydrothermally-activated graphite felt GF-9</td><td>Pt sheet</td><td>pH: 3, catalyst concentration: 0.15 mM, Na<sub>2</sub>SO<sub>4</sub>: 0.05 M, current density: 8 mA.cm<sup>-2</sup>, O<sub>2</sub> flow rate: 0.2 L.min<sup>-1</sup><br/>Catalyst: Fe<sup>2+</sup></td><td>Complete degradation in 10 min</td><td>(Liu et al., 2020a)</td></tr><tr><td>Sulfamethazine<br/>C<sub>12</sub>H<sub>14</sub>N<sub>4</sub>O<sub>2</sub>S<br/>10 mg/L</td><td>Carbon black modified graphite felt</td><td>Dimensionally-stable anode with IrO<sub>2</sub> and RuO<sub>2</sub> coating</td><td>pH: 4, catalyst concentration: 0.224 g/L, Na<sub>2</sub>SO<sub>4</sub>: 0.05 M, current: 50 mA<br/>Catalyst: Fe<sup>2+</sup> Co-catalyst: MoS<sub>2</sub> (catalyst)<br/>0.224 g/L</td><td>Complete removal in 10 min</td><td>(Tian et al., 2021)</td></tr><tr><td>Tetracycline<br/>hydrochloride<br/>C<sub>22</sub>H<sub>25</sub>ClN<sub>2</sub>O<sub>8</sub><br/>50 mg/L</td><td>NPC—CNTs/ACF</td><td>Pt</td><td>pH: 3, catalyst concentration: 0.4 mM, Na<sub>2</sub>SO<sub>4</sub>: 0.05 M, current density: 20 mA.cm<sup>-2</sup>, temperature: 25 °C<br/>Catalyst: Fe<sup>2+</sup></td><td>Total degradation in 90 min<br/>96% TOC removal</td><td>(Tao et al., 2020)</td></tr><tr><td>Tetracycline<br/>C<sub>22</sub>H<sub>24</sub>N<sub>2</sub>O<sub>8</sub><br/>20 mg/L</td><td>Cu-doped Fe@Fe<sub>2</sub>O<sub>3</sub>-Ni foam</td><td>Ti-TiO<sub>2</sub>-RuO<sub>2</sub></td><td>pH: 3, Na<sub>2</sub>SO<sub>4</sub>: 0.05 M, current density: 40 mA/cm<sup>2</sup>, air flow rate: 0.1 L/min, temperature: 25 °C</td><td>98.1% removal in 120 min</td><td>(Luo et al., 2020)</td></tr><tr><td>Sulfamethazine<br/>C<sub>12</sub>H<sub>14</sub>N<sub>4</sub>O<sub>2</sub>S<br/>50 mg/L</td><td>Pt</td><td>OCNTs-PBA</td><td>pH: 6.5, Na<sub>2</sub>SO<sub>4</sub>: 0.05 mol/L, O<sub>2</sub> flow rate: 100 mL/min, potential: -0.5 V</td><td>100% removal in 30 min</td><td>(Cheng et al., 2020)</td></tr><tr><td>Sulfamethazine<br/>C<sub>12</sub>H<sub>14</sub>N<sub>4</sub>O<sub>2</sub>S<br/>10 mg/L</td><td>CB-CF</td><td>Ti-RuO<sub>2</sub></td><td>pH: 3, catalyst concentration: 0.05 g/L, Na<sub>2</sub>SO<sub>4</sub>: 0.05 M, current: 25 mA<br/>Catalyst: Fe/Fe<sub>3</sub>C@CP</td><td>99% removal in 30 min</td><td>(Du et al., 2020a)</td></tr><tr><td>Ofloxacin<br/>C<sub>18</sub>H<sub>20</sub>FN<sub>3</sub>O<sub>4</sub><br/>36.1 mg/L</td><td>Fe<sup>II</sup>Fe<sup>III</sup>LDH-CF</td><td>BDD</td><td>pH: 7, Na<sub>2</sub>SO<sub>4</sub>: 0.05 M, current density: 12.5 mA/cm<sup>2</sup>, air flow rate: 0.75 L/min</td><td>100% removal in 30 min<br/>100% TOC degradation (480 min)</td><td>(Yang et al., 2020)</td></tr><tr><td>Cephalexin<br/>C<sub>16</sub>H<sub>17</sub>N<sub>3</sub>O<sub>4</sub>S<br/>50 mg/L</td><td>Air diffusion cathode</td><td>IrO<sub>2</sub></td><td>pH: 3, catalyst concentration: 1 g/L, Na<sub>2</sub>SO<sub>4</sub>: 0.05 M, current: 125 mA, air flow rate: 0.6 L/min<br/>Catalyst: Chalcopyrite</td><td>94% removal in 15 min<br/>44% TOC degradation (300 min)</td><td>(Droguett et al., 2020)</td></tr><tr><td>Sulfamethizole<br/>C<sub>9</sub>H<sub>10</sub>N<sub>4</sub>O<sub>2</sub>S<sub>2</sub><br/>25 mg/L</td><td>Carbone felt</td><td>BDD</td><td>pH: 6, Na<sub>2</sub>SO<sub>4</sub>: 0.01 M, current: 300 mA, air flow rate: 1 L/min<br/>Catalyst: FeCl<sub>3</sub>-modified perlite</td><td>100% removal in 15 min<br/>65% TOC decay (120 min)</td><td>(Puga et al., 2020)</td></tr><tr><td>Nafcillin<br/>C<sub>21</sub>H<sub>22</sub>N<sub>2</sub>O<sub>5</sub>S<br/>36 mg/L</td><td>Carbon-PTFE air diffusion</td><td>BDD</td><td>pH: 7, catalyst concentration: 1 g/L, Na<sub>2</sub>SO<sub>4</sub>: 0.05 M, current density: 5 mA/cm<sup>2</sup><br/>Catalyst: Fe/Cu bimetallic nanoparticles</td><td>100% removal in 7 min</td><td>(Campos et al., 2020)</td></tr><tr><td>Cefazolin<br/>C<sub>14</sub>H<sub>14</sub>N<sub>8</sub>O<sub>4</sub>S<sub>3</sub><br/>20 mg/L</td><td>CUFeNLDH-CNTs-graphite</td><td>Pt sheet</td><td>pH: 6, Na<sub>2</sub>SO<sub>4</sub>: 0.02 M, current: 300 mA, air flow rate: 10 L/h</td><td>89.9% removal in 100 min<br/>70.1 COD degradation (300 min)</td><td>(Ghasemi et al., 2020)</td></tr><tr><td>Cefotaxime<br/>C<sub>16</sub>H<sub>17</sub>N<sub>5</sub>O<sub>7</sub>S<sub>2</sub><br/>50 mg/L</td><td>Ni@NCNT</td><td>Ti/CNT/SnO<sub>2</sub>-Sb-Er</td><td>pH: 3, catalyst concentration: 1 mM, Na<sub>2</sub>SO<sub>4</sub>: 50 mM, current density: 30 mA.cm<sup>-2</sup>, persulfate PS: 1 mM<br/>Catalyst: Fe<sup>2+</sup></td><td>Complete degradation in 60 min<br/>81.6% TOC removal in 240 min</td><td>(Lei et al., 2020)</td></tr><tr><td>Sulfathiazole<br/>C<sub>9</sub>H<sub>9</sub>N<sub>3</sub>O<sub>2</sub>S<sub>2</sub><br/>500 mg/L</td><td>Nitrogen-doped CN-3</td><td>BDD</td><td>pH: 3, catalyst concentration: 100 µM, Na<sub>2</sub>SO<sub>4</sub>: 0.05 M, current: 50 mA, air flow rate: 0.4 L.min<sup>-1</sup><br/>Catalyst: Fe<sup>2+</sup></td><td>98.25 ± 0.14% removal in 180 min<br/>70.57 ± 0.27% TOC removal in 180 min</td><td>(Zhu et al., 2020)</td></tr><tr><td>Sulfathiazole<br/>C<sub>9</sub>H<sub>9</sub>N<sub>3</sub>O<sub>2</sub>S<sub>2</sub><br/>300 mg/L</td><td>Waste-wood-derived biochar (WDC)-900</td><td>BDD</td><td>pH: 8, catalyst concentration: 4 mM, current: 100 mA, air flow rate: 500 mL/min, pyrophosphate:50 mM<br/>Catalyst: Fe<sup>2+</sup></td><td>Approximately 92.6% degradation in 360 min</td><td>(Deng et al., 2019)</td></tr></tbody></table>

(continued on next page)
<table><caption>Table 5 (continued)</caption><thead><tr><th>Antibiotic</th><th>Cathode material</th><th>Anode material</th><th>Experimental conditions</th><th>Removal efficiency</th><th>Refs.</th></tr></thead><tbody><tr><td>Gentamicin<br>C21H43N5O7<br>20 mg/L</td><td>Graphite plate</td><td>Pt sheet</td><td>pH: 6, catalyst concentration: 1.25 g/L, Na2SO4: 0.05 M, current: 400 mA, air flow rate: 10 L/h<br>Catalyst: Cu–Fe–NLDH</td><td>91.3% removal in 100 min</td><td>(Ghasemi et al., 2019)</td></tr><tr><td>Ciprofloxacin<br>C17H18FN3O3<br>50 mg/L</td><td>RGO–Ce/WO3/CF<br>(Carbon felt modified with RGO–Ce/WO3)</td><td>Pt</td><td>pH: 3, catalyst concentration: 0.1 mM, Na2SO4: 0.05 M, current: 300 mA, O2 flow rate: 100 mL·min<sup>-1</sup>, conductivity: 0.08 mS·cm<sup>-1</sup><br>Catalyst: Fe2+</td><td>94.73% degradation in 10 min<br>100% after 60 min</td><td>(Mi et al., 2019)</td></tr><tr><td>Sulfadiazine<br>C10H10N4O2S<br>10 mg/L</td><td>Graphene-modified<br>EEGr</td><td>Dimensionally-stable<br>a</td><td>pH: 7.13, catalyst concentration: 0.4 mmol/L, current: 50 mA, air flow rate: 10.5 mL/min<br>Catalyst: Fe2+</td><td>Complete degradation in 30 min</td><td>(Ren et al., 2019)</td></tr><tr><td>Sulfathiazole<br>C10H10N4O2S<br>50 mg/L</td><td>Porous graphite</td><td>Ni</td><td>pH: 3, catalyst concentration: 1 g/L, Na2SO4: 0.05 M, current density: 7.5 mA/cm<sup>2</sup>, air flow rate: 1.5 L·min<sup>-1</sup>, conductivity: 0.1 µS/cm<br>Catalyst: Zero-valent iron NPs</td><td>100% removal in 5 min</td><td>(Chen et al., 2019b)</td></tr><tr><td>Metronidazole<br>C6H9N3O3<br>80 mg/L</td><td>Carbon felt</td><td>Pt gauze</td><td>pH: 3, catalyst concentration: 0.5 g/L, Na2SO4: 0.05 M, current: 300 mA, air flow rate: 10 L/h<br>Catalyst: Fe3O4–GO</td><td>100% removal in 15 min<br>73% TOC degradation (300 min)</td><td>(Görmez et al., 2019)</td></tr><tr><td>Ciprofloxacin<br>C17H18FN3O3<br>33.1 mg/L</td><td>MnCo2O4–CF</td><td>Pt plate</td><td>pH: 3, catalyst concentration: 0.1 mM, Na2SO4: 0.05 M, current: 300 mA, O2 flow rate: 50 mL/min<br>Catalyst: Fe2+</td><td>100% removal in 90 min</td><td>(Mi et al., 2019b)</td></tr><tr><td>Metronidazole<br>C6H9N3O3<br>200 mg/L</td><td>P-CNTs</td><td>IrO2</td><td>pH: 3.7, catalyst concentration: 0.78 g/L, Na2SO4: 0.1 M, potential: -0.7 V<br>Catalyst: Fe2+</td><td>99.9% removal in 30 min</td><td>(Xia et al., 2019)</td></tr><tr><td>Ciprofloxacin<br>C17H18FN3O3<br>50 mg/L</td><td>Ce3ZrFe4O14-x/CF<br>(3.8 wt%)</td><td>Pt</td><td>pH: 3, Na2SO4: 50 mM, current: 100 mA, O2 flow rate: 100 mL·min<sup>-1</sup>, temperature: 25 ± 2 °C, O2 flow rate: 100 mL·min<sup>-1</sup></td><td>100% removal in 180 min (96.9% at 2 min)<br>92.0% TOC degradation after 420 min</td><td>(Yi Li et al., 2019b)</td></tr><tr><td>Levofloxacin<br>C18H20FN3O4<br>50 mg/L<br>Trimethoprim<br>C14H18N4O3<br>50 mg/L</td><td>Graphite felt</td><td>DSA<br>(Ti/RuO2–IrO2)</td><td>pH: 3, catalyst concentration: 50 mg/L, Na2SO4: 50 mM, current: 200 mA, air flow rate: 0.5 L/min<br>Catalyst: Pd–Fe3O4 nanoparticles</td><td>Total removal of LEV in 30 min<br>Total removal of TMP in 120 min<br>99% TOC degradation in 480 min</td><td>(Zhou et al., 2018b)</td></tr><tr><td>Amoxicillin<br>C16H19N3O5S<br>20 mg/L</td><td>Graphite felt</td><td>Pt sheet</td><td>pH: 3, catalyst concentration: 1 g/L, Na2SO4: 0.01 M, current: 300 mA, air flow rate: 1 L/min<br>Catalyst: Nano–Fe2O3</td><td>98.2% removal in 60 min</td><td>(Kalantary et al., 2018)</td></tr><tr><td>Metronidazole<br>C6H9N3O3<br>70 mg/L</td><td>Graphite felt</td><td>Pt sheet</td><td>pH: 3, catalyst concentration: 1 g/L, Na2SO4: 0.05 M, current: 200 mA, air flow rate: 1 L/min<br>Catalyst: Nano–Fe2O3</td><td>92.3% removal in 30 min</td><td>(Rahmatinia and Rahmatinia, 2018)</td></tr><tr><td>Sulfamethoxazole<br>C10H11N3O3S<br>50,7 mg/L</td><td>FeIIFeIIILDH–CF</td><td>Ti4O7</td><td>pH: 3, Na2SO4: 0.05 M, current density: 7.5 mA/cm<sup>2</sup>, air flow rate: 1 L/min</td><td>100% removal in 40 min<br>97% TOC degradation in 480 min</td><td>(Ganiyu et al., 2018a)</td></tr><tr><td>Tetracycline<br>C22H24N2O8<br>50 mg/L</td><td>Fe3O4/GDE<br>(Fe3O4/gas diffusion electrode)</td><td>Pt</td><td>pH: 3, Na2SO4: 0.05 M, potential: -0.8 V</td><td>94.2% removal in 100 min (total removal in 120 min)</td><td>(Zhang et al., 2017)</td></tr><tr><td>Ciprofloxacin<br>C17H18FN3O3<br>50 mg/L</td><td>3D CeO2/RGO</td><td>Pt</td><td>pH: 3, catalyst concentration: 0.1 mM, Na2SO4: 0.05 M, current: 400 mA, O2 flow rate: 100 mL·min<sup>-1</sup>, temperature: 23 ± 2 °C<br>Catalyst: Fe2+</td><td>Total removal<br>91% TOC degradation in 390 min</td><td>(Zhang et al., 2017b)</td></tr><tr><td>Tetracycline<br>C22H24N2O8<br>88.8 mg/L</td><td>Carbon felt</td><td>BDD</td><td>pH: 3, catalyst concentration: 2 g/L, Na2SO4: 0.05 M, current: 300 mA, air flow rate: 1 L/min<br>Catalyst: Pyrite</td><td>100% removal in 20 min<br>96% TOC degradation in 480 min</td><td>(Barhoumi et al., 2017)</td></tr><tr><td>Enoxacin<br>C15H17FN4O3<br>80 mg/L</td><td>Carbon felt</td><td>BDD</td><td>pH: 3, catalyst concentration: 1.7 g/L, Na2SO4: 0.05 M, current: 300 mA, air flow rate: 1 L/min<br>Catalyst: Fe2O3–modified kaolin</td><td>100% removal in 15 min<br>98% TOC degradation in 420 min</td><td>(Özcan et al., 2017)</td></tr><tr><td>Tetracycline<br>C22H24N2O8<br>89 mg/L</td><td>Carbon felt</td><td>BDD</td><td>pH: 5.94, catalyst concentration: 1 g/L, Na2SO4: 0.05 M, current: 300 mA, air flow rate: 0.6 L/min, room temperature<br>Catalyst: Chalcopyrite</td><td>100% removal in 7 min<br>85% TOC degradation in 120 min</td><td>(Barhoumi et al., 2017a)</td></tr></tbody></table>

(continued on next page)
<table><caption>**Table 5 (continued)**</caption><thead><tr><th>Antibiotic</th><th>Cathode material</th><th>Anode material</th><th>Experimental conditions</th><th>Removal efficiency</th><th>Refs.</th></tr></thead><tbody><tr><td>Levofloxacin<br>C<sub>18</sub>H<sub>20</sub>FN<sub>3</sub>O<sub>4</sub><br>50 mg/L</td><td>Modified<br>graphite felt<br>GF-10</td><td>Ti/RuO<sub>2</sub>-IrO<sub>2</sub> (DSA)</td><td>pH: 3, catalyst concentration: 0.2 mM,<br>Na<sub>2</sub>SO<sub>4</sub>: 50 mM, current: 400 mA<br>Catalyst: Fe<sup>2+</sup></td><td>90% removal in<br>15 min</td><td>(Liu et al., 2017)</td></tr><tr><td>Sulfamethazine<br>C<sub>12</sub>H<sub>14</sub>N<sub>4</sub>O<sub>2</sub>S<br>55.6 mg/L</td><td>Carbon felt</td><td>BDD</td><td>pH: 3, catalyst concentration: 2 g/L,<br>Na<sub>2</sub>SO<sub>4</sub>: 0.05 M, current:<br>300 mA, air flow rate:<br>1 L/min, room temperature<br>Catalyst:<br>Pyrite</td><td>100% removal<br>in 40 min</td><td>(Barhoumi et al.,<br>2016)</td></tr></tbody></table>

**Fe-GAC:** iron-loaded granular activated carbon; **CB-Bio-FeMnOx:** carbon black-biogenic Fe-Mn oxides; **Pt:** platinum; **LDH:** layered double hydroxide; **BDD:** boron doped-diamond; **NDB:** nitrogen-doped biochar; **Fe/Co/Zn@C—NCNFs-800:** Fe/Co/Zn-tri-metal co-doped carbon nanofibers; **GF:** graphite felt, **NBDC:** 2-aminotерephthalic acid; **MMO:** mixed metal oxide; **CB:** carbon black; **CFP:** carbon fiber paper **ACF:** activated carbon fiber; **CNT:** carbon nanotube; **NPC:** nitrogen-doped porous carbon; **CF:** carbon felt; **PC:** porous carbon; **Fe<sup>II</sup>Fe<sup>III</sup> LDH—CF:** Fe<sup>II</sup>Fe<sup>III</sup>-layered double hydroxide modified carbon felt; **NLDH:** nano-layered double hydroxide; **CBAC:** carbon black air diffusion.

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Sources of antibiotics in the environment.</figcaption></figure>

antibiotic degradation was explored using Scopus data. The search specifically focused on the term string “Advanced AND oxidation AND processes AND antibiotic AND removal”. Only research papers and reviews written in English were collected; other sources such as book chapters, conference papers, and data papers were excluded. This study covered the last five years, i.e. 2019 to 2023.

Based on the results output of VOSviewer software (Fig. 2), the analysis provided evidence for homogeneous and heterogeneous AOPs including ozonation, electro-Fenton, photo-Fenton and Fenton processes. The presence of heterogeneous catalytic processes was confirmed specifically with hematite, nanoscale zero-valent iron, and zero-valent iron. The occurrence of antibiotics in the environment, such as in rivers and potable water, was linked to municipal and urban wastewater.

## 4. The conventional Fenton reaction for antibiotics removal

Fenton processes represent a major subset of AOPs for the treatment of organic pollutants. They involve using a mixture of two reagents, ferrous ion $Fe^{2+}$ and hydrogen peroxide $H_2O_2$. These two reagents, known as ‘Fenton reagents’, react with each other to generate hydroxide radicals $OH^*$ and ferric ions $Fe^{3+}$. In the presence of $H_2O_2$, $Fe^{3+}$ can reduce to $Fe^{2+}$ (Brillas, 2022b) (Eqs. (1)–(3)). The oxidation mechanism of the Fenton process is shown in Fig. 3.

$$Fe^{2+} + H_2O_2 \rightarrow OH^- + OH^* + Fe^{3+} \quad (1)$$

$$Fe^{3+} + H_2O_2 \rightarrow Fe^{2+} + OH_2^* + H^+ \quad (2)$$

$$Fe^{3+} + OH_2^* \rightarrow Fe^{2+} + O_2 + H^+ \quad (3)$$

The performance and efficiency of the Fenton process depends on the concentration of ferrous ion and hydrogen peroxide and on the pH of the
<figure><img src="image_4.png" /><figcaption>Fig. 2. VOSviewer analysis of abstract keywords associated with advanced oxidation processes for antibiotic removal.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 3. Reaction pathway of the Fenton process (Zhang et al., 2019).</figcaption></figure>

medium (a pH range of 2 to 3 is largely recommended). Fenton-based processes are classified as homogeneous or heterogeneous according to the form of Fe used in the Fenton reaction.

The homogeneous Fenton process is based on the use of ferrous salts including ferrous sulfate in acidic medium. Verma and Haritash (2019) reported that ferrous sulfate effectively degraded 10 mg/L of
Heterogeneous Fenton catalysts

<figure><img src="image_6.png" /><figcaption>Fig. 4. Heterogeneous Fenton catalysts (Nidheesh, 2015b).</figcaption></figure>

amoxicillin. Complete degradation was achieved in 12 min at pH 3 using 375 mg/L of

$H_2O_2$. This process thus demonstrated high degradation efficiency and operational simplicity. However, it also generated a large amount of iron-containing sludge, thus posing a serious challenge for disposal. To overcome this disadvantage, heterogeneous Fenton catalysts are employed (Wang and Zhuan, 2020a).

As shown in Fig. 4, there are a variety of heterogeneous Fenton catalysts used in Fenton processes to remove various antibiotics. Heterogeneous catalysts such as iron minerals, zero-valent iron, iron and iron oxide loaded materials, soil, and waste materials can be integrated into the Fenton process to improve the antibiotic degradation process (Nidheesh et al., 2017). Iron minerals including magnetite (Fe₃O₄), hematite (Fe₂O₃), and pyrite (FeS₂) are generally considered as non-toxic, biodegradable, eco-friendly catalyst materials that are common in soil and extensively employed for the removal of emerging pollutants, as they offer inherent stability, reusability, and easier separation than the other ferrous salts (Thomas et al., 2021). Hassani et al. (2018) investigated the degradation of ciprofloxacin using magnetite nanoparticles. They produced magnetite nanoparticles using a mechanical ball mill, which has numerous advantages such as ease of operation, replicability, cost-effectiveness, and the ability to produce substantial quantities of nanoparticles. Ciprofloxacin was chosen as the specific target as it is a recalcitrant contaminant. The findings revealed that 89% of 10 mg/L of ciprofloxacin was removed under optimum conditions, i.e. pH 3, 12 mM $H_2O_2$ and 1.75 g/L of catalyst. Nie et al. (2022) synthesized hexapod-like pyrite nanosheets as a heterogeneous Fenton catalyst using a simple hydrothermal method. The rationale for

using synthesized pyrite was to avoid the presence of large quantities of impurities and harmful heavy metals such as metalloids in the natural pyrite that could potentially release hazardous ions. They selected ciprofloxacin as a representative antibiotic to examine the heterogeneous Fenton catalytic activity of the synthesized pyrite. They tested a wide pH interval (in the range 2–11) with 1 g/L of the prepared pyrite and 2 mmol/L of $H_2O_2$. The maximum ciprofloxacin degradation rate was obtained at an initial pH of 4, leading to complete degradation within 10 min (Table 1). Other heterogeneous Fenton processes applied on antibiotics have reaction times ranging from 5 min to 4 h (see Table 1). The hexapod-like pyrite nanosheet mineral clusters function as a dual catalyst. They offer a high surface area for ciprofloxacin adsorption, provide surface catalysis at the solid–liquid interface, and contribute to a homogeneously-catalyzed process through leached/dissolved $Fe^{2+}/Fe^{3+}$ leading to the removal of both adsorbed and aqueous-phase ciprofloxacin. In a related study, Sang et al. (2022) found that pyrite, at a concentration of 1.5 g/L, along with 5 mM of $H_2O_2$ at pH 3, led to an 83.2% degradation of ciprofloxacin within 20 min.

Zero-valent iron (ZVI) has been shown to be effective in treating emerging pollutants such as antibiotics. It has a standard E potential ($Fe^{2+}/Fe^{0}$) of –440 mV and the ability to release electrons in the presence of $H_2O_2$ or $O_2$, resulting in the generation of Fe(II) species as indicated in Eqs. (4) and (5) (Zhou et al., 2019).

$$Fe^{0} + O_{2} + 2H^{+} \rightarrow Fe(II) + H_{2}O_{2} \quad (4)$$

$$Fe^{0} + H_{2}O_{2} + 2H^{+} \rightarrow Fe(II) + 2H_{2}O \quad (5)$$
<figure><img src="image_7.png" /><figcaption>Fig. 5. Mechanisms of the homogeneous photo-Fenton reaction (Gou et al., 2021a).</figcaption></figure>

Zhang and Zhou (2019) reported that nanoscale zero-valent iron (nZVI) modified with CeO₂ (Fe⁰/CeO₂) could effectively mitigate the agglomeration of nZVI and potentially enhance the catalytic activity of the composite material via synergistic effects. The modification was performed using a precipitation method. The authors evaluated the material's catalytic performance based on the removal of 100 mg/L of tetracycline. The results showed that 0.1 g/L of Fe⁰/CeO₂ was able to degrade more than 80% of tetracycline whereas nZVI only degraded 45% at pH 5.8, 25 °C, and 100 mM H₂O₂. However, at pH 3, both the nZVI and Fe⁰/CeO₂ systems led to the best degradation efficiency of 93% removal in 60 min.

Iron and iron loaded materials have demonstrated significant effectiveness as heterogeneous Fenton catalysts for the degradation of various organic pollutants. A diverse array of materials, including activated carbon, biosorbents, clay, alumina and silica, have been employed as efficient supports for iron and iron oxides, enhancing the overall efficiency of the heterogeneous Fenton process. He et al. (2022) demonstrated the effectiveness of iron-loaded granular activated carbon in tetracycline degradation (10 mg/L), achieving 92.6% within 60 min at pH 3, with 9 mmol/g of H₂O₂. Additionally, Wang et al. (2020) showed that Fe²⁺-zeolite facilitated a 10% degradation of sulfamethoxazole (1.27 mg/L) within 5 min.

Fly ash is another promising material recognized for its high specific surface area and porous structure, making it a suitable candidate for use as heterogeneous Fenton catalyst (Nidheesh, 2015a). Although most studies have focused on its application in photo-Fenton processes (Ramírez-Franco et al., 2019), there is a notable gap in the literature regarding its effectiveness in heterogeneous Fenton reaction, specially targeting antibiotic degradation. Despite this fly ash has demonstrated potential for the degradation of various organic pollutants (Nidheesh, 2015b).

Moreover, solid materials such as montmorillonite and bentonite have also proven to be effective heterogeneous Fenton catalysts. (Ahmad et al., 2020a) found that montmorillonite, when used at pH 7 with a concentration of 0.75 g/L and 5 mL/L of H₂O₂, achieved a 60% degradation of ofloxacin (50 mg/L) within 120 min. Furthermore, modified Fe₂O₃-montmorillonite synthesized via a co-precipitation method exhibited a significantly higher degradation rate of 81% under the same conditions. This enhancement suggests that the modification of

montmorillonite whit iron oxide improves catalytic performance.

## 5. The photo-Fenton process for antibiotics removal

### 5.1. Homogeneous photo-Fenton processes

The homogeneous photo-Fenton process is based on photochemical catalysis of the Fenton reaction. The presence of UV/visible radiation increases the rate of hydroxyl radical formation and improves pollutant degradation efficiency compared to the Fenton reaction due to the reduction of Fe³⁺ to Fe²⁺; Xavier et al., 2016) (Fig. 5). The basic mechanisms of the homogeneous photo-Fenton reaction are described in the following equations:

$$ \mathrm{Fe}^{2+} + \mathrm{H}_{2}\mathrm{O}_{2} \rightarrow \mathrm{OH}^{-} + \mathrm{OH}^{\bullet} + \mathrm{Fe}^{3+} \quad k_{1} = 63-76 \ \mathrm{M}^{-1}\mathrm{s}^{-1} \quad (6) $$

$$ \mathrm{Fe}^{3+} + \mathrm{H}_{2}\mathrm{O} + \mathrm{h}\theta \rightarrow \mathrm{Fe}^{2+} + \mathrm{OH}^{\bullet} + \mathrm{H}^{+} \quad k_{2} = 3.33 \times 10^{-6} \ \mathrm{M}^{-1}\mathrm{s}^{-1} \quad (7) $$

$$ \mathrm{H}_{2}\mathrm{O}_{2} + \mathrm{h}\theta \rightarrow 2\mathrm{OH}^{\bullet} \quad k_{3} = 4.13 \times 10^{-5} \ \mathrm{s}^{-1} \quad (8) $$

$$ \text{Organics} + \text{OH}^{\bullet} \rightarrow \text{H}_2\text{O} + \text{degradation product} \rightarrow \text{H}_2\text{O} + \text{CO}_2 \quad (9) $$

Eq. (6) represents the reaction between H₂O₂ and Fe²⁺ to form Fe³⁺ and OH·. The Fe³⁺ formed in the first reaction is reduced to Fe²⁺, which is the rate-limiting step of the Fenton process as shown in Eq. (7). Eq. (8) shows that H₂O₂ is photolyzed to produce more OH· under the action of light (Gou et al., 2021a; Zhu et al., 2018). The OH· hydroxyl radicals produced react with the pollutants to degrade them (Eq. (9)).

Homogeneous photo-Fenton processes have been proven to effectively remove antibiotics, but their performances are dependent on various factors including pH, temperature, type of catalyst, concentration of hydroxide peroxide, and concentration of target compound. Optimal effectiveness is at a pH value of 3 and room temperature, and the process only requires ferrous salts and hydrogen peroxide. The primary source of iron catalyst in homogeneous photo-Fenton processes is FeSO₄·7H₂O, which is both convenient and inexpensive. Table 2 recaps the latest findings in homogeneous photo-Fenton processes. Rodrigues-Silva et al. (2022) improved the performance of a homogeneous photo-Fenton process for the degradation of isoniazid (an antibiotic used in tuberculosis treatment), and achieved approximately 99% removal of
<figure><img src="image_8.png" /><figcaption>Fig. 6. Mechanisms of the heterogeneous photo-Fenton process (Gou et al., 2021a).</figcaption></figure>

isoniazid in 10 min under optimum conditions, i.e. pH 3, 10 mg/L of ferric sulfate, and 125 mg/L of hydrogen peroxide. Ioannou-Ttofa et al. (2019) applied the homogeneous photo-Fenton process to effluent doped with 100 µg/L of ampicillin, and reported that the homogeneous photo-Fenton process was able to degrade 99.9% of ampicillin within 5 min, using 50 mg/L of H₂O₂ and maintaining the inherent pH. This approach prevents acidification of the effluent and reduces the consumption of reagents for pH adjustment, thus decreasing the operational costs of the process. The treatment was achieved using a lamp similar to solar irradiation, which presents another advantage for homogeneous photo-Fenton processes.

## 5.2. Heterogeneous photo-Fenton processes

The heterogeneous photo-Fenton process is an alternative strategy based on the use of various heterogeneous catalysts such as iron-based composite catalysts, iron oxide catalysts, iron-based semiconductors, and other typical catalysts to degrade organic pollutants such as antibiotics (Fig. 6) (Hang et al., 2022). In this approach, due to photoexcitation transfers the electron from the valence band of the catalysts to the conduction band, which results in the generation of various species on the catalyst surface, i.e. OH⁻, e⁻, electron holes (h⁺) and O₂•, that promote the degradation of organic compounds such as antibiotics. These reactions are described by Eqs. (10) to (14).

$$ \text{Catalysts} + h\theta \rightarrow h^{+} + e^{-} \quad (10) $$

$$ \text{H}_2\text{O}_2 + e^{-} \rightarrow \text{OH}^{-} + \text{OH}^{\bullet} \quad (11) $$

$$ \text{Fe}^{3+} + e^{-} \rightarrow \text{Fe}^{2+} \quad (12) $$

$$ \text{O}_2 + e^{-} \rightarrow \text{O}_2^{\bullet} \quad (13) $$

$$ h^{+} + \text{H}_2\text{O} \rightarrow \text{H}^{+} + \text{OH}^{\bullet} \quad (14) $$

Iron-based catalysts have been used to degrade antibiotics. Iron oxides including magnetite (Fe₂O₃) have attracted much interest in heterogeneous photo-Fenton processes due to their low toxicity and chemical inertness. Under irradiation, Fe₂O₃ is able to synthesize photo-generated carriers facilitating the activation of H₂O₂ and subsequent antibiotic degradation (Li et al., 2022a). α-Fe₂O₃ is an iron-based catalyst prepared by thermal decomposition of [Fe(Ofloxacin)₂Cl₂] at 600 °C under atmosphere for 2 h α-Fe₂O₃ nanoparticles have a diameter of around 60 nm and are able to degrade 59% of ciprofloxacin at low concentration of 0.16 g/L. The performance of this catalyst depends on

its morphology, structure, and crystalline phase (Hakimi and Alikhani, 2020).

The catalytic efficiency of iron oxides is unsatisfactory due to the aggregation and agglomeration of iron oxide nanoparticles. An effective solution to this problem is to immobilize iron oxides on various supports. Multi-walled carbon nanotubes (MWCNT) are among the most widely used supports due to their large specific surface area. NiFe₂O₄ MWCNTs synthesized in a single step by hydrothermal treatment showed excellent performance in the photo-Fenton degradation of sulfamethoxazole (99% in 120 min). The NiFe₂O₄-MWCNT composite was shown to have exceptional magnetic properties, allowing easy separation using an external magnet and providing the added advantage of making the material recyclable (Nawaz et al., 2020).

The main challenge in heterogeneous photo-catalysis and the photo-Fenton process is the reduction in free-radical generation due to the rapid recombination of photo-generated carriers, which in turn leads to a drop in catalytic efficiency. Associating iron oxides with semiconductors could promote the creation of heterojunction structures to separate the light-generated charge carriers and help improve the efficiency of the photo-Fenton degradation process. Recent studies have revealed that coupling titanium dioxide (TiO₂) with iron oxide (Fe₃O₄) leads to good magnetic response and high photocatalytic capacity. Fe₃O₄@void@TiO₂ nanoparticles were synthesized by successively coating amorphous SiO₂ and TiO₂ shells around a Fe₃O₄ core and applying the sol-gel strategy to form Fe₃O₄@SiO₂@TiO₂. TiO₂ was crystallized and SiO₂ was removed by calcination and ultrasonic ammonia treatment-etching treatment. The Fe₃O₄@void@TiO₂ composite was demonstrated to be an efficient catalyst with an improved antibiotic degradation rate. For instance, 0.25 g/L of Fe₃O₄@void@TiO₂ nanoparticles were able to completely degrade 40 mg/L tetracycline in 6 min at pH 3 (Du et al., 2017). Table 3 provides additional examples of heterogeneous photo-Fenton degradation of antibiotics.

# 6. The electro-Fenton process for antibiotics removal

## 6.1. Homogeneous electro-Fenton processes

The following equations describe the traditional mechanism of homogeneous electro-Fenton (EF) systems:

### Cathode

$$ \text{O}_2 + 2\text{H}^+ + 2\text{e}^- \rightarrow \text{H}_2\text{O}_2 \quad (15) $$

$$ \text{H}_2\text{O}_2 + 2\text{H}^+ + 2\text{e}^- \rightarrow 2\text{H}_2\text{O} \quad (16) $$
$$O_2 + 4H^+ + 4e^- \rightarrow 2H_2O \quad (17)$$

$$Fe^{3+} + e^{-} \rightarrow Fe^{2+} \quad (18)$$

$$2H^+ + 2e^- \rightarrow H_2 \quad (19)$$

**Anode**

$$2H_2O \rightarrow O_2 + 4H^+ + 4e^- \quad (20)$$

On the cathode, $H_2O_2$ is generated continuously by the reduction of dissolved oxygen $O_2$ (Eq. (15)). The $H_2O_2$ produced is then decomposed to free radicals by the Fenton reaction (Eq. (21)) through the addition of $Fe^{2+}$ that can be regenerated by the reduction of $Fe^{3+}$ (Eq. (18)) (Brillas, 2022a).

$$Fe^{2+} + H_2O_2 \rightarrow Fe^{3+} + OH^- + OH^* \quad (21)$$

Homogeneous electro-Fenton treatment is considered the most effective route for antibiotics remediation and mineralization, and it has been applied for both real and synthetic wastewater (Brillas, 2022b). Table 4 presents the best results obtained for homogeneous electro-Fenton treatments with a description of the experimental conditions applied. Research has tested homogeneous electro-Fenton treatment on various families of antibiotics, including fluroquinolones, β-lactams, cephalosporines, sulfonamides, and tetracyclines. Most of these studies used sodium sulfate ($Na_2SO_4$) and examined the impact of the operating parameters.

Sodium sulfate ($Na_2SO_4$) is frequently employed as an electrolyte in homogeneous electro-Fenton processes due to the low reactivity of $SO_4^{2-}$ (Liu et al., 2018). $Na_2SO_4$ and other supporting electrolytes such as sodium chloride (NaCl) and sodium nitrate (NaNO$_3$) were tested by Özcan et al. (2016) under the same conditions (0.1 mM $Fe^{3+}$, 300 mA I) for norfloxacin degradation (0.25 mM). The highest rate of norfloxacin mineralization was achieved in the presence of $Na_2SO_4$. The supporting electrolyte NaCl can be converted into active species such as $Cl_2$ and HClO according to Eqs. (22) and (23) (Jojoa-Sierra et al., 2017). These species have a lower oxidizing power than hydroxyl radicals, and thus decrease the rate of norfloxacin mineralization. Furthermore, HClO may react with hydrogen peroxide to increase the efficiency of hydroxyl radical production.

$$2Cl^- \rightarrow Cl_2 + 2e^- \quad (22)$$

$$Cl_2 + H_2O \rightarrow H^+ + Cl^- + HClO \quad (23)$$

The anode is critical, as it can produce hydroxyl radicals by oxidizing water (Eq. (24)). Sopaj et al. (2016) conducted excellent work on the impact of the anode on the treatment of 0.2 mM of sulfamethazine. They tested different anodes, i.e. platinum, boron-doped diamond (BDD), dimensionally-stable anode (DSA, Ti/RuO$_2$-IrO$_2$) and graphite felt (GF), and compared their oxidation efficiencies at pH 3 and room temperature, with 0.05 M of $Na_2SO_4$, 0.2 mM $Fe^{2+}$, in a cell using a graphite felt cathode. The study aimed to investigate the impact of current intensity in the range 2.08 mA/cm$^2$ to 20.83 mA/cm$^2$. The graphite felt showed the best performance at low current densities. Using the competition kinetics method, they determined the absolute rate constant $K_{SMT} = 2.9 \times 10^{-9} M^{-1}S^{-1}$ for the oxidation of sulfamethazine by $OH^*$. Graphite felt can only be used at low current densities (around 50 mA) due to the risk of burning at higher currents. The BDD anode proved exceptionally efficient at removing total organic carbon (TOC), mainly because BDD has a high $O_2$ evolution overpotential and low adsorption ($OH^*$). These results were confirmed by HPLC showing the release of inorganic ions at almost the theoretically expected concentrations of $SO_4^{2-}$, $NO_3^-$ and $NH_4^+$.

$$M + H_2O \rightarrow M(OH^*) + H^+ + e^- \quad (24)$$

Yahya et al. (2017) examined the effects of catalyst concentrations in the range 0.1 mM to 1 mM and current density between 60 and 500 mA

on the homogeneous electro-Fenton process. The experiment was carried out using an electrochemical cell equipped with two electrodes, a platinum anode and a large-surface-area three-dimensional carbon felt cathode. The process treated 200 mL air-saturated solution containing 0.15 mM moxifloxacin with 0.05 M $Na_2SO_4$ and 0.5 mM $Fe^{2+}$ at pH 3 at room temperature. Increasing currents from 60 to 400 mA increased the moxifloxacin degradation rate, and there was a good correlation between $OH^*$ production in the medium and the rise in concentration of generated hydrogen peroxide. Total time required to remove moxifloxacin was 60, 30, 25 and 11 min at 60, 100, 300, and 400 mA, respectively. The results also showed the impact of catalyst concentration on mixofloxacin degradation rate.

Shoorangiz et al. (2019a) used response surface methodology to study the effect of several parameters, including current density, treatment time, initial ciprofloxacin concentration, and initial pH, on the removal of both ciprofloxacin and chemical oxygen demand (COD). A trifle stainless steel plate (AISI 304) was used as sacrificial anode to provide ferrous catalyst, and a graphite felt sheet was used as cathode to continuously electro-generate $H_2O_2$. The responses of both ciprofloxacin and COD removal were significantly affected by all independent variables. Under optimum conditions (current density of 75 mA, reaction time of 20 min, initial ciprofloxacin concentration of 45 mg/L, and initial pH of 5), 95.1% of ciprofloxacin and 57% COD removal were obtained. Validation of the model was confirmed by three tests repeated under these experimental conditions.

In addition to the controlled studies conducted on synthetic solutions, tests were also performed in real wastewater to evaluate the process under practical environmental conditions. The homogeneous electro-Fenton has proven highly effective for the remediation of real wastewater, particularly for ciprofloxacin in sewage. Liu et al. (2020a) demonstrated up to 99% degradation and 81% total organic carbon reduction after 120 min of electrolysis using a ZnO—CeO$_2$/carbon felt cathode and a platinum sheet anode in a stirred tank reactor. The experiment treated 65 ml of a 50 mg/L antibiotic solution with 0.1 mM $Fe^{2+}$ at a pH range 3–7 and a cell voltage of 4 V. LC-MS analysis identified nine by-products and a reaction sequence for the antibiotic degradation was proposed. A more interesting investigation was presented by Gonzaga et al. (2021), who employed a stirred Ti/RuO$_2$-IrO$_2$ anode and carbon felt cathode to degrade 50 mg/L of penicillin G in urine, with the addition of 0.5 mM $Fe^{2+}$ at pH 3. The study operated at a current density of 120 mA.cm$^{-2}$ and identified two clear kinetic phases in the degradation process. The first phase exhibited a rapid decline in penicillin G concentration due to the action of $OH^*$ radicals produced from the Fenton reaction, followed by a slower phase associated with the breakdown of the Fe(III)-penicillin G complex. After 480 min of electrolysis, only 60% chemical oxygen demand and 50% total organic carbon reductions were achieved, as the treatment also oxidized other components present in the urine. Significant removal of urea, uric acid, and creatinine demonstrated the process's ability to oxidize various pollutants. Furthermore, the study revealed the formation of active species, such as $ClO^-$, $ClO_3^-$ and $ClO_4^-$, along with chloramines through the oxidation of $Cl^-$ present in the wastewater (Eqs. (22), (23), (25) and (26)). These by-products which are more toxic and undesirable, must be removed from treated wastewater before disposal. Active chlorine species can react with organic pollutants, competing with $OH^*$ or $M(OH^*)$, leading to the formation of chlorinated derivatives that are often more toxic and persistent than the parent compound (Brillas, 2022a).

$$HClO + 2H_2O \rightarrow ClO_3^- + 5H^+ + 4e^- \quad (25)$$

$$ClO_3^- + H_2O \rightarrow ClO_4^- + 2H^+ + 2e^- \quad (26)$$

## 6.2. The heterogeneous electro-Fenton process

The fundamental reaction mechanism of the heterogeneous electro-Fenton process is based on the Haber-Weiss chain reaction. At the
<figure><img src="image_9.png" /><figcaption>Fig. 7. Schematic representation of the heterogeneous electro-Fenton (Wang et al., 2024).</figcaption></figure>

cathode surface, free radicals are generated from the reaction between hydrogen peroxide and iron ions such as ≡ Fe<sup>III</sup>-OH and ≡ Fe<sup>II</sup>-OH to produce the complex ≡ Fe<sup>III</sup>-OH(H<sub>2</sub>O<sub>2</sub>)<sub>(s)</sub> (Ganiyu et al., 2018b).

This complex can subsequently transfer electrons to form ≡ Fe<sup>II</sup>-OH and HO<sub>2</sub><sup>*</sup>, after which ≡ Fe<sup>II</sup>-OH catalyzes the generation of hydroxyl free radicals that degrade and mineralize organic pollutants into CO<sub>2</sub> and H<sub>2</sub>O (Eqs. (27)–(31)). The critical steps of heterogeneous electro-Fenton are illustrated in Fig. 7.

$$ \equiv \mathrm{Fe}^{\mathrm{III}-\mathrm{OH}} + \mathrm{e}^{-} \rightarrow \mathrm{Fe}^{\mathrm{II}-\mathrm{OH}} \qquad (27) $$

$$ \equiv \mathrm{Fe}^{\mathrm{III}-\mathrm{OH}} + \mathrm{H}_{2}\mathrm{O}_{2} \rightarrow \equiv \mathrm{Fe}^{\mathrm{III}-\mathrm{OH}}(\mathrm{H}_{2}\mathrm{O}_{2})_{(\mathrm{s})} \qquad (28) $$

$$ \equiv \mathrm{Fe}^{\mathrm{III}-\mathrm{OH}}(\mathrm{H}_{2}\mathrm{O}_{2})_{(\mathrm{s})} \rightarrow \equiv \mathrm{Fe}^{\mathrm{II}-\mathrm{OH}}(\mathrm{HO}_{2}^{*})_{(\mathrm{s})} + \mathrm{H}^{+} \qquad (29) $$

$$ \equiv \mathrm{Fe}^{\mathrm{II}-\mathrm{OH}}(\mathrm{HO}_{2}^{*})_{(\mathrm{s})} \rightarrow \equiv \mathrm{Fe}^{\mathrm{II}-\mathrm{OH}} + \mathrm{HO}_{2}^{*} + \mathrm{H}^{+} \qquad (30) $$

$$ \equiv \mathrm{Fe}^{\mathrm{II}-\mathrm{OH}} + \mathrm{H}_{2}\mathrm{O}_{2} \rightarrow \equiv \mathrm{Fe}^{\mathrm{III}-\mathrm{OH}} + \mathrm{OH}^{*} + \mathrm{OH}^{-} \qquad (31) $$

Heterogeneous electro-Fenton processes have been developed to address key challenges associated with homogeneous electro-Fenton, such as the generation of iron sludge that necessitates further treatment and the increase in operating pH toward neutral values (Wang et al., 2022b). To overcome these challenges, researchers have proposed the use of iron catalysts and iron-functionalized cathodes, which offer distinct advantages in enhancing the efficiency of the process.

### 6.2.1. Heterogeneous electro-Fenton processes using catalysts

The decomposition of H<sub>2</sub>O<sub>2</sub> to OH<sup>*</sup> is driven by the integration of natural or synthetic iron-based heterogeneous catalysts. Natural heterogeneous catalysts such as magnetite (Fe<sub>3</sub>O<sub>4</sub>), geothite (α-FeOOH), hematite (α-Fe<sub>3</sub>O<sub>4</sub>), pyrite (FeS<sub>2</sub>) and chalcopyrite have shown interesting catalytic efficiency. Several authors have evaluated the catalytic performance of these catalysts. Barhoumi et al. (2016)) found that 2 g/L pyrite is able to completely degrade 89 mg/L of tetracycline in 20 min and 56 mg/L of sulfamethazine in 40 min in the presence of a carbon felt cathode and a boron-doped diamond anode, under optimum conditions (pH 3, 0.05 M Na<sub>2</sub>SO<sub>4</sub> and 300 mA). The efficiency of EF-pyrite treatment is due to the autoregulation of Fe<sup>2+</sup> concentration and pH from

pyrite, according to the following equations (Eqs. 32, 33 and 34).

$$ 2\mathrm{FeS}_{2}+7\mathrm{O}_{2}+2\mathrm{H}_{2}\mathrm{O} \rightarrow 2\mathrm{Fe}^{2+}+4\mathrm{SO}_{4}^{2-}+4\mathrm{H}^{+} \qquad (32) $$

$$ 2\mathrm{FeS}_{2}+15\mathrm{H}_{2}\mathrm{O}_{2} \rightarrow 2\mathrm{Fe}^{3+}+14\mathrm{H}_{2}\mathrm{O}+4\mathrm{SO}_{4}^{2-}+2\mathrm{H}^{+} \qquad (33) $$

$$ \mathrm{FeS}_{2}+14\mathrm{Fe}^{3+}+8\mathrm{H}_{2}\mathrm{O} \rightarrow 15\mathrm{Fe}^{2+}+2\mathrm{SO}_{4}^{2-}+16\mathrm{H}^{+} \qquad (34) $$

Barhoumi et al. (2017) tested the viability of chalcopyrite as a natural catalyst and iron source to degrade and mineralize tetracycline and found that 1 g/L of chalcopyrite totally degraded tetracycline in 7 min, and removed 85% TOC in 120 min, at a pH 5.94, i.e. similar to the pH of wastewater. Droguett et al. (2020) used the same quantity of catalyst (1 g/L chalcopyrite) to degrade 50 mg/L of cephalexin and achieved satisfactory results of 94% degradation in 15 min and 44% TOC removal in 300 min. The best mineralization was obtained in the chalcopyrite-catalyzed electro-Fenton process, suggesting that chalcopyrite is a suitable iron source for heterogeneous electro-Fenton processes.

In addition to natural minerals, studies have been carried out on supported solid catalysts, synthetic iron oxides, and iron-containing nanoparticles to treat organic pollutants accumulated in water. Synthesis techniques for these catalysts differ for each solid catalyst. Recently, Zhou et al. (2018) synthesized a heterogeneous Pd-Fe<sub>2</sub>O<sub>3</sub> catalyst characterized by high catalytic power. Pd supported on Fe<sub>2</sub>O<sub>3</sub> enabled complete removal of levofloxacin and trimethoprim in 30 min and 120 min, respectively, as well as near-total (>99%) mineralization of both antibiotics. Applying Pd-Fe<sub>2</sub>O<sub>3</sub> in the heterogeneous electro-Fenton process was shown to reduce the potential risks associated with toxic intermediates produced during the degradation process.

Another relevant investigation by Tian et al. (2021) focused on the use of Fe and MoS<sub>2</sub>/Fe catalysts for degrading sulfamethazine in sulfate medium and real wastewater. The authors tested the oxidation of 200 mL of air-saturated solutions containing 10 mg/L of the antibiotic in 0.05 M Na<sub>2</sub>SO<sub>4</sub>, using 0.224 g/L of Fe and/or 0.02 g/L of MoS<sub>2</sub> at pH 4, in a stirred tank reactor, operating at I = 50 mA for 30 min. The performance of the system was diminished in real wastewater compared to
the sulfate solution due to the concurrent degradation of other organic pollutants. Nonetheless, the combination of MoS₂ and Fe as co-catalysts proved more effective than Fe alone, achieving complete degradation of sulfamethazine within 15 min for the MoS₂/Fe catalyst and 30 min for Fe, yielding rate constants (k₁) of 0.13 min⁻¹ and 0.06 min⁻¹, respectively. Furthermore, the lower energy consumption of 0.012 kWh (g TOC)⁻¹ for the MoS₂/Fe catalyst indicated a favorable operational cost of 1.22 $US (kg TOC)⁻¹. However, a major limitation of this process was the low reusability of the catalyst due to corrosion of the Fe component.

### 6.2.2. Heterogeneous electro-Fenton processes with an iron-functionalized cathode

Recently, researchers have focused on developing functionalized cathode materials for heterogeneous electro-Fenton processes. These materials are designed to function simultaneously as both electrode and catalyst. Researchers have concentrated on modifying these cathodes through the addition of iron oxide and/or transition metals, or by immobilizing an iron source and/or chemically modifying the carbon-source material. Such modifications enable efficient in situ generation of hydrogen peroxide and facilitate the subsequent degradation and mineralization of various contaminants, including antibiotics.

The effectiveness of iron-functionalized cathode materials in heterogeneous electro-Fenton processes depends on several crucial proprieties. Notably these materials must exhibit chemical stability in acidic environments, as their ability to withstand such conditions is essential for maintaining catalyst activity and ensuring long term reliability, such as the Fe/N-graphene cathode, which demonstrated stability across a wide pH range, achieving 98% chloramphenicol degradation efficiency at pH 1, 92% at pH 4, and 82% at pH 13 (Song et al., 2021). Additionally, high surface area and porosity are important characteristics for optimal hydrogen peroxide generation and catalytic loading. Moreover, effective cathode materials should enable high rates of H₂O₂ production and facilitate its activation, thereby enhancing the degradation of target pollutants.

#### 6.2.2.1. Modification strategies for iron-functionalized cathodes.
Several methods have been developed to prepare iron-containing support materials or iron-coated cathode materials that are thermally stable, reusable, and capable of efficiently generating hydrogen peroxide and ferric ion. Techniques for coating cathode materials fall into two broad methods: wet chemical coating and dry chemical coating.

**Wet chemical coating methods.** Wet chemical coating methods include sol-gel, co-precipitation, and hydrothermal/solvothermal synthesis, among others.

(i) Sol-gel is a widely used method for preparing coated materials using an immersion coater equipped with an adjustable motor to control the extraction speed. This coater immerses the material in a solution containing precursor and solvent, and removes it at a set speed. Ahmadi et al. (2021) synthesized an iron(II)-doped carbonaceous aerogel as a three-dimensional cathode electrode for the degradation of ceftazidime. The synthesis involved the preparation of resorcinol-formaldehyde polymers containing iron (II) salt using the sol-gel method. Following polymerization, the material was dried at ambient temperature and carbonized in a furnace under an inert nitrogen atmosphere at 800 °C. These material exhibited a degradation efficiency of 95.34% for 40 mg/L of ceftazidime within 110 min when operated at 400 mA and a pH 4. The mineralization current efficiency was measured at 60.66% over 200 min, with energy consumption for the electro-Fenton process amounting to 1.838 kWh/kg COD removed (pH 3, ceftazidime concentration of 30 mg/L, and an applied current of 200 mA). The iron(II)-doped carbonaceous aerogel cathode demonstrated high reusability and stability

during electro-Fenton process, with the concentration of leached iron measured at less than 0.19 mg/L after eight cycles.

(ii) Co-precipitation involves adding a precipitating agent to the immersion solution, resulting in the formation of a layer on the material after it has undergone heat treatment.

(iii) Solvothermal synthesis coats cathode materials using a Teflon-lined autoclave that keeps the temperature of the precursor-plus-solvent solution higher than the boiling point of the solvent.

(iv) Hydrothermal treatment uses water as solvent. Xu et al. (2023) used hydrothermal treatment to prepare a coated cathode material based on carbon felt. The CoFeCe-LDH@ carbon felt cathode is characterized by a very strong oxygen reduction activity and strong ability to degrade doxycycline (about 95.2% of doxycycline was degraded in 180 min with 64.5% TOC degradation at pH 3). The CoFeCe-LDH@ carbon felt material also shows good reusability and the intermediates resulting from doxycycline degradation show low toxicity to typical aquatic organisms. Other authors have used the same method to fabricate a bifunctional cathode capable of generating hydrogen peroxide in situ and activating it at the same time to produce free radicals that degrade sulfamethoxazole. The electrode (Cu-Fe₂O₃/EGF) was synthesized in a single step without incorporating polymers (binders) to grow metals in-situ on the electrode surface of etched graphite felt. Qi et al. (2022) used etched graphite felt as a matrix because of its porous structure, high specific surface area, and excellent electrochemical properties. They succeeded in preparing an electrode of excellent stability and reusability capable of completely degrading sulfamethoxazole within 45 min. Yao et al. (2021) integrated a solvothermal treatment to prepare a FeII FeIII layered double hydroxide/carbon felt electrode (FeIIFeIII-LDH/CF) that they applied in an electro-Fenton system to degrade and mineralize ciprofloxacin (66 mg/L).

**Dry chemical coating.** The dry coating method is a chemical process that consists of forming a non-volatile thin film on the surface of the cathode material by reacting ions/species in the vapor state, followed by a deposition step, typically chemical vapor deposition (CVD).

## 7. Energy consumption in photo-Fenton and electro-Fenton systems

Energy consumption is a critical factor in evaluating the viability and sustainability of advanced oxidation processes, particularly in photo-Fenton and electro-Fenton processes. As these methods gain prominence in the remediation of organic pollutants, understanding their energy requirements is essential for assessing their overall efficiency and practicality for large-scale application. In the context of photo-Fenton processes, selecting an appropriate light source is critical for the effective generation of hydroxyl radicals, which are vital for organic pollutants degradation. The 300 W xenon lamp is commonly utilized due to its ability to facilitate rapid pollutant removal. Du et al. (2017) demonstrated complete degradation of tetracycline within 1 min using this lamp. Similarly, Cheng et al. (2019) reported a notable 95.42% removal of sulfamethoxazole in only 30 min, while Xu et al. (2023) achieved an 88.48% removal of oxytetracycline in 60 min, both using a 300 W xenon lamp. However, the high energy consumption associated with such powerful lamps raises significant concerns regarding the sustainability and operational costs of the process.

To address these challenges, exploring alternative irradiation sources becomes curial. Cahino et al. (2018) demonstrated the effectiveness of sunlight for tetracycline degradation, achieving 88.7% degradation in 120 min. This finding illustrates the potential of natural light as a sustainable and efficient option for the removal of contaminants, contributing to reduced energy consumption.

Energy consumption represents a significant portion of the operational costs associated with electro-Fenton process, often surpassing chemical expenses (Ismail et al., 2021). This high energy demand is
<figure><img src="image_10.png" /><figcaption>Fig. 8. Proposed degradation routes for tetracycline during the homogeneous electro-Fenton process (Chen et al., 2019a).</figcaption></figure>

mainly due to electrogeneration of H₂O₂, a curial component of the process. The overall costs can vary widely based on the design of the electro-Fenton system, the specific type of wastewater being treated, and the pollutants involved.

However, optimizing energy costs is possible through careful management of operational parameters, chemical usage, and electrode selection, which can enhance the economic feasibility of the process. Anode selection, in particular, has a major influence on energy consumption. Oturan et al. (2021) evaluated various anodes including boron-doped diamond, PbO₂, Ti₄O₇, Platinum, and dimensionally stable anode for their effectiveness in removing of the antibiotic p-amino-salicylic acid. Boron-doped diamond anode demonstrated the best

performance, achieving a maximum mineralization current efficiency of 4.8% and a low energy consumption of 0.62 kWh/g TOC removed at a current density of 50 mA. Moreover, operational conditions in electro-Fenton process affect energy consumption. Sopaj et al. (2016) assessed energy consumption during the electro-Fenton process with different anodes, such as platinum, dimensionally stable anode, boron-doped diamond, and graphite felt, alongside a graphite felt cathode, under varying currents densities of 4.16 to 41.66 mA/cm². The study revealed that energy consumption increase with higher current densities in all the anodes tested.
<figure><img src="image_11.png" /><figcaption>Fig. 9. Proposed degradation routes for tetracycline during the homogeneous photo-Fenton process. (Han et al., 2020b).</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Fig. 10. Proposed degradation routes for ciprofloxacin during the homogeneous electro-Fenton process (Shoorangiz et al., 2019b).</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>**Fig. 11.** Proposed degradation mechanisms for ciprofloxacin during the homogeneous photo-Fenton process. (Gupta and Garg, 2018; Villegas-Guzman et al., 2017; Giri and Golder, 2014).</figcaption></figure>

# 8. Antibiotic degradation products and pathways

## 8.1. Homogeneous photo-Fenton and electro-Fenton degradation products and pathways

Each antibiotic has its own degradation mechanism, but antibiotics from the same family tend to share a similar mechanism. In the homogeneous electro-Fenton process, tetracycline is dehydrated and deaminated to obtain anhydrotetracycline (P2) and P5, respectively. Tetracycline deamination is obtained due to the low N—C bond energy, and P3 and P4 are intermediates resulting from the hydroxylation and N-demethylation of tetracycline. The free radical hydroxyls thus generated continue to react with the intermediates produced until the tetracycline benzene rings open and cleave, leading to the formation of low-molecular-weight compounds that are subsequently transferred to inorganic compounds (H₂O, CO₂ and NO₃; Fig. 8; (Chen et al., 2019a)).

Different pathways of tetracycline degradation by homogeneous photo-Fenton have been described. The first route proposed is hydroxylation (1), where a hydroxyl group is added to the tetracycline molecule at the C11a-C12 bond (P2). Then dehydration follows at the C5a-C6 bond (P3). The second tetracycline degradation pathway is dehydration (C5a-C6) and N-dealkylation to produce P4, where the final degradation

pathway is an aldehyde to give P5 (Fig. 9; (Han et al., 2020b)).

Ciprofloxacin (fluoroquinolone family) was degraded using a homogeneous electro-Fenton process (Fig. 10; (Shoorangiz et al., 2019b)). As shown in Fig. 10, ciprofloxacin can undergo degradation by several routes, including oxidation of the piperazine ring, hydroxylation of the quinolone ring, defluorination, and hydroxyl substitution. These degradation mechanisms generally occur due to the generation of hydroxyl radicals during oxidation. LC-MS/MS analysis showed the presence of four products (Fig. 8).

The mechanism of ciprofloxacin degradation by homogeneous photo-Fenton processes involves three main pathways: piperazine ring opening, defluorination, and decarboxylation. The hydroxyl radical generated by the Fenton reaction attacks the ciprofloxacin molecule (P1), specifically the piperazine ring, leading to partial ring opening, formation of an amino group, and a loss of ethylene (P2). The piperazine cycle undergoes direct hydroxylation to form 1-cyclopropyl-6-fluoro-7-[(hydroxymethyl){2[(hydroxymethyl)amino]ethyl}amino]-4-oxo-1,4-dihydroquinoline-3-carboxylic acid (P3), which is considered another mechanism for cycle opening. The hydroxyl radical attacks the carboxyl group, which is a highly active site, to yield a decarboxylated and hydroxylated product, i.e. 1-cyclopropyl 6-fluoro 4-oxo 3-hydroxy 7-piperazine 1,4-dihydroquinoline (P4). Degradation products can also be
<figure><img src="image_14.png" /><figcaption>Fig. 12. Proposed degradation routes for tetracycline during the heterogeneous electro-Fenton process. (Barhoumi et al., 2017b).</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>Fig. 13. Proposed degradation mechanisms for tetracycline during the heterogeneous photo-Fenton process. (Lina Wang et al., 2023).</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>Fig. 14. Proposed degradation routes for ciprofloxacin during the heterogeneous electro-Fenton process. (Yao et al., 2021).</figcaption></figure>

obtained through direct defluorination of the ciprofloxacin ring or defluorination followed by hydroxylation (P5) (Fig. 11; (Gupta and Garg, 2018; Villegas-Guzman et al., 2017; Giri and Golder, 2014)).

## 8.2. Heterogeneous photo-Fenton and electro-Fenton degradation and pathways

The mechanism for heterogeneous electro-Fenton degradation of tetracycline is initiated by multiple hydroxylation to produce the first compound (P1), followed by demethylation of the amino group to produce P2. This compound can undergo decarboxylation at the CH3-NR position followed by demethylation. P3 and P4 are obtained by the amino group demethylation reaction. The opening of the P4 aromatic ring (A) generates product P5. The other ring continues to open to form low-molecular-weight substances (P6, P7, P8, P9, and P10) that are subsequently transferred to short-chain carboxylic acids. These acids are considered final products prior to total mineralization into CO₂, H₂O, and inorganic ions (Fig. 12; (Barhoumi et al., 2017b)).

Fig. 13 illustrates different degradation pathways for heterogeneous photo-Fenton degradation of tetracycline. 1,3-dipolar cycloaddition of the C11a-C12 double bond with radicals and rearrangement of the hydroxyl at C12 gives compound P1 (the first mechanism), which is oxidized to two compounds P2 and P4. P2 undergoes dehydration and aromatic ring attack at C15-C17 to produce P3. Dimethylation of

tetracycline in the C4 position produces P6 (the second mechanism) due to the weak C-N bond. Deamination of P6 in C2 gives P7, which is transferred by dehydroxylation in C1 and oxidation of eNHCH3 in C2 to the intermediate P8. P8 undergoes aromatic ring opening to form the low-molecular-weight intermediates P9 and P10, which are decomposed to CO₂ and H₂O. N-dealkylation of tetracycline produces P11 (the third mechanism). The hydroxyl radical can attack two positions: (i) the first position is the C2-C3 bond of P11 to give the compound P12. Cleavage of the P12 carbon ring gives P13 that undergoes radical attack to generate P14 and P15, which then produce P16 and P17, respectively; (ii) the second position is the acylamino group which produces the intermediate P18 (Usman et al., 2022). Tetracycline undergoes a dehydration reaction (the fourth mechanism) producing P22. P22 then loses its amino and methyl groups to generate P23 and P24, which undergo further oxidation to give P25. The intermediates obtained during oxidation can decompose into H₂O and CO₂ (Fig. 13; (Lina Wang et al., 2023)).

Fig. 14 shows the mechanism and intermediates of ciprofloxacin degradation by the heterogeneous electro-Fenton process. The degradation of ciprofloxacin is achieved by two routes: (i) decarboxylation of ciprofloxacin, leading to an intermediate P5 (m/z = 301), and (ii) opening of the ciprofloxacin piperazine ring by OH⁺ radicals, generating the product at peak m/z = 362 (P6). The product at peak m/z = 334 (P7) is obtained by P6 losing a formaldehyde group from the P6 group. For
<figure><img src="image_17.png" /><figcaption>Fig. 15. Proposed degradation mechanisms for ciprofloxacin during the heterogeneous photo-Fenton process. (Giri and Golder, 2019c; Gomes Júnior et al., 2018).</figcaption></figure>

the intermediate at peak *m/z* = 306 (P11) obtained by defluorination of P10 (*m/z* = 334), oxidation of P11 (*m/z* = 306) gives the compounds P12 (*m/z* = 291) and P13 (*m/z* = 263) (Yao et al., 2021).

The main degradation pathways of the fluoroquinolone ciprofloxacin by heterogeneous photo-Fenton are defluorination, decarboxylation, hydroxylation, and opening of the quinolone, cyclopropyl, and piperazine rings (Fig. 15). Here the presence of irradiation generates more free radicals than in a homogeneous system. In addition, the C–C bond of the piperazine ring is cleaved and then hydroxylated to form another compound. The cyclopropyl group of ciprofloxacin is an unstable group that would be attacked by hydroxyl radicals to produce an intermediate. The carboxyl group of the quinolone ring undergoes dehydroxylation by hydroxyl radicals, quinolone ring cleavage, and decarboxylation followed by hydroxylation, which is considered another pathway of ciprofloxacin degradation by a heterogeneous photo-Fenton process (Wang and Zhuan, 2020b; Giri and Golder, 2019c; Gomes Júnior et al., 2018).

## 9. Challenges and future directions

There is an increasing interest in overcoming the key limitations associated with Fenton processes, as referred in the numerous approaches documented in the literature. While laboratory-scale studies often achieve high efficiency, scaling these methods for industrial applications frequently uncovers significant obstacles, including technical limitations and high costs. A critical examination of existing technologies is essential for identifying these challenges. In this article, we point out areas that require further investigation in each of these approaches (Table 6).

## 10. Conclusions and future perspectives

This work investigated the degradation of antibiotics by different homogeneous and heterogeneous Fenton processes, including Fenton, photo-Fenton and electro-Fenton system conditions. We reviewed the impact of the experimental conditions and parameters on the efficiency
<caption>**Table 6**</caption>

Challenges and future research directions in advanced oxidation processes. (Saravanan et al., 2022b; Ziembowicz and Kida, 2022; Oller and Malato, 2021; Puga et al., 2021; Ribeiro and Nunes, 2021; Bello et al., 2019; Zhang and Zhou, 2019; Ganiyu et al., 2017; Ouiriemmi et al., 2017; Nidheesh and Gandhimathi, 2012).

<table><caption>**Table 6 (continued)**</caption><thead><tr><th>Advanced oxidations processes</th><th>Challenges</th><th>Directions</th></tr></thead><tbody><tr><td>Homogeneous Fenton</td><td><ul><li>Increased production of ferrous sludge</li><li>complex pH conditions</li><li>Risks associated with the storage and handling of reagents</li></ul></td><td><ul><li>Employing heterogeneous catalysts</li></ul></td></tr><tr><td>Heterogeneous Fenton</td><td><ul><li>Complex synthesis routes for catalysts</li><li>Significant expenses related to catalyst synthesis</li><li>Diminished catalytic activity</li><li>Harsh, synthetic catalytic conditions</li></ul></td><td><ul><li>Exploration of cost-effective and stable catalysts</li><li>Utilization of waste materials as catalysts</li></ul></td></tr><tr><td>Homogeneous photo-Fenton</td><td><ul><li>Significant energy consumption with UV reactors</li><li>High costs associated</li><li>Short operational lifespan of artificial UV sources</li><li>Narrow optimal pH operating range</li><li>Variability and limited availability of solar radiation</li><li>Low efficiency in separating photo-generated electron-hole pairs</li><li>High risk of environmental pollution</li><li>Fenton reaction complexes absorb light, which increases the generation of OH*</li><li>Substantial economic costs</li><li>considerable costs and risks related to handling</li><li>Difficulties in transporting and storing reagents</li></ul></td><td><ul><li>Incorporating renewable energy sources into the photo-Fenton process</li><li>Carrying out the process at a neutral pH</li><li>Replacing iron with other metal alternatives</li><li>Fixing iron on solid surfaces (heterogeneous photo-Fenton)</li><li>Utilizing iron oxides</li><li>Introducing new LED photoreactors</li><li>Examining the relationship between transformation by-products and toxicity along with further risk assessments</li><li>Substituting Fe<sup>2+</sup> with a metal free catalyst</li></ul></td></tr><tr><td>Heterogeneous photo-Fenton</td><td><ul><li>Insufficient performance of naturally produced iron oxides in photo-Fenton due to slow Fe<sup>2+</sup>/Fe<sup>3+</sup> redox recycling</li><li>Low efficiency of heterogeneous catalysts</li><li>Slightly higher impurity removal efficiency compared to homogeneous process</li><li>Limitations and challenges in real-world implementation</li><li>Decreased catalyst efficiency in subsequent cycles due to wear</li><li>Leaching of catalyst components into the medium</li></ul></td><td><ul><li>Development of efficient heterogeneous catalysts</li><li>Iron oxides supported on graphitic or other carbon materials</li><li>Structural iron oxide incorporated into inert microporous materials</li><li>Use of natural iron oxide semiconductors</li><li>Magnetic particle mixtures with high proportions of Fe<sub>2</sub>O<sub>3</sub> and small amounts of additional metal elements</li><li>Photo-Fenton ceramic membranes and alternative materials to iron oxides</li><li>Bimetallic catalysts, such as Cu-Fe oxide, operating at neutral pH</li><li>Integration of the bioelectro-Fenton process</li><li>Creation of a three-dimensional electro-Fenton system</li></ul></td></tr><tr><td>Homogeneous electro-Fenton</td><td><ul><li>Insufficient H<sub>2</sub>O<sub>2</sub> production</li><li>Limited throughput of the unit cell body</li><li>Low current density</li><li>Poor conductivity</li></ul></td><td></td></tr></tbody></table>

<table><thead><tr><th>Advanced oxidations processes</th><th>Challenges</th><th>Directions</th></tr></thead><tbody><tr><td>Heterogeneous electro-Fenton</td><td><ul><li>The addition of excessive catalyst can lead to aggregation in the bulk solution, which subsequently decrease the active surface area of the catalyst</li><li>High current levels may encourage side reactions, such as the decomposition of H<sub>2</sub>O<sub>2</sub> and the conversion of produced protons into hydrogen gas.</li><li>There is a risk of catalyst components leaching into the solution.</li><li>There is a possibility that more toxic organic compound may be produced than the original pollutant.</li></ul></td><td><ul><li>Utilization of a gas diffusion electrode with a porous structure to facilitate rapid oxygen or air access to the electrode</li><li>Improvement of solar-driven processes</li><li>Progressing these technologies towards large-scale implementation</li><li>The advancement of alternative catalysts that exhibit minimal metal leaching and enhanced stability</li></ul></td></tr></tbody></table>

of antibiotic degradation, and concluded that it is vital to optimize the key parameters used in antibiotic degradation—such as catalyst concentration, pH and hydrogen peroxide concentration—in order to improve the performance of the Fenton degradation processes. The use of heterogeneous catalysts is equally crucial for promoting the production of the free radicals needed to break down antibiotics, and thus enhance the overall degradation efficiency. Furthermore, the strategy of integrating Fenton processes with biological treatments offers perspectives for enhancing the efficiency of antibiotic degradation and mineralization. Employing a biological treatment after chemical treatment offers a potentially synergistic system of methods to mitigate the toxicity of the by-products generated during Fenton processes, and ultimately further improve the efficiency of water remediation.

## Funding information

No funding is applicable for this paper.

## CRediT authorship contribution statement

**Hasna Befenzi:** Writing – original draft, Software, Formal analysis, Data curation, Conceptualization.
**Amine Ezzariai:** Software, Investigation, Data curation.
**Tahar Mechichi:** Visualization, Data curation.
**Lamfeddal Kousni:** Validation, Supervision, Software, Data curation.
**Mohamed Hafidi:** Visualization, Supervision, Investigation, Conceptualization.
**Eric Record:** Writing – review & editing, Writing – original draft, Supervision, Resources, Investigation, Conceptualization.
**Loubna EL Fels:** Writing – review & editing, Writing – original draft, Supervision, Investigation, Data curation, Conceptualization.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence
the work reported in this paper.

## Acknowledgments

The authors thank the Marocco Ministry of Higher Education, Scientific Research and Innovation and the National Centre for Scientific and Technical Research (CNRST) for providing financial support through the "PhD-Associate Scholarship-PASS" program for the first author (H.B.). The authors also thank Campus France for providing funding through the PHC-Maghreb project 'AntiBioPol' (PHC Maghreb code: 23MAG31. Campus France code: 48644XM).

## Data availability

Data will be made available on request.

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