Role of pH in the Transformation of Perfluoroalkyl Carboxylic Acids by Activated Persulfate: Implications from the Determination of Absolute Electron-Transfer Rates and Chemical Computations

Peroxysulfur species-mediated enhanced oxidation of micropollutants by ferrate(VI): Peroxymonosulfate versus peroxydisulfate

Many studies have shown that Peroxymonosulfate (PMS) works synergistically with ferrate (Fe(VI)) to remove refractory organic compounds in a few minutes. However, little has been reported on the combined effects of peroxydisulfate (PDS) and Fe(VI). Since PDS is stable and cost effective, it is of practical significance to study the reaction mechanism and conditions of the PDS/Fe(VI) system. The results of the study indicate that the intermediate Fe(II) is formed during the decomposition of Fe(VI), which is then rapidly oxidized. Due to the asymmetry of the PMS molecular structure, PMS can rapidly trap Fe(II) (kPMS/Fe(II)= 3 × 104 M-1∙s-1), whereas PDS cannot (kPDS/Fe(II)= 26 M-1∙s-1). Hydroxylamine hydrochloride (HA) can reduce Fe(VI) and Fe(III) to Fe(II) to excite PDS to produce SO4•-. Acetate helps to detect Fe(II), but does not help PDS to trap Fe(II). Active species such as SO4•-, •OH, 1O2, and Fe(IV), Fe(V) are present in both systems, but in different amounts. In the PMS/Fe(Ⅵ) system, all these active species react with ibuprofen (IBP) and degrade IBP within several minutes. The effects of the initial pH, PMS or Fe(VI) dosage, and different amounts of IBP on the removal rate of IBP were investigated. According to the intermediates detected by the GC-MS, the degradation process of IBP includes hydroxylation, demethylation and single bond breakage. The degradation pathways of IBP were proposed. The degradation of IBP in tap water and Songhua River was also investigated. In actual water treatment, the dosage needs to be increased to achieve the same results. This study provides a basis and theoretical support for the application of PMS/Fe(Ⅵ) and PDS/Fe(VI) system in water treatment.

Kinetics and mechanism analysis of advanced oxidation degradation of PFOA/PFOS by UV/Fe3+ and persulfate: A DFT study

UV/Fe3+ and persulfate are two promising advanced oxidative degradation systems for in situ remediation of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS), yet a lack of comprehensive understanding of the degradation mechanisms. For the first time, we used density functional theory (DFT) to calculate the entire reaction pathways of the degradation of PFOA/PFOS in water by UV/Fe3+ and persulfate. In addition, we have deeply explored the different attack pathways driven by •OH and SO4-•, and found that SO4-• determines PFOA/PFOS to obtain PFOA/PFOS free radicals through single electron transfer to initiate the degradation reaction, while •OH determines the speed of PFOA/PFOS degradation reaction. Both degradation reactions were thermodynamically advantageous and kinetically feasible under calculated conditions. Based on the thermodynamic data, persulfate was found to be more favorable for the advanced oxidative degradation of Perfluorinated compounds (PFCs). Moreover, for SO4-• and •OH co-existing in the persulfate system, pH will affect the presence and concentration of these two types of free radicals, and low pH is not necessary for the degradation of PFOA/PFOS in the persulfate system. These results can considerably advance our understanding of the PFOA/PFOS degradation process in advanced oxidation processes (AOPs), which is driven by •OH and SO4-•. This study provides a DFT calculation process for the mechanism calculation of advanced oxidation degradation of other types of PFCs pollutants, hoping to elucidate the future development of PFCs removal. Further research should focus on determining the advanced oxidation degradation pathways of other types of PFCs, to support the development of computational studies on the advanced oxidation degradation of PFCs.

The dissolution characteristics of cadmium containing birnessite produced from paddy crusts

The cadmium (Cd) accumulates in birnessite as it forms on the surface of paddy crusts (PC). The stability of Cd-containing birnessite is influenced by environmental factors, and destabilized birnessite releases dissolved Cd. We report the effects of pH, oxalic acid, and light on the dissolution of Cd-containing birnessite. We found that at pH 4.0, with light and 0.20 mol/L oxalic acid, the ratio of dissolved Cd and manganese (Mn) peaked after 24 h at 2978.0 μg/g and 326.8 mg/g, respectively. The three environmental factors affected the dissolution of Cd-containing birnessite in the following order: pH > oxalic acid > light. During dissolution process, Cd and Mn did not dissolve simultaneously, and the dissolved Cd/Mn ratio in the solution was significantly lower than that of the pristine mineral (33.5 × 10-3). Compared with Mn, Cd dissolution was inhibited by strong acidity (pH 4.0-5.0), and the dissolved Cd/Mn ratio was 5-10 × 10-3. Mild acidity (pH 6.0) was weakly inhibitory, with a Cd/Mn ratio of 6-15 × 10-3. In an alkaline (pH 8.0) oxalate environment, light illumination inhibited Cd dissolution, and the Cd/Mn ratio decreased over time due to the stability of the products formed by oxalate and carbonate, with Cd being more stable than those formed by Mn. Our findings would provide insights into the migration and transformation of PC-associated Cd in paddy fields.

Oxidation of Per- and Polyfluoroalkyl Ether Acids and Other Per- and Polyfluoroalkyl Substances by Sulfate and Hydroxyl Radicals: Kinetic Insights from Experiments and Models

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 (SO4•-) and hydroxyl (•OH) radicals can oxidize some PFAS, but the behavior of per- and polyfluoroalkyl ether acids (PFEAs) in processes involving SO4•- 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 SO4•- and •OH. Among the studied PFAS, 6:2 fluorotelomer sulfonate reacted most readily with •OH [k•OH = (1.1-1.2) × 107 M-1 s-1], while polyfluoroalkyl ether acids containing an -O-CFH- moiety reacted more slowly [k•OH = (0.5-1.0) × 106 M-1 s-1]. In the presence of SO4•-, polyfluoroalkyl ether acids with an -O-CFH- moiety reacted more rapidly [kSO4•- = (0.89-4.6) × 106 M-1 s-1] than perfluoroalkyl ether carboxylic acids (PFECAs) and a chloro-perfluoro-polyether carboxylic acid (ClPFPECA) [kSO4•- = (0.85-9.5) × 104 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. SO4•- 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 SO4•- attack was the -O-CFH- moiety. Perfluoroalkyl ether sulfonic acids were not oxidized by SO4•- and •OH under the conditions evaluated in this study.

Merits and Limitations of Radical vs. Nonradical Pathways in Persulfate-Based Advanced Oxidation Processes

Urbanization and industrialization have exerted significant adverse effects on water quality, resulting in a growing need for reliable and eco-friendly treatment technologies. Persulfate (PS)-based advanced oxidation processes (AOPs) are emerging as viable technologies to treat challenging industrial wastewaters or remediate groundwater impacted by hazardous wastes. While the generated reactive species can degrade a variety of priority organic contaminants through radical and nonradical pathways, there is a lack of systematic and in-depth comparison of these pathways for practical implementation in different treatment scenarios. Our comparative analysis of reaction rate constants for radical vs. nonradical species indicates that radical-based AOPs may achieve high removal efficiency of organic contaminants with relatively short contact time. Nonradical AOPs feature advantages with minimal water matrix interference for complex wastewater treatments. Nonradical species (e.g., singlet oxygen, high-valent metals, and surface activated PS) preferentially react with contaminants bearing electron-donating groups, allowing enhancement of degradation efficiency of known target contaminants. For byproduct formation, analytical limitations and computational chemistry applications are also considered. Finally, we propose a holistically estimated electrical energy per order of reaction (EE/O) parameter and show significantly higher energy requirements for the nonradical pathways. Overall, these critical comparisons help prioritize basic research on PS-based AOPs and inform the merits and limitations of system-specific applications.

Recent advances in the remediation of perfluoroalkylated and polyfluoroalkylated contaminated sites

Per- and polyfluoroalkyl substances (PFASs) are compounds used since 1940 in various formulations in the industrial and consumer sectors due to their high chemical and thermal stability. In recent years, PFASs have caused global concern due to their presence in different water and soil matrices, which threatens the environment and human health. These compounds have been reported to be linked to the development of serious human diseases, including but not limited to cancer. For this reason, PFASs have been considered as persistent organic compounds (COPs) and contaminants of emerging concern (CECs). Therefore, this work aims to present the advances in remediation of PFASs-contaminated soil and water by addressing the current literature. The performance and characteristics of each technique were addressed deeply in this work. The reviewed literature found that PFASs elimination studies in soil and water were carried out at a laboratory and pilot-scale in some cases. It was found that ball milling, chemical oxidation and thermal desorption are the most efficient techniques for the removal of PFASs in soils, however, phyto-microbial remediation is under study, which claims to be a promising technique. For the remediation of PFASs-contaminated water, the processes of electrocoagulation, membrane filtration, ozofractionation, catalysis, oxidation reactions - reduction, thermolysis and destructive treatments with plasma have presented the best results. It is noteworthy that hybrid treatments have also proved to be efficient techniques in the removal of these contaminants from soil and water matrices. Therefore, the improvisation and implication of existing techniques on a field-scale are greatly warranted to corroborate the yields obtained on a pilot- and laboratory-scale.

Atomistic insights into the hydrodefluorination of PFAS using silylium catalysts

Fluorochemicals are a persistent environmental contaminant that require specialized techniques for degradation and capture. In particular, recent attention on per- and poly-fluoroalkyl substances (PFAS) has led to numerous explorations of different techniques for degrading the super-strong C-F bonds found in these fluorochemicals. In this study, we investigated the hydrodefluorination mechanism using silylium-carborane salts for the degradation of PFAS at the density functional theory (DFT) level. We find that the degradation process involves both a cationic silylium (Et₃Si⁺) and a hydridic silylium (Et₃SiH) to facilitate the defluorination and hydride-addition events. Additionally, the role of carborane ([HCB₁₁H₅F₆]^-) is to force unoccupied anti-bonding orbitals to be partially occupied, weakening the C-F bond. We also show that changing the substituents on carborane from fluorine to other halogens weakens the C-F bond even further, with iodic carborane ([HCB₁₁H₅I₆]^-) having the greatest weakening effect. Moreover, our calculations reveal why the C-F bonds are resistant to degradation, and how the silylium-carborane chemistry is able to chemically transform these bonds into C-H bonds. We believe that our results are further applicable to other halocarbons, and can be used to treat either our existing stocks of these chemicals or to treat concentrated solutions following filtration and capture.

Biological and Chemical Transformation of the Six-Carbon Polyfluoroalkyl Substance N-Dimethyl Ammonio Propyl Perfluorohexane Sulfonamide (AmPr-FHxSA)

Sites impacted by aqueous film-forming foam (AFFF) contain co-contaminants that can stimulate biotransformation of polyfluoroalkyl substances. Here, we compare how microbial enrichments from AFFF-impacted soil amended with diethyl glycol monobutyl ether (found in AFFF), aromatic hydrocarbons (present in co-released fuels), acetate, and methane (substrates used or formed during bioremediation) impact the aerobic biotransformation of an AFFF-derived six-carbon electrochemical fluorination (ECF) precursor N-dimethyl ammonio propyl perfluorohexane sulfonamide (AmPr-FHxSA). We found that methane- and acetate-oxidizing cultures resulted in the highest yields of identifiable products (38 and 30%, respectively), including perfluorohexane sulfonamide (FHxSA) and perfluorohexane sulfonic acid (PFHxS). Using these data, we propose and detail a transformation pathway. Additionally, we examined chemical oxidation products of AmPr-FHxSA and FHxSA to provide insights on remediation strategies for AmPr-FHxSA. We demonstrate mineralization of these compounds using the sulfate radical and test their transformation during the total oxidizable precursor (TOP) assay. While perfluorohexanoic acid accounted for over 95% of the products formed, we demonstrate here for the first time two ECF-based precursors, AmPr-FHxSA and FHxSA, that produce PFHxS during the TOP assay. These findings have implications for monitoring poly- and perfluoroalkyl substances during site remediation and application of the TOP assay at sites impacted by ECF-based precursors.

Enhanced semi-volatile DNAPL accessibility at sub-boiling temperature during electrical resistance heating in heterogeneous porous media

Successful remediation of semi-volatile contaminants using electrical resistance heating (ERH) coupled technologies requires a deep understanding of contaminant migration and accessibility, especially with stratigraphic heterogeneity and dense nonaqueous phase liquid (DNAPL) occurrence. Here, we chose nitrobenzene (NB) as a model contaminant of semi-volatile DNAPL and uniquely demonstrated that temperature variation during ERH could induce NB DNAPL migration out of the low permeability zone (LPZ) even below water boiling temperature. When heating the system using alternating current (AC) of 140 V to a temperature range of 50-79 °C, obvious DNAPL migration was visually observed. The upward migration of DNAPL would considerably increase the mass of accessible contaminant by other remediation measures. The downstream cumulative NB mass of 1092 mg in 140 V system raised 56-folds compared to that of 19 mg in the control experiment with only groundwater flow. This migration was mainly attributed to a complex natural convection caused by temperature gradient. Comparing with traditional AC heating, ERH powered by pulsed direct current (PDC-ERH) showed a higher and more uneven heating pattern, resulting in a stronger convection at the same voltage that enhanced the DNAPL migration out of LPZ. These results revealed the importance of natural convection in the ERH process, which could be further optimized to improve the energy efficiency of remediation.

Role of Explicit Hydration in Predicting the Aqueous Standard Reduction Potential of Sulfate Radical Anion by DFT and Insight into the Influence of pH on the Reduction Potential

Sulfate radical anion (SO₄^•-) is a potent oxidant capable of destroying recalcitrant environmental contaminants such as perfluoroalkyl carboxylic acids. In addition, it is thought to participate in important atmospheric reactions. Its standard reduction potential (E°) is fundamental to its reactivity. Using theoretical methods to accurately predict the aqueous phase E° requires solvation with explicit water molecules. Herein, using density functional theory, we calculated the aqueous E° of SO₄^•- and evaluated sensitivity to explicit water count. The E° increased considerably with more waters until ca. 24 were included, after which change in E° was small. When a proton was added to these systems, the E° was similar regardless of the explicit water count and this value was similar to the E° for systems with a large number of explicit waters but no proton. This result agrees with literature evidence that the E° is pH independent. Natural Bond Orbital natural population analysis indicated that in the case of both SO₄^2- and SO₄^•-, considerable charge was donated from the SO₄ center to the explicit solvation shells.

Mechanistic Understanding of Superoxide Radical-Mediated Degradation of Perfluorocarboxylic Acids

Perfluorocarboxylic acids (PFCAs) exhibit strong persistence in sunlit surface waters and in radical-based treatment processes, where superoxide radical (O₂^•-) is an important and abundant reactive oxygen species. Given that the role of O₂^•- during the transformation of PFCAs remains largely unknown, we investigated the kinetics and mechanisms of O₂^•--mediated PFCAs attenuation through complementary experimental and theoretical approaches. The aqueous-phase rate constants between O₂^•- and C3-C8 PFCAs were measured using a newly designed in situ spectroscopic system. Mechanistically, bimolecular nucleophilic substitution (S_N2) is most likely to be thermodynamically feasible, as indicated by density functional theory calculations at the CBS-QB3 level of theory. This pathway was then investigated by ab initio molecular dynamics simulation with free-energy samplings. As O₂^•- approaches PFCA, the C-F bond at the alpha carbon is spontaneously stretched, leading to the bond cleavage. The solvation mechanism for O₂^•--mediated PFCA degradation was also elucidated. Our results indicated that although the less polar solvent enhanced the nucleophilicity of O₂^•-, it also decreased the desolvation process of PFCAs, resulting in reduced kinetics. With these quantitative and mechanistic results, we achieved a defined picture of the O₂^•--initiated abatement of PFCAs in natural and engineered waters.