Carbon dioxide radical anion-based UV/S2O82−/HCOOH reductive process for carbon tetrachloride degradation in aqueous solution

Carbon dioxide radical anion mediated dehalogenation kinetics and mechanisms of halogenated alkanes

Carbon dioxide radical anion (CO2•-) recently becomes appreciated in halogenated contaminants elimination; nevertheless, its application has been restricted by insufficient mechanistic understanding. Herein, we provided a quantitative insight into the kinetics and mechanisms of CO2•- mediated dehalogenation of halogenated alkanes. A CO2•- dominated UV254/H2O2/HCOO- system has been successfully established and demonstrated for effective elimination of 7 kinds of halogenated alkanes (71.3 % to 100 % of removal). Using a laser flash photolysis technology, the second-order rate constants of CO2•- ( [Formula: see text] ) reacting with CCl4, CHCl3 and CH2Cl2 were firstly reported, to be 2.5 × 108, 6.2 × 107 and 5.8 × 106 M-1s-1, respectively. [Formula: see text] presented a significant negative correlation with the lowest unoccupied molecular orbital energy (ELUMO) of chlorinated alkanes, proving that the enhanced dehalogenation of CO2•- was attributed by direct electron transfer mechanism. A fitting model was developed accordingly for [Formula: see text] prediction. This study also demonstrated that the CO2•- mediated ARP effectively removed halogenated alkanes regardless of pH condition (6.0∼9.0) and bicarbonate concentrations. These findings are significant in advancing the scientific understanding of CO2•- mediated ARP. This reductive process a promising control strategy for halogenated contaminants, such as polyfluoroalkyl substances (PFAS) and halogenated pharmaceuticals.

Surfactant enhanced persulfate system for the synergistic oxidation and reduction of mixed chlorinated hydrocarbons

Surfactant-enhanced in-situ chemical oxidation (S-ISCO) is widely applied in soil and groundwater remediation. However, the role of surfactants in the reactive species (RSs) transformation remains inadequately explored. This work introduced nonionic surfactant Tween-80 (TW-80) into a nano zero-valent iron (nZVI) activated persulfate (PS) system. The findings indicate that PS/nZVI/TW-80 system can realize the concurrent removal of trichloroethylene (TCE), tetrachloroethene (PCE), and carbon tetrachloride (CT), whereas CT cannot be eliminated without TW-80 presence. Further analysis unveiled that hydroxyl (HO•) and sulfate radicals (SO4－•) were the primary species for TCE and PCE degradation, while CT was reductively eliminated by surfactant radicals generated from TW-80. Moreover, the surfactant radicals were found to accelerate Fe(III)/Fe(II) cycle, reduce the production of iron sludge, and increase PS decomposition. The possible degradation routes of mixed chlorinated hydrocarbons (CHCs) and the decomposition pathways of TW-80 were proposed through the density function theory (DFT) calculation and intermediates analysis. Additionally, the effects of other nonionic surfactants on the simultaneous removal of TCE, PCE, and CT, and the practical applications using the actual contaminated groundwater were also evaluated. This study provides theoretical support for the simultaneous removal of CHCs, particularly those containing perchlorinated contaminants, using the S-ISCO techniques.

Electrochemical Redox Conversion of Formate to CO via Coupling Fe-Co Layered Double Hydroxides and Au Catalysts

Formate has been considered an inactive molecule and thus cannot be further reduced under CO2 reduction conditions, which limits its widespread application as feedstock. Here we present an electrochemical redox conversion of formate to CO through the potential-dependent generation of carbon dioxide radical anions (CO2 ⋅- ) on Fe-Co layered double hydroxides (Fe-Co LDHs) and the subsequent reduction of CO2 ⋅- to CO on Au catalysts. We present an electrodeposition protocol for the synthesis of Fe-Co LDHs with precise composition control and find that Fe1 Co4 exhibits a promising potential window for CO2 ⋅- formation between 1.14 and 1.4 V and an optimized potential at 1.24 V at a neutral pH condition. We further determined the formation of CO2 ⋅- at 1.24 V via electron paramagnetic resonance and CO2 at >1.4 V through differential electrochemical mass spectrometry. This work provides a redox chemistry route for converting formate into CO through a coupled slit parallel-plate electrode system.

Enhanced photocarrier separation in Br substitution-induced [W(VI)O6-x] units for highly efficient photocatalytic nitrate reduction under alkaline conditions

Photocatalytic nitrate (NO3-) reduction is considered a promising green and non-polluting technology to solve the nitrate pollution of groundwater and surface water. Herein, a novel Br-substituted Bi2WO6-x ultrathin nanosheets were prepared by a simple hydrothermal method in a strong acid environment containing sixteen alkyl three methyl bromide (CTAB). The catalytic system solves the problems of low carrier separation efficiency, poor performance under alkaline conditions, and a hard-to-activate N = O bond, achieving efficient NO3- removal under alkaline conditions along with high N2 selectivity. It was confirmed that Br-substituted Bi2WO6-x produced the [W(VI)O6-x] units with a strong electron-withdrawing property by changing the polarity of the O-W-O bond. As a result, the effective space charge separation caused by the change of the W valence state and the spontaneous fracture behavior of the N = O bond improved the carriers utilization efficiency and distinctly reduced the reaction energy consumption, synergistically achieving excellent performance.

Decreasing dissolved oxygen enhances in situ curtailment of intermediate Cr(VI) during photo-oxidative decomplexation of Cr(III)-EDTA

Cr(III)-organic complexes are stably presented in tanning, electroplating, and other industrial wastewaters, and their safe and efficient removal remains a current challenge. Available oxidation processes can remove Cr(III) complexes but readily result in highly toxic Cr(VI) accumulation. Herein, negligible Cr(VI) accumulation was achieved during photo-oxidation of Cr(III) complexes using a simple strategy of decreasing dissolved oxygen (DO). At the DO concentration of 5.0 mg·L^-1 or less, the in-process formation of intermediate Cr(VI) was totally abated by in situ formed reductive species, and total Cr was reduced from 9.0-11.0 mg·L^-1 to below 1.0 mg·L^-1. A complete curtailment of Cr(VI) was observed after 30-60 min at pH 6.0-9.0. Increasing Cr(III)-EDTA concentration and decreasing pH value facilitated the in situ reduction of intermediate Cr(VI). Based on the identification of intermediates and additional Cr(II) and quenching experiments, the possible key species involved in intermediate Cr(VI) reduction were the photogenerated Cr(II) and some C-centered radicals from Cr(III)-EDTA decomplexation, and the possible mechanisms of Cr(III)-EDTA decomplexation and intermediate Cr(VI) reduction were thus proposed. The process also showed efficient treatment on other Cr(III) complexes (citrate, oxalate, and tartrate) and realistic Cr(III) complexed wastewater. This study would provide an insignificant Cr(VI)-accumulated alternative for efficient and safe removal of Cr(III) complexes from contaminated water.

Reductive Removal and Recovery of As(V) and As(III) from Strongly Acidic Wastewater by a UV/Formic Acid Process

The removal of arsenic (As(V) and As(III)) from strongly acidic wastewater using traditional neutralization or sulfuration precipitation methods produces a large amount of arsenic-containing hazardous wastes, which poses a potential threat to the environment. In this study, an ultraviolet/formic acid (UV/HCOOH) process was proposed to reductively remove and recover arsenic from strongly acidic wastewater in the form of valuable elemental arsenic (As(0)) products to avoid the generation of hazardous wastes. We found that more than 99% of As(V) and As(III) in wastewater was reduced to highly pure solid As(0) (>99.5 wt %) by HCOOH under UV irradiation. As(V) can be efficiently reduced to As(IV) (H₂AsO₃ or H₄AsO₄) by hydrogen radicals (H^•) generated from the photolysis of HCOOH through dehydroxylation or hydrogenation. Then, As(IV) is reduced to As(III) by H^• or through its disproportionation. The reduction of As(V) to H₄AsO₄ by H^• and the disproportionation of H₄AsO₄ are the main reaction processes. Subsequently, As(III) is reduced to As(0) not only by H^• through stepwise dehydroxylation but also through the disproportionation of intermediate arsenic species As(II) and As(I). With additional density functional theory calculations, this study provides a theoretical foundation for the reductive removal of arsenic from acidic wastewater.

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.

Overlooked Formation of H2O2 during the Hydroxyl Radical-Scavenging Process When Using Alcohols as Scavengers

Hydroxyl radical (•OH) is an active species widely reported in studies across many scientific fields, and hence, its reliable analysis is vitally important. Currently, alcohols are commonly used as scavengers for •OH determination. However, the impacts of alcohols on the reliability of •OH detection remain unknown. In this study, we found that adding different types and different amounts of alcohols in water samples treated with ultraviolet irradiation undesirably produced substantial amounts of hydrogen peroxide (H₂O₂), which is a known •OH precursor. This means that the conventional •OH determination method using alcohols is likely unreliable or even misleading. Through careful investigation, we revealed an overlooked reaction pathway during H₂O₂ and •OH transformations. Varying oxygen concentrations, pHs, alcohol dosages, and types altered H₂O₂ formation, which can affect •OH determination accuracy. Among alcohols, n-butanol is the best scavenger because it quenches •OH rapidly but re-forms little H₂O₂.

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.

Electrostatic self-assembly to form unique LiNbO3/ZnS core-shell structure for photocatalytic nitrate reduction enhancement

Photocatalytic NO₃^- reduction in water has been regarded as a promising route due to its high efficiency and green feature. Several limiting factors, such as lack of catalytic sites, insufficient light collection, and spatial charge separation capacity photocatalytic denitrification, still need to be overcome for the practical applications. Herein, an innovative LiNbO₃/ZnS heterojunction with a unilateral opening core-shell structure was constructed. ZnS was tightly anchored on the surface of LiNbO₃ by modified electrostatic self-assembly method. High nitrate removal rate (98.84%) and N₂ selectivity (98.92%) were achieved with a molar ratio of LiNbO₃ and ZnS of 1:5 (1:5L-ZS) using formic acid as a hole scavenge. The LiNbO₃/ZnS degradation kinetics of NO₃^- was corresponding to the first-order kinetics equation. The nitrate removal rate and N₂ selectivity remained stable after three cycles in such photocatalytic NO₃^- reduction. The outstanding photocatalyst performance can be ascribed to the improved surface active sites, the well-matched band structure, and the unique core-shell structure. It provides an effective strategy for controllable fabrication of core-shell photocatalyst with strong light-harvesting ability and charge separation efficiency to enhance the removal rate of nitrate in water.

Non-innocent Radical Ion Intermediates in Photoredox Catalysis: Parallel Reduction Modes Enable Coupling of Diverse Aryl Chlorides

We describe a photocatalytic system that elicits potent photoreductant activity from conventional photocatalysts by leveraging radical anion intermediates generated in situ. The combination of an isophthalonitrile photocatalyst and sodium formate promotes diverse aryl radical coupling reactions from abundant but difficult to reduce aryl chloride substrates. Mechanistic studies reveal two parallel pathways for substrate reduction both enabled by a key terminal reductant byproduct, carbon dioxide radical anion.

A microwave irradiation-persulfate-formate system for achieving the detoxification and alkali-activated composite geopolymerization of the chromate-contaminated soil

A microwave (MA) irradiation-persulfate-formate system was constructed to detoxify Cr contamination and solidify the geopolymerization of the alkali-activated composite material. Three series of experiments were correspondingly conducted to evaluate the treatment for the chromate-contaminated soil. The changes in the molar ratios of formate to persulfate and the mass rates of fortifier to soil led to a significantly greater reduction of Cr^VI in the detoxification experiments. The increase of blast furnace slag from 50% to 80% in the composite cementitious materials (CCM) intensified the immobilization efficiencies of chromate and the compressive strengths of geopolymer blocks. MA irradiation potentially enhanced the binding of Ca cations to the aluminosilicate compounds. The degree of reaction in the phenomenological kinetics model mathematically verified the geopolymerization process. Ettringite was formed within the structure of the geopolymer in the coupling system. Sulfate radicals released from persulfate not only contributed to the detoxification process but also strengthened the immobilization process.

Photoreductive degradation of CCl4 by UV-Na2SO3: influence of various factors, mechanism and application

Due to the strong electron-withdrawing nature of Cl atom in CCl₄, CCl₄ could not readily be degraded by oxidation process. In the present study, aqueous electron (e_aq ^-), a powerful reducing agent generated in UV-Na₂SO₃ system, was applied to reductively degradation of CCl₄. The effects of several crucial factors (e.g. Na₂SO₃ concentration, solution pH, inorganic ions and NOM) on CCl₄ degradation as well as degradation mechanism and pathway were systematically investigated. Results indicated that CCl₄ was efficiently degraded in UV-Na₂SO₃ system and the process could be well described by pseudo-first order kinetic model. The degradation rate increased with the elevated Na₂SO₃ concentration (0-10 mmol/L) and solution pH (6.0-8.0), while remained approximately constant in alkaline conditions (pH = 8.0, 9.0 and 10.0). Nevertheless, O₂, inorganic ions and NOM exerted a negative effect on CCl₄ degradation and the removal efficiency of CCl₄ in groundwater was only 31.7%. Mechanistic study implied that degradation of CCl₄ was primarily induced by e_aq ^-. CCl₄ (10 mg/L) was almost completely dechlorinated within 60 min and the predominant intermediate products were CHCl₃, C₂Cl₄ and C₂HCl₃. CHCl₃ and CH₂Cl₂ were also rapidly degraded in the UV-Na₂SO₃ system.

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

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

Carbon dioxide anion radical as a tool to enhance lignin valorization

Lignin is particularly recalcitrant for valorization via the existing pretreatment methods due to its complex cross-linking polymeric network. The aim of this study is to develop a novel integrated pretreatment strategy to exploit the potential of lignocellulosic biomass as resource for production of biofuels and aromatic chemicals. In this regard, a novel UV/TiO₂/HCOOH reaction was proposed to systematically generate hydroxyl radical (OH) and carbon dioxide radical anion (CO₂^-) to depolymerize lignin. Usage of 2,3-dihydrobenzofuran as a simple probe molecule showed cleavage β-O-4 linkage occurred via H abstraction mechanism. The addition of methyl viologen as CO₂^- scavengers proved the presence of CO₂^- in this UV/TiO₂/HCOOH reaction. Lignin and wheat straw were used to investigate the effect of different parameters, including formic acid concentration and TiO₂ dosage, on the efficiency of the reaction. At optimized conditions, the highest phenolic concentrations attained were 173.431 and 66.802 mg/g lignin and wheat straw, respectively. A cycle test was designed with the aim to favor the complete consumption of formic acid through more pretreatment cycles for producing the highest possible Total Phenolic Compounds (TPC) in the liquid phase. After the third consecutive cycle, 103.651 ± 5.964 mg-TPC/g, was obtained. Meanwhile it was found the remaining wheat straw solid fibers used for biogas production, showed 11.0% increase biogas production and increased degradation rate compared to the untreated wheat straw.

Enhanced redox degradation of chlorinated hydrocarbons by the Fe(II)-catalyzed calcium peroxide system in the presence of formic acid and citric acid

Two carboxylic acids (formic acid (FA) and citric acid (CIT)) enhanced the Fenton process using Fe(II)-activated calcium peroxide (CP) to develop a hydroxyl (HO) and carbon dioxide radical (CO₂^-) coexistence process for the simultaneous redox-based degradation of three chlorinated hydrocarbons (CHs), namely carbon tetrachloride (CT), tetrachloroethene (PCE), and trichloroethene (TCE), was investigated. The experimental results showed that CT removal was increased while PCE and TCE degradation were decreased with the addition of FA to the Fe(II)/CP system. However, addition of CIT to the Fe(II)/CP/FA system enhanced the removal efficiency of all three contaminants. For example, 81.7%, 79.4%, and 96.1% of CT, PCE, and TCE, respectively, were removed simultaneously under the optimal molar ratio of 12/12/12/12/1 of CIT/CP/Fe(II)/FA/CHs. Mechanism study confirmed the specific roles of HO and secondarily generated CO₂^- radical. PCE and TCE were degraded oxidatively by HO while CT was degraded via reductive dechlorination by CO₂^-. Carbonate reduced PCE and TCE degradation in actual groundwater as it consumed reactive oxygen species, whereas humic acid and neutral pH had minimal impact on contaminant removal. These results can help us better understand the synergistic effects of carboxylic acids in the modified Fenton process for the redox degradation of refractory chlorinated hydrocarbons.

Mechanism of carbon tetrachloride reduction in ferrous ion activated calcium peroxide system in the presence of methanol

This study investigated the reductive initiation for the depletion of highly oxidized/perhalogenated pollutants, specifically the degradation of carbon tetrachloride (CT) was induced by adding methanol (MeOH) into a ferrous ion (Fe(II)) activated calcium peroxide (CaO₂) system. The results indicated that CT could be completely degraded within 20 min at CaO₂/Fe(II)/MeOH/CT molar ratio of 30/40/10/1 in this system. Scavenging tests suggested that both superoxide radical anion (O₂ ^•-) and carbon dioxide radical anion (CO₂ ^•-) were predominant reactive species responsible for CT destruction. Hydroxymethyl radicals (^•CH₂OH), an intermediate in the transformation of MeOH, could also initiate CT degradation by reducing C-Cl bond. GC/MS analysis identified CHCl₃, C₂Cl₄, and C₂Cl₆ as the intermediates accompanied by CT destruction, and a reduction mechanism for CT degradation was proposed accordingly. In addition, the impact of solution matrix and initial solution pH were evaluated, and the results showed that Cl^-, NO₃ ^-, and HCO₃ ^- had adverse effects on CT degradation. Moreover, the alkaline condition was unfavorable to CT depletion. In conclusion, the results obtained in the actual groundwater tests encouragingly demonstrated that the CaO2/Fe(II)/MeOH process is a highly promising technique for the remediation of CT-contaminated groundwater.

Highly Efficient Degradation of Tartrazine with a Benzoic Acid/TiO2 System

The roles of benzoic acid and its derivatives in the photocatalytic degradation of tartrazine (TZ) by titanium dioxide have been studied. A series of comparative experiments were carried out, such as the experimental comparisons of concentrations, pH values, effects on the para-position of benzoic acid, gas atmospheres, and different target pollutants. It should be noted that the degradation rate of TZ solution without benzoic acid and benzoic acid after degradation for 90 min was 28.69 and 99.08%, respectively. The reason for the above results is that benzoic acid acts as an electron donor to react with photogenerated holes, suppressing the recombination of photogenerated holes and electrons, and thus causing a significant increase in the degradation rate. Moreover, the degradation process is mainly induced by O₂ ^•- and photogenerated holes (h⁺). It is the first time that the benzoic acid/TiO₂ system has been used to degrade the TZ dye. In addition, the benzoic acid/TiO₂ system is also suitable for the degradation of other organic dyes such as methyl orange, rhodamine B, methylene blue, and methyl violet.

Degradation of naphthenic acid model compounds in aqueous solution by UV activated persulfate: Influencing factors, kinetics and reaction mechanisms

Naphthenic acids (NAs) are one of the constituents of concerns in oil sands process water (OSPW) because of their persistence and recalcitrance. Herein, we investigated the degradation of five model NA compounds by UV-activated persulfate (UV/persulfate) process under medium-pressure UV lamp irradiation at pH 8.0. UV/persulfate process showed higher degradation efficiency towards cyclohexanoic acid (CHA) compared to UV/H₂O₂ process under the same experimental conditions. CHA (0.39 mM) was completely removed within 30 min when 2 mM persulfate was used as oxidant, while more than 60 min were needed for the UV/H₂O₂ process. The removal of CHA decreased from 100% to 10% when 300 mM tert-butyl alcohol (TBA) was used as the scavenger, indicating that hydroxyl radical (OH) was responsible for the CHA degradation in the UV/persulfate process. Sulfate (SO₄^-) radicals reacted slowly with CHA in the UV/persulfate process with a second-order rate constant of k = 5.3 × 10⁷ M^-1s^-1. Relative kinetics studies using binary mixtures of model NA compounds showed similar structure-reactivity to that under UV/H₂O₂ process. NAs with long carbon chain, cyclic ring, and aromatic ring were more reactive in the UV/persulfate process. The presence of high concentration of chloride ions dramatically inhibited the reaction. The OH radicals in the UV/persulfate process were generated by capturing OH^- in solutions, as evidenced by the decrease of the pH value from 8.0 to 2.8 before and after treatment, respectively, in a pure water matrix. Primary intermediate products (oxy-CHA, hydroxyl-CHA, and dihydroxyl-CHA) of UV/persulfate process were confirmed by UPLC-MS.

Contribution of alcohol radicals to contaminant degradation in quenching studies of persulfate activation process

Alcohols such as ethanol (EtOH) and tert-butanol (TBA) are frequently used as quenching agents to identify the primary radical species in the persulfate (PS)-based oxidation processes. However, the contribution of alcohol radicals (ARs) to contaminant degradation in this process has rarely been assessed. In this study, trichloroacetic acid (TCA), phenol, and carbon tetrachloride were selected as probes to test the role of ARs in the thermally activated PS system. It was found that the degradation rates of these compounds were largely depended on their reactivities with ARs and the concentration of dissolved oxygen in the reaction system. In the PS/alcohol system, TCA was degraded efficiently under anaerobic conditions, while it was hardly degraded in the presence of oxygen. The results of electron paramagnetic resonance, reducing radical quenching studies, and the analysis of PS consumption suggested that ARs were the dominant reactive species contributing to TCA degradation in the PS/EtOH system under anaerobic conditions. Further studies indicated that ARs could significantly degrade CCl₄ through dechlorination but not phenol. CCl₄ was also degraded efficiently by ARs when oxygen in the reaction solution was completely consumed by ARs. This study highlights the important role of alcohol radicals in the degradation of contaminants during quenching studies in PS-activated processes.

Enhanced effect of HAH on citric acid-chelated Fe(II)-catalyzed percarbonate for trichloroethene degradation

This work demonstrates the impact of hydroxylamine hydrochloride (HAH) addition on enhancing the degradation of trichloroethene (TCE) by the citric acid (CA)-chelated Fe(II)-catalyzed percarbonate (SPC) system. The results of a series of batch-reactor experiments show that TCE removal with HAH addition was increased from approximately 57 to 79% for a CA concentration of 0.1 mM and from 89 to 99.6% for a 0.5 mM concentration. Free-radical probe tests elucidated the existence of hydroxyl radical (HO^•) and superoxide anion radical (O₂^•-) in both CA/Fe(II)/SPC and HAH/CA/Fe(II)/SPC systems. However, higher removal rates of radical probe compounds were observed in the HAH/CA/Fe(II)/SPC system, indicating that HAH addition enhanced the generation of both free radicals. In addition, increased contribution of O₂^•- in the HAH/CA/Fe(II)/SPC system compared to the CA/Fe(II)/SPC system was verified by free-radical scavengers tests. Complete TCE dechlorination was confirmed based on the total mass balance of the released Cl^- species. Lower concentrations of formic acid were produced in the later stages of the reaction for the HAH/CA/Fe(II)/SPC system, suggesting that HAH addition favors complete TCE mineralization. Studies of the impact of selected groundwater matrix constituents indicate that TCE removal in the HAH/CA/Fe(II)/SPC system is slightly affected by initial solution pH, with higher removal rates under acidic and near neutral conditions. Although HCO₃^- was observed to have an adverse impact on TCE removal for the HAH/CA/Fe(II)/SPC system, the addition of HAH reduced its inhibitory effect compared to the CA/Fe(II)/SPC system. Finally, TCE removal in actual groundwater was much significant with the addition of HAH to the CA/Fe(II)/SPC system. The study results indicate that HAH amendment has potential to enhance effective remediation of TCE-contaminated groundwater.