Reactivity of selected volatile organic compounds (VOCs) toward the sulfate radical (SO4?)

ZIF-8 derived defect-rich nitrogen-doped carbon with enhanced catalytic activity for efficient non-radical activation of peroxydisulfate

The non-radical oxidation processes of persulfate activation by carbon materials have shown great potential for industrial and saline wastewater treatment. Recently, metal-organic frameworks (MOFs) as an emerging precursor have been widely used for fabricating functional carbon materials. Herein, we reported ZIF-8 derived defect-rich nitrogen-doped carbon (ZCNs) via NaCl-assisted pyrolysis for efficient non-radical activation of peroxydisulfate to degrade rhodamine B (RB). All samples exhibited excellent catalytic activity, and ZCN-900 (pyrolyzed at 900 °C) was found to be the most active, able to degrade 96 % of RB quickly within 10 min. Quenching tests and electron paramagnetic resonance (EPR) analyses suggested that the singlet oxygen (¹O₂) dominated the degradation process by a non-radical pathway. Furthermore, the effect of anions and water quality on RB oxidation were investigated, and ZCN-900/PDS system showed great resistance to the anions and natural organic matters (NOM). This work may provide a significant addition to MOF-based functional materials for environmental remediation based on the results above.

New insights into FeS/persulfate system for tetracycline elimination: Iron valence, homogeneous-heterogeneous reactions and degradation pathways

In this study, complete tetracycline (TTC) and above 50% of total organic carbon (TOC) were removed by FeS/PS after 30 min under optimized conditions. Although free radicals and high-valent iron ions were identified to generate in the process, the apparent similarity between intermediate products of FeS/PS, Fe/PS, and UV/PS systems demonstrated that the degradation of TTC was due to sulfate radicals (SO₄^⋅-) and hydroxyl radicals (⋅OH). Based on the reaction between free radicals and organic matter, we speculated that TTC in the FeS/PS system was decomposed and mineralized by dehydration, dehydrogenation, hydroxyl addition, demethylation, substitution, E-transfer, and ring-opening. Furthermore, a new understanding of FeS-mediated PS activation based on stoichiometry and kinetic analysis showed that there were both homogeneous and heterogeneous reactions that occurred in the entire progress. However, due to the effect of pH on the dissolution of iron ions, the homogeneous reaction became the principal process with iron ions concentration exceeding 1.35 mg/L. This work provides a theoretical basis for the study of the degradation of TTC-containing wastewater by the iron-based advanced oxidation process.

Accelerated oxidation of the emerging brominated flame retardant tetrabromobisphenol S by unactivated peroxymonosulfate: The role of bromine catalysis and formation of disinfection byproducts

Tetrabromobisphenol S (TBBPS) is an emerging brominated flame retardant (BFR) that can cause endocrinological abnormalities in aquatic species and is neurotoxic and cytotoxic to humans. Herein, we investigated the oxidation of TBBPS by unactivated peroxymonosulfate (PMS) in aqueous solution. Results show that PMS was capable of oxidizing TBBPS without activation, and the transformation of TBBPS was pH-dependent. Interestingly, the unactivated PMS oxidation of TBBPS exhibited an autocatalytic behavior. Radical quenching experiments and electron paramagnetic resonance (EPR) analyzes ruled out the involvement of hydroxyl radical (HO•) and sulfate radical (SO₄^•‑) as reactive species. While the generation of singlet oxygen (¹O₂₎ was confirmed in PMS solution, it was also not responsible for TBBPS oxidation. The bromine substituents are believed to be responsible for the autocatalysis observed during PMS oxidation. We propose that the initial oxidation of TBBPS by PMS resulted in the release of bromide ions (Br^-) via debromination, which could be rapidly oxidized to hypobromous acid (HOBr). 3,5-Dimethyl-1H-pyrazole (DMPZ) trapping coupled with liquid chromatography-mass spectrometry (LC-MS) analysis evidenced the formation of HOBr in PMS/TBBPS system. Therefore, the presence of Br^-, albeit at trace level, could significantly accelerate the oxidation of TBBPS in PMS solution via HOBr formation. The intermediate products of TBBPS were identified by solid phase extraction (SPE) coupled with high resolution-mass spectrometry (HR-MS). The oxidation of TBBPS by unactivated PMS was likely initiated through a single electron transfer mechanism, and the transformation pathways included β-scission, debromination, and cross-coupling reactions. Further oxidation and ring-opening of the intermediates yielded three brominated disinfection byproducts (Br-DBPs), including bromoform (CHBr₃), mono-, and di-bromoacetic acids (MBAA and DBAA), as quantified by gas chromatography (GC). The presence of natural organic matter (NOM) inhibited the oxidation of TBBPS and reduced the yields of Br-DBPs. Our results indicate that unactivated PMS was efficient in the abatement of TBBPS in aqueous solution due to the accelerated oxidation by bromine catalysis; however, the formation of brominated intermediate products and Br-DBPs should be scrutinized due to their potential carcinogenicity and mutagenicity.

Contrasting hydrogen peroxide- and persulfate-driven oxidation systems: Impact of radical scavenging on treatment efficiency and cost

For the first time, the fate of radicals generated in heterogeneous chemical oxidation treatment systems has been accounted for and used to assess treatment performance in three reaction compartments; reaction with the target compound, rhodamine B (RhB), the aqueous phase scavengers, and the solid phase scavengers. Radicals formed during the ultra-violet (UV) activation of hydrogen peroxide (H₂O₂) (UV-AHP) and persulfate (S₂O₈ ^2-) (UV-APS) include hydroxyl (•OH) and sulfate radicals (SO₄ ^•-), respectively. •OH and SO₄ ^•-, used in oxidation treatment systems to degrade a broad spectrum of environmental contaminants, may also react with non-target chemical species (scavengers) that limit treatment efficiency. UV-AHP and UV-APS treatment systems were amended with solid phase alumina to assess scavenging by solid surfaces. The overall rate of reaction and rate of radical scavenging was greater for •OH than SO₄ ^•-. Scavenging by dissolved constituents was dominated by the oxidant used (H₂O₂, S₂O₈ ^2-); and the rate of radical scavenging by alumina was greater than the rate of RhB oxidation in all cases. Treatment efficiency was lower in the UV-AHP than in the UV-APS treatment system and was attributed to greater aqueous and solid phase scavenging rates. The cost of commercially available H₂O₂ ($0.031 mol^-1) and PS ($0.24 mol^-1) was used in conjunction with the overall treatment efficiency to assess specific cost of treatment. The specific cost to treat the probe compound with UV-AHP was greater than UV-APS and was attributed to the much lower treatment efficiency with UV-AHP. The much-desired high reaction rate constants between •OH and environmental contaminants, relative to SO₄ ^•-, may come at the cost of greater combined scavenging rates, and consequently lower treatment efficiency.

Sulfate Radical Scavenging by Mineral Surfaces in Persulfate-Driven Oxidation Systems: Reaction Rate Constants and Implications

Activated persulfate (PS) is a common method used to generate sulfate radicals (SO₄^•-), a powerful oxidant capable of degrading a broad array of environmental contaminants. The reaction of SO₄^•- 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 SO₄^•- scavenging rate constant (k_≡S) for alumina, a naturally occurring mineral in soil and aquifer materials. SO₄^•- 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 SO₄^•- activity. k_≡S for alumina was 2.42 × 10⁴ and 2.03 × 10⁴ m^-2 s^-1 for UV-APS and T-APS oxidative treatment systems, respectively. At [alumina] >5 g L^-1, the reaction of SO₄^•- with solid phase media increased over the aqueous phase reactions with RhB and aqueous scavengers. SO₄^•- 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 SO₄^•-.

Aqueous Reactions of Sulfate Radical-Anions with Nitrophenols in Atmospheric Context

Nitrophenols, hazardous environmental pollutants, react promptly with atmospheric oxidants such as hydroxyl or nitrate radicals. This work aimed to estimate how fast nitrophenols are removed from the atmosphere by the aqueous-phase reactions with sulfate radical-anions. The reversed-rates method was applied to determine the relative rate constants for reactions of 2-nitrophenol, 3-nitrophenol, 4-nitrophenol, 2,4-dinitrophenol, and 2,4,6-trinitrophenol with sulfate radical-anions generated by the autoxidation of sodium sulfite catalyzed by iron(III) cations at ~298 K. The constants determined were: 9.08 × 108, 1.72 × 109, 6.60 × 108, 2.86 × 108, and 7.10 × 107 M−1 s−1, respectively. These values correlated linearly with the sums of Brown substituent coefficients and with the relative strength of the O–H bond of the respective nitrophenols. Rough estimation showed that the gas-phase reactions of 2-nitrophenol with hydroxyl or nitrate radicals dominated over the aqueous-phase reaction with sulfate radical-anions in deliquescent aerosol and haze water. In clouds, rains, and haze water, the aqueous-phase reaction of 2-nitrophenol with sulfate radical-anions dominated, provided the concentration of the radical-anions was not smaller than that of the hydroxyl or nitrate radicals. The results presented may be also interesting for designers of advanced oxidation processes for the removal of nitrophenol.

Rate constants of sulfate radical anion reactions with organic molecules: A review

The rate constants of sulfate radical anion reaction (k_SO4-) with about 230 organic molecules of environmental interest are tabulated and discussed, together with both the methods of rate constant determinations and the reaction mechanisms. k_SO4-'s were collected from the original publications. The highest values in the ∼10⁹ M^-1 s^-1 range are published for aromatic molecules. There is a tendency that electron donating substituents increase and electron withdrawing substituents decrease these values. There are just a few compounds with rate constants established using different techniques in different laboratories. k_SO4-'s determined in different laboratories by the direct techniques, pulse radiolysis or laser flash photolysis, in most cases agree reasonably. The values determined by competitive experimental techniques, by complex kinetics calculations, or by modelling show a large scatter. Some of these techniques seem to be questionable for k_SO4- determination. The sulfate radical anion reacts with ketone and amine moieties of molecules by electron transfer. The same mechanism is also suggested for the reaction with aromatic rings. However, in a few cases addition to the double bond and sulfate anion elimination reactions were distinguished. A typical reaction with the aliphatic parts of the molecule is H-abstraction.

Thermochemical degradation of furfural by sulfate radicals in aqueous solution: optimization and synergistic effect studies

In this study, thermochemical degradation of furfural by sulfate radical has been investigated to find the best-operating conditions. For this purpose, the response surface methodology (RSM) based on central composite design (CCD) was applied to optimize the five independent variables of thermally activated persulfate (TAP)/nZVI oxidation process including pH, PS concentration, furfural concentration, nZVI dosage, and heat. The ANOVA results ("P > F value" < 0.0001 and [Formula: see text] = 0.9701) showed the obtained quadratic model is acceptable to predict furfural removal. Based on the reduced quadratic model PS concentration, nZVI dosage, and heat revealed the positive effects on removal efficiency, while pH and furfural concentration had a negative effect. Accordingly, 98.4% of furfural could be removed within 60 min of reaction under the optimum conditions: pH 5.26, PS concentration of 20.52 mM, furfural concentration of 84.32 mg/L, nZVI dosage of 1.15 mg/L, and a temperature of 79 °C. In such circumstances, the furfural removal efficiency for TAP, PS/nZVI, PS, and nZVI was 94.5, 9, 3, and 2%, respectively. Therefore, based on the synergy index (SI) values, the combination of PS, nZVI, and heat can lead to a synergistic effect in the performance of the thermochemical process.

Quantitative Structure--Activity Relationship (QSAR) for the Oxidation of Trace Organic Contaminants by Sulfate Radical

The sulfate radical anion (SO4•–) based oxidation of trace organic contaminants (TrOCs) has recently received great attention due to its high reactivity and low selectivity. In this study, a meta-analysis was conducted to better understand the role of functional groups on the reactivity between SO4•– and TrOCs. The results indicate that compounds in which electron transfer and addition channels dominate tend to exhibit a faster second-order rate constants (kSO4•–) than that of H–atom abstraction, corroborating the SO4•– reactivity and mechanisms observed in the individual studies. Then, a quantitative structure activity relationship (QSAR) model was developed using a sequential approach with constitutional, geometrical, electrostatic, and quantum chemical descriptors. Two descriptors, ELUMO and EHOMO energy gap (ELUMO–EHOMO) and the ratio of oxygen atoms to carbon atoms (#O:C), were found to mechanistically and statistically affect kSO4•– to a great extent with the standardized QSAR model: ln kSO4•– = 26.8–3.97 × #O:C – 0.746 × (ELUMO–EHOMO). In addition, the correlation analysis indicates that there is no dominant reaction channel for SO4•– reactions with various structurally diverse compounds. Our QSAR model provides a robust predictive tool for estimating emerging micropollutants removal using SO4•– during wastewater treatment processes.

Sulfate radical-based water treatment in presence of chloride: formation of chlorate, inter-conversion of sulfate radicals into hydroxyl radicals and influence of bicarbonate

Sulfate radical (SO4(-)) based oxidation is discussed as a potential water treatment option and is already used in ground water remediation. However, the complex SO4(-) chemistry in various matrices is poorly understood. In that regard, the fast reaction of SO4(-) with Cl(-) is of high importance since Cl(-) belongs to the main constituents in aqueous environments. This reaction yields chlorine atoms (Cl) as primary products. Cl initiate a cascade of subsequent reactions with a pH dependent product pattern. At low pH (<5) formation of chlorine derived oxidation products such as chlorate (ClO3(-)) is favoured. This is undesired because ClO3(-) may reveal adverse effects on the environment and human health. At pH > 5 Cl mainly react with water yielding hydroxyl radicals. Thus, at moderate Cl(-) concentrations (mM range) the SO4(-)-based process may be converted into a conventional (hydroxyl radical -based) advanced oxidation process. The conversion of SO4(-) into OH, however, is interrupted in presence of bicarbonate by scavenging of Cl.

Degradation of chlorotriazine pesticides by sulfate radicals and the influence of organic matter

Atrazine, propazine, and terbuthylazine are chlorotriazine herbicides that have been frequently used in agriculture and thus are potential drinking water contaminants. Hydroxyl radicals produced by advanced oxidation processes can degrade these persistent compounds. These herbicides are also very reactive with sulfate radicals (2.2-3.5 × 10(9) M(-1) s(-1)). However, the dealkylated products of chlorotriazine pesticides are less reactive toward sulfate radicals (e.g., desethyl-desisopropyl-atrazine (DEDIA; 1.5 × 10(8) M(-1) s(-1))). The high reactivity of the herbicides is largely due to the ethyl or isopropyl group. For example, desisopropyl-atrazine (DIA) reacts quickly (k = 2 × 10(9) M(-1) s(-1)), whereas desethyl-atrazine (DEA) reacts more slowly (k = 9.6 × 10(8) M(-1) s(-1)). The tert-butyl group does not have a strong effect on reaction rate, as shown by the similar second order reaction rates between desethyl-terbuthylazine (DET; k = 3.6 × 10(8) M(-1) s(-1)) and DEDIA. Sulfate radicals degrade a significant proportion of atrazine (63%) via dealkylation, in which deethylation significantly dominates over deisopropylation (10:1). Sulfate and hydroxyl radicals react at an equally fast rate with atrazine (k (hydroxyl radical + atrazine) = 3 × 10(9) M(-1) s(-1)). However, sulfate and hydroxyl radicals differ considerably in their reaction rates with humic acids (k (sulfate radical + humic acids) = 6.8 × 10(3) L mgC(-1) s(-1) (mgC = mg carbon); k (hydroxyl radical + humic acids) = 1.4 × 10(4) L mgC(-1) s(-1)). Thus, in the presence of humic acids, atrazine is degraded more efficiently by sulfate radicals than by hydroxyl radicals.

Direct α-arylation of ethers through the combination of photoredox-mediated C-H functionalization and the Minisci reaction

The direct α-arylation of cyclic and acyclic ethers with heteroarenes has been accomplished through the design of a photoredox-mediated CH functionalization pathway. Transiently generated α-oxyalkyl radicals, produced from a variety of widely available ethers through hydrogen atom transfer (HAT), were coupled with a range of electron-deficient heteroarenes in a Minisci-type mechanism. This mild, visible-light-driven protocol allows direct access to medicinal pharmacophores of broad utility using feedstock substrates and a commercial photocatalyst.

Aqueous-phase secondary organic aerosol and organosulfate formation in atmospheric aerosols: a modeling study

We have examined aqueous-phase secondary organic aerosol (SOA) and organosulfate (OS) formation in atmospheric aerosols using a photochemical box model with coupled gas-phase chemistry and detailed aqueous aerosol chemistry. SOA formation in deliquesced ammonium sulfate aerosol is highest under low-NO(x) conditions, with acidic aerosol (pH = 1) and low ambient relative humidity (40%). Under these conditions, with an initial sulfate loading of 4.0 μg m(-3), 0.9 μg m(-3) SOA is predicted after 12 h. Low-NO(x) aqueous-aerosol SOA (aaSOA) and OS formation is dominated by isoprene-derived epoxydiol (IEPOX) pathways; 69% or more of aaSOA is composed of IEPOX, 2-methyltetrol, and 2-methyltetrol sulfate ester. 2-Methyltetrol sulfate ester comprises >99% of OS mass (66 ng m(-3) at 40% RH and pH 1). In urban (high-NO(x)) environments, aaSOA is primarily formed via reversible glyoxal uptake, with 0.12 μg m(-3) formed after 12 h at 80% RH, with 20 μg m(-3) initial sulfate. OS formation under all conditions studied is maximum at low pH and lower relative humidities (<60% RH), i.e., when the aerosol is more concentrated. Therefore, OS species are expected to be good tracer compounds for aqueous aerosol-phase chemistry (vs cloudwater processing).

Involvements of chloride ion in decolorization of Acid Orange 7 by activated peroxydisulfate or peroxymonosulfate oxidation

The effects of chloride anion (Cl-) (up to 1.0 mol/L) on the decolorization of a model compound, azo dye Acid Orange 7 (AO7), by sulfate radical (SO4-*) based-peroxydisulfate (PS) or peroxymonosulfate (PMS) oxidation under various activated conditions (UV254 nm/PS, Thermal (70 degrees C/PS, UV254 nm/PMS, Co2+/PMS) were investigated. Methanol and NH4+ were used as quenching reagents to determine the contributions of active chlorine species (dichloride radical (Cl2-*) and hypochlorous acid (HClO)). The results indicated that the effects of Cl- on the reaction mechanism were different under various activated conditions. For UV/PS and Thermal/PS, the inhibition tendency became more clear as the Cl- concentration increased, probably due to the reaction between Cl- and SO4-* and the generation of Cl2-* or HCIO. For UV/PMS, Cl- did not exhibit inhibition when the concentration was below 0.1 mol/L. As Cl- concentration reached to 1.0 mol/L, the decolorization rate of AO7 was, however, accelerated, possibly because PMS directly reacts with Cl- to form HClO. For Co2+/PMS, Cl- exhibited a significant inhibiting effect even at low concentration (< or = 0.01 mol/L). When Cl- concentration exceeded 0.1 mol/L, the activation of PMS by Co2+ was almost completely inhibited. Under this condition, HClO maybe played a major role in decolorization of AO7. The results implicated that chloride ion is an important factor in SO4(-*) -based degradation of organic contamination in chloride-containing water.

Aqueous Phase Reactivity of Nitrate Radicals (NO3) Toward Dicarboxylic Acids

Laser photolysis technique was used to study the reactivity of nitrate radical towards four dicarboxylic acids. The temperature dependence of the reactions was investigated in the range from 278K to 318K. The effect of the acid-base equilibrium was examined by measuring the activation parameters of the dissociated and undissociated acids at different pH. For droplets or aerosols under moderate acidic conditions, the charge exchange reaction at the carboxylate group might compete with the oxidation by the hydroxyl radical. On liquid particles enriched in nitrate (such as a deliquescent ammonium nitrate), the lifetime of carboxylic groups towards the NO3 radical may be as short as few hours (or less).

Decomposition of polycyclic aromatic hydrocarbons in atmospheric aqueous droplets through sulfate anion radicals: an experimental and theoretical study

Polycyclic aromatic hydrocarbons (PAHs) are environmental pollutants that have received considerable attention because of their carcinogenic and mutagenic effects. PAHs can be degraded by sulfate anion radicals in atmospheric aqueous droplets. This study was to investigate the mechanism and degradation products of sulfate anion radical reaction with anthracene (ANT) by experimental and quantum chemical approaches. From these observations of the experiments, the sulfate anion radical is capable of oxidizing ANT rapidly and three intermediates anthraquinone (ATQ), 1-hydroxyanthraquinone (1-hATQ), and 1,4-dihydroxyanthraquinone (1,4-dhATQ) were detected as degradation products by GC-MS. The proposed one-electronic transfer mechanism of sulfate anion radical reaction with ANT was modeled using hybrid density function theory (BHandHLYP) methods. Geometry optimization and vibrational frequency analysis calculation were performed for reactants, transition states, intermediates, and products. The potential energy surfaces of these reactions are explored to establish structures and relative energies of reactants, intermediates, transition states, and products. Computational results suggest that initial electron transfer step is predicted to have activation energy of -3.35 kcal/mol in water, indicating that ANT can be oxidized quickly in atmospheric aqueous droplets. The reaction pathways have been proposed on the basis of these experimental and theoretical findings. The results may provide useful information for a better understanding of the sulfate anion radical-initiated reactions in atmospheric aqueous droplets such as clouds, rains or fogs.