Advanced Reduction Processes: A New Class of Treatment Processes

Enhanced sorption and destruction of PFAS by biochar-enabled advanced reduction process

The biochar-enabled advanced reduction process (ARP) was developed for enhanced sorption (by biochar) and destruction of PFAS (by ARP) in water. First, the biochar (BC) was functionalized by iron oxide (Fe3O4), zero valent iron (ZVI), and chitosan (chi) to produce four biochars (BC, Fe3O4-BC, ZVI-chi-BC, and chi-BC) with improved physicochemical properties (e.g., specific surface area, pore structure, hydrophobicity, and surface functional groups). Batch sorption experimental results revealed that compared to unmodified biochar, all modified biochars showed greater sorption efficiency, and the chi-BC performed the best for PFAS sorption. The chi-BC was then selected to facilitate reductive destruction and defluorination of PFAS in water by ARP in the UV-sulfite system. Adding chi-BC in UV-sulfite ARP system significantly enhanced both degradation and defluorination efficiencies of PFAS (up to ∼100% degradation and ∼85% defluorination efficiencies). Radical analysis using electron paramagnetic resonance (EPR) spectroscopy showed that sulfite radicals dominated at neutral pH (7.0), while hydrated electrons (eaq-) were abundant at higher pH (11) for the efficient destruction of PFAS in the ARP system. Our findings elucidate the synergies of biochar and ARP in enhancing PFAS sorption and degradation, providing new insights into PFAS reductive destruction and defluorination by different reducing radical species at varying pH conditions.

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.

Photoreduction of atrazine from aqueous solution using sulfite/iodide/UV process, degradation, kinetics and by-products pathway

Due to its widespread use in agriculture, atrazine has entered aquatic environments and thus poses potential risks to public health. Therefore, researchers have done many studies to remove it. Advanced reduction process (ARP) is an emerging technology for degrading organic contaminants from aqueous solutions. This study was aimed at evaluating the degradation of atrazine via sulfite/iodide/UV process. The best performance (96% of atrazine degradation) was observed in the neutral pH at 60 min of reaction time, with atrazine concentration of 10 mg/L and concentration of sulfite and iodide of 1 mM. The kinetic study revealed that the removal of atrazine was matched with the pseudo-first-order model. Results have shown that reduction induced by e aq - and direct photolysis dominated the degradation of atrazine. The presence of anions (Cl - , CO 3 2 - and SO 4 2 - ) did not have a significant effect on the degradation efficiency. In optimal conditions, COD and TOC removal efficiency were obtained at 32% and 4%, respectively. Atrazine degradation intermediates were generated by de-chlorination, hydroxylation, de-alkylation, and oxidation reactions. Overall, this research illustrated that Sulfite/iodide/UV process could be a promising approach for atrazine removal and similar contaminants from aqueous solutions.

Modeling the Role in pH on Contaminant Sequestration by Zerovalent Metals: Chromate Reduction by Zerovalent Magnesium

The role of pH in sequestration of Cr(VI) by zerovalent magnesium (ZVMg) was characterized by global fitting of a kinetic model to time-series data from unbuffered batch experiments with varying initial pH values. At initial pH values ranging from 2.0 to 6.8, ZVMg (0.5 g/L) completely reduced Cr(VI) (18.1 μM) within 24 h, during which time pH rapidly increased to a plateau value of ∼10. Time-series correlation analysis of the pH and aqueous Cr(VI), Cr(III), and Mg(II) concentration data suggested that these conditions are controlled by combinations of reactions (involving Mg0 oxidative dissolution and Cr(VI) sequestration) that evolve over the time course of each experiment. Since this is also likely to occur during any engineering applications of ZVMg for remediation, we developed a kinetic model for dynamic pH changes coupled with ZVMg corrosion processes. Using this model, the synchronous changes in Cr(VI) and Mg(II) concentrations were fully predicted based on the Langmuir-Hinshelwood kinetics and transition-state theory, respectively. The reactivity of ZVMg was different in two pH regimes that were pH-dependent at pH < 4 and pH-independent at the higher pH. This contrasting pH effect could be ascribed to the shift of the primary oxidant of ZVMg from H+ to H2O at the lower and higher pH regimes, respectively.

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 new perspective on predicting the reaction rate constants of hydrated electrons for organic contaminants: Exploring molecular structure characterization methods and ambient conditions

Hydrated electrons (eaq-) exhibit rapid degradation of diverse persistent organic contaminants (OCs) and hold great promise as a formidable reducing agent in water treatment. However, the diverse structures of compounds exert different influences on the second-order rate constant of hydrated electron reactions (keaq-), while the same OCs demonstrate notable discrepancies in keaq- values across different pH levels. This study aims to develop machine learning (ML) models that can effectively simulate the intricate reaction kinetics between eaq- and OCs. Furthermore, the introduction of the pH variable enables a comprehensive investigation into the impact of ambient conditions on this process, thereby improving the practicality of the model. A dataset encompassing 701 keaq- values derived from 351 peer-reviewed publications was compiled. To comprehensively investigate compound properties, this study introduced molecular descriptor (MD), molecular fingerprint (MF), and the integration of both (MD + MF) as model variables. Furthermore, 60 sets of predictive models were established utilizing two variable screening methodologies (MLR and RF) and ten prominent algorithms. Through statistical parameter analysis, it was determined that descriptors combined with MD and MF, the RF screening method, and the symbolism algorithm exhibited the best predictive efficacy. Importantly, the combination of descriptor models exhibited significantly superior performance compared to individual MF and MD models. Notably, the optimal model, denoted as RF - (MF + MD) - LGB, exhibited highly satisfactory predictive results (R2tra = 0.967, Q2tra = 0.840, R2ext = 0.761). The mechanistic explanation study based on Shapley Additive Explanations (SHAP) values further elucidated the crucial influences of polarity, pH, molecular weight, electronegativity, carbon-carbon double bonds, and molecular topology on the degradation of OCs by eaq-. The proposed modeling approach, particularly the integration of MF and MD, alongside the introduction of pH, may furnish innovative ideas for advanced reduction or oxidation processes (ARPs/AOPs) and machine learning applications in other domains.

A review of innovative approaches for onsite management of PFAS-impacted investigation derived waste

The historic use of aqueous film-forming foam (AFFF) has led to widespread detection of per- and polyfluoroalkyl substance (PFAS) in groundwater, soils, sediments, drinking water, wastewater, and receiving aquatic systems throughout the United States (U.S.). Prior to any remediation activities, in order to identify the PFAS-impacted source zones and select the optimum management approach, extensive site investigations need to be conducted. These site investigations have resulted in the generation of considerable amount of investigation-derived waste (IDW) which predominantly consists of well purging water and drill fluid, equipment washing residue, soil, drill cuttings, and residues from the destruction of asphalt and concrete surfaces. IDW is often impacted by varying levels of PFAS which poses a substantial challenge concerning disposal to prevent potential mobilization of PFAS, logistical complexities, and increasing requirement for storage as a result of accumulation of the associated wastes. The distinct features of IDW involve the intermittent generation of waste, substantial volume of waste produced, and the critical demand for onsite management. This article critically focuses on innovative technologies and approaches employed for onsite treatment and management of PFAS-impacted IDW. The overall objective of this study centers on developing and deploying end-of-life treatment technology systems capable of facilitating unrestricted disposal, discharge, and/or IDW reuse on-site, thereby reducing spatial footprints and mobilization time.

Photolysis Products of Fluorinated Pharmaceuticals: A Combined Fluorine Nuclear Magnetic Resonance Spectroscopy and Mass Spectrometry Approach

The aqueous photolysis of four pharmaceuticals with varying fluorinated functional groups was assessed under neutral, alkaline, advanced oxidation, and advanced reduction conditions with varying light sources. Solar simulator quantum yields were 2.21 × 10-1  mol Ei-1 for enrofloxacin, 9.36 × 10-3  mol Ei-1 for voriconazole, and 1.49 × 10-2  mol Ei-1 for flecainide. Florfenicol direct photolysis was slow, taking 150 h for three degradation half-lives. Bimolecular rate constants between pharmaceuticals and hydroxyl radicals were 109 to 1010  M-1  s-1 . Using a combined quantitative fluorine nuclear magnetic resonance spectroscopy (19 F-NMR) and mass spectrometry approach, fluorine mass balances and photolysis product structures were elucidated. Enrofloxacin formed a variety of short-lived fluorinated intermediates that retained the aryl F motif. Extended photolysis time led to complete aryl F mineralization to fluoride. The aliphatic F moiety on florfenicol was also mineralized to fluoride, but the resulting product was a known antibiotic (thiamphenicol). For voriconazole, the two aryl Fs contributed more to fluoride production compared with the heteroaromatic F, indicating higher stability of the heteroaromatic F motif. The two aliphatic CF3 moieties in the flecainide structure remained intact under all conditions, further supporting the stability of these moieties found in per- and polyfluoroalkyl substances under a variety of conditions. The advanced treatment conditions generating hydroxyl radicals or hydrated electrons accelerated the degradation, but not the defluorination, of flecainide. The combination of 19 F-NMR and mass spectrometry proved powerful in allowing identification of fluorinated products and verifying the functional groups present in the intermediates and products. The results found in the present study will aid in the understanding of which fluorinated functional groups should be incorporated into pharmaceuticals to ensure organofluorine byproducts are not formed in the environment and help determine the water-treatment processes that effectively remove specific pharmaceuticals and more generally fluorinated motifs. Environ Toxicol Chem 2023;00:1-12. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Selective and effective oxidation of ammonium to dinitrogen in MgO/Na2SO3/K2S2O8 system

The oxidation of ammonium (NH₄⁺) to dinitrogen (N₂) with high selectivity and high efficiency is still a challenge. Herein, a novel sunlight induced persulfate (PS)-based AOPs process (MgO/Na₂SO₃/PS/hv) was proposed by introducing solid base (MgO) and hydrated electron (e_aq^-), to selectively oxidize NH₄⁺ to N₂, with high selectivity and high efficiency at a wide range of pH value. The deprotonation of NH₄⁺ into NH₃ by MgO and the generation of •OH and SO₄^-• by PS activation were responsible for the high efficiency of NH₄⁺ oxidation. The buffering capacity provided by MgO to proton released from PS activation made the NH₄⁺ oxidation possible at a wide pH range. The e_aq^- from the Na₂SO₃/hv process was the main active specie to reduce NO₂^-and NO₃^- (NO_x^-) into N₂, responsible for high N₂ selectivity of NH₄⁺ oxidation. 100% NH₄⁺ could be oxidized within 30 min, and N₂ selectivity exceeded 96% at the initial pH range of 3-11 and the initial concentration of NH₄⁺ of 30 mg N/L. This work could offer an efficient AOPs process for selective NH₄⁺ oxidation, which is promising for the chemical denitrification of wastewater ….

Nanomaterial-Based Advanced Oxidation/Reduction Processes for the Degradation of PFAS

This review focuses on a critical analysis of nanocatalysts for advanced reductive processes (ARPs) and oxidation processes (AOPs) designed for the degradation of poly/perfluoroalkyl substances (PFAS) in water. Ozone, ultraviolet and photocatalyzed ARPs and/or AOPs are the basic treatment technologies. Besides the review of the nanomaterials with greater potential as catalysts for advanced processes of PFAS in water, the perspectives for their future development, considering sustainability, are discussed. Moreover, a brief analysis of the current state of the art of ARPs and AOPs for the treatment of PFAS in water is presented.

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.

Boosting degradation and defluorination efficiencies of PFBS in a vacuum-ultraviolet/S(Ⅳ) process with iodide involvement

Perfluorobutane sulfonate (PFBS) is considered to be a promising alternative of perfluorooctane sulfonates (PFOS), while it is also hazardous. The UV/S (Ⅳ) system has been confirmed to be effective for PFOS removal from water, while it is inefficient for PFBS decomposition. A hybrid vacuum-ultraviolet (VUV)/S (Ⅳ)/KI process was investigated for the degradation of PFBS in aqueous solution. With KI involvement, the degradation rate of PFBS was boosted from 1.8802 μg h^-1 up to 3.5818 μg h^-1 in the VUV/S (Ⅳ) process. Alkaline conditions significantly increased the degradation efficiency of PFBS, which can be explained that S (Ⅳ) was dominated by SO₃^2- rather than HSO₃^- and H₂SO₃ in alkaline conditions. Cl^-, HCO₃^-, NO₃^-, NO₂^-, and HA would inhibit the performance of the VUV/S (Ⅳ)/KI process via various reactions. In addition, the toxicity of PFBS was significantly reduced by the VUV/S (Ⅳ)/KI process. Even in actual waters, the VUV/S (Ⅳ)/KI process also presented a satisfying performance in the degradation of PFBS.

Performance and mechanisms for tetrabromobisphenol A efficient degradation in a novel homogeneous advanced treatment based on S2O42- activated by Fe3

Tetrabromobisphenol A (TBBPA), a representative brominated flame retardant (BFR), generally could be debrominated and degraded effectively in photolysis systems with the high energy consumption. In this study, the novel sulfate radical (SO₄^•-) generation resource of dithionite (S₂O₄^2-), activated by the common transition metal of Fe³⁺, has been applied for establishing an innovative homogeneous advance treatment system for BFR treatment in water. When coupling Fe³⁺ with S₂O₄^2-, TBBPA degradation efficiency could be remarkably improved from 38.7% to 93.8% with the debromination and mineralization efficiency of 83.9% and 18.5% in 60 min, respectively. The primary reactive species also have been identified as SO₃^•-, SO₄^•- and •OH responsible for TBBPA treatment and the contributions of SO₄^•- and •OH have been calculated as 43.8% and 28.4% for TBBPA degradation, respectively. In Fe³⁺/S₂O₄^2- system, TBBPA was effectively degraded in a wide initial pH range (3.0-9.0), whose activation energy was calculated as 32.01 kJ mol^-1. Due to the only operation of reagents dosing, the energy consumption and cost could be decreasing significantly without any light energy input and reaction conditions (e.g., pH and dissolved oxygen) adjustment compared with the general photolysis process. Moreover, some possible degradation approaches of TBBPA also have been proposed via GC-MS including debromination, hydroxylation, methylation, and mineralization in Fe³⁺/S₂O₄^2- system. And these probable degradation pathways also have been confirmed with the decreased Gibbs free energy (ΔG) based on density functional theory (DFT). This study has revealed that it was promising of Fe³⁺/S₂O₄^2- system for BFRs degradation and detoxification efficiently through the simple operation and mild condtions.

Insights into the degradation of diphenhydramine - An emerging SARS-CoV-2 medicine by UV/Sulfite

As Diphenhydramine (DPH) has been considered as a drug to treat SARS-CoV-2, the degradation of DPH from water was investigated and evaluated in this study by adopting an advanced oxidation/advanced reduction process – the UV/sulfite process. The UV/sulfite system was able to eliminate DPH within 6 mins under UV_254nm and 1.0 mM sulfite. It was observed that the presence of NO3-, NO2-, Cl-, HCO3-, and SO42- anions in water can affect the performance of UV/Sulfite degradation system. The mechanism of UV/sulfite/anions was evaluated which the presence of NO3- in UV/sulfite process has revealed faster initial decay rate but lower final DPH removal. It was observed that the UV/Sulfite process was extremely sensitive to pH as the dissociation of ion species varied among pH. The reaction became sluggish in acidic solution due to the dissociation of less reactive species such as HSO₃^-. In alkaline solution, SO₃^2- was the dominant species, producing powerful SO3∙- and eaq- when activated by UV at 254 nm. By conducting LC/MS analysis, the degradation pathway was proposed and can be summarized into four main pathways: hydroxylation, side chain cleavage, losing aromatic ring or ring opening. Scavenging tests were also carried out and validated the presence of various radicals contributing to the reaction, including eaq-, H^˙, OH^˙, SO₃^˙-, O₂^•- and SO₄^˙-.

Sources, occurrence, and treatment techniques of per- and polyfluoroalkyl substances in aqueous matrices: A comprehensive review

Per- and polyfluoroalkyl substances (PFAS), a class of synthetic organic pollutants, have prompted concerns about their global prevalence and possible health effects. This review consolidates the most recent data on different aspects of PFAS, such as their occurrence, and prominent sources. The current literature analysis of PFAS occurrence suggests significant variation in their concentration ranging from 0.025 to 1.2 × 10⁸ ng/L in wastewater, 0.01 to 8.9 × 10⁵ ng/L in surface water, and <0.01 to 1.3 × 10⁴ ng/L in groundwater globally. Since conventional treatment techniques are inadequate in remediating PFAS, innovative treatment approaches based on their removal or mineralization mechanism have been comprehensively reviewed. Advanced treatment technologies have shown degradation or removal of PFAS to be around 6 and > 99.9% in different aqueous matrices. However, due to significant drawbacks in their applicability in wastewater treatment plants (WWTPs), a novel treatment train approach has emerged as an effective alternative. This approach synergistically integrates multiple remediation techniques while addressing the impediments of individual treatments. Furthermore, nanofiltration (NF270) combined with electrochemical degradation has been demonstrated to be the most efficient (>98%) treatment train approach in PFAS remediation. If implemented in WWTPs, nanofiltration followed by adsorption using activated carbon is also a viable method for PFAS removal.

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.

Recent progress and challenges on the removal of per- and poly-fluoroalkyl substances (PFAS) from contaminated soil and water

Currently, due to an increase in urbanization and industrialization around the world, a large volume of per- and poly-fluoroalkyl substances (PFAS) containing materials such as aqueous film-forming foam (AFFF), protective coatings, landfill leachates, and wastewater are produced. Most of the polluted wastewaters are left untreated and discharged into the environment, which causes high environmental risks, a threat to human beings, and hampered socioeconomic growth. Developing sustainable alternatives for removing PFAS from contaminated soil and water has attracted more attention from policymakers and scientists worldwide under various conditions. This paper reviews the recent emerging technologies for the degradation or sorption of PFAS to treat contaminated soil and water. It highlights the mechanisms involved in removing these persistent contaminants at a molecular level. Recent advances in developing nanostructured and advanced reduction remediation materials, challenges, and perspectives in the future are also discussed. Among the variety of nanomaterials, modified nano-sized iron oxides are the best sorbents materials due to their specific surface area and photogenerated holes and appear extremely promising in the remediation of PFAS from contaminated soil and water.

Degradation of Per- and Polyfluoroalkyl Substances with Hydrated Electrons: A New Mechanism from First-Principles Calculations

Per- and polyfluoroalkyl substances (PFASs) are synthetic contaminants found in drinking groundwater sources and a wide variety of consumer products. Because of their adverse environmental and human health effects, remediation of these persistent compounds has attracted significant recent attention. To gain mechanistic insight into their remediation, we present the first ab initio study of PFAS degradation via hydrated electrons─a configuration that has not been correctly considered in previous computational studies up to this point. To capture these complex dynamical effects, we harness ab initio molecular dynamics (AIMD) simulations to probe the reactivities of perfluorooctanoic (PFOA) and perfluorooctane sulfonic acid (PFOS) with hydrated electrons in explicit water. We complement our AIMD calculations with advanced metadynamics sampling techniques to compute free energy profiles and detailed statistical analyses of PFOA/PFOS dynamics. Although our calculations show that the activation barrier for C-F bond dissociation in PFOS is three times larger than that in PFOA, all the computed free energy barriers are still relatively low, resulting in a diffusion-limited process. We discuss our results in the context of recent studies on PFAS degradation with hydrated electrons to give insight into the most efficient remediation strategies for these contaminants. Most importantly, we show that the degradation of PFASs with hydrated electrons is markedly different from that with excess electrons/charges, a common (but largely incomplete) approach used in several earlier computational studies.

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.

Tracking Fluorine during Aqueous Photolysis and Advanced UV Treatment of Fluorinated Phenols and Pharmaceuticals Using a Combined 19F-NMR, Chromatography, and Mass Spectrometry Approach

Fluorine incorporation into organic molecules has increased due to desirable changes in the molecular physiochemical properties. Common fluorine motifs include: aliphatic fluorines and -CF₃, or -F containing groups bonded directly onto an aromatic (Ar-CF₃ and Ar-F) or heteroaromatic ring. Photolysis of these compounds, either in natural or engineered systems, is a potential source of new fluorinated byproducts. Given the potential persistence and toxicity of fluorinated byproducts, monitoring of product formation during photolysis of various fluorinated motifs is needed. ¹⁹F-NMR is a means to detect and quantify these species. Ar-CF₃ and Ar-F model compounds (2-, 3-, and 4-(trifluoromethyl)phenol, 2-, 3-, 4-fluorophenol, and 2,6-, 3,5-difluorophenol) were photolyzed under a variety of aqueous conditions: pH 5, pH 7, pH 10, 1 mM H₂O₂ at pH 7 to form •OH, and 0.5 mM SO₃ ^2- at pH 10 to form e_aq ^-. Pharmaceuticals with the Ar-CF₃ (fluoxetine) and Ar-F plus pyrazole-CF₃ (sitagliptin) motifs were treated similarly. Parent molecule concentrations were monitored with high pressure liquid chromatography with a UV detector. Fluorine in the parent and product molecules was quantified with ¹⁹F-NMR and complete fluorine mass balances were obtained. High resolution mass spectrometry was used to further explore product identities. The major product for Ar-F compounds was fluoride. The Ar-CF₃ model compounds led to fluoride and organofluorine products dependent on motif placement and reaction conditions. Trifluoroacetic acid was a product of 4-(trifluoromethyl)phenol and fluoxetine. Additional detected fluoxetine products identified using mass spectrometry resulted from addition of -OH to the aromatic ring, but a dealkylation product could not be distinguished from fluoxetine by ¹⁹F-NMR. Sitagliptin formed multiple products that all retained the pyrazole-CF₃ motif while the Ar-F motif produced fluoride. ¹⁹F-NMR, mass spectrometry, and chromatography methods provide complementary information on the formation of fluorinated molecules by modification or fragmentation of the parent structure during photolysis, allowing screening for fluorinated photoproducts and development of fluorine mass balances.

Degradation of Benzotriazole UV-stabilizers in the presence of organic photosensitizers and visible light: A time-resolved mechanistic study

Benzotriazole UV-stabilizers (BUVSs) are commonly used in industry as solar filters, due to their strong UV light absorption. Because of their extended usage, environmental contamination of waters due to BUVSs constitutes a growing concern. Direct photodegradation of BUVSs is not efficient due to their intrinsic thermal pathways to release the absorbed light. Nevertheless, their abatement in natural environments could be assisted by chromophoric dissolved organic matter. Among the BUVSs, three representative candidates were selected, UV-326, UV-327 and UV-328, to demonstrate the potential of Riboflavin (RF) as a natural visible-light absorbing photocatalyst for the abatement of these recalcitrant pollutants under reductive conditions. The use of visible light and DABCO, as a model sacrificial electron donor, generates the radical anion RFTA^.-. This key species reacts with the solar filters, achieving their reductive abatement from the medium. Moreover, the participation of every potential reactive species has been investigated by photophysical techniques, together with determination of the quenching rate constant for every reaction pathway. Consequently, evidence supported the main role of the reductive photodegradation pathway, being RFTA^.- the key species in the abatement of BUVSs.

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.

Selective and Efficient Reduction of Nitrate to Gaseous Nitrogen from Drinking Water Source by UV/Oxalic Acid/Ferric Iron Systems: Effectiveness and Mechanisms

Nitrate (NO3−) reduction in water has been receiving increasing attention in water treatment due to its carcinogenic and endocrine-disrupting properties. This study employs a novel advanced reduction process, the UV/oxalic acid/ferric iron systems (UV/C2O42−/Fe3+ systems), in reducing NO3− due to its high reduction efficiency, excellent selectivity, and low treatment cost. The UV/C2O42−/Fe3+ process reduced NO3− with pseudo-first-order reaction rate constants of 0.0150 ± 0.0013 min−1, minimizing 91.4% of 60 mg/L NO3− and reaching 84.2% of selectivity for gaseous nitrogen after 180 min at pHini. 7.0 and 0.5 mg/L dissolved oxygen (DO). Carbon dioxide radical anion (CO2•−) played a predominant role in reducing NO3−. Gaseous nitrogen and NH4+, as well as CO2, were the main nitrogen- and carbon-containing products, respectively, and reduction pathways were proposed accordingly. A suitable level of oxalic acids (3 mM) and NO3− (60 mg/L) was recommended; increasing initial iron concentrations and UV intensity increased NO3− reduction. Instead, increasing the solution pH decreased the reduction, and 0.5–8.0 mg/L DO negligibly affected the process. Moreover, UV/C2O42−/Fe3+ systems were not retarded by 0.1–10 mM SO42− or Cl− or 0.1–1.0 mM HCO3− but were prohibited by 10 mM HCO3− and 30 mg-C/L humic acids. There was a lower reduction of NO3− in simulated groundwater (72.8%) than deionized water after 180 min at pHini. 7.0 and 0.5 mg/L DO, which meets the drinking water standard (<10 mg/L N-NO3−). Therefore, UV/C2O42−/Fe3+ systems are promising approaches to selectively and efficiently reduce NO3− in drinking water.

Effectiveness of UV/SO32- advanced reduction process for degradation and mineralization of trichlorfon pesticide in water: identification of intermediates and toxicity assessment

This study aimed to investigate the degradability, mineralization, proposed decomposition pathway, intermediate products, and toxicity of effluent from trichlorfon (TCF) degradation in water by UV/sulfite-advanced reduction process (UV/S-ARP). This study was experimentally performed in a photochemical reactor as a batch operation. The source of light was a UV lamp. Sulfite ion was used as the reducing agent. After the treatment, the residual concentration of TCF was measured by liquid chromatography equipped with tandem mass spectrometry (LC-MS/MS). UV/S-ARP had the highest performance at an initial pH of 7, a sulfite ion concentration of 120 mg/L, a contact time of 60 min, and a TCF concentration of 10 mg/L. Under such conditions, the degradation efficiency of TCF was 96.0%, and the amount of mineralization based on the removal of TOC and COD was 74.6% and 79.5%, respectively. The results of the degradation mechanism showed that eaq- and SO3•- have played the greatest role in dechlorination and transformation of TCF. Based on the identified intermediates, more complex compounds are transformed into compounds with simpler structures by UV/S-ARP. Evaluating the toxicity of TCF by-products via ECOSAR bioassay showed that as-generated intermediates do not have acute and chronic adverse effects on fish. The results of our study indicated that the advanced reduction process could be an effective process for the purification of TCF-contaminated water.

NO3- photolysis-induced advanced reduction process removes NO3- and 2, 4, 6-tribromophenol

Reductive processes are an important type of pollutant removal technology, particularly for organic halogens. NO₃^- is an anion and pollutant that is commonly present in wastewater. In this study, a novel advanced reduction process (ARP) induced by NO₃^- photolysis was developed to remove 2,4,6-tribromophenol (TBP) and NO₃^-. The UV/NO₃^-/formate acid (FA) process achieved NO₃^- removal and improved the debromination of TBP (initial TBP concentration = 0.1 mM) (up to 97.8%), however, their coexistence adversely affected the reductive removal of each component. Acidic conditions (pH 3 in this study) benefited the removal of NO₃^- and the debromination of TBP. Cl^- promoted NO₃^- removal in UV/NO₃^-/FA, however, it decreased the debromination effect of TBP by 27.8%. Humic acid, a typical dissolved organic matter, suppressed NO₃^- removal, TBP degradation and debromination under all experimental conditions. Methyl viologen significantly inhibited the performance of ARP, and this verified the role of CO₂^•- in this ARP. Insufficient reduction and over-reduction of NO₃^- were observed under different conditions and a greater amount of NH₄⁺ was formed under the influence of TBP. The data also indicated that as much as 80% of the removed NO₃^- was converted to NO₂^-, and this is noteworthy. Due to the reductive radicals generated from the oxidation of FA, both oxidative and reductive products of TBP were detected in the effluent. The results of this study provide a potential technology for the reductive removal of organic halogens from NO₃^--rich wastewater.

Degradation of Reactive Brilliant Red X-3B by Photo-Fenton-like Process: Effects of Water Chemistry Factors and Degradation Mechanism

Azo dye wastewater belongs to the highly concentrated organic wastewater, which is difficult to be treated by traditional biological processes. The oxidation efficiency of a single physicochemical method is not considerable. Recent research indicated that the advanced oxidation processes (AOPs) based on the highly reactive hydroxyl radical (∙OH) became one of the preferred methods in dealing with such dye wastewater. In this paper, the typical azo dye, reactive brilliant red X-3B, was employed as the target pollutant, and the transition metal Mn and hydrogen peroxide as the catalysts. A photo-Fenton-like process, UV/Mn2+-H2O2 system, was established, which enables a combination of various technologies to improve azo dye degradation efficiency while reducing disposal costs. The results indicated that the UV/Mn2+-H2O2 system had the synergism of Mn2+/H2O2 and UV/H2O2, which was 2.6 times greater than the sum of the two individual effects. And the degradation of X-3B reached the optimum under the conditions of 0.59 mmol/L of the Mn2+, 10 mmol/L of the H2O2, pH = 6 and a high level of DO. The ∙OH, generated from chem-catalytic and photocatalytic decomposition of H2O2, played the predominant role in the decolorization of X-3B and mineralization of its intermediates. The ∙OH tended to attack and break the chromophore group, resulting in the rapid decolorization of X-3B. The azo bond in X-3B was easy to be decomposed in the form of N2, while the triazinyl group was recalcitrant for ring opening. The degradation process of the UV/Mn2+-H2O2 system preferred to be conducted at an acidic condition and appropriate concentrations of Mn2+ and H2O2. The alkaline condition would decrease the utilization of H2O2, and excessive H2O2 would also quench the ∙OH.

The degradation mechanisms of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) by different chemical methods: A critical review

Per- and polyfluoroalkyl substances (PFASs) are a class of artificial compounds comprised of a perfluoroalkyl main chain and a terminal functional group. With them being applied in a wide range of applications, PFASs have drawn increasing regulatory attention and research interests on their reductions and treatments due to their harmful effects on environment and human beings. Among numerous studies, chemical treatments (e.g., photochemical, electrochemical, and thermal technologies) have been proved to be important methods to degradation PFASs. However, the pathways and mechanisms for the degradation of PFASs through these chemical methods still have not been well documented. This article therefore provides a comprehensive review on the degradation mechanisms of two important PFASs (perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS)) with photochemical, electrochemical and thermal methods. Different decomposition mechanisms of PFOA and PFOS are reviewed and discussed. Overall, the degradation pathways of PFASs are associated closely with their head groups and chain lengths, and H/F exchange and chain shortening were found to be predominant degradation mechanisms. The clear study on the degradation mechanisms of PFOA and PFOS should be very useful for the complete degradation or mineralization of PFASs in the future.

3D Self-Supported Nitrogen-Doped Carbon Nanofiber Electrodes Incorporated Co/CoOx Nanoparticles: Application to Dyes Degradation by Electro-Fenton-Based Process

We developed free-standing nitrogen-doped carbon nanofiber (CNF) electrodes incorporating Co/CoO_x nanoparticles (NPs) as a new cathode material for removing Acid Orange 7 (AO7; a dye for wool) from wastewater by the heterogeneous electro-Fenton reaction. We produced the free-standing N-doped CNF electrodes by electrospinning a polyacrylonitrile (PAN) and cobalt acetate solution followed by thermal carbonation of the cobalt acetate/PAN nanofibers under a nitrogen atmosphere. We then investigated electro-Fenton-based removal of AO7 from wastewater with the free-standing N-doped-CNFs-Co/CoO_x electrodes, in the presence or not of Fe²⁺ ions as a co-catalyst. The electrochemical analysis showed the high stability of the prepared N-doped-CNF-Co/CoO_x electrodes in electrochemical oxidation experiments with excellent degradation of AO7 (20 mM) at acidic to near neutral pH values (3 and 6). Electro-Fenton oxidation at 10 mA/cm² direct current for 40 min using the N-doped-CNF-Co/CoO_x electrodes loaded with 25 wt% of Co/CoO_x NPs led to complete AO7 solution decolorization with total organic carbon (TOC) removal values of 92.4% at pH 3 and 93.3% at pH 6. The newly developed N-doped-CNF-Co/CoO_x electrodes are an effective alternative technique for wastewater pre-treatment before the biological treatment.

Reduction of bromate from drinking water by sulfite/ferric ion systems: Efficacy and mechanisms

In this work, we investigate radical-based chemical reduction systems for BrO₃^- reduction. Sulfite/ferric ions [S(IV)/Fe(III)] systems eliminated 85% of 0.1 mM BrO₃^- in 1 min at pH_ini. 7.0, and 2.2 mg/L dissolved oxygen (DO) with rate constants (2.104 ± 0.090 min^-1) orders of magnitude faster than previously published advanced reduction processes. Increasing S(IV) and Fe(III) concentrations and decreasing DO and pH contributed to faster BrO₃^- reduction likely due to enhanced reactive species. Lower anionic levels (0.5 mM) did not hinder BrO₃^- reduction but further increases in the levels to 1.0 mM decreased their reduction. Notably, S(IV)/Fe(III) process reduced 68% of 25.6 μg/L BrO₃^- in 3 min from tap water at pH_ini. 7.0, 0.02 mM Fe(III), and 1.0 mM S(IV), meeting drinking water standard (<10 μg/L). Also, ~99% of irons could be removed after the following activated carbon treatment. Moreover, sulfite radicals formed by S(IV) activation with Fe(III) were significant reactive species in the S(IV)/Fe(III) system. Reduction pathways were proposed accordingly. Collectively, S(IV)/Fe(III) process is a viable technology for reductively decontaminating realistic waters, especially when applied between the ozone-activated carbon process (i.e., the ozone-S(IV)/Fe(III)-activated carbon process).

Roles and Knowledge Gaps of Point-of-Use Technologies for Mitigating Health Risks from Disinfection Byproducts in Tap Water: A Critical Review

Due to rising concerns about water pollution and affordability, there is a rapidly-growing public acceptance and global market for a variety of point-of-use (POU) devices for domestic uses. However, the efficiencies and mechanisms of POU technologies for removing regulated and emerging disinfection byproducts (DBPs) are still not systematically known. To facilitate the development of this field, we summarized performance trends of four common technologies (i.e., boiling, adsorption, membrane filtration, and advanced oxidation) on mitigating preformed DBPs and identified knowledge gaps. The following highest priority knowledge gaps include: 1) data on DBP levels at the tap or cup in domestic applications; 2) certainty regarding the controls of DBPs by heating processes as DBPs may form and transform simultaneously; 3) standards to evaluate the performance of carbon-based materials on varying types of DBPs; 4) long-term information on the membrane performance in removing DBPs; 5) knowledge of DBPs' susceptibility toward advanced redox processes; 6) tools to monitor/predict the toxicity and diversity of DBPs formed in waters with varying precursors and when implementing different treatment technologies; and 7) social acceptance and regulatory frameworks of incorporating POU as a potential supplement to current centralized-treatment focused DBP control strategies. We conclude by identifying research needs necessary to assure POU systems protect the public against regulated and emerging DBPs.

Review on UV/sulfite process for water and wastewater treatments in the presence or absence of O2

Based on previous reports, UV/sulfite process is generally used as an advanced reduction process (ARP) since e_aq^- and/or ∙H, both with strong reduction potential, could be substantially generated herein. Very recently, the combination of UV and sulfite as an advanced oxidation process (AOP) or an oxidation-reduction coupling process has attracted increasing interest due to the yield of SO₄^∙- and/or HO∙. Herein, the application of UV/sulfite as an ARP and AOP (or oxidation-reduction coupling process) during water and wastewater treatments is reviewed respectively. (1) In the absence of O₂, UV/sulfite works as an ARP. The generation mechanism of reactive reduction species and various contaminants removal (including degradation kinetics and efficiency, decomposition mechanisms, effects of some factors, etc.) is summarized in detail and systematically. Moreover, both the application of different types of UV lights and the economic evaluation are summarized systematically. (2) In the presence of O₂, UV/sulfite could be used as an AOP or oxidation-reduction coupling process. The generation mechanism of reactive oxidation species and influencing factors is also presented in detail. Moreover, two ways (including homogeneous and heterogeneous activation) used to enhance the UV/sulfite oxidation potential are summarized respectively. Moreover, several knowledge gaps and research needs for further research are proposed. Overall, this review provides an overview for in-depth understanding of UV/sulfite as an ARP or AOP (oxidation-reduction coupling process) during water and wastewater treatments.

Perchlorate adsorption onto epichlorohydrin crosslinked chitosan hydrogel beads

Perchlorate (ClO₄^-) in water is an emerging contaminant that threatens human health by inhibiting the uptake of iodine in the thyroid gland. Biopolymer adsorbents including chitosan hydrogel beads (CSBs) have attracted increasing attentions in water treatment for their low costs, ease in preparation, and environmental friendliness. However, the adsorption capacity for ClO₄^- by several crosslinked CSBs has been shown to be low. To overcome this, epichlorohydrin (ECH) crosslinked CSBs (ECH-CSBs) that preserved -NH₂ functional groups as potential sites for adsorption are synthesized and characterized, followed by batch adsorption experiments to evaluate adsorption and desorption reactions. The point of zero charge is determined to be 5.1 ± 0.1. Both XPS spectra and DFT calculations support that electrostatic interaction between ClO₄^- and protonated -NH₃⁺ functional groups is responsible for adsorption that reaches a capacity of 63.4 to 76.3 mg/g between pH of 4.0-10.0 at 303.15 K that follows Langmuir isotherm. ECH crosslinking also enhances hydrophilicity of CSBs to allow for increased adsorption for ClO₄^-. Adsorption of ClO₄^- (10 and 100 mg/L) follows a pseudo-first order kinetics with equilibrium time of 2-6 h but is limited by intra-particle diffusion. Anions common in natural waters exhibit interference effects due to similar electrostatic attraction mechanism, thus HCO₃^- and SO₄^2- with high abundance in natural waters need pre-treatment. Regeneration of the adsorbents to 100% of its adsorption capacity by rinsing with 0.1 M NaOH is demonstrated for 12 cycles due to complete desorption of ClO₄^- via electrostatic repulsion, assuring reusability.

Perchlorate Contamination: Sources, Effects, and Technologies for Remediation

Perchlorate is a persistent pollutant, generated via natural and anthropogenic processes, that possesses a high potential for endocrine disruption in humans and biota. It inhibits iodine fixation, a major reason for eliminating this pollutant from ecosystems. Remediation of perchlorate can be achieved with various physicochemical treatments, especially at low concentrations. However, microbiological approaches using microorganisms, such as those from the genera Dechloromonas, Serratia, Propionivibrio, Wolinella, and Azospirillum, are promising when perchlorate pollution is extensive. Perchlorate-reducing bacteria, isolated from harsh environments, for example saline soils, mine sediments, thermal waters, wastewater treatment plants, underground gas storage facilities, and remote areas, including the Antarctica, can provide removal yields from 20 to 100%. Perchlorate reduction, carried out by a series of enzymes, such as perchlorate reductase and superoxide chlorite, depends on pH, temperature, salt concentration, metabolic inhibitors, nutritional conditions, time of contact, and cellular concentration. Microbial degradation is cost-effective, simple to implement, and environmentally friendly, rendering it a viable method for alleviating perchlorate pollution in the environment.

Photo enhanced degradation of polyfluoroalkyl and perfluoroalkyl substances

The increase in the presence of highly recalcitrant poly- and per- fluoroalkyl substances (PFAS) in the environment, plant tissues and animals continues to pose serious health concerns. Several treatment methods such as physical, biological and chemical processes have been explored to deal with these compounds. Current trends have shown that the destructive treatment processes, which offer degradation and mineralization of PFASs, are the most desirable process among researchers and policy makers. This article, therefore, reviews the degradation and defluorination processes, their efficiencies and the degradation mechanism of photon-based processes. It shows that high degradation and defluorination efficiency of PFASs could be achieved by photon driven processes such as photolysis, photochemical, photocatalysis and photoreduction. The efficiency of these processes is greatly influenced by the nature of light and the reactive radical generated in the system. The limitation of these processes, however, include the long reaction time required and the use of anoxic reaction conditions, which are not obtainable at ambient conditions.

Removal of Poly- and Per-Fluorinated Compounds from Ion Exchange Regenerant Still Bottom Samples in a Plasma Reactor

"High-concentration" and "low-concentration" bench-scale batch plasma reactors were used to effectively degrade per- and polyfluoroalkyl substances (PFAS) at a high concentration (∼100 mg/L) and a low concentration (<1 μg/L), respectively, in ion exchange (IX) regenerant still bottom (SB) solutions. In the SBs, numerous PFAS were detected with a wide concentration range (∼0.01 to 100 mg/L; total oxidizable precursors (TOP) ∼4000 to 10000 mg/L). In the "high-concentration" plasma reactor, the concentrations of PFAS precursors and long-chain perfluoroalkyl acids (PFAAs) (≥6C for PFSAs and ≥8C for perfluorocarboxylic acids (PFCAs)) were decreased by >99.9% in 2 h, and short-chain PFAAs (<6C for perfluorocarboxylic acids (PFSAs) and <8C PFCAs) were decreased by >99% in 6 h of treatment. Subsequently, a "low concentration" plasma reactor was used to remove additional PFAAs. In this reactor, the addition of CTAB (cetrimonium bromide, a cationic surfactant) caused short-chain PFAAs, other than PFBA, to be removed to below detection limits in 90 min of treatment time. Overall, >99% of the TOP present in SBs was removed during the treatment. Fluorine recovery of 47 to 117% was obtained in six SB samples. Energy requirement (EE/O) for the treatment of PFOA and PFOS from SBs ranged from 380 to 830 kWh/m³.

Critical Perspective on Advanced Treatment Processes for Water and Wastewater: AOPs, ARPs, and AORPs

Emerging contaminants’ presence in water, wastewater, and aquatic environments has been widely reported. Their environmental and health-related effects, and the increasing tendency towards wastewater reuse require technology that could remove to a greater degree, or even mineralize, all these contaminants. Currently, the most commonly used process technologies for their removal are advanced oxidation processes (AOPs); however, recent advances have highlighted other advanced treatment processes (ATPs) as possible alternatives, such as advanced reduction processes (ARPs) and advanced oxidation-reduction processes (AORPs). Although they are not yet widely diffused, they may remove contaminants that are not readily treatable by AOPs, or offer better performance than the former. This paper presents an overview of some of the most common or promising ATPs for the removal of contaminants from water and wastewater, and their application, with discussion of their limitations and merits. Issues about technologies’ costs and future perspectives in the water sector are discussed.

Destruction of Per- and Polyfluoroalkyl Substances (PFAS) with Advanced Reduction Processes (ARPs): A Critical Review

Advanced reduction processes (ARPs) have emerged as a promising method for destruction of persistent per- and polyfluoroalkyl substances (PFAS) in water due to the generation of short-lived and highly reductive hydrated electrons (e_aq^-). This study provides a critical review on the mechanisms and performance of reductive destruction of PFAS with e_aq^-. Unique properties of e_aq^- and its generation in different ARP systems, particularly UV/sulfite and UV/iodide, are overviewed. Different degradation mechanisms of PFAS chemicals, such as perfluorooctanoic acid (PFOA), perfluorooctanesulfonate (PFOS), and others (e.g., short chain perfluorocarboxylic acids (PFCAs) and perfluorosulfonic acids (PFSAs), per- and polyfluoro dicarboxylic acids, and fluorotelomer carboxylic acids), are reviewed, discussed, and compared. The degradation pathways of these PFAS chemicals rely heavily upon their head groups. For specific PFAS types, fluoroalkyl chain lengths may also affect their reductive degradation patterns. Degradation and defluorination efficiencies of PFAS are considerably influenced by solution chemistry parameters and operating factors, such as pH, dose of chemical solute (i.e., sulfite or iodide) for e_aq^- photoproduction, dissolved oxygen, humic acid, nitrate, and temperature. Furthermore, implications of the state-of-the-art knowledge on practical PFAS control actions in water industries are discussed and the priority research needs are identified.

Treatment train approaches for the remediation of per- and polyfluoroalkyl substances (PFAS): A critical review

Per- and polyfluoroalkyl substances (PFAS) have recently drawn great attention due to their ubiquitous presence in aquatic environments and potential toxicity to human health and the environment. A number of recent studies have demonstrated that "passive" removal approaches, such as adsorption, filtration, and reverse osmosis or "active" degradation technologies, such as enhanced photolysis, electrochemical oxidation, and sonochemical destruction, are all able to individually conduct remedial measures for PFAS contamination at some level. However, drawbacks, specifically high energy consumption, low cost-efficiency, and extreme operating conditions, are commonly observed from these studies which significantly suppress the future for commercialization of these innovative technologies. Since 2015, a new trend of PFAS remediation has emerged that uses multiple synergetic technologies simultaneously (known as treatment train processes) to effectively achieve in-situ remediation of PFAS. This paper provides new insight of the recently reported treatment train studies selected from approximately 150 different publications with regards to the remediation of PFAS and discusses their innovative designs, remediation performances, present limits, and possible improvements. Based on a comprehensive review of the current treatment train studies, this review work proposes a new design that consists of three individual technologies, namely, nanofiltration, electrochemical anodic oxidation, and electro-Fenton degradation, to maximize economic and environmental benefits of PFAS remedial measures.

Removal of urea from swimming pool water by UV/VUV: The roles of additives, mechanisms, influencing factors, and reaction products

To discover an applicable technology for urea abatement from swimming pool water (SPW), this study compared the performances of seven ultraviolet (UV)-based technologies on urea removal, including UV alone, UV coupled with hydrogen peroxide (UV/H₂O₂), sulfite (UV/Na₂SO₃), potassium persulfate (UV/K₂S₂O₈), a combination of UV and vacuum UV (UV/VUV), and UV/VUV in tandem with either H₂O₂ (VUV/H₂O₂) or potassium persulfate (VUV/K₂S₂O₈). Among them, UV and UV/Na₂SO₃ showed little removal ability, and UV/H₂O₂ removed only 12.8% of urea within 3-h experiments, while UV/VUV degraded 71.7% of urea without introducing substantial total dissolved solids (TDS). Therefore, UV/VUV was considered as a promising technology for further exploration. In comparison, although UV/K₂S₂O₈ exhibited higher urea removal than UV/VUV, it caused dramatic increases of TDS, which made the regulatory threshold for the TDS increment difficult to maintain. Within UV/VUV studies, some common components in SPW (e.g., cyanuric acid, humic acid, nitrate, and bicarbonate) inhibited the removal process, whereas chloride and sulfate facilitated it, while free chlorine at doses ≤ 3 mg-Cl₂/L and pH levels from 6.8 to 8.0 imposed little impact on urea degradation. Overall, UV/VUV degraded 40.0% and 22.2% of urea from tap water and SPW, respectively; both were lower than the efficiency observed in ultrapure water. As for reaction byproducts, urea phototransformation via UV/VUV yielded nitrate and ammonia as the key products with the mass balance of nitrogen element being met. However, the contents of organic carbon decreased at a rate slightly lower than urea degradation, suggesting that urea was mostly mineralized and slightly converted to unknown organic compounds. The results hence demonstrate that UV/VUV is an effective alternative for urea removal from SPW.

Cathode-Introduced Atomic H* for Fe(II)-Complex Regeneration to Effective Electro-Fenton Process at a Natural pH

Promotion of iron solubility using ligands is the preliminary step in the homogeneous electro-Fenton (EF) process at a mild pH, but the chelate efficiencies of most organic ligands are unsatisfactory, resulting in insufficient Fe(II) availability. In this study, atomic H* was, for the first time, introduced to the EF process to accelerate the regeneration of the Fe(II)-complex at a mild pH using a Ni-deposited carbon felt (Ni-CF) cathode. The introduction of atomic H* significantly elevated total organic carbon (TOC) abatement of ciprofloxacin (CIP) from 42% (CF) to 81% (Ni-CF) at a natural pH. In the presence of humic acids (HAs), atomic H* introduced via Ni-CF enhanced the CIP degradation rate to 10 times that of the CF at a mild pH. The electron spin resonance (ESR), density functional theory (DFT) calculations, electrochemical characterization, and in situ electrochemical Raman study clearly demonstrated that the atomic H* generated from the Ni-CF cathode was highly efficient at reducing Fe(III)-complexes at a natural pH. Additionally, the Ni-CF could generate atomic H* without significant nickel leaching. Thus, the atomic H* could continuously facilitate iron cycling and, consequently, enhance pollutant mineralization via the homogeneous EF process at a mild pH in an environmentally friendly manner.

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.