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

The self-boosting ultrafast removal of Cr(VI) and organic dye in textile wastewater through sulfite-induced redox processes

The treatment of textile wastewater containing harmful metal ions poses a significant challenge in industrial applications due to its environmental impact. In this study, the use of sulfite for treating simulated dye wastewater containing New Coccine (NC) and Cr(VI) was investigated. The removal of NC was influenced by the redox reaction between Cr(VI) and sulfite, demonstrating a strong self-boosting effect of Cr(VI) on NC removal. Remarkable NC decoloration (95%) and Cr(VI) reduction (90%) were achieved within 1 min, highlighting the effectiveness of the treatment. Quenching experiments and electron paramagnetic resonance (EPR) technology confirmed that singlet oxygen (1O2) was the main oxidative agent for organic dye removal and SO4•-, •OH and Cr(V) were also identified as key contributors to NC degradation. The Cr(VI)/sulfite system exhibited higher efficiency in degrading azo dyes, such as NC and Congo Red (CR), compared to non-azo dyes like Methylene Blue (MB). This superiority may be attributed to the action of Cr(V) on azo groups. Additionally, the COD removal experiments were conducted on the actual dye wastewater, showing the excellent performance of the Cr(VI)/Sulfite system in treating industrial textile wastewater. This approach presents a promising strategy for effective "waste control by waste", offering great potential for addressing challenges related to dye wastewater treatment and environmental pollution control in practical industrial scenarios.

Positive profile of natural small molecule organic matters on emerging antivirus pharmaceutical elimination in advance reduction process: A deep dive into the photosensitive mechanism of triplet excited state compounds

Natural small molecular organic matter (NSOM), ubiquitous in natural waters and distinct from humic acid or fulvic acid, is a special type of dissolved organic matter (DOM) which is characterized as strong photosensitivity and simple molecular structure. However, little study had been directed on the role of NSOM in eliminating emerging contaminants in advanced reduction process (ARP). This study took three small molecular isomeric organic acids (p-hydroxybenzoic acid, pHBA; salicylic acid, SA; m-hydroxybenzoic acid, mHBA) as the representative substances of NSOM to explore these mechanisms on promoting Ribavirin (RBV, an anti COVID-19 medicine) degradation in ultraviolet activated sulfite (UV/Sulfite) process. The results demonstrated that the observed degradation rate constant of RBV (kobs-RBV) was 7.56 × 10-6 s-1 in UV/Sulfite process, indicating that hydrated electron (eaq-) from UV/Sulfite process could not effectively degrade RBV, while it increased by 178 and 38 times when pHBA and SA were introduced into UV/Sulfite process respectively, suggesting that pHBA and SA strongly promoted RBV degradation while mHBA had no promotion on RBV abatement in UV/Sulfite process. Transient absorption spectra and reactive intermediates scavenging experiment indicated that the triplet excited state pHBA and SA (3pHBA* and 3SA*) contributed to the degradation of RBV through non-radical process. Notably, eaq- played the role of key initiator in transforming pHBA and SA into their triplet states. The difference of kobs-RBV in UV/Sulfite/pHBA and UV/Sulfite/SA process was attributed to different generation pathways of 3pHBA* and 3SA* (high molar absorptivity at the wavelength of 254 nm and photosensitive cycle, respectively) and their second order rate constants towards RBV (kRBV-3pHBA* = 8.60 × 108 M-1 s-1 and kRBV-3SA* = 6.81 × 107 M-1 s-1). mHBA could not degrade RBV for its lack of intramolecular hydrogen bond and low molar absorptivity at 254 nm to abundantly transform into its triplet state. kobs-RBV increased as pH increased from 5.0 to 11.0 in UV/Sulfite/SA process, due to the high yield of eaq- in alkaline condition which promoted the generation of 3SA* and the stable of the absorbance of SA at 254 nm. By contrast, kobs-RBV underwent a process of first increasing and then decreasing in UV/Sulfite/pHBA process as the increase of pH, and its highest value achieved in a neutral condition. This lied in the exposure of eaq- increased as the increase of pH which promoted the generation of 3pHBA*, while the molar absorptivity of pHBA at 254 nm decreased as the increase of pH in an alkaline condition which inhibited the yield of 3pHBA*. The RBV degradation pathways and products toxicity assessment indicated that UV/Sulfite/pHBA had better detoxification performance on RBV than UV/Sulfite/SA process. This study disclosed a novel mechanism of emerging contaminants abatement through non-radical process in NSOM mediated ARP, and provide a wide insight into positive profile of DOM in water treatment process, instead of only taking DOM as a quencher of reactive intermediates.

Direct aqueous photochemistry of methylglyoxal and its effect on sulfate formation

In recent years, among many oxidation pathways studied for atmospheric sulfate formation, the aqueous phase oxidation pathways of H2O2 and organic hydroperoxides (ROOHs) have attracted great scientific attention. Higher concentrations of H2O2 and ubiquitous ROOHs have been observed in atmospheric aqueous phase environments (cloud water, fog droplets, etc.). However, there are still some gaps in the study of their aqueous phase generation and their influences on sulfate formation. In this study, the aqueous phase photochemical reaction of methylglyoxal, a ubiquitous organic substance in the atmospheric aqueous phase, was studied under UV irradiation, and the generation of H2O2 and ROOHs in this system was investigated. It is found for the first time that the aqueous phase photolysis of methylglyoxal not only produces H2O2 but also produces ROOHs, and UV light and O2 are necessary for the formation of H2O2 and ROOHs. Based on the experimental results, the possible mechanism of aqueous phase photochemistry of methylglyoxal and the generation of H2O2 and ROOHs were proposed. The effect of aqueous phase photolysis of methylglyoxal on sulfate formation under different conditions was also investigated. The results show that the aqueous phase photolysis of methylglyoxal significantly promoted SO2 oxidation and sulfate formation, in which SO2 oxidation was realized by the generated H2O2, ROOHs and •OH radicals, and the importance of the formed ROOHs cannot be ignored. These results fill some gaps in the field of aqueous phase H2O2 and ROOHs production, and to a certain extent confirm the important roles of the aqueous phase photolysis of methylglyoxal and the formed H2O2 and ROOHs in the production of sulfate. The study reveals the new sources of H2O2 and ROOHs, and provides a new insight into the heterogeneous aqueous phase oxidation pathways and mechanisms of SO2 in cloud and fog droplets and haze particles.

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.

Degradation of iopamidol in the permanganate/sulfite process: Evolution of iodine species and effect on the subsequent formation of disinfection by-products

Permanganate/sulfite (Mn(VII)/S(IV)) process is a promising pre-oxidation technology for sequestering the emerging organic contaminants in drinking water treatment plant. Iopamidol (IPM), a representative of iodinated X-ray contrast media, has been widely detected in water sources and has the risk of forming iodinated disinfection byproducts (I-DBPs) in water treatment system. In this study, we investigated the evolution of iodine species during the IPM degradation by the Mn(VII)/S(IV) process and its effect on the subsequent formation of I-DBPs during chlorination at pH 7.0 and 8.0. IPM could be effectively degraded in the Mn(VII)/S(IV) process at environmentally relevant pH (pH 7.0 and 8.0). The results of quenching and competitive oxidation kinetic experiments revealed that SO4·- was the major reactive oxidizing species contributing to the degradation of IPM whereas the contributions of HO· and reactive manganese species were negligible in the Mn(VII)/S(IV) process. I- and IO3- were generated while no HOI was detected during the degradation of IPM in the Mn(VII)/S(IV) process. The effects of IPM oxidation by Mn(VII)/S(IV) on the subsequent formation of chlorinated disinfection by-products (Cl-DBPs) during chlorination were related to the category of Cl-DBPs. The pre-oxidation of IPM by Mn(VII)/S(IV) resulted in the generation of I-DBPs during the disinfection process although no I-DBPs were detected if no pre-oxidation was applied. The finding of this study suggested that attention should be paid to the toxicity of DBPs when water containing iodinated organic contaminants is treated by Mn(VII)/S(IV) process or other pre-oxidation technologies.

Experimental determination and QSAR analysis of the rate constants for SO5•- reactions with aromatic micropollutants in water

S(IV)-based systems used for advanced oxidation processes (AOPs) have been constructed for the degradation of organic contaminants via oxysulfur radicals, including SO3•-, SO4•-, and SO5•-. Although SO5•- is proposed as an active species in AOPs processes, research on the reactivity of SO5•- has remained unclear. In this work, 53 target aromatic micropollutants (AMPs), including 13 phenols, 27 amines, and 13 PPCPs were selected to determine the second-order reaction rate constants for SO5•- using the competitive kinetics method, in which the [Formula: see text] values, observed at pH 4 ranged from (2.44 ± 0.00) × 105 M-1 s-1 to (4.41 ± 0.28) × 107 M-1 s-1. Quantitative structure-activity relationship (QSAR) models for the oxidation of AMPs by SO5•- were developed based on 40 [Formula: see text] values of amines and phenols, and their molecular descriptors, using the stepwise multiple linear regression method. This comprehensive model exhibited the excellent goodness-of-fit (Radj2 = 0.802), robustness (QLOO2 = 0.749), and predictability (Qext2 = 0.656), and the one-electron oxidation potential (Eox), energy of the highest occupied molecular orbital energy (EHOMO), and most positive net atomic charge on the carbon atoms (qC+) were considered the most influential descriptors for the comprehensive model, indicating that SO5•- oxidizes pollutants via single electron transfer reaction and exhibits a strong oxidation capacity, especially for pollutants containing electron-donating groups. Moreover, the [Formula: see text] values of 13 PPCPs were predicted using this comprehensive model, which suggested the practical application significance of the QSAR model. This study emphasizes the direct oxidation capacity of SO5•-, which is important to evaluate and simulate AOPs based on S(IV).

New Insights on Designing the Next-Generation Materials for Electrochemical Synthesis of Reactive Oxidative Species Towards Efficient and Scalable Water Treatment: A Review and Perspectives

Electrochemical water remediation technologies offer several advantages and flexibility for water treatment and degradation of contaminants. These technologies generate reactive oxidative species (ROS) that degrade pollutants. For the implementation of these technologies at an industrial scale, efficient, scalable, and cost-effective in-situ ROS synthesis is necessary to degrade complex pollutant mixtures, treat large amount of contaminated water, and clean water in a reasonable amount of time and cost. These targets are directly dependent on the materials used to generate the ROS, such as electrodes and catalysts. Here, we review the key design aspects of electrocatalytic materials for efficient in-situ ROS generation. We present a mechanistic understanding of ROS generation, including their reaction pathways, and integrate this with the key design considerations of the materials and the overall electrochemical reactor/cell. This involves tunning the interfacial interactions between the electrolyte and electrode which can enhance the ROS generation rate up to ~ 40% as discussed in this review. We also summarized the current and emerging materials for water remediation cells and created a structured dataset of about 500 electrodes and 130 catalysts used for ROS generation and water treatment. A perspective on accelerating the discovery and designing of the next generation electrocatalytic materials is discussed through the application of integrated experimental and computational workflows. Overall, this article provides a comprehensive review and perspectives on designing and discovering materials for ROS synthesis, which are critical not only for successful implementation of electrochemical water remediation technologies but also for other electrochemical applications.

Degradation of metoprolol by UV/sulfite as an advanced oxidation or reduction process: The significant role of oxygen

The degradation of metoprolol (MTP) by the UV/sulfite with oxygen as an advanced reduction process (ARP) and that without oxygen as an advanced oxidation process (AOP) was comparatively studied herein. The degradation of MTP by both processes followed the first-order rate law with comparable reaction rate constants of 1.50×10^-3sec^-1 and 1.20×10^-3sec^-1, respectively. Scavenging experiments demonstrated that both e_aq^- and H• played a crucial role in MTP degradation by the UV/sulfite as an ARP, while SO₄^•- was the dominant oxidant in the UV/sulfite AOP. The degradation kinetics of MTP by the UV/sulfite as an ARP and AOP shared a similar pH dependence with a minimum rate obtained around pH 8. The results could be well explained by the pH impacts on the MTP speciation and sulfite species. Totally six transformation products (TPs) were identified from MTP degradation by the UV/sulfite ARP, and two additional ones were detected in the UV/sulfite AOP. The benzene ring and ether groups of MTP were proposed as the major reactive sites for both processes based on molecular orbital calculations by density functional theory (DFT). The similar degradation products of MTP by the UV/sulfite process as an ARP and AOP indicated that e_aq^-/H• and SO₄^•- might share similar reaction mechanisms, primarily including hydroxylation, dealkylation, and H abstraction. The toxicity of MTP solution treated by the UV/sulfite AOP was calculated to be higher than that in the ARP by the Ecological Structure Activity Relationships (ECOSAR) software, due to the accumulation of TPs with higher toxicity.

The overlooked role of UV185 induced high-energy excited states in the dephosphorization of organophosphorus pesticide by VUV/persulfate

Vacuum ultraviolet (VUV) based advanced oxidation processes (AOPs) recently attracted widespread interests. However, the role of UV₁₈₅ in VUV is only considered to be generating a series of active species, while the effect of photoexcitation has long been overlooked. In this work, the role of UV₁₈₅ induced high-energy excited state for the dephosphorization of organophosphorus pesticides was studied using malathion as a model. Results showed malathion degradation was highly related to radical yield, while its dephosphorization was not. It was UV₁₈₅ rather than UV₂₅₄ or radical yield that was responsible for malathion dephosphorization by VUV/persulfate. DFT calculation results demonstrated that the polarity of P-S bond was further increased during UV₁₈₅ excitation, favoring dephosphorization while UV₂₅₄ did not. The conclusion was further supported by degradation path identification. Moreover, despite the fact that anions (Cl^-, SO₄^2- and NO₃^-) considerably affected radical yield, only Cl^- and NO₃^- with high molar extinction coefficient at 185 nm significantly affected dephosphorization. This study shed light on the crucial role of excited states in VUV based AOPs and provided a new idea for the development of mineralization technology of organophosphorus pesticides.

The efficient degradation of diclofenac by ferrate and peroxymonosulfate: performances, mechanisms, and toxicity assessment

In this study, the degradation efficiency and reaction mechanisms of diclofenac (DCF), a nonsteroidal anti-inflammatory drug, by the combination of ferrate (Fe(VI) and peroxymonosulfate (PMS) (Fe(VI)/PMS) were systematically investigated. The higher degradation efficiency of DCF in Fe(VI)/PMS system can be obtained than that in alone persulfate (PS), Fe(VI), PMS, or the Fe(VI)/PS process at pH 6.0. DCF was efficiently removed in Fe(VI)/PMS process within a wide range of pH values from 4.0 to 8.0, with higher degradation efficiency in acidic conditions. The increasing reaction temperature (10 to 30 ℃), Fe(VI) dose (6.25 to 100 µM), or PMS concentration (50 to 1000 µM) significantly enhanced the DCF degradation. The existences of HCO₃¯, Cl¯, and humic acid (HA) obviously inhibited the DCF removal. Electron paramagnetic resonance (EPR), free radical quenching, and probing experiments confirmed the existence of sulfate radicals (SO₄^•¯), hydroxyl radicals (^•OH), and Fe(V)/ Fe(IV), which are responsible for DCF degradation in Fe(VI)/PMS system. The variations of TOC removal ratio reveal that the adsorption of organics with ferric particles, formed in the reduction of Fe(VI), also were functioned in the removal process. Sixteen DCF transformation byproducts were identified by UPLC-QTOF/MS, and the toxicity variation was evaluated. Consequently, eight reaction pathways for DCF degradation were proposed. This study provides theoretical basis for the utilization of Fe(VI)/PMS process.

Ferrous-Iron-Activated Sulfite-Accelerated Short-Chain Fatty Acid Production from Waste-Activated Sludge Fermentation: Process Assessment and Underlying Mechanism

To break the bottlenecks of slow hydrolysis and low acid production efficiency of waste-activated sludge (WAS) in the traditional anaerobic fermentation process, this study investigated the employment of ferrous-iron (Fe(II))-activated sulfite to produce hydroxyl, sulfate, and other highly oxidizing radicals on WAS floc cracking and short-chain fatty acid (SCFAs) production during anaerobic fermentation. The effect of the dosage ratio of Fe(II)/S(IV) was also studied. Results showed that the combined pretreatment of Fe(II)-activated sulfite significantly promoted the exfoliation of extracellular polymers and the subsequent SCFAs production. The highest concentration of SCFAs reached 7326.5 mg COD/L under the optimal dosage of 1:2 for Fe(II)/S(IV), which was 1.1~2.1 times higher than that of other research groups. Meanwhile, the analysis by 3D fluorescence spectroscopy and EPR (electron paramagnetic resonance) showed that Fe(II)-activated sulfite had a synergistic effect on the rupture of sludge cells and the stripping of extracellular polymers, with SO4− and OH as the key radicals generated and being much stronger in the 1:1 and 1:2 groups. High-throughput sequencing showed that the Fe(II)-activated sulfite system significantly changed the functional microbial diversity. The anaerobic fermentation bacteria and sulfate-reducing bacteria were significantly enriched. The underlying mechanism of Fe(II)-activated sulfite oxidation and molecular ecological network of key microbiomes were unveiled.

Treatment of bromate in UV/sulfite autoxidation process enhances formation of dibromoacetonitrile during chlorination

The integration of UV/sulfite autoxidation process (USAP, i.e., UV activation of sulfite in the presence of 5 ∼ 10 mg/L O₂) into conventional water to degrade micropollutants rises extensive attention, but its impact on water quality, and especially the formation of disinfection byproducts is still unclear. Herein, the formation of dibromoacetonitrile (DBAN) from bromate (BrO₃^-) upon treatment with USAP followed by chlorination was evaluated, in the presence of amino acids (AAs) selected as representative organic matter in drinking water. Results revealed that hydrated electrons (e_aq^-) produced during USAP contribute to the reduction of BrO₃^- to Br^-, which is then converted into HBrO/BrO^- during post-chlorination. At the same time, sulfate radicals (SO₄^•-) and hydroxyl radicals (•OH) generated in USAP mediated AAs' conversion via α-hydrogen abstraction and NH₂-hydrogen abstraction reactions to produce HN=C(CH₃)‒COOH, CH₃‒CH=NH, and CH₃‒CN, which are released into the post-chlorination stage and therefore, enhance the bromine utilization factor (BUF) value and DBAN formation. The effects of the USAP treatment time, BrO₃^- concentration, AA concentration, pH, and real waters were also evaluated. Although 63.5% of BrO₃^- was eliminated by USAP followed by chlorination, the toxicity index (TI) was increased by 1.5-fold due to the formation of the all brominated CX₃R-type nitrogenous disinfection byproducts (N-DBPs), demonstrating the potential risk of applying USAP as a treatment process in BrO₃^- containing waters.

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.

Efficient degradation and deiodination of iopamidol by UV/sulfite process: Assessment of typical process parameters and transformation paths

Iopamidol (IPM) is widely used in medical clinical examination and treatment and has immeasurable harm to the ecological environment. The combination of UV and sulfite (UV/sulfite) process was developed to degrade IPM in this study. In contrast to that almost no removal of IPM was observed under sulfite reduction alone, the UV/sulfite process could efficiently reductively degrade IPM with the observed rate constant (k_obs) of 2.08 min^-1, which was nearly 4 times that of UV irradiation alone. The major active species in the UV/sulfite process were identified as hydrated electrons (e_aq^-) by employing active species scavengers. The influence of the initial pH, sulfite dosage, IPM concentration, UV intensity and common water matrix were evaluated. The degradation of IPM reached nearly 100% within only 2.5 min at pH 9, and k_obs increased at higher initial sulfite dosages and greater UV intensities. HCO₃^- had a limited effect on the degradation of IPM, while humic acid (HA) was found to be a strong inhibitor in the UV/sulfite process. With the synergistic action of UV/sulfite, most of the iodine in IPM was found to release in the form of iodide ions (up to approximately 98%), and a few formed iodide-containing organic compounds, reducing significantly the toxicity of degradation products. Under direct UV irradiation and free radical reduction (mainly e_aq^-), 15 transformation intermediates of IPM were produced by amide hydrolysis, deiodination, hydroxyl radical addition and hydrogen abstraction reactions, in which free radical attack accounted for the main part. Consequently, the UV/sulfite process has a strong potential for IPM degradation in aquatic environments.

Degradation of iopamidol by UV365/NaClO: Roles of reactive species, degradation mechanism, and toxicology

The degradation of iopamidol (IPM) was investigated using a UV₃₆₅/NaClO system. The reactive species (HO·, ClO·, ozone, Cl·, and Cl₂^-·) in the system were identified, and the changing trends of the percentage contributions of these reactive species to IPM removal under various conditions were systematically evaluated. The results showed that ClO· and HO· played the most significant roles in the apparent pseudo-first-order rate constants of IPM degradation (k_obs, min^-1) in the control experiment, and their percentage contributions to k_obs were 41.31% and 34.45%, respectively. In addition, Cl· and Cl₂^-· together contributed 22% to the k_obs. Furthermore, the contribution of ozone to the IPM removal could be neglected. The concentrations of these species increased significantly when the concentration of NaClO was increased from 50 µM to 200 µM, while the percentage contribution of ClO· to k_obs was greatly increased. The concentrations and percentage contributions of HO· and ClO· decreased significantly as the solution pH increased from 5 to 9, with Cl₂^-· playing a greater role in the degradation of IPM under alkaline conditions. While Cl^- or HCO₃^-/CO₃^2- significantly promoted the generation of Cl₂^-· or CO₃^-·, neither had an obvious effect on k_obs, suggesting that Cl₂^-· and CO₃^-· should have a certain reactivity with IPM. Compared with that of Cl₂^-·, the percentage contribution of ClO· and Cl· to k_obs was more likely to be inhibited by NOM. In addition, the organic and inorganic oxidation products of IPM were detected. The oxidation mechanisms of IPM degradation in the UV₃₆₅/NaClO system, such as the H-extraction reaction, deiodination, substitution reaction, amide hydrolysis, and amine oxidation, were proposed according to the obtained 15 organic products. No effect on acute toxicity towards Vibrio fischeri and Photobacterium phosphoreum was detected during the oxidation of IPM by the UV₃₆₅/NaClO system. Furthermore, the engineering feasibility of the oxidation system was demonstrated, by the effective degradation of IPM in actual water. However, HOI rapidly accumulated during the removal of IPM in the UV₃₆₅/NaClO system, which poses certain environmental risks and will needs to be investigated.

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.

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.

Making waves: Defining advanced reduction technologies from the perspective of water treatment

Studies related to advanced reduction technologies (ARTs) have grown exponentially since the term was first coined in 2013. Despite recent interests in ARTs, the conditions and requirements for these processes have yet to be defined and clarifed. In comparision to well defined advanced oxidation technologies/processes (AOTs/AOPs) which involve the generation of hydroxyl radical as the common characteristic, ARTs function by electron donation from a variety of reducing agents and activators. Based on an extensive literature review, we propose that ARTs be defined as processes employing strong chemical reductants with E° ≤ -2.3 V vs. normal hydrogen electrode at 25 ºC. While extensive studies have revealed critical fundamental details of AOTs/AOPs mediated processes, there are still significant gaps in elucidation of the mechanistic details of reductive degradation/transformation of highly toxic compounds by ARTs. A significant number of pollutants and toxins resistant to AOTs/AOPs treatment are effectively degraded by ARTs. A great leap is needed on understanding ARTs to fully utilize their potential to efficiently remediate recalcitrant compounds of different sources and structures.

Transformation mechanisms of iopamidol by iron/sulfite systems: Involvement of multiple reactive species and efficiency in real water

Although the ability of iron/sulfite system for decontamination purposes has been investigated, the complex reactive species generated and the underlying transformation mechanisms remain elusive. Here, we have comprehensively examined the transformation of iopamidol (IPM), a representative of iodinated X-ray contrast media, by iron catalyzed sulfite oxidation process under different water chemistry conditions. Multiple reactive intermediates including Fe(IV), SO₄^•-, and SO₅^•- were identified by conducting a series of experiments. Eight transformation products were detected by mass spectrometry analysis, and correlation with the nature of involved reactive species has been made. Further, the transformation pathways including amide hydrolysis, deiodination, amino and hydroxyl groups oxidation were proposed. Interestingly, these transformation products could be removed through adsorption to iron precipitates formed via pH adjustment. Combining Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy, we revealed an effective way to reduce the amount of transformation products in the treated solutions. Since the iron/sulfite process appears to be less sensitive to natural organic matter, it exhibited very good efficiency for IPM removal in real water samples, even with a high organic carbon loading. These findings may have strong implications in the development of novel oxidation process based on the sulfite/iron systems for wastewater treatment.

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.