Kinetics and efficiency of the hydrated electron-induced dehalogenation by the sulfite/UV process

Photo-reductive decomposition of perfluorooctane sulfonate (PFOS) and its common alternatives by UV/VUV/sulfite process: Mechanism, kinetic modeling, and water matrix effects

The present study investigated the photo-reduction of perfluorooctane sulfonate (PFOS) and its alternatives, focusing on decomposition mechanisms, active species involvement, the influence of background water constituents, and kinetic model development. The decomposition and defluorination rates followed the order of PFOS > PFHxS > 6:2 FTSA > PFBS, with shorter chains and CH2 linkers enhancing the resistance of PFOS alternatives against the attack of hydrated electrons (eaq-). Two primary pathways were identified during the photodegradation of PFAS: (i) H/F exchange at CF bonds with the lowest bond dissociation energies (BDEs) and (ii) functional group cleavage followed by short-chain PFCAs formation, with OH playing a crucial role in transforming intermediates. Adding iodide and elevated temperatures demonstrated a synergistic effect on PFBS decomposition and defluorination, with high temperatures promoting functional group cleavage as the preferred defluorination pathway. The study examined the impact of background water constituents in different aqueous environments, from surface waters to wastewater streams and ion-exchange brine concentrates. Chloride exhibited a concentration-based dual impact on the UV/VUV/sulfite process: promotive effects at low dosages (1-10 mM) by acting as a secondary eaq- mediator, and adverse effects at high dosages (20-500 mM) due to the scavenging effect of generated chlorine radicals (Cl). High ionic strength adversely affected eaq- quantum efficiency. Additionally, bicarbonate and natural organic matter (NOM) had opposing effects on PFOS photo-reduction, primarily through eaq- scavenging and pH alteration. Kinetic modeling revealed reaction rate constants of the studied PFAS with eaq- ranging from 1.8 × 106 to 1.3 × 109 M-1 s-1, corroborating the concentration profiles of active species and highlighting the reductive nature of sulfite-mediated processes.

Insight into the Efficient Selective Reduction of Cr(VI) in Sulfite/UV Process under Near-Neutral Conditions: The Critical Role of In Situ-Generated Sulfite Radical

Efficient removal of contaminants in complex water matrices under mild conditions is highly desirable but still challenging. In this study, we unraveled the overlooked but crucial role of sulfite radical (SO3·-) in the efficient selective reduction of toxic Cr(VI) under near-neutral conditions. Fast removal of Cr(VI) at around pH 7 in sulfite/UV was found to be attributable to high reactivity of SO3·- toward HCrO4- (∼5.3 × 106 M-1 s-1). Furthermore, SO3·- was fast generated in situ via one-electron oxidation of S(IV) by transient reactive protonated Cr(V) and Cr(IV) intermediates. Therefore, the specific reactivity of SO3·- and its in situ generation together resulted in the surprisingly positive effect of nitrate and the efficient reduction of Cr(VI) in authentic surface water and industrial wastewater. A mathematical model was developed to simulate Cr(VI) removal in the process, and thus quantitatively demonstrated the roles of reactive species, i.e., SO3·- contributed to ∼93% of Cr(VI) reduction in surface water. Overall, this study provides an insight into the pivotal role of SO3·- in Cr(VI) reduction, and underscores its significance in selective reduction and detoxification of contaminants.

Carbon Adsorbent Properties Impact Hydrated Electron Activity and Perfluorocarboxylic Acid (PFCA) Destruction

Carbon-based adsorbents used to remove recalcitrant water contaminants, including perfluoroalkyl substances (PFAS), are often regenerated using energy-intensive treatments that can form harmful byproducts. We explore mechanisms for sorbent regeneration using hydrated electrons (eaq -) from sulfite ultraviolet photolysis (UV/sulfite) in water. We studied the UV/sulfite treatment on three carbon-based sorbents with varying material properties: granular activated carbon (GAC), carbon nanotubes (CNTs), and polyethylenimine-modified lignin (lignin). Reaction rates and defluorination of dissolved and adsorbed model perfluorocarboxylic acids (PFCAs), perfluorooctanoic acid (PFOA) and perfluorobutanoic acid (PFBA), were measured. Monochloroacetic acid (MCAA) was employed to empirically quantify eaq - formation rates in heterogeneous suspensions. Results show that dissolved PFCAs react rapidly compared to adsorbed ones. Carbon particles in solution decreased aqueous reaction rates by inducing light attenuation, eaq - scavenging, and sulfite consumption. The magnitude of these effects depended on adsorbent properties and surface chemistry. GAC lowered PFOA destruction due to strong adsorption. CNT and lignin suspensions decreased eaq - formation rates by attenuating light. Lignin showed high eaq - quenching, likely due to its oxygenated functional groups. These results indicate that desorbing PFAS and separating the adsorbent before initiating PFAS degradation reactions will be the best engineering approach for adsorbent regeneration using UV/sulfite.

Perspectives for Photocatalytic Decomposition of Environmental Pollutants on Photoactive Particles of Soil Minerals

The literature shows that both in laboratory and in industrial conditions, the photocatalytic oxidation method copes quite well with degradation of most environmental toxins and pathogenic microorganisms. However, the effective utilization of photocatalytic processes for environmental decontamination and disinfection requires significant technological advancement in both the area of semiconductor material synthesis and its application. Here, we focused on the presence and "photocatalytic capability" of photocatalysts among soil minerals and their potential contributions to the environmental decontamination in vitro and in vivo. Reactions caused by sunlight on the soil surface are involved in its normal redox activity, taking part also in the soil decontamination. However, their importance for decontamination in vivo cannot be overstated, due to the diversity of soils on the Earth, which is caused by the environmental conditions, such as climate, parent material, relief, vegetation, etc. The sunlight-induced reactions are just a part of complicated soil chemistry processes dependent on a plethora of environmental determinates. The multiplicity of affecting factors, which we tried to sketch from the perspective of chemists and environmental scientists, makes us rather skeptical about the effectiveness of the photocatalytic decontamination in vivo. On the other hand, there is a huge potential of the soils as the alternative and probably cheaper source of useful photocatalytic materials of unique properties. In our opinion, establishing collaboration between experts from different disciplines is the most crucial opportunity, as well as a challenge, for the advancement of photocatalysis.

New insight into wastewater treatment by activation of sulfite with humic acid under visible light irradiation

Sulfite (S(IV)), as an alternative to persulfate, has demonstrated its cost-effectiveness and environmentally friendly nature, garnering increasing attention in Advanced Oxidation Processes (AOPs). Dissolved organic matter (DOM) commonly occurred in diverse environments and was often regarded as an interfering factor in sulfite-based AOPs. However, less attention has been paid to the promotion of the activation of sulfite by excited DOM, which could produce various reactive intermediates. The study focused on the activation of sulfite using visible light (VL) - excited humic acid (HA) to efficiently degrade many common organic pollutants, which was better than peroxydisulfate (PDS) and peroxymonosulfate (PMS) systems. Quenching experiments and electron paramagnetic resonance (EPR) analysis revealed that the triplet states of HA (3HA*) activated sulfite through energy transfer, resulting in the production of SO4·-, O2·-, and 1O2. The most significant active species found in the degradation of roxarsone (ROX) was 1O2, which was a non-radical pathway and exhibits high selectivity for pollutant degradation. This non-radical pathway was not commonly observed in traditional sulfite-based AOPs. Additionally, the coexistence of various inorganic anions, such as NO3-, Cl-, SO42-, CO32-, and PO43-, had little effect on the degradation of ROX. Furthermore, DOM from different natural water demonstrated efficient activation of S(IV) under light conditions, opening up new possibilities for applying sulfite-based advanced oxidation to the remediation of organic pollution in diverse sites and water bodies. In summary, this research offered promising insights into the potential application of sulfite-based AOPs, facilitated by photo-excited HA, as a new strategy for efficiently degrading organic pollutants in various environmental settings.

Kinetic Model for Predicting Perfluoroalkyl Acid Degradation During UV-Sulfite Treatment

Hydrated electron (eaq-) treatment processes show great potential in remediating recalcitrant water contaminants, including perfluoroalkyl and polyfluoroalkyl substances (PFAS). However, treatment efficacy depends upon many factors relating to source water composition, UV light source characteristics, and contaminant reactivity. Here, we provide critical insights into the complex roles of solution parameters on contaminant abatement through application of a UV-sulfite kinetic model that incorporates first-principles information on eaq- photogeneration and reactivity. The model accurately predicts decay profiles of short-chain perfluoroalkyl acids (PFAAs) during UV-sulfite treatment and facilitates quantitative interpretation of the effects of changing solution composition on PFAS degradation rates. Model results also confirm that the enhanced degradation of PFAAs observed under highly alkaline pH conditions results from changes in speciation of nontarget eaq- scavengers. Reverse application of the model to UV-sulfite data collected for longer chain PFAAs enabled estimation of bimolecular rate constants (k2, M-1 s-1), providing an alternative to laser flash photolysis (LFP) measurements that are not feasible due to the water solubility limitations of these compounds. The proposed model links the disparate means of investigating eaq- processes, namely, UV photolysis and LFP, and provides a framework to estimate UV-sulfite treatment efficacy of PFAS in diverse water sources.

Effects of Halides on Organic Compound Degradation during Plasma Treatment of Brines

Plasma has been proposed as an alternative strategy to treat organic contaminants in brines. Chemical degradation in these systems is expected to be partially driven by halogen oxidants, which have been detected in halide-containing solutions exposed to plasma. In this study, we characterized specific mechanisms involving the formation and reactions of halogen oxidants during plasma treatment. We first demonstrated that addition of halides accelerated the degradation of a probe compound known to react quickly with halogen oxidants (i.e., para-hydroxybenzoate) but did not affect the degradation of a less reactive probe compound (i.e., benzoate). This effect was attributed to the degradation of para-hydroxybenzoate by hypohalous acids, which were produced via a mechanism involving halogen radicals as intermediates. We applied this mechanistic insight to investigate the impact of constituents in brines on reactions driven by halogen oxidants during plasma treatment. Bromide, which is expected to occur alongside chloride in brines, was required to enable halogen oxidant formation, consistent with the generation of halogen radicals from the oxidation of halides by hydroxyl radical. Other constituents typically present in brines (i.e., carbonates, organic matter) slowed the degradation of organic compounds, consistent with their ability to scavenge species involved during plasma treatment.

Iodide and sulfite synergistically accelerate the photo-reduction and recovery of As(V) and As(III) in sulfite/iodide/UV process: Efficiency and mechanism

Photo-reduction of arsenic (As) by hydrated electron (eaq-) and recovery of elemental arsenic (As(0)) is a promising pathway to treat As-bearing wastewater. However, previously reported sulfite/UV system needs large amounts of sulfite as the source of eaq-. This work suggests a sulfite/iodide/UV approach that is more efficient and consumes much less chemical reagents to remove As(III) and As(V) and recover valuable As(0) from wastewater, hence preventing the production of large amounts of As-containing hazardous wastes. Our results showed that more than 99.9% of As in the aqueous phase was reduced to highly pure solid As(0) (>99.5 wt%) by sulfite/iodide/UV process under alkaline conditions. Sulfite and iodide worked synergistically to enhance reductive removal of As. Compared with sulfite/UV, the addition of iodide had a substantially greater effect on As(III) (over 200 times) and As(V) (approximately 30 times) removals because of its higher absorptivity and quantum yield of eaq-. Furthermore, more than 90% of the sulfite consumption was decreased by adding a small amount of iodide while maintaining similar reduction efficiency. Hydrated electron (eaq-) was mainly responsible for As(III) and As(V) reductions and removals under alkaline conditions, while both SO3•- and reactive iodine species (e.g., I•, I2, I2•-, and I3-) may oxidize As(0) to As(III) or As(V). Acidic circumstances caused sulfite protonation and the scavenging of eaq- by competing processes. Dissolved oxygen (O2) and CO32- prevented As reduction by light blocking or eaq- scavenging actions, but Cl-, Ca2+, and Mg2+ showed negligible impacts. This study presented an efficient method for removing and recovering As from wastewater.

Highly efficient reduction of bromate by vacuum UV/sulfite system

Bromate (BrO3-), a worldwide regulated by-product after ozone disinfection, is often detected in bromide-containing water, and has a strict limit of 10 μg L-1 in potable water. BrO3- degradation by advanced reduction processes (ARPs) has gained much attention because of efficient removal and easy integration with ultraviolet disinfection (UV at 254 nm). In the vacuum UV (VUV, 185/254 nm)/sulfite system, the elimination kinetics of BrO3- increased by 9-fold and 15-fold comparing with VUV alone and UV/sulfite system. This study further demonstrated the hydrated electron (eaq-) works as the dominant species in BrO3- degradation in alkaline solution, while in the acidic solution the H• became a secondary reactive species besides eaq-. Hence, the influences of pH, sulfite concentration, dissolved gas and water matrix on effectiveness of degradation kinetics of BrO3- was explored in details. With increasing pH, the proportion of SO32- species increased and even became the major ones, which also correlated well with the kobs (min-1) of BrO3- degradation. The stability of eaq- also climbs with increasing pH, while that of H• drops significantly. Higher sulfite dosage favored a more rapid degradation of BrO3-. The presence of dissolved oxygen inhibited BrO3- removal due to the scavenging effect of O2 toward eaq- and transformed VUV/sulfite-based ARP to an advanced oxidation process (AOP), which was ineffective for BrO3- removal. BrO3- removal was inhibited to varying degrees after anions (e.g., bicarbonate (HCO3-), chloride (Cl-), nitrate (NO3-)) and humic acid (HA) being added.

A review of factors affecting the formation and roles of primary and secondary reactive species in UV254-based advanced treatment processes

The presence of organic micropollutants (OMPs) in water has been threatening human health and aquatic ecosystems worldwide. Ultraviolet-based advanced treatment processes (UV-ATPs) are one of the most effective and promising technologies to transform OMPs in water; therefore, an increasing number of emerging UV-ATPs are proposed. However, appropriate selection of UV-ATPs for practical applications is challenging because each UV-ATP generates different types and concentrations of reactive species (RSs) that may not be sufficient to degrade specific types of OMPs. Furthermore, the concentrations and types of RSs are highly influenced by anions and dissolved organic matter (DOM) coexisting in real waters, making systematic understandings of their interfering mechanisms difficult. To identify and address the knowledge gaps, this review provides a comparison of the generations and variations of various types of RSs in different UV-ATPs. These analyses not only prove the importance of water matrices on formation and consumption of primary and secondary RSs under different conditions, but also highlight the non-negligible roles of optical properties and reactivities of DOM and anions. For example, different UV-ATPs may be applicable to different target OMPs under different conditions; and the concentrations and roles of secondary RSs may outperform those of primary RSs in OMP degradation for real applications. With continuous progress and outstanding achievements in the UV-ATPs, it is hoped that the findings and conclusions of this review could facilitate further research and application of UV-ATPs.

Fast degradation of atrazine by nZVI-Cu0/PMS: Re-evaluation and quantification of reactive species, generation pathways, and application feasibility

Additive metal to zero-valent iron (ZVI) could enhance the reduction ability and the additive Cu0 was incorporated to ZVI to accelerate PMS activation with atrazine (ATZ) as target compound. The efficiencies of ATZ degradation and PMS decomposition climbed up firstly and then declined as Cu0 loading increased from 0.01 to 1.00 wt% with the maximums at 0.10 wt%. SO4•-, HO•, Fe(IV), O2•- and 1O2 were generated by nZVI-Cu0/PMS based on the results of electron paramagnetic resonance (EPR) and simultaneous degradation of nitrobenzene, ATZ, and methyl phenyl sulfoxide (PMSO). The rate constant of Fe(IV) and ATZ was estimated as 7 × 104 M-1∙s-1 via the variation of methyl phenyl sulfone (PMSO2)formation at different ATZ concentrations. However, Fe(IV) contributed negligibly to ATZ degradation due to the strong scavenging of Fe(IV) by PMS. SO4•- and HO• were the reactive species responsible for ATZ degradation and the yield ratio of SO4•- and HO• was about 8.70 at initial stage. Preliminary thermodynamic calculation on the possible activation ways revealed that the dominant production of SO4•- might originate from the atomic H reduction of PMS in the surface layer of nZVI-Cu0. Ten products of ATZ degradation were identified by HPLC/ESI/QTOF and the possible degradation pathways were analyzed combined with theoretical calculation on ATZ structure. The decrease of temperature or increase of solution pH led to the decline of ATZ degradation, as well as the individual addition of common ions (HCO3-, Cl-, SO42-, NH4+, NO3- and F-) and natural organic matters (NOM). In real water, ATZ was still efficiently degraded with the decontamination efficiency decreasing in the sequence of tap water > surface water > simulated wastewater > groundwater. For the treatment of ATZ-polluted continuous flow, nZVI-Cu0 in double-layer layout had a higher capacity than the single-layer mode. Meanwhile, the leaching TFe and TCu were limited. The results indicate nZVI-Cu0/PMS is applicable and the multiple-layer layout of nZVI-Cu0 is suggested for ATZ-polluted ground water and soil remediation.

Reactivity of Dissolved Organic Matter with the Hydrated Electron: Implications for Treatment of Chemical Contaminants in Water with Advanced Reduction Processes

Advanced reduction processes (ARP) have garnered increasing attention for the treatment of recalcitrant chemical contaminants, most notably per- and polyfluoroalkyl substances (PFAS). However, the impact of dissolved organic matter (DOM) on the availability of the hydrated electron (eaq-), the key reactive species formed in ARP, is not completely understood. Using electron pulse radiolysis and transient absorption spectroscopy, we measured bimolecular reaction rates constant for eaq- reaction with eight aquatic and terrestrial humic substance and natural organic matter isolates ( kDOM,eaq-), with the resulting values ranging from (0.51 ± 0.01) to (2.11 ± 0.04) × 108 MC-1 s-1. kDOM,eaq- measurements at varying temperature, pH, and ionic strength indicate that activation energies for diverse DOM isolates are ≈18 kJ mol-1 and that kDOM,eaq- could be expected to vary by less than a factor of 1.5 between pH 5 and 9 or from an ionic strength of 0.02 to 0.12 M. kDOM,eaq- exhibited a significant, positive correlation to % carbonyl carbon for the isolates studied, but relationships to other DOM physicochemical properties were surprisingly more scattered. A 24 h UV/sulfite experiment employing chloroacetate as an eaq- probe revealed that continued eaq- exposure abates DOM chromophores and eaq- scavenging capacity over a several hour time scale. Overall, these results indicate that DOM is an important eaq- scavenger that will reduce the rate of target contaminant degradation in ARP. These impacts are likely greater in waste streams like membrane concentrates, spent ion exchange resins, or regeneration brines that have elevated DOM concentrations.

Trade-off effect of dissolved organic matter on degradation and transformation of micropollutants: A review in water decontamination

The degradation of micropollutants by various treatments is commonly affected by the ubiquitous dissolved organic matter (DOM) in the water environment. To optimize the operating conditions and decomposition efficiency, it is necessary to consider the impacts of DOM. DOM exhibits varied behaviors in diverse treatments, including permanganate oxidation, solar/ultraviolet photolysis, advanced oxidation processes, advanced reduction process, and enzyme biological treatments. Besides, the different sources (i.e., terrestrial and aquatic, etc) of DOM, and operational circumstances (i.e., concentration and pH) fluctuate different transformation efficiency of micropollutants in water. However, so far, systematic explanations and summaries of relevant research and mechanism are rare. This paper reviewed the "trade-off" performances and the corresponding mechanisms of DOM in the elimination of micropollutants, and summarized the similarities and differences for the dual roles of DOM in each of the aforementioned treatments. Inhibition mechanisms typically include radical scavenging, UV attenuation, competition effect, enzyme inactivation, reaction between DOM and micropollutants, and intermediates reduction. Facilitation mechanisms include the generation of reactive species, complexation/stabilization, cross-coupling with pollutants, and electron shuttle. Moreover, electron-drawing groups (i.e., quinones, ketones functional groups) and electron-supplying groups (i.e., phenols) in the DOM are the main contributors to its trade-off effect.

Revisiting UV/sulfite exposed to air: A redox process for reductive dechlorination and oxidative mineralization

2,4-Dichlorophenol (2,4-DCP) widely exists in many industrial wastewaters and is considered a priority pollutant due to its high toxicity. In this work, we proposed a UV/sulfite process without aeration for high-efficiency dechlorination and enhanced mineralization. The UV/sulfite system significantly improved the removal of 2,4-DCP (93.33 % in 60 min) and dechlorination rate (85.13 % in 60 min) compared with UV alone and sulfite alone, and the synergistic factor was 6.59. The UV/sulfite system without aeration improved the mineralization rate (49.80 %) compared with the nitrogen aeration system. UV/sulfite was a reduction-oxidation coupled process and was more suitable for neutral and alkaline conditions. Reducing species (hydrated electrons and H^•) made 2,4-DCP dechlorinate to form dechlorination products such as p-chlorophenol (4-CP), o-chlorophenol (2-CP), and phenol, which were further mineralized by oxidized species (SO₄^•-). UV/sulfite remained highly efficient in the presence of coexisting ions and under different water quality conditions. This process was also suitable for removing a wide range of chlorinated organic compounds. The UV/sulfite process without aeration can achieve high dechlorination and enhanced mineralization with simple operation and low cost (1.78 $·m^-3 order^-1), which has a broad and cost-effective application prospect in removing refractory halogenated organic pollutants.

Ultra-short chain fluorocarboxylates exhibit wide ranging reactivity with hydrated electrons

Recent reports demonstrate that technologies generating hydrated electrons (e_aq^-; e.g., UV-sulfite) are a promising strategy for destruction of per- and polyfluoroalkyl substances, but fundamental rate constants are lacking. This work examines the kinetics and mechanisms of e_aq^- reactions with ultra-short chain (C2-C4) fluorocarboxylates using experimental and theoretical approaches. Laser flash photolysis (LFP) was used to measure bimolecular rate constants (k₂; M^-1 s^-1) for e_aq^- reactions with thirteen per-, and for the first time, polyfluorinated carboxylate structures. The measured k₂ values varied widely from 5.26 × 10⁶ to 1.30 × 10⁸ M^-1s^-1, a large range considering the minor structural changes among the target compounds. Molecular descriptors calculated using density functional theory did not reveal correlation between k₂ values and individual descriptors when considering the whole dataset, however, semiquantitative correlation manifests when grouping by similar possible initial reduction event such as electron attachment at the α-carbon versus β- or γ-carbons along the backbone. From this, it is postulated that fluorocarboxylate reduction by e_aq^- occurs via divergent mechanisms with the possibility of non-degradative pathways being prominent. These mechanistic insights provide rationale for contradictory trends between LFP-derived k₂ values and apparent degradation rates recently reported in UV-sulfite constant irradiation treatment experiments.

Role of trace TEMPO as electron shuttle in enhancing chloroquine phosphate elimination in UV-LED-driven persulfate activation process

Chloroquine Phosphate (CP) is an antiviral drug used for treatment of COVID-19. It is released into wastewater and eventually contaminates natural water. This study reports an effective homogeneous catalysis way for CP degradation by the 2,2,6,6-Tetramethylpiperidine-N-oxyl (TEMPO) enhanced persulfate (PDS) activation under UVB-LEDs irradiation at 305 nm. TEMPO at a low concentration (0.1 μM) enhanced CP degradation in UV₃₀₅/PDS process in deionized water at different pHs, in different anions and different molecular weight dissolved organic matter solutions and in real surface water. The enhancement was verified to be attributed to the electron shuttle role of TEMPO, which promoted the yield of SO₄ ^•- by enhancing electron donating capacity of the reacting system. The degradation products of CP and their acute toxicities suggested that UV₃₀₅/PDS/TEMPO process has better performance on CP detoxification than UV₃₀₅/PDS process. This study provides a new way to tackle the challenge of pharmaceutical pollutions in homogeneous photocatalysis process for natural water and sewage restoration.

Nitrate promoted defluorination of perfluorooctanoic acid in UV/sulfite system: Coupling hydrated electron/reactive nitrogen species-mediated reduction and oxidation

A significantly accelerated defluorination of recalcitrant perfluorooctanoic acid (PFOA) was explored with the co-present nitrate (20 mg L^-1) by UV/sulfite treatment (UV/sulfite-nitrate). The deep defluorination of PFOA and complete denitrification of nitrate were simultaneously achieved in UV/sulfite-nitrate system. At the initial 30 min, PFOA defluorination exhibited an induction period, exactly corresponding to the removal of the co-existed nitrate. Upon the induction period passed, an accelerated removal of PFOA (5 mg L^-1) occurred, nearly 100% defluorination ratio reached within 2 h. Compared with those in UV/sulfite, the kinetics of PFOA decay, defluorination, and transformation product formations were greatly enhanced in UV/sulfite-nitrate system. Reactive nitrogen species (RNS) generated from e_aq^--induced reduction of nitrate were found to play significant roles on the promoted defluorination apart from e_aq^--mediated reductive defluorination. The investigations on solution pH (7.0-11.0) confirmed that the reductive defluorination of PFOA was more efficient under alkaline conditions, however, the presence of nitrate can promote the defluorination even under neutral pH. Theoretical calculations of Fukui function demonstrated that RNS could easily launch electrophilic attack toward H-rich moieties of fluorotelomer carboxylates (FTCAs, C_nF_2n+1-(CH₂)_m-COO^-), more persistent intermediates (formed via H/F exchange), and convert FTCAs into shorter-chain perfluorinated carboxylic acids, thus facilitating the deep defluorination. Along with the analysis on the denitrification products, the liberation of fluoride ions and generated intermediates, possible decomposition pathways were proposed. This work highlights the indispensable synergy from e_aq^-/RNS with integrated reduction and oxidation on PFOA defluorination and will advance remediation technologies of perfluorinated compound contaminated water.

Hydrated electron based photochemical processes for water treatment

Hydrated electron (e_aq^-) based photochemical processes have emerged as a promising technology for contaminant removal in water due to the mild operating conditions. This review aims to provide a comprehensive and up-to-date summary on e_aq^- based photochemical processes for the decomposition of various oxidative contaminants. Specifically, the characteristics of different photo-reductive systems are first elaborated, including the environment required to generate sufficient e_aq^-, the advantages and disadvantages of each system, and the comparison of the degradation efficiency of contaminants induced by e_aq^-. In addition, the identification methods of e_aq^- (e.g., laser flash photolysis, scavenging studies, chemical probes and electron spin resonance techniques) are summarized, and the influences of operating conditions (e.g., solution pH, dissolved oxygen, source chemical concentration and UV type) on the performance of contaminants are also discussed. Considering the complexity of contaminated water, particular attention is paid to the influence of water matrix (e.g., coexisting anions, alkalinity and humic acid). Moreover, the degradation regularities of various contaminants (e.g., perfluorinated compounds, disinfection by-products and nitrate) by e_aq^- are summarized. We finally put forward several research prospects for the decomposition of contaminants by e_aq^- based photochemical processes to promote their practical application in water treatment.

Treasuring industrial sulfur by-products: A review on add-value to reductive sulfide and sulfite for contaminant removal and hydrogen production

Reductive sulfur-containing by-products (S-BPs) released from industrial process mainly exist in the simple form of sulfide and sulfite. In this study, recent advances to remove and make full use of reductive S-BPs to achieve efficient contaminant removal and hydrogen production are critically reviewed. Sulfide, serves as both reductant and nucleophile, can form intermediates with the catalyst surface functional group through chemical interaction, efficiently promoting the catalytic reduction process to remove contaminants. Sulfite assisted catalytic process could be classified to the advanced reduction processes (ARPs) and advanced oxidation processes (AOPs), mainly depending on the presence of dissolved oxygen (DO) in the solution. During ARPs, sulfite could generate reductive active species including hydrated electron (e_aq^-), hydrogen radical (H·), and sulfite radical (SO₃^•-) under the irradiation of UV light, leading to the efficient reduction removal of a variety of contaminants. During AOPs, sulfite could first produce SO₃^•- under the action of the catalyst or energy, initiating a series of reactions to produce oxysulfur radicals. Various contaminants could be effectively removed under the role of these oxidizing active species. Sulfides and sulfites could also be removed along with promoting hydrogen production via photocatalytic and electrocatalytic processes. Besides, the present limitations and the prospects for future practical applications of the process with these S-BPs are proposed. Overall, this review gives a comprehensive summary and aims to provide new insights and thoughts in promoting contaminant removal and hydrogen production through taking full advantage of reductive S-BPs.

Finding Fluorine: Photoproduct Formation during the Photolysis of Fluorinated Pesticides

The photolysis of pesticides with different fluorine motifs was evaluated to quantify the formation of fluorinated products in buffered aqueous systems, advanced oxidation (AOP) and reduction processes (ARP), and river water. Simulated sunlight quantum yields at pH 7 were 0.0033, 0.0025, 0.0015, and 0.00012 for penoxsulam, florasulam, sulfoxaflor, and fluroxypyr, respectively. The bimolecular rate constants with hydroxyl radicals were 2 to 5.7 × 1010 M-1 s-1 and, with sulfate radicals, 1.6 to 2.6 × 108 M-1 s-1 for penoxsulam, florasulam, and fluroxypyr, respectively. The rate constants of sulfoxaflor were 100-fold lower. Using quantitative 19F-NMR, complete fluorine mass balances were obtained. The maximum fluoride formation was 53.4 and 87.4% for penoxsulam and florasulam under ARP conditions, and 6.1 and 100% for sulfoxaflor and fluroxypyr under AOP conditions. Heteroaromatic CF3 and aliphatic CF2 groups were retained in multiple fluorinated photoproducts. Aryl F and heteroaromatic F groups were readily defluorinated to fluoride. CF3 and CF2 groups formed trifluoroacetate and difluoroacetate, and yields increased under oxidizing conditions. 19F-NMR chemical shifts and coupling analysis provided information on hydrogen loss on adjacent bonds or changes in chirality. Mass spectrometry results were consistent with the observed 19F-NMR products. These results will assist in selecting treatment processes for specific fluorine motifs and in the design of agrochemicals to reduce byproduct formation.

Effects of UV/Fe(II)/sulfite pre-treatment on NOM-enhanced Ca2+ scaling during nanofiltration treatment: Fouling mitigation, mechanisms, and correlation analysis of membrane resistance

This study was aimed to evaluate the effects of a pre-treatment involving sulfite (S(IV)) synergistically activated by ultraviolet (UV)/Fe(II) on natural organic matter (NOM)-enhanced Ca²⁺ scaling during nanofiltration treatment. Based on the variations in the physicochemical properties and correlation analyses of irreversible resistance, the intrinsic fouling mechanisms were revealed from two aspects: bulk crystallization (interaction between NOM and inorganic ions) and surface crystallization (morphology of surface crystallization and a change in the Ca²⁺ concentration in the scaling layer). Furthermore, the degradation contribution rates of different free radicals during the UV/Fe(II)/S(IV) (UFS) treatment process were evaluated. During the reactions in the UFS, three free radicals (SO·-4, OH^·- and e- aq) were generated, and in-situ Fe(III) was formed in-situ. The carboxyl groups of the NOM were attacked by the free radicals, resulting in decreased of carboxyl concentration and density. In addition, the bond between Ca²⁺ and NOM weakened, and hydrophobic (HPO) substances were mineralized. However, the Fe(III) formed in-situ was active and electropositive, competing with Ca²⁺ for the complexation active sites on the NOM. The synergy effect of bulk crystallization and surface crystallization led to a significant decrease in the particle size of feed solution. The crystal size and roughness of membrane surface also decreased, which was conducive to reducing the membrane irreversible resistance. Correlation analysis revealed that the HPO ratio, carboxyl density and particle size (> 100 nm) ratio were effective characterization parameters for predicting irreversible resistance. This study not only provides guidance for alleviating membrane fouling caused by NOM-enhanced Ca²⁺ scaling during the nanofiltration process, but also presents the rationality of irreversible resistance during nanofiltration process and various indicators with strong linear correlation.

Photochemical degradation of short-chain chlorinated paraffins in aqueous solution by hydrated electrons and hydroxyl radicals

Short-chain chlorinated paraffins (SCCPs) are a complex mixture of polychlorinated alkanes (C₁₀-C₁₃, chlorine content 40-70%), and have been categorized as persistent organic pollutants. However, there are knowledge gaps about their environmental degradation, particularly the effectiveness and mechanism of photochemical degradation in surface waters. Photochemically-produced hydrated electrons (e^-_(aq)) have been shown to degrade highly chlorinated compounds in environmentally-relevant conditions more effectively than hydroxyl radicals (·OH), which can degrade a wide range of organic pollutants. This study aimed to evaluate the potential for e^-_(aq) and ·OH to degrade SCCPs. To this end, the degradation of SCCP model compounds was investigated under laboratory conditions that photochemically produced e^-_(aq) or ·OH. Resulting SCCP degradation rate constants for e^-_(aq) were on the same order of magnitude as well-known chlorinated pesticides. Experiments in the presence of ·OH yielded similar or higher second-order rate constants. Trends in e^-_(aq) and ·OH degradation rate constants of the investigated SCCPs were consistent with those of other chlorinated compounds, with higher chlorine content producing in higher rate constants for e^-_(aq) and lower for ·OH. Above a chlorine:carbon ratio of approximately 0.6, the e^-_(aq) second-order rate constants were higher than rate constants for ·OH reactions. Results of this study furthermore suggest that SCCPs are likely susceptible to degradation in sunlit surface waters, facilitated by dissolved organic matter as a source of photochemically produced e^-_(aq) and ·OH.

Reductive removal of As(V) and As(III) from aqueous solution by the UV/sulfite process: Recovery of elemental arsenic

The removal of arsenic (As(V) and As(III)) from contaminated water has attracted great attention. However, the generation of arsenic-containing hazardous waste by traditional methods has become an inevitable environmental problem. Herein, a UV/sulfite advanced reduction method was proposed to remove As(V) and As(III) from aqueous solution in the form of valuable elemental arsenic (As(0)), thus avoiding the generation of arsenic-containing hazardous waste. The results showed that greater than 99.9% of As(V) and As(III) were reduced to the high purity As(0) (> 99.5 wt%) with the residual arsenic concentration below 10 μg L^-1. The hydrated electrons (e_aq^-), H^• and SO₃^•- radicals are generated by the UV/sulfite process, of which e_aq^- and H^• serve as reductants of As(V) and As(III) while the SO₃^•- radicals inhibit arsenic reduction by oxidizing arsenic. The effective quantum efficiency (Φ) for the formation of As(0) in the As(V) and As(III) removal process is approximately 0.0078 and 0.0055 mol/Einstein, respectively. The reduction of arsenic is favorable under alkaline conditions (pH > 9.0) due to the higher photolysis efficiency of SO₃^2- than HSO₃^- (pK_a = 7.2) and higher stability of e_aq^-/H^• under alkaline conditions. The presence of dissolved oxygen (O₂), NO₂^-, NO₃^-, CO₃^2-, PO₄^3- and humic acid (HA) inhibited arsenic reduction through light blocking or e_aq^-/H^• scavenging effects while Cl^-, SO₄^2-, Ca²⁺ and Mg²⁺ had negligible effects on arsenic reduction. The proposed method can effectively remove and recover arsenic from contaminated water at a low cost, demonstrating feasibility for practical application. This study provides a novel technology for the reductive removal and recovery of arsenic from contaminated water.

Efficient removal of Cr(VI) at alkaline pHs by sulfite/iodide/UV: Mechanism and modeling

Efficient removal of toxic hexavalent chromium (Cr(VI)) under alkaline conditions is still a challenge due to the relatively low reactivity of CrO₄^2-. This study proposed a new sulfite/iodide/UV process to remove Cr(VI). The removal of Cr(VI) followed pseudo-zero-order kinetics at alkaline pHs, and was enhanced by sulfite and iodide with synergy. Compared with sulfite/UV, iodide in sulfite/iodide/UV showed about 40 times higher concentration-normalized enhancement for Cr(VI) removal, and reduced the requirement of sulfite ([S(IV)]₀/[Cr(VI)]₀ of about 2.1:1) by more than 90%. The Cr(VI) removal was accelerated by decreasing pH and by increasing temperature, and was slightly influenced by dissolved oxygen, carbonate, and humic acid. The process was still effective in real surface water and industrial wastewater. Mechanism and pathways of Cr(VI) removal were revealed by quenching experiments, competition kinetic analysis, product identification and quantification, and mass and electron balance. Both e_aq^- and SO₃^•- were responsible for Cr(VI) removal, making contributions of about 75% and 25%, respectively. When e_aq^- mainly reacted with Cr(VI), SO₃^•- participated in reduction of Cr(V) and Cr(IV) intermediates, with Cr(III), S₂O₆^2-, and SO₄^2- as the final products. A model was developed to predict removal kinetics of Cr(VI), and well interpreted the roles of S(IV) and iodide in the process. This study sheds light on mechanism of Cr(VI) removal at alkaline pHs by kinetic modeling, and thus advances the applicability of this promising process for water decontamination.

Quantifying Hydrated Electron Transformation Kinetics in UV-Advanced Reduction Processes Using the Re-,UV Method

Ultraviolet advanced reduction processes (UV-ARP) have garnered significant attention recently for the degradation of several hard to treat contaminants, including recalcitrant per- and polyfluoroalkyl substances (PFAS). The rate of contaminant degradation in UV-ARP is directly related to the available hydrated electron concentration ([e_aq^-]). However, reports of [e_aq^-] and other parameters typically used to characterize photochemical systems are not widely reported in the UV-ARP literature. Deploying monochloroacetate as a probe compound, we developed a method (R_e-,UV) to quantify the time-based hydrated electron concentration ([e_aq]_t) available for contaminant degradation relative to inputted UV fluence. Measured [e_aq]_t was then used to understand the impact of e_aq^- rate of formation and scavenging capacity on the degradation of two contaminants─nitrate and perfluorooctane sulfonate (PFOS)─in four source waters with varying background water quality. The results show that the long-term treatability of PFOS by UV-ARP is not significantly impacted by the initial e_aq^- scavenging conditions but rather is influenced by the presence of e_aq^- scavengers like dissolved organic carbon and bicarbonate. Lastly, using [e_aq]_t, degradation of nitrate and PFOS was modeled in the source waters. We demonstrate that the R_e-,UV method provides an effective tool to assess UV-ARP treatment performance in a variety of source waters.

Detection and Stability of Cyanogen Bromide and Cyanogen Iodide in Drinking Water

This study systematically summarized the factors affecting the stability of CNXs, providing a reference for better control and elimination of CNXs. A method for the detection of CNBr and CNI in solution was established using a liquid–liquid extraction/gas chromatography/electron capture detector. Specifically, the method was used to investigate the stability of CNBr and CNI in drinking water, especially in the presence of chlorine and sulfite, and it showed good reproducibility (relative standard deviation <3.05%), high sensitivity (method detection limit <100 ng/L), and good recovery (91.49–107.24%). Degradation kinetic studies of cyanogen halides were conducted, and their degradation rate constants were detected for their hydrolysis, chlorination, and sulfite reduction. For hydrolysis, upon increasing pH from 9.0 to 11.0, the rate constants of CNCl, CNBr, and CNI changed from 8 to 155 × 10−5 s−1, 1.1 to 34.2 × 10−5 s−1, and 1.5 to 6.2 × 10−5 s−1, respectively. In the presence of 1.0 mg/L chlorine, upon increasing pH from 7.0 to 10.0, the rate constants of CNCl, CNBr, and CNI changed from 36 to 105 × 10−5 s−1, 15.8 to 49.0 × 10−5 s−1, and 1.2 to 24.2 × 10−5 s−1, respectively. In the presence of 3 μmol/L sulfite, CNBr and CNI degraded in two phases. In the first phase, they degraded very quickly after the addition of sulfite, whereas, in the second phase, they degraded slowly with rate constants similar to those for hydrolysis. Owing to the electron-withdrawing ability of halogen atoms and the nucleophilic ability of reactive groups such as OH− and ClO−, the rate constants of cyanogen halides increased with increasing pH, and they decreased in the order of CNCl > CNBr > CNI during hydrolysis and chlorination. The hydrolysis and chlorination results could be used to assess the stability of cyanogen halides in water storage and distribution systems. The sulfite reduction results indicate that quenching residual oxidants with excess sulfite could underestimate the levels of cyanogen halides, especially for CNBr and CNI.

Critical Review of UV-Advanced Reduction Processes for the Treatment of Chemical Contaminants in Water

UV-advanced reduction processes (UV-ARP) are an advanced water treatment technology characterized by the reductive transformation of chemical contaminants. Contaminant abatement in UV-ARP is most often accomplished through reaction with hydrated electrons (e_aq ^-) produced from UV photolysis of chemical sensitizers (e.g., sulfite). In this Review, we evaluate the photochemical kinetics, substrate scope, and optimization of UV-ARP. We find that quantities typically reported in photochemical studies of natural and engineered systems are under-reported in the UV-ARP literature, especially the formation rates, scavenging capacities, and concentrations of key reactive species like e_aq ^-. The absence of these quantities has made it difficult to fully evaluate the impact of operating conditions and the role of water matrix components on the efficiencies of UV-ARP. The UV-ARP substrate scope is weighted heavily toward contaminant classes that are resistant to degradation by advanced oxidation processes, like oxyanions and per- and polyfluoroalkyl substances. Some studies have sought to optimize the UV-ARP treatment of these contaminants; however, a thorough evaluation of the impact of water matrix components like dissolved organic matter on these optimization strategies is needed. Overall, the data compilation, analysis, and research recommendations provided in this Review will assist the UV-ARP research community in future efforts toward optimizing UV-ARP systems, modeling the e_aq ^--based chemical transformation kinetics, and developing new UV-ARP systems.

Accelerated Degradation of Perfluorosulfonates and Perfluorocarboxylates by UV/Sulfite + Iodide: Reaction Mechanisms and System Efficiencies

The addition of iodide (I^-) in the UV/sulfite system (UV/S) significantly accelerated the reductive degradation of perfluorosulfonates (PFSAs, C_nF_2n+1SO₃^-) and perfluorocarboxylates (PFCAs, C_nF_2n+1COO^-). Using the highly recalcitrant perfluorobutane sulfonate (C₄F₉SO₃^-) as a probe, we optimized the UV/sulfite + iodide system (UV/S + I) to degrade n = 1-7 PFCAs and n = 4, 6, 8 PFSAs. In general, the kinetics of per- and polyfluoroalkyl substance (PFAS) decay, defluorination, and transformation product formations in UV/S + I were up to three times faster than those in UV/S. Both systems achieve a similar maximum defluorination. The enhanced reaction rates and optimized photoreactor settings lowered the EE/O for PFCA degradation below 1.5 kW h m^-3. The relatively high quantum yield of e_aq^- from I^- made the availability of hydrated electrons (e_aq^-) in UV/S + I and UV/I two times greater than that in UV/S. Meanwhile, the rapid scavenging of reactive iodine species by SO₃^2- made the lifetime of e_aq^- in UV/S + I eight times longer than that in UV/I. The addition of I^- also substantially enhanced SO₃^2- utilization in treating concentrated PFAS. The optimized UV/S + I system achieved >99.7% removal of most PFSAs and PFCAs and >90% overall defluorination in a synthetic solution of concentrated PFAS mixtures and NaCl. We extended the discussion over molecular transformation mechanisms, development of PFAS degradation technologies, and the fate of iodine species.

Efficient reductive and oxidative decomposition of haloacetic acids by the vacuum-ultraviolet/sulfite system

Haloacetic acids (HAAs), as a representative category of halogenated disinfection byproducts, are widely detected in disinfected water. In this work, the vacuum ultraviolet (VUV)/sulfite process under N₂ saturated conditions was proposed to eliminate a series of HAAs (i.e., monochloroacetic acid (MCAA), difluoroacetic acid (DFAA), trifluoroacetic acid (TFAA), dichloroacetic acid (DCAA), etc.). The in situ generated hydrated electron (e_aq^-) demonstrated to be the main species to fulfill the initial degradation and dechlorination of MCAA, while hydroxyl radicals (˙OH) were in charge of the mineralization of MCAA. This means that the VUV/sulfite system is a combination of advanced reduction and oxidation processes (ARPs and AOPs). A significant enhancement of MCAA removal was observed with increasing pH values from 6.0 to 10.0, and surprisingly, k_obs correlated well with the proportion of SO₃^2- as the pH changed. This can be explained by the production of e_aq^- from VUV irradiation of SO₃^2- rather than HSO₃^- and also due to e_aq^- being more stable under alkaline conditions. Increasing the sulfite dosage also elevated the degradation of MCAA. However, the addition of certain anions (i.e., chloride (Cl^-), bicarbonate (HCO₃^-), and nitrate (NO₃^-)) and dissolved organic matter (DOM) inhibited the removal of MCAA to varying degrees. The VUV/sulfite system was effective toward various types of halogenated disinfection byproducts, supporting its broad applicability. Nevertheless, even in real waters, the VUV/sulfite system was also promising for the simultaneous abatement of HAAs and other oxyanions.

Insights into the hydrated electron generation from UV/aniline: Mechanism and quantum efficiency

The insight into the neglected reduction process accompanied by UV-based oxidation process may provide new ideas for the development of advanced oxidation and reduction technologies. In this study, aniline was comprehensively investigated as an unexpected indicator of hydrated electron (e_aq^-) under UV irradiation. Monochloroacetic acid (MCAA) was selected as the probe of e_aq^- and the balance of chloride ions indicated the reduction of MCAA. Further, laser flash photolysis experiments demonstrated the generation of e_aq^- in the UV/aniline process and the half-life period of formed e_aq^- was demonstrated to be 0.13 μs. The photolysis of aniline along with the decay of the excited state of aniline was responsible for the e_aq^- generation. Besides, the hydrogen atom (H) generated from the photolysis can subsequently reacted with OH^- to generate e_aq^-. The photolysis pathways of aniline were proposed by the results of GC-MS. Aniline was abstracted of H in solution to the formation of aniline radical (PhNH) or form aminophenol in three different isomers (orto-, meta- and para-aminophenol). Moreover, UV/aniline showed a higher reducing capacity of MCAA compared with other organic electron donors and sustained a highly reducing ability in a wide pH. And the calculation results of quantum efficiency (Φ) showed that excessive aniline was not conducive to the elevation of Φ. This study introduced a novel pathway of e_aq^- generation during the photolysis of aniline and provided a new perspective for e_aq^--based advanced reduction processes.

Advanced reduction process to achieve efficient degradation of pyridine

Pyridine and its derivatives are widely consumed and detected in the environment persistently, which can cause potential adverse impacts on environment and human health. Considering the fact that pyridine could absorb UV light at 254 nm to generate excited one, which could react with reductive radicals, promoting its structural changes, we proposed that one typical efficient advanced reduction process (ARP) which combines UV irradiation with sulfite could be used to eliminate pyridine quickly. Sulfite/UV process showed a higher pyridine removal rate with a pseudo-first-order reaction rate constant of 0.1439 min^-1, which was 3 times of that in UV irradiation and 1.3 times in UV/H₂O₂ process. This was primarily due to reductive radicals (e_aq^-, H^• and SO₃^•-) produced by UV irradiation. The removal rate of pyridine was highest in slightly alkaline environment. And the presence of oxygen, as well as certain concentration of humid acid just showed slight inhibition, indicating the possibility of application in practical environment. A positive impact was observed with increasing sulfite dosage, but it was gradually inhabited when the dosage was over 5 mM. The present study may provide an alternative efficient technology for the degradation of pyridine ring-containing substances.

Reductive degradation of perfluorooctanoic acid in complex water matrices by using the UV/sulfite process

Hydrated electrons (e^-_aq,E= -2.9 V) generated by advanced reduction processes (ARPs) have been proved to be a promising approach to eliminate various per- and polyfluoroalkyl substances (PFASs) in water. In this study, the decomposition of perfluorooctanoic acid (PFOA) in a complex water matrix by e^-_aq generated from the UV/sulfite process was investigated. The effect of pH (9-12) and co-existing compounds (chloride, nitrate, phosphate, carbonate and humic acid) on PFOA degradation efficiency was studied. In addition, the intermediates and possible degradation pathways were analyzed by ultra-high performance liquid chromatography-tandem mass spectrometry (UPLC-MS). The results showed that the concentration of PFOA was below the detection limit (10 μg/L) after 1 h (conditions: C₀ 10 mg/L, initial pH = 10, sulfite 10 mM) while 89% defluorination was achieved after 24 h. Using a higher initial pH (pH = 12) greatly enhanced the PFOA degradation as 100% degradation and 98% defluorination were achieved after 24 h. The presence of carbonate (> 5 mM), nitrate (> 2 mM) and humic acid (> 25 mg/L) showed a significant negative effect on PFOA degradation via a UV blocking effect or quenching of hydrated electrons while the presence of chloride and phosphate had a smaller effect on PFOA degradation. Even at extremely high concentrations of chloride (1.709 M, pH = 11.25), the defluorination ratio reached 97% after 24 h of reaction time. During the process, short-chain perfluorinated carboxylic acids (PFCAs, C < 7) and hydrogen substituted compounds were detected, which implies that chain-shortening and H/F change reactions had occurred. Moreover, this confirmed the generation of sulfonated and unsaturated intermediates during the process, which disclosed valuable new mechanistic insights into PFOA degradation.

Strengthening arsenite oxidation in water using metal-free ultrasonic activation of sulfite

Although sulfite-based advanced oxidation processes (AOPs) have received renewed attention due to the production of oxysulfur radicals, the feasibility of using ultrasound (US) to activate sulfite remains unknown. In this work, low frequency ultrasound has been applied for the first time to develop a novel sulfite activation process (US-S(IV)) for enhanced oxidation of arsenite (As(III)). Our results showed that the US-S(IV) process with 1 mM sulfite addition and 20 kHz 650 W ultrasound can achieve approximately 2.9-fold increase in As(III) oxidation rate compared to the US process at pH 7. The mechanisms underpinning the US-S(IV) process have been probed through radical-scavenging experiments and electron spin resonance (ESR) spectrometry. Direct ultrasonolysis of sulfite has been demonstrated to be the predominant pathway producing the primary sulfite radical (SO₃⁻) in the US-S(IV) process. Besides, the US-S(IV) process also works well in the treatment process of natural water, suggesting that this process could be promising in commercial scale application. This work not only provides a new application of ultrasound in sulfite-based AOP, but also provides further insights into how sulfite impacts the US process.

Degradation of 17β-estradiol by UV/persulfate in different water samples

Sulfate radical (•SO₄^-)-based advanced oxidation processes are widely used for wastewater treatment. This study explored the potential use of UV/persulfate (UV/PS) system for the degradation of 17β-estradiol (E2). The pH of the reaction system can affect the degradation rate of E2 by UV/PS and the optimum pH was 7.0; Br^- and Cl^- in water can promote the degradation rate, HCO₃^- has an inhibitory effect on the reaction, SO₄^2- and cations (Na⁺, Mg²⁺, K⁺) have no effect on the degradation rate. The degradation of E2 by UV/PS was a mineralization process, with the mineralization rate reaching 90.97% at 8 h. E2 in the UV/PS system was mainly degraded by hydroxylation, deoxygenation, and hydrogenation. E2 reaction sites were mainly located on benzene rings, mainly carbonylation on quinary rings, and bond breakage between C10 and C5 resulted in the removal of benzene rings and carboxyl at C2 and C3 sites. In the presence of halogen ions, halogenated disinfection by-products were not formed in the degradation process of E2 by UV/PS. E2 in the UV/PS system could inhibit the formation of bromate. The results of this study suggest that UV/PS is a safe and reliable method to degrade E2.

Efficient photochemical denitrification by UV/sulfite system: Mechanism and applications

Denitrification is an effective strategy to control eutrophication caused by excessive nitrate in water. However, the comparatively low efficiency of nitrate removal and N₂ selectivity remains a challenge in the denitrification process. Herein, this study proposed a novel photochemical denitrification process by introducing hydrated electron (e_aq^-) to reduce nitrate in UV/sulfite system. The results indicated that the optimized UV/sulfite system could effectively reduce nitrate to N₂ with a nearly 100% denitrification efficiency in 90 min. e_aq^- was identified as the mainly reactive species to achieve rapid removal of nitrate and nitrite, and the result was verified by quenching and laser flash photolysis tests. Benefiting from the high dispersion of e_aq^- in water and the rapid reaction rate between e_aq^- and the target, the generated N₂O is susceptible to be reduced, leading to a high selectivity of N₂ that was confirmed by ¹⁵N-isotopic. Besides, thermodynamic results based on the density functional theory (DFT) calculations suggested that the photochemical denitrification process was exothermic process and tend to transform to N₂. Significantly, UV/sulfite system applied in the nickel-plating wastewater showed high denitrification efficiency, demonstrating that the novel photochemical denitrification process is promising for practical wastewater treatment.

Efficient destruction of emerging contaminants in water by UV/S(IV) process with natural reoxygenation: Effect of pH on reactive species

UV/sulfite systems with oxygen have recently been considered as advanced oxidation processes in view of the participation of oxysulfur radicals. However, the contribution of •OH and the efficiency of destructing emerging contaminants (ECs) in water remain largely unclear. Here, the UV/S(IV) process was applied with natural reoxygenation to degrade two typical ECs, diethyl phthalate (DEP) and bisphenol A (BPA) showing different properties. Solution pH played the key role in determining the reactive species, and both DEP and BPA were more favorably degraded at more alkaline conditions with higher utilization efficiency of SO₃^2-. Specifically, the H•, O₂^•-, •OH and SO₃^•- were identified at acidic condition, but the amount of •OH accumulated significantly with the elevation of pH. Competitive quenching experiments showed that e_aq^- and •OH dominated the degradation of DEP and BPA at alkaline condition, respectively. Besides, DEP showed higher quantum efficiency for the indirect photolysis and mineralization degree than that of BPA at pH 9.2 mainly due to the direct use of the primary photoproduct. The possible transformation mechanisms of S(IV) and mineralization routes of both pollutants were proposed. This study may provide new insights into the mechanisms involved in UV/S(IV) process and a promising alternative for efficient removal of ECs in water.

Sulfite enhanced transformation of iopamidol by UV photolysis in the presence of oxygen: Role of oxysulfur radicals

UV/sulfite process in the absence of oxygen was previously applied as an advanced reduction process for the removal of many halogenated organics and inorganics in water and wastewater. Here, it was found that UV/sulfite process in the presence of oxygen could act as an advanced oxidation process. Specifically, the oxysulfur radicals (including sulfate radical (SO₄^·-) and sulfite/peroxomonosulfate radicals (SO₃^·-/SO₅^·-)) played important roles on the degradation of iopamidol (IPM) as a typical iodinated contrast media (ICM). Furthermore, the contribution of SO₄^·- on IPM removal gradually increased as pH increased from 5 to 7 and that of SO₃^·-/SO₅^·- decreased. Besides, all water quality parameters (i.e., chloride (Cl^-), iodide (I^-) and natural organic matter (NOM)) investigated here exhibited inhibitory effect on IPM removal. Three inorganic iodine species (i.e., I^-, reactive iodine species and iodate (IO₃^-)) were detected in UV/sulfite process in the presence of oxygen, while only I^- was detected in that without oxygen. During UV/sulfite/ethanol, UV photolysis and UV/peroxydisulfate (PDS)/tert-butyl alcohol (TBA) processes, thirteen transformation products including eleven deiodinated products of IPM were identified by ultra HPLC quadrupole time of flight-mass spectrometry (UPLC-Q-TOF-MS). Besides, these products generated by direct UV photolysis, SO₄^·- and SO₃^·-/SO₅^·- were further distinguished. The acute toxicity assay of Vibrio fischeri indicated that transformation products by UV/sulfite under aerobic conditions were less toxic than that by direct UV photolysis.

Effective in-situ reduction of Cr(VI) from leather wastewater by advanced reduction process based on CO2·- with visible-light photocatalyst

Advanced reduction process (ARP) has drawn an increasing interest as a new manner for removing oxidative pollutants in water. In this paper, we demonstrate the possibility of in-situ reduction of Cr(VI) by CO₂^·- produced from formate originally existing in leather wastewater by visible-light-driven ARP containing black TiO₂ photocatalyst. The prepared black TiO₂ with nanotube structure achieves remarkable enhanced the reduction rate of Cr(VI) as high as 96.2% (k = 0.0114 min^-1) in the presence of formate, which is approximately 4.75 times than that of 56.3% (k = 0.0024 min^-1) in the absence under 120 min visible-light irradiate at unadjusted pH. The results exhibit a distinct contrast with commercial TiO₂ (P25). A series of control experiments are also performed, indicating that formate is able to convert the oxidative environment into a highly reductive one, and the formate concentration, black TiO₂ dosage and pH may greatly impact on the Cr(VI) reduction rate. According to the electron spin resonance (ESR) measurement, CO₂^·- radicals can be directly verified as dominate radical in this system. Moreover, this system appears to be attractive for creating photochemical systems where in-situ production of CO₂^·- radicals may be realized by using formate. Then this in-situ ARP system will provide a new perspective for the Cr(VI) removing, which makes leather wastewater treatment much easier and more sustainable in the future.

Enhanced photocatalytic degradation of perfluorooctanoic acid by Ti3C2 MXene-derived heterojunction photocatalyst: Application of intercalation strategy in DESs

In-situ construction of heterojunction photocatalyst on two-dimensional (2D) Ti₃C₂ MXene substrate has been proved to be a feasible method to enhance the photocatalytic degradation of organic pollutants. However, the limited interlayer spacing of 2D Ti₃C₂ hinders the in-situ growth of TiO₂ photocatalyst. Herein, the intercalation strategy was developed in deep eutectic solvents (DESs) method to achieve interlayer expansion of Ti₃C₂ and improve Ti₃C₂-derived photocatalyst performance. Because of the intercalation of choline cations, the DESs method synthesized Ti₃C₂ (Ti₃C₂-DES) had the larger c-lattice parameter than that of traditional HF method synthesized Ti₃C₂ (Ti₃C₂-HF). The interlayer space of Ti₃C₂-DES could be intercalated with more water molecule for oxidization of the Ti atoms, which remarkably promoted the in-situ growth of TiO₂ crystals. The formed heterojunction between (001) and (101) facets enhanced carriers separation. The Ti₃C₂ substrate with excellent conductivity further promoted carriers transfer. As a result, Ti₃C₂/TiO₂ photocatalyst exhibited superior perfluorooctanoic acid (PFOA) removal performance (almost 100% removal efficiency and 49% defluorination efficiency within 16 h) compared with the traditional Ti₃C₂-HF/TiO₂ (22% removal efficiency and 12% defluorination efficiency within 16 h). This study provides a feasible strategy for enhancing photocatalytic degradation of PFOA by Ti₃C₂ MXene-derived heterojunction photocatalyst.

Photolysis and photocatalysis of haloacetic acids in water: A review of kinetics, influencing factors, products, pathways, and mechanisms

Haloacetic acids (HAAs) are a group of pollutants ubiquitous in natural environment and anthropogenic systems, and therefore in need of control. Photolysis and photocatalysis techniques via ultraviolet (UV)-based technologies have held promise for decades in degrading organic molecules in water, but their capacities in removing HAAs remain to be explored. To better understand the trends in the existing literature and to identify the knowledge gaps that may merit further exploration, this review compares the HAAs photodegradation kinetics, influencing factors, reaction products, pathways, and mechanisms for a variety of UV technologies. The selected UV processes are classified into three types: UV-only photolysis, photooxidation, and photoreduction. Overall, although trends vary significantly depending upon many factors, the photo-susceptibility of HAAs always increases with rising molecular weight of substituted halogen atom(s), with those chlorinated HAAs being the most refractory species. Notably, while many processes proved hydroxyl radical (OH) as the forcing driver, the patterns of kinetics among HAAs were not consistent among processes, suggesting that OH was not the only driver. Compared to earlier studies focusing on specific technologies to treat numerous contaminants through a material perspective, this review commits to understanding the commonalities and differences among multiple UV-based technologies in treating only one group of compound mainly via a chemistry viewpoint.

Some issues limiting photo(cata)lysis application in water pollutant control: A critical review from chemistry perspectives

For decades, photolysis and photocatalysis have been touted as promising environment-benign and robust technologies to degrade refractory pollutants from water. However, extensive, large-scale engineering applications remain limited now. To facilitate the technology transfer process, earlier reviews have advocated to developing more cost-effective and innocuous materials, maximizing efficiency of photon usage, and optimizing photoreactor systems, mostly from material and reactor improvement perspectives. However, there are also some fundamental yet critical chemistry issues in photo(cata)lysis processes demanding more in-depth understanding and more careful consideration. Hence, this review summarizes some of these challenges. Of them, the first and paramount issue is the interference of coexisting compounds, including dissolved organic matter, anions, cations, and spiked additives. Secondly, considerable concerns are pointed to the formation of undesirable reaction by-products, such as halogenated, nitrogenous, and sulfur-containing compounds, which might increase instead of reduce toxicity of water if inadequate fluence and catalyst/additive are supplied due to time and cost constraints. Lastly, a critical issue lies in the uncertainty of current approaches used for identifying and quantifying radicals, especially when multiple radicals coexist together under changing and interconvertible conditions. The review hence highlights the needs to better understand these fundamental chemistry issues and meanwhile calls for more delicate design of experiments in future studies to overcome these barriers.

Research progress on the removal of hazardous perfluorochemicals: A review

Perfluorinated substances are global and ubiquitous pollutants. The persistent organic pollution of perfluorochemicals (PFCs) have drawn attentions worldwide. In view of the current need for sustainable development, many researchers began to study the remediation techniques for PFCs. Due to its unique hydrophobic and oil-phobic characteristics, the requirements for the PFCs removal process are different, so that their remediation techniques are still under continuous exploration. Hence, this review summarized the removal behaviors of various PFCs on different materials which supply a good foundation for future investigations in this field. It is evident from previous literature that every remediation techniques for PFCs has its own advantages. Among various currently evaluated removal methods, adsorption seems to be one of the most commonly used and recognized techniques for PFCs pollution control. Other innovative and promising techniques, such as physical and/or chemical methods, have also been tested for their effectiveness in removing perfluorinated compounds.

Removal of F-53B as PFOS alternative in chrome plating wastewater by UV/Sulfite reduction

Chrome mist suppressants are key chemicals used in the chrome plating industry to reduce exposure of workers by inhalation to airborne chromic acid pollution. Perfluoroalkyl sulfonated compounds are excellent mist suppressants, thanks to their chemical stability and surface activity. Therefore, despite mounting evidence for their persistence, bioaccumulation and toxicity, it is likely that such chemicals will continue to be used for the foreseeable future because of their importance and lack of alternatives. The present study is aimed at assessing the feasibility of advanced reduction as an effective technology to treat chrome plating industry wastewater. In particular, wastewater containing a chlorinated polyfluorinated ether sulfonate (i.e. F-53B), an alternative to perfluorooctanesulfonate (PFOS) used to prepare chrome mist suppressant in China, was treated by UV-activated sulfite. Results demonstrates that in ultrapure water F-53B can be easily degraded within 1 min-much faster than PFOS. Stoichiometric fluoride recovery was also achieved, confirming significant defluorination of the pollutant. Such superior reducibility was due to the presence of chlorine atoms, as corroborated by quantum chemical calculations. F-53B degradation was also achieved in chrome plating industrial wastewater, which yielded results were slower than those achieved in the laboratory nonetheless obtained complete abatement within 60 min. These results suggest that the proposed advanced reduction process is one of the safest options to control PFAS discharge in the environment and reduce the related risks to ecosystems.

Nanosecond transient absorption studies of the pH-dependent hydrated electron quenching by HSO3. Photochem Photobiol Sci 2019;18:1526-1532. [PMID: 30984955 DOI: 10.1039/c9pp00063a]

The large standard reduction potential of an aqueous solvated electron (eaq-, E° = -2.9 V) makes it an attractive candidate for reductive treatment of wastewater contaminants. Using transient absorption spectroscopy, the nanosecond to microsecond dynamics of eaq- generated from 10 mM solutions of Na2SO3 at pH 4 to 11 in H2O and D2O are characterized, resulting in the determination that between pH 4 and 9 it is the HSO3-, and not H+ as previously postulated by others, that effectively quenches eaq-. The observed bimolecular quenching rate constant (k = 1.2 × 108 M-1 s-1) for eaq- deactivation by HSO3- is found to be consistent with a Brønsted acid catalysis mechanism resulting in formation of H˙ and SO32-. A large solvent isotope effect is observed from the lifetimes of the eaq- in H2O compared to D2O (kH2O/kD2O = 4.4). In addition, the bimolecular rate constant for eaq- deactivation by DSO3- (k = 2.7 × 107 M-1 s-1) is found to be an order of magnitude lower than by HSO3-. These results highlight the role of acids, such as HSO3-, in competition with organic contaminant targets for eaq- and, by extension, that knowledge of the pKa of eaq- sources can be a predictive measure of the effective pH range for the treatment of wastewater contaminants.

Autocatalytic Decomplexation of Cu(II)-EDTA and Simultaneous Removal of Aqueous Cu(II) by UV/Chlorine

Traditional processes usually cannot enable efficient water decontamination from toxic heavy metals complexed with organic ligands. Herein, we first reported the removal of Cu(II)-EDTA by a UV/chlorine process, where the Cu(II)-EDTA degradation obeyed autocatalytic two-stage kinetics, and Cu(II) was simultaneously removed as CuO precipitate. The scavenging experiments and EPR analysis indicated that Cl^• accounted for the Cu(II)-EDTA degradation at diffusion-controlled rate (∼10¹⁰ M^-1 s^-1). Mechanism study with mass spectrometry evidence of 11 key intermediates revealed that the Cu(II)-EDTA degradation by UV/chlorine was an autocatalytic successive decarboxylation process mediated by the Cu(II)/Cu(I) redox cycle. Under UV irradiation, Cu(I) was generated during the photolysis of the Cl^•-attacked complexed Cu(II) via ligand-to-metal charge transfer (LMCT). Both free and organic ligand-complexed Cu(I) could form binary/ternary complexes with ClO^-, which were oxidized back to Cu(II) via metal-to-ligand charge transfer (MLCT) with simultaneous production of Cl^•, resulting in the autocatalytic effect on Cu(II)-EDTA removal. Effects of chlorine dosage and pH were examined, and the technological practicability was validated with authentic electroplating wastewater and other Cu(II)-organic complexes. This study shed light on a new mechanism of decomplexation by Cl^• and broadened the applicability of the promising UV/chlorine process in water treatment.

A novel combined process for efficient removal of Se(VI) from sulfate-rich water: Sulfite/UV/Fe(III) coagulation

The efficient removal of Se(VI) from sulfate-rich water is challenging as most reported processes last for hours to days. In this study, a combined sulfite/UV/Fe(III) coagulation process was proposed for efficient Se(VI) removal from sulfate-rich water within a short time (∼1 h). In the presence of sulfate (1000 mg L^-1), over 99% of Se(VI) (initially at 10 mg L^-1) could be reduced by sulfite (5.0 mM) with a UV dose of 16 J cm^-2 (within 20 min) into Se(IV) as the sole observed product. An alkaline pH (>9) was required for the reduction process, which was naturally obtained with the addition of sulfite. Scavenging experiments with N₂O and NO₃^- both indicated that hydrated electrons (e_aq^-) were responsible for Se(VI) reduction by sulfite/UV. The presence of chloride, sulfate, phosphate, and carbonate (up to 10 mM) showed negligible influence on Se(VI) reduction, whereas nitrate and humic acid inhibited Se(VI) reduction to different extents depending on their concentrations. By Fe(III) coagulation, Se(IV) in the co-presence of sulfite and sulfate was efficiently removed at an OH^-/Fe molar ratio of 1.8-2.8. The removal of Se(IV) by Fe(III) coagulation responded insignificantly to chloride, nitrate, or sulfate (up to 10 mM), whereas it was adversely affected at high levels of carbonate (10 mM) and phosphate (1 mM). The combined sulfite/UV/Fe(III) coagulation process was validated for the efficient removal of Se(VI) from synthetic sulfate-rich solution, simulated wastewater, and authentic smelting wastewater (in 1.1 h). The introduced sulfite underwent minor consumption during UV irradiation and was almost (∼90%) removed after coagulation.

Effects of process variables and kinetics on the degradation of 2,4-dichlorophenol using advanced reduction processes (ARP)

This study aims at investigating the efficiency and kinetics of 2,4-DCP degradation via advanced reduction processes (ARP). Using UV light as activation method, the highest degradation efficiency of 2,4-DCP was obtained when using sulphite as a reducing agent. The highest degradation efficiency was observed under alkaline conditions (pH = 10.0), for high sulphite dosage and UV intensity, and low 2,4-DCP concentration. For all process conditions, first-order reaction rate kinetics were applicable. A quadratic polynomial equation fitted by a Box-Behnken Design was used as a statistical model and proved to be precise and reliable in describing the significance of the different process variables. The analysis of variance demonstrated that the experimental results were in good agreement with the predicted model (R² = 0.9343), and solution pH, sulphite dose and UV intensity were found to be key process variables in the sulphite/UV ARP. Consequently, the present study provides a promising approach for the efficient degradation of 2,4-DCP with fast degradation kinetics.