# Green Power Generation by Microbial Fuel Cells Using Pharmaceutical Wastewater as Substrate and Electroactive Biofilms (Bacteria/Biocarbon)

Authors: Ivonne L. Alonso-Lemus; Carlos Cobos-Reyes; Mayra Figueroa-Torres; Beatriz Escobar-Morales; K. Kunhiraman Aruna; Prabhu Akash; Fabian Fernández-Luqueño; Javier Rodríguez-Varela
Journal: Journal of Chemistry
 (2022)

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## Abstract

In this work, electroactive biofilms ofBacillus subtilis (B. subtilis) or Escherichia coli (E. coli) were supported on functionalized biocarbon (AB7-F), which was synthesized from waste leather and was used as catalysts to develop bioanodes for microbial fuel cells (MFCs). This way, bioanodes were fabricated and further evaluated in a three-electrode cell using pharmaceutical wastewater (PWW) as substrate. The electrochemical measurements showed a higher performance of the bioanode based on AB7-f+ B. subtilis to oxidize organic matter from PWW. The polarization curves in the dual-chamber MFC showed that AB7-f+ B. subtilis bioanode can generate an open circuit voltage of 602 mV and a power density of 77 mW m−2. During long-term tests of the MFC, a variation in performance was observed, with a maximum of 96.3 mW m−2 on day 7. Such variation was attributed to the development of more stable biofilm as well as consumption of some compounds metabolized by bacteria grown on the bioanode. The results showed that AB7-f+ B. subtilis can be used as bioanode for MFCs with PWW as substrate removing around 45% of the chemical oxygen demand (COD).


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## Body

## 1. Introduction

Water pollution has emerged as a life-threating global problem due to rapid growth of urbanization, industrialization, and irrational utilization of water resources. All the effluents from different types of industries like fertilizers, textiles, diary processing, petrochemical, beverages, and pharmaceutical have toxicated the water to such an extent that all lives on this planet are affected. In this respect, industrial wastewater treatment is a big challenge issue. It is believed that nearly 80–90% of all industrial wastewater in developing countries is discharged untreated, causing severe environmental damage [1]. Among the various types of effluents, the one from the pharmaceutical must be treated with utmost vigilance due to the presence of organic pollutants and drug components [2]. In addition, the presence of pharmaceuticals in wastewater does not only come from industrial effluents, it is also found in domestic wastewater [3]. Even though there are various methods for the treatment of pharmaceutical wastewater (PWW), which can be classified as physical, physicochemical, chemical, and biological methods, but all are accompanied with various challenges with low efficiency or economic infeasibility [4]. On the other hand, PWW treatment and simultaneously generating energy from it will support the waste management system, alongside sustainable means of energy generation. A vast number of research is going on bioelectrochemical systems (BES) [5]. Microbial fuel cell (MFC) has attained considerable remark due to the advantage of cost-effectiveness, high-biodegradability, and eco-friendly.The MFCs transform the chemical energy into electrical energy by electrochemical active microorganism, known as exoelectrogens. In the anode chamber, organic matter is oxidized, producing electrons and protons. Electrons are transferred to the cathode chamber through an external circuit, where they react with oxygen and protons to form water [6]. Since electroactive bacteria can remove organic matter while generating electrical energy simultaneously, researchers focus the MFC as an alternative electrochemical technology for wastewater treatment and a sustainable energy resource. Despite this, MFCs’ actual deployment has been hampered by their limited power output and expensive fabrication costs [7]. Recent research proves that modification in anaerobic anode chamber (e.g., active microorganism, bioanode configuration, and use of carbon-based catalysts) improve the total output power and performance. Since, the wastewater substrate is present in the anode chamber, the bioanode must be extremely active and electrochemically stable in wide range of pH. In this regard, a comprehensive variety of carbon material-based bioanodes has been evaluated for MFC applications [8, 9]. Biomass-derived biocarbons are rich in carbon with self-doped heteroatom, inexpensive, and eco-friendly with interesting surface chemistry and structural advantage over other precious metal material that have been used as catalysts in this application. Biocarbons from bamboo, cotton textiles, bananas, neem wood, silk cocoon, egg, and bread have been successfully demonstrated in bioanodes for MFC [8].On the other hand, the growing interest in the development of high-efficient BES has led to the study of various exoelectrogenic cultures [9]. For example, anaerobic bacteria such as Rhodoferax ferrireducens and Geobacter sulfurreducens are studied as active microorganisms for MFC applications. When they meet exocellular solid substrates, they use outer membrane redox proteins (e.g., cytochromes) to mediate electron transfer [10]. Moreover, other bacteria like Pseudomonas aeruginosa and Shewanella putrefaciens create redox molecules as phenazine and quinones as electron carriers [11]. On the other hand, Escherichia coli (E. coli) create artificial redox molecules for extracellular electron transfer to solid substrates, and Bacillus subtilis (B. subtilis) has the advantage to be electrochemically active towards oxidation on wide range of pH in addition to being resistant to the presence of several antibiotics [12].In our research group we have carried out previous studies for the treatment of PWW usingB. subtilis growing on carbon-based catalyst, which improves the overall performance of MFC in terms of electricity generation and COD removal. In this concern, Duarte-Urbina et al. [12] reported decrease in chemical and biological oxygen demand (COD & BOD), total dissolved solids (TDS), and pH from 9.2–8.7 in PWW for 14 days of treatment in a dual-chamber MFC. In that work, bioanode is composed of onion waste-derived biocarbon+ B. subtilis showing a maximum power density (Pcell) of 30.72 mW m−2. Meanwhile, García–Mayagoitia et al. [13] reported stable bioanodes composed of functionalized ordered mesoporous carbon (f-OMC) + B. subtilis using PWW as a substrate (pH = 9.6). With this bioanode configuration, an open circuit voltage up to 0.62 V is obtained, with maximum current density and power density (Pcell) of 854 mA m−2 and 105 mW m−2, respectively.In this work, we report the performance of dual-chambered H-type MFC, using a novel bioanode made with functionalized waste leather-derived biocarbon (AB7-F) plusE. coli and B. subtilis used separately as electrochemically active microorganisms and PWW as organic substrate. Whilst in the cathode chamber, a commercial Pt-based catalyst electrode and electrolyte of O2-satured aqueous KOH solution (pH = 9.7) were used. An alkalinized Nafion membrane was used as a separator between anode and cathode chambers.


## 2. Experimental

### 2.1. Biocarbon Synthesis (Catalyst)

The biocarbon was obtained based on a previous methodology reported by Alonso–Lemus et al. [14]. First, 10 g of leather waste slices were cleaned with ethanol (C2H5OH, Jalmek, 96%), then they were pyrolyzed at 700°C for 90 minutes at a heat rate of 10°C min−1 in N2 atmosphere (Infra, 99.99%). The carbonized slices were pulverized and mixed with potassium hydroxide (KOH, Fermont, 87.5%) in a weight ratio of 2 : 1. Afterwards, this mixture was activated at 750°C for 90 minutes, at a heat rate of 8°C min−1 in N2 atmosphere and cooled down to room temperature. Then, a black powder was recovered to wash it with an aqueous solution of 1 mol L−1 of hydrochloric acid (HCl, Sigma Aldrich, 37%) and dried overnight at 80°C. The biocarbon obtained was labeled as AB7.AB7 was functionalized by intermittent microwave heating (IMH) to improve its biocompatibility [13] as follows: 300 mg of AB7 were dispersed in 30 mL of 0.15 mol L−1 methanol (CH3OH, Sigma Aldrich, 99.8%) aqueous solution by sonication for 30 minutes. Then, the dispersion was stirred for one hour more and transferred to a modified homemade microwave maintaining magnetic stirring. The thermal treatment by IMH was for 8 minutes with 25 s on/15 s off pulses. The functionalized biocarbon was labeled as AB7-f.


### 2.2. Bioanodes Construction

As shown in Figure1, the bioanodes were composed of three layers: support, catalytic layer, and biofilm. Carbon cloth was used as a support with a geometric area of 2 cm2 (1 × 2 cm). A copper wire was fixed on its back using silver ink, then an area of 1 cm2 of the carbon cloth was delimited with epoxy resin. Afterwards, the catalytic layer was deposited on the delimited area by drop-by-drop method (catalyst loading = 5 mg cm-2). The catalytic ink had the following composition: 5 mg of catalyst, 64 µL of Nafion® 117 solution (Aldrich 5 wt. %), and 64 µL of 2-propanol (C3H7OH, Sigma Aldrich, 99.5%). The anodes (support + catalytic layer) were stored in a desiccator until to use them.Figure 1
Bioanode side view with a configuration of support+ catalysts + biofilm.For the biofilm layer two bacteria were used separately:Bacillus subtilis (B. subtilis), which was obtained from the Cinvestav National Collection of Microbial Strains and Cell Cultures, whilst Escherichia coli (E. coli) ATCC® 25922™ was supplied by Microbiologics®. Inoculation of the strains from their reception until their use for the growth of the biofilm can be consulted in supplementary information. For the biofilm grown on the anodes, first, bacteria were inoculated at 30°C for 3 days using an orbital incubator (Vichi, INC-C); then the anodes were immersed into 50 mL of sterile broth media contained in an Erlenmeyer flask and inoculated for 6 days at 30°C in the orbital incubator (Figure S1(a)).


### 2.3. Pharmaceutical Wastewater (PWW) Substrate

The PWW was provided by a local pharmaceutical industry. The PWW was collected and stored at -18°C to preserve it until its use. The physicochemical features of the PWW are shown in Table1.Table 1
Physicochemical parameters of the PWW used as substrate.
ParameterValuepH9.2Conductivity2870 mS m−1Chemical oxygen demand (COD)27,603 mg L−1Biochemical oxygen demand (BOD)10,433 mg L−1Nitrate nitrogen (N–NO3)6.13 mg L−1Total phosphorus93.91 mg L−1Sulfate ion (SO2−4)1,161 mg L−1Total alkalinity3,574 mg L−1 CaCO3Chloride10,616 mg L−1Total dissolved solids (TDS)30,540 mg L−1Total suspended solids (TSS)3,320 mg L−1


### 2.4. Physicochemical Characterization

Morphology and elemental composition of the bioanodes were determined using a scanning electron microscope (SEM) Philips XL 30 ESEM with an EDAX detector. Moreover, the structural features of the catalysts were analyzed using a DXR-RAMAN thermo scientific spectrometer (He–Ne laser of 633 nm, 0.3 mW). Meanwhile, their functional groups were determined using a Bruker Tensor II equipment with ATR, in a range of 4000 to 500 cm−1 with a resolution of 4 cm−1. In addition, an ASAP 2050 equipment (Micrometrics, USA) was used to determine the textural properties of the catalysts. Prior to the measurements, the samples were degassed for 14 h at 220°C, then nitrogen was used as absorbate.


### 2.5. Electrochemical Measurements

#### 2.5.1. Half-Cell

The electrochemical measurements were performed using a Bio-Logic VSP-300 potentiostat in a three-electrode cell configuration. A platinum wire and a Ag/AgCl (NaCl 3 mol L−1, 0.209 V vs. SHE) were used as counter and reference electrodes, respectively. The work electrodes (WE) were the anodes (support + catalyst) and bioanodes (support + catalyst + bacteria) previously made. Prior to the electrochemical test, the WE were stabilized for 18 hours into the N2-saturated PW at room temperature, which was used as substrate. The Cyclic Voltammetry (CV) curves were obtained in a potential range of -0.70 to 0.45 V vs. the reversible hydrogen electrode (RHE) at a scan rate of 20 mV s−1. For comparison purposes, CVs of the support and support + bacteria were obtained.All the potential reported in this work were converted to the RHE voltage (ERHE) using the following equation [15]:(1)ERHE=E+0.059pH+Eref,where the pH of the PWW is 9.2, Eref is the potential of the reference electrode versus the standard hydrogen electrode (SHE), and E in the potential that is wanted to convert to ERHE.


#### 2.5.2. Microbial Fuel Cell Test

The most active bioanode was tested in a dual-chambered H-type MFC (FigureS1(b)). The anode chamber contained 240 mL of N2-satured PW and the bioanode previously stabilized for 18 hours. Whilst in the cathode chamber a commercial Pt-based catalyst electrode (Etek, 20 wt. % Pt/C, catalyst load = 2 mg cm-2) and electrolyte of O2-satured aqueous KOH solution (pH = 9.7) were used. A Nafion® 117 (DuPont) membrane was used as separator, which was previously alkalinized as was described in supplementary information [16]. First, the open circuit potential (EOC) was determined, then, the cell potential (Ecell) was registered at different external resistance values (Rext = 0.01, 1, 1.2, 5, and 10 kΩ). The cell current (Icell) was calculated by the Ohm’s law (2)(2)Icell=EcellRext.Then, the current density (jcell) was calculated normalizing the Icell respect to the geometrical bioanode area (Abio). Thus, the polarization curve (Ecell vs. jcell) plots were obtained. Finally, the power density (Pcell) was calculated as follows (3):(3)Pcell=Ecell2Rext⋅Abio.Both, the polarization (Ecell vs. jcell) and power density (Pcell vs. jcell) plots were obtained at day 0, 1, 7, and 14.


## 2.1. Biocarbon Synthesis (Catalyst)

The biocarbon was obtained based on a previous methodology reported by Alonso–Lemus et al. [14]. First, 10 g of leather waste slices were cleaned with ethanol (C2H5OH, Jalmek, 96%), then they were pyrolyzed at 700°C for 90 minutes at a heat rate of 10°C min−1 in N2 atmosphere (Infra, 99.99%). The carbonized slices were pulverized and mixed with potassium hydroxide (KOH, Fermont, 87.5%) in a weight ratio of 2 : 1. Afterwards, this mixture was activated at 750°C for 90 minutes, at a heat rate of 8°C min−1 in N2 atmosphere and cooled down to room temperature. Then, a black powder was recovered to wash it with an aqueous solution of 1 mol L−1 of hydrochloric acid (HCl, Sigma Aldrich, 37%) and dried overnight at 80°C. The biocarbon obtained was labeled as AB7.AB7 was functionalized by intermittent microwave heating (IMH) to improve its biocompatibility [13] as follows: 300 mg of AB7 were dispersed in 30 mL of 0.15 mol L−1 methanol (CH3OH, Sigma Aldrich, 99.8%) aqueous solution by sonication for 30 minutes. Then, the dispersion was stirred for one hour more and transferred to a modified homemade microwave maintaining magnetic stirring. The thermal treatment by IMH was for 8 minutes with 25 s on/15 s off pulses. The functionalized biocarbon was labeled as AB7-f.


## 2.2. Bioanodes Construction

As shown in Figure1, the bioanodes were composed of three layers: support, catalytic layer, and biofilm. Carbon cloth was used as a support with a geometric area of 2 cm2 (1 × 2 cm). A copper wire was fixed on its back using silver ink, then an area of 1 cm2 of the carbon cloth was delimited with epoxy resin. Afterwards, the catalytic layer was deposited on the delimited area by drop-by-drop method (catalyst loading = 5 mg cm-2). The catalytic ink had the following composition: 5 mg of catalyst, 64 µL of Nafion® 117 solution (Aldrich 5 wt. %), and 64 µL of 2-propanol (C3H7OH, Sigma Aldrich, 99.5%). The anodes (support + catalytic layer) were stored in a desiccator until to use them.Figure 1
Bioanode side view with a configuration of support+ catalysts + biofilm.For the biofilm layer two bacteria were used separately:Bacillus subtilis (B. subtilis), which was obtained from the Cinvestav National Collection of Microbial Strains and Cell Cultures, whilst Escherichia coli (E. coli) ATCC® 25922™ was supplied by Microbiologics®. Inoculation of the strains from their reception until their use for the growth of the biofilm can be consulted in supplementary information. For the biofilm grown on the anodes, first, bacteria were inoculated at 30°C for 3 days using an orbital incubator (Vichi, INC-C); then the anodes were immersed into 50 mL of sterile broth media contained in an Erlenmeyer flask and inoculated for 6 days at 30°C in the orbital incubator (Figure S1(a)).


## 2.3. Pharmaceutical Wastewater (PWW) Substrate

The PWW was provided by a local pharmaceutical industry. The PWW was collected and stored at -18°C to preserve it until its use. The physicochemical features of the PWW are shown in Table1.Table 1
Physicochemical parameters of the PWW used as substrate.
ParameterValuepH9.2Conductivity2870 mS m−1Chemical oxygen demand (COD)27,603 mg L−1Biochemical oxygen demand (BOD)10,433 mg L−1Nitrate nitrogen (N–NO3)6.13 mg L−1Total phosphorus93.91 mg L−1Sulfate ion (SO2−4)1,161 mg L−1Total alkalinity3,574 mg L−1 CaCO3Chloride10,616 mg L−1Total dissolved solids (TDS)30,540 mg L−1Total suspended solids (TSS)3,320 mg L−1


## 2.4. Physicochemical Characterization

Morphology and elemental composition of the bioanodes were determined using a scanning electron microscope (SEM) Philips XL 30 ESEM with an EDAX detector. Moreover, the structural features of the catalysts were analyzed using a DXR-RAMAN thermo scientific spectrometer (He–Ne laser of 633 nm, 0.3 mW). Meanwhile, their functional groups were determined using a Bruker Tensor II equipment with ATR, in a range of 4000 to 500 cm−1 with a resolution of 4 cm−1. In addition, an ASAP 2050 equipment (Micrometrics, USA) was used to determine the textural properties of the catalysts. Prior to the measurements, the samples were degassed for 14 h at 220°C, then nitrogen was used as absorbate.


## 2.5. Electrochemical Measurements

### 2.5.1. Half-Cell

The electrochemical measurements were performed using a Bio-Logic VSP-300 potentiostat in a three-electrode cell configuration. A platinum wire and a Ag/AgCl (NaCl 3 mol L−1, 0.209 V vs. SHE) were used as counter and reference electrodes, respectively. The work electrodes (WE) were the anodes (support + catalyst) and bioanodes (support + catalyst + bacteria) previously made. Prior to the electrochemical test, the WE were stabilized for 18 hours into the N2-saturated PW at room temperature, which was used as substrate. The Cyclic Voltammetry (CV) curves were obtained in a potential range of -0.70 to 0.45 V vs. the reversible hydrogen electrode (RHE) at a scan rate of 20 mV s−1. For comparison purposes, CVs of the support and support + bacteria were obtained.All the potential reported in this work were converted to the RHE voltage (ERHE) using the following equation [15]:(1)ERHE=E+0.059pH+Eref,where the pH of the PWW is 9.2, Eref is the potential of the reference electrode versus the standard hydrogen electrode (SHE), and E in the potential that is wanted to convert to ERHE.


### 2.5.2. Microbial Fuel Cell Test

The most active bioanode was tested in a dual-chambered H-type MFC (FigureS1(b)). The anode chamber contained 240 mL of N2-satured PW and the bioanode previously stabilized for 18 hours. Whilst in the cathode chamber a commercial Pt-based catalyst electrode (Etek, 20 wt. % Pt/C, catalyst load = 2 mg cm-2) and electrolyte of O2-satured aqueous KOH solution (pH = 9.7) were used. A Nafion® 117 (DuPont) membrane was used as separator, which was previously alkalinized as was described in supplementary information [16]. First, the open circuit potential (EOC) was determined, then, the cell potential (Ecell) was registered at different external resistance values (Rext = 0.01, 1, 1.2, 5, and 10 kΩ). The cell current (Icell) was calculated by the Ohm’s law (2)(2)Icell=EcellRext.Then, the current density (jcell) was calculated normalizing the Icell respect to the geometrical bioanode area (Abio). Thus, the polarization curve (Ecell vs. jcell) plots were obtained. Finally, the power density (Pcell) was calculated as follows (3):(3)Pcell=Ecell2Rext⋅Abio.Both, the polarization (Ecell vs. jcell) and power density (Pcell vs. jcell) plots were obtained at day 0, 1, 7, and 14.


## 2.5.1. Half-Cell

The electrochemical measurements were performed using a Bio-Logic VSP-300 potentiostat in a three-electrode cell configuration. A platinum wire and a Ag/AgCl (NaCl 3 mol L−1, 0.209 V vs. SHE) were used as counter and reference electrodes, respectively. The work electrodes (WE) were the anodes (support + catalyst) and bioanodes (support + catalyst + bacteria) previously made. Prior to the electrochemical test, the WE were stabilized for 18 hours into the N2-saturated PW at room temperature, which was used as substrate. The Cyclic Voltammetry (CV) curves were obtained in a potential range of -0.70 to 0.45 V vs. the reversible hydrogen electrode (RHE) at a scan rate of 20 mV s−1. For comparison purposes, CVs of the support and support + bacteria were obtained.All the potential reported in this work were converted to the RHE voltage (ERHE) using the following equation [15]:(1)ERHE=E+0.059pH+Eref,where the pH of the PWW is 9.2, Eref is the potential of the reference electrode versus the standard hydrogen electrode (SHE), and E in the potential that is wanted to convert to ERHE.


## 2.5.2. Microbial Fuel Cell Test

The most active bioanode was tested in a dual-chambered H-type MFC (FigureS1(b)). The anode chamber contained 240 mL of N2-satured PW and the bioanode previously stabilized for 18 hours. Whilst in the cathode chamber a commercial Pt-based catalyst electrode (Etek, 20 wt. % Pt/C, catalyst load = 2 mg cm-2) and electrolyte of O2-satured aqueous KOH solution (pH = 9.7) were used. A Nafion® 117 (DuPont) membrane was used as separator, which was previously alkalinized as was described in supplementary information [16]. First, the open circuit potential (EOC) was determined, then, the cell potential (Ecell) was registered at different external resistance values (Rext = 0.01, 1, 1.2, 5, and 10 kΩ). The cell current (Icell) was calculated by the Ohm’s law (2)(2)Icell=EcellRext.Then, the current density (jcell) was calculated normalizing the Icell respect to the geometrical bioanode area (Abio). Thus, the polarization curve (Ecell vs. jcell) plots were obtained. Finally, the power density (Pcell) was calculated as follows (3):(3)Pcell=Ecell2Rext⋅Abio.Both, the polarization (Ecell vs. jcell) and power density (Pcell vs. jcell) plots were obtained at day 0, 1, 7, and 14.


## 3. Results and Discussion

### 3.1. Catalysts Physicochemical Properties

Figure2(a) shows the chemical composition of the catalysts and the raw material used for their synthesis (leather), which was determined by energy-dispersive X-ray spectroscopy (EDS). As can be seen, leather was composed mainly of carbon (69.62 wt. %) and oxygen (18.89 wt. %), in addition to some heteroatoms like nitrogen (4.13 wt. %) and sulfur (3.10 wt. %). Also, other elements in less concentration were detected (e.g., Ca, Na, Mg, and Si). As it was expected, the carbon concentration of both AB7 and AB7-f catalysts increased (C ∼ 89 wt. %) and the oxygen decreased (O ∼ 7 wt. %) respect to leather due to pyrolysis treatment. Further, it was observed that the remaining concentration of nitrogen (N ∼ 2 wt. %) and sulfur (S ∼ 0.1 wt. %) was lower for both catalysts than leather, which implies that by the synthesis route proposed in this work, heteroatom self-doped catalysts could be obtained. On the other hand, it was observed that elements contained in leather in low concentration were removed from both AB7 and AB7-f, where only Mg was detected in concentrations of 0.17 and 0.14 wt. %, respectively. The chemical composition values also can be found in Table S2; meanwhile, Figure S2 shows the elemental mapping of the catalysts which demonstrate that both catalysts had a homogeneous distribution of the elements.Figure 2
(a) Chemical composition of leather and catalysts determined by EDS and morphology of (b) leather, (c) AB7, and (d) AB7-f.
(a)(b)(c)(d)Figure2(b) shows the typical morphology of leather’s collagen fibers about a micron in diameter. Interestingly, the morphology of the collagen fibers was lost after the carbonization and activation treatment to which AB7 was subjected, as shown in Figure 2(c). As can be seen, collagen fibers sinterized until forming 3D particles interconnected by macropores. However, AB7-f revealed carbon particles of smaller size than the AB7 particles, which indicates that microwave heating could promote the reduction in particle size, as has been reported [12, 13].On the other hand, the catalysis’ structural and textural features were studied (Figure3). Raman spectroscopy of AB7 and AB7-f are shown in Figures 3(a) and 3(b), respectively. Two broad bands normally observed in amorphous carbon materials were detected, the first one at ≈1330 cm−1, named D-band, which can be associated to defects and disorder in the graphitic lattice. The second signal at ≈1600 cm−1, named G-band, is normally associated with sp2 hybridization. The ID/IG ratio was calculated from the deconvoluted peaks showing little variation between the values found for AB7 (ID/IG = 1.58) and AB7-f (ID/IG = 1.50). However, when the ratio between D2 interband (≈1200 cm−1) and G band was calculated, a difference was observed between AB7 (ID2/IG = 1.12) and AB7-f (ID2/IG = 0.46). The D2 interband, also called D∗ has been attributed sp2-sp3 bond in carbon lattice [17], and when its value decreases it can be assumed that the crystallinity in the nanodomains increases, therefore, AB7-f has a more crystalline nanodomain structure than AB7. This occurs because microwave heating promotes the release of carbon and oxygen from the carbon material, generating small conjugated graphitic domains, which improves crystallinity [18]. In addition, D3 interband (≈1500 cm−1) can be related to small functional groups and SP2-bonded forms, and it has been reported that when D3 interband intensity decrease the crystallinity increase [19], which is in good agreement with our results.Figure 3
Structural and textural features of the catalysts. Raman spectroscopy of (a) AB7 and (b) AB7-f, (c) adsorption/desorption isotherms, and (d) Fourier-transform infrared spectra of the catalysts.
(a)(b)(c)(d)Figure3(c) shows the nitrogen adsorption/desorption isotherms of the catalyst. According to the international union of pure and applied chemistry (IUPAC) classification [20], the adsorption isotherms of both AB7 and AB7-f are type IV(a), which are commonly observed for mesoporous materials with pores greater than 4 nm. In addition, both isotherms show a hysteresis loop H4 type, indicating the filling of micropores. Therefore, it can be assumed that AB7 and AB7-f are micro-mesoporous carbon materials [21]. The specific surface area was calculated by Brunauer–Emmett–Teller method (SSABET). As it was expected, high surface area values were obtained for AB7 (SSABET = 1890 m2g−1) and AB7-f (SSABET = 1984 m2g−1) because KOH was used as a chemical activating agent and it mainly promotes the formation of micropores generating high surface area carbons. Moreover, the average pore size of AB7 and AB7-f was of 4.4 nm and 5.5 nm, respectively. The slight increase of 4% in SSABET of AB7-f with respect to AB7, and the increase in the average pore size is because, as already mentioned, during microwave heating, carbon and oxygen were expelled, generating the formation of additional pores and the increase in the average pore size. On the other hand, Figure 3(d) shows the Fourier–transform infrared (FTIR) spectra of the catalysts. As can be seen, there are two broad peaks in the range of 1400–1600 cm−1 and 1650–1900 cm−1. In the first region, the signal associated to the combined stretching of C=C and C–C of aromatic compounds was observed. Meanwhile, in the second region, the corresponding C=O bond can be found. The presence of C–H bond at ≈ 2600 cm−1 generated by aldehyde type bond also was detected [22, 23].


### 3.2. Bioanodes Morphology and Performance in Half-Cell

Figure4 shows the morphology of bioanodes with different configurations. The micrographs in Figures 4(a) and 2(b) show the bioanodes with S + AB7 +  E. coli and S + AB7-f + E. coli configurations, respectively. As can be seen, groups of bacteria are observed on the surface of the catalysts with a quasi-spherical shape, which may indicate that they are in an early stage of development, since the characteristic morphology of E. coli is that of bacilli (smaller than those of B. subtilis). Despite the presence of these bacteria, an inhomogeneous growth of biofilm on the surface of the catalytic layer has been observed. However, for the bioanode with the S + AB7-f + E. coli configuration, a greater growth of the bacteria is observed compared to the non-functionalized catalyst (Figure 4(b)). Meanwhile, the bioanode with S + AB7 + B. subtilis configuration (Figure 4(c)) shows the presence of some isolated bacilli on a smooth surface. That change in surface texture can be attributed to polysaccharides excreted during biofilm formation [24]. Figure 4(d) shows the bioanode with S + AB7-f + B. subtilis configuration. It should be noted the outstanding biocompatibility of the bioanode, the growth of the B. subtilis biofilm on this catalyst is abundant, forming a reticular structure that almost completely covers the surface of the catalyst. Interestingly, these results indicate that functionalized catalysts improve biocompatibility, which is in good agreement with observations from our previous studies [12, 13].Figure 4
Bioanodes morphology with different configurations. (a)S + AB7 + E. coli, (b) S + AB7-f + E. coli, (c) S + AB7 + B. subtilis, and(d) S + AB7-f + B. subtilis.The electrochemical performance of all bioanodes was evaluated in half-cell system. First, the cyclic voltammetry (CV) curves of support (S) + bacteria using N2-saturated PWW as substrate were obtained (Figure 5(a)). In general, the electrochemical behavior of the supports with and without biofilm is very similar. The sweep in the cathodic direction had shown an increase in the current density (j) near to 0.45 V/RHE, which is more notable for S + B. subtilis. These results confirm the need to use a catalyst to promote the generation of redox couples.Figure 5
Half-cell characterization of the bioanodes with different configurations using N2-saturated PWW as substrate at a scan rate of 20 mV s−1 and as support (S) carbon cloth, (a) S + bacteria, (b) S + AB7 + bacteria, and (c) S + AB7-f + bacteria.
(a)(b)(c)In Figure5(b) the CV of the bioanode S + AB7 has shown a similar shape to those CVs of Figure 5(a), although with highest values of j in the same potential range, this increase in j can be attributed to the effect of the catalyst AB7, which increases the electrode-electrolyte interaction due to its high surface area [25]. Whilst for S + AB7 + B. subtilis bioanode j decreases suggesting low electroactivity. Furthermore, the VC of the S + AB7 + E. coli bioanode shows a quasi-capacitive behavior indicating a better bioelectrochemical interaction with organic matter of PWW than the previous two bioanodes.In Figure5(c), the VC of the anode S + AB7-f also shows a quasicapacitive behavior, which suggests catalytic activity of AB7-f in PWW. Remarkably, the bioanode S + AB7-f + B. subtilis generates the highest values of j, with a capacitive behavior. This indicates a high catalytic activity of this bioanode to oxidize the organic matter in the PWW. In addition, this result suggests a positive effect of the functionalization treatment by modifying the surface of the catalyst, and thus, improving the biocompatibility. In contrast, the VC of the bioanode S + AB7-f + E. coli has lower j values than S + AB7-f + B. subtilis, due mainly to the low biocompatibility between AB7-f and E. coli, which implies a low electroactivity to oxidize PWW organic matter. In the case of this study, the electrochemical behaviors observed in Figure 5 can be attributed to the nature of both catalysts (chemical composition, morphology, specific area, structure, and texture) since it has been reported that there is not a direct relationship between the amount of biofilm and the current density generated [26]. It has also been reported that the presence of oxygenated groups in the catalysts where the biofilm grows can increase the catalyst-bacteria biocompatibility [27, 28].


### 3.3. Bioanodes Morphology and Performance in Half-Cell

With the results obtained from the electrochemical characterization in half-cell, it was decided to use the bioanodeS + AB7-f + B. subtilis for the electrochemical characterization and PWW remediation tests in a dual-chamber MFC. Figure 6(a) shows the Ecell-j polarization curve (black triangle) and the Pcell-j curve (red square) obtained at the beginning of the long-term test. The initial Ecell was of 0.60 V, with a maximum value of Pcell = 77 mW m−2 and j = 254 mA m−2. The linear tendence of the Ecell-j polarization curve indicates a good mass transport to the electrode, which suggests that the bioanode S + AB7-f + B. subtilis biofilm reduces the diffusional and electrochemical limitations [29]. In addition, no significant voltage drops were observed at the beginning of the test, which implies that activation losses to carry out the redox reaction was low [30, 31], maybe due to synergic interaction of several characteristics of the bioanodes such as high specific surface area of the catalysts (AB7-f), good biocompatibility between AB7-f and B. subtilis and good performance in PWW substrate [30]. Figure 6(b) shows Pcell-j curves obtained during the long-term test (initial, day 1, 7, and 14). As can be seen, the Pcell generates by the MFC varies over time. The better performance was observed on day 7 with a maximum of Pcell = 96.3 mW m−2. Meanwhile, the oscillating Pcell generation in the MFC is because it is a dynamic system, where the biofilm of B. subtilis could be modified in the presence of PWW (formation/shedding of other bacteria) [32].Figure 6
Long-term test of the MFC. Anode chamber:S + AB7-f + B. subtilis bioanode and PWW as substrate (pH = 9.2). Cathode chamber: 20% Pt/C catalysts in KOH (pH = 9.7) electrolyte. Nafion® 117 alkalinized membrane was used as separator. (a) Ecell-j and Pcell-j curves obtained at the initial of the test, (b) Pcell-j curves obtained during the initial, days 1, 7, and 14, and (c) open circuit voltage of the MFC vs. time using a Rext = 10 kΩ.
(a)(b)(c)Table2 summarized the electrochemical parameters obtained from Pcell-j curves at different times. The Ecell values at initial and day 7 were similar (≈0.6 V) and is consistent with previously reported MFC voltages with similar arrays (Ecell between 0.5 and 0.8 V) [33]. Therefore, it can be assumed that in these periods of time, the system was in optimal conditions.Table 2
Electrochemical parameters of the MFC during the long-term test.
TimeEcell (V)Maximum Pcell (mW m−2)j (mA m−2)Pv (mW m−3)Initial0.6077.425438.7Day 10.3119.31279.6Day 70.5396.328348.1Day 140.3848.128324.0On the other hand, the maximum Pcell values obtained in this work are comparable with other previously reported (Table S3). In this work, we used PWW (as received from the industry) and S + AB7-f + B. subtilis as bioanode, meanwhile, other reports operating the MFC with more controllable parameters (e.g., synthetic substrates and mixed cultures). However, the comparison between different types of MFC systems is complicated, since there are many variables, such as the substrate, electrodes, use of catalysts, cell configuration, temperature, pH, and bacteria [34]. Therefore, with this comparison we want to highlight that S + AB7-f + B. subtilis bioanode is promising to generate bioenergy even in recalcitrant media as the PWW.Figure6(c) shows the Ecell-time curve applying a Rext of 10 kΩ to the MFC during the 14 days of testing. Under this condition, the initial Ecell(10 kΩ) was of 360 mV, which continuously decreases until 83 mV, the lowest Ecell(10 kΩ) value recorded at 25 h. This voltage drop may be caused by the processes of stabilization and adaptation in the bioelectrochemical system. In a period from 25 to 130 hours, a constant increase in voltage was observed until reaching voltages above 300 mV; where interestingly it can be seen that there are periods where the voltage slightly decreases, which coincides with the night periods. After 130 h, the voltage is relatively constant until 160 h, then, the voltage decreases to 224 mV. These results confirm that the best performance is between 130 and 168 h (5–7 days). Meanwhile, the voltage decreases towards the end of the long-term test may be due to decrease in nutrients assimilable by the bacteria near to the bioanode [35]. It is important to note that this is a batch process without agitation, so the diffusion of nutrients to the bioanode may be limited. This behavior has also been reported for MFC with complex organic matter such as aromatic compounds [36], paper recycling [37], and wastewater from the coke production process [38].Finally, Table3 shows the physicochemical parameters of PWW during the long-term test. As can be seen, there was a gradual decrease in pH from 9.2 to 8.5, while the electrical conductivity remains relatively constant. Regarding the chemical and biochemical oxygen demand, the removal of these in this work was of 45 and 15% of COD and BOD, respectively. The sulfates ion (SO−4) increases almost three times on day 14 compared to its initial value. Meanwhile, total dissolved and suspended solids decrease 8.5 and 50.6%, respectively. Other chemical parameters such as nitrates, phosphorous, and alkalinity remains constant.Table 3
Physicochemical parameters of PWW at different times during the long-term test. No determined.
ParameterUnitsLong-term testInitial (0 h)Day 1 (24 h)Day 7 (168 h)Day 14 (336 h)pH9.209.108.608.50ConductivitymS m−12,8702,8302,8102,770Chemical oxygen demand (COD)mg L−127,60322,69421,38415,181Biochemical oxygen demand (BOD)mg L−110,433——8,900Nitrates (N–NO3)mg L−16.13——6.76Total phosphorousmg L−193.91——91.15Sulfate ion (SO2-4)mg L−11,161——3,592Alkalinitymg L−1 CaCO33,574——3,772Chloridesmg L−110,616——9,466Total dissolved solids (TDS)mg L−130,540——27,900Total suspended solids (TSS)mg L−13,320——1,640


## 3.1. Catalysts Physicochemical Properties

Figure2(a) shows the chemical composition of the catalysts and the raw material used for their synthesis (leather), which was determined by energy-dispersive X-ray spectroscopy (EDS). As can be seen, leather was composed mainly of carbon (69.62 wt. %) and oxygen (18.89 wt. %), in addition to some heteroatoms like nitrogen (4.13 wt. %) and sulfur (3.10 wt. %). Also, other elements in less concentration were detected (e.g., Ca, Na, Mg, and Si). As it was expected, the carbon concentration of both AB7 and AB7-f catalysts increased (C ∼ 89 wt. %) and the oxygen decreased (O ∼ 7 wt. %) respect to leather due to pyrolysis treatment. Further, it was observed that the remaining concentration of nitrogen (N ∼ 2 wt. %) and sulfur (S ∼ 0.1 wt. %) was lower for both catalysts than leather, which implies that by the synthesis route proposed in this work, heteroatom self-doped catalysts could be obtained. On the other hand, it was observed that elements contained in leather in low concentration were removed from both AB7 and AB7-f, where only Mg was detected in concentrations of 0.17 and 0.14 wt. %, respectively. The chemical composition values also can be found in Table S2; meanwhile, Figure S2 shows the elemental mapping of the catalysts which demonstrate that both catalysts had a homogeneous distribution of the elements.Figure 2
(a) Chemical composition of leather and catalysts determined by EDS and morphology of (b) leather, (c) AB7, and (d) AB7-f.
(a)(b)(c)(d)Figure2(b) shows the typical morphology of leather’s collagen fibers about a micron in diameter. Interestingly, the morphology of the collagen fibers was lost after the carbonization and activation treatment to which AB7 was subjected, as shown in Figure 2(c). As can be seen, collagen fibers sinterized until forming 3D particles interconnected by macropores. However, AB7-f revealed carbon particles of smaller size than the AB7 particles, which indicates that microwave heating could promote the reduction in particle size, as has been reported [12, 13].On the other hand, the catalysis’ structural and textural features were studied (Figure3). Raman spectroscopy of AB7 and AB7-f are shown in Figures 3(a) and 3(b), respectively. Two broad bands normally observed in amorphous carbon materials were detected, the first one at ≈1330 cm−1, named D-band, which can be associated to defects and disorder in the graphitic lattice. The second signal at ≈1600 cm−1, named G-band, is normally associated with sp2 hybridization. The ID/IG ratio was calculated from the deconvoluted peaks showing little variation between the values found for AB7 (ID/IG = 1.58) and AB7-f (ID/IG = 1.50). However, when the ratio between D2 interband (≈1200 cm−1) and G band was calculated, a difference was observed between AB7 (ID2/IG = 1.12) and AB7-f (ID2/IG = 0.46). The D2 interband, also called D∗ has been attributed sp2-sp3 bond in carbon lattice [17], and when its value decreases it can be assumed that the crystallinity in the nanodomains increases, therefore, AB7-f has a more crystalline nanodomain structure than AB7. This occurs because microwave heating promotes the release of carbon and oxygen from the carbon material, generating small conjugated graphitic domains, which improves crystallinity [18]. In addition, D3 interband (≈1500 cm−1) can be related to small functional groups and SP2-bonded forms, and it has been reported that when D3 interband intensity decrease the crystallinity increase [19], which is in good agreement with our results.Figure 3
Structural and textural features of the catalysts. Raman spectroscopy of (a) AB7 and (b) AB7-f, (c) adsorption/desorption isotherms, and (d) Fourier-transform infrared spectra of the catalysts.
(a)(b)(c)(d)Figure3(c) shows the nitrogen adsorption/desorption isotherms of the catalyst. According to the international union of pure and applied chemistry (IUPAC) classification [20], the adsorption isotherms of both AB7 and AB7-f are type IV(a), which are commonly observed for mesoporous materials with pores greater than 4 nm. In addition, both isotherms show a hysteresis loop H4 type, indicating the filling of micropores. Therefore, it can be assumed that AB7 and AB7-f are micro-mesoporous carbon materials [21]. The specific surface area was calculated by Brunauer–Emmett–Teller method (SSABET). As it was expected, high surface area values were obtained for AB7 (SSABET = 1890 m2g−1) and AB7-f (SSABET = 1984 m2g−1) because KOH was used as a chemical activating agent and it mainly promotes the formation of micropores generating high surface area carbons. Moreover, the average pore size of AB7 and AB7-f was of 4.4 nm and 5.5 nm, respectively. The slight increase of 4% in SSABET of AB7-f with respect to AB7, and the increase in the average pore size is because, as already mentioned, during microwave heating, carbon and oxygen were expelled, generating the formation of additional pores and the increase in the average pore size. On the other hand, Figure 3(d) shows the Fourier–transform infrared (FTIR) spectra of the catalysts. As can be seen, there are two broad peaks in the range of 1400–1600 cm−1 and 1650–1900 cm−1. In the first region, the signal associated to the combined stretching of C=C and C–C of aromatic compounds was observed. Meanwhile, in the second region, the corresponding C=O bond can be found. The presence of C–H bond at ≈ 2600 cm−1 generated by aldehyde type bond also was detected [22, 23].


## 3.2. Bioanodes Morphology and Performance in Half-Cell

Figure4 shows the morphology of bioanodes with different configurations. The micrographs in Figures 4(a) and 2(b) show the bioanodes with S + AB7 +  E. coli and S + AB7-f + E. coli configurations, respectively. As can be seen, groups of bacteria are observed on the surface of the catalysts with a quasi-spherical shape, which may indicate that they are in an early stage of development, since the characteristic morphology of E. coli is that of bacilli (smaller than those of B. subtilis). Despite the presence of these bacteria, an inhomogeneous growth of biofilm on the surface of the catalytic layer has been observed. However, for the bioanode with the S + AB7-f + E. coli configuration, a greater growth of the bacteria is observed compared to the non-functionalized catalyst (Figure 4(b)). Meanwhile, the bioanode with S + AB7 + B. subtilis configuration (Figure 4(c)) shows the presence of some isolated bacilli on a smooth surface. That change in surface texture can be attributed to polysaccharides excreted during biofilm formation [24]. Figure 4(d) shows the bioanode with S + AB7-f + B. subtilis configuration. It should be noted the outstanding biocompatibility of the bioanode, the growth of the B. subtilis biofilm on this catalyst is abundant, forming a reticular structure that almost completely covers the surface of the catalyst. Interestingly, these results indicate that functionalized catalysts improve biocompatibility, which is in good agreement with observations from our previous studies [12, 13].Figure 4
Bioanodes morphology with different configurations. (a)S + AB7 + E. coli, (b) S + AB7-f + E. coli, (c) S + AB7 + B. subtilis, and(d) S + AB7-f + B. subtilis.The electrochemical performance of all bioanodes was evaluated in half-cell system. First, the cyclic voltammetry (CV) curves of support (S) + bacteria using N2-saturated PWW as substrate were obtained (Figure 5(a)). In general, the electrochemical behavior of the supports with and without biofilm is very similar. The sweep in the cathodic direction had shown an increase in the current density (j) near to 0.45 V/RHE, which is more notable for S + B. subtilis. These results confirm the need to use a catalyst to promote the generation of redox couples.Figure 5
Half-cell characterization of the bioanodes with different configurations using N2-saturated PWW as substrate at a scan rate of 20 mV s−1 and as support (S) carbon cloth, (a) S + bacteria, (b) S + AB7 + bacteria, and (c) S + AB7-f + bacteria.
(a)(b)(c)In Figure5(b) the CV of the bioanode S + AB7 has shown a similar shape to those CVs of Figure 5(a), although with highest values of j in the same potential range, this increase in j can be attributed to the effect of the catalyst AB7, which increases the electrode-electrolyte interaction due to its high surface area [25]. Whilst for S + AB7 + B. subtilis bioanode j decreases suggesting low electroactivity. Furthermore, the VC of the S + AB7 + E. coli bioanode shows a quasi-capacitive behavior indicating a better bioelectrochemical interaction with organic matter of PWW than the previous two bioanodes.In Figure5(c), the VC of the anode S + AB7-f also shows a quasicapacitive behavior, which suggests catalytic activity of AB7-f in PWW. Remarkably, the bioanode S + AB7-f + B. subtilis generates the highest values of j, with a capacitive behavior. This indicates a high catalytic activity of this bioanode to oxidize the organic matter in the PWW. In addition, this result suggests a positive effect of the functionalization treatment by modifying the surface of the catalyst, and thus, improving the biocompatibility. In contrast, the VC of the bioanode S + AB7-f + E. coli has lower j values than S + AB7-f + B. subtilis, due mainly to the low biocompatibility between AB7-f and E. coli, which implies a low electroactivity to oxidize PWW organic matter. In the case of this study, the electrochemical behaviors observed in Figure 5 can be attributed to the nature of both catalysts (chemical composition, morphology, specific area, structure, and texture) since it has been reported that there is not a direct relationship between the amount of biofilm and the current density generated [26]. It has also been reported that the presence of oxygenated groups in the catalysts where the biofilm grows can increase the catalyst-bacteria biocompatibility [27, 28].


## 3.3. Bioanodes Morphology and Performance in Half-Cell

With the results obtained from the electrochemical characterization in half-cell, it was decided to use the bioanodeS + AB7-f + B. subtilis for the electrochemical characterization and PWW remediation tests in a dual-chamber MFC. Figure 6(a) shows the Ecell-j polarization curve (black triangle) and the Pcell-j curve (red square) obtained at the beginning of the long-term test. The initial Ecell was of 0.60 V, with a maximum value of Pcell = 77 mW m−2 and j = 254 mA m−2. The linear tendence of the Ecell-j polarization curve indicates a good mass transport to the electrode, which suggests that the bioanode S + AB7-f + B. subtilis biofilm reduces the diffusional and electrochemical limitations [29]. In addition, no significant voltage drops were observed at the beginning of the test, which implies that activation losses to carry out the redox reaction was low [30, 31], maybe due to synergic interaction of several characteristics of the bioanodes such as high specific surface area of the catalysts (AB7-f), good biocompatibility between AB7-f and B. subtilis and good performance in PWW substrate [30]. Figure 6(b) shows Pcell-j curves obtained during the long-term test (initial, day 1, 7, and 14). As can be seen, the Pcell generates by the MFC varies over time. The better performance was observed on day 7 with a maximum of Pcell = 96.3 mW m−2. Meanwhile, the oscillating Pcell generation in the MFC is because it is a dynamic system, where the biofilm of B. subtilis could be modified in the presence of PWW (formation/shedding of other bacteria) [32].Figure 6
Long-term test of the MFC. Anode chamber:S + AB7-f + B. subtilis bioanode and PWW as substrate (pH = 9.2). Cathode chamber: 20% Pt/C catalysts in KOH (pH = 9.7) electrolyte. Nafion® 117 alkalinized membrane was used as separator. (a) Ecell-j and Pcell-j curves obtained at the initial of the test, (b) Pcell-j curves obtained during the initial, days 1, 7, and 14, and (c) open circuit voltage of the MFC vs. time using a Rext = 10 kΩ.
(a)(b)(c)Table2 summarized the electrochemical parameters obtained from Pcell-j curves at different times. The Ecell values at initial and day 7 were similar (≈0.6 V) and is consistent with previously reported MFC voltages with similar arrays (Ecell between 0.5 and 0.8 V) [33]. Therefore, it can be assumed that in these periods of time, the system was in optimal conditions.Table 2
Electrochemical parameters of the MFC during the long-term test.
TimeEcell (V)Maximum Pcell (mW m−2)j (mA m−2)Pv (mW m−3)Initial0.6077.425438.7Day 10.3119.31279.6Day 70.5396.328348.1Day 140.3848.128324.0On the other hand, the maximum Pcell values obtained in this work are comparable with other previously reported (Table S3). In this work, we used PWW (as received from the industry) and S + AB7-f + B. subtilis as bioanode, meanwhile, other reports operating the MFC with more controllable parameters (e.g., synthetic substrates and mixed cultures). However, the comparison between different types of MFC systems is complicated, since there are many variables, such as the substrate, electrodes, use of catalysts, cell configuration, temperature, pH, and bacteria [34]. Therefore, with this comparison we want to highlight that S + AB7-f + B. subtilis bioanode is promising to generate bioenergy even in recalcitrant media as the PWW.Figure6(c) shows the Ecell-time curve applying a Rext of 10 kΩ to the MFC during the 14 days of testing. Under this condition, the initial Ecell(10 kΩ) was of 360 mV, which continuously decreases until 83 mV, the lowest Ecell(10 kΩ) value recorded at 25 h. This voltage drop may be caused by the processes of stabilization and adaptation in the bioelectrochemical system. In a period from 25 to 130 hours, a constant increase in voltage was observed until reaching voltages above 300 mV; where interestingly it can be seen that there are periods where the voltage slightly decreases, which coincides with the night periods. After 130 h, the voltage is relatively constant until 160 h, then, the voltage decreases to 224 mV. These results confirm that the best performance is between 130 and 168 h (5–7 days). Meanwhile, the voltage decreases towards the end of the long-term test may be due to decrease in nutrients assimilable by the bacteria near to the bioanode [35]. It is important to note that this is a batch process without agitation, so the diffusion of nutrients to the bioanode may be limited. This behavior has also been reported for MFC with complex organic matter such as aromatic compounds [36], paper recycling [37], and wastewater from the coke production process [38].Finally, Table3 shows the physicochemical parameters of PWW during the long-term test. As can be seen, there was a gradual decrease in pH from 9.2 to 8.5, while the electrical conductivity remains relatively constant. Regarding the chemical and biochemical oxygen demand, the removal of these in this work was of 45 and 15% of COD and BOD, respectively. The sulfates ion (SO−4) increases almost three times on day 14 compared to its initial value. Meanwhile, total dissolved and suspended solids decrease 8.5 and 50.6%, respectively. Other chemical parameters such as nitrates, phosphorous, and alkalinity remains constant.Table 3
Physicochemical parameters of PWW at different times during the long-term test. No determined.
ParameterUnitsLong-term testInitial (0 h)Day 1 (24 h)Day 7 (168 h)Day 14 (336 h)pH9.209.108.608.50ConductivitymS m−12,8702,8302,8102,770Chemical oxygen demand (COD)mg L−127,60322,69421,38415,181Biochemical oxygen demand (BOD)mg L−110,433——8,900Nitrates (N–NO3)mg L−16.13——6.76Total phosphorousmg L−193.91——91.15Sulfate ion (SO2-4)mg L−11,161——3,592Alkalinitymg L−1 CaCO33,574——3,772Chloridesmg L−110,616——9,466Total dissolved solids (TDS)mg L−130,540——27,900Total suspended solids (TSS)mg L−13,320——1,640


## 4. Conclusions

### 4.1. The most important findings of this work are the following

Amorphous carbon-based catalysts were successfully obtained from waste leather labeled as AB7 and AB7-f, both with a carbon content greater than 90% and high SBET (up to 1984 m2 g−1). Furthermore, the results prove that subjecting the catalysts to a microwave intermittent heating (MIH) treatment improves the biocompatibility between it and the bacteria.The bioanodeS + AB7-f + B. subtilis has an outstanding performance in half-cell and MFC systems for the generation of energy and remediation of PWW. The maximum Pcell of 96.3 mW m−2 was observed on day 7, achieving a removal of up to 45% of COD on day 14. Therefore, the development of catalysts made from leather waste in conjunction with PWW as a substrate and B. subtilis as an electroactive microorganism for use in MFCs is considered promising [39, 40].


## 4.1. The most important findings of this work are the following

Amorphous carbon-based catalysts were successfully obtained from waste leather labeled as AB7 and AB7-f, both with a carbon content greater than 90% and high SBET (up to 1984 m2 g−1). Furthermore, the results prove that subjecting the catalysts to a microwave intermittent heating (MIH) treatment improves the biocompatibility between it and the bacteria.The bioanodeS + AB7-f + B. subtilis has an outstanding performance in half-cell and MFC systems for the generation of energy and remediation of PWW. The maximum Pcell of 96.3 mW m−2 was observed on day 7, achieving a removal of up to 45% of COD on day 14. Therefore, the development of catalysts made from leather waste in conjunction with PWW as a substrate and B. subtilis as an electroactive microorganism for use in MFCs is considered promising [39, 40].



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*Source: 1963973-2022-08-28.xml*