Generation of hydroxyl radical during chlorination of hydroxyphenols and natural organic matter extracts

FePO4/WB as an efficient heterogeneous Fenton-like catalyst for rapid removal of neonicotinoid insecticides: ROS quantification, mechanistic insights and degradation pathways

Iron-based catalysts for peroxymonosulfate (PMS) activation hold considerable potential in water treatment. However, the slow conversion of Fe(III) to Fe(II) restricts its large-scale application. Herein, an iron phosphate tungsten boride composite (FePO4/WB) was synthesized by a simple hydrothermal method to facilitate the Fe(III)/Fe(II) redox cycle and realize the efficient degradation of neonicotinoid insecticides (NEOs). Based on electron paramagnetic resonance (EPR) characterization, scavenging experiments, chemical probe approaches, and quantitative tests, both radicals (HO• and SO4⋅-) and non-radicals (1O2 and Fe(IV)) were produced in the FePO4/WB-PMS system, with relative contributions of 3.02 %, 3.58 %, 6.24 %, and 87.16 % to the degradation of imidacloprid (IMI), respectively. Mechanistic studies revealed that tungsten boride (WB) promoted the reduction of FePO4, and the generated Fe(II) dominantly activated PMS through a two-electron transfer to form Fe(IV), while a minority of Fe(II) engaged in a one-electron transfer with PMS to produce SO4⋅-, HO•, and 1O2. In addition, four degradation pathways of NEOs were proposed by analyzing the byproducts using UPLC-Q-TOF-MS/MS. Besides, seed germination experiments revealed the biotoxicity of NEOs was significantly reduced after degradation via the FePO4/WB-PMS system. Meanwhile, the recycling experiments and continuous flow reactor experiments showed that FePO4/WB exhibited high stability. Overall, this study provided a new perspective on water remediation by Fenton-like reaction. ENVIRONMENTAL IMPLICATION: Neonicotinoids (NEOs) are a type of insecticide used widely around the world. They've been found in many aquatic environments, raising concerns about their possible negative effects on the environment and health. Iron-based catalysts for peroxymonosulfate (PMS) activation hold great promise for water purification. However, the slow conversion of Fe(III) to Fe(II) restricts its large-scale application. Herein, iron phosphate tungsten boride composite (FePO4/WB) was synthesized by a simple hydrothermal method to facilitate the Fe(III)/Fe(II) redox cycle and realize the efficient degradation of NEOs. The excellent stability and reusability provided a great prospect for water remediation.

Were Persulfate-Based Advanced Oxidation Processes Really Understood? Basic Concepts, Cognitive Biases, and Experimental Details

Persulfate (PS)-based advanced oxidation processes (AOPs) for pollutant removal have attracted extensive interest, but some controversies about the identification of reactive species were usually observed. This critical review aims to comprehensively introduce basic concepts and rectify cognitive biases and appeals to pay more attention to experimental details in PS-AOPs, so as to accurately explore reaction mechanisms. The review scientifically summarizes the character, generation, and identification of different reactive species. It then highlights the complexities about the analysis of electron paramagnetic resonance, the uncertainties about the use of probes and scavengers, and the necessities about the determination of scavenger concentration. The importance of the choice of buffer solution, operating mode, terminator, and filter membrane is also emphasized. Finally, we discuss current challenges and future perspectives to alleviate the misinterpretations toward reactive species and reaction mechanisms in PS-AOPs.

Photochemical Transformation of Monochloramine Induced by Triplet State Dissolved Organic Matter

The photoexcited dissolved organic matter (DOM) could produce reactive intermediates, affecting chemical oxidant transformation in UV based advanced oxidation processes (AOPs). This study confirmed the critical role of triplet state DOM (3DOM*), generated from DOM photoexcitation, in the transformation of monochloramine (NH2Cl), a commonly used chemical oxidant and disinfectant in water treatment. NH2Cl (42.25 μM, as Cl2) was decayed by 17.4-73.4 % within 60 min, primarily due to 3DOM* , in DOM (2-30 mgC L-1) solutions irradiated by 365 nm, where NH2Cl has no absorption. The second-order quenching rate constants of triplet state model photosensitizers by NH2Cl were determined to be 0.95(± 0.04)-4.49(± 0.04)× 108 M-1 s-1 by using laser flash photolysis. As a reductant, 3DOM* reacted with NH2Cl through one-transfer mechanism, leading to amino radical (NH2•) generation, which then transferred to ammonia (NH4+, pKa 9.25) through H-abstraction by the phenolic moieties in DOM. Additionally, the intermediate product of 3DOM* oxidized by NH2Cl or those triplet state quinones can hydrolyze to form phenolic moieties, elevating NH4+ yield to higher than 99% upon 365 nm irradiation. These findings suggest that the widespread DOM can be applied to convert NH2Cl via 3DOM* with minimal toxic risks.

Preparation of novel Fe-containing zeolite-A for KN-R decolorization by Fenton-like reaction

Herein, novel catalysts of Fe-containing zeolite-A (Fe/zeolite-A) were synthesized by exchanging iron ions into zeolite-A framework, and short-chain organic acids (SCOAs) were employed as chelating agents. Reactive Brilliant Blue KN-R (KN-R) was used as a model pollutant to evaluate the performance of these catalysts based on the heterogeneous Fenton reaction. The results showed that Fe-OA/3A, which applied zeolite-3A as the supporter and oxalic as the chelating agent, presented the most prominent KN-R decolorization efficiency. Under the initial pH of 2.5, 0.4 mM KN-R could be totally decolorized within 20 min. However, the mineralization efficiency of KN-R was only 58.2%. Therefore, anthraquinone dyes were introduced to modify zeolite-3A. As a result, the mineralization efficiency of KN-R was elevated to 92.7% when using Alizarin Violet (AV) as the modifier. Moreover, the modified catalysts exhibited excellent stability, the KN-R decolorization efficiency could be maintained above 95.0% within 20 min after operating for nine cycles. The mechanism revealed that the Fe(II)/Fe(III) cycle was accelerated by AV-modified catalyst thus prompting the KN-R decolorization in Fenton-like system. These findings provide new insights for preparing catalysts with excellent activity and stability for dye wastewater treatment.

Efficient treatment of actual glyphosate wastewater via non-radical Fenton-like oxidation

Compared to radical oxidative pathway, recent research revealed that non-radical oxidative pathway has higher selectivity, higher adaptability and lower oxidant requirement. In this work, we have designed and synthesized Cu2O/Cu nanowires (CuNWs), by pyrolysis of copper chloride and urea, to selectively generate high-valent copper (CuIII) upon H2O2 activation for the efficient treatment of actual glyphosate wastewater. The detailed characterizations confirmed that CuNWs nanocomposite was comprised of Cu0 and Cu2O, which possessed a nanowire-shaped structure. The electron paramagnetic resonance (EPR) analysis, in situ Raman spectra, chronoamperometry and liner sweep voltammetry (LSV) verified CuIII, which mainly contributed to glyphosate degradation, was selectively generated from CuNWs/H2O2 system. In particular, CuI is mainly oxidized by H2O2 into CuIIIvia dual-electron transfer, rather than simultaneously releasing OH• via single electron transfer. More importantly, CuNWs/H2O2 system exhibited the excellent potential in the efficient treatment of actual glyphosate wastewater, with 96.6% degradation efficiency and chemical oxygen demand (COD) dropped by 30%. This novel knowledge gained in the work helps to apply CuNWs into heterogeneous Fenton-like reaction for environmental remediation and gives new insights into non-radical pathway in H2O2 activation.

Reaction Mechanisms of Chlorine Dioxide with Phenolic Compounds─Influence of Different Substituents on Stoichiometric Ratios and Intrinsic Formation of Free Available Chlorine

Chlorine dioxide (ClO2) is an oxidant applied in water treatment processes that is very effective for disinfection and abatement of inorganic and organic pollutants. Thereby phenol is the most important reaction partner of ClO2 in reactions of natural organic matter (NOM) and in pollutant degradation. It was previously reported that with specific reaction partners (e.g., phenol), free available chlorine (FAC) could form as another byproduct next to chlorite (ClO2-). This study investigates the impact of different functional groups attached to the aromatic ring of phenol on the formation of inorganic byproducts (i.e., FAC, ClO2-, chloride, and chlorate) and the overall reaction mechanism. The majority of the investigated compounds reacted with a 2:1 stoichiometry and formed 50% ClO2- and 50% FAC, regardless of the position and kind of the groups attached to the aromatic ring. The only functional groups strongly influencing the FAC formation in the ClO2 reaction with phenols were hydroxyl- and amino-substituents in ortho- and para-positions, causing 100% ClO2- and 0% FAC formation. Additionally, this class of compounds showed a pH-dependent stoichiometric ratio due to pH-dependent autoxidation. Overall, FAC is an important secondary oxidant in ClO2 based treatment processes. Synergetic effects in pollutant control and disinfection might be observable; however, the formation of halogenated byproducts needs to be considered as well.

A self-cascade system based on Ag nanoparticle/single-walled carbon nanotube nanocomposites as an enzyme mimic for ultrasensitive detection of L-cysteine

L-Cysteine, widely used in medicine and the food industry, is of great essentiality to organisms and the food quality. Given that current detection approaches require exacting lab conditions and tedious sample treatment, there is a pressing demand for developing a method that possesses advantages of user friendliness, prominent performance, and cost-effectiveness. Herein, a self-cascade system was developed for the fluorescence detection of L-cysteine based on the ingenious performance of Ag nanoparticle/single-walled carbon nanotube nanocomposites (AgNP/SWCNTs) and DNA-templated Ag nanoclusters (DNA-AgNCs). The fluorescence of DNA-AgNCs could be quenched on account of the adsorption of DNA-AgNCs on AgNP/SWCNTs by π-π stacking. With the cooperation of Fe²⁺, AgNP/SWCNTs with oxidase and peroxidase-like activities could catalyze the oxidation of L-cysteine to produce cystine and hydrogen peroxide (H₂O₂) and then break the O-O bond of H₂O₂ to generate a hydroxyl radical (·OH), which could cleave the DNA strand into different sequence fragments which subsequently peeled off from the AgNP/SWCNTs, resulting in a "turn-on" fluorescence response. In this paper, AgNP/SWCNTs with multi-enzyme activities was synthesized enabling the reaction to proceed in just one step. The successful preliminary applications for the L-cysteine detection in pharmaceutical, juice beverage, and serum samples indicated that the developed method exhibited great potential in medical diagnosis, food monitoring, and the biochemical field, which also broadened the horizon for follow-up research.

Bacterial inactivation processes in water disinfection - mechanistic aspects of primary and secondary oxidants - A critical review

Water disinfection during drinking water production is one of the most important processes to ensure safe drinking water, which is gaining even more importance due to the increasing impact of climate change. With specific reaction partners, chemical oxidants can form secondary oxidants, which can cause additional damage to bacteria. Cases in point are chlorine dioxide which forms free available chlorine (e.g., in the reaction with phenol) and ozone which can form hydroxyl radicals (e.g., during the reaction with natural organic matter). The present work reviews the complex interplay of all these reactive species which can occur in disinfection processes and their potential to affect disinfection processes. A quantitative overview of their disinfection strength based on inactivation kinetics and typical exposures is provided. By unifying the current data for different oxidants it was observable that cultivated wild strains (e.g., from wastewater treatment plants) are in general more resistant towards chemical oxidants compared to lab-cultivated strains from the same bacterium. Furthermore, it could be shown that for selective strains chlorine dioxide is the strongest disinfectant (highest maximum inactivation), however as a broadband disinfectant ozone showed the highest strength (highest average inactivation). Details in inactivation mechanisms regarding possible target structures and reaction mechanisms are provided. Thereby the formation of secondary oxidants and their role in inactivation of pathogens is decently discussed. Eventually, possible defense responses of bacteria and additional effects which can occur in vivo are discussed.

Formation of halogenated macromolecular organics induced by Br- and I- during plasma oxidation/chlorination of DOM: Highlighting competitive mechanisms

Understanding the effects of halogens on the production of macromolecular disinfection byproducts (DBPs) is critical for drinking water safety. The effects of Br^- and I^- on the chemical diversity of dissolved organic matter (DOM) during plasma preoxidation and the subsequent formation of macromolecular halogenated DBPs after chlorination were deciphered. Plasma preoxidation changed DOM diversity from aromatic component-oriented to lignin and tannin component-oriented, resulting in 62.0% and 21.2% decreases in N-DBPs (C_kH_nO_mN_zCl_x formulas) and C-DBPs (C_kH_nO_mCl_x formulas) after chlorination, respectively. Br^- could induce the formation of organobromine compounds (OBrCs) during plasma oxidation; however, the intensities of OBrCs decreased by 56.3% (CHO formulas) and 75.2% (CHON formulas) after further chlorination. OBrCs still accounted for 79.8% of the total organohalogen compounds (OXCs, X=Cl or Br) due to the higher substitutability of bromine. I^-promoted OIC production in the DOM preoxidation process, and OICs acted as intermediates to form OClCs during chlorination. When Br^-and I^-coexisted, Br^- promoted OIC production in the DOM preoxidation process; therefore, more OBrCs and OClCs were generated due to intermediates of OICs in subsequent chlorination. Connections between OXCs and their precursors were established using network computation. The precursors of OClCs were located in the aromatic structure region (0.2 < H/C ≤ 0.7; O/C ≤ 0.67); those of OBrCs and OICs were located in the lignin (0.7 < H/C ≤ 1.5; 0.1 < O/C < 0.67) and tannin (0.6 ≤ H/C ≤ 1.5, 0.67 < O/C < 1.0) regions with relatively greater H/C and O/C ratios, respectively.

Dominant Dissolved Oxygen-Independent Pathway to Form Hydroxyl Radicals and the Generation of Reactive Chlorine and Nitrogen Species in Breakpoint Chlorination

Due to the complexities of the interactions between ammonia, chlor(am)ine, and intermediate species such as ONOOH, the radical formation in breakpoint chlorination and the consequential removal of micropollutants remain largely unexplored. In this study, the dominant generation pathway of HO^•, as a primary radical in breakpoint chlorination, was examined, and the generations of HO^•, reactive chlorine species (RCS), and reactive nitrogen species (RNS) were quantitatively evaluated. A dissolved oxygen (DO)-independent pathway was verified by ¹⁸O labeling and contributed over 90% to HO^• generation. The commonly believed pathway, the decomposition of ONOOH involving DO, contributed only 7% to HO^• formation in breakpoint chlorination. The chlorine to nitrogen (Cl/N) ratio and pH greatly affected the generations and speciations of the reactive species. An optimum Cl/N mass ratio for HO^•, Cl₂^•-, and RNS generations occurred at the breakpoint (i.e., Cl/N mass ratio = 9), whereas excessive free chlorine shifted the radical speciation toward ClO^• at Cl/N mass ratios above the breakpoint. Basic conditions inhibited the generations of HO^• and RNS but significantly promoted that of ClO^•. These findings improved the fundamental understanding of the radical chemistry of breakpoint chlorination, which can be extended to estimate the degradations of micropollutants of known rate constants toward the reactive species with influences from the Cl/N ratio and pH in real-world applications.

Formation of Larger Molecular Weight Disinfection Byproducts from Acetaminophen in Chlorine Disinfection

Acetaminophen is widely used to treat mild to moderate pain and to reduce fever. Under the worldwide COVID-19 pandemic, this over-the-counter pain reliever and fever reducer has been drastically consumed, which makes it even more abundant than ever in municipal wastewater and drinking water sources. Chlorine is the most widely used oxidant in drinking water disinfection, and chlorination generally causes the degradation of organic compounds, including acetaminophen. In this study, a new reaction pathway in the chlorination of acetaminophen, i.e., oxidative coupling reactions via acetaminophen radicals, was investigated both experimentally and computationally. Using an ultraperformance liquid chromatograph coupled to an electrospray ionization-triple quadrupole mass spectrometer, we detected over 20 polymeric products in chlorinated acetaminophen samples, some of which have structures similar to the legacy pollutants "polychlorinated biphenyls". Both C-C and C-O bonding products were found, and the corresponding bonding processes and kinetics were revealed by quantum chemical calculations. Based on the product confirmation and intrinsic reaction coordinate computations, a pathway for the formation of the polymeric products in the chlorination of acetaminophen was proposed. This study suggests that chlorination may cause not only degradation but also upgradation of a phenolic compound or contaminant.

CaO2-based electro-Fenton-oxidation of 1,2-dichloroethane in groundwater

It has been well recognized that the Fenton reaction requires a rigorous pH control and suffers from the fast self-degradation of H₂O₂. In an effort to resolve the technical demerits of the conventional Fenton reaction, particular concern on the use of CaO₂-based Fenton reaction was paid in this study. To realize the practical use of CaO₂ in the Fenton reaction for groundwater remediation, it could be of great importance to control its reaction rate in the subsurface. As such, this study laid great emphasis on the combined process of electrochemical oxidation and CaO₂-based Fenton oxidation, using 1,2-dichloroethane (1,2-DCA) as a model compound. It was hypothesized that the reaction rate is also highly contingent on the formation of Fe(II) (stemmed from iron anode oxidation). Eighty percent of 1,2-DCA were degraded by the CaO₂-based Fenton reaction. The final pH was neutral, inferring that the reaction could be a viable option for the subsurface environment. Moreover, the supply of electric current in an iron anode expedited 1,2-DCA degradation efficiency from 35 % to 62 % via electrically generated Fe(II), which donated electrons to H₂O₂, producing more hydroxyl radicals. An anode-cathode configuration from the single-well system enhanced the degradation of 1,2-DCA, with less amount of energy consumption than the double-well system. Based on results, CaO₂-based electro-Fenton oxidation can remove well 1,2-DCA in groundwater and can be a strategic measure for groundwater remediation.

Photodynamic inactivation of multidrug-resistant bacteria in wastewater effluent using green phytochemicals as a natural photosensitizer

The control of multidrug-resistant bacteria (MDRB) is a great challenge in the 21st century. Photodynamic treatment (PDT) is one of the promising approaches to control MDRB. In the process, powerful oxidants such as reactive oxygen species (ROS) are produced, which cause cytotoxic damage and cell death of bacteria. This study examined a new and environment-friendly strategy for the photodynamic inactivation of two MDRB (Escherichia coli and Staphylococcus aureus) and total coliform (TC) in wastewater effluent using two phytochemicals, pyrogallol (PGL) and terpinolene (TPN), along with white and blue light-emitting diode (LED) light. Fourier-transform infrared spectroscopy (FTIR) of the phytochemicals confirmed the presence of different phenolic and aromatic compounds, which can enhance the generation of ROS alongside inactivating the bacterial cells. In the PDT process, white LED light was more active in controlling MDRB than blue LED light. After 80 min irradiation with white LED light (17 mW/cm²), the MDRB bacteria were eradicated completely at a minimum inhibitory concentration (MIC) dose (0.156 mg/mL for E. coli and 0.078 mg/mL for S. aureus) of PGL. In addition, light intensity was an important parameter in photodynamic disinfection. The TC in the secondary effluent was inactivated completely by both phytochemicals after 60 min of exposure to white LED light with an intensity of 80 mW/cm². The photosensitizing activity of phytochemicals was analyzed by a bactericidal and imidazole-RNO assay. These assays showed that PGL contributed to the generation of •OH radicals, whereas TPN produced ¹O₂ in the PDT process. Transmission electron microscopy (TEM) confirmed bacterial cell disruption after treatment. Overall, PDT using the phytochemicals as PS is a sustainable approach to control the MDRB and TC in wastewater successfully.

Simultaneous redox transformation and removal of Cr(Ⅵ) and As(Ⅲ) by polyethyleneimine modified magnetic mesoporous polydopamine nanocomposite: Insights into synergistic effects and mechanisms

Chromium(Ⅵ) and arsenic(Ⅲ) as typical anionic heavy metal pollutants normally coexist in the environment, greatly aggravating their environmental risks and elevating the difficulty of remediation. Here, a novel polyethyleneimine modified magnetic mesoporous polydopamine nanocomposite (Fe₃O₄ @mesoPDA/PEI) with abundant active functional groups was exploited as the synchronous adsorbent of Cr(Ⅵ) and As(Ⅲ). The results showed that Cr(Ⅵ) and As(Ⅲ) could mutually promote their conversions and adsorptions on Fe₃O₄ @mesoPDA/PEI. The adsorption mechanisms of Fe₃O₄ @mesoPDA/PEI were primarily redox chemistry and also involved electrostatic interactions and coordination. Cr(Ⅵ) was mainly reduced by reductive catechol, while As(Ⅲ) was oxidized to As(Ⅴ) by oxidative active substances (e.g., H₂O₂, •OH, and quinone). Meanwhile, active intermediate (semiquinone radicals) generated during the Cr(Ⅵ) reduction and As(Ⅲ) oxidation could constitute redox microcirculation with Cr(Ⅵ) and As(Ⅲ) to further accelerate redox reactions of Cr(Ⅵ) and As(Ⅲ) on Fe₃O₄ @mesoPDA/PEI, thereby exhibiting a synergistic effect. Moreover, newly immobilized Cr(Ⅲ) onto Fe₃O₄ @mesoPDA/PEI became extra active sites for As adsorption through cation bridges and then recovered by magnetic separation in favor of diminishing the environmental hazards of Cr and As. These findings also provide new inspirations for the roles of redox-active functional groups in the remediation of multiple redox-sensitive heavy metals including Cr(Ⅵ) and As(Ⅲ).

Enhanced photocatalytic degradation of malachite green using manganese oxide doped graphene oxide/zinc oxide (GO-ZnO/Mn2O3) ternary composite under sunlight irradiation

In this work, photocatalytic degradation of malachite green (MG) dye using manganese oxide doped graphene oxide/zinc oxide (GO-ZnO/Mn₂O₃) ternary composite under sunlight irradiation is studied. GO-ZnO/Mn₂O₃ is a novel composite, which is non-toxic and of low cost prepared by the conventional solvothermal route. The synthesized composite was characterized using Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), RAMAN spectra, photoluminescence (PL) spectroscopy, Diffuse reflectance spectroscopy (UV-Vis. DRS), and Barrett-Joyner-Halenda (BJH) pore size distribution. The GO-ZnO/Mn₂O₃ composite has a moderately large surface area of 75.35 m²/g and bandgap energy of 1.6 eV. Due to more pollutants adsorbed onto the photocatalyst surface and the reduction in bandgap energy, resulted less recombination rate and shifted the light absorption into the visible region to effectively utilize the sunlight and enhanced degradation of 98.75% is achieved within 30 min of sunlight irradiation for GO-ZnO/Mn₂O₃ composite. Using various scavengers, the main oxidizing radicals for the degradation of MG dye are identified as hydroxyl (^•OH) and superoxide radical (^•O₂^-). The synthesized composite is stable and reproducible as after five successful cycles, there was a small reduction in its degradation efficiency of 11.65%. Keywords: Graphene oxide; Photocatalyst; Malachite green; Bandgap; Scavengers.

Facilely tuning the intrinsic catalytic sites of the spinel oxide for peroxymonosulfate activation: From fundamental investigation to pilot-scale demonstration

Heterogeneous peroxymonosulfate (PMS)-based advanced oxidation processes (AOPs) have shown a great potential for pollutant degradation, but their feasibility for large-scale water treatment application has not been demonstrated. Herein, we develop a facile coprecipitation method for the scalable production (∼10 kg) of the Cu-Fe-Mn spinel oxide (CuFeMnO). Such a catalyst has rich oxygen vacancies and symmetry-breaking sites, which endorse it with a superior PMS-catalytic capacity. We find that the working reactive species and their contributions are highly dependent on the properties of target organic pollutants. For the organics with electron-donating group (e.g., -OH), high-valent metal species are mainly responsible for the pollutant degradation, whereas for the organics with electron-withdrawing group (e.g., -COOH and -NO2), hydroxyl radical (•OH) as the secondary oxidant also plays an important role. We demonstrate that the CuFeMnO-PMS system is able to achieve efficient and stable removal of the pollutants in the secondary effluent from a municipal wastewater plant at both bench and pilot scales. Moreover, we explore the application prospect of this PMS-based AOP process for large-scale wastewater treatment. This work describes an opportunity to scalably prepare robust spinel oxide catalysts for water purification and is beneficial to the practical applications of the heterogeneous PMS-AOPs.

Revisiting the Oxidizing Capacity of the Periodate-H2O2 Mixture: Identification of the Primary Oxidants and Their Formation Mechanisms

This study reexamined the mechanisms for oxidative organic degradation by the binary mixture of periodate and H₂O₂ (PI/H₂O₂) that was recently identified as a new advanced oxidation process. Our findings conflicted with the previous claims that (i) hydroxyl radical (^•OH) and singlet oxygen (¹O₂) contributed as the primary oxidants, and (ii) ^•OH production resulted from H₂O₂ reduction by superoxide radical anion (O₂^•-). PI/H₂O₂ exhibited substantial oxidizing capacity at pH < 5, decomposing organics predominantly by ^•OH. The likelihood of a switch in the major oxidant under varying pH conditions was revealed. IO₄^- as the major PI form under acidic conditions underwent one-electron reduction by H₂O₂ to yield radical intermediates, whereas H₂I₂O₁₀^4- preferentially occurring at pH > 7 caused ¹O₂ generation through two-electron oxidation of H₂O₂. PI reduction by O₂^•- was suggested to be a key reaction in ^•OH production, on the basis of the electron paramagnetic resonance detection of methyl radicals in the dimethyl sulfoxide solutions containing PI and KO₂, and the absence of deuterated and ¹⁸O-labeled hydroxylated intermediates during PI activation using D₂O and H₂¹⁸O₂. Finally, simple oxyanion mixing subsequent to electrochemical PI and H₂O₂ production achieved organic oxidation, enabling a potential strategy to minimize the use of chemicals.

Removal of 1,2-dichloroethane in groundwater using Fenton oxidation

Among the chlorinated aliphatic hydrocarbons, 1,2-dichloroethane (1,2-DCA) is widely used for the synthesis of vinyl chloride monomers. Despite the high demand for 1,2-DCA, it poses a risk to the environment because it is persistent and carcinogenic. Therefore, in this study, several reagents (dithionite, hydrosulfide, sulfite, persulfate, sulfate radicals, and hydroxyl radicals) were evaluated for the degradation of 1,2-DCA. Among these, the hydroxyl radicals generated by the Fenton reaction were the most suitable oxidant, decomposing 92% of 1,2-DCA. Chloride, one of the final oxidized products, was observed, which supported the oxidation reaction. Moreover, with an increasing concentration of hydroxyl radicals, the degradation of 1,2-DCA increased. Furthermore, sufficient amounts of hydrogen peroxide were more important than Fe(II) in the decomposition of 1,2-DCA. The radical reaction can generate larger molecules via the degradation of 1,2-DCA, which are degraded over time. The applicability of Fenton oxidation was evaluated using real 1,2-DCA-contaminated groundwater. Although the degradation of target contaminant was lowered due to the alkaline pH and the presence of chloride and bicarbonate ions in groundwater, the Fenton reaction was still efficient to oxidize 1,2-DCA. These results indicate that Fenton oxidation is an effective technique for the treatment of 1,2-DCA in contaminated groundwater.

Humic Acid and Fulvic Acid Hinder Long-Term Weathering of Microplastics in Lake Water

We investigated the photoaging of polypropylene (PP) microplastics (MPs) in lake water. The results showed that photoaging of PP MPs was significantly inhibited in lake water compared with ultrapure water after 12 d of ultraviolet (UV) irradiation, and humic acid and fulvic acid, rather than carbonate (CO₃^2-), nitrate (NO₃^-), or chloride (Cl^-) ions, were identified as the primary contributors to the observed inhibition. Mechanisms for the roles of humic acid (Suwannee River humic acid) and fulvic acid (Pony Lake fulvic acid) in reducing the rates of photodegradation showed that humic acid and fulvic acid acted as both reactive oxygen species (ROS) scavengers (e.g., of ^•OH) (dominant contribution) and optical light filters. As ROS scavengers, humic acid and fulvic acid significantly decreased the capacity for the formation of ^•OH and O₂^•- by PP MPs under irradiation. In addition, the chromophores in humic acid and fulvic acid competed for photons with MPs through the light-shielding effect, thereby causing less fragmentation of PP particles and changes in other properties (melting temperature, contact angle, and surface zeta potential). The proposed mechanisms for inhibition by humic acid and fulvic acid will aid our efforts to assess the duration of aging and alterations of MP properties during long-term weathering in natural waters.

Characteristics and transformation pathways of venlafaxine degradation during disinfection processes using free chlorine and chlorine dioxide

Venlafaxine, a representative antidepressant, has been detected frequently in aquatic environments. The treatment of venlafaxine by free chlorine (NaOCl) and chlorine dioxide (ClO₂) was investigated in this study. The effects of operational variables and the water matrix on venlafaxine degradation were evaluated. The transformation pathways of venlafaxine were also studied. The results indicated that venlafaxine was removed efficiently during disinfection processes, especially when reacted with ClO₂. A higher dosage of disinfectant and mildly alkaline conditions (pH 9) enhanced the degradation of venlafaxine. The reactions were impacted when the tests were conducted in real water matrices, especially in secondary effluent. The presence of chloride and low concentrations of fulvic acid enhanced venlafaxine decomposition. The presence of Br^- also accelerated the reaction between venlafaxine and NaOCl. However, NO₂^- inhibited venlafaxine removal in both disinfection processes. Six intermediates were identified during venlafaxine degradation by ultrahigh-performance liquid chromatography with quadrupole-time-of-flight mass spectrometry, and the main reactions included dehydration and demethylation.

Evaluation of hydroxyl radical and reactive chlorine species generation from the superoxide/hypochlorous acid reaction as the basis for a novel advanced oxidation process

The reaction of hypochlorous acid (HOCl) with superoxide radical (O₂^•-) - a source of hydroxyl radical (HO^•) and various reactive chlorine species (RCS) - was investigated as the basis for a novel non-photochemical advanced oxidation process (AOP). Moderately stable (t_1/2 ~ minutes) aqueous O₂^•- stocks were prepared by several approaches at pH>12 and either (a) added directly to aqueous free available chlorine (FAC; i.e., HOCl/OCl^-) at circumneutral pH, or (b) premixed with alkaline FAC and then acidified to pH 7, to degrade various organic probe compounds via in situ generated HO^• and RCS. Radical production was optimal at [HO₂^•/O₂^•-]₀/[FAC]₀ ~ 2, with ~0.8 mol HO^• formed/mol FAC consumed, and HO^• and RCS exposures reaching ~5×10^-10 and ~10^-9 M×s, respectively. Similar trends were observed in natural waters and organic matter-amended phosphate buffer containing up to 5 mgC/L of dissolved organic carbon. Direct formation of oxyhalides, trihalomethanes (THMs), and haloacetic acids (HAAs), was minimal, though THM and HAA formation was moderately enhanced during post-chlorination of O₂^•-/FAC-treated solutions. This process could provide a beneficial addition to the range of available AOPs due to its high radical exposures, simplicity, rapid time-scales, potential for on-site O₂^•- generation, and widespread accessibility of FAC and other reagents.

Enhancement of photocatalytic degradation of Malachite Green using iron doped titanium dioxide loaded on oil palm empty fruit bunch-derived activated carbon

Iron-doped titanium dioxide loaded on activated carbon (Fe-TiO₂/AC) was successfully synthesized from oil palm empty fruit bunch (OPEFB) using sol-gel method. The properties of the synthesized pure TiO₂, Fe-doped TiO₂, AC, TiO₂/AC and Fe-TiO₂/AC were examined by various techniques such as field emission scanning electron microscopy (FE-SEM), Fourier-transform infrared spectroscopy (FT-IR), UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) and nitrogen adsorption-desorption analyses at 77 K. FE-SEM revealed that Fe-doped TiO₂ particles were dispersed homogeneously on the AC surface. FT-IR demonstrated high surface hydroxylation after Fe doping on TiO₂ and UV-Vis DRS showed that Fe-TiO₂/AC had the lowest band gap energy. Catalytic performance results proved that Fe dopants could restrict the recombination rate of hole and electron pairs, whereas AC support improved the Malachite Green (MG) adsorption sites and active sites of the hybrid catalyst. Photocatalytic degradation of 100 mg/L MG in the presence of 1.0 g/L 15 wt% Fe-TiO₂ incorporated with 25 wt% AC, initial solution pH of 4 and 3 mM H₂O₂ could achieve the highest removal efficiency of 97% after 45 min light irradiation. This work demonstrates a promising approach to synthesis an inexpensive and efficient Fe-TiO₂/AC for the photocatalytic degradation of organic dye.

Treating disinfection byproducts with UV or solar irradiation and in UV advanced oxidation processes: A review

This review focuses on the degradation kinetics and mechanisms of disinfection byproducts (DBPs) under UV and solar irradiation and in UV-based advanced oxidation processes (AOPs). A total of 59 such compounds are discussed. The processes evaluated are low pressure, medium pressure and vacuum UV irradiation, solar irradiation together with UV/hydrogen peroxide, UV/persulfate and UV/chlorine AOPs. Under UV and solar irradiation, the photodegradation rates of N-nitrosamines are much higher than those of halogenated DBPs. Among halogenated DBPs, those containing iodine are photodegraded more rapidly than those containing bromine or chlorine. This is due to differences in their bond energies (E_N-N < E_C-I < E_C-Br < E_C-Cl). Molar absorption coefficients at 254 nm and energy gaps can be used to predict the photodegradation rates of DBPs under low pressure UV irradiation. But many DBPs of interest cannot be degraded to half their original concentration with less than a 500 mJ cm^-2 dose of low pressure UV light. HO^• generally contributes to less than 30% of the degradation of DBPs except iodo-DBPs in UV/H₂O₂ AOPs. Reaction mechanisms under UV irradiation and in HO^•-mediated oxidation are also summarized. N-N bond cleavage initiates their direct UV photolysis of N-nitrosamines as C-X cleavage does among halogenated compounds. HO^• generally initiates degradation via single electron transfer, addition and hydrogen abstraction pathways. Information on the reaction rate constants of SO₄^•- and halogen radicals with DBPs is rather limited, and little information is available about their reaction pathways. Overall, this review provides improved understanding of UV, solar and AOPs.