Deciphering the degradation/chlorination mechanisms of maleic acid in the Fe(II)/peroxymonosulfate process: An often overlooked effect of chloride

Impact of metal ions on PMS/Cl- disinfection efficacy: Enhancing or impeding microbial inactivation?

Peroxymonosulfate (PMS) is an eco-friendly disinfectant gaining attention. This study examined the influence of metal ions (Co(II), Cu(II), Fe(II)) on PMS disinfection with chloride ions (Cl-) against waterborne microorganisms, encompassing both bacteria and fungal spores. The findings elucidated that metal ions augment the inactivation of bacteria in the PMS/Cl- system while concurrently impeding the inactivation of fungal spores. Specifically, the PMS/Co(II)/Cl- process increased E. coli inactivation rates by 2.25 and 2.75 times compared to PMS/Co(II) and PMS/Cl-, respectively. Conversely, PMS/Me(II)/Cl- generally exhibited a diminished inactivation capacity against the three fungal spores compared to PMS/Cl-, albeit surpassing the efficacy of PMS/Me(II). For instance, the inactivation levels of A. niger by PMS/Cl-, PMS/Cu(II)/Cl-, and PMS/Cu(II) are 4.47-log, 1.92-log, and 0.11-log, respectively. Notably, fungal spores demonstrated a substantially higher resistance to disinfectants compared to bacteria. Differences in microbial susceptibility were linked to cell wall structure, composition, antioxidant defenses, and reactive species generation, such as hydroxyl radicals (•OH), sulfate radicals (SO4•-), and reactive chlorine species (RCS). This study demonstrated the novel and unique phenomenon of metal ions' dual role in modulating the PMS/Cl- disinfection process, which has not been reported before and has important implications for the field of water treatment.

Chloride-mediated enhancement in Cu(II)-catalyzed Fenton-like reaction: The overlooked reactive chlorine species

The practical application of Cu(II)-catalyzed Fenton-like reaction (Cu(II)/H2O2) exhibits a low efficiency in the degradation of refractory compounds of wastewater. The impact of chloride ions (Cl-) on Fenton-like reactions have been investigated, but the influence mechanism is still unclear. Herein, the presence of Cl- (5 mM) significantly accelerated the degradation of benzoic acid (BA) under neutral conditions. The degradation of BA follows pseudo-first-order kinetics, with a degradation rate 7.3 times higher than the Cu(II)/H2O2 system. Multiple evidences strongly demonstrated that this reaction enables the production of reactive chlorine species (RCS) rather than HO• and high-valent copper (Cu(III)). The kinetic model revealed that Cl- could shift reactive species from the key intermediate (Cu(III)-chloro complexes) to RCS. Dichlorine radicals (Cl2•-) was discovered to play a crucial role in BA degradation, which was largely overlooked in previous reports. Although the reaction rate of Cl2•- with BA (k = 2.0 × 106 M-1 s-1) is lower than that of other species, its concentration is 10 orders of magnitude higher than that of Cu(III) and HO•. Furthermore, the exceptional efficacy of the Cu(II)/H2O2 system in BA degradation was observed in saline aquatic environments. This work sheds light on the previously unrecognized role of the metal-chloro complexes in production the RCS and water purification.

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.

Activation of peroxymonosulfate by Co-Mg-Fe layered doubled hydroxide for efficient degradation of Rhodamine B

Reactive species serve as a key to remediate the contamination of refractory organic contaminants in advanced oxidation processes. In this study, a novel heterogeneous catalyst, CoMgFe-LDH layered doubled hydroxide (CoMgFe-LDH), was prepared for an efficient activation of peroxymonosulfate (PMS) to oxidize Rhodamine B (RhB). The characterization results showed that CoMgFe-LDH had a good crystallographic structure. Correspondingly, the CoMgFe-LDH/PMS process exhibited good capacity to remove RhB, which was equivalent to degradation performance as homogeneous Co(II)/PMS process. The RhB oxidation in the CoMgFe-LDH/PMS process was well described with pseudo-first-order kinetic model. Additionally, the oxidation process presented an excellent stability, and only 0.9% leaching rate was detected after six sequential reaction cycles at pH 5.0. The effects of initial pH, CoMgFe-LDH dosage, PMS concentration, RhB concentration, and inorganic anions on the RhB degradation were discussed in detail. Quenching experiments showed that sulfate radicals (SO₄^•-) acted as the dominant reactive species. Further, the removal of RhB from simulated wastewater was explored. The removal efficiency of RhB (90 μM) could reach 94.3% with 0.8 g/L of catalyst and 1.2 mM of PMS addition at pH 5.0, which indicated the CoMgFe-LDH/PMS process was also effective in degrading RhB in wastewater.

Fe-g-C3N4/reduced graphene oxide lightless application for efficient peroxymonosulfate activation and pollutant mineralization: Comprehensive exploration of reactive sites

To overcome the shortcomings of homogeneous Fe ion activating peroxymonosulfate (PMS), such as high pH-dependence, limited cycling of Fe(III)/Fe(II) and sludge production, graphite carbon nitride (g-C₃N₄) is chosen as a support for Fe ions, and reduced graphene oxide (rGO) is employed to facilitate the electron transfer process, thereby enhancing catalysis. Herein, a ternary catalyst, Fe-g-C₃N₄/rGO, is first applied under lightless condition for PMS activation, which exhibits ideal performance for contaminant mineralization. 82.5 % of the total organic carbon (TOC) in 100 mL of 5 mg/L bis-phenol A (BPA) was removed within 20 min by the optimal catalyst named 30%rFe_0.2CN, which shows a strong pH adaptability over the range of 3-11 compared with a common Fenton-like system. Moreover, the highly stable Fe-g-C₃N₄/rGO/PMS catalytic system resists complex water matrices, especially those with high turbidity. To unveil the mechanism of PMS activation and pollutant degradation, the physicochemical properties of the as-prepared catalysts are comprehensively characterized by multiple techniques. The Fe(III) contained in both the Fe-N group and α-Fe₂O₃ component of 30%rFe_0.2CN not only directly reacts with PMS to produce sulfate radicals (SO₄^-) and hydroxyl radicals (OH), but also combines with PMS to form the essential [Fe(III)OOSO₃]⁺ active complex, thereby generating superoxide radicals (O₂^-) and singlet oxygen (¹O₂). Among the various reactive oxidizing species, ¹O₂ plays an important role in pollutant removal, which is additionally generated by the CO moiety of the catalyst activating PMS as well as PMS self-oxidation, indicating the dominance of the non-radical pathway in the pollutant degradation process. Due to the advantages of high efficiency, wide pH adaptability and stability, the proposed lightless Fe-g-C₃N₄/rGO/PMS catalytic system represents a promising avenue for practical wastewater purification.

Fate of typical organic halogen compounds in the coexistence of endogenic chlorine atoms and exogenic X. CHEMOSPHERE 2022;309:136761. [PMID: 36220428 DOI: 10.1016/j.chemosphere.2022.136761]

The transformation of halogenated organics in advanced oxidation processes (AOPs) has been extensively investigated. However, we currently know little about the fate of halogenated pollutants in the presence of exogenic halides (Cl^- or Br^-). Herein, the degradability, mineralization rate, and accumulation capacity of adsorbable organic halogen (AOX) for chlorophenols (2-chlorophenol (2-CP), 3-chlorophenol (3-CP), 4-chlorophenol (4-CP), and 2,4,6-trichlorophenol (TCP)) were compared in the Fe²⁺/persulfate (PS) process with the addition of exogenic halides. Results indicate that exogenic X^- can lead to a decrease in chlorophenols degradation and mineralization rate, undesirable accumulation of AOX, and generation of halogenated by-products which are more toxic than precursor chlorophenols. Results of kinetics modeling show that Cl₂^•- plays more important role than SO₄^•- with an addition of Cl^-, while SO₄^•-, Br₂^•-, and Br₂ are responsible for the effect of Br^-. As well, the effect of endogenic chlorine atoms on chlorophenols reveals that the degradability and AOX formation potential of 3-CP are highest while that of TCP are the lowest. This study demonstrates the significant influence of endogenic chlorine atoms and exogenic X^- on the fate of typical organic halogen compounds. Consequently, the X^- level and position/number of halogen atoms should be considered simultaneously when treating organohalogen compounds.

Application of combined EO/PMS/Me2+ process in organic matter and true color removal from paint manufacturing industry wastewater

Treatment of paint manufacturing industry wastewater by electrooxidation (EO) process in which peroxymonosulfate (PMS) and transition metals are added was investigated. In the EO/PMS process, graphite was the cathode while different anode materials (Ti/IrO₂, Ti/RuO_2, and Ti/SnO₂) were used. The anode with the highest chemical oxygen demand (COD) and true color removal efficiency was selected. To determine the catalyst effect on the process, different transition metals (Fe²⁺, Cu²⁺, Zn²⁺) were added and Fe²⁺ was chosen as the catalyst which provided higher removal efficiency and lower cost. The central composite design was applied for the optimization of the process variables of the EO/PMS/Fe²⁺ process. Current density, PMS dose, Fe²⁺ dose, and reaction time were process variables whereas COD and true color removal efficiency were system responses. Under optimum conditions (200 A/m² current density, 14 mM PMS dose, 2.5 mM Fe²⁺ dose, 60 min reaction time), the estimated COD and true color removal efficiency by the model were 74.89% and 99.86%, respectively. The experimentally obtained COD and true color removal efficiencies as a result of validation studies were 74.28% and 99.03%, respectively. Quenching experiments showed that hydroxyl and sulfate radicals were both involved in the process.

Formation and transformation of reactive species in the Fe2+/peroxydisulfate/Cl- system

The influence of Cl^- on the formation mechanism of active components is often neglected in the Fe²⁺/peroxydisulfate (PDS) system containing a large amount of ferryl ion reactive specie (Fe(Ⅳ)). In the current investigation, the effects of Cl^- concentration on the removal of methyl phenyl sulfoxide (PMSO), the formation of methyl phenyl sulfone (PMSO₂), the transformation of reactive species and oxidation products were investigated under different reaction conditions that included Fe²⁺ dosage, PDS dosage, and pH₀. The results showed that Cl^- complexing Fe²⁺ increased the formation path of sulfate radical (SO₄^·-) in the Fe²⁺/PDS system. Fe²⁺ dosage and pH₀ value affected the content and morphology of Fe²⁺-Cl^- complex, thus affecting the composition of reactive species. According to the experiment of free radical steady-state concentration, it was found that low concentration of Cl^- reacted with SO₄^·- and increased the steady-state concentration of chlorine radicals (8.09 × 10^-13 M [·Cl]_ss at 1.41 mM Cl^-), while at high concentration of Cl^-, the contents of SO₄^·-, hydroxyl radical (·OH) and dichloride anion radicals (Cl₂^·-) increased and the contents of Fe(Ⅳ) and ·Cl decreased. ·Cl had strong reactivity with PMSO, and PMSO and its oxidation products were chlorinated under the combined action of ·Cl and Cl₂^·-. This work reveals the reaction mechanism and environmental application risks of Fe²⁺/PDS technology and lays the groundwork for subsequent industrial application of Fe²⁺/PDS system.

Activation of peroxymonosulfate by pyrophosphate for the degradation of AO7 at neutral pH

In the present study, pyrophosphate (PP) was used to activate peroxymonosulfate (PMS) for acid orange 7 (AO7) removal under neutral pH conditions. The removal rate of AO7 (20 mg/L) was 84% within the reaction time with a rate constant value of 0.0165 min^-1 under optimum conditions. Additionally, the effects of the concentrations of PMS and PP in solutions with various pH values and the coexisting inorganic anions on AO7 removal were measured. In addition, the performance of phosphate (P(V)) on PMS activation was compared with that of phosphite (P(III)) species. In contrast to P(III), the concentration of P(V) showed a positive correlation with the efficiency of AO7 decolorization. PMS activation in different types of buffer solutions was also examined, and the results indicated that the decolorization efficiency of AO7 induced by PP addition, and the buffer solution also contributed to PMS self-decomposition. Singlet oxygen (¹O₂) might be the primary reactive oxygen species (ROS) in the PP/PMS system in which AO7 is decolorized at an initial pH of 7.06, as indicated by quenching experiments and electron spin resonance (ESR) tests. Therefore, PP/PMS systems may be promising technologies for removing organic contaminants, particularly for PP-rich electroplating wastewater.

Visible-light-assisted peroxymonosulfate activation by metal-free bifunctional oxygen-doped graphitic carbon nitride for enhanced degradation of imidacloprid: Role of non-photochemical and photocatalytic activation pathway

Bifunctional oxygen-doped graphitic carbon nitride (OCN) was fabricated to activate peroxymonosulfate (PMS) for degrading imidacloprid (IMD). The modulated electronic structure of OCN promoted the adsorption, electron transfer, and formation of the redox site of PMS. The light absorption capacity, and the separation and migration speed of photogenerated carriers of OCN were increased. Consequently, 94.5% of IMD (3.0 mg/L) was removed by OCN-10/PMS process in 2.0 h. Compared with g-C₃N₄/PMS (0.048 h^-1), the IMD degradation rate constant of OCN-10/Vis/PMS system (1.501 h^-1) was increased by 30.3 times. The PMS oxidation on electron-deficient C atoms and holes, the PMS reduction around electron-rich O atoms and photogenerated electrons, and the multiple reactions of superoxide radical were the sources of the main active species singlet oxygen. Moreover, even under different pH conditions, coexisting anions, humic acid, and other neonicotinoid pesticides, the OCN-10/Vis/PMS system still showed acceptable applicability. Finally, mass spectrometry identified that hydroxylation and N-dealkylation of amines were the primary degradation pathways of IMD. This paper demonstrates an environmental-friendly combined activation strategy of PMS that can be operated day and night with low energy consumption, aiming to pave the way for developing metal-free photocatalysts for high-efficient environmental purification based on advanced oxidation coupling technology.

Amlodipine removal via peroxymonosulfate activated by carbon nanotubes/cobalt oxide (CNTs/Co3O4) in water

Amlodipine (AML) is an effective drug that has been widely used for hypertension and angina. However, AML is frequently detected in aqueous environments, posing potential risks to human and ecological health. In this study, the degradation of AML via peroxymonosulfate (PMS) activated by CNTs/Co₃O₄ was investigated. CNTs/Co₃O₄ was prepared via a facile method, and multiple characterizations suggested that Co₃O₄ were uniformly dispersed on the surface of MWCNTs-COOH. Experimental results indicated that complete removal of 10 μM AML was achieved within 30 min by using 2 mg/L CNTs/Co₃O₄ and 4 μM PMS at 25 °C in PBS buffered solution (pH 7.0). The observed pseudo-first-order rate constant was calculated to be 0.1369 min^-1. Interestingly, the presence of 100 mM Cl^- resulted in a slight enhancement of AML removal rate from 0.0528 to 0.0642 min^-1. The addition of 100 mM HCO₃^-, 5 mg/L Pony Lake fulvic acid (PLFA), or Suwannee River humic acid (SRHA) retarded AML degradation by 15.5, 0.7, and 1.6 times, respectively. As per the quenching experiments, SO₄^⦁- rather than ⦁OH were verified to be the dominant reactive oxygen species (ROS). Additionally, ten major intermediates were identified using TOF-LC-MS and three associated reaction pathways including ether bond broken, H-abstraction, and hydroxylation were proposed. We outlook these findings to advance the feasibility of organic contaminants removal via CNTs/Co₃O₄ + PMS systems that have extremely low-level PMS.

Ozone oxidation of 2,4,6-TCP in the presence of halide ions: Kinetics, degradation pathways and toxicity evaluation

2,4,6-Trichlorophenol (2,4,6-TCP) is extensively consumed in industrial production and may cause environmental damages. The effect of halide ions on the decomposition of 2,4,6-TCP has often been overlooked. In this study, the bromide ion was found to have a stronger negative impact on 2,4,6-TCP degradation than chloride ion in the O₃ system, and led to the formation of adsorbable organic halogens (AOX). Kinetic modeling demonstrated that the concentration of various radicals was largely depended on the solution pH, and stronger basicity not only contributed to the mineralization of 2,4,6-TCP, but also inhibited the formation of halogenated by-products. Combining the intermediate identification and quantum chemical calculation, the degradation pathways of 2,4,6-TCP during ozone oxidation process were proposed. The toxicity test and ECOSAR simulation demonstrated that the acute toxicity of some 2,4,6-TCP degradation intermediates was relatively higher than their parent compound. With high concentrations of halide ions, the ozone-treated solution showed greater toxicity than the originator 2,4,6-TCP solution. These results illustrate that the ozone treatment of the halide-containing wastewater may cause potential ecological hazards and its application needs to be more cautious.

Misconceptions about the Chemistry of Aqueous Chlorine Atoms and HClOH•(aq), and a Revised Mechanism for the Photochemical Peroxydisulfate/Chloride Reaction

It is widely considered that aqueous chlorine atoms (Cl•) convert to the species HClOH• with a half life of about 3 µs and that this species plays an important role...

A combined radical and non-radical oxidation processes for efficient degradation of Acid Orange 7 in the homogeneous Cu(II)/PMS system: important role of chloride

Trace copper ion (Cu(II)) in water and wastewater can trigger peroxymonosulfate (PMS) activation to oxidize organic compounds, but it only works under alkaline conditions. In this work, we found that the presence of chloride could significantly accelerate the oxidation of Acid Orange 7 (AO7) by the Cu(II)/PMS process at a wide pH range (4.0-9.0). The observed pseudo-first-order rate constant k for AO7 oxidation was linearly correlated with the increased Cl^- concentration (0-300 mM). An increase in mineralization rate was observed in the presence of Cl^-, while the overall mineralization was quite low. Decomposition of PMS facilitated when Cl^- concentration or pH value increased. Based on the scavenger experiments and electron paramagnetic resonance (EPR) measurement, the mechanism of Cu(II)-catalyzed PMS oxidation process in the presence of Cl^- was proposed as both the radical and non-radical pathway, and ¹O₂ was the reactive oxygen species in the Cu(II)/PMS system. Finally, a possible degradation pathway of AO7 was elucidated. The feasibility of in situ utilizing high salinity and trace cupric species to accelerate the degradation of organic pollutants by the Cu(II)/PMS process in water and wastewater was demonstrated. However, the identification of undesired chlorinated by-products reminds us of cautiousness in assessing the application of Cu(II)/PMS system under chloride-rich environment. The findings of this work provide a simple and efficient approach to apply PMS in the remediation of refractory organic contaminants in the presence of trace cupric species under a high salinity environment with a wide range of pH.

Degradation and chlorination mechanism of fumaric acid based on SO4•-: an experimental and theoretical study

It is well known that chloride ions could affect the oxidation kinetics and mechanism of contaminant based on SO₄^•- in the wastewater. Here, the degradation of an organic acid, fumaric acid (FA), was investigated in the presence of chloride (0-300 mM) by the Fe(II)/peroxymonosulfate (Fe(II)/PMS) system. A negative impact of chloride was observed on the rates of FA degradation. The degree of inhibitory effect was higher in Fe(II)/PMS addition order. Some chlorinated byproducts were identified during the FA oxidation process in the presence of Cl^- by the ultraperformance liquid chromatography and quadrupole-time of flight mass spectrometer (UPLC-QTOF-MS). With the increasing content of Cl^-, an accumulation of adsorbable organic halogen (AOX), an increase in acute toxicity, and an inhibition of mineralization were observed. According to the results of kinetic modeling, the production and transformation of oxidative species were dependent on Cl^- dosage and reaction time. SO₄^•- was supposed to be the main radical for FA degradation with Cl^- concentration below 5 mM, whereas Cl₂^•- was primarily responsible for the depletion of FA at [Cl^-] > 5 mM. A possible degradation pathway of FA was discussed. This study reveals the potential environmental risk of organic acid and is necessary to explore useful strategies for ameliorating the treatment of chloride-rich wastewater.

Abiotic oxidation of arsenite in natural and engineered systems: Mechanisms and related controversies over the last two decades (1999-2020)

Abiotic oxidation of toxic As(III) to As(V) is being deemed as a necessary step for the overall arsenic decontamination in both natural and engineered systems. Direct oxidation of As(III) by chemical oxidants, such as ozone, permanganate, ferrate, chlorine and chloramine, or naturally occurring minerals like Mn, Fe oxides, seems straightforward. Both O₂ and H₂O₂ are ineffective for arsenite oxidation, but they can be activated by reducing substances like Fe²⁺, Fe⁰ to increase the oxidation rates. Photo-induced oxidation of As(III) has been demonstrated effective in Fe complexes or minerals, NO₃^-/NO₂^-, dissolved organic matter (DOM), peroxygens and TiO₂ systems. Although a variety of oxidation methods have been developed over the past two decades, there remain many scientific and technical challenges that must be overcome before the rapid progress in basic knowledge can be translated into environmental benefits. To better understand the trends in the existing data and to identify the knowledge gaps, this review describes in detail the complicated mechanisms for As(III) oxidation by various methods and emphasizes on the conflicting data and explanation. Some prevailing concerns and challenges in the sphere of As(III) oxidation are also pointed out so as to appeal to researchers for further investigations.

Treatment of simulated textile sludge using the Fenton/Cl- system: The roles of chlorine radicals and superoxide anions on PAHs removal

The main content of this work is to investigate the removal of polycyclic aromatic hydrocarbons (PAHs: phenanthrene, anthracene, and fluoranthene) from simulated sludge solid phase employing an Fenton/Cl^- system under various Cl^- contents and pH values. The steady-state concentrations of the hydroxyl, chlorine, and dichloride anion radicals ([·OH]_ss, [·Cl]_ss, and [Cl₂^·-]_ss) in heterogeneous system were first measured using tert-butanol, nitrobenzene, and benzoic acid. The outcomes exhibited that increasing the Cl^- content from 50 to 2000 mg/L (pH = 3.0) or raising the pH from 3.0 to 5.0 (1000 mg/L Cl^-) caused [·OH]_ss to continuously decrease and [Cl₂^·-]_ss and the concentration of superoxide anions (HO₂·/O₂^·-) to continuously increase. When the pH was 3.0 and the Cl^- concentration was 1000 mg/L, [·Cl]_ss had a maximum value of 9.27 × 10^-14 M. Combining the results of PAH removal, radical quenching, and product analysis, it was found that ·Cl in the Fenton/Cl^- system promoted the oxidative degradation of phenanthrene without forming chlorination byproducts. Furthermore, HO₂·/O₂^·- was helpful in removing anthracene and fluoranthene. Under the environment of high Cl^- content (≥1000 mg/L), PAHs could be removed more effectively by using HO₂·/O₂^·-. This investigation underpins further study on the regulation of reactive species and the efficient degradation of target organic matter in Fenton/Cl^- system, and provides a basis for studying the formation of chlorinated or toxic byproducts in the process of treating textile dyeing sludge by Fenton.

Overlooked enhancement of chloride ion on the transformation of reactive species in peroxymonosulfate/Fe(II)/NH2OH system

Though hydroxylamine (NH₂OH) is effective for accelerating pollutants degradation in Fenton and Fenton-like systems, the effect of anions simultaneously introduced by the hydroxylamine salts have always been ignored. Herein, effect of two commonly used hydroxylamine salts, hydroxylamine hydrochloride (NH₂OH·HCl) and hydroxylamine sulfate [(NH₂OH)₂·H₂SO₄], for the degradation of dimethyl phthalate (DMP) in peroxymonosulfate (PMS)/Fe(II) system was comparatively investigated. Degradation efficiency of DMP with NH₂OH·HCl was 1.6 times of that with same dosages of (NH₂OH)₂·H₂SO₄. SO₄^·-, Fe(IV) and ·OH formed in the PMS/Fe(II)/NH₂OH system, but ·OH was the major species for DMP degradation. Addition of Cl^- significantly improved the production of ·OH and Cl·, and the exposure dose of ·OH (CT_·OH) was more than 10 times that of CT_Cl· as the concentration of Cl^- increased to 1 mM. Calculations based on branching ratios of Cl· and ·OH indicated that the reactions of Cl^- with SO₄^·- and Cl· with H₂O were not the only production sources of ·OH in the system. Further experiments with methyl phenyl sulfoxide (PMSO) as the probe indicated that Cl^- would facilitate the shift of reactive species from Fe(IV) to radicals (SO₄^·- or ·OH) in the system. Both hydroxylation and nitration intermediate products were detected in the oxidation of DMP. Cl^- promoted the formation of hydroxylation intermediates and reduced the formation of nitration intermediates. This study revealed for the first time that Cl^- could shift reactive species from Fe(IV) to radicals in PMS/Fe(II) system, raising attention to the influence of the coexisting anions (especially Cl^-) for pollutants oxidation in iron-related oxidation processes.

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.

Electron-transfer-dominated non-radical activation of peroxydisulfate for efficient removal of chlorophenol contaminants by one-pot synthesized nitrogen and sulfur codoped mesoporous carbon

Synergistic adsorption and oxidative degradation (via persulfate activation) on metal-free carbonaceous materials are expected to be environmentally friendly and highly efficient approach toward contaminants removal. Herein, nitrogen and sulfur codoped mesoporous carbon (NSDMC) were firstly synthesized via co-carbonization of calcium citrate and thiourea without any templates. NSDMC samples exhibit remarkably enhanced adsorption capacity and oxidative degradation (by activating PDS) for chlorophenols elimination. Increased S_BET and introduced N-containing functional groups are beneficial for chlorophenols adsorption, PDS accessibility and successive activation. Doped sulfur species (especially for thiophenic S) can enhance the electron-transport performance of NSDMC, further promoting PDS activation and chlorophenols degradation. It can be ascribed to the synergistic effect of N and S codoping. NSDMC-30 (containing 5.83 at.% nitrogen and 2.15 at.% sulfur, and possessing S_BET of 1935.9 m² g^-1) exhibits the optimal adsorption and catalytic oxidation capability for 4-CP removal. Degradation rate constant of NSDMC-30 is 0.125 min^-1, which is 3.0 times and 7.8 times higher than nitrogen-doped MC and pristine MC, respectively. Radicals quenching experiments and EPR tests demonstrate that non-radical pathways play dominant role for PDS activation and chlorophenols degradation. Based on the influences of catalyst loading, initial 4-CP concentration, and PDS dosage on degradation kinetics of 4-CP, the pre-adsorption is unveiled to be the critical step determining oxidation rate of chlorophenols. More importantly, the results of in-situ Raman and electrochemical tests show that the surface-confined and activated PDS complex (carbon-PDS*) and continuous electron transfer from co-adsorbed 4-CP are mainly responsible for the oxidative degradation of chlorophenols. The intermediate products and TOC removal indicate that chlorophenols can be efficiently degraded and mineralized by as-synthesized NSDMC via activating PDS. Besides, the present NSDMC/PDS system is also applicable for purification of actual polluted water samples. This work provides in-depth knowledge of carbon-driven nonradical process for PDS activation and contaminants remediation.

Influence of inorganic ions on degradation capability of Fe-based metallic glass towards dyeing wastewater remediation

Metallic glasses (MGs) are promising candidates for catalysts with high efficiency for dyeing wastewater remediation, due to their metastable nature, disordered structure, and large residual stresses. However, dyeing wastewater usually contains a high concentration of inorganic ions which may have adverse effects on the degradation process, while the impacts of these ions on MGs' degradation capability have often been overlooked and still remain unknown. Thus, the roles of inorganic ions (Cl^-, NO₃^-, SO₄^2-, and H₂PO₄^-) on the degradation of azo dye by Fe-based MG with nominal composition of Fe₈₁Si₄B₁₄Cu₁ were systematically investigated. The results showed that the inorganic ions have significant influence on MG's surface morphology, degradation capability, mineralization and durability. All these aspects need to be considered prior to application of MGs for azo dyes degradation in real natural contaminated water or saline wastewater.

Comparison of the Fe2+/H2O2 and Fe2+/PMS systems in simulated sludge: Removal of PAHs, migration of elements and formation of chlorination by-products

In this study, polycyclic aromatic hydrocarbons (PAHs) at practical concentrations in the simulated sludge treated by the Fe²⁺/H₂O₂ and Fe²⁺/peroxymonosulfate (PMS) systems were evaluated in terms of the PAHs (phenanthrene, anthracene, fluoranthene) removal, element migration, Cl^- effect, and chlorination by-product formation. The results indicated that according to the removal rate of PAHs, the optimal dosage of the Fe²⁺/PMS system (∑PAHs removal rate was 64.66 ± 2.82 %) was 1/30 of that for the Fe²⁺/H₂O₂ system (∑PAHs removal rate was 78.63 ± 0.38 %). The elemental contents in the simulated sludge were mainly affected by the extent of advanced oxidation and the amount of generated iron flocs. By studying the PAHs removal, free chlorine formation, total organochlorine content, and PAHs products in Fe²⁺/H₂O₂/Cl^- and Fe²⁺/PMS/Cl^- systems, it was found that chlorine radicals (·Cl) had high reactivity with phenanthrene and fluoranthene, whereas dichloride anion radicals (Cl₂^·-) exhibited the opposite behavior. Furthermore, PAHs were oxidized by ·Cl and hydroxyl radical in the Fe²⁺/H₂O₂/Cl^- system, whereas PAHs and their products were chlorinated by free chlorine and ·Cl in the Fe²⁺/PMS/Cl^- system to six chlorinated by-products such as Cl-PAHs (9-Cl-phenanthrene, 2-Cl-anthracene, 9,10-Cl₂-anthracene, 3-Cl-fluoranthene). These results provide some useful suggestions for the safe advanced oxidation process treatment of textile dyeing sludge.

Enhancing peroxymonosulfate activation of Fe-Al layered double hydroxide by dissolved organic matter: Performance and mechanism

In this study, peroxymonosulfate (PMS) activation of FeAl layered double hydroxide (FeAl-LDH) was enhanced by compounding dissolved organic matter (DOM). The characterization and catalytic performance of FeAl-LDH and DOM-LDH were investigated. The results revealed that the physicochemical properties of DOM-LDH were superior to FeAl-LDH: (i) The higher proportion of Fe(II) was found in DOM-LDH, mainly existed in the form of trans-coordinated octahedral Fe(II); (ii) DOM-LDH showed a flower-like morphology with larger specific surface area, pore width and pore volume; (iii) More functional groups and surface oxygen vacancies were found in DOM-LDH. Moreover, DOM promoted the process of PMS activation by accelerating Fe(III) reduction with humic acid-like compounds. The results of electron paramagnetic resonance (EPR) and quenching experiments indicated that more reactive oxygen species (ROS) were generated in DOM-LDH/PMS system, •OH was considered as the dominant ROS for Bisphenol A (BPA) degradation. As a result, the degradation efficiency for BPA (20 mg L^-1) in FeAl-LDH/PMS system was increased from 60% to 93% within 60 min after the introduction of DOM. This work is expected to facilitate the design and application of Fe(II)/PMS system for environmental protection.

Peroxymonosulfate Activation by Fe-Co-O-Codoped Graphite Carbon Nitride for Degradation of Sulfamethoxazole

Graphite carbon nitride (g-C₃N₄) has a stable structure but poor catalytic capability for activating peroxymonosulfate (PMS). In this study, the codoping of g-C₃N₄ with bimetallic oxides (iron and cobalt) and oxygen was investigated to enhance its catalytic capability. The results showed that iron, cobalt, and oxygen codoped g-C₃N₄ (Fe-Co-O-g-C₃N₄) was successfully prepared, which was capable of completely degrading sulfamethoxazole (SMX) (0.04 mM) within 30 min, with a reaction rate of 0.085 min^-1, indicating the superior catalytic activity of Fe-Co-O-g-C₃N₄. The mineralization efficiency of SMX was 22.1%. Sulfate radicals and singlet oxygen were detected during the process of PMS activation. However, the role that singlet oxygen played in degrading SMX was not obvious. Surface-bound reactive species and sulfate radicals were responsible for SMX degradation, in which sulfate radicals contributed to 46.6% of SMX degradation. The superior catalytic activity was due to the synergistic effect of metal oxides and O-g-C₃N₄, in which O-g-C₃N₄ could act as a carrier and an activator as well as an electron mediator to promote the conversion of Fe(III) to Fe(II) and Co(III) to Co(II). Four main steps of SMX degradation were proposed, including direct oxidation of SMX, bond fission of N-C, bond fission of N-S, and bond fission of S-C. The effect of the pH, temperature, PMS concentration, chloridion, bicarbonate, and humic acids on SMX degradation was investigated. Cycling experiments demonstrated the good stability of Fe-Co-O-g-C₃N₄. This study first reported the preparation of bimetallic oxide and oxygen codoped g-C₃N₄, which was an effective PMS activator for degradation of toxic organic pollutants.

Efficient inactivation of bacteria in ballast water by adding potassium peroxymonosulfate alone: Role of halide ions

In recent years, ballast water disinfection has been paid much more attention due to the untreated discharged ballast water posing threaten of biological invasion and health related consequences. In this study, an effective and simple approach for ballast water disinfection by just adding potassium peroxymonosulfate (PMS) was assessed, and the role of halide ions in seawater on the enhancement of inactivation was revealed. The reactive species responsible for inactivation, the leakage of intracellular materials, and changes of cellular morphology after inactivation were evaluated to explore the inactivation mechanism. The results showed that Escherichia coli and Bacillus subtilis in ballast water could be totally inactivated within 10 min by adding 0.2 mM PMS alone. The inactivation of bacteria in ballast water fitted to the delayed Chick-Watson model. Chloride and bromide ion in seawater were found to play a crucial role in inactivating bacteria, while the effect of iodide ion could be negligible due to its relative lower concentration in seawater. Chlorine and bromine, produced by the reaction of PMS with chloride and bromide ion, were proved to be the main reactive components that were responsible for the inactivation of bacteria. The extracellular ATP and total nitrogen concentration increased after inactivation which indicated that cell membrane was destroyed by reactive oxidants produced by the reaction between PMS and halide ions. The change of cell morphology confirmed that bacteria were seriously damaged after inactivation. The results suggest that PMS is an attractive alternative disinfectant for ballast water disinfection and this application deserved further research.

Efficient removal of triclosan via peroxymonosulfate activated by a ppb level dosage of Co(II) in water: Reaction kinetics, mechanisms and detoxification

Triclosan (TCS), an extensively used broad-spectrum antimicrobial agent, has raised significant environmental concerns regarding its widespread occurrence in waters. In this study, the removal of TCS in aqueous solution via peroxymonosulfate (PMS) activated by an extremely low-level Co²⁺ (0.02 μM) was systematically investigated. During preliminary test, TCS (10 μM) was totally degraded in 30 min by using 0.1 μM Co²⁺ and 40 μM PMS at pH 7.0 with a degradation rate constant of 0.1219 min^-1. A first-order apparent degradation rate of TCS was found with respect to the PMS concentrations. At extremely low dosage of Co²⁺ (0.02 μM), the presence of NO₃^-, HCO₃^-, PLFA, and SRHA within test concentrations significantly inhibited TCS removal, while a dual effect of Cl^- on the degradation rate of TCS was observed. The quenching experiments verified that SO₄^- was the dominant reactive oxygen species (ROS) rather than OH. Six major intermediates were identified using TOF-LC-MS, based on which we proposed three associated reaction pathways including hydroxylation, ether bond breakage, and dechlorination. Toxicity predictions by ECOSAR software exhibited aquatic toxicity reduction of TCS after Co²⁺/PMS treatment. We outlook these findings to advance the feasibility of organic contaminants removal via Co²⁺/PMS system with Co²⁺ at extremely low levels.

Significant role of high-valent iron-oxo species in the degradation and detoxification of indomethacine

A novel high-valent iron-oxo species (Fe(IV) = O) generated from Iron hexadecachlorophthalocyanine (FePcCl₁₆)-mediated peroxymonosulfate (PMS) activation under visible light illumination for the degradation of a special group of compounds, indomethacine (IDM), containing methoxy, carboxyl, chloro, and amide groups was investigated. The experimental results indicate that Fe(IV) = O was able to selectively attack the carbonyl C-N bond on twisted amide groups, which exerts a strong toxic effect, and could therefore, effectively degrade and detoxify IDM and its byproducts. Twelve byproducts were identified by HPLC/MS/MS and calculation of frontier electron densities (FEDs), with all amide-group breakage products detected, and the possible pathways were deduced, which mainly consisted of Fe(IV) = O-induced cleavage of amide groups and radicals-induced reactions. Ecological risk assessment further confirmed a decrease in toxicity towards IDM degradation, which provides a promising Fe(IV) = O species for selective oxidation and detoxification of destabilized ground-state amides in drinking-water and wastewater treatment.

Mechanisms of Advanced Oxidation Processes, the Principle of Detailed Balancing, and Specifics of the UV/Chloramine Process

Advanced oxidation processes tend to have very complex reaction mechanisms, and models containing over 150 steps have been developed to describe the chemistry. Without the aid of automation, it is extremely difficult to avoid the development of kinetic mechanisms that violate the principle of detailed balancing. Here, we apply DETBAL, a computer application, to systematically identify many violations of the principle of detailed balancing in a model proposed for the UV/chloramine process. We then show that these violations can also be found in dozens of other proposed models for advanced oxidation processes. Suggested repairs to these violations are provided. These repairs lead to no significant changes in the model predictions because the illegal loops include steps that are unnecessary under the conditions modeled. The model omits certain steps that do have significant effects on the model predictions.

Is addition of reductive metals (Mo, W) a panacea for accelerating transition metals-mediated peroxymonosulfate activation?

The interaction of reductive metal ions and peroxymonosulfate (PMS) is necessary for the generation of sulfate radials (SO₄^-), however, this process is greatly restrained by the sluggish reduction of high-valent metal ions. Here we report that commercially available reductive metal (Mo or W) powders are capable of unlocking this kinetic constraint. The reduction of Fe(III) to Fe(II), decomposition of PMS, and degradation/mineralization of 4-chlorophenol (4-CP) are all accelerated in the Mo/Fe²⁺/PMS process at a very low Fe²⁺/PMS ratio (Fe²⁺/PMS = 1/10). In such an accelerated system, common adverse effects of natural water constituents such as chloride and humic acid are largely mitigated. According to the fluorescence measurement and scavenging tests, sulfate and hydroxyl radicals dominate in Mo/Fe²⁺/PMS process. The addition of Mo or W is further confirmed to favor Cu²⁺/PMS process, but this is not the case for other metal ions (Mn²⁺, Ni²⁺, Ce³⁺ and Co²⁺). Reductive zero-valence and four-valence active sites (Mo⁰ and Mo⁴⁺; W⁰ and W⁴⁺) play key roles in overall redox reaction. Overall, our present work provides an alternative route for expediting redox cycling of transition metals in advanced oxidation processes, without useless consumption of PMS and increase of total organic carbon.

Resolving the kinetic and intrinsic constraints of heat-activated peroxydisulfate oxidation of iopromide in aqueous solution

Iopromide (IOP) has been identified as one of the most persistent pharmaceuticals in wastewater treatment processes, however, kinetic and intrinsic factors constraining its fast removal in advanced oxidation processes (AOPs) are yet to be resolved. Here oxidation of IOP by heat-activated peroxydisulfate (PDS) was investigated both experimentally and theoretically. Rates of IOP degradation were enhanced by elevating solution temperature and acidity. An apparent kinetic rate equation was developed, based on the pseudo-first-order reaction model and assumption of steady state of SO₄^-. The common water constituents showed inhibitory effects on IOP decomposition to various extent. An insufficient supply of SO₄^- was considered as the major kinetic constraint. Eight byproducts were identified and most of which had intact triiodinated benzene ring. O-demethylation, oxidation of amino moiety and oxidation/elimination of alcohol groups are proposed as the primary degradation pathways, in accordance with the incomplete mineralization and non-detectable release of inorganic iodine. Quantum chemical calculations predict that oxidation of alkyl chains of IOP preferentially occurs and IOP byproducts with shorter side chains and intact triiodinated ring are more reactive than IOP. By virtue of the identified kinetic and intrinsic constraints, strategies to maximize degradation efficiency of IOP are proposed.

Efficient degradation of refractory organic contaminants by zero-valent copper/hydroxylamine/peroxymonosulfate process

Degradation of naproxen, bisphenol S and ibuprofen in a hydroxylamine enhanced zero-valent copper (Cu⁰) catalyzed peroxymonosulfate system was investigated for the first time. We found that hydroxylamine addition accelerated the reduction of Cu²⁺ to Cu⁺ as well as the corrosion of Cu⁰, and environmental friendly gas nitrogen was the main product of hydroxylamine. Additionally, hydroxyl radical and sulfate radical were identified to be the dominant reaction species by competitive experiments. The degradation of naproxen, bisphenol S and ibuprofen kept highly efficient in the pH range of 3.0-7.0 in Cu⁰/hydroxylamine/peroxymonosulfate process, with their degradation products identified by HPLC-MS, which showed that Cu⁰/hydroxylamine/peroxymonosulfate system could be an alternative to remove non-steroidal antiinflammatory drugs or plasticizers in wastewater. Furthermore, the effects of Cu⁰, hydroxylamine and peroxymonosulfate dosage were studied and optimized by a BBD based response surface model. This study provided a method to solve the disadvantages of Cu⁰/peroxymonosulfate systems, and gave a promising method to enhance the efficiencies of ZVMs activated system such as iron, cobalt and copper.

Accelerated oxidation of 2,4,6-trichlorophenol in Cu(II)/H2O2/Cl- system: A unique "halotolerant" Fenton-like process?

The roles of chloride in enhanced oxidative degradation of refractory organic pollutants are recently identified in the Cu(II)/H₂O₂/Cl^- system, but the identity of the reactive oxidants and potential conversion of inorganic chloride to organochlorine in such oxidizing environment remain obscure. Here we report that Cu(II)/H₂O₂/Cl^- system is a unique "halotolerant" Fenton-like process that works most efficiently in saline water among the five tested redox-active metals ions (i.e. Cr(VI), Ce(III), Co(II), Mn(II) and Cu(II)). The observed pseudo first-order rate constant for 2,4,6-trichlorophenol (TCP) degradation was linearly correlated with the elevated Cl^- content. The TCP degradation rate at [Cl^-]₀ = 1000 mM by the Cu(II)/H₂O₂ system was approximately 46-fold higher than that at [Cl^-]₀ = 5 mM. The optimal mineralization rate of TCP and percentage of absorbable organic halogens (AOX) decrease were 31.6% and 63.8%, respectively, in the tested Cu(II)/H₂O₂/Cl^- system. However, the detection of fused chlorinated byproducts (i.e. chloro-anthracene-pentaol, dioxine, chlorinated dibenzofuran) reminds us of cautiousness in evaluating the applicability of Cu(II)-catalyzed Fenton-like reaction, particularly while it is to be applied to the treatment of wastewater contaminated with chlorophenols. Two independent models (i.e. "Cu(III) model" and "OH model") were developed to describe the kinetics of Cu(II)/H₂O₂/Cl^- system. The failure of "OH model" to rationalize the observed AOX decay has disproved the "OH model" through reductio ad absurdum. The ability of "Cu(III) model" to adequately explain the experimental data demonstrates that Cu(III)-chloro complexes, rather than OH, is the major product resulting from reactions between Cu(I)-chloro complexes and H₂O₂ at neutral pH.

Physico-chemical processes

The review scans research articles published in 2018 on physico-chemical processes for water and wastewater treatment. The paper includes eight sections, that is, membrane technology, granular filtration, flotation, adsorption, coagulation/flocculation, capacitive deionization, ion exchange, and oxidation. The membrane technology section further divides into six parts, including microfiltration, ultrafiltration, nanofiltration, reverse osmosis/forward osmosis, and membrane distillation. PRACTITIONER POINTS: Totally 266 articles on water and wastewater treatment have been scanned; The review is sectioned into 8 major parts;  Membrane technology has drawn the widest attention from the research community.

Degradation of triphenyl phosphate (TPhP) by CoFe2O4-activated peroxymonosulfate oxidation process: Kinetics, pathways, and mechanisms

The aryl organophosphate flame retardant triphenyl phosphate (TPhP) has been frequently detected in environment and biota, and the potential risks of TPhP to aquatic organisms have also been demonstrated. The degradation of TPhP by CoFe₂O₄ activated peroxymonosulfate (PMS) was studied in this work. At initial pH of 7.0, 10 μM TPhP could be removed by 99.5% with 0.25 g/L CoFe₂O₄ and 0.5 mM PMS after 6 min oxidation, indicating the excellent performance of CoFe₂O₄ activated PMS process on the treatment of TPhP. The influence of PMS and CoFe₂O₄ dosage, initial pH, humic acid (HA), and anions (Cl^-, NO₃^-, and HCO₃^-) on TPhP degradation were investigated systematically. Results showed that the degradation of TPhP was enhanced with increasing PMS concentrations from 0.1 to 1 mM, while it reduced as CoFe₂O₄ dosage increased. TPhP degradation efficiencies depended on solution pH with neutral pH showing the optimum degradation conditions. Recycling experiment indicated that the CoFe₂O₄ nanoparticles (NPs) possessed high potential for reusability. The radical identification experiments were performed and SO₄•^- was confirmed as the dominant radicals in TPhP degradation, and activation mechanism of PMS by CoFe₂O₄ NPs was hence explained. Humic acids (HA) (2-20 mg/L) as the representative organic natural matter existing in environment inhibited TPhP removal. Anions including Cl^-, NO₃^-, and HCO₃^- all reduced TPhP degradation. In addition, TPhP degradation products were identified by liquid chromatography-mass spectrometry, and the degradation pathways of TPhP were proposed accordingly.

An often-overestimated adverse effect of halides in heat/persulfate-based degradation of wastewater contaminants

Halides (X^-) in the industrial wastewater are usually thought to adversely affect the degradation kinetics and mineralization rates in several SO₄^--based advanced oxidation processes. However, their unfavorable effects might be overestimated, particularly the heat/persulfate (PS) system as tested in the present study. Here the degradation of phenol, benzoic acid, coumarin and acid orange 7 (AO7) was examined with the presence of chloride or bromide in a heat/PS process. Cl^- was found to have a dual effect (inhibition followed by enhancement) on the decomposition rates of organic pollutants, whereas the effects of Br^- are insignificant within the tested concentration (0-0.2 mM). However, some chlorinated or brominated compounds were still identified in this heat/PS system. Unexpectedly, the mineralization rates of AO7, phenol, benzoic acid and coumarin were not apparently inhibited. In addition, the formation of adsorbable organic halogen (AOX) in the heat/PS system was much less than those in the peroxymonosulfate (PMS)/Cl^- or PMS/Br^- systems. According to the results of kinetic modeling, SO₄^- was the dominating radical for AO7 degradation without Cl^- or Br^-, but Cl₂^- was the main oxidant in the presence of Cl^-, SO₄^-, Br and Br₂- were responsible for the oxidation of AO7 in the presence of Br^-. The present study assumes that X₂/HOX, rather than halogen radicals, is responsible for the enhanced formation of organohalogens. These findings are meaningful to evaluate the PS-based technologies for the high-salinity wastewater and to develop useful strategies for mitigating the negative effects of halides in advanced oxidation processes (AOPs).

Chlorine incorporation into dye degradation by-product (coumarin) in UV/peroxymonosulfate process: A negative case of end-of-pipe treatment

Recently, UV/peroxymonosulfate (PMS) seems as a panacea for the treatment of recalcitrant organic pollutants; however, the presence of high concentration of chloride in saline wastewater indeed complicates this end-of-pipe technology. Here a negative case of UV/PMS for the treatment of one of secondary degradation byproducts of dyes (coumarin, COU) is demonstrated. The removal rate of COU is reduced by addition of Cl^- (0-10 mM). Further increase in Cl^- content favors a rapid COU degradation, whereas Cl^- involvement seems to open a "Pandora's box": 1) a variety of chlorinated organic intermediates such as 4-chloroisocoumarin and 5-chloro-2-hydroxy-benzaldehyde are identified; 2) Accumulation and relative increase of absorbable organic halogen (AOX) with reaction time in the presence of high levels of chloride are observed; 3) the acute toxicity of the treated COU solution increases; 4) mineralization rate of COU decreases with the increasing [Cl^-]. The fluorescence intensity in the UV/PMS/COU system declines with the addition of Cl^-, implying the scavenging effects of chloride on hydroxyl radicals. The possible reaction pathways of COU are discussed. These findings highlight the imperativeness of minimizing auxiliary salt dosages in dyeing processes (i.e., source reduction) and developing new end-of-pipe technologies that can work in a saline environment.

Enhanced degradation of sulfadiazine by novel β-alaninediacetic acid-modified Fe3O4 nanocomposite coupled with peroxymonosulfate

Magnetic nanocomposite β-alaninediacetic acid-modified Fe₃O₄ (β-ADA@Fe₃O₄) was prepared, characterized and evaluated to activate peroxymonosulfate (PMS) for improved degradation of sulfadiazine (SD). The results reveal that β-ADA@Fe₃O₄ express more efficient catalytic activity in PMS inducement compared with Fe₃O₄, and the observed pseudo first rate constant (k_obs) of SD degradation is enhanced from 1.05 × 10^-2 to 7.02 × 10^-2 min^-1 when Fe₃O₄ is replaced by β-ADA@Fe₃O₄. The highest removal rate 54.0% occurs when [PMS]₀ and m(β-ADA@Fe₃O₄)₀ was 0.3 mM and 0.8 g/L at neutral pH. High intensity of hydroxyl radicals (OH) and relatively low intensity of sulfate radicals (SO₄^-) are distinguished in system by scavenging experiments and electron paramagnetic resonance (EPR) tests. Results point that β-ADA would significantly promote the circulation of Fe²⁺-Fe³⁺ on the surface of β-ADA@Fe₃O₄, producing more radicals (OH, SO₄^-). The findings herein imply that β-ADA@Fe₃O₄ is an efficient and green catalyst in activation of peroxymonosulfate under neutral environment.