Transformation of bromide in thermo activated persulfate oxidation processes

Underestimated role of hydroxyl radicals for bromate formation in persulfate-based advanced oxidation processes

In persulfate-based advanced oxidation processes (PS-AOPs), sulfate radicals (SO4•-) have been recognized to play more important roles in inducing bromate (BrO3-) formation rather than hydroxyl radicals (HO•) because of the stronger oxidation capacity of the former. However, this study reported an opposite result that HO• indeed dominated the formation of bromate instead of SO4•-. Quenching experiments were coupled with electron paramagnetic resonance (EPR) detection and chemical probe identification to elucidate the contributions of each radical species. The comparison of different thermal activated persulfates (PDS and PMS) demonstrated that the significant higher bromate formation in HEAT/PMS ([BrO3-]/[Br-]0 = 0.8), as compared to HEAT/PDS ([BrO3-]/[Br-]0 = 0.2), was attributable to the higher concentration of HO• radicals in HEAT/PMS. Similarly, the bromate formation in UV/PDS ([BrO3-]/[Br-]0 = 1.0), with a high concentration of HO•, further underscored the dominant role of HO•. As a result, we quantified that HO• and SO4•- radicals accounted 66.7% and 33.3% for bromate formation. This controversial result can be reconciled by considering the critical intermediate, hypobromic acid/hypobromate (HOBr/BrO-), involved in the transformation of Br- to BrO3-. HO• radicals have the chemical preference to induce the formation of HOBr/BrO- intermediates (contributing ∼ 60%) relative to SO4•- radicals (contributing ∼ 40%). This study highlighted the dominant role of HO• in the formation of bromate rather than SO4•- in PS-AOPs and potentially offered novel insights for reducing disinfection byproduct formation by controlling the radical species in AOPs.

Formation of nitrated naphthalene in the sulfate radical oxidation process in the presence of nitrite

Polycyclic aromatic hydrocarbons (PAHs) have become a global environmental concern due to their potential hazardous implication for human health. In this study, we found that sulfate radical (SO4•-) could effectively degrade naphthalene (NAP), a representative PAH in groundwaters, generating 1-naphthol. This intermediate underwent further degradation, yielding ring-opening products including phthalic acid and salicylic acid. However, the presence of nitrite (NO2-), a prevalent ion in subsurface environments, was observed to compete with NAP for SO4•-, thus slowing down the NAP degradation. The reaction between NO2- and SO4•- generated a nitrogen dioxide radical (NO2•). Concurrently, in-situ formed 1-naphthol underwent further oxidization to the 1-naphthoxyl radical by SO4•-. The coupling of 1-naphthoxyl radicals with NO2• gave rise to a series of nitrated NAP, namely 2-nitro-1-naphthol, 4-nitro-1-naphthol, and 2,4-dinitro-1-naphthol. In addition, the in-situ formed phthalic acid and salicylic acid also underwent nitration, generating nitrophenolic products, although this pathway appeared less prominent than the nitration of 1-naphthol. When 10 μΜ NAP was subjected to heat activated peroxydisulfate oxidation in the presence of 10 μΜ NO2-, the total yield of nitrated products reached 0.730 μΜ in 120 min. Overall, the presence of NO2- dramatically altered the behavior of NAP degradation by SO4•- oxidation and contributed to the formation of toxic nitrated products. These findings raise awareness of the potential environmental risks associated with the application of SO4•--based oxidation processes for the remediation of PAHs-polluted sites in presence of NO2-.

Discriminating the Active Ru Species Towards the Selective Generation of Singlet Oxygen from Peroxymonosulfate: Nanoparticles Surpass Single-Atom Catalysts

Singlet oxygen (1O2) is an exceptional reactive oxygen species in advanced oxidation processes for environmental remediation. Despite single-atom catalysts (SACs) representing the promising candidate for the selective generation of 1O2 from peroxymonosulfate (PMS), the necessity to meticulously regulate the coordination environment of metal centers poses a significant challenge in the precisely-controlled synthetic method. Another dilemma to SACs is their high surface free energy, which results in an inherent tendency for the surface migration and aggregation of metal atoms. We here for the first time reported that Ru nanoparticles (NPs) synthesized by the facile pyrolysis method behave as robust Fenton-like catalysts, outperforming Ru SACs, towards efficient activation of PMS to produce 1O2 with nearly 100 % selectivity, remarkably improving the degradation efficiency for target pollutants. Density functional theory calculations have unveiled that the boosted PMS activation can be attributed to two aspects: (i) enhanced adsorption of PMS molecules onto Ru NPs, and (ii) decreased energy barriers by offering adjacent sites for promoted dimerization of *O intermediates into adsorbed 1O2. This study deepens the current understanding of PMS chemistry, and sheds light on the design and optimization of Fenton-like catalysts.

Degrading surface-water-based natural organic matter and mitigating haloacetonitrile formation during chlorination: Comparison of UV/persulfate and UV/hydrogen peroxide pre-treatments

Haloacetonitriles (HANs) are unregulated disinfection by-products that are more toxic than regulated species. Therefore, efficient decomposition of HAN precursors prior to disinfection is crucial for allaying the potential HAN-induced health risks. This study investigated the key roles of ultraviolet-activated persulfate (UV/PS) treatment in alleviating HAN formation. The effects of UV/PS treatment were evaluated by correlating with the characteristics of organic matter in surface water and comparing with conventional UV/H2O2 treatment. Upon irradiating raw water samples and a Suwannee River humic acid solution spiked with 10 mM PS or H2O2 with 254 nm UV light, UV/PS treatment was found to be more potent than UV/H2O2 in mitigating the HAN production and degrading organic substances; moreover, UV/PS treatment effectively decreased the dissolved organic nitrogen (DON) content. In contrast, UV/H2O2 treatment did not induce any noticeable reduction in DON level. Furthermore, both UV/PS and UV/H2O2 treatments reduced the dichloroacetonitrile (DCAN) formation potential (FP), leading to strong correlations with the degradation of aromatic and humic-acid-like compounds. Notably, UV/PS treatment efficiently decreased the FP of bromochloroacetonitrile (BCAN) and dramatically reduced that of dibromoacetonitrile (DBAN) after a sharp increase; however, UV/H2O2 treatment gradually increased the DBAN-FP. Bromide was activated by sulfate radicals during UV/PS treatment, negatively correlating with the BCAN-FP and DBAN-FP, indicating that the formation of reactive bromine species increased the DBAN-FP; however, excessive oxidation possibly led to the recovery of inorganic bromine for decreasing the BCAN-FP and DBAN-FP. Additionally, UV/PS treatment effectively suppressed toxicity owing to its high reduction rate for brominated HANs; in contrast, UV/H2O2 treatment resulted in less significant BCAN and DBAN reductions, leading to minimal net reduction in toxicity. Overall, UV/PS treatment was remarkably effective at diminishing the toxicity of brominated HANs, underscoring its potential to mitigate drinking-water-related health risks.

Degradation of Tetrabromobisphenol S by thermo-activated Persulphate Oxidation: reaction Kinetics, transformation Mechanisms, and brominated By-products

Brominated flame retardants (BFRs) are a group of contaminants of emerging environmental concern. In this study, systematic exploration was carried out to investigate the degradation of tetrabromobisphenol S (TBBPS), a typical emerging BFRs, by thermally activated persulfate (PDS) oxidation. The removal of 5.0 μM TBBPS was 100% after 60 min oxidation treatment under 60°C. Increasing the temperature or initial PDS concentration facilitated the degradation efficiency of TBBPS. The quenching test indicated that TBBPS degradation occurred via the attack of both sulphate radicals and hydroxyl radicals. Natural organic matter (NOM) decreased the removal rate, however, complete disappearance of TBBPS could still be obtained. Six intermediate products were formed during reactions between TBBPS and radicals. Transformation pathways including debromination, β-Scission, and cross-coupling were proposed. Brominated disinfection by-products (DBPs) in situ formed during the degradation of TBBPS were also investigated, such as bromoform and dibromoacetic acid. The presence of NOM reduced the formation rates of brominated DBPs. Results reveal that although thermo-activated PDS is a promising method for TBBPS-contaminated water, it can lead to potential brominated DBPs risks, which should be paid more attention to when SO4•--based oxidation technology is applied.

In Situ Monitoring of the Polymerization Kinetics of Organic Pollutants during Persulfate-Based Advanced Oxidation Processes Using Plasmonic Colorimetry

Using sulfate radicals to initiate polymer production in persulfate-based advanced oxidation processes (AOPs) is an emerging strategy for organics removal. However, our understanding of this process remains limited due to a dearth of efficient methods for in situ and real time monitoring of polymerization kinetics. This study leverages plasmonic colorimetry to monitor the polymerization kinetics of an array of aromatic pollutants in the presence of sulfate radicals. We observed that the formation of polymer shells on the surfaces of gold nanoparticles (AuNPs) led to an increase and red shift in their localized surface plasmon resonance (LSPR) band as a result of an increased refractive index surrounding the AuNP surfaces. This observation aligns with Mie theory simulations and transmission electron microscopy-electron energy loss spectroscopy characterizations. Our study demonstrated that the polymerization kinetics exhibits a significant reliance on the electrophilicity and quantity of benzene rings, the concentration of aromatic pollutants, and the dosage of oxidants. In addition, we found that changes in LSPR band wavelength fit well into a pseudo-first-order kinetic model, providing a comprehensive and quantitative insight into the polymerization kinetics involving diverse organic compounds. This technique holds the potential for optimizing AOP-based water treatment by facilitating the polymerization of aromatic pollutants.

Overlooked Cytotoxicity and Genotoxicity to Mammalian Cells Caused by the Oxidant Peroxymonosulfate during Wastewater Treatment Compared with the Sulfate Radical-Based Ultraviolet/Peroxymonosulfate Process

Byproduct formation (chlorate, bromate, organic halogen, etc.) during sulfate radical (SO₄^•-)-based processes like ultraviolet/peroxymonosulfate (UV/PMS) has aroused widespread concern. However, hypohalous acid (HOCl and HOBr) can form via two-electron transfer directly from PMS, thus leading to the formation of organic halogenated byproducts as well. This study found both PMS alone and UV/PMS can increase the toxicity to mammalian cells of wastewater, while the UV/H₂O₂ decreased the toxicity. Cytotoxicity of two wastewater samples increased from 5.6-8.3 to 15.7-29.9 mg-phenol/L, and genotoxicity increased from 2.8-3.1 to 5.8-12.8 μg 4-NQO/L after PMS treatment because of organic halogen formation. Organic halogen formation from bromide rather than chloride was found to dominate the toxicity increase. The SO₄^•--based process UV/PMS led to the formation of both organic halogen and inorganic bromate and chlorate. However, because of the very low concentration (<20 μg/L) and relatively low toxicity of bromate and chlorate, contributions of inorganic byproducts to toxicity increase were negligible. PMS would not form chlorate and bromate, but it generated a higher concentration of total organic halogen, thus leading to a more toxic treated wastewater than UV/PMS. UV/PMS formed less organic halogen and toxicity because of the destruction of byproducts by UV irradiation and the removal of byproduct precursors. Currently, many studies focused on the byproducts bromate and chlorate during SO₄^•--based oxidation processes. This work revealed that the oxidant PMS even needs more attention because it caused higher toxicity due to more organic halogen formation.

New Insights into the Role of Nitrite in the Degradation of Tetrabromobisphenol S by Sulfate Radical Oxidation

Tetrabromobisphenol S (TBBPS) is a brominated flame retardant and a contaminant of emerging concern. Several studies found that sulfate radical (SO₄^•-) oxidation is effective to degrade TBBPS. Here, we demonstrate that the presence of nitrite (NO₂^-) at environmentally relevant levels causes dramatic changes in the kinetics and pathways of TBBPS degradation by SO₄^•-. Initially, NO₂^- suppresses the reaction by competing with TBBPS for SO₄^•-. At the same time, SO₄^•- oxidizes NO₂^- to form nitrogen dioxide radicals (NO₂^•), which actively react with some key TBBPS degradation intermediates, thus greatly altering the transformation pathway. As a result, 2,6-dibromo-4-nitrophenol (DBNP) becomes the primary TBBPS product. As TBBPS undergoes degradation, the released bromide (Br^-) is oxidized by SO₄^•- to form bromine radicals and free bromine. These reactive bromine species immediately combine with NO₂^• or NO₂^- to form nitryl bromide (BrNO₂) that in turn attacks the parent TBBPS, resulting in its accelerated degradation and increased formation of toxic nitrophenolic byproducts. These results show that nitryl halides (e.g., BrNO₂ or ClNO₂) are likely formed yet inadequately recognized when SO₄^•- is applied to remediate halogenated pollutants in the subsurface environment where NO₂^- is ubiquitously found. These insights further underscore the potential risks of the application of SO₄^•- oxidation for the remediation of halogenated compounds in realistic environmental conditions.

Multiple Roles of Dissolved Organic Matter in Advanced Oxidation Processes

Advanced oxidation processes (AOPs) can degrade a wide range of trace organic contaminants (TrOCs) to improve the quality of potable water or discharged wastewater effluents. Their effectiveness is impacted, however, by the dissolved organic matter (DOM) that is ubiquitous in all water sources. During the application of an AOP, DOM can scavenge radicals and/or block light penetration, therefore impacting their effectiveness toward contaminant transformation. The multiple ways in which different types or sources of DOM can impact oxidative water purification processes are critically reviewed. DOM can inhibit the degradation of TrOCs, but it can also enhance the formation and reactivity of useful radicals for contaminants elimination and alter the transformation pathways of contaminants. An in-depth analysis highlights the inhibitory effect of DOM on the degradation efficiency of TrOCs based on DOM's structure and optical properties and its reactivity toward oxidants as well as the synergistic contribution of DOM to the transformation of TrOCs from the analysis of DOM's redox properties and DOM's transient intermediates. AOPs can alter DOM structure properties as well as and influence types, mechanisms, and extent of oxidation byproducts formation. Research needs are proposed to advance practical understanding of how DOM can be exploited to improve oxidative water purification.

Ultraviolet light-activated peroxymonosulfate (UV/PMS) system for humic acid mineralization: Effects of ionic matrix and feasible application in seawater reverse osmosis desalination

The use of membrane-based technology has evolved into an important strategy for supplying freshwater from seawater and wastewater to overcome the problems of water scarcity around the world. However, the presence of natural organic matter (NOM), including humic substances affects the performance of the process. Here, we present a systematic report on the mineralization of humic acid (HA), as a model for NOM, in high concentration of salts using the ultraviolet light-activated peroxymonosulfate (UV/PMS) system as a potential alternative for HA elimination during membrane-based seawater desalination and water treatment processes. Effects of various parameters such as PMS concentration, solution type, pH, anions, and anion-cation matrix on HA mineralization were assessed. The results show that 100%, 78% and 58% of HA (2 mg/L TOC) were mineralized with rate constants of 0.085 min^-1, 0.0073 min^-1, and 0.0041 min^-1 after 180 min reaction time at pH 7 when 0.5 mM PMS was used in deionized water, sodium chloride solution (35,000 ppm) and synthetic seawater, respectively. The reduced efficiency under saline conditions was attributed to the presence of anions in the system that acted as sulfate and hydroxyl radicals' scavengers. Furthermore, the safety of the treated synthetic seawater was evaluated by analyzing the residual transformed products. Overall, pretreatment with the UV/PMS system mitigated fouling on the RO membrane.

Formation of brominated by-products during the degradation of tetrabromobisphenol S by Co2+/peroxymonosulfate oxidation

Tetrabromobisphenol S (TBBPS), an emerging brominated flame retardant, can cause neurotoxic and cytotoxic effects to human physiology. In this study, the degradation of TBBPS in Co²⁺ activated peroxymonosulfate (PMS) oxidation process was explored. In particular, brominated by-products formed during the degradation of the TBBPS were examined. It was found that TBBPS could be effectively removed in the Co²⁺/PMS oxidation process. The pseudo-first-order rate constants were 0.13 min^-1 at 0.2 mM PMS and 0.5 μM Co²⁺ initially. It appeared that TBBPS degradation occurred via and HO attacks, but played a dominant role. The presence of natural organic matter (NOM) greatly inhibited the transformation of the TBBPS, which can be explained by the scavenging of the radical species. β-Scission, debromination, and cross-coupling were identified as the main reaction pathways of TBBPS degradation in the Co²⁺/PMS system. Further oxidation and ring-opening of the intermediates generated brominated by-products including bromoform, monobromoacetic acid, and dibromoacetic acid. The formation of the brominated by-products increased gradually in approximately 48 h. But, the presence of NOM reduced the yields of the brominated -by-products. The findings of this study indicate that organic bromine contaminants can be effectively removed but lead to brominated by-products in the activated PMS oxidation process, which should be taken into consideration when -based oxidation technology is applied.

Differentiation of Pathways of Nitrated Byproduct Formation from Ammonium and Nitrite During Sulfate Radical Oxidation

Recent studies found that both nitrite (NO₂^-) and ammonium (NH₄⁺) lead to nitrophenolic byproducts in SO₄^•- oxidation processes, during which NO₂^• generated through the oxidation of the inorganic nitrogen by SO₄^•- is the key nitrating agent. This study demonstrates that the formation of phenoxy radicals to which NO₂^• can be incorporated immediately is another governing factor. Two types of sites having distinct reactivities in natural organic matter (NOM) molecules can be transformed to phenoxy radicals upon SO₄^•- oxidation. Fast sites associated with phenolic functionalities are primarily targeted in the reaction sequence involving NO₂^-, because both are preferentially oxidized. Following the depletion of NO₂^-, NH₄⁺ becomes the main precursor of NO₂^• that interacts with slow sites associated with the carboxylic functionalities. Experimental data show that the formation of total organic nitrogen in 24 h reached 6.28 μM during SO₄^•- oxidation of NOM (4.96 mg/L organic carbon) in the presence of both NO₂^- (0.1 mM) and NH₄⁺ (1.0 mM), while the sum of those formed in the presence of each alone was only 3.52 μM. Results of this study provide further insights into the mechanisms of nitrated byproduct formation when SO₄^•- is applied for environmental remediation.

Persulfate-nitrogen doped graphene mixture as an oxidant for the synthesis of 3-nitro-4-aryl-2H-chromen-2-ones from aryl alkynoate esters and nitrite

A series of 3-nitro-4-aryl-2H-chromen-2-ones in good yields have directly been obtained from aryl alkynoate esters and nitrite by employing a mixture of K₂S₂O₈-nitrogen doped graphene as an oxidant in a watery medium at room temperature. A plausible mechanism for the reaction is also reported. It reveals that the product is formed through a cascade of nitro radical addition, spirocyclization, and ester migration. When compared to known methods for the synthesis of 3-nitro-4-aryl-2H-chromen-2-ones from aryl alkynoate esters, this protocol is environmentally friendly, sustainable, practical and energy efficient and does not use a harmful nitro source. Furthermore, nitrogen doped graphene used in this approach can be easily recovered and reused at least four times without losing its activity.

Inactivation of E. coli and Streptococcus agalactiae by UV/persulfate during marine aquaculture disinfection

Sulfate radical (•SO₄^-)-based advanced oxidation processes have attracted a great deal of attention for use in water disinfection because of their strong oxidation ability toward electron-rich moieties on microorganism molecules. However, a few studies have focused on the effects of •SO₄^- on pathogenic microorganism inactivation in marine aquaculture water containing various inorganic anions. We employed the gram-negative bacteria E. coli and gram-positive bacteria S. agalactiae as representatives to evaluate the application of UV/persulfate (S₂O₈^2-, PDS), to the disinfection of marine aquaculture water in a comprehensive manner. Total inactivation of 4.13ˍlog of E. coli cells and 4.74ˍlog of S. agalactiae cells was reached within 120 s in the UV/PDS system. The inactivation of pathogenic bacteria in marine aquaculture water increased with the increasing PDS concentration and UV intensity. An acidic pH was beneficial for UV/PDS inactivation. Halogen-free radicals showed a strong influence on the inactivation. Anions in seawater, including Cl^-, Br^-, and HCO₃^- inhibited the disinfection. The inactivation rates of pathogenic bacteria followed the order seawater < marine aquaculture water < freshwater. Pathogenic bacteria could also be effectively inactivated in actual marine aquaculture water and reservoir water. The analysis of the inactivation mechanisms showed that S₂O₈^2- was activated by UV to produce •SO₄^-, which damaged the cell membranes. In addition, antioxidant enzymes, including SOD and CAT, were induced. The genomic DNA was also damaged. Inorganic disinfection byproducts such as chlorate and bromate were not formed during the disinfection of marine aquaculture water, which indicated that UV/PDS was a safe and efficient disinfection method.

Bromine Radical (Br• and Br2•-) Reactivity with Dissolved Organic Matter and Brominated Organic Byproduct Formation

Dissolved organic matter (DOM) is a major scavenger of bromine radicals (e.g., Br^• and Br₂^•-) in sunlit surface waters and during oxidative processes used in water treatment. However, the literature lacks quantitative measurements of reaction rate constants between bromine radicals and DOM and lacks information on the extent to which these reactions form brominated organic byproducts. Based on transient kinetic analysis with different fractions and sources of DOM, we determined reaction rate constants for DOM with Br^• ranging from <5.0 × 10⁷ to (4.2 ± 1.3) × 10⁸ M_C^-1 s^-1, which are comparable with those of HO^• but higher than those with Br₂^•- (k = (9.0 ± 2.0) × 10⁴ to (12.4 ± 2.1) × 10⁵ M_C^-1 s^-1). Br^• and Br₂^•- attack the aromatic and antioxidant moieties of DOM via the electron transfer mechanism, resulting in Br^- release with minimal substitution of bromine into DOM. For example, the total organic bromine was less than 0.25 μM (as Br) at environmentally relevant bromine radicals' exposures of ∼10^-9 M·s. The results give robust evidence that the scavenging of bromine radicals by DOM is a crucial step to prevent inorganic bromine radical chemistry from producing free bromine (HOBr/OBr^-) and subsequent brominated byproducts.

Transformation of bromide and formation of brominated disinfection byproducts in peracetic acid oxidation of phenol

Peracetic acid (PAA) has attracted increasing attention in wastewater treatment as a disinfectant. However, the transformation of bromide (Br^-) during PAA oxidation of bromide-containing wastewater has not been fully explored. This study showed that Br^- could be oxidized by PAA to free bromine which reacted with phenol to form organic bromine. At pH 7.0, more than 35.2% inorganic Br^- was converted to organic bromines in 4 h. At acidic conditions, the conversion ratio was even higher, reaching 69.9% at pH 2.8. Most of the organic bromines were presented as bromophenols (i.e., 2-bromophenol, 4-bromophenol, and 2,4-dibromophenol), while regulated brominated disinfection byproducts (Br-DBPs, i.e., bromoform and bromoacetic acids) only accounted for a tiny fraction of total organic bromine. Similar results were observed when PAA was applied to natural organic matter (NOM) or wastewater in presence of Br^-. The organic bromine yield reached 56.6 μM in the solution containing 0.1 mM Br^- and 2 mg/L NOM initially. Among them, only 1.00 μM bromoform and 0.16 μM dibromoacetic acid were found. Similarly, regulated Br-DBPs only accounted for 28.3% of the organic bromine in a real wastewater effluent treated with PAA. All these data show that monitoring regulated DBPs cannot fully indicate the potential environmental risk of the application of PAA to wastewater.

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.

Transformation of ammonium to nitrophenolic byproducts by sulfate radical oxidation

Sulfate radical (SO₄^•-) based oxidation shows great promise in wastewater treatment and subsurface remediation. For the first time, we demonstrated that SO₄^•- could induce the transformation of ammonium (NH₄⁺) to nitrophenolic byproducts. Using high-resolution mass spectrometry in combination with ¹⁵N labeling, mono-nitro and di-nitro phenolic byproducts were identified in a sample containing 1 mM NH₄⁺ and 10 mg/L natural organic matter (NOM) following heat activated peroxydisulfate (PDS) oxidation. At PDS dose of 1 mM, the formation of p-nitrophenol and 5-nitrosalicylic acid reached 0.21 and 0.30 μM, respectively, in 12 h and then decreased; the formation of 2,4-dinitrophenol and 3,5-dinitrosalicylic acid increased monotonically, reaching 0.37 and 0.62 μM, respectively, in 24 h. One-electron oxidation of NH₄⁺ to form aminyl radicals (^•NH₂) was the first step of the transformation. The reaction of ^•NH₂ with oxygen was a key step in propagating radical chain reactions, leading to nitrogen dioxide radicals (NO₂^•) as a key nitrating agent. The reactive sites susceptible to nitrating in NOM molecules are not limited to phenolic moieties. We found that aromatic carboxylate moieties could be in situ transformed to phenolics by SO₄^•-, thus contributed to nitrophenolic byproducts formation as well. Considering the ubiquitous presence of NH₄⁺ in the environment, formation of nitrophenolic byproducts will be widespread when SO₄^•- is applied for onsite remediation, which should be taken into consideration when evaluating the feasibility of this technology.

Ferrous-activated persulfate oxidation of triclosan in soil and groundwater: The roles of natural mineral and organic matter

Contamination of antimicrobial agents such as Triclosan (TCS) in soil and groundwater possess high risk to human health and ecological systems. Present study systematically studied the degradation of TCS in soil and groundwater by Fe²⁺ activated persulfate (Fe²⁺/PS) oxidation process and special attention was paid on revealing the influence of remediation process on soil physicochemical and microbial characteristics. Experimental results demonstrated that TCS was readily degraded in soil upon Fe²⁺/PS oxidation system. Higher Fe²⁺/PS concentration and lower pH value may promote the TCS degradation. Besides added Fe²⁺, the naturally present Fe (III)-O and dissolved Fe from iron containing minerals may also activate PS for TCS degradation. SO₄^•-, HO•, R^• and ¹O₂ were identified to be involved in the reaction system while addition of Fe²⁺-chelating agents, e.g., oxalic acid and ethylene diamine tetraacetic acid (EDTA) may slightly promote the degradation. Low concentration of Cl^- facilitated TCS degradation and high concentration of Cl^- slowed down the degradation. The presence of HCO₃^- may inhibit the degradation. Fe²⁺/PS oxidation process may partly reduce the soil organic matter content and diversely affect the composition of various C functional groups on soil. It also induced the breakdown of large soil aggregates and reduced the soil porosity, especially at macroporosity region. Phospholipid Fatty Acid test indicated that soil microbial community structure has been altered and the actinomycetes, fungi and Gram-negative bacteria decreased largely. The feasibility of remediation of TCS using Fe²⁺/PS oxidation in various natural groundwater samples was evaluated. Finally, five degradation intermediates of TCS by Fe²⁺/PS oxidation in soil were enriched by solid phase extraction and were identified by liquid chromatography-triple quadrupole mass spectrometry for proposing detailed transformation pathways.

Performance of UV/acetylacetone process for saline dye wastewater treatment: Kinetics and mechanism

Futility of traditional advanced oxidation processes (AOPs) in saline wastewater treatment has stimulated the quest for novel "halotolerant" chemical oxidation technology. Acetylacetone (AA) has proven to be a potent photo-activator in the degradation of dyes, but the applicability of UV/AA for saline wastewater treatment needs to be verified. In this study, degradation of crystal violet (CV) was investigated in the UV/AA system in the presence of various concentrations of exogenic Cl^- or Br^-. The results reveal that degradation, mineralization and even accumulation of adsorbable organic halides (AOX) were not significantly affected by the addition of Cl^- or Br^-. Rates of CV degradation were enhanced by elevating either AA dosage or solution acidity. An apparent kinetic rate equation was developed as r = -d[CV]/dt = k[CV]^a[AA]^b = (7.34 × 10^-4 mM^1-(a+b) min^-1) × [CV]^a=0.16 [AA]^b=0.97. In terms of results of radical quenching experiments, direct electron/energy transfer is considered as the major reaction mechanism, while either singlet oxygen or triplet state (³(AA)*) might be involved. Based on identification of degradation byproducts, a possible degradation pathway of CV in the UV/AA system is proposed.

Diphenamid photodegradation using Fe(III) impregnated N-doped TiO2/sulfite/visible LED process: Influence of wastewater matrix, kinetic modeling, and toxicity evaluation

Sulfite-based photocatalysis has been recently employed as a promising technology for the treatment of organic pollutants via the generation of reactive radicals. In this contribution, the effect of wastewater matrix constituents and toxicity evaluation were systematically investigated in the Fe^III impregnated N-doped TiO₂ (FeN-TiO₂)/sulfite/visible LED (Vis LED) process in the presence of diphenamid (DPA) as a model organic pollutant. The results showed that the presence of HCO₃^-, SO₄^2-, NO₃^-, and F^- had no detrimental effect on DPA degradation. Conversely, the presence of Cr(VI), NO₂^-, Cl^-, and Br^- caused a stronger retardation effect. The effect of natural organic matter such as humic acid (HA) was inert at normal concentrations. Interestingly, the retardation effect of inorganic ions can be quantified at any given ion concentration based on the linear correlations between the DPA decay (first-order kinetic constants) and concentration of ion species. Toxicity tests on Synechocystis sp., Microcystis flos-aquae, and Nostoc sp. algae revealed that higher toxicity was noticed at 240 min treatment time accompanied by lower toxicity with prolonging the treatment time for all selected algae except for Microcystis flos-aquae. In addition, novel two-phase mathematical models were successfully proposed to predict the accumulation of intermediates depending on their evolution profile.

Change of disinfection byproducts formation potential of natural organic matter after exposure to persulphate and bicarbonate

Activated persulphate (PS) oxidation is a promising in situ remediation technology for groundwater and soils. Application of this technology to contaminated zones may result in a large quantity of PS residue in the subsurface environment due to inefficient activation and repeated injection. In this study, we demonstrated that natural organic matter (NOM) molecules could be reconfigured due to exposure to unactivated PS and bicarbonate, resulting in reduced disinfection byproducts formation potential in post chlorination process. Fourier transformed inferred spectrometry (FTIR), size exclusive chromotraghy (SEC), and mass spectrometry (MS) analysis revealed that hydroxylation and carboxylation of NOM occurred, followed by inter-molecular coupling via ether bonds. The change of both the reactivity toward free chlorine and molecular structure of NOM during PS/bicarbonate treatment was well mimicked by 3,5-dihydroxylbenzoic acid, suggesting that phenolic moieties in NOM molecules were the main sites underwent transformation in the PS/bicarbonate system. We propose that peroxymonocarbonate (HCO₄^-) formed upon the reaction between PS and bicarbonate was the main reactive species responsible for the reconfiguration of NOM. It selectively attacked the phenolic moieties via single-electron abstracting mechanism, leading to phenoxy radical intermediates which couple to each other via C-O-C bonds. The findings of this study shed light on the environmental behaviors and impacts of PS in groundwater environment.

Metal-Free Electro-Activated Sulfite Process for As(III) Oxidation in Water Using Graphite Electrodes

Transition-metal-activated sulfite [S(IV)] processes for water decontamination have recently received intense attention in the field of decontamination by advanced oxidation processes (AOPs). However, the drawback with respect to the secondary metal sludge contamination involved in various AOPs has been argued often. In this work, we developed a novel electro-sulfite (ES) process using stable and low-cost graphite electrodes to address that concern. Arsenite [As(III)] was used as the target compound for removal by the ES process because of its wide presence and high toxicity. Parameters, including cell voltage, S(IV) concentration, solution pH, and water matrix, and the mechanisms for reactions on anode and cathode were investigated in electrolytic cells containing one or two compartments, respectively. The results show that the ES process using 1 mM S(IV) and 2 V cell voltage oxidizes 5 μM As(III) at a rate of 0.127 min^-1, which is 15-fold higher than mere electrolysis without S(IV) addition (0.008 min^-1) at pH 7. Further studies using radical scavengers and electron spin resonance assays demonstrated that oxysulfur radicals (i.e., SO₅^•- and SO₄^•-) and HO^• are responsible for As(III) oxidation in the ES process. However, HO₂^• produced via the oxygen reduction reaction in the EO process plays a major role in As(III) oxidation, which explains the lower reaction rate in the absence of S(IV). The effectiveness of the ES process was moreover evidenced by 60-82% As(III) oxidation in field water within 40 min. Overall, this work realizes the metal-free activation of S(IV) and significantly leverages the S(IV)-based water treatment technologies.

Formation of chloronitrophenols upon sulfate radical-based oxidation of 2-chlorophenol in the presence of nitrite

Sulfate radical (SO₄^-)-based advanced oxidation processes (SR-AOPs) are promising in-situ chemical oxidation technologies widely applied for soil/groundwater remediation. The presence of non-target water constituents may interfere the abatement of contaminants by SR-AOPs as well as result in the formation of unintended byproducts. Herein, we reported the formation of toxic chloronitrophenols during thermally activated persulfate oxidation of 2-chlorophenol (2CP) in the presence of nitrite (NO₂^-). 2-Chloro-4-nitrophenol (2C4NP) and 2-chloro-6-nitrophenol (2C6NP) were identified as nitrated byproducts of 2CP with total yield up to 90%. The formation of nitrated byproducts is a result of coupling reaction between 2CP phenoxyl radical (ClPhO) and nitrogen dioxide radical (NO₂). As a critical step, the formation of ClPhO was supported by density functional theory (DFT) computation. Both 2C4NP and 2C6NP could convert to 2-chloro-4,6-dinitrophenol (2C46DNP) upon further treatment via a denitration-renitration process. The formation rate of 2C4NP and 2C6NP was closely dependent on the concentration of NO₂^-, solution pH, and natural water constituents. ECOSAR calculation suggests that chloronitrophenols are generally more hydrophobic and ecotoxic than 2CP. Our result therefore reveals the potential risks in the abatement of chlorophenols by SR-AOP, particularly when high level of NO₂^- is present in water matrix.

Formation and control of bromate in sulfate radical-based oxidation processes for the treatment of waters containing bromide: A critical review

Sulfate radical-based advanced oxidation processes (SR-AOPs) show a good prospect for effective elimination of organic contaminants in water due to the powerful oxidation capability and good adaptability of sulfate radical (SO₄^•-). However, great concerns have been raised on occurrence of the carcinogenic byproduct bromate (BrO₃^-) in SR-AOPs. The present article aims to provide a critical review on BrO₃^- formation during bromine (Br)-containing water oxidation by various SR-AOPs. Potential reaction mechanisms are elaborated, mainly involving the sequential oxidation of bromide (Br^-) by SO₄^•- to Br-containing radicals (e.g., bromine atom (Br•)) and then to hypobromous acid/hypobromite (HOBr/OBr^-), which acts as the requisite intermediate for BrO₃^- formation. Some key influencing factors on BrO₃^- formation are discussed. Particularly, dissolved organic matter (DOM) as a component ubiquitously present in aquatic environments shows a significant suppression effect on BrO₃^- formation, primarily attributed to the reduction of Br• by DOM to Br^-. The reaction of Br• with DOM can hardly produce organic brominated byproducts, while their formation is mainly due to the bromination of HOBr/OBr^- generated through nonradical pathways such as the direct reaction of Br^- with oxidants (e.g., peroxymonosulfate (PMS)) or other reactive species derived from catalytic activators (e.g., Co(III) in the Co(II)/PMS process). The debromination of brominated pollutants during their oxidation by SO₄^•- results in the release of Br^-, which, however, is not further transformed to BrO₃^- until coexisting organic matters are mineralized nearly completely. Furthermore, possible strategies for control of BrO₃^- formation in SR-AOPs as well as the future research needs are proposed.

Application of activated persulfate for the inactivation of fecal bacterial indicators in water

Activated persulfate, as a member of the broad group of Advanced Oxidation Processes (AOPs), has emerged as a promising method for the elimination of microorganisms in aqueous matrices. This study evaluates the disinfection efficiency of this technique with respect to the inactivation of Escherichia coli and Enterococcus faecalis in water samples, as representative Gram negative and Gram positive bacterial indicators, respectively. In this perspective, various activators were employed, namely, ferric ion, heating, ultrasound application and UVA irradiation, which exhibited different bactericidal effect, depending on the operating conditions and the structural properties of each species. The highest disinfection rates were achieved with 200 mg/L of persulfate and ferric ion or heating as activators. For instance, 6 Log reductions were recorded within only 10-15 min when 30 mg/L of iron were applied, whereas the same bacterial removal was noted upon heat-activation at 50 °C, but in longer periods (i.e. 45-60 min). Nevertheless, in all cases E. faecalis was more resistant than E. coli, which was readily inactivated in shorter treatment periods. The overall process activity was deteriorated above the limit of 200 mg/L of persulfate. Ultrasound application exhibited lower performance, as even more prolonged treatment was required (120-150 min) for the same bacterial decay with the persulfate concentration not affecting substantially the process. In an attempt to improve the ultrasound activity, it was combined together with iron but with no synergistic results, as no actual enhancement of the method was observed. Finally, UVA did not seem to serve as an activator under the applied conditions, taking into account that it resulted in negligible loss of bacterial viability. Based on the current results, activated persulfate may be used successfully for disinfection purposes; however, the appropriate establishment of process variables is mostly required, considering the various resistance levels of aquatic microorganisms under stressed conditions.

Ultraviolet/peroxydisulfate degradation of ofloxacin in seawater: Kinetics, mechanism and toxicity of products

The ultraviolet/peroxydisulfate (UV/PDS) system was used to degrade ofloxacin (OFL) in fresh water, synthetic marine aquaculture water and synthetic seawater. The comparison of the reaction degradation rate constants proved that the order of reaction rate was the following: synthetic seawater (0.77 min^-1) > synthetic marine aquaculture water (0.74 min^-1) > freshwater (0.30 min^-1). Bromide (Br^-) and bicarbonate (HCO₃^-) promote the degradation of OFL, whereas chloride (Cl^-) inhibits the degradation. The piperazine ring of OFL was the main reactive group, and atoms N1, C6, C7 and N2 were identified as the reaction sites. Based on the intermediate and final products, the possible degradation pathways of OFL in the three kinds of water were proposed. Additionally, during the UV/PDS treatment of synthetic marine aquaculture water containing Cl^- and Br^-, the oxidation products of OFL showed a slight toxicity to Chlorella pyrenoidosa (C. pyrenoidosa) and Priacanthus tayenus (P. tayenus). The maximum growth inhibition rate of the products to C. pyrenoidosa was 9.72%. The products also caused liver cells of P. tayenus to be damaged and reduced the species richness and diversity of intestinal microorganism. Nevertheless, compared with the products degraded by traditional disinfection methods using NaClO, the biological toxicities were much lower. UV/PDS can be used for seawater as a new alternative disinfection method.

Synthesis of polyimide-modified carbon nanotubes as catalyst for organic pollutant degradation via production of singlet oxygen with peroxymonosulfate without light irradiation

Polyimide-modified carbon nanotubes (PI/CNTs) were synthesized via a solvent-free thermal method and used as a metal-free catalyst to activate peroxymonosulfate for organic contaminant degradation without light irradiation. The characterization results suggested that PI was loaded onto the surface of CNTs. The catalytic ability of the PI/CNTs was strongly correlated with the content of PI in the catalysts. The PI/CNTs (22% of PI) showed the highest catalytic efficiency for organic pollutant degradation at room temperature. The degradation efficiency of acid orange 7 (AO7) dye was significantly enhanced to 98.9% within 15 min, compared to the efficiency of 2.2% exhibited by pure PI. The radical quenching tests and electron paramagnetic resonance spectrometry proved that singlet oxygen, instead of hydroxyl radicals or sulfate radicals, played a dominant role during the catalytic oxidation of AO7. The influences of operation parameters including temperature and catalyst amount were investigated. The PI/CNTs metal-free catalyst exhibited high catalytic activity under a broad range of pH values. The recycling study of four repeated reactions demonstrated good stability of the PI/CNTs. This work provided a promising metal-free catalyst for degradation of organic pollutants in aqueous solutions, contributing to the development of green materials for sustainable remediation.

Degradation of imidacloprid by UV-activated persulfate and peroxymonosulfate processes: Kinetics, impact of key factors and degradation pathway

UV-activated persulfate (UV/PS) and peroxymonosulfate (UV/PMS) processes as alternative methods for removal of imidacloprid (IMP) were conducted for the first time. The reaction rate constants between IMP and the sulfate or hydroxyl radical were calculated as 2.33×10⁹  or 2.42×10¹⁰ M^-1 s^-1, respectively. The degradation of IMP was greatly improved by UV/PS and UV/PMS compared with only UV or oxidant. At any given dosage, UV/PS achieved higher IMP removal rate than UV/PMS. The pH range affecting the degradation in the UV/PS and UV/PMS systems were different in the ranges of 6-8 and 9 to 10. SO₄^2-, F^- and NO₃^- had no obvious effect on the degradation in the UV/PS and UV/PMS systems. CO₃^2- and PO₄^3- inhibited the degradation of IMP in the UV/PS system, while they enhanced the degradation in the UV/PMS system. Algae organic matters (AOM) were used to consider the impact of the degradation of IMP for the first time. The removal of IMP were restrained by both AOM and natural organic matters. The higher removal rate of IMP demonstrated that both UV/PS and UV/PMS were suitable for treating the water containing IMP, while UV/PS was cost-effective than UV/PMS based on the total cost calculation. Finally, the degradation pathways of IMP were proposed.

An overview of bromate formation in chemical oxidation processes: Occurrence, mechanism, influencing factors, risk assessment, and control strategies

Chemical oxidation processes have been extensively utilized in disinfection and removal of emerging organic contaminants in recent decades. Some undesired byproducts, however, are produced in these processes. Of them, bromate has attracted the most intensive attention. It was previously regarded as a byproduct that typically occurred in ozone-based oxidation processes. However, for the past decade, bromate formation has been detected in other oxidation processes such as CuO-catalyzed chlorination, SO₄^--based oxidation, and ferrate oxidation processes. This review summarizes the occurrences, mechanisms, influencing factors, risk assessment, and control strategies of bromate formation in the four oxidation processes, i.e., ozone-based oxidation, chlorine-based oxidation, SO₄^--based oxidation, and ferrate oxidation. Besides, some unresolved issues for future studies are provided: (1) Clarification of the relative contributions of SO₄^- and Br to the oxidation of bromine for bromate formation in SO₄^--based oxidation processes; (2) evaluation of the role of different reactive species in the bromate formation in the process of UV/HOCl; (3) quantification of the dual role of alkalinity in bromate formation during ozonation; (4) assessment of the risks of bromate formation in SO₄^--based oxidation processes for practical applications; and (5) exploration of strategies for inhibiting bromate formation in SO₄^--based oxidation, UV/chlorine, and metal oxide-catalyzed chlorination processes.

Oxidation byproducts from the degradation of dissolved organic matter by advanced oxidation processes - A critical review

Advanced oxidation processes (AOPs) have been increasingly used for the treatment of source waters and wastewaters. AOPs characteristically produce oxidation byproducts (OBPs) from the partial degradation of dissolved organic matter (DOM) and/or the transformation of inorganic ions (especially, halides) into highly toxic substances including bromate and halogenated organic OBPs (X-OBPs). However, despite the enormous health and environmental risks posed by X-OBPs, an integral understanding of the complex OBP formation mechanisms during AOPs is lacking, which limits the development of safe and effective AOP-based water treatment schemes. The present critical and comprehensive review was intended to fill in this important knowledge gap. The study shows, contrary to the hitherto prevailing opinion, that the direct incorporation of halide atoms (X^•) into DOM makes an insignificant contribution to the formation of organic X-OBPs. The principal halogenating agent is hypohalous acid/hypohalite (HOX/XO^-), whose control is, therefore, critical to the reduction of both organic and inorganic X-OBPs. Significant generation of X-OBPs has been observed during sulfate radical AOPs (SR-AOPs), which arises principally from the oxidizing effects of the unactivated oxidant and/or the applied catalytic activator rather than the sulfate radical as is commonly held. A high organic carbon/X^- molar ratio (>5), an effective non-catalytic activator such as UV or Fe²⁺, a low oxidant concentration, and short treatment time are suggested to limit the accumulation of HOX/XO^- and, thus, the generation of X-OBPs during SR-AOPs. At present, there are no established techniques to prevent the formation of X-OBPs during UV/chlor(am)ine AOPs because the maintenance of substantial amounts of active halogen is essential to these processes. The findings and conclusions reached in this review would advance the research and application of AOPs.

Application of persulfate salts for enhancing UV disinfection in marine waters

Over the years, industrial activities that generate high salinity effluents have been intensifying; this has relevant potential for causing organic and microbiological pollution which damages both human and ocean health. The development of new regulations, such as ballast water convention, encourage the development of treatment systems that can be feasible for treating seawater effluents. Accordingly, an approach based on the UV activation of persulfate salts has been assessed. In this scenario, two different persulfate sources (S₂O₈^2- and HSO₅^-) were evaluated under UV-C irradiation for disinfection purposes. An optimization process was performed with low chemical doses (<1 mM). In order to extensively examine the applicability on seawater, different water matrices were tested as well as different microorganisms including both fecal and marine bacteria. An enhancement of UV-inactivation with the addition of persulfate salts was achieved in all cases, kinetic rate constant has been accelerated by up to 79% in seawater. It implies a UV-dose saving up to 45% to achieve 4-log reductions. Best efficiencies were obtained with [HSO₅^-] = 0.005 mM and [S₂O₈^2-] = 0.5 mM. Higher effectiveness was obtained with the use of HSO₅^- due to its low stability and interaction with chloride. Also, different responses were obtained according to the specific microorganisms by achieving faster disinfection in Gram-negative than in Gram-positive bacteria, the sensitivity observed was Vibrio spp. > E. coli > E. faecalis ≈ Marine Heterotrophic Bacteria. With an evaluation of regrowth after treatment, greater cell damage was detected with the addition of persulfate salts. The major ability of regrowth for marine bacteria encourages the use of a residual disinfectant after disinfection processes.

Carbon Materials Inhibit Formation of Nitrated Aromatic Products in Treatment of Phenolic Compounds by Thermal Activation of Peroxydisulfate in the Presence of Nitrite

Recent studies have reported that toxic nitrated aromatic products are generated during treatment of phenolic compounds by thermally activated peroxydisulfate (thermal/PDS) in the presence of nitrite (NO₂^-). This work explored the potential of carbon materials on controlling the formation of nitrated aromatic products using phenol as a model compound. In the presence of selected carbon materials including diverse carbon nanotubes (CNT) and powdered activated carbon (PAC), the transformation kinetics of phenol was significantly enhanced, primarily attributed to nonradical activation of PDS by carbon materials. Nitrophenols (NPs) including 2-NP and 4-NP were formed in phenol oxidation by the thermal/PDS/NO₂^- process, due to the reaction of phenol with reactive nitrogen species generated from NO₂^- oxidation. The addition of carbon materials obviously inhibited NPs formation under various experimental conditions. The bonding of nitro groups on the CNT surface was clearly confirmed by means of various characterizations, probably resulting from the competitive reaction of reactive nitrogen species with CNT vs phenol. The controlling effect of carbon materials was also verified in the cases of other phenolic compounds. Therefore, the addition of carbon materials may be a promising approach to control the formation of undesirable nitrated byproducts by the thermal/PDS process in the presence of NO₂^-.

Interactions between chlorophenols and peroxymonosulfate: pH dependency and reaction pathways

A non-radical reaction between peroxysulfates and phenolic compounds, as important structural moieties of natural organic matters, has been reported recently, implying new opportunities for environmental remediation without need for catalyst or energy input. However, this approach seems to be ineffective for halogenated aromatic compounds, an important disinfection by-products (DBPs). Here, we shed light on the interactions between peroxymonosulfate (PMS) and chlorophenols and the influential factors. The results show that the chlorophenols transformation kinetics were highly dependent on the solution pH and chlorophenol species: raising the pH significantly accelerated the chlorophenols degradation, and at alkaline pH the removal rates of different chlorophenols were in the order of trichlorophenol > dichlorophenol > chlorophenol > tetrachlorophenol. The faster degradation of pollutants with more chlorine groups was mainly due to their relatively higher dissociation degree, which favors a direct pollutant-PMS interaction to generate radicals for their degradation. The chlorophenol degradation intermediate (i.e. benzoquinone) further mediated the generation of singlet oxygen at alkaline pH, thereby contributing to accelerated pollutant removal. The slower degradation of tetrachlorophenol than other chlorophenols was likely due to its strong electrostatic epulsion to PMS which restricted the reaction. Our work unveils the chlorophenols degradation mechanisms in PMS reaction system, which may facilitate a better understanding and optimization of advanced oxidation processes for pollution control to reduce potential DBPs accumulation.

Formation of Nitrophenolic Byproducts during Heat-Activated Peroxydisulfate Oxidation in the Presence of Natural Organic Matter and Nitrite

Sulfate radical (SO₄^•-)-based advanced oxidation is a viable in situ remediation technology for degrading organic contaminants in the subsurface. In this study, we demonstrated that SO₄^•- could induce the activation of nitrite, an anion commonly present in the subsurface environment, leading to the formation of nitrophenolic byproducts. Fourier-transform infrared spectroscope and ¹⁵N nuclear magnetic resonance analysis revealed that the inorganic nitrite was incorporated into natural organic matter (NOM) to form organic nitrogen upon SO₄^•- oxidation. Nitrophenolic byproducts, including 2-hydroxy-5-nitrobenzoic acid, 4-nitrophenol, and 2,4-dinitrophenol, were identified using high-resolution mass spectrometry in combination with a ¹⁵N labeling technique. Formation of nitrated byproducts was ascribed to the scavenging of SO₄^•- by nitrite, which not only generated the nitrating agent NO₂^• but also inhibited the degradation of organic compounds, making them more available to the reactions with NO₂^•. The phenolic moieties in NOM served as the main reactive sites for NO₂^• attack. The nitration begins with H abstraction on the phenoxy oxygen, followed by the addition of another NO₂^• to its ortho or para site. Decarboxylation followed by NO₂^• addition can also generate nitrophenolic byproducts. To the best of our knowledge, this is the first study reporting the nitration of NOM and formation of toxic nitrophenolic byproducts during SO₄^•--based oxidation. It sheds light on the potential risks of this technology in subsurface remediation practices.

Characterization and biological removal of organic compounds from hydraulic fracturing produced water

Hydraulic fracturing generates large volumes of produced water, and treatment of produced water may be necessary for disposal or reuse. Biological treatment of produced water is a potential approach to remove organic constituents and reduce fouling, in conjunction with other treatment processes. This study investigates the biological treatability of produced water samples from the Utica and Bakken Shales using engineered biofilms. Observed total dissolved organic carbon (DOC) removal varied between 1-87% at normalized total dissolved solids concentrations, suggesting that the composition of produced water, including organic constituents and trace elements such as nutrients and metals, is an important driver of biological treatment performance. Mass spectrometric analyses of the DOC composition revealed various alkanes in all samples, but differences in non-ionic surfactant, halogenated, and acidic compound content. Statistical data reduction approaches suggest that the latter two groups are correlated with reduced biodegradation kinetics. These results demonstrate that the combination of biodegradation performance and organic speciation can guide the assessment of the biological treatment of produced water.

Transformation of antimicrobial agent sulfamethazine by peroxymonosulfate: Radical vs. nonradical mechanisms

Peroxymonosulfate (PMS) is increasingly used as an oxidant for in situ remediation of organic contaminants in soil and groundwater. In this study we demonstrated that sulfamethazine (SMZ) could be transformed by PMS in the absence of any activators. Such transformation was ascribed to the oxidation by PMS per se, rather than free radicals (SO₄^- or HO), superoxide (O₂^-), or singlet oxygen (¹O₂). The aniline moiety of SMZ molecule was the reactive site for PMS oxidation, leading to the formation of nitrated products. This nitration pathway in fact played a significant role in the removal of SMZ in activated PMS oxidation processes. For instance, it contributed 26% of the total SMZ transformation, while SO₄^- contributed the other 74% during the removal of SMZ, in Co(II)/PMS oxidation process with initial PMS and Co(II) concentrations of 1.0 mM and 0.1 μM, respectively. Similar nitration reaction also occurred to other sulfonamide antibiotics bearing an aniline moiety upon the reaction with PMS. Since nitrated sulfonamide antibiotics appear more persistent than the parent compounds and may cause other environmental problems, such a pathway should not be desired. Therefore, PMS might not be an ideal oxidant for the treatment of sulfonamide antibiotics and other compounds having aniline moieties, especially in subsurface remediation practices where efficient activation of PMS represents a major challenge.

Halogen Radical Oxidants in Natural and Engineered Aquatic Systems

Photochemical reactions contribute to the transformation of contaminants and biogeochemically important substrates in environmental aquatic systems. Recent research has demonstrated that halogen radicals (e.g., Cl^•, Br^•, Cl₂^•-, BrCl^•-, Br₂^•-) impact photochemical processes in sunlit estuarine and coastal waters rich in halides (e.g., chloride, Cl^-, and bromide, Br^-). In addition, halogen radicals participate in contaminant degradation in some engineered processes, including chlorine photolysis for drinking water treatment and several radical-based processes for brine and wastewater treatment. Halogen radicals react selectively with substrates (with bimolecular rate constants spanning several orders of magnitude) and via several potential chemical mechanisms. Consequently, their role in photochemical processes remains challenging to assess. This review presents an integrative analysis of the chemistry of halogen radicals and their contribution to aquatic photochemistry in sunlit surface waters and engineered treatment systems. We evaluate existing data on the generation, speciation, and reactivity of halogen radicals, as well as experimental and computational approaches used to obtain this data. By evaluating existing data and identifying major uncertainties, this review provides a basis to assess the impact of halogen radicals on photochemical processes in both saline surface waters and engineered treatment systems.

Chlorate Formation Mechanism in the Presence of Sulfate Radical, Chloride, Bromide and Natural Organic Matter

Halides and natural organic matter (NOM) are inevitable in aquatic environment and influence the degradation of contaminants in sulfate radical (SO₄^•-)-based advanced oxidation processes. This study investigated the formation of chlorate in the coexposure of SO₄^•-, chloride (Cl^-), bromide (Br^-) and/or NOM in UV/persulfate (UV/PDS) and cobalt(II)/peroxymonosulfate (Co/PMS) systems. The formation of chlorate increased with increasing Cl^- concentration in the UV/PDS system, however, in the Co/PMS system, it initially increased and then decreased. The chlorate formation involved the formation of hypochlorous acid/hypochlorite (HOCl/OCl^-) as an intermediate in both systems. The formation was primarily attributable to SO₄^•- in the UV/PDS system, whereas Co(III) played a significant role in the oxidation of Cl^- to HOCl/OCl^- and SO₄^•- was important for the oxidation of HOCl/OCl^- to chlorate in the Co/PMS system. The pseudo-first-order rate constants ( k') of the transformation from Cl^- to HOCl/OCl^- were 3.32 × 10^-6 s^-1 and 9.23 × 10^-3 s^-1 in UV/PDS and Co/PMS, respectively. Meanwhile, k' of HOCl/OCl^- to chlorate in UV/PDS and Co/PMS were 2.43 × 10^-3 s^-1 and 2.70 × 10^-4 s^-1, respectively. Br^- completely inhibited the chlorate formation in UV/PDS, but inhibited it by 45.2% in Co/PMS. The k' of SO₄^•- reacting with Br^- to form hypobromous acid/hypobromite (HOBr/OBr^-) was calculated to be 378 times higher than that of Cl^- to HOCl/OCl^-, but the k' of Co(III) reacting with Br^- to form HOBr/OBr^- was comparable to that of Cl^- to HOCl/OCl^-. NOM also significantly inhibited the chlorate formation, due to the consumption of SO₄^•- and reactive chlorine species (RCS, such as Cl·, ClO· and HOCl/OCl^-). This study demonstrated the formation of chlorate in SO₄^•--based AOPs, which should to be considered in their application in water treatment.

Influence of water matrix species on persulfate oxidation of phenol: reaction kinetics and formation of undesired degradation byproducts

The present study explored the influence of Cl^-, Br^-, CO₃^2-, HCO₃^-, PO₄^3-, HPO₄^2-, NO₃^-, SO₃^2- and natural organic matter (NOM) on the reaction kinetics and the formation of undesired degradation byproducts during phenol oxidation by heat-activated persulfate (PS). CO₃^2- and PO₄^3- promoted the phenol degradation, because the hydrolysis of CO₃^2- and PO₄^3- created basic pH conditions which were conducive to enhanced PS oxidation rate. Br^- promoted the reaction by reacting with sulfate radicals (SO₄^•-) to produce bromine radicals that can selectively react with electron-rich phenol. NOM scavenged reactive SO₄^•-, thus inhibiting the reaction. As a strong reducing agent, SO₃^2- rapidly reduced PS, thus completely suppressing the reaction. HCO₃^-, HPO₄^2-, Cl^-, and NO₃^- had negligible impact on PS oxidation of phenol. Six intermediates were detected in the no anion control using gas chromatography-mass spectrometry (GC-MS). Various toxic halogenated phenols and halogenated hydroquinones were detected in the treatment containing Cl^- and Br^-. In contrast, in the treatment containing CO₃^2-, HCO₃^-, PO₄^3-, HPO₄^2-, and NO₃^-, no new intermediates were identified except for the intermediates already detected in the control treatment. Based on intermediates identified, reaction pathways for PS oxidation of phenol without anions and in the presence of halides were proposed respectively.

Formation of brominated oligomers during phenol degradation on boron-doped diamond electrode

In this research, the brominated oligomers formed during phenol degradation with boron-doped diamond (BDD) anode had been initially studied at three different concentrations of bromide (1, 10 and 100mM). The results from LC/MS analysis indicated that, brominated monomer, dimer and trimer of phenols resulting from electrophilic substitution and coupling reactions were the important reaction by-products. Specifically, the trimer by-products were generated only in bromide-rich systems. The reaction mechanisms concerning oligomer formations were proposed in detail accordingly. The above results were in well accordance with those recorded in the degradation experiments. As a whole, bromides and chlorides demonstrated quite different effects toward phenol degradation, which deepened our understanding on the reactions involved in BDD anode cells.

Electrolysis assisted persulfate with annular iron sheet as anode for the enhanced degradation of 2, 4-dinitrophenol in aqueous solution

Annular iron sheet (AIS), playing a dual role of anode electrode and source of activator, was combined with electrolysis technology for the activation of persulfate to improve the degradation of 2, 4-dinitrophenol (DNP) in aqueous solution. In this study, effects of current density (0-10.0mA/cm²), persulfate (PS) dosage (0-8.0mM), initial pH (3.0-11.0), reaction temperature (25-60°C) and reaction time (0-30min) on COD removal of DNP in aqueous solution were investigated, respectively. COD removal reached its maximal value (63.4%) after 15min treatment due to the synergistic effect in electro/AIS/PS system under the optimal conditions. Furthermore, comparative studies of 7 different experimental processes were setup. In addition, the reasonable DNP degradation pathway was proposed based on intermediates detected by HPLC. According to characterization analysis of SEM-EDS, XRD and XPS of the generated flocculation in electro/AIS/PS system, the possible reaction mechanism was proposed in detail. In a word, the electrolysis process coupled with annular iron sheet as anode activating persulfate technology shows a significant synergetic effect in enhancing degradation of DNP in aqueous solution.

Impacts of inorganic anions and natural organic matter on thermally activated persulfate oxidation of BTEX in water

The present study investigated the impacts of water matrix constituents (CO₃^2-, HCO₃^-, Cl^-, Br^-, PO₄^3-, HPO₄^2-, H₂PO₄^-, NO₃^-, SO₄^2- and natural organic matters (NOM) on the oxidation of a mixture of benzene, toluene, ethylbenzene, and xylenes (BTEX) by thermally activated persulfate (PS). In the absence of matrix constituents, the BTEX oxidation rates decreased in the following order: xylenes > toluene ≈ ethylbenzene > benzene. HCO₃^-/CO₃^2- and NOM inhibited the BTEX oxidation and the inhibiting effects became more pronounced as the HCO₃^-/CO₃^2-/NOM concentration increased. SO₄^2-, NO₃^-, PO₄^3- and H₂PO₄^- did not affect the BTEX oxidation while HPO₄^2- slightly inhibited the reaction. The impacts of Cl^- and Br^- were complex. Cl^- inhibited the benzene oxidation while 100 mM and 500 mM of Cl^- promoted the oxidation of m-xylene and p-xylene. Br^- completely suppressed the benzene oxidation while 500 mM of Br^- strongly promoted the oxidation of xylenes. Detailed explanations on the influence of each matrix constituent were discussed. In addition, various halogenated degradation byproducts were detected in the treatments containing Cl^- and Br^-. Overall, this study indicates that some matrix constituents such as NOM, HCO₃^-, CO₃^2-, H₂PO₄^-, Cl^- and Br^- may reduce the BTEX removal efficiency of sulfate radical-based advanced oxidation process (SR-AOP) and the presence of Cl^- and Br^- may even lead to the formation of toxic halogenated byproducts.

The role of nitrite in sulfate radical-based degradation of phenolic compounds: An unexpected nitration process relevant to groundwater remediation by in-situ chemical oxidation (ISCO)

As promising in-situ chemical oxidation (ISCO) technologies, sulfate radical-based advanced oxidation processes (SR-AOPs) are applied in wastewater treatment and groundwater remediation in recent years. In this contribution, we report for the first time that, thermally activated persulfate oxidation of phenol in the presence of nitrite (NO₂^-), an anion widely present in natural waters, could lead to the formation of nitrated by-products including 2-nitrophenol (2-NP), 4-nitrophenol (4-NP), 2,4-dinitrophenol (2,4-DNP), and 2,6-dinitrophenol (2,6-DNP). Nitrogen dioxide radical (NO₂^•), arising from SO₄^•- scavenging by NO₂^-, was proposed to be involved in the formation of nitrophenols as a nitrating agent. It was observed that nitrophenols accounted for approximately 70% of the phenol transformed under reaction conditions of [NO₂^-] = 200 μM, [PS] = 2 mM and temperature of 50 °C. Increasing the concentration of NO₂^- remarkably enhanced the formation of nitrophenols but did not affect the transformation rate of phenol significantly. The degradation of phenol and the formation of nitrophenols were significantly influenced by persulfate dosage, solution pH and natural organic matter (NOM). Further studies on the degradation of other phenolic compounds, including 4-chlorophenol (4-CP), 4-hydroxybenzoic acid (4-HBA), and acetaminophen (ATP), verified the formation of their corresponding nitrated by-products as well. Therefore, formation of nitrated by-products is probably a common but overlooked phenomenon during SO₄^•--based oxidation of phenolic compounds in the presence of NO₂^-. Nitroaromatic compounds are well known for their carcinogenicity, mutagenicity and genotoxicity, and are potentially persistent in the environment. The formation of nitrated organic by-products in SR-AOPs should be carefully scrutinized, and risk assessment should be carried out to assess possible health and ecological impacts.

Comparative study of the formation of brominated disinfection byproducts in UV/persulfate and UV/H2O2 oxidation processes in the presence of bromide

The objective of this research was to compare the transformation of Br^- and formation of brominated byproducts in UV/persulfate (PS) and UV/H₂O₂ processes. It was revealed that Br^- was efficiently transformed to free bromine which reacted with humic acid (HA) or dihydroxybenzoic acid resulting in the formation of brominated byproducts such as bromoacetic acids (BAAs) in UV/PS system. In contrast, no free bromine and brominated byproducts could be detected in UV/H₂O₂ system, although the oxidization of Br^- was evident. We presumed that the oxidation of Br^- by hydroxyl radicals led to the formation of bromine radicals. However, the bromine radical species could be immediately reduced back to Br^- by H₂O₂ before coupling to each other to form free bromine, which explains the undetection of free bromine and BAAs in UV/H₂O₂. In addition to free bromine, we found that the phenolic functionalities in HA molecules, which served as the principal reactive sites for free chlorine attack, could be in situ generated when HA was exposed to free radicals. This study demonstrates that UV/H₂O₂ is more suitable than UV/PS for the treatment of environmental matrices containing Br^-. Graphical abstract Graphical abstract.

Oxidation Kinetics of Bromophenols by Nonradical Activation of Peroxydisulfate in the Presence of Carbon Nanotube and Formation of Brominated Polymeric Products

This work demonstrated that bromophenols (BrPs) could be readily oxidized by peroxydisulfate (PDS) activated by a commercial carbon nanotube (CNT), while furfuryl alcohol (a chemical probe for singlet oxygen (¹O₂)) was quite refractory. Results obtained by radical quenching experiments, electron paramagnetic resonance spectroscopy, and Fourier transform infrared spectroscopy further confirmed the involvement of nonradical PDS-CNT complexes rather than ¹O₂. Bicarbonate and chloride ion exhibited negligible impacts on BrPs degradation by the PDS/CNT system, while a significant inhibitory effect was observed for natural organic matter. The oxidation of BrPs was influenced by solution pH with maximum rates occurring at neutral pH. Linear free energy relationships (LFERs) were established between the observed pseudo-first-order oxidation rates of various substituted phenols and the classical descriptor variables (i.e., Hammett constant σ⁺, and half-wave oxidation potential E_1/2). Products analyses by liquid chromatography tandem mass spectrometry clearly showed the formation of hydroxylated polybrominated diphenyl ethers and hydroxylated polybrominated biphenyls on CNT surface. Their formation pathway possibly involved the generation of bromophenoxyl radicals from BrPs one-electron oxidation and their subsequent coupling reactions. These results suggest that the novel nonradical PDS/CNT oxidation technology is a good alternative for selectively eliminating BrPs with alleviating toxic byproducts in treated water effluent.

Transformation of iodide and formation of iodinated by-products in heat activated persulfate oxidation process

Formation of halogenated disinfection by-products (DBPs) in sulfate radical-based advanced oxidation processes (SR-AOPs) have attracted considerable concerns recently. Previous studies have focused on the formation of chlorinated and brominated DBPs. This research examined the transformation of I^- in heat activated PS oxidation process. Phenol was employed as a model compound to mimic the reactivity of dissolved natural organic matter (NOM) toward halogenation. It was found that I^- was transformed to free iodine which attacked phenol subsequently leading to iodinated DBPs such as iodoform and iodoacetic acids. Iodophenols were detected as the intermediates during the formation of the iodoform and triiodoacetic acid (TIAA). However, diiodoacetic acid (DIAA) was formed almost concomitantly with iodophenols. In addition, the yield of DIAA was significantly higher than that of TIAA, which is distinct from conventional halogenation process. Both the facts suggest that different pathway might be involved during DIAA formation in SR-AOPs. Temperature and persulfate dose were the key factors governing the transformation process. The iodinated by-products can be further degraded by excessive SO₄^- and transformed to iodate. This study elucidated the transformation pathway of I^- in SR-AOPs, which should be taken into consideration when persulfate was applied in environmental matrices containing iodine.