Aqueous Iron(IV)-Oxo Complex: An Emerging Powerful Reactive Oxidant Formed by Iron(II)-Based Advanced Oxidation Processes for Oxidative Water Treatment.

High-valent iron(IV)-oxo complexes are of great significance as reactive intermediates implicated in diverse chemical and biological systems. The aqueous iron(IV)-oxo complex (FeaqIVO2+) is the simplest but one of the most powerful ferryl ion species, which possesses a high-spin state, high reduction potential, and long lifetime. It has been well documented that FeaqIVO2+ reacts with organic compounds through various pathways (hydrogen-atom, hydride, oxygen-atom, and electron transfer as well as electrophilic addition) at moderate reaction rates and show selective reactivity toward inorganic ions prevailing in natural water, which single out FeaqIVO2+ as a superior candidate for oxidative water treatment. This review provides state-of-the-art knowledge on the chemical properties and oxidation mechanism and kinetics of FeaqIVO2+, with special attention to the similarities and differences to two representative free radicals (hydroxyl radical and sulfate radical). Moreover, the prospective role of FeaqIVO2+ in Feaq2+ activation-initiated advanced oxidation processes (AOPs) has been intensively investigated over the past 20 years, which has significantly challenged the conventional recognition that free radicals dominated in these AOPs. The latest progress in identifying the contribution of FeaqIVO2+ in Feaq2+-based AOPs is thereby reviewed, highlighting controversies on the nature of the reactive oxidants formed in several Feaq2+ activated peroxide and oxyacid processes. Finally, future perspectives for advancing the evaluation of FeaqIVO2+ reactivity from an engineering viewpoint are proposed.

New Insights into the Combination of Permanganate and Bisulfite as a Novel Advanced Oxidation Process: Importance of High Valent Manganese-Oxo Species and Sulfate Radical.

Recently, it has been reported that the combination of permanganate (Mn(VII)) and bisulfite can lead to a rapid degradation of organic contaminants, where soluble Mn(III) is proposed to be responsible. Interestingly, in this work, we demonstrated the involvement of high-valent Mn-oxo species (possibly Mn(V)) as well as sulfate radical in the Mn(VII)/bisulfite system, by using methyl phenyl sulfoxide (PMSO) as a chemical probe. It was found that the combination of Mn(VII) and bisulfite resulted in appreciable degradation of PMSO under various conditions, while negligible PMSO was degraded by manganese dioxide (MnO2) in the presence of bisulfite under similar conditions. This result indicated that Mn(III) intermediate formed in situ in both Mn(VII)/bisulfite and MnO2/bisulfite systems as proposed in literature exhibited sluggish reactivity toward PMSO. In parallel, the formation of methyl phenyl sulfone (PMSO2) product in the Mn(VII)/bisulfite system was observed, suggesting the role of high-valent Mn-oxo species as an oxygen-atom donor in conversion of PMSO to PMSO2. Moreover, the yield of PMSO2 (i.e., mole of PMSO2 produced per mole of PMSO degraded) was quantified to be 20-100%, strongly depending on the [Mn(VII)]/[bisulfite] ratio as well as solution pH. The competitive contribution of sulfate radical, which oxidized PMSO to hydroxylated and/or polymeric products but not to PMSO2, accounted for the yield of PMSO2 less than 100%. This work advances the fundamental understanding of a novel class of oxidation technology based on the combination of Mn(VII) and bisulfite for environmental decontamination.

Removal of Organoarsenic with Ferrate and Ferrate Resultant Nanoparticles: Oxidation and Adsorption.

Many investigations focused on the capacity of ferrate for the oxidation of organic pollutant or adsorption of hazardous species, while little attention has been paid on the effect of ferrate resultant nanoparticles for the removal of organics. Removing organics could improve microbiological stability of treated water and control the formation of disinfection byproducts in following treatment procedures. Herein, we studied ferrate oxidation of p-arsanilic acid ( p-ASA), an extensively used organoarsenic feed additive. p-ASA was oxidized into As(V), p-aminophenol ( p-AP), and nitarsone in the reaction process. The released As(V) could be eliminated by in situ formed ferric (oxyhydr) oxides through surface adsorption, while p-AP can be further oxidized into 4,4'-(diazene-1,2-diyl) diphenol, p-nitrophenol, and NO3-. Nitarsone is resistant to ferrate oxidation, but mostly adsorbed (>85%) by ferrate resultant ferric (oxyhydr) oxides. Ferrate oxidation (ferrate/ p-ASA = 20:1) eliminated 18% of total organic carbon (TOC), while ferrate resultant particles removed 40% of TOC in the system. TOC removal efficiency is 1.6 to 38 times higher in ferrate treatment group than those in O3, HClO, and permanganate treatment groups. Besides ferrate oxidation, adsorption of organic pollutants with ferrate resultant nanoparticles could also be an effective method for water treatment and environmental remediation.

Does Soluble Mn(III) Oxidant Formed in Situ Account for Enhanced Transformation of Triclosan by Mn(VII) in the Presence of Ligands?

In previous studies, we interestingly found that several ligands (e.g., pyrophosphate, nitrilotriacetate, and humic acid) could significantly accelerate the oxidation rates of triclosan (TCS; the most widely used antimicrobial) by aqueous permanganate (Mn(VII)) especially at acid pH, which was ascribed to the contribution of ligand-stabilized Mn(III) (defined Mn(III)L) formed in situ as a potent oxidant. In this work, it was found that the oxidation of TCS by Mn(III)L resulted in the formation of dimers, as well as hydroxylated and quinone-like products, where TCS phenoxy radical was likely involved. This transformation pathway distinctly differed from that involved in Mn(VII) oxidation of TCS, where 2,4-dichlorophenol (DCP) was the major product with a high yield of ∼80%. Surprisingly, we found that the presence of various complexing ligands including pyrophosphate, nitrilotriacetate, and humic acid, as well as bisulfite slightly affected the yields of DCP, although they greatly enhanced the oxidation kinetics of TCS by Mn(VII). This result could not be reasonably explained by taking the contribution of Mn(III)L into account. Comparatively, the degradation of TCS by manganese dioxide (MnO2) was also greatly enhanced in the presence of these ligands with negligible formation of DCP, which could be rationalized by the contribution of Mn(III)L. In addition, it was demonstrated that DCP could not be generated from Mn(VII) oxidation of unstable phenoxy radical intermediates and stable oxidation products formed from TCS by Mn(III)L. These findings indicate that manganese intermediates other than Mn(III) are likely involved in the Mn(VII)/TCS/ligand systems responsible for the high yields of DCP product.

Transformation of Flame Retardant Tetrabromobisphenol A by Aqueous Chlorine and the Effect of Humic Acid.

In this work, it was found that the most widely used brominated flame retardant tetrabromobisphenol A (TBrBPA) could be transformed by free chlorine over a wide pH range from 5 to 10 with apparent second-order rate constants from 138 to 3210 M(-1)·s(-1). A total of eight products, including one quinone-like compound (i.e., 2,6-dibromoquinone), two dimers, and several simple halogenated phenols (e.g., 4-(2-hydroxyisopropyl)-2,6-dibromophenol, 2,6-dibromohydroquinone, and 2,4,6-tribromophenol), were detected by high-performance liquid chromatography tandem mass spectrometry (HPLC-MS/MS) using a novel precursor ion scan (PIS) approach. A tentative reaction pathway was proposed: chlorine initially oxidized TBrBPA leading to the formation of a phenoxy radical, and then this primary radical and its secondary intermediates (e.g., 2,6-dibromo-4-isopropylphenol carbocation) formed via beta-scission subsequently underwent substitution, dimerization, and oxidation reactions. Humic acid (HA) considerably inhibited the degradation rates of TBrBPA by chlorine even accounting for oxidant consumption. A similar inhibitory effect of HA was also observed in permanganate and ferrate oxidation. This inhibitory effect was possibly attributed to the fact that HA competitively reacted with the phenoxy radical of TBrBPA and reversed it back to parent TBrBPA. This study confirms that chlorine can transform phenolic compounds (e.g., TBrBPA) via electron transfer rather than the well-documented electrophilic substitution, which also have implications on the formation pathway of halo-benzoquinones during chlorine disinfection. These findings can improve the understanding of chlorine chemistry in water and wastewater treatment.

Understanding the role of manganese dioxide in the oxidation of phenolic compounds by aqueous permanganate.

Recent studies have shown that manganese dioxide (MnO2) can significantly accelerate the oxidation kinetics of phenolic compounds such as triclosan and chlorophenols by potassium permanganate (Mn(VII)) in slightly acidic solutions. However, the role of MnO2 (i.e., as an oxidant vs catalyst) is still unclear. In this work, it was demonstrated that Mn(VII) oxidized triclosan (i.e., trichloro-2-phenoxyphenol) and its analogue 2-phenoxyphenol, mainly generating ether bond cleavage products (i.e., 2,4-dichlorophenol and phenol, respectively), while MnO2 reacted with them producing appreciable dimers as well as hydroxylated and quinone-like products. Using these two phenoxyphenols as mechanistic probes, it was interestingly found that MnO2 formed in situ or prepared ex situ greatly accelerated the kinetics but negligibly affected the pathways of their oxidation by Mn(VII) at acidic pH 5. The yields (R) of indicative products 2,4-dichlorophenol and phenol from their respective probes (i.e., molar ratios of product formed to probe lost) under various experimental conditions were quantified. Comparable R values were obtained during the treatment by Mn(VII) in the absence vs presence of MnO2. Meanwhile, it was confirmed that MnO2 could accelerate the kinetics of Mn(VII) oxidation of refractory nitrophenols (i.e., 2-nitrophenol and 4-nitrophenol), which otherwise showed negligible reactivity toward Mn(VII) and MnO2 individually, and the effect of MnO2 was strongly dependent upon its concentration as well as solution pH. These results clearly rule out the role of MnO2 as a mild co-oxidant and suggest a potential catalytic effect on Mn(VII) oxidation of phenolic compounds regardless of their susceptibility to oxidation by MnO2.

Activation of Peroxymonosulfate by Benzoquinone: A Novel Nonradical Oxidation Process.

The reactions between peroxymonosulfate (PMS) and quinones were investigated for the first time in this work, where benzoquinone (BQ) was selected as a model quinone. It was demonstrated that BQ could efficiently activate PMS for the degradation of sulfamethoxazole (SMX; a frequently detected antibiotic in the environments), and the degradation rate increased with solution pH from 7 to 10. Interestingly, quenching studies suggested that neither hydroxyl radical (•OH) nor sulfate radical (SO4•-) was produced therein. Instead, the generation of singlet oxygen (1O2) was proved by using two chemical probes (i.e., 2,2,6,6-tetramethyl-4-piperidinol and 9,10-diphenylanthracene) with the appearance of 1O2 indicative products detected by electron paramagnetic resonance spectrometry and liquid chromatography mass spectrometry, respectively. A catalytic mechanism was proposed involving the formation of a dioxirane intermediate between PMS and BQ and the subsequent decomposition of this intermediate into 1O2. Accordingly, a kinetic model was developed, and it well described the experimental observation that the pH-dependent decomposition rate of PMS was first-order with respect to BQ. These findings have important implications for the development of novel nonradical oxidation processes based on PMS, because 1O2 as a moderately reactive electrophile may suffer less interference from background organic matters compared with nonselective •OH and SO4•-.

ABTS as an Electron Shuttle to Enhance the Oxidation Kinetics of Substituted Phenols by Aqueous Permanganate.

In this study, it was, interestingly, found that 2,2'-azino-bis(3-ethylbenzothiazoline)-6-sulfonate (ABTS), a widely used electron shuttle, could greatly accelerate the oxidation of substituted phenols by potassium permanganate (Mn(VII)) in aqueous solutions at pH 5-9. This was attributed to the fact that these substituted phenols could be readily oxidized by the stable radical cation (ABTS(•+)), which was quickly produced from the oxidation of ABTS by Mn(VII). The reaction of Mn(VII) with ABTS exhibited second-order kinetics, with stoichiometries of ∼5:1 at pH 5-6 and ∼3:1 at pH 7-9, and the rate constants varied negligibly from pH 5 to 9 (k = (9.44 ± 0.21) × 10(4) M(-1) s(-1)). Comparatively, the reaction of ABTS(•+) with phenol showed biphasic kinetics. The second-order rate constants for the reactions of ABTS(•+) with substituted phenols obtained in the initial phase were strongly affected by pH, and they were several orders of magnitude higher than those for the reactions of Mn(VII) with substituted phenols at each pH. Good Hammett-type correlations were found for the reactions of ABTS(•+) with undissociated (log(k) = 2.82-4.31σ) and dissociated phenols (log(k) = 7.29-5.90σ). The stoichiometries of (2.2 ± 0.06):1 (ABTS(•+) in excess) and (1.38 ± 0.18):1 (phenol in excess) were achieved in the reaction of ABTS(•+) with phenol, but they exhibited no pH dependency.

Oxidation of bromophenols and formation of brominated polymeric products of concern during water treatment with potassium permanganate.

The extensive use of bromophenols (BrPs) in industrial products leads to their occurrence in freshwater environments. This study explored the oxidation kinetics of several BrPs (i.e., 2-BrP, 3-BrP, 4-BrP, 2,4-diBrP, and 2,6-diBrP) and potential formation of brominated polymeric products of concern during water treatment with potassium permanganate [Mn(VII)]. These BrPs exhibited appreciable reactivity toward Mn(VII) with the maxima of second-order rate constants (kMn(VII)) at pH near their pKa values, producing bell-shaped pH-rate profiles. The unusual pH-dependency of kMn(VII) was reasonably explained by a tentative reaction model, where the formation of an intermediate between Mn(VII) and dissociated BrP was likely involved. A novel and powerful precursor ion scan (PIS) approach was used for selective detection of brominated oxidation products by liquid chromatography/electrospray ionization-triple quadrupole mass spectrometry. Results showed that brominated dimeric products such as hydroxylated polybrominated diphenyl ethers (OH-PBDEs) and hydroxylated polybrominated biphenyls (OH-PBBs) were readily produced. For instance, 2'-OH-BDE-68, one of the most naturally abundant OH-PBDEs, could be formed at a relatively high yield possibly via the coupling between bromophenoxyl radicals generated from the one-electron oxidation of 2,4-diBrP by Mn(VII). Given the altered or enhanced toxicological effects of these brominated polymeric products compared to the BrP precursors, it is important to better understand their reactivity and fate before Mn(VII) is applied by water utilities for the oxidative treatment of BrP-containing waters.

Oxidation of flame retardant tetrabromobisphenol a by aqueous permanganate: reaction kinetics, brominated products, and pathways.

In this work, the most widely used brominated flame retardant tetrabromobisphenol A (TBrBPA) was shown to exhibit appreciable reactivity toward potassium permanganate [Mn(VII)] in water over a wide pH range of 5-10 with the maxima of second-order rate constants (kMn(VII) = 15-700 M(-1) s(-1)) at pH near its pKa values (7.5/8.5). A novel precursor ion scan (PIS) approach using negative electrospray ionization-triple quadrupole mass spectrometry (ESI-QqQMS) was adopted and further optimized for fast selective detection of brominated oxidation products of TBrBPA by Mn(VII). By setting PIS of m/z 79 and 81, two major products (i.e., 4-(2-hydroxyisopropyl)-2,6-dibromophenol and 4-isopropylene-2,6-dibromophenol) and five minor ones (including 2,6-dibromophenol, 2,6-dibromo-1,4-benzoquinone, and three dimers) were detected and suggested with chemical structures from their product ion spectra and bromine isotope patterns. Reaction pathways mainly involving the initial one-electron oxidation of TBrBPA and subsequent release and further reactions of 2,6-dibromo-4-isopropylphenol carbocation intermediate were proposed. The effectiveness of Mn(VII) for treatment of TBrBPA in real waters was confirmed. It is important to better understand the reactivity and toxicity of primary brominated products before Mn(VII) can be applied for treatment of TBrBPA-contaminated wastewater and source water.

Transformation of hydroxylamine to nitrosated and nitrated products during advanced oxidation process.

Recently, hydroxylamine (HAm) was introduced to drive advanced oxidation processes (AOPs) for removing organic contaminants. However, we found that HAm-driven Cu(II)/peroxymonosulfate oxidation of phenol produced p-nitrosophenol, 2-nitrophenol and 4-nitrophenol. The total nitro(so) products accounted for approximately 25.0 % of the phenol transformation at certain condition. SO4•- and •OH were identified as the primary and second significant oxidants, respectively. Reactive nitrogen species (RNS) were involved in phenol transformation. The pathway and mechanism of HAm transformation in HAm-driven transition metal ion-catalyzed AOPs were proposed for the first time in this study. The product of HAm via twice single-electron oxidation by Cu(II) is nitroxyl (HNO/NO-), which is a critical oxidation intermediate of HAm. Further oxidation of HNO by SO4•- or •OH is the initial step in propagating radical chain reactions, leading to nitric oxide radical (•NO) and nitrogen dioxide radical (•NO2) as the primary RNS. HAm is a critical intermediate in natural nitrogen cycle, suggesting that HAm can drive the oxidation processes of pollutants in natural environments. Nitro(so) products will be readily produced when AOPs are applied for ecological remediation. This study highlights the formation of toxic nitrosated and nitrated products in HAm-driven AOPs, and the requirement of risk assessments to evaluate the possible health and ecological impacts.

Oxidation of phenolic endocrine disrupting chemicals by potassium permanganate in synthetic and real waters.

In this study, five selected environmentally relevant phenolic endocrine disrupting chemicals (EDCs), estrone, 17ß-estradiol, estriol, 17α-ethinylestradiol, and 4-n-nonylphenol, were shown to exhibit similarly appreciable reactivity toward potassium permanganate [Mn(VII)] with a second-order rate constant at near neutral pH comparable to those of ferrate(VI) and chlorine but much lower than that of ozone. In comparison with these oxidants, however, Mn(VII) was much more effective for the oxidative removal of these EDCs in real waters, mainly due to the relatively high stability of Mn(VII) therein. Mn(VII) concentrations at low micromolar range were determined by an ABTS [2,2-azino-bis(3-ethylbenzothiazoline)-6-sulfonic acid diammonium] spectrophotometric method based on the stoichiometric reaction of Mn(VII) with ABTS [Mn(VII) + 5ABTS → Mn(II) + 5ABTS(•+)] forming a stable green radical cation (ABTS(•+)). Identification of oxidation products suggested the initial attack of Mn(VII) at the hydroxyl group in the aromatic ring of EDCs, leading to a series of quinone-like and ring-opening products. The background matrices of real waters as well as selected model ligands including phosphate, pyrophosphate, NTA, and humic acid were found to accelerate the oxidation dynamics of these EDCs by Mn(VII). This was explained by the effect of in situ formed dissolved Mn(III), which could readily oxidize these EDCs but would disproportionate spontaneously without stabilizing agents.

Oxidation of sulfoxides and arsenic(III) in corrosion of nanoscale zero valent iron by oxygen: evidence against ferryl ions (Fe(IV)) as active intermediates in Fenton reaction.

Previous studies have shown that the corrosion of zerovalent iron (ZVI) by oxygen (O(2)) via the Fenton reaction can lead to the oxidation of various organic and inorganic compounds. However, the nature of the oxidants involved (i.e., ferryl ion (Fe(IV)) versus hydroxyl radical (HO(•))) is still a controversial issue. In this work, we reevaluated the relative importance of these oxidants and their role in As(III) oxidation during the corrosion of nanoscale ZVI (nZVI) in air-saturated water. It was shown that Fe(IV) species could react with sulfoxides (e.g., dimethyl sulfoxide, methyl phenyl sulfoxide, and methyl p-tolyl sulfoxide) through a 2-electron transfer step producing corresponding sulfones, which markedly differed from their HO(•)-involved products. When using these sulfoxides as probe compounds, the formation of oxidation products indicative of HO(•) but no generation of sulfone products supporting Fe(IV) participation were observed in the nZVI/O(2) system over a wide pH range. As(III) could be completely or partially oxidized by nZVI in air-saturated water. Addition of scavengers for solution-phase HO(•) and/or Fe(IV) quenched As(III) oxidation at acidic pH but had little effect as solution pH increased, highlighting the importance of the heterogeneous iron surface reactions for As(III) oxidation at circumneutral pH.

Enhanced peroxymonosulfate activation via complexed Mn(II): A novel non-radical oxidation mechanism involving manganese intermediates.

In recent years, the activation of persulfates (peroxydisulfate (PDS) and peroxymonosulfate (PMS)) via transition metal ions for contaminants degradation has received extensive attention in water treatment. There has been growing interest on the mechanism (radical versus non-radical pathway) of activation processes. Interestingly, in contrast to copper, iron or cobalt ions regarded as effective activators for persulfates, manganese ion (Mn(II)) is inefficient for persulfates activation. Inspired by the enhanced stability of manganese species by ligands, this study for the first time systematically investigated the Mn(II)/persulfates with different ligands as a novel oxidation technology. UV-vis spectrometry, chemical probing method and mass spectrometry were used to explore the reactive intermediate (free radical versus high-valent manganese species) therein. It was surprisingly found that the oxidation efficiency of Mn(II)/ligand/persulfates system was highly dependent on the nature of persulfates and ligands. Mn(II) chelated by amino ligands such as ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetate (NTA) could efficiently trigger the oxidation of contaminants (e.g., recalcitrant compounds nitrophenol, benzoic acid and atrazine) by PMS, suggesting a promising Mn(II)/ligand/PMS technology for environmental decontamination especially under manganese-rich conditions. High-valent Mn species (Mn(V)) but not free radicals was demonstrated to be the dominant reactive intermediate, where Mn(III) species played a vital role in Mn(V) generation. The formation of Mn(III) species was found to be affected by the reactivity of persulfates and the type of ligands, thus influencing its further oxidation to Mn(V) species. This study presents a new oxidation process based on the combination of PMS and Mn(II) complex and broadens the knowledge of persulfates activation as well as manganese chemistry for decontamination in water treatment.

A comparison study of levofloxacin degradation by peroxymonosulfate and permanganate: Kinetics, products and effect of quinone group.

Permanganate (Mn(VII)) as a selective oxidant has been widely used in water treatment process. Recently, peroxymonosulfate (PMS) was recognized as an emerging selective oxidant, which showed appreciable reactivity toward organic compounds containing electron-rich functional groups. In this study, the oxidation of a model fluoroquinolone antibiotic levofloxacin (LEV) by Mn(VII) and PMS was comparatively investigated. Degradation of LEV by PMS followed second-order kinetics and showed strong pH dependency with apparent second-order rate constants (kapp) of 0.15-26.52 M-1 s-1 at pH 5.0-10.0. Oxidation of LEV by Mn(VII) showed autocatalysis at pH 5.0-7.0, while no autocatalysis was observed at pH 8.0-10.0 (kapp = 2.23-4.16 M-1 s-1). Such unusual oxidation kinetics was attributed to the in-situ formed MnO2 from Mn(VII) consumption. The performance of PMS and Mn(VII) for the degradation of LEV was also examined in real waters. PMS primarily react with the aliphatic N4 amine on the piperazine ring of LEV, and Mn(VII) reacted with both the aliphatic N4 amine and aromatic N1 amine. Both PMS and Mn(VII) could efficiently eliminate the antibiotic activity of LEV. Benzoquinone showed activating effect on both PMS and Mn(VII) oxidation, but their activation mechanisms were totally different.

Oxidative transformation of emerging organic contaminants by aqueous permanganate: Kinetics, products, toxicity changes, and effects of manganese products.

Permanganate (Mn(VII)) has been widely studied for removal of emerging organic contaminants (EOCs) in water treatment and in situ chemical oxidation process. Studies on the reactive intermediate manganese products (e.g., Mn(III) and manganese dioxide (MnO2)) generated from Mn(VII) reduction by EOCs in recent decades shed new light on Mn(VII) oxidation process. The present work summarizes the latest research findings on Mn(VII) reactions with a wide range of EOCs (including phenols, olefins, and amines) in detailed aspects of reaction kinetics, oxidation products, and toxicity changes, along with special emphasis on the impacts of intermediate manganese products (mainly Mn(III) and MnO2) in-situ formed. Mn(VII) shows appreciable reactivities towards EOCs with apparent second-order rate constants (kapp) generally decrease in the order of olefins (kapp = 0.3 - 2.1 × 104 M-1s-1) > phenols (kapp = 0.03 - 460 M-1s-1) > amines (kapp = 3.5 × 10-3 - 305.3 M-1s-1) at neutral pH. Phenolic benzene ring (for phenols), (conjugated) double bond (for olefins), primary amine group and the N-containing heterocyclic ring (for amines) are the most reactive sites towards Mn(VII) oxidation, leading to the formation of products with different structures (e.g., hydroxylated, aldehyde, carbonyl, quinone-like, polymeric, ring-opening, nitroso/nitro and C-N cleavage products). Destruction of functional groups of EOCs (e.g., benzene ring, (conjugated) double bond, and N-containing heterocyclic) by Mn(VII) tends to decrease solution toxicity, while oxidation products with higher toxicity than parent EOCs (e.g., quinone-like products in the case of phenolic EOCs) are sometimes formed. Mn(III) stabilized by model or unknown ligands remarkably accelerates phenolic EOCs oxidation by Mn(VII) under acidic to neutral conditions, while MnO2 enhances the oxidation efficiency of phenolic and amine EOCs by Mn(VII) at acidic pH. The intermediate manganese products participate in Mn(VII) oxidation process most likely as both oxidants and catalysts with their generation/stability/reactivity affecting by the presence of NOM, ligand, cations, and anions in water matrices. This work presents the state-of-the-art findings on Mn(VII) oxidation of EOCs, especially highlights the significant roles of manganese products, which advances our understanding on Mn(VII) oxidation and its application in future water treatment processes.

Formation of nitrosated and nitrated aromatic products of concerns in the treatment of phenols by the combination of peroxymonosulfate and hydroxylamine.

Recently, the combination of peroxymonosulfate (PMS) and hydroxylamine (HA) has been proposed as a green and efficient sulfate radical ()-based advanced oxidation process (AOP) for eliminating organic contaminants. However, we found that toxic nitrosated and nitrated aromatic compounds were generated during the treatment of phenolic compounds by PMS/HA system, indicating the involvement of reactive nitrogen species (RNS) during the interaction of PMS with HA. Specifically, considerable production of p-nitrosophenol (p-NSP) and mononitrophenol were obtained when phenol was oxidized by PMS/HA system under various conditions. At the molar ratio between HA and PMS of 1.0 and pH 5.0, sum of the yields of p-NSP and nitrophenols reached their maxima (around 50%). Moreover, production of p-NSP was inhibited while that of nitrophenols was promoted when applied NH2OH1/2H2SO4 was replaced by NH2OHHCl, which was possibly related to the formation of secondary reactive species induced by the reaction of with chloride ion. Further, formation of undesirable nitrosated and nitrated aromatic products was obtained in the treatment of other phenolic compounds including acetaminophen, bisphenol A, and bisphenol S by PMS/HA system. Considering the toxicity of nitrosated and nitrated aromatic compounds, practical application of PMS/HA system for environmental decontamination should be scrutinized.

Hydroxylamine driven advanced oxidation processes for water treatment: A review.

Hydroxylamine (HA) driven advanced oxidation processes (HAOPs) for water treatment have attracted extensive attention due to the acceleration of reactive intermediates generation and the improvement on the elimination effectiveness of target contaminants. In this review, HAOPs were categorized into three parts: (1) direct reaction of HA with oxidants (e.g., hydrogen peroxide (H2O2), peroxymonosulfate (PMS), ozone (O3), ferrate (Fe(VI)), periodate (IO4-)); (2) HA driven homogeneous Fenton/Fenton-like system (Fe(II)/peroxide/HA system, Cu(II)/O2/HA system, Cu(II)/peroxide/HA system, Ce(IV)/H2O2/HA system); (3) HA driven heterogeneous Fe/Cu-Fenton/Fenton-like system (iron-bearing material/peroxide/HA system, copper-bearing material/peroxide/HA system, bimetallic composite/peroxide/HA system). Degradation efficiency of the target pollutant, reactive intermediates, and effective pH range of various HAOPs were summarized. Further, corresponding reaction mechanism was elaborated. For the direct reaction of HA with oxidants, improvement of pollutants degradation was achieved through the generation of secondary reactive intermediates which had higher reactivity compared with the parent oxidant. For HA driven homogeneous and heterogeneous Fe/Cu-Fenton/Fenton-like system, improvement of pollutants degradation was achieved mainly via the acceleration of redox cycle of Fe(III)/Fe(II) or Cu(II)/Cu(I) and subsequent generation of reactive intermediates, which avoided the drawbacks of classical Fenton/Fenton-like system. In addition, HA driven homogeneous Fe/Cu-Fenton/Fenton-like system with heterogeneous counterpart were compared. Further, formation of oxidation products from HA in various HAOPs was summarized. Finally, the challenges and prospects in this field were discussed.

Transformation of X-ray contrast media by conventional and advanced oxidation processes during water treatment: Efficiency, oxidation intermediates, and formation of iodinated byproducts.

X-ray contrast media (ICM), as the most widely used intravascular pharmaceuticals, have been frequently detected in various environmental compartments. ICM have attracted increasingly scientific interest owing to their role as an iodine contributor, resulting in the high risk of forming toxic iodinated byproducts (I-BPs) during water treatment. In this review, we present the state-of-the-art findings relating to the removal efficiency as well as oxidation intermediates of ICM by conventional and advanced oxidation processes. Moreover, formation of specific small-molecular I-BPs (e.g., iodoacetic acid and iodoform) during these processes is also summarized. Conventional oxidants and disinfectants including chlorine (HOCl) and chloramine (NH2Cl) have low reactivities towards ICM with HOCl being more reactive. Iodinated/deiodinated intermediates are generated from reactions of HOCl/NH2Cl with ICM, and they can be further transformed into small-molecular I-BPs. Types of disinfectants and ICM as well as solution conditions (e.g., presence of bromide (Br-) and natural organic matters (NOM)) display significant impact on formation of I-BPs during chlor(am)ination of ICM. Uncatalyzed advanced oxidation process (AOPs) involving ozone (O3) and ferrate (Fe(VI)) exhibit slow to mild reactivities towards ICM, usually leading to their incomplete removal under typical water treatment conditions. In contrast, UV photolysis and catalyzed AOPs including hydroxyl radical (HO•) and/or sulfate radical (SO4.-) based AOPs (e.g., UV/hydrogen peroxide, UV/persulfate, UV/peroxymonosulfate (PMS), and CuO/PMS) and reactive chlorine species (RCS) involved AOPs (e.g., UV/HOCl and UV/NH2Cl) can effectively eliminate ICM under various conditions. Components of water matrix (e.g., chloride (Cl-), Br-, bicarbonate (HCO3-), and NOM) have great impact on oxidation efficiency of ICM by catalyzed AOPs. Generally, similar intermediates are formed from ICM oxidation by UV photolysis and AOPs, mainly resulting from a series reactions of the side chain and/or C-I groups (e.g. cleavage, dealkylation, oxidation, and rearrange). Further oxidation or disinfection of these intermediates leads to formation of small-molecular I-BPs. Pre-oxidation of ICM-containing waters by AOPs tends to increase formation of I-BPs during post-disinfection process, while this trend also depends on the oxidation processes applied and solution conditions. This review summarizes the latest research findings relating to ICM transformation and (by)products formation during disinfection and AOPs in water treatment, which has great implications for the practical applications of these technologies.