# The arms race of environmental scientists to purify contaminated water

Eric Lichtfouse, Virender K Sharma, Dionysios D Dionysiou

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Eric Lichtfouse, Virender K Sharma, Dionysios D Dionysiou. The arms race of environmental scientists to purify contaminated water. Environmental Chemistry Letters, 2024, 22, pp.2607-2609. 10.1007/s10311-024-01775-7. hal-04719049

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# The arms race of environmental scientists to purify contaminated water

Eric Lichtfouse<fnref n="1" /> Ⓓ Virender K. Sharma<fnref n="2" /> Ⓓ Dionysios D. Dionysiou<fnref n="3" />

Almost all Earth water resources are now contaminated at various levels by toxic compounds (Morin-Crini et al. 2022). Indeed, as the years go by, research is disclosing the presence in ecosystems of both recently-designed industrial compounds such as engineered nanoparticles, and old industrial compounds that have occurred in ecosystems since a long time but have been detected only recently, such as microplastics, and per- and poly-fluoro alkyl substances (PFAS), named ‘forever chemicals’ (Banu et al. 2021, Anand et al. 2023, Wang et al. 2023, Dadashi Firouzjaei et al. 2022, He et al. 2024). This issue is inducing an arms race between the ever-renewed pollution and environmental scientists who must find innovative techniques to remove contaminants in order to provide drinkable water and to lessen ecosystem pollution. Here, we narrate the history of the major treatment techniques with emphasis on newly-developped ultraviolet irradiation-based advanced oxidation processes.

## Forever chemicals

A major global challenge for living organisms is having access to clean and adequate water in the context of the rising population and industrialization, which has induced a rapid increase of the number of contaminants since the last century (Yang et al. 2022; Gu et al. 2023). Indeed, researchers and water supply experts face many hurdles to produce water that is devoid of organic contaminants such as toxins, antibiotics and pesticides. Moreover, emerging contaminants such as PFAS and nanoplastics can penetrate living cells, and are by structure very likely to escape conventional oxidation and filtration methods for water treatment (Sharma et al. 2023a; Savage 2023).

## Chlorination

Conventional treatments for the production of water for consumption are not adequate for remediating the wide range of organic contamination that is usually found in water. Moreover, chlorination, a major engineering achievement of the last century for human health, is now largely criticized due to the formation of toxic halogenated disinfection byproducts from the reaction of chlorine with organic contaminants and common water constituents such as bromide (Br⁻), dissolved organic matter, fulvic acids, and humic acids (Li and Mitch 2018, Dejaeger et al. 2022) (Fig. 1). This led to the alternative use of chloramine as disinfection reagent, yet research rapidly found that chloramine application also induced the formation nitrogenous disinfection byproducts (Shah and Mitch 2012).

## Ozonation

Similarly, the use of ozone as a disinfectant leads to the formation of bromate, a toxic ion, though bromate levels can be minimized by controlling the dose of the oxidant, and by

<footnote n="1">¹ State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, 28 Xianning West Rd, Xi’an 710049, Shaanxi, People’s Republic of China</footnote>

<footnote n="2">² Program for Environment and Sustainability, Department of Environment and Occupational Health, School of Public Health, Texas A&M University, 212 Adriance Lab Rd., College Station, Texas 77843, USA</footnote>

<footnote n="3">³ Environmental Engineering and Science Program, Department of Chemical and Environmental Engineering (ChEE), University of Cincinnati, Cincinnati, OH 45221, USA</footnote>
<figure><img src="image_1.png" /><figcaption>**Fig. 1** The reactions of commonly-used disinfectants with water constituents such as bromide and dissolved organic matter induce the formation of toxic disinfection byproducts</figcaption></figure>

adding or generating hydrogen peroxide ($H_2O_2$, Richardson and Ternes 2022). Levels of disinfection byproducts also can be reduced by removing their organic precursors by adsorption on activated carbon, yet the efficiency of adsorption methods is still limited. Overall, both conventional and alternative disinfection methods generate toxic byproducts, thus calling for more efficient treatments such as the recently developed advanced oxidation processes (AOP, Shannon et al. 2008; Morrison et al. 2023; Marbaniang et al. 2023).

## Advanced oxidation processes

Advanced oxidation processes could solve the current issues of water purification because chemical reactions involve more efficient oxidants, e.g. reactive oxygen species such as hydroxyl radicals (·OH), superoxide ($O_2^−$), single oxygen ($^1O_2$) and sulfate radicals ($SO_4^{2−}$). For instance, these reactive species are generated during ultraviolet irradiation-based advanced oxidation processes using hydrogen peroxide ($H_2O_2$), ozone ($O_3$), and persulfate ($S_2O_8^{2−}$, Fig. 2). Other reagents such as chlorine, chlorine dioxide ($ClO_2$), ozone, peroxyacetic acid, and high-valent metal oxidants, e.g. permanganate ($Mn(VII)$), and ferrate(VI), allow to generate other reactive species such as reactive chlorine species, i.e. $Cl^-$, $Cl_2^-$, and $ClO^-$, peroxyacetyl radicals ($CH_3C(O)OO^-$), and high-valent intermediates ($Mn^V$, $Fe^V$, and $Fe^{IV}$), that enhance the treatment efficiency (Sharma et al. 2023b; Wen et al. 2022). In particular, the UV/chlorine process is currently gaining momentum in disinfection and oxidation treatments of water, as discussed below (Guo et al. 2022).

## Ultraviolet/chlorine process

The UV/chlorine process shows a higher efficiency and lower energy consumption compared to ·OH-based advanced oxidation processes in the abatement of contaminants and in disinfection, because the radical chemistry of reactive chlorine species is more efficient than oxidation by ·OH (Wen et al. 2022). Furthermore, reactive chlorine species are less affected by water matrix

<figure><img src="image_2.png" /><figcaption>**Fig. 2** Ultraviolet-based advanced oxidation processes that generate reactive species, radicals and non-radicals, to oxidize contaminants in water. The primary reactive species reacts with organic and inorganic constituents of water to generate secondary radical species, which may aid in abatement of targeted contaminants or influencing the main reactions between primary reactive species with the contaminants. PAA, peracetic acid; PFAS, per- and poly-fluoro alkyl substances; NOM, natural organic matter; FA, fulvic acids HA: humic acids</figcaption></figure>

components than ·OH-based processes. Additionally, the reactivity of $Cl^-$ is more selective with similar or higher second-order rate constants than ·OH for a wide range of organic compounds. Last, reactive chlorine species-based processes can be optimized to target specific contaminants.

Nonetheless, though the UV/chlorine reaction is highly effective, it tends to produce chlorinated disinfection byproducts by reactions with organic matter, requiring prior removal of organic matter by adsorption. Alternatively, singlet oxygen, $^1O_2$, appears as one of the best oxidative species, given its tolerance to ions and organic matter, and longer lifetime compared to ·OH and $SO_4^{2−}$ (Morrison et al. 2023). Catalytic production of $^1O_2$ from $O_2$ is also regarded as a green and cost-effective method for large scale
applications. Nevertheless, the slow reactivity of $ ^{1}O_{2} $ with some organic contaminants needs to be addressed.

Future research should focus on the role of secondary radicals produced from the reactions of radicals of advanced oxidation processes with constituents of water such as natural organic matter (NOM), fulvic acids, humic acids, bromide ($Br^{-}$), iodide, ($I^{-}$), carbonates ($HCO_{3}^{-}/CO_{3}^{2-}$), and phosphates ($H_{2}PO_{4}^{-}/HPO_{4}^{2-}$) (Wen et al. 2022; Sharma et al. 2023b). Indeed, these constituents are known to decrease the efficiency of treatments by consuming reactive species. Little is known about the role of secondary radicals in abatement as they have their own steady-state concentrations, half-lives, and reactivity with organic compounds. Moreover, more research should be done on the structure, properties and dynamics of natural organic matter at the molecular level because, so far, few compounds have been identified (Mitschke et al. 2023; Lichtfouse 2024). Last, more research is needed using real conditions with real wastewater because most publications report experiments in either ultrapure water or simulated wastewater, which are often irrelevant of real conditions where micropollutants occur only as trace compounds in complex organo-mineral solid-liquid media.

Declaration

EL and VKS declare that they are Editors of Environmental Chemistry Letters.

References

Anand U, Dey S, Bontempi E et al (2023) Biotechnological methods to remove microplastics: a review. Environ Chem Lett 21:1787-1810.
https://doi.org/10.1007/s10311-022-01552-4

Banu AN, Kudesia N, Raut AM et al (2021) Toxicity, bioaccumulation, and transformation of silver nanoparticles in aqua biota: a review. Environ Chem Lett 19:4275-4296. https://doi.org/10.1007/s10311-021-01304-w

DadashiFirouzjaei M, Zolghadr E, Ahmadalipour S et al (2022) Chemistry, abundance, detection and treatment of per- and polyfluoroalkyl substances in water: a review. Environ Chem Lett 20:661-679. https://doi.org/10.1007/s10311-021-01340-6

Dejaeger K, Criquet J, Vanoppen M et al (2022) Identification of disinfection by-product precursors by natural organic matter fractionation: a review. Environ Chem Lett 20:3861-3882. https://doi.org/10.1007/s10311-022-01478-x

Gu B, Zhang X, Lam SK et al (2023) Cost-effective mitigation of nitrogen pollution from global croplands. Nature 613:77-84. https://doi.org/10.1038/s41586-022-05481-8

Guo K, Wu Z, Chen C et al (2022) UV/Chlorine process: an efficient advanced oxidation process with multiple radicals and functions in water treatment. Acc Chem Res 55:286-297. https://doi.org/10.1021/acs.accounts.1c00269

He K, Feerick A, Jin H et al (2024) Retention of per- and polyfluoroalkyl substances by syringe filters. Environ Chem Lett 22:1569-1579.
https://doi.org/10.1007/s10311-024-01718-2

Li X-F, Mitch WA (2018) Drinking water disinfection byproducts (DBPs) and human health effects: multidisciplinary challenges and opportunities. Environ Sci Technol 52:1681-1689. https://doi.org/10.1021/acs.est.7b05440

Lichtfouse E (2024) Single sample molecular chronology. Acc Chem Res 57:514-520. https://doi.org/10.1021/acs.accounts.3c00665

Marbaniang CV, Sathiyan K, McDonald TJ et al (2023) Metal ion-induced enhanced oxidation of organic contaminants by ferrate: a review. Environ Chem Lett 21:1729-1743. https://doi.org/10.1007/s10311-023-01584-4

Mitschke N, Vemulapalli SPB, Dittmar T (2023) NMR spectroscopy of dissolved organic matter: a review. Environ Chem Lett 21:689-723.
https://doi.org/10.1007/s10311-022-01528-4

Morin-Crini N, Lichtfouse E, Liu G et al (2022) Worldwide cases of water pollution by emerging contaminants: a review. Environ Chem Lett 20:2311-2338. https://doi.org/10.1007/s10311-022-01447-4

Morrison CM, Hogard S, Pearce R et al (2023) Critical review on bromate formation during ozonation and control options for its minimization. Environ Sci Technol 57:18393-18409. https://doi.org/10.1021/acs.est.3c00538op

Reid E, Igou T, Zhao Y et al (2023) The minus approach can redefine the standard of practice of drinking water treatment. Environ Sci Technol 57:7150-7161. https://doi.org/10.1021/acs.est.2c09389

Richardson SD, Temes TA (2022) Water analysis: emerging contaminants and current issues. Anal Chem 94:382-416. https://doi.org/10.1021/acs.analchem.1c04640

Savage N (2023) How to take ‘forever’ out of forever chemicals. Nature.
https://doi.org/10.1038/d41586-023-03876-9

Shah AD, Mitch WA (2012) Halonitroalkanes, halonitriles, haloamides, and N-nitrosamines: a critical review of nitrogenous disinfection byproduct formation pathways. Environ Sci Technol 46:119-131.
https://doi.org/10.1021/es203312s

Shannon M, Bohn P, Elimelech M et al (2008) Science and technology for water purification in the coming decades. Nature 452:301-310.
https://doi.org/10.1038/nature06599

Sharma VK, Ma X, Lichtfouse E et al (2023a) Nanoplastics are potentially more dangerous than microplastics. Environ Chem Lett 21:1933-1936. https://doi.org/10.1007/s10311-022-01539-1

Sharma VK, Ma X, Zboril R (2023b) Single atom catalyst-mediated generation of reactive species in water treatment. Chem Soc Rev 52:7673-7686. https://doi.org/10.1039/D3CS00627A

Wang Y, Okochi H, Tani Y et al (2023) Airborne hydrophilic microplastics in cloud water at high altitudes and their role in cloud formation. Environ Chem Lett 21:3055-3062. https://doi.org/10.1007/s10311-023-01626-x

Wen Y, Sharma VK, Ma X (2022) Activation of peroxymonosulfate by phosphate and carbonate for the abatement of atrazine: roles of radical and nonradical species. ACS EST Water 2:635-643. https://doi.org/10.1021/acsestwater.1c00482

Yang Y, Zhang X, Jiang J et al (2022) Which micropollutants in water environments deserve more attention globally? Environ Sci Technol 56:13-29. https://doi.org/10.1021/acs.est.1c04250

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