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Advanced oxidation processes (AOPs), such as hydroxyl radical (HO • )- and sulfate radical (SO 4 •- )-mediated oxidation, are attractive technologies used in water and wastewater treatments. To evaluate the treatment efficiencies of AOPs, monitoring the primary radicals (HO • and SO 4 •- ) as well as the secondary radicals generated from the reaction of HO • /SO 4 •- with water matrices is necessary. Therefore, we developed a novel chemical probe method to examine five key radicals simultaneously, including HO • , SO 4 •- , Cl • , Cl 2 •- , and CO 3 •- . Five probes, including nitrobenzene, para -chlorobenzoic acid, benzoic acid, 2,4,6-trimethylbenzoic acid, and 2,4,6-trimethylphenol, were selected in this study. Their bimolecular reaction rate constants with diverse radicals were first calibrated under the same conditions to minimize systematic errors. Three typical AOPs (UV/H 2 O 2 , UV/S 2 O 8 2- , and UV/HSO 5 - ) were tested to obtain the radical steady-state concentrations. The effects of dissolved organic matter, Br - , and the probe concentration were inspected. Our results suggest that the five-probe method can accurately measure radicals in the HO • - and SO 4 •- -mediated AOPs when the concentration of Br - and DOM are less than 4.0 μM and 15 mg C L -1 , respectively. Overall, the five-probe method is a practical and easily accessible method to determine multiple radicals simultaneously.

Per- and polyfluoroalkyl substances (PFAS) are widely used anthropogenic chemicals. Because of the strength of the carbon-fluorine bond, PFAS are not destroyed in typical water treatment processes. Sulfate (SO 4 •- ) and hydroxyl ( • OH) radicals can oxidize some PFAS, but the behavior of per- and polyfluoroalkyl ether acids (PFEAs) in processes involving SO 4 •- and • OH is poorly understood. In this study, we determined second-order rate constants ( k ) describing the oxidation of 18 PFAS, including 15 novel PFEAs, by SO 4 •- and • OH. Among the studied PFAS, 6:2 fluorotelomer sulfonate reacted most readily with • OH [ k •OH = (1.1-1.2) × 10 7 M -1 s -1 ], while polyfluoroalkyl ether acids containing an -O-CFH- moiety reacted more slowly [ k •OH = (0.5-1.0) × 10 6 M -1 s -1 ]. In the presence of SO 4 •- , polyfluoroalkyl ether acids with an -O-CFH- moiety reacted more rapidly [ k SO4 •- = (0.89-4.6) × 10 6 M -1 s -1 ] than perfluoroalkyl ether carboxylic acids (PFECAs) and a chloro-perfluoro-polyether carboxylic acid (ClPFPECA) [ k SO4 •- = (0.85-9.5) × 10 4 M -1 s -1 ]. For homologous series of perfluoroalkyl carboxylic acids, linear and branched monoether PFECAs, and multiether PFECAs, PFAS chain length had little impact on second-order rate constants. SO 4 •- reacted with the carboxylic acid headgroup of perfluoroalkyl carboxylic acids and PFECAs. In contrast, for polyfluoroalkyl ether carboxylic and sulfonic acids with an -O-CFH- moiety, the site of SO 4 •- attack was the -O-CFH- moiety. Perfluoroalkyl ether sulfonic acids were not oxidized by SO 4 •- and • OH under the conditions evaluated in this study.

Current techniques for identifying and quantifying sulfate radicals (SO 4 · - ) in SO 4 · - -based advanced oxidation processes (SR-AOPs) are unsatisfactory due to their low selectivity, poor reliability, and limited feasibility for real-time quantification. In this study, naproxen (NAP) was employed as a turn-on luminescent probe for real-time quantification of SO 4 · - in SR-AOPs. The chemiluminescence(CL) yield (Φ CL ) of the reaction of NAP with SO 4 · - was first determined to be 1.49 × 10 -5 E mol -1 with the bisulfite activation by cerium(IV) [Ce(IV)/BS] process. Then, the maximum peak concentrations of SO 4 · - in the Ce(IV)/BS-NAP process was quantified to be ∼10 -11 M based on the derived equation. Since Φ CL of the reaction of NAP with SO 4 · - was much greater than that with other reactive oxidizing species (ROS), the developed CL method worked well in selective quantification of SO 4 · - in various SR-AOPs (e.g., the activation of peroxymonosulfate and persulfate by iron processes). Finally, the electron transfer from NAP to SO 4 · - was proposed to be the critical step for CL production. This work provides a novel CL method for real-time quantification of SO 4 · - , which facilitates the development of SR-AOPs and their application in water and wastewater treatment.

Changes in sulfate (SO 4 2- ) deposition have been linked to changes in mercury (Hg) methylation in peatlands and water quality in freshwater catchments. There is little empirical evidence, however, of how quickly methyl-Hg (MeHg, a bioaccumulative neurotoxin) export from catchments might change with declining SO 4 2- deposition. Here, we present responses in total Hg (THg), MeHg, total organic carbon, pH, and SO 4 2- export from a peatland-dominated catchment as a function of changing SO 4 2- deposition in a long-term (1998-2011), whole-ecosystem, control-impact experiment. Annual SO 4 2- deposition to half of a 2-ha peatland was experimentally increased 6-fold over natural levels and then returned to ambient levels in two phases. Sulfate additions led to a 5-fold increase in monthly flow-weighted MeHg concentrations and yields relative to a reference catchment. Once SO 4 2- additions ceased, MeHg concentrations in the outflow streamwater returned to pre-SO 4 2- addition levels within 2 years. The decline in streamwater MeHg was proportional to the change in the peatland area no longer receiving experimental SO 4 2- inputs. Importantly, net demethylation and increased sorption to peat hastened the return of MeHg to baseline levels beyond purely hydrological flushing. Overall, we present clear empirical evidence of rapid and proportionate declines in MeHg export from a peatland-dominated catchment when SO 4 2- deposition declines.

Dissolved organic matter (DOM) scavenges sulfate radicals (SO 4 •- ), and SO 4 •- -induced DOM transformations influence disinfection byproduct (DBP) formation when chlorination follows advanced oxidation processes (AOPs) used for pollutant destruction during water and wastewater treatment. Competition kinetics experiments and transient kinetics experiments were conducted in the presence of 19 DOM fractions. Second-order reaction rate constants for DOM reactions with SO 4 •- ( k DOM,SO 4 •- ) ranged from (6.38 ± 0.53) × 10 6 M -1 s -1 to (3.68 ± 0.34) × 10 7 M C -1 s -1 . k DOM,SO 4 •- correlated with specific absorbance at 254 nm (SUVA 254 ) ( R 2 = 0.78) or total antioxidant capacity ( R 2 = 0.78), suggesting that DOM with more aromatics and antioxidative moieties reacted faster with SO 4 •- . SO 4 •- exposure activated DBP precursors and increased carbonaceous DBP (C-DBP) yields (e.g., trichloromethane, chloral hydrate, and 1,1,1-trichloropropanone) in humic acid and fulvic acid DOM fractions despite the great reduction in their organic carbon, chromophores, and fluorophores. Conversely, SO 4 •- -induced reactions reduced nitrogenous DBP yields (e.g., dichloroacetonitrile and trichloronitromethane) in wastewater effluent organic matter and algal organic matter without forming more C-DBP precursors. DBP formation as a function of SO 4 •- exposure (concentration × time) provides guidance on optimization strategies for SO 4 •- -based AOPs in realistic water matrices.

Excessive emissions of gaseous pollutants such as SO 2 , NO x , heavy metals (Hg, As, etc.), H 2 S, VOCs, etc. have triggered a series of environmental pollution incidents. Sulfate radical (SO 4 • - )-based advanced oxidation technologies (AOTs) are one of the most promising gaseous pollutants removal technologies because they can not only produce active free radicals with strong oxidation ability to simultaneously degrade most of gaseous pollutants, but also their reaction processes are environmentally friendly. However, so far, the special review focusing on gaseous pollutants removal using SO 4 • - -based AOTs is not reported. This review reports the latest advances in removal of gaseous pollutants (e.g., SO 2 , NO x , Hg, As, H 2 S, and VOCs) using SO 4 • - -based AOTs. The performance, mechanism, active species identification and advantages/disadvantages of these removal technologies using SO 4 • - -based AOTs are reviewed. The existing challenges and further research suggestions are also commented. Results show that SO 4 • - -based AOTs possess good development potential in gaseous pollutant control field due to simple reagent transportation and storage, low product post-treatment requirements and strong degradation ability of refractory pollutants. Each SO 4 • - -based AOT possesses its own advantages and disadvantages in terms of removal performance, cost, reliability, and product post-treatment. Low free radical yield, poor removal capacity, unclear removal mechanism/contribution of active species, unreliable technology and high cost are still the main problems in this field. The combined use of multiactivation technologies is one of the promising strategies to overcome these defects since it may make up for the shortcomings of independent technology. In order to improve free radical yield and pollutant removal capacity, enhancement of mass transfer and optimization design of reactor are critical issues. Comprehensive consideration of catalytic materials, removal chemistry, mass transfer and reactor is the promising route to solve these problems. In order to clarify removal mechanism, it is essential to select suitable free radical sacrificial agents, probes and spin trapping agents, which possess high selectivity for target specie, high solubility in water, and little effect on activity of catalyst itself and mass transfer/diffusion parameters. In order to further reduce investment and operating costs, it is necessary to carry out the related studies on simultaneous removal of more gaseous pollutants.

Perfluoroalkyl carboxylic acids (PFCAs) are ubiquitous contaminants known for their bioaccumulation, toxicological harm, and resistance to degradation. Remediating PFCAs in water is an ongoing challenge with existing technologies being insufficient or requiring additional disposal. An emergent approach is using activated persulfate, which degrades PFCAs through sequential scission of CF 2 equivalents yielding shorter-chain homologues, CO 2 and F - . This transformation is thought to be initiated by single electron transfer (SET) from the PFCA to the activate oxidant, SO 4 •- . A pronounced pH effect has been observed for thermally activated persulfate PFCA transformation. To evaluate the role of pH during SET, we directly determined absolute rate constants for perfluorobutanoic acid and trifluoroacetic acid oxidation by SO 4 •- in the pH range of 0.5-4.0 using laser flash photolysis. The average of the rate constants for both substrates across all pH values was 9 ± 2 × 10 3 M -1 s -1 (±2σ), implying that acid catalysis of thermal persulfate activation may be the primary culprit of the observed pH effect, instead of pH influencing the SET step. In addition, density functional theory was used to investigate if SO 4 •- protonation might enhance PFCA transformation kinetics. We found that when calculations include explicit water molecules, direct SO 4 •- protonation does not occur.

Sulfur amendment of zerovalent iron (ZVI) materials has been shown to improve the reactivity and selectivity of ZVI toward a select group of organohalide contaminants in groundwater, most notably trichloroethene (TCE). In previous studies, chemical or mechanochemical sulfidation methods were used; however, the potential of using sulfate-reducing bacteria (SRB) to enable sulfur amendment has not been closely examined. In this study, lab-synthesized nanoscale ZVI (nZVI) and Peerless iron particles (ZVI PLS ) were treated in a sulfate-reducing monoculture ( D. desulfuricans ) and an enrichment culture derived from freshwater sediments (AMR-1) prior to reactivity assessments with TCE as the model contaminant. ZVI conditioned in both cultures exhibited higher dechlorination efficiencies compared to unamended ZVIs. Remarkably, nZVI and ZVI PLS exposed to AMR-1 attained similar TCE dechlorination rates as their counterparts receiving chemical sulfidation (i.e., S-nZVI) using previously reported method. Product distribution data show that, in the SRB-ZVI system, abiotic dechlorination is the dominant TCE reduction pathway. In addition to dissolved sulfide, biogenic or synthesized FeS particles can enhance nZVI reactivity even as nZVI and FeS were not in direct contact, implying that SRB may influence the reactivity of ZVI via multiple mechanisms in different remediation situations. A shift in Archaea abundance in AMR-1 with nZVI amendment was observed but not with ZVI PLS . Overall, the synergy exhibited in the SRB-ZVI system may offer a valuable remediation strategy to overcome limitations of standalone biological or abiotic dechlorination approaches for chlorinated solvent abatement.

Methylmercury (MeHg) is a bioaccumulative neurotoxin produced by certain sulfate-reducing bacteria and other anaerobic microorganisms. Because microorganisms differ in their capacity to methylate mercury, the abundance and distribution of methylating populations may determine MeHg production in the environment. We compared rates of MeHg production and the distribution of hgcAB genes in epilimnetic sediments from a freshwater lake that were experimentally amended with sulfate levels from 7 to 300 mg L -1 . The most abundant hgcAB sequences were associated with clades of Methanomicrobia , sulfate-reducing Deltaproteobacteria , Spirochaetes , and unknown environmental sequences. The hgcAB + communities from higher sulfate amendments were less diverse and had relatively more Deltaproteobacteria , whereas the communities from lower amendments were more diverse with a larger proportion of hgcAB sequences affiliated with other clades. Potential methylation rate constants varied 52-fold across the experiment. Both potential methylation rate constants and % MeHg were the highest in sediments from the lowest sulfate amendments, which had the most diverse hgcAB + communities and relatively fewer hgcAB genes from clades associated with sulfate reduction. Although pore water sulfide concentration covaried with hgcAB diversity across our experimental sulfate gradient, major changes in the community of hgcAB + organisms occurred prior to a significant buildup of sulfide in pore waters. Our results indicate that methylating communities dominated by diverse anaerobic microorganisms that do not reduce sulfate can produce MeHg as effectively as communities dominated by sulfate-reducing populations.

Sulfate radical (SO 4 •- )- and hydroxyl radical (HO • )-based advanced oxidation processes (AOPs) are effective for the removal of organic pollutants in water treatment. This study compared the interactions of SO 4 •- and HO • for the transformation of phenol in UV/peroxydisulfate (PDS) and UV/H 2 O 2 with the presence of NO 2 - , which is widely present in aquatic environments and transforms SO 4 •- and HO • to • NO 2 . By using laser flash photolysis, the products of phenol reacting with SO 4 •- and HO • were demonstrated to be phenoxy radical and phenol-HO-adduct radical, respectively. This result, along with density functional theory (DFT) calculations, indicate that the predominant reaction mechanisms of phenol with SO 4 •- and HO • with phenol are electron transfer and addition, respectively. The different mechanisms induced the much higher formation of nitrophenols by SO 4 •- than HO • in the presence of NO 2 - through the fast combination of phenoxy radicals and • NO 2 . The conversion yields of phenol to nitrophenols (including 2-nitrophenol and 4-nitrophenol), were 47.5% by SO 4 •- versus 5.3% by HO • at the experimental conditions. Increasing PDS/H 2 O 2 dosages from 0.2 to 1 mM resulted in a 61.9% increase of nitrophenol conversion yield in UV/PDS/NO 2 - but a 35.4% decrease of that in UV/H 2 O 2 /NO 2 - . In addition, the significant formation of phenoxy radicals by SO 4 •- also induced many nitrated polymers in UV/PDS/NO 2 - , while those induced in UV/H 2 O 2 /NO 2 - were negligible. The significant formation of nitrophenols and nitrated polymers increased the mutagenicity by 860.5% when the removal rate of phenol was 98% by UV/PDS/NO 2 - . This is the first study to demonstrate the different mechanisms of phenol transformation by SO 4 •- and HO • in the presence of NO 2 - .

Mixing of acid mine drainage (AMD) and hydraulic fracturing flowback fluids (HFFF) could represent an efficient management practice to simultaneously manage two complex energy wastewater streams while reducing freshwater resource consumption. AMD discharges offer generally high sulfate concentrations, especially from the bituminous coal region of Pennsylvania; unconventional Marcellus shale gas wells generally yield HFFF enriched in alkaline earth metals such as Sr and Ba, known to cause scaling issues in oil and gas (O&G) production. Mixing the two waters can precipitate HFFF-Ba and -Sr with AMD-SO 4 , therefore removing them from solution. Four AMD discharges and HFFF from two unconventional Marcellus shale gas wells were characterized and mixed in batch reactors for 14 days. Ba could be completely removed from solution within 1 day of mixing in the form Ba x Sr 1- x SO 4 and no further significant precipitation occurred after 2 days. Total removal efficiencies of Ba + Sr + SO 4 and the proportion of Ba and Sr in Ba x Sr 1- x SO 4 depended upon the Ba/Sr ratio in the initial HFFF. A geochemical model was calibrated from batch reactor data and used to identify optimum AMD-HFFF mixing ratios that maximize total removal efficiencies (Ba + Sr + SO 4 ) for reuse in O&G development. Increasing Ba/Sr ratios can enhance total removal efficiency but decrease the efficiency of Ra removal. Thus, treatment objectives and intended beneficial reuse need to be identified prior to optimizing the treatment of HFFF with AMD.

Reports that promote persulfate-based advanced oxidation process (AOP) as a viable alternative to hydrogen peroxide-based processes have been rapidly accumulating in recent water treatment literature. Various strategies to activate peroxide bonds in persulfate precursors have been proposed and the capacity to degrade a wide range of organic pollutants has been demonstrated. Compared to traditional AOPs in which hydroxyl radical serves as the main oxidant, persulfate-based AOPs have been claimed to involve different in situ generated oxidants such as sulfate radical and singlet oxygen as well as nonradical oxidation pathways. However, there exist controversial observations and interpretations around some of these claims, challenging robust scientific progress of this technology toward practical use. This Critical Review comparatively examines the activation mechanisms of peroxymonosulfate and peroxydisulfate and the formation pathways of oxidizing species. Properties of the main oxidizing species are scrutinized and the role of singlet oxygen is debated. In addition, the impacts of water parameters and constituents such as pH, background organic matter, halide, phosphate, and carbonate on persulfate-driven chemistry are discussed. The opportunity for niche applications is also presented, emphasizing the need for parallel efforts to remove currently prevalent knowledge roadblocks.

Activated persulfate (PS) is a common method used to generate sulfate radicals (SO 4 •- ), a powerful oxidant capable of degrading a broad array of environmental contaminants. The reaction of SO 4 •- with nontarget species (i.e., scavenging) contributes significantly to treatment inefficiency. Radical scavenging in this manner has been quantified for nontarget chemical species in the aqueous phase but has never been quantified for solid phase media. Kinetic analysis and laboratory methods were developed to quantify the SO 4 •- scavenging rate constant ( k ≡S ) for alumina, a naturally occurring mineral in soil and aquifer materials. SO 4 •- were generated in UV and thermally activated persulfate (UV-APS, T-APS) batch systems, and the loss of rhodamine B (RhB) served as an indicator of SO 4 •- activity. k ≡S for alumina was 2.42 × 10 4 and 2.03 × 10 4 m -2 s -1 for UV-APS and T-APS oxidative treatment systems, respectively. At [alumina] >5 g L -1 , the reaction of SO 4 •- with solid phase media increased over the aqueous phase reactions with RhB and aqueous scavengers. SO 4 •- scavenging by solid surfaces was orders of magnitude greater than the reaction with the target compound and scavengers in the aqueous phase, underscoring the significant role of solid surfaces in scavenging SO 4 •- .

Purines are the most widely occurring heterocyclic-N compounds. The degradation behaviors of purine derivatives, theophylline (TPL) and adenine (ADN) as representatives, in both single-component and mixture systems during UV/peroxydisulfate (PDS) treatment were explored. In the mixture system when the concentrations of SO 4 •- and HO • were reduced by more than half in comparison to a single-component system, the observed first-order rate constant of TPL was reduced by 11%, whereas ADN degradation was almost completely inhibited. An ADN "revert" pathway, that is, back transformation of ADN(-H) • to ADN by TPL via single electron-transfer reactions, was found. The second-order rate constant of ADN(-H) • with TPL was determined to be (1.94 ± 0.21) × 10 8 M -1 s -1 . A kinetic model was developed, which successfully quantifies the contribution of each reaction pathway at various target compound concentrations. In the copresence of 0.1 μM TPL, 58% ADN (3.0 μM) was reduced back to ADN at the PDS dose of 290 μM. The ADN revert pathway's effectiveness is governed by the relative reduction potentials of the reactants. Purines and phenols with lower reduction potentials are able to react via the ADN revert pathway. These findings improve the understanding of the removal of mixture pollutants from real water media in advanced oxidation processes.

Hexafluoropropylene oxide dimer acid (HFPO-DA, trade name GenX) is a perfluoroalkyl ether carboxylic acid (PFECA) that has been detected in watersheds around the world. Similar to other per- and polyfluoroalkyl substances (PFASs), few processes are able to break HFPO-DA's persistent carbon-fluorine bonds. This study provides both experimental and computational lines of evidence for HFPO-DA mineralization during electrochemical oxidation at a boron-doped diamond anode with a low potential for the generation of stable organofluorine intermediates. Our density functional theory calculations consider the major operative mechanism, direct electron transfer, throughout the entire pathway. Initial oxidative attack does not break the ether bond, but leads to stepwise mineralization of the acidic side chain. Our mechanistic investigations reveal that hydroxyl radicals are unreactive toward HFPO-DA, while electrochemically activated sulfate facilitates its oxidation. Furthermore, we demonstrate that an NF90 membrane is capable of removing 99.5% of HFPO-DA from contaminated water. Electrochemical treatment of the nanofiltration rejectate is shown to reduce both energy and electrode costs by more than 1 order of magnitude compared to direct electrochemical treatment of the raw water. Overall, a nanofiltration-electrochemical oxidation treatment train is a sustainable destructive approach for the cost-effective elimination of HFPO-DA and other PFASs from contaminated water.

Dissolved organic matter (DOM) has been shown to inhibit the oxidation of aromatic amines initiated by excited triplet states, an effect that was attributed to the reduction of oxidation intermediates back to their parent compounds. The present study focuses on the quantification of an analogous inhibitory effect of DOM on aqueous oxidations induced by the sulfate radical (SO 4 · - ). Second-order rate constants for the SO 4 · - -induced transformation of selected anilines and sulfonamide antibiotics were determined by competition kinetics in the presence and absence of DOM from three different isolates at pH 8. In the presence of 1 mg C L -1 of DOM, a significant reduction in the rate constant was observed for most of the compounds compared to DOM-free solutions, but for two electron-rich anilines, increases in the rate constant were measured. For 4-cyanoaniline and sulfamethoxazole, the DOM concentration dependence of the rate constant consisted of a sharp decrease up to ∼1.0 mg C L -1 of DOM followed by a region of slight changes or even increases for higher DOM concentrations (up to 5 mg C L -1 ). This behavior was attributed to the occurrence of the aforementioned inhibitory effect and a counteracting accelerated transformation of the contaminants due to reactions with secondary radical oxidants resulting from DOM oxidation by SO 4 · - . Both effects of inhibition and secondary oxidants should be considered when assessing the abatement of aromatic amines in SO 4 · - -based advanced oxidation processes.

Electrochemical oxidation based on SO 4 •- and • OH generated from sulfate electrolyte is a cost-effective method for degradation of persistent organic pollutants (POPs). However, sulfate activation remains a great challenge due to lack of active and robust electrodes. Herein, a B/N codoped diamond (BND) electrode is designed for electrochemical degradation of POPs via sulfate activation. It is efficient and stable for perfluorooctanoic acid (PFOA) oxidation with first-order kinetic constants of 2.4 h -1 and total organic carbon removal efficiency of 77.4% (3 h) at relatively low current density of 4 mA cm -2 . The good activity of BND mainly originates from a B and N codoping effect. The PFOA oxidation rate at sulfate electrolyte is significantly enhanced (2.3-3.4 times) compared with those at nitrate and perchlorate electrolytes. At sulfate, PFOA oxidation rate decreases slightly in the presence of • OH quencher while it declines significantly with SO 4 •- and • OH quenchers, indicate both SO 4 •- and  • OH contribute to PFOA oxidation but SO 4 •- contribution is more significant. On the basis of intermediates analysis, a proposed mechanism for PFOA degradation is that PFOA is oxidized to shorter chain perfluorocarboxylic acids gradually by SO 4 •- and • OH until it is mineralized.

This study determined the sorption affinity to artificial phospholipid membranes ( K MW ) for series of perfluorinated carboxylates (PFCAs), perfluorinated sulfonates (PFSAs), alkyl sulfates (C x SO 4 ), and 1-alkanesulfonates (C x SO 3 ). A sorbent dilution assay with solid supported lipid membranes (SSLM) showed consistent CF 2 unit increments of 0.59, and CH 2 unit increments of 0.53, for the log K MW of perfluorinated and hydrogenated anions, respectively. PFSAs sorbed 0.90 log units stronger than analogue PFCAs; C x SO 4 sorbed 0.75 log units stronger than analogue C x SO 3 anions. The log K MW values for the octyl analogues increase in the order H(CH 2 ) 8 SO 3 - (1.74) < H(CH 2 ) 8 SO 4 - (2.58) < F(CF 2 ) 8 CO 2 - (PFNA, 4.04) < F(CF 2 ) 8 SO 3 - (PFOS, 4.88). Intrinsic partition ratios determined on a phospholipid coated HPLC column (IAM-HPLC) closely aligned with SSLM K MW values. COSMO-RS based molecular calculations of K MW aligned with SSLM K MW values for hydrogenated anions with C 8 -C 14 alkyl chains but strongly underestimated CF 2 and CH 2 unit increments for C 4 -C 8 based anions. Dividing the critical narcotic membrane burden of 100 mmol/kg by the experimental K MW predicts lethal baseline toxicity concentrations (LC 50,narc ). The LC 50,narc coincides with the lowest reported acute LC 50 values for several anionic surfactants but were on average about an order of magnitude lower.

Hexafluoropropylene oxide dimer acid (HFPO-DA, ammonium salt with trade name: GenX) has been recently detected in river water worldwide. There are significant concerns about its persistence, and potential adverse effects to the biota. In this study, the degradability of GenX by typical advanced redox technologies (UV/persulfate and UV/sulfate) is investigated. Results demonstrate that <5% GenX is oxidized after 3 h in UV/persulfate system, which is much lower than ∼27% for PFOA. In comparison, GenX can be readily degraded and defluorinated by hydrated electron (e aq - ) generated by UV/sulfite system. Specifically, GenX is not detectable after 2 h, and >90% of fluoride ion is recovered 6 h later. This is attributed to the accumulation and subsequent degradation of CF 3 CF 2 COOH and CF 3 COOH, which are stable intermediates of GenX degradation. Mechanistic investigations suggest that the etheric bond in the molecule is a favorable attack point for the e aq - . Such finding is corroborated by quantum chemical calculations. The side CF 3 - at the α-carbon probably acts as an effective barrier that prevents GenX from being cleaved by SO 4 - • or OH• at its most sensible point (i.e. the carboxyl group). This study illustrates that reduction by UV/sulfite might be a promising technology to remove GenX from contaminated water.

Despite that persulfate radical (S 2 O 8 •- ) is an important radical species formed from the persulfate (PS) activation process, its reactivity toward contaminant degradation has rarely been explored. In this study, we found that S 2 O 8 •- efficiently degrades the contaminant hexachloroethane (HCA) under anaerobic conditions, whereas HCA degradation is negligible in the presence of oxygen. We observed dechlorination products such as pentachloroethane, tetrachloroethylene, and Cl - during HCA degradation, which suggest that HCA degradation is mainly a reductive process under anaerobic conditions. Using free radical quenching and electron paramagnetic resonance (EPR) experiments, we confirmed that S 2 O 8 •- forms from the reaction between sulfate radical (SO 4 •- ) and S 2 O 8 2- , which are the dominant reactive species in HCA degradation. Density functional theory (DFT) calculations were used to elucidate the pathways of HCA degradation and S 2 O 8 •- radical decomposition. Further investigation showed that S 2 O 8 •- can efficiently degrade HCA and DDTs in soil via reduction during the thermal activation of PS under anaerobic conditions. The finding of this study provide a novel strategy for the reductive degradation of contaminant when PS-based in situ chemical oxidation used in the remediation of soil and groundwater, particularly those contaminated with highly halogenated compounds.

Halides and natural organic matter (NOM) are inevitable in aquatic environment and influence the degradation of contaminants in sulfate radical (SO 4 •- )-based advanced oxidation processes. This study investigated the formation of chlorate in the coexposure of SO 4 •- , chloride (Cl - ), bromide (Br - ) and/or NOM in UV/persulfate (UV/PDS) and cobalt(II)/peroxymonosulfate (Co/PMS) systems. The formation of chlorate increased with increasing Cl - concentration in the UV/PDS system, however, in the Co/PMS system, it initially increased and then decreased. The chlorate formation involved the formation of hypochlorous acid/hypochlorite (HOCl/OCl - ) as an intermediate in both systems. The formation was primarily attributable to SO 4 •- in the UV/PDS system, whereas Co(III) played a significant role in the oxidation of Cl - to HOCl/OCl - and SO 4 •- was important for the oxidation of HOCl/OCl - to chlorate in the Co/PMS system. The pseudo-first-order rate constants ( k') of the transformation from Cl - to HOCl/OCl - were 3.32 × 10 -6 s -1 and 9.23 × 10 -3 s -1 in UV/PDS and Co/PMS, respectively. Meanwhile, k' of HOCl/OCl - to chlorate in UV/PDS and Co/PMS were 2.43 × 10 -3 s -1 and 2.70 × 10 -4 s -1 , respectively. Br - completely inhibited the chlorate formation in UV/PDS, but inhibited it by 45.2% in Co/PMS. The k' of SO 4 •- reacting with Br - to form hypobromous acid/hypobromite (HOBr/OBr - ) was calculated to be 378 times higher than that of Cl - to HOCl/OCl - , but the k' of Co(III) reacting with Br - to form HOBr/OBr - was comparable to that of Cl - to HOCl/OCl - . NOM also significantly inhibited the chlorate formation, due to the consumption of SO 4 •- and reactive chlorine species (RCS, such as Cl·, ClO· and HOCl/OCl - ). This study demonstrated the formation of chlorate in SO 4 •- -based AOPs, which should to be considered in their application in water treatment.

Hydrogen storage in geological formations is one of the most promising technologies for balancing major fluctuations between energy supply from renewable energy plants and energy demand of customers. If hydrogen gas is stored in a porous medium or if it leaks into a shallow aquifer, redox reactions can oxidize hydrogen and reduce electron acceptors such as nitrate, Fe III and Mn IV (hydro)oxides, sulfate, and carbonate. These reactions are of key significance, because they can cause unintentional losses in hydrogen stored in porous media and they also can cause unwanted changes in the composition of protected potable groundwater. To represent an aquifer environment enclosing a hydrogen plume, laboratory experiments using sediment-filled columns were constructed and percolated by groundwater in equilibrium with high (2-15 bar) hydrogen partial pressures. Here, we show that hydrogen is consumed rapidly in these experiments via sulfate reduction (18 ± 5 μM h -1 ) and acetate production (0.030 ± 0.006 h -1 ), while no methanogenesis took place. The observed reaction rates were independent from the partial pressure of hydrogen and hydrogen consumption only stopped in supplemental microcosm experiments where salinity was increased above 35 g L -1 . The outcomes presented here are implemented for planning the sustainable use of the subsurface space within the ANGUS+ project.

Sulfate inputs to the Florida Everglades stimulate sulfidic conditions in freshwater wetland sediments that affect ecological and biogeochemical processes. An unexplored implication of sulfate enrichment is alteration of the content and speciation of sulfur in dissolved organic matter (DOM), which influences the reactivity of DOM with trace metals. Here, we describe the vertical and lateral spatial dependence of sulfur chemistry in the hydrophobic organic acid fraction of DOM from unimpacted and sulfate-impacted Everglades wetlands using X-ray absorption spectroscopy and ultrahigh-resolution mass spectrometry. Spatial variation in DOM sulfur content and speciation reflects the degree of sulfate enrichment and resulting sulfide concentrations in sediment pore waters. Sulfur is incorporated into DOM predominantly as highly reduced species in sulfidic pore waters. Sulfur-enriched DOM in sediment pore waters exchanges with overlying surface waters and the sulfur likely undergoes oxidative transformations in the water column. Across all wetland sites and depths, the total sulfur content of DOM correlated with the relative abundance of highly reduced sulfur functionality. The results identify sulfate input as a primary determinant on DOM sulfur chemistry to be considered in the context of wetland restoration and sulfur and trace metal cycling.

The advanced oxidation process (AOP) based on SO 4 •- radicals has been receiving growing attention in water and wastewater treatment. Producing SO 4 •- radicals by activation of peroxymonosulfate or persulfate faces the challenges of high operational cost and potential secondary pollution. In this study, we report the in situ photochemical activation of sulfate (i-PCAS) to produce SO 4 •- radicals with bismuth phosphate (BPO) serving as photocatalyst. The prepared BPO rod-like material could achieve remarkably enhanced degradation of 2,4-dichlorophenol (2,4-DCP) in the presence of sulfate, indicated by the first-order kinetic constant (k = 0.0402 min -1 ) being approximately 2.1 times that in the absence (k = 0.019 min -1 ) at pH-neutral condition. This presented a marked contrast with commercial TiO 2 (P25), the performance of which was always inhibited by sulfate. The impact of radical scavenger and electrolyte, combined with electron spin resonance (ESR) measurement, verified the formation of •OH and SO 4 •- radicals during i-PCAS process. According to theoretical calculations, BPO has a sufficiently high valence band potential making it thermodynamically favorable for sulfate oxidation, and weaker interaction with SO 4 •- radicals resulting in higher reactivity toward target organic pollutant. The concept of i-PCAS appears to be attractive for creating new photochemical systems where in situ production of SO 4 •- radicals can be realized by using sulfate originally existing in aqueous environment. This eliminates the need for extrinsic chemicals and pH adjustment, which makes water treatment much easier, more economical, and more sustainable.

Three one-dimensional MnO2 nanoparticles with different crystallographic phases, α-, β-, and γ-MnO2, were synthesized, characterized, and tested in heterogeneous activation of Oxone for phenol degradation in aqueous solution. The α-, β-, and γ-MnO2 nanostructured materials presented in morphologies of nanowires, nanorods, and nanofibers, respectively. They showed varying activities in activation of Oxone to generate sulfate radicals for phenol degradation depending on surface area and crystalline structure. α-MnO2 nanowires exhibited the highest activity and could degrade phenol in 60 min at phenol concentrations ranging in 25-100 mg/L. It was found that phenol degradation on α-MnO2 followed first order kinetics with an activation energy of 21.9 kJ/mol. The operational parameters, such as MnO2 and Oxone loading, phenol concentration and temperature, were found to influence phenol degradation efficiency. It was also found that α-MnO2 exhibited high stability in recycled tests without losing activity, demonstrating itself to be a superior heterogeneous catalyst to the toxic Co3O4 and Co(2+).

Enhancement of in situ anaerobic biodegradation of BTEX compounds was demonstrated at a petroleum-contaminated aquifer in Seal Beach, CA. Specifically, combined injection of nitrate and sulfate into the contaminated aquifer was used to accelerate BTEX removal as compared to remediation by natural attenuation. An array of multi-level sampling wells was used to monitor the evolution of the in situ spatial distributions of the electron acceptors and the BTEX compounds. Nitrate was utilized preferentially over sulfate and was completely consumed within a horizontal distance of 4-6 m from the injection well; sulfate reduction occurred in the region outside the denitrifying zone. By combining injection of both nitrate and sulfate, the total electron acceptor capacity was enhanced without violating practical considerations that limit the amount of nitrate or sulfate that can be added individually. Degradation of total xylene appears linked to sulfate utilization, indicating another advantage of combined injection versus injection of nitrate alone. Benzene degradation also appears to have been stimulated by the nitrate and sulfate injection close to the injection well but only toward the end of the 15-month demonstration. The results are consistent with the hypothesis that benzene can be biodegraded anaerobically after other preferentially degraded hydrocarbons have been removed.