Reactivity and Role of SO5•- Radical in Aqueous Medium Chain Oxidation of Sulfite to Sulfate and Atmospheric Sulfuric Acid Generation

UV/sulfite chemistry to reduce N-nitrosodimethylamine formation in chlor(am)inated water

The disinfection by-product N-nitrosodimethylamine (NDMA) is a major concern in water quality management due to its carcinogenicity. Thus, a proper pretreatment is necessary to mitigate NDMA formation upon periodic chloramination by removing precursors, such as ranitidine (RNT). This study investigated the effect of UV/sulfite pretreatment on NDMA formation from an RNT-spiked tap and chloraminated synthetic swimming pool (SSP) water. At UVC intensity of 2.1 mW cm^-2 and 0.5 mM of sulfite, UV/sulfite chemistry showed complete degradation of 20 µM RNT within 30 min. It was found that SO₄^•- primarily reduced the NDMA formation potential (FP) of RNT, while hydrated electrons effectively mitigated the pre-formed NDMA in the SSP water. The UV/sulfite pretreatment alleviated NDMA formation during post-chloramination (24 h) by up to 82%, outperforming the commonly employed advanced oxidation processes such as UV/H₂O₂. However, in the presence of bromide ions, the effectiveness of UV/sulfite pretreatment was seriously deteriorated, although the bromide ion itself was found to inhibit the NDMA formation from RNT especially at pH < 8 during chloramination. Mass spectrometric analysis indicated that the NDMA-FP of RNT could be removed by UV/sulfite principally via N-methylation, dealkylation, and oxygen transfer pathways. Consequently, UV/sulfite could be used as an alternative unit process for water treatment with reduced NDMA formation.

Degradation of orange II by Fe@Fe2O3 core shell nanomaterials assisted by NaHSO3

Fe@Fe₂O₃ core shell nanomaterials with different Fe₂O₃ shell thickness were synthesized and the Fe@Fe₂O₃/NaHSO₃ Fenton-like system was used for the decomposition of Orange II. The consequences are compared with traditional Fenton Fe@Fe₂O₃/H₂O₂ system. The Fe@Fe₂O₃/NaHSO₃ system showed extremely good applicability under both strongly acidic and alkaline conditions. The new Fe@Fe₂O₃-(2)/NaHSO₃ system led to more than 99% degradation in 30 s when the pH was 3, which indicated that the Fe@Fe₂O₃ material was not corroded during the process even under strongly acidic condition. The above Fe@Fe₂O₃-(2) material was prepared from nano-zero-valent iron aged in solution for 2 h to synthesize the Fe₂O₃ shell. The reaction mechanism of Fe@Fe₂O₃/NaHSO₃ Fenton-like system was also concluded. The oxidation efficiency was highly improved due to rapid electron transfer between Fe core and Fe₂O₃ shell, which promoted the direct recycling of ≡Fe³⁺ and ≡Fe²⁺ and thus accelerated the generation of SO₄^- and OH radicals.

Relative contribution of ferryl ion species (Fe(IV)) and sulfate radical formed in nanoscale zero valent iron activated peroxydisulfate and peroxymonosulfate processes

Activation of persulfates (i.e., peroxydisulfate (PDS) and peroxymonosulfate (PMS)) by nanoscale zero-valent iron (nZVI) is reported to be effective in oxidative treatment of environmental contaminants. It has been widely accepted in numerous literature that sulfate radical (SO₄^•-) formed from the decomposition of persulfates activated by aqueous Fe(II) released from nZVI corrosion is responsible for the oxidative performance in nZVI/persulfates systems. In this work, by employing methyl phenyl sulfoxide (PMSO) as a probe, we demonstrated that the activation of persulfates by nZVI through electron transfer led to SO₄^•- formation, while the homogeneous activation of persulfate by the released Fe(II) resulted in ferryl ion species (Fe(IV)) generation in nZVI/persulfates systems. Similarly, nanoscale zero-valent aluminum (nZVAl) and zinc (nZVZn) were also demonstrated to be able to donate electron to persulfates leading to SO₄^•- formation. However, the insulative aluminum oxide shell hindered the electron transfer leading to the poor persulfates decomposition, while the conductive iron and zinc oxide shell enabled the electron transfer process resulting in a continuous generation of SO₄^•-. Further, it was obtained that the relative contribution of SO₄^•- and Fe(IV) in nZVI/persulfates systems was independent of the initial concentration of nZVI and PDS, but was positively correlated with PMS concentration. In addition, the increase of pH from 3 to 7 led to the decrease of the relative contribution of Fe(IV), which was rationalized by the decrease of availability of aqueous Fe(II) at higher pH. Our findings not only shed lights on the nature of the reactive intermediate formed in the nZVI/persulfates systems, but also unprecedentedly distinguished the surface activation of persulfates from the homogeneous catalysis process.

Mechanistic insight into the generation of reactive oxygen species in sulfite activation with Fe(III) for contaminants degradation

Since the reactive species during the sulfite activation by Fe(III) (Fe(III)/sulfite process) had not been directly determined and the role of in-situ generated Fe(II) was overlooked, this study evaluated the oxidation performance of the Fe(III)/sulfite process, identified the reactive species, and investigated the role of in-situ generated Fe(II) in this process. The results demonstrated that carbamazepine (CBZ) could be degraded at different sulfite concentrations. Compared to the single-dosing mode, sulfite applied with multiple-dosing mode was beneficial to CBZ removal in this process when the same amount of sulfite was dosed. Fe(II) was rapidly generated and then decayed in this process, which were consistent with the trends of CBZ degradation and sulfite consumption. Electron paramagnetic resonance and scavenging experiments showed that SO₄^- was a major oxidant, while HO also played a significant role in CBZ degradation in this process. The tert-butyl alcohol assay indicated that the generation and re-oxidation of Fe(II) was accompanied with the generation of reactive species. Besides sulfite dosage, CBZ degradation was also affected by initial pH, Fe(III) dosage, and CBZ concentration. Cl^- showed little inhibition on CBZ degradation while humic acid inhibited CBZ degradation in this process. This study advances the application of this oxidation system.

Activation of peroxymonosulfate by Fe doped g-C3N4 /graphene under visible light irradiation for Trimethoprim degradation

Fe-doped g-C₃N₄ / graphene (rGO) composites were investigated as catalysts for the activation of peroxymonosulfate (PMS) to degrade Trimethoprim (TMP) under visible light irradiation. The rapid recombination of photogenerated electron-hole pairs in g-C₃N₄ may be suppressed by doping with Fe and incorporating rGO. The TMP degradation efficiency using 0.2% Fe-g-C₃N₄/2 wt% rGO/PMS was 3.8 times than that of g-C₃N₄/PMS. The degradation efficiency of TMP increased with higher catalyst dosages and PMS concentrations. Acidic condition (pH = 3) was observed to significantly enhance the TMP degradation efficiency from 61.4% at pH = 6 to nearly 100%. By quenching experiments and electron spin resonance (ESR), O₂^- was found to play an important role for the activation of PMS to accelerate the generation of reactive radicals for the TMP degradation. A total of 8 intermediates derived from hydroxylation, demethoxylation and carbonylation were identified through theoretical calculations and the HRAM/LC-MS-MS technique, and transformation pathways of TMP oxidation were proposed. TOC removal rate of TMP increased as reaction time was prolonged. Acute toxicity estimation by quantitative structure-active relationship analysis indicated that most of the less toxic intermediates were generated. The aim of this study was to elucidate and validate the functionality of a promising polymeric catalyst for the environmental remediation of organic contaminants.

Influence of [sulfite]/[Fe(VI)] molar ratio on the active oxidants generation in Fe(VI)/sulfite process

Although several groups have made efforts to study micropollutants degradation by Fe(VI)/sulfite process, the mechanism is far from clear and warrants further investigation. Herein, the degradation kinetics and mechanism of selected micropollutants by sulfite (SO₃^2-)-activated Fe(VI) oxidation were systematically investigated. The oxidation rates of enrofloxacin (ENR) and phenol in Fe(VI)/sulfite process ranged from 0.151 s^-1 to 6.18 s^-1 at pH 6.5 and 8.0. Sulfite applied in multiple-addition mode improved the degradation efficiency of micropollutants with electron-rich moieties compared to the single-addition mode. Based on results of the quenching experiments and kinetic simulation, Fe(V) was identified as the predominant active oxidant at [SO₃^2-]/[Fe(VI)] molar ratio of 0.1 to 0.3. However, both Fe(V) and SO₄^-/OH contributed to micropollutants oxidation at [SO₃^2-]/[Fe(VI)] molar ratio ≥ 0.4 and their contributions were strongly dependent on the properties of micropollutants. The different degradation products of ENR in Fe(VI)/sulfite process at different sulfite dosages further supported the contribution of different active oxidants at different [SO₃^2-]/[Fe(VI)] molar ratios. The toxicity of the reaction products of ENR towards Vibrio qinghaiensis sp.-Q67 decreased dramatically after Fe(VI)/sulfite treatment. The results of this work may promote the application of sulfite-activated Fe(VI) oxidation in water treatment.

The impact of wastewater matrix on the degradation of pharmaceutically active compounds by oxidation processes including ultraviolet radiation and sulfate radicals

Ultraviolet radiation (UV)-activated peroxydisulfate (PDS) and peroxymonosulfate (PMS) advanced oxidation processes were examined for their capacity to remove nine pharmaceutically active compounds (PhACs) from secondary effluent. The effect of operational parameters (initial oxidant concentration, UV exposure time, pH, common coexisting anions and effluent organic matter (EfOM)) on UV/PDS and UV/PMS treatment efficiency was investigated in a collimated beam device housing a low-pressure mercury UV lamp emitting light at 253.7 nm. Both AOPs achieved high removals (>90%) when applied to pure water. Under otherwise similar conditions the removal percentage fell by 20-30% due to the scavenging of effluent organic matter (EfOM) in secondary effluent. Finally, eliminating EfOM but maintaining the inorganic composition, the radical scavenging effect was reduced and 98.3% and 85.6% average removals were obtained by UV/PDS and UV/PMS, respectively. Increasing pH improved degradation of several PhACs containing amine groups. Higher oxidant dosages created only a significant benefit in UV/PDS. The chloride anion produced a negligible effect on both processes, while higher nitrate concentrations increased removal percentage but did not affect degradation rate constants. Finally and surprisingly, the addition of bicarbonate had the strongest positive impact on the degradation kinetics observed, even stronger than the elimination of EfOM from secondary effluent.

A potential novel approach for in situ chemical oxidation based on the combination of persulfate and dithionite

Although persulfate (PS) activation has been commonly applied to remove organic contaminants on the subsurface, it is valuable to further explore PS activation methods. In this study, a novel combined process based on PS coupled with dithionite was investigated using trichloroethene (TCE) as a typical organic contaminant. PS/dithionite was demonstrated to be an effective system for TCE degradation depending on the operating parameters such as the initial PS and dithionite dosages. The optimal molar ratio of PS/dithionite/TCE was 5/5/1. Sulfate radicals (SO₄•^-) were the dominant reactive species responsible for TCE degradation in the PS/dithionite system. Two pathways for SO₄•^- generation were proposed in the PS/dithionite system. The generation of SO₄•^- increased in the presence of oxygen but was still effective in an anaerobic environment. This study is the first to report a novel combined process based on PS coupled with dithionite, which is expected to be an efficient and environmentally friendly approach for in situ chemical oxidation (ISCO) remediation of contaminated soil and groundwater, whether in aerobic or anaerobic environments.

Ordered Mesoporous Cobalt Containing Perovskite as a High-Performance Heterogeneous Catalyst in Activation of Peroxymonosulfate

An ordered mesoporous perovskite, La₂CoMnO_6-δ (MLCMO), was synthesized for the first time using a facile method of evaporation-induced self-assembly. The N₂-sorption, scanning electron microscopy, and transmission electron microscopy measurements indicated that the optimized MLCMO possessed a high specific surface area (58.7 m²/g) and was uniformly mesoporous (11.6 nm). The MLCMO exhibited superior catalytic performance in peroxymonosulfate (PMS) activation for atrazine (ATZ) degradation. From a comparison view, the catalytic activity of the mesoporous MLCMO outperformed that of the bulk La₂CoMnO_6-δ (LCMO) and other common PMS activators, including α-MnO₂, Co₃O₄, and CoFe₂O₄. The mechanisms of PMS activation by the MLCMO were investigated by X-ray photoelectron spectroscopy, electron spin resonance, and quenching tests. SO₄^•-, ^•OH, ¹O₂, and O₂^•- were identified as main reactive oxygen species generated from PMS activation. The Co and Mn in MLCMO were the active sites responsible for active radical generation. The lattice oxygen reversible redox sites (O_L^-/O_L^2-), which were involved in the electron transfer of the Mn^III/Mn^IV cycle, were demonstrated as redox partners to the cation active sites. In addition, the SO₄^•-/^•OH radical conversion was promoted at pH 11, which accelerated the consumption of PMS and seriously inhibited the degradation of ATZ.

Surface treatment by the Fe(III)/sulfite system for flotation separation of hazardous chlorinated plastics from the mixed waste plastics

A novel advanced oxidation process by a combination of Fe(III) and sulfite for surface treatment of waste plastic mixtures is proposed. The Fe(III)/sulfite system has been found to enhance hydrophilicity of the mixed waste plastics, including acrylonitrile butadiene styrene (ABS), polystyrene (PS) and polycarbonate (PC), while it has little effect on hazardous polyvinyl chloride (PVC), thus promoting separation of PVC from the mixed waste plastics by flotation. Radical scavenging experiments indicate that sulfate radicals are the main reactive species. Fourier transform infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS) results imply the formation of CO or CO groups on the treated plastics surface except for PVC and a plausible mechanism for oxidizing plastics with sulfate radicals is proposed. PVC with 100.00% recovery and 99.84% purity is achieved under optimum surface treatment conditions of sodium sulfite concentration 10 mM, ferric sulfate concentration 0.4 mM, pH 6.0, temperature 25 °C and treatment time 15 min. Consequently, surface treatment by the Fe(III)/sulfite system is an effective technology for separating hazardous PVC from the mixed waste plastics by flotation.

Nitrate-Enhanced Oxidation of SO2 on Mineral Dust: A Vital Role of a Proton

Heterogeneous oxidation of SO₂ on mineral dust is a significant source of sulfate in the atmosphere. Given that a large fraction of nitrate is deposited on the mineral aerosols, the determination of the effect of nitrate on the SO₂ oxidation on mineral dust and its in-depth mechanism are much desired. In this work, we report nitrate-enhanced SO₂ oxidation on authentic mineral dust. By comparing the SO₂ uptake behaviors on Arizona test dust (ATD, a typical proxy of mineral dust) with or without nitrate, we found that although nitrate hinders the initial SO₂ uptake, it substantially accelerates SO₂ uptake and oxidation after a pronounced induction period. In other words, a hindering-then-accelerating feature in the SO₂ uptake profile was observed on nitrate-containing ATD (N-ATD) particles. In addition, HONO was released in the accelerating period as the reduction product of nitrate. The accumulation of protons (H⁺) from SO₂ oxidation during the induction period plays a key role in the acceleration of SO₂ oxidation. Our work suggests that the nitrate-participating SO₂ oxidation on mineral dust can be one of the important contributions of the sulfate source in the atmosphere.

Degradation of dye in wastewater by Homogeneous Fe(VI)/NaHSO3 system

The homogeneous Fe(VI)/Na₂SO₃ system has been proposed for highly efficient degradation of recalcitrant contaminants, in which sulfite could significantly enhance the transformation of organic substrate by Fe(VI). Also, the Fe(VI)/NaHSO₃ system could show high efficiency across a wide range of pH conditions. The degradation rates reached up to 80% within 2 min and 70% within 5 min in strongly acidic and alkaline conditions, respectively. Unexpectedly, a faster removal rate was obtained in Fe(VI)/NaHSO₃ system than that in Fe(VI)/Na₂SO₃ system for the degradation of methylene blue (MB). A reasonable dye degradation mechanism was proposed and verified by a series of experiments. The high oxidation potential of Fe(VI) and other species such as sulfate and hydroxyl radicals were responsible for the outstanding capabilities of Fe(VI)/NaHSO₃ system, which could significantly improve the treatment of organics in wastewater under a very wide range of pH conditions.

Degradation of nitrilotris-methylenephosphonic acid (NTMP) antiscalant via persulfate photolysis: Implications on desalination concentrate treatment

Nitrilotris-methylenephosphonic acid (NTMP) has been widely used as an antiscalant in reverse osmosis (RO) desalination and other industrial processes to inhibit scaling from calcium and other hardness ions. Removal of NTMP from RO concentrate can induce the precipitation of oversaturated scale-forming substances, enable additional water recovery from RO concentrate, and reduce the risk of eutrophication after brine disposal. This study investigated the kinetics and mechanisms of oxidative degradation of NTMP by UV photolysis of persulfate at 254 nm. Results showed that NTMP was effectively degraded by persulfate photolysis and the reaction followed pseudo first-order kinetics. The degradation of NTMP was favorable at circumneutral pHs but significantly inhibited in highly alkaline conditions (e.g., pH of 11.5), mainly due to the reduced concentration of SO₄^•-. Using a competition reaction kinetics approach, the second-order rate constants of NTMP with SO₄^•- and HO^• were determined to be (2.9 ± 0.6) × 10⁷ M^-1s^-1 and (1.1 ± 0.1) × 10⁸ M^-1s^-1, respectively. SO₄^•- had a predominant contribution to NTMP degradation (62%-95%), because the steady-state concentration of SO₄^•- was 11-54 times higher than that of HO^• at pHs between 4 and 11.5. NTMP degradation rate increased with an increase in persulfate dosage and a decrease in NTMP concentration. In the real RO concentrate, NTMP degradation rate was impacted by the presence of chloride and bicarbonate. The degradation of NTMP started with the cleavage of C-N bonds, and then generated intermediates including iminodi(methylene)phosphonate, hydroxymethylphosphonic acid and aminotris(methylenephosphonic acid), which were eventually mineralized into ammonia, phosphate and carbon dioxide. This study demonstrated that UV/persulfate is a promising technology to remove phosphonate antiscalants from RO concentrate.

Degradation of tris(2-chloroethyl) phosphate (TCEP) in aqueous solution by using pyrite activating persulfate to produce radicals

Organophosphorus esters (OPEs), as one kind of emerging and toxic contaminant are ubiquitous in various environments. This study investigated the degradation of tris(2-chloroethyl) phosphate (TCEP) as a category OPEs by pyrite (FeS₂)-activated persulfate (PS). The result shows that near-100% degradation of TCEP was achieved after 120 min in FeS₂-PS system. The important role of Fe²⁺ in the activation mechanism was confirmed by the introduction of Fe²⁺ into the PS only system. Radical scavengers experiment and electron paramagnetic resonance (EPR) confirmed the presence of SO₄^·- and ·OH，which revealed that ·OH and SO₄^·- played major roles in TCEP degradation. The effect of various environmental factors, including pyrite and oxidant dosage, inorganic ions and pH were investigated. The result indicated that Fe³⁺ and Cl^- can accelerate the degradation rate of TCEP and the reaction between TCEP and FeS₂-PS favors acidic conditions (pH>9). In addition, due to the acidification of pyrite, this system can be applied with a wide pH range. Besides, two oxidation products, C₄H₉Cl₂O₄P and C₂H₆ClO₄P were identified, which suggest that hydroxylation was probably the main mechanism. This study greatly improves our understanding on TCEP removal in FeS₂-PS system.

Mechanism and Kinetic Analysis of Degradation of Atrazine by US/PMS

The degradation effect, degradation mechanism, oxidation kinetics, and degradation products of Atrazine (ATZ) by Ultrasound/Peroxymonosulfate (US/PMS) in phosphate buffer (PB) under different conditions were studied. It turned out that the degradation rate of US/PMS to ATZ was 45.85% when the temperature of the reaction system, concentration of PMS, concentration of ATZ, ultrasonic intensity, and reaction time were 20 °C, 200 μmol/L, 1.25 μmol/L, 0.88 W/mL, and 60 min, respectively. Mechanism analysis showed that PB alone had no degradation effect on ATZ while PMS alone had extremely weak degradation effect on ATZ. HO• and SO₄⁻• coexist in the US/PMS system, and the degradation of ATZ at pH7 is dominated by free radical degradation. Inorganic anion experiments revealed that Cl⁻, HCO₃⁻, and NO₃⁻ showed inhibitory effects on the degradation of ATZ by US/PMS, with Cl⁻ contributing the strongest inhibitory effect while NO₃⁻ showed the weakest suppression effect. According to the kinetic analysis, the degradation kinetics of ATZ by US/PMS was in line with the quasi-first-order reaction kinetics. ETA with concentration of 1 mmol/L reduced the degradation rate of ATZ by US/PMS to 10.91%. Product analysis indicated that the degradation of ATZ by US/PMS was mainly achieved by dealkylation, dichlorination, and hydroxylation, but the triazine ring was not degraded. A total of 10 kinds of ATZ degradation intermediates were found in this experiment.

Degradation of Phenol Using Peroxymonosulfate Activated by a High Efficiency and Stable CoMgAl-LDH Catalyst

In this study, we report on an active and stable CoMgAl layered double hydrotalcite (LDH) catalyst for phenol degradation by heterogeneous activation of peroxymonosulfate (PMS). The CoMgAl-LDH catalyst was synthesized by hydrothermal method. The PMS/CoMgAl-LDH system overcomes the drawbacks of traditional Fenton processes. Various effects, e.g., scavengers, chloride ion, catalyst dosage, PMS concentration, temperature, and pH, were also inspected to evaluate the system. The results indicated that the PMS/CoMgAl-LDH system had extremely high efficiency for phenol degradation; 0.1 mM phenol could be completely degraded by 0.3 g/L catalyst and 3 mM PMS within 60 min at 30 °C. The CoMgAl-LDH catalyst appeared to possess outstanding reusability and stability. After four rounds of recycling, nearly 100% of the phenol was removed within 80 min by the PMS/CoMgAl-LDH system, with only 0.05 mg/L Co2+ leaching. A sulfate radical was the main oxidation species in the PMS/Co-LDH system. The degradation rate of phenol was influenced by temperature, and the activation energy was 65.19 kJ/mol. These advantages proved the PMS/CoMgAl-LDH system is an effective strategy for the treatment of organic contaminants.

Iron-Catalyzed Oxidation of Sulfite: From Established Results to a New Understanding

A survey is made of the iron-catalyzed oxidation of sulfite describing a conceptual framework to explain the key processes involved, with a focus on kinetics. Perhaps most importantly, the incorporation of the HSO5- + Fe(II) step into the regeneration of catalytically active ferric ions which does not deplete its role over the iron redox cycle. The radical-radical recombination SO5-• + SO5-•, which terminates the cycling between ferric and ferrous ions, represents a gross but not a net loss of the chain-carriers, because nearly all of them are reformed through the branching step HSO5- + Fe(II) → Fe2+ + H2O + SO4-•, [Formula: see text] in just a few seconds or somewhat longer. A branching mechanism is thus the only possible means of allowing the catalytic process to reach a stationary state. Observations that may be considered as evidence (fingerprints) of rate variations in sulfite depletion due to the branching mechanism are explored in detail, and the related dynamics of the chain-carriers and metal ions cycles are discussed. In particular, the most important is found to be the aspect related to the intrinsic limitation of the cycle of metal ions. This limitation governs the extent of the oxidative/reducing potential of sulfite solutions with respect to the Fe(III/II) couple, thereby governing the quasi-state partioning between ferric and ferrous ions. Such a view enables examination of those conditions under which the limitation to the rate of the catalytic reaction is controlled by the reduction or re-oxidation of ferric ions. Readily applicable kinetic criteria and kinetic diagrams to delimit the conditions are given. In such a framework, the majority of known anomalies of the catalytic reaction receive an explanation.

Role of Ferrate(IV) and Ferrate(V) in Activating Ferrate(VI) by Calcium Sulfite for Enhanced Oxidation of Organic Contaminants

Although the Fe(VI)-sulfite process has shown great potential for the rapid removal of organic contaminants, the major active oxidants (Fe(IV)/Fe(V) versus SO₄^•-/^•OH) involved in this process are still under debate. By employing sparingly soluble CaSO₃ as a slow-releasing source of SO₃^2-, this study evaluated the oxidation performance of the Fe(VI)-CaSO₃ process and identified the active oxidants involved in this process. The process exhibited efficient oxidation of a variety of compounds, including antibiotics, pharmaceuticals, and pesticides, at rates that were 6.1-173.7-fold faster than those measured for Fe(VI) alone, depending on pH, CaSO₃ dosage, and the properties of organic contaminants. Many lines of evidence verified that neither SO₄^•- nor ^•OH was the active species in the Fe(VI)-CaSO₃ process. The accelerating effect of CaSO₃ was ascribed to the direct generation of Fe(IV)/Fe(V) species from the reaction of Fe(VI) with soluble SO₃^2- via one-electron steps as well as the indirect generation of Fe(IV)/Fe(V) species from the self-decay of Fe(VI) and Fe(VI) reaction with H₂O₂, which could be catalyzed by uncomplexed Fe(III). Besides, the Fe(VI)-CaSO₃ process exhibited satisfactory removal of organic contaminants in real water, and inorganic anions showed negligible effects on organic contaminant decomposition in this process. Thus, the Fe(VI)-CaSO₃ process with Fe(IV)/Fe(V) as reactive oxidants may be a promising method for abating various micropollutants in water treatment.

New insight into the reactivity of Mn(III) in bisulfite/permanganate for organic compounds oxidation: The catalytic role of bisulfite and oxygen

A recently discovered bisulfite(HSO₃^-)/permanganate(MnO₄^-) system was reported to produce highly reactive free Mn(III) that can oxidize organic compounds in milliseconds. However, this characteristic reactivity was not found in all other known reaction systems that can also produce free Mn(III). Why can Mn(III) in NaHSO₃/KMnO₄ be so active? Here, we found NaHSO₃ and O₂ acted as catalysts for the reaction between Mn(III) and organic compounds. Without O₂, 0% of organic compounds were oxidized in NaHSO₃/KMnO₄, indicating the absence of O₂ inactivated Mn(III) reactivity. When the reaction between NaHSO₃ and KMnO₄ was monitored in air, Mn(III) catalyzed rapid oxidation of NaHSO₃ by O₂. Then, the Mn(III) that could oxidize organic compounds was found to be the ones involved in the catalytic reaction between NaHSO₃ and O₂, thus the link between O₂ and Mn(III) reactivity was established. Finally, NaHSO₃/O₂ can be viewed as catalysts for the reaction between Mn(III) and organic compounds because 1) when Mn(III) was involved in oxidizing organic compounds, it stopped being the catalyst for the reaction between NaHSO₃ and O₂ so that they were consumed to a much smaller extent; and 2) without NaHSO₃ and O₂, Mn(III) lost its oxidation ability. To the best of our knowledge, this is the first report on "catalytic role exchange" where Mn(III) is the catalyst for NaHSO₃/O₂ reaction while NaHSO₃/O₂ are the catalysts for Mn(III)/organic compounds reaction. Understanding the critical role of oxygen in NaHSO₃/KMnO₄ will enable us to apply this technology more efficiently toward contaminant removal.

Efficient removal of organic and bacterial pollutants by Ag-La0.8Ca0.2Fe0.94O3-δ perovskite via catalytic peroxymonosulfate activation

Removal of toxic organics and bacterial disinfection are important tasks in wastewater treatment. Most heavy metal-based catalysts for degradation of aqueous organic pollutants in heterogeneous Fenton-like processes suffer from the toxicity of leached metals. The present work reports environmentally benign systems for both degradation of organics and bacterial disinfection. Calcium substituted LaFeO_3-δ perovskite was demonstrated as an efficient catalyst to activate peroxymonosulfate (PMS) for degradation of phenol, methylene blue and rhodamine 6 G. Compared to LaFeO_3-δ and nanocrystal Fe₃O₄, the lattice oxygen vacancies in B-site cation-deficient perovskite of La_0.8Ca_0.2Fe_0.94O₃-_δ (LaCaFeO_3-δ) particles renders this material a greatly improved catalytic performance. Electron paramagnetic resonance (EPR) suggested that both sulfate (SO₄-) and hydroxyl radicals (OH) played critical roles in the advanced oxidation processes. Moreover, silver doped perovskite (Ag-LaCaFeO_3-δ)/PMS successfully inhibited the growth of waterborne pathogen Escherichia coli and Methicillin-resistant Staphylococcus aureus (MRSA) at a lower dose than silver ions, proving a synergetic effect between free radicals and Ag⁺ in killing the bacteria. Therefore, Ag-LaCaFeO_3-δ/PMS would be promising for practical wastewater treatment.

Oxidation of cefalexin by thermally activated persulfate: Kinetics, products, and antibacterial activity change

While the widely used β-lactam antibiotics, such as cephalosporins, are known to be susceptible to oxidation by sulfate radical (SO₄^-), comprehensive study about SO₄^--induced oxidation of cephalosporins is still limited, such as the impact of water matrices, and the structure and antibacterial activity of transformation products. Herein, the oxidation of cefalexin (CFX), a most frequently detected cephalosporin, was systematically investigated by thermally activated persulfate (PS). CFX oxidation followed pseudo-first-order kinetics, and SO₄^- dominantly contributed to the overall oxidation of CFX. The impact of water matrices, such as Cl^-, HCO₃^- and natural organic matter, on CFX degradation was predicted using a pseudo-steady-state kinetic model. The secondary reactive species, such as chlorine and carbonate radicals, were found to contribute to CFX degradation. Product analysis indicated oxidation of CFX to six products (molecular weight of 363), with two stereoisomeric sulfoxides as the primary oxidation products. It was thus suggested that the primary amine on the side chain, and the thioether sulfur and double bond on the six-membered ring were the reactive sites of CFX towards SO₄^- oxidation. Antibacterial activity assessment showed that the biological activity of CFX solution was significantly diminished after treatment by the thermally activated PS.

Oxidation of Organic Compounds in Water by Unactivated Peroxymonosulfate

Peroxymonosulfate (HSO₅^- and PMS) is an optional bulk oxidant in advanced oxidation processes (AOPs) for treating wastewaters. Normally, PMS is activated by the input of energy or reducing agent to generate sulfate or hydroxyl radicals or both. This study shows that PMS without explicit activation undergoes direct reaction with a variety of compounds, including antibiotics, pharmaceuticals, phenolics, and commonly used singlet-oxygen (¹O₂) traps and quenchers, specifically furfuryl alcohol (FFA), azide, and histidine. Reaction time frames varied from minutes to a few hours at pH 9. With the use of a test compound with intermediate reactivity (FFA), electron paramagnetic resonance (EPR) and scavenging experiments ruled out sulfate and hydroxyl radicals. Although ¹O₂ was detected by EPR and is produced stoichiometrically through PMS self-decomposition, ¹O₂ plays only a minor role due to its efficient quenching by water, as confirmed by experiments manipulating the ¹O₂ formation rate (addition of H₂O₂) or lifetime (deuterium solvent isotope effect). Direct reactions with PMS are highly pH- and ionic-strength-sensitive and can be accelerated by (bi)carbonate, borate, and pyrophosphate (although not phosphate) via non-radical pathways. The findings indicate that direct reaction with PMS may steer degradation pathways and must be considered in AOPs and other applications. They also signal caution to researchers when choosing buffers as well as ¹O₂ traps and quenchers.

Reinvestigating the role of reactive species in the oxidation of organic co-contaminants during Cr(VI) reactions with sulfite

Experimental work was undertaken in this study to re-investigate the mechanisms and active species responsible for oxidation of co-contaminants in the Cr(VI)/HSO₃^- reaction system. Batch experiments showed that the degradation rates of 4-chlorophenol (4-CP) correlated well with the rates of Cr(VI) reduction by sulfite in the same solutions, and that O_2(aq) was necessary for the oxidation of 4-CP. Multiple lines of evidences indicate that Cr(VI)/HSO₃^- reaction is a SO₄^--based oxidation process. SO₃^- was generated in Cr(VI)/HSO₃^- system based on the electron spin resonance spectra, which could be transformed to secondary radicals (SO₄^-, SO₅^-, and HO). The contribution of SO₅^- was ruled out through almost complete inhibition of methanol (MeOH) on 4-CP degradation. Considering the negligible inhibition of tert-butanol (TBA) on 4-CP degradation, SO₄^- was identified to be reactive species in Cr(VI)/HSO₃^- process. This result was further verified by almost no degradation of nitrobenzene and the inhibiting effect of Cl^- in Cr(VI)/HSO₃^- process. This mechanism is beneficial to application of Cr(VI)/HSO₃^- system in wastewater treatment.

Degradation of atrazine by electrochemically activated persulfate using BDD anode: Role of radicals and influencing factors

A novel advanced oxidation process using boron-doped diamond (BDD) anode to activate persulfate (PS) with low concentration of electrolyte was systematically investigated in this study. Compared to direct electrochemical oxidation of atrazine (ATZ) using BDD anode, the addition and activation of PS significantly declined the demand for electrolytes. It was confirmed by scavenger experiments that both radical and non-radical oxidation occurred in this system. Degradation of ATZ was enhanced with the increase of current density and dosage of PS, and decrease of initial pH. However, the increase of current density can also lead to the decrease of current efficiency, then increase of energy consumption. Besides, the inhibitory effect of anions on the degradation of ATZ followed the order of HCO₃^->H₂PO₄^->NO₃^-, while the presence of Cl^- accelerated the degradation of ATZ. Furthermore, the degradation products mainly resulting from de-alkylation, de-chlorination, and hydroxylation were detected. Due to the distinctive preference to ethyl group in BDD/PS system, the formation of deethyl-atrazine was quicker than that of deisopropyl-atrazine. The study aims to provide a comprehensive understanding on the potential application of BDD/PS system in water treatment.

Removal of sulfamethoxazole by electrochemically activated sulfate: Implications of chloride addition

Electrochemical oxidation is considered to be an attractive alternative to chemical oxidation for the treatment of polluted water. Given the of ability of boron-doped diamond (BDD) electrodes to generate hydroxyl radicals (OH), they are often selected for the degradation of persistent organic contaminants. Recently, BDD anodes have been demonstrated to form strong oxidants, sulfate radicals (SO₄^-), directly from sulfate ions. In this study, electrochemical activation of sulfate to SO₄^- at BDD anodes enhanced the removal of an antibiotic sulfamethoxazole (SMX). The rate of SMX oxidation was 6 times higher in sulfate anolyte compared to inert nitrate anolyte. Addition of chloride accelerated the disappearance of SMX in both anolytes due to electrochlorination. Yet, mineralization efficiency was decreased, particularly in Na₂SO₄ anolyte due to the scavenging of SO₄^- by Cl^-. Electrogenerated SO₄^- yielded nitroso- and nitro-derivatives, which were not observed in the absence of sulfate. The peak intensities of chlorinated TPs were three orders of magnitude lower in Na₂SO₄ than in NaNO₃ anolyte, suggesting that addition of sulfate may lower the formation of chlorinated organics. However, attention should be paid to the formation of inorganic byproducts, as the formation rates of toxic chlorate and in particular perchlorate were higher in Na₂SO₄ anolyte.

Mechanisms on the Impacts of Alkalinity, pH, and Chloride on Persulfate-Based Groundwater Remediation

Persulfate (S₂O₈^2-)-based in situ chemical oxidation (ISCO) has gained more attention in recent years due to the generation of highly reactive and selective sulfate radical (SO₄^•-). This study examined the effects of important groundwater chemical parameters, i.e., alkalinity, pH, and chloride on benzene degradation via heterogeneous persulfate activation by three Fe(III)- and Mn(IV)-containing aquifer minerals: ferrihydrite, goethite, and pyrolusite. A comprehensive kinetic model was established to elucidate the mechanisms of radical generation and mineral surface complexation. Results showed that an increase of alkalinity up to 10 meq/L decreased the rates of persulfate decomposition and benzene degradation, which was associated with the formation of unreactive surface carbonato complexes. An increase in pH generally accelerated persulfate decomposition due to enhanced formation of reactive surface hydroxo complexation. A change in the chloride level up to 5 mM had a negligibly effect on the reaction kinetics. Kinetics modeling also suggested that SO₄^•- was transformed to hydroxyl radical (HO^•) and carbonate radical (CO₃^•-) at higher pHs. Furthermore, the yields of two major products of benzene oxidation, i.e., phenol and aldehyde, were positively correlated with the branching ratio of SO₄^•- reacting with benzene, but inversely correlated with that of HO^• or CO₃^•-, indicating that SO₄^•- preferentially oxidized benzene via pathways involving fewer hydroxylation steps compared to HO^• or CO₃^•-.

Sulfate radical oxidation of aromatic contaminants: a detailed assessment of density functional theory and high-level quantum chemical methods

Advanced oxidation processes that utilize highly oxidative radicals are widely used in water reuse treatment. In recent years, the application of sulfate radical (SO₄˙^-) as a promising oxidant for water treatment has gained increasing attention. To understand the efficiency of SO₄˙^- in the degradation of organic contaminants in wastewater effluent, it is important to be able to predict the reaction kinetics of various SO₄˙^--driven oxidation reactions. In this study, we utilize density functional theory (DFT) and high-level wavefunction-based methods (including computationally-intensive coupled cluster methods), to explore the activation energies of SO₄˙^--driven oxidation reactions on a series of benzene-derived contaminants. These high-level calculations encompass a wide set of reactions including 110 forward/reverse reactions and 5 different computational methods in total. Based on the high-level coupled-cluster quantum calculations, we find that the popular M06-2X DFT functional is significantly more accurate for OH^- additions than for SO₄˙^- reactions. Most importantly, we highlight some of the limitations and deficiencies of other computational methods, and we recommend the use of high-level quantum calculations to spot-check environmental chemistry reactions that may lie outside the training set of the M06-2X functional, particularly for water oxidation reactions that involve SO₄˙^- and other inorganic species.

Kinetic Study of Hydroxyl and Sulfate Radical-Mediated Oxidation of Pharmaceuticals in Wastewater Effluents

Advanced oxidation processes (AOPs), such as hydroxyl radical (HO^•)- and sulfate radical (SO₄^•-)-mediated oxidation, are alternatives for the attenuation of pharmaceuticals and personal care products (PPCPs) in wastewater effluents. However, the kinetics of these reactions needs to be investigated. In this study, kinetic models for 15 PPCPs were built to predict the degradation of PPCPs in both HO^•- and SO₄^•--mediated oxidation. In the UV/H₂O₂ process, a simplified kinetic model involving only steady state concentrations of HO^• and its biomolecular reaction rate constants is suitable for predicting the removal of PPCPs, indicating the dominant role of HO^• in the removal of PPCPs. In the UV/K₂S₂O₈ process, the calculated steady state concentrations of CO₃^•- and bromine radicals (Br^•, Br₂^•- and BrCl^•-) were 600-fold and 1-2 orders of magnitude higher than the concentrations of SO₄^•-, respectively. The kinetic model, involving both SO₄^•- and CO₃^•- as reactive species, was more accurate for predicting the removal of the 9 PPCPs, except for salbutamol and nitroimidazoles. The steric and ionic effects of organic matter toward SO₄^•- could lead to overestimations of the removal efficiencies of the SO₄^•--mediated oxidation of nitroimidazoles in wastewater effluents.

A novel heterogeneous system for sulfate radical generation through sulfite activation on a CoFe2O4 nanocatalyst surface

Heterogeneous catalytic activation is important for potential application of new sulfate-radical-based advanced oxidation process using sulfite as source of sulfate radical. We report herein a heterogeneous system for sulfite activation by CoFe₂O₄ nanocatalyst for metoprolol removal. Factors that influence metoprolol removal were investigated, including pH and initial concentrations of components. The CoFe₂O₄ nanocatalyst was characterized by X-ray diffractometry (XRD) and transmission electron microscopy (TEM), and the catalytic stability was tested by consecutive runs. Radicals generated in the CoFe₂O₄S(IV)O₂ system were identified through radical quenching experiments and by electron spin resonance (ESR). The catalytic mechanism was elucidated further by X-ray photoelectron spectroscopy (XPS). The catalytic process was dependent on initial pH, and more than 80% of the metoprolol can be removed at pH 10.0 following the Langmubir-Hinshelwood equation. The generation of Co-OH complexes on the CoFe₂O₄ surface was crucial for sulfite activation. SO₄^- was verified to be the main oxidative species responsible for metoprolol degradation. Other organic pollutants, such as sulfanilamide, sulfasalazine, 2-nitroaniline, sulfapyridine, aniline, azo dye X-3B and 4-chloroaniline, could also be removed in this CoFe₂O₄S(IV)O₂ system. The results suggest that the CoFe₂O₄S(IV)O₂ system has good application prospects in alkaline organic wastewater treatment.

Factors affecting formation of deethyl and deisopropyl products from atrazine degradation in UV/H2O2 and UV/PDS

Formation of deethyl products (DEPs) (i.e., atrazine amide and deethylatrazine) and deisopropyl product (i.e., deisopropylatrazine (DIA)) from parent atrazine (ATZ) degraded in UV/H₂O₂ and UV/PDS processes under various conditions was monitored.

Sulphate radical generation through interaction of peroxymonosulphate with Co(II) for in-line sample preparation aiming at spectrophotometric flow-based determination of phosphate and phosphite in fertilizers

An advanced oxidative process relying on the interaction of peroxymonosulphate and cobalt(II) was implemented for generating the sulphate radicals in flow analysis, in order to accomplish in-line sample preparation thus improving the spectrophotometric determination of phosphate and phosphite in liquid foliar fertilizers. To this end, a flow-batch system with a heated chamber was designed. The sample was handled twice, with and without the step of phosphite oxidation to phosphate, and the formed orthophosphate was quantified after interaction with the vanadate-molybdate reagent. Phosphite was determined as the difference in analytical signals corresponding to sample handling with and without the oxidation step. Influence of Co(II) on the peroxymonosulphate activation, reagent concentrations and added volumes, acidity, temperature and heating time were investigated like aiming at to improve analytical recovery and measurement repeatability, as well as the and system ruggedness. The 6.6-20.0mgL(-1) P2O5 standards were in-line prepared from a single stock solution. Detection limits were estimated as 0.8 and 0.1mgL(-1) for P2O5 and P-PO4. Twenty-four samples are were run per hour, and results are were in agreement with those obtained by the official procedure.

Oxidation of the odorous compound 2,4,6-trichloroanisole by UV activated persulfate: Kinetics, products, and pathways

The transformation efficiency and products of an odorous compound 2,4,6-trichloroanisole (TCA) at the wavelength of 254 nm in the presence of persulfate were investigated for the first time. The effects of water matrix (i.e., natural organic matter (NOM), pH, carbonate/bicarbonate (HCO3(-)/CO3(2-)), and chloride ions (Cl(-))) were evaluated. The second order rate constant of TCA reacting with sulfate radical (SO4(-)) was determined to be (3.72 ± 0.10) × 10(9) M(-1) s(-1). Increasing dosage of persulfate increased the observed pseudo-first-order rate constant for TCA degradation (kobs), and the contribution of SO4(-) to TCA degradation was much higher than that of HO at each experimental condition. Degradation rate of TCA decreased with pH increasing from 4.0 to 9.0, which could be explained by the lower radical scavenging effect of dihydrogen phosphate than hydrogen phosphate in acidic condition (pH < 6). NOM significantly decreased kobs due to the effects of radical scavenging and UV absorption with the former one being dominant. kobs decreased from 2.32 × 10(-3) s(-1) to 0.92 × 10(-3) s(-1) with the CO3(2-)/HCO3(-) concentration increased from 0.5 mM to 10 mM in the UV/persulfate process, while kobs slightly decreased from 2.54 × 10(-3) s(-1) in the absence of Cl(-) to 2.10 × 10(-3) s(-1) in the presence of 10 mM Cl(-). Most of these kinetic results could be described by a steady-state kinetic model. Furthermore, liquid chromatography/electrospray ionization-triple quadrupole mass spectrometry at powerful precursor ion scan approach was used to selectively detect oxidation products of TCA. It was found that 2,4,6-trichorophenol (TCP) was the major oxidation product (i.e., the initial yield of TCP was above 90%). The second order rate constant between TCP and SO4(-) was estimated to be (4.16 ± 0.20) × 10(9) M(-1) s(-1). In addition, three products (i.e., 2,6-dichloro-1,4-benzoquinone and two aromatic ring-opening products) were detected in the reaction of TCP with SO4(-), which also appeared in the oxidation of TCA in the UV/persulfate process. A tentative pathway was proposed, where the initial one-electron oxidation of TCA by SO4(-) and further reactions (e.g., ipso-hydroxylation and aromatic ring-cleavage) of the formed cation intermediate TCA were involved.

Oxidation of Benzene by Persulfate in the Presence of Fe(III)- and Mn(IV)-Containing Oxides: Stoichiometric Efficiency and Transformation Products

Sulfate radical (SO4(•-)) is a strong, short-lived oxidant that is produced when persulfate (S2O8(2-)) reacts with transition metal oxides during in situ chemical oxidation (ISCO) of contaminated groundwater. Although engineers are aware of the ability of transition metal oxides to activate persulfate, the operation of ISCO remediation systems is hampered by an inadequate understanding of the factors that control SO4(•-) production and the overall efficiency of the process. To address these shortcomings, we assessed the stoichiometric efficiency and products of transition metal-catalyzed persulfate oxidation of benzene with pure iron- and manganese-containing minerals, clays, and aquifer solids. For most metal-containing solids, the stoichiometric efficiency, as determined by the loss of benzene relative to the loss of persulfate, approached the theoretical maximum. Rates of production of SO4(•-) or hydroxyl radical (HO(•)) generated from radical chain reactions were affected by the concentration of benzene, with rates of S2O8(2-) decomposition increasing as the benzene concentration increased. Under conditions selected to minimize the loss of initial transformation products through reaction with radicals, the production of phenol only accounted for 30%-60% of the benzene lost in the presence of O2. The remaining products included a ring-cleavage product that appeared to contain an α,β-unsaturated aldehyde functional group. In the absence of O2, the concentration of the ring-cleavage product increased relative to phenol. The formation of the ring-cleavage product warrants further studies of its toxicity and persistence in the subsurface.

Kinetics of the Benzaldehyde-Inhibited Oxidation of Sulfite by Chlorine Dioxide

There has been steady interest in the aqueous reaction of ClO2• with sulfur(IV) since the 1950s, and a wide variety of rate laws and mechanisms have been proposed. In neutral-to-alkaline media, the reaction is challenging to study because of its great rate. Here it is shown that benzaldehyde can be used as an additive to slow the reaction and make its rates more amenable to study. The rates can be quantitatively modeled by a mechanism that includes reversible binding of sulfur(IV) by benzaldehyde and a rate-limiting mixed second-order reaction of ClO2• with SO3(2-). The latter reaction occurs through parallel electron transfer from SO3(2-) to ClO2• and oxygen-atom transfer from ClO2• to SO3(2-).

Removal of Persistent Organic Contaminants by Electrochemically Activated Sulfate

Solutions of sulfate have often been used as background electrolytes in the electrochemical degradation of contaminants and have been generally considered inert even when high-oxidation-power anodes such as boron-doped diamond (BDD) were employed. This study examines the role of sulfate by comparing electro-oxidation rates for seven persistent organic contaminants at BDD anodes in sulfate and inert nitrate anolytes. Sulfate yielded electro-oxidation rates 10-15 times higher for all target contaminants compared to the rates of nitrate anolyte. This electrochemical activation of sulfate was also observed at concentrations as low as 1.6 mM, which is relevant for many wastewaters. Electrolysis of diatrizoate in the presence of specific radical quenchers (tert-butanol and methanol) had a similar effect on electro-oxidation rates, illustrating a possible role of the hydroxyl radical ((•)OH) in the anodic formation of sulfate radical (SO4(•-)) species. The addition of 0.55 mM persulfate increased the electro-oxidation rate of diatrizoate in nitrate from 0.94 to 9.97 h(-1), suggesting a nonradical activation of persulfate. Overall findings indicate the formation of strong sulfate-derived oxidant species at BDD anodes when polarized at high potentials. This may have positive implications in the electro-oxidation of wastewaters containing sulfate. For example, the energy required for the 10-fold removal of diatrizoate was decreased from 45.6 to 2.44 kWh m(-3) by switching from nitrate to sulfate anolyte.

Simulation and comparative study on the oxidation kinetics of atrazine by UV/H₂O₂, UV/HSO₅⁻ and UV/S₂O₈²⁻

This study comparatively investigated atrazine (ATZ) degradation by irradiation at the wavelength of 254 nm in the presence of peroxides including hydrogen peroxide (H2O2), peroxymonosulfate (HSO5(-)), and persulfate (S2O8(2-)) at various initial ATZ concentrations and oxidant dosages. The effects of water matrix, such as carbonate/bicarbonate (HCO3(-)/CO3(2-)), chloride ions (Cl(-)), and natural organic matter (NOM), were evaluated on these three advanced oxidation processes. A simple steady-state kinetic model was developed based on the initial rates of ATZ destruction, which could well describe the apparent pseudo-first-order rate constants (k(app), s(-1)) of ATZ degradation in these three processes. The specific roles of reactive species (i.e., HO·, SO4(-·), CO3(-·), and Cl2(-·)) under various experimental conditions were quantitatively evaluated based on their steady-state concentrations obtained from this model. Modeling results showed that the steady-state concentrations of HO· and SO4(-·) decreased with the increase of CO3(2-)/HCO3(-) concentration, and the relative contribution of HO· to ATZ degradation significantly decreased in UV/H2O2 and UV/HSO5(-) systems. On the other hand, the scavenging effect of HCO3(-)/CO3(2-) on the relative contribution of SO4(-·) to ATZ degradation was lower than that on HO·. The presence of Cl(-) (0.5-10 mM) significantly scavenged SO4(-·) but had slightly scavenging effect on HO· at the present experimental pH, resulting in greater decrease of k(app) in the UV/S2O8(2-) than UV/H2O2 and UV/HSO5(-) systems. Higher levels of Cl2(-·) were generated in the UV/S2O8(2-) than those in the UV/H2O2 and UV/HSO5(-) systems at the same Cl(-) concentrations. NOM significantly decreased k(app) due to its effects of competitive UV absorption and radical scavenging with the latter one being dominant. These results improve the understanding of the effects of water constituents for ATZ degradation in the UV-based oxidation processes.

Production of sulfate radical and hydroxyl radical by reaction of ozone with peroxymonosulfate: a novel advanced oxidation process

In this work, simultaneous generation of hydroxyl radical (•OH) and sulfate radical (SO4•−) by the reaction of ozone (O3) with peroxymonosulfate (PMS; HSO5−) has been proposed and experimentally verified. We demonstrate that the reaction between the anion of PMS (i.e.,SO52−) and O3 is primarily responsible for driving O3 consumption with a measured second order rate constant of (2.12 ± 0.03) × 10(4) M(-1) s(-1). The formation of both •OH and SO4•− from the reaction between SO52− and O3 is confirmed by chemical probes (i.e., nitrobenzene for •OH and atrazine forb oth •OH and SO4•−). The yields of •OH and SO4•− are determined to be 0.43 ± 0.1 and 0.45 ± 0.1 per mol of O3 consumption, respectively. An adduct,−O3SOO− + O3 → −O3SO5−, is assumed as the first step, which further decomposes into SO5•− and O3•−. The subsequent reaction of SO5•− with O3is proposed to generate SO4•−, while O3•− converts to •OH. A definition of R(ct,•OH) and R(ct,SO4•−) (i.e., respective ratios of •OH and SO4•− exposures to O3 exposure) is adopted to quantify relative contributions of •OH and SO4•−. Increasing pH leads to increases in both values of R(ct,•OH) and R(ct,SO4•−) but does not significantly affect the ratio of R(ct,SO4•−) to R(ct,•OH) (i.e., R(ct,SO4•−)/R(ct,•OH)), which represents the relative formation of SO4•− to •OH. The presence of bicarbonate appreciably inhibits the degradation of probes and fairly decreases the relative contribution of •OH for their degradation, which may be attributed to the conversion of both •OH and SO4•− to the more selective carbonate radical (CO3•−).Humic acid promotes O3 consumption to generate •OH and thus leads to an increase in the R(ct,•OH) value in the O3/PMS process,w hile humic acid has negligible influence on the R(ct,SO4•−) value. This discrepancy is reasonably explained by the negligible effect of humic acid on SO4•− formation and a lower rate constant for the reaction of humic acid with SO4•− than with •OH. In addition, the efficacy of the O3/PMS process in real water is also confirmed.

In situ chemical oxidation of contaminated groundwater by persulfate: decomposition by Fe(III)- and Mn(IV)-containing oxides and aquifer materials

Persulfate (S2O8(2-)) is being used increasingly for in situ chemical oxidation (ISCO) of organic contaminants in groundwater, despite an incomplete understanding of the mechanism through which it is converted into reactive species. In particular, the decomposition of persulfate by naturally occurring mineral surfaces has not been studied in detail. To gain insight into the reaction rates and mechanism of persulfate decomposition in the subsurface, and to identify possible approaches for improving its efficacy, the decomposition of persulfate was investigated in the presence of pure metal oxides, clays, and representative aquifer solids collected from field sites in the presence and absence of benzene. Under conditions typical of groundwater, Fe(III)- and Mn(IV)-oxides catalytically converted persulfate into sulfate radical (SO4(•-)) and hydroxyl radical (HO(•)) over time scales of several weeks at rates that were 2-20 times faster than those observed in metal-free systems. Amorphous ferrihydrite was the most reactive iron mineral with respect to persulfate decomposition, with reaction rates proportional to solid mass and surface area. As a result of radical chain reactions, the rate of persulfate decomposition increased by as much as 100 times when benzene concentrations exceeded 0.1 mM. Due to its relatively slow rate of decomposition in the subsurface, it can be advantageous to inject persulfate into groundwater, allowing it to migrate to zones of low hydraulic conductivity where clays, metal oxides, and contaminants will accelerate its conversion into reactive oxidants.

Comparison of halide impacts on the efficiency of contaminant degradation by sulfate and hydroxyl radical-based advanced oxidation processes (AOPs)

The effect of halides on organic contaminant destruction efficiency was compared for UV/H2O2 and UV/S2O8(2-) AOP treatments of saline waters; benzoic acid, 3-cyclohexene-1-carboxylic acid, and cyclohexanecarboxylic acid were used as models for aromatic, alkene, and alkane constituents of naphthenic acids in oil-field waters. In model freshwater, contaminant degradation was higher by UV/S2O8(2-) because of the higher quantum efficiency for S2O8(2-) than H2O2 photolysis. The conversion of (•)OH and SO4(•-) radicals to less reactive halogen radicals in the presence of seawater halides reduced the degradation efficiency of benzoic acid and cyclohexanecarboxylic acid. The UV/S2O8(2-) AOP was more affected by Cl(-) than the UV/H2O2 AOP because oxidation of Cl(-) is more favorable by SO4(•-) than (•)OH at pH 7. Degradation of 3-cyclohexene-1-carboxylic acid, was not affected by halides, likely because of the high reactivity of halogen radicals with alkenes. Despite its relatively low concentration in saline waters compared to Cl(-), Br(-) was particularly important. Br(-) promoted halogen radical formation for both AOPs resulting in ClBr(•-), Br2(•-), and CO3(•-) concentrations orders of magnitude higher than (•)OH and SO4(•-) concentrations and reducing differences in halide impacts between the two AOPs. Kinetic modeling of the UV/H2O2 AOP indicated a synergism between Br(-) and Cl(-), with Br(-) scavenging of (•)OH leading to BrOH(•-), and further reactions of Cl(-) with this and other brominated radicals promoting halogen radical concentrations. In contaminant mixtures, the conversion of (•)OH and SO4(•-) radicals to more selective CO3(•-) and halogen radicals favored attack on highly reactive reaction centers represented by the alkene group of 3-cyclohexene-1-carboxylic acid and the aromatic group of the model compound, 2,4-dihydroxybenzoic acid, at the expense of less reactive reaction centers such as aromatic rings and alkane groups represented in benzoic acid and cyclohexanecarboxylic acid. This effect was more pronounced for the UV/S2O8(2-) AOP.

Microbial sulfite respiration

Despite its reactivity and hence toxicity to living cells, sulfite is readily converted by various microorganisms using distinct assimilatory and dissimilatory metabolic routes. In respiratory pathways, sulfite either serves as a primary electron donor or terminal electron acceptor (yielding sulfate or sulfide, respectively), and its conversion drives electron transport chains that are coupled to chemiosmotic ATP synthesis. Notably, such processes are also seen to play a general role in sulfite detoxification, which is assumed to have an evolutionary ancient origin. The diversity of sulfite conversion is reflected by the fact that the range of microbial sulfite-converting enzymes displays different cofactors such as siroheme, heme c, or molybdopterin. This chapter aims to summarize the current knowledge of microbial sulfite metabolism and focuses on sulfite catabolism. The structure and function of sulfite-converting enzymes and the emerging picture of the modular architecture of the corresponding respiratory/detoxifying electron transport chains is emphasized.

Kinetic modeling and synergy quantification in sono and photooxidative treatment of simulated dyehouse effluent

The aim of this work was to explore the application of sulfate radical based advanced oxidation processes: photooxidation (UV/PMS/PS), sonooxidation (US/PMS/PS) and combined sono-photooxidation (US/UV/PMS/PS) for the mineralization of simulated dyehouse effluent (WW); using peroxymonosulfate (PMS) and persulfate (PS) as oxidants. Experiments were performed in a reaction vessel of a defined geometry and axially positioned source of UV-C radiation, all placed in the ultrasonic bath (35 kHz). Mathematical model of the process was developed according to the proposed degradation scheme. Decomposition of dyestuff (C.I. Reactive Violet 2, RV2 and C.I. Reactive Blue 7, RB7), surfactant (linear alkylbenzene sulfonate; hereafter: LAS) and auxiliary organic components was explored in three types of model wastewater: WW, simulated effluent excluding inorganic species (WW-IS) and model solution that consists of a specific compound (hereafter: compound model solutions). The influence of inorganic matrix (Cl(-), CO(3)(2-)/HCO(3)(-)) was studied due to the corresponding quenching affinity toward HO and SO(4)(-) radicals. The efficiency of applied processes was evaluated and the response to combined phenomena (cavitation and irradiation) was quantified as synergy index, f(Syn). Sono-photooxidative treatment (US/UV/PMS/PS) of WW resulted in a partial mineralization and partial decolourization; approximately 40% of initial TOC and 30% of initial RB7 remained after 60 min of treatment, while RV2 and LAS molecule were completely decomposed. Circumstantially, the combined process increased the mineralization efficiency by a factor of 3 (f(Syn) = 3.026).

Evaluation of activated carbon for remediating benzene contamination: adsorption and oxidative regeneration

This study investigated the potential usage of activated carbon (AC) as a permeable reactive barrier material for the adsorption of benzene contaminant. Sodium persulfate (SPS) or pyrite (FeS(2)) activated SPS oxidation was used for the regeneration of spent AC. Results indicate that persulfate oxidation of AC caused a loss of electrons and a reduction in adsorptive capacity due to the formation of acidic functional groups on the AC. Concerning the reactants that can be used for oxidation of the benzene contaminants, SPS/FeS(2)/AC, as oppose to SPS/AC, can achieve benzene destruction in both the aqueous and the sorbed phases. Furthermore, regeneration of benzene spent AC by SPS or SPS/FeS(2) revealed that SPS oxidation resulted primarily in desorption of benzene over direct oxidation of AC sorbed benzene. In contrast, the SPS/FeS(2) system achieved complete oxidation of desorbed benzene in the aqueous phase while also oxidizing sorbed benzene. Results of re-adsorption show that oxidative regeneration recovered around 70% of the AC adsorption sites and the remaining capacity was mostly occupied by the residual benzene on the AC. This study demonstrates that SPS or FeS(2) activated SPS oxidation is an effective alternative method for the regeneration of spent AC.