Impact of Chloride Ions on UV/H2O2 and UV/Persulfate Advanced Oxidation Processes

Persulfate-Based Advanced Oxidation: Critical Assessment of Opportunities and Roadblocks

Reports that promote persulfate-based advanced oxidation process (AOP) as a viable alternative to hydrogen peroxide-based processes have been rapidly accumulating in recent water treatment literature. Various strategies to activate peroxide bonds in persulfate precursors have been proposed and the capacity to degrade a wide range of organic pollutants has been demonstrated. Compared to traditional AOPs in which hydroxyl radical serves as the main oxidant, persulfate-based AOPs have been claimed to involve different in situ generated oxidants such as sulfate radical and singlet oxygen as well as nonradical oxidation pathways. However, there exist controversial observations and interpretations around some of these claims, challenging robust scientific progress of this technology toward practical use. This Critical Review comparatively examines the activation mechanisms of peroxymonosulfate and peroxydisulfate and the formation pathways of oxidizing species. Properties of the main oxidizing species are scrutinized and the role of singlet oxygen is debated. In addition, the impacts of water parameters and constituents such as pH, background organic matter, halide, phosphate, and carbonate on persulfate-driven chemistry are discussed. The opportunity for niche applications is also presented, emphasizing the need for parallel efforts to remove currently prevalent knowledge roadblocks.

Synergistic activation of peroxymonosulfate and persulfate by ferrous ion and molybdenum disulfide for pollutant degradation: Theoretical and experimental studies

Activation of peroxymonosulfate (PMS) and persulfate (PS) by Fe²⁺ is widely used for oxidizing organic pollutants. However, their application is limited by the slow conversion rate of Fe³⁺ to Fe²⁺ and the accumulation of Fe³⁺. Here, we introduce commercial molybdenum disulfide (MoS₂) to promote the activation of PMS and PS by Fe²⁺, and explore the mechanism of this promotion using experimental and theoretical methods. The Fe²⁺/PMS/MoS₂ and Fe²⁺/PS/MoS₂ systems achieved faster rate of PMS and PS conversion and also higher degradation efficiency toward pollutants. About 94.7% and 87.6% of rhodamine B (RhB) could be degraded in Fe²⁺/PMS/MoS₂ (54 μM Fe²⁺, 1 mM PMS) and Fe²⁺/PS/MoS₂ (54 μM Fe²⁺, 0.25 mM PS) system, respectively. MoS₂ addition simultaneously promoted the Fe³⁺/Fe²⁺ cycle, the PMS and PS conversion, and the RhB mineralization. As a co-catalyst, MoS₂ exhibited excellent stability for eight successive cycles of use. The predominant oxidant was identified as SO₄^- in Fe²⁺/PMS/MoS₂ and Fe²⁺/PS/MoS₂ systems. Theoretical calculations and a kinetic model were employed to evaluate the catalytic performance of the systems. These novel findings indicate that the combination of a commercially available MoS₂ catalyst with a low dosage of Fe²⁺ is a promising and effective approach for efficient activation of PMS and PS to produce SO₄^- and OH.

Application of Cobalt/Peracetic Acid to Degrade Sulfamethoxazole at Neutral Condition: Efficiency and Mechanisms

An advanced oxidation process of combining cobalt and peracetic acid (Co/PAA) was developed to degrade sulfamethoxazole (SMX) in this study. The formed acetylperoxy radical (CH₃CO₃^•) through the activation of PAA by Co (Co²⁺) was the dominant radical responsible for SMX degradation, and acetoxyl radical (CH₃CO₂^•) might also have played a role. The efficient redox cycle of Co³⁺/Co²⁺ allows good removal efficiency of SMX even at quite low dosage of Co (<1 μM). The presence of H₂O₂ in the Co/PAA process has a negative effect on the degradation of SMX due to the competition for reactive radicals. The SMX degradation in the Co/PAA process is pH dependent, and the optimum reaction pH is near-neutral. Humic acid and HCO₃^- can inhibit SMX degradation in the Co/PAA process, while the presence of Cl^- plays a little role in the degradation of SMX in this system. Although transformation products of SMX in the Co/PAA system show higher acute toxicity, the low Co dose and SMX concentration in aquatic solution can efficiently weaken the acute toxicity. After reaction in the Co/PAA process, numerous carbon sources that could be provided for bacteria and algae growth can be produced, suggesting that the proposed Co/PAA process has good potential when combined with the biotreatment processes.

Cu(II)-enhanced degradation of acid orange 7 by Fe(II)-activated persulfate with hydroxylamine over a wide pH range

The activation of persulfate by Fe(II) coupled with hydroxylamine (the HA/Fe(II)/PS system) was highly effective for the degradation of refractory organic contaminants under acidic pH conditions. However, owing to the precipitation of ferric hydroxide and/or the slow reduction from Fe(III) to Fe(II), the HA/Fe(II)/PS system was invalid under neutral and alkaline pH conditions. In this study, it was observed that the degradation of acid orange 7 (AO7) was strongly enhanced over the wide pH range of 2-9 when trace Cu(II) (0.5-5 μM) was spiked into the HA/Fe(II)/PS system. It was evident that Cu(I) was generated via the reduction of Cu(II) by HA in the bimetallic system at both pH 3 and pH 8, and the steady concentration of Fe(II) in the bimetallic system was much higher than that in the HA/Fe(II)/PS system due to the rapid reaction between Fe(III) and Cu(I). Quenching experiments using tert-butyl alcohol, methanol and sodium bromide as the scavengers and electron spin resonance experiments confirmed that the primary reactive species responsible for AO7 degradation were sulfate radical at both pH 3 and pH 8, rather than hydroxyl radical and Cu(III). Nevertheless, sulfate radical was mainly produced by Fe(II)-activated PS at pH 3, while both Cu(I) and Fe(II) made important contributions to the generation of sulfate radical at pH 8. The bimetallic system was also highly effective in degrading other organic contaminants, such as phenol, diclofenac, reactive red 2 and orange G. This study might provide a promising idea based on Fe(II)-activated PS for degrading organic contaminants over a wide pH range.

Development of a Three-Dimensional Electrochemical System Using a Blue TiO2/SnO2-Sb2O3 Anode for Treating Low-Ionic-Strength Wastewater

Reducing energy use is crucial to commercialize electrochemical oxidation technologies. We developed a three-dimensional (3-D) electrochemical system that can significantly reduce the applied voltage and effectively degrade organic contaminants in low-ionic-strength wastewaters. The 3-D system consisted of a composite wire mesh anode (composed of blue TiO₂ nanotubes covered with SnO₂-Sb₂O₃), a proton exchange membrane, and a stainless-steel wire mesh cathode, which were compressed firmly together. For the 3-D system, we placed the anode of a 3-D electrode toward the wastewater that flowed past the anode. Both the two-dimensional (2-D) and 3-D systems had the same anode and cathode. We found that the 3-D system could reduce the applied voltage by 75.7% and reduce the electrical efficiency per log order reduction (EE/O) by 73% for 0.001 M Na₂SO₄. For Na₂SO₄ concentrations greater than 0.05 M, the 2-D system had a slightly lower EE/O. We also compared the EE/O of electrochemical advanced oxidation processes (EAOPs) with that of other advanced oxidation processes (UV/H₂O₂, UV/persulfate, O₃/H₂O₂, UV/ TiO₂, and UV/chlorine). We found that EAOPs have a much higher EE/O for low BA concentrations (20 mg/L) and a much lower EE/O for high BA concentrations (2000 mg/L).

Phenanthrene degradation using Fe(III)-EDDS photoactivation under simulated solar light: A model for soil washing effluent treatment

In this work, for the first time, the nonionic surfactant polyoxyethylene-(20)-sorbitan monooleate (Tween 80, C₆₄H₁₂₄O₂₆) aided soil washing effluent was treated by enhanced activation of persulfate (PS) using Fe(III)-EDDS (EDDS: ethylenediamine-N, N-disuccinic acid) complexes under simulated solar light irradiation. The performance of this system was followed via the production and reactivity of radical species (SO₄^-, HO, Cl₂^-) and degradation of phenanthrene (PHE) used as a model pollutant in soils. Different physico-chemical parameters such as the concentration of reactive species and pH were investigated through the PHE degradation efficiency. The second-order rate constants of the reactions for generated radicals with PHE and Tween 80 in solution were identified through competitive reaction experiments under steady-state conditions and application of nanosecond laser flash photolysis (LFP) as well. A kinetic approach was applied to assess the selectivity and reactivity of photo-generated radicals in aqueous medium in order to explain the observed degradation trends. This work proposes an innovative technology of management of soil washing solutions using Fe(III)-EDDS complexes and solar light for the activation of persulfate.

Complexation Enhances Cu(II)-Activated Peroxydisulfate: A Novel Activation Mechanism and Cu(III) Contribution

While aqueous free Cu(II) ion is known to be ineffective to activate peroxydisulfate (PDS), here we report for the first time that Cu(II) complexes are potentially effective activators for PDS when the coordination involves suitable ligands. Using cefalexin (CFX) as a representative, studies show that the complex of Cu(II) with CFX can efficiently activate PDS to induce rapid degradation of CFX. Transformation products of CFX by PDS/Cu(II) differ substantially from those generated from the typical radical oxidation process, for example, PDS/Ag(I), but quite resemble the products from oxidation of CFX by Cu(III). Complexation with CFX increases the electron density of Cu(II), favoring electron transfer from Cu(II) to PDS to generate radicals and Cu(III). The produced Cu(III), rather than radicals, plays the primary role in the overall CFX degradation and regenerates Cu(II) in a catalytic cycle. This novel activation process can occur for a wide range of contaminants (cephalosporin, penicillin, and tetracycline antibiotics) and ligands when coordinated with Cu(II), and N-containing functional groups (e.g. amines) were found to form effective Cu(II) complexes for PDS activation. The new findings of this study further broaden the knowledge on PDS activation by aqueous Cu(II), and verify the contribution of Cu(III) to contaminant elimination.

Spectrophotometric determination of hydrogen peroxide in water by oxidative decolorization of azo dyes using Fenton system

In this study, based on the oxidative decolorization of three azo dyes (Orange G (OG), Acid Orange 7 (AO7) and Reactive Black 5 (RB5)) with hydroxyl radicals generated in Fenton system, we have successfully established three types of azo dyes spectrophotometric methods for measuring aqueous hydrogen peroxide (H₂O₂) concentration. The decolorization extent of OG, AO7 and RB5 at the corresponding characteristic wavelengths of 478 nm, 484 nm and 597 nm are proportion to the concentration of H₂O₂ in aqueous solutions. Under the selected reaction conditions, three well linear correlations between the depletion of azo dyes and the H₂O₂ concentration are established in the range of 0.45-175 μmol L^-1 of OG, 0.36-120 μmol L^-1 of AO7 and 0.44-175 μmol L^-1 of RB5, respectively. These proposed spectrophotometric methods are enough accurate to measure low concentrations of H₂O₂ in practical water samples and monitor the variations of H₂O₂ concentration during the phenol degradation in the Cu(II)/HCO₃^-/H₂O₂ process.

Degradation of imipramine by vacuum ultraviolet (VUV) system: Influencing parameters, mechanisms, and variation of acute toxicity

Degradation of imipramine (IMI) in the VUV system (VUV₁₈₅ + UV₂₅₄) was firstly evaluated in this study. Both HO^• oxidation and UV₂₅₄ direct photolysis accounted for IMI degradation. The quantum yields of UV₂₅₄ direct photolysis of deprotonated and protonated IMI were 1.31×10^-2 and 3.31×10^-3, respectively, resulting in the higher degradation efficiency of IMI at basic condition. Increasing the initial IMI concentration lowered the degradation efficiency of IMI. While elevating reaction temperature significantly improved IMI degradation efficiency through the promotion of both the quantum yields of HO^• and the UV₂₅₄ direct photolysis rate. The apparent activation energy was calculated to be about 26.6 kJ mol^-1. Negative-linear relationships between the k_obs of IMI degradation and the concentrations of HCO₃^-/CO₃^2-, NOM and Cl^- were obtained. The degradation pathways were proposed that cleavage of side chain and hydroxylation of iminodibenzyl and methyl groups were considered as the initial steps for IMI degradation in the VUV system. Although some high toxic intermediate products would be produced, they can be further transformed to other lower toxic products. The good degradation efficiency of IMI under realistic water matrices further suggests that the VUV system would be a good method to degrade IMI in aquatic environment.

Cooperative Pollutant Adsorption and Persulfate-Driven Oxidation on Hierarchically Ordered Porous Carbon

This study presents a 3D hierarchically ordered porous carbon material (HOPC) that simultaneously achieves efficient adsorption of a range of water pollutants as well as catalytic oxidation of adsorbed pollutants. High adsorption capacity and rapid adsorption kinetics are attributed to the hydrophobic nature of the carbon substrate, the large surface area due to high porosity, and the relatively uniform size of pores that comprise the structure. The oxidative degradation is achieved by efficient mediation of electron transfer from pollutants to persulfate through the sp²-hybridized carbon and nitrogen network. As the persulfate activation and pollutant oxidation do not involve reactive radicals, oxidative degradation of the adsorbent is prevented, which has been a primary concern when adsorption and oxidation are combined either to regenerate adsorbate or to enhance oxidation performance. Batch tests showed that near complete removal of various recalcitrant micropollutants can be achieved within a short time (less than 1 min) even when treating a complex water matrix, as pollutants are concentrated on the surface of HOPC, where their oxidation is catalyzed.

Insights into the oxidation of organic contaminants by iron nanoparticles encapsulated within boron and nitrogen co-doped carbon nanoshell: Catalyzed Fenton-like reaction at natural pH

Iron nanoparticles encapsulated within boron and nitrogen co-doped carbon nanoshell (B/N-C@Fe) were synthesized through a novel and green pyrolysis process using melamine, boric acid, and ferric nitrate as the precursors. The surface morphology, structure, and composition of the B/N-C@Fe materials were thoroughly investigated. The materials were employed as novel catalysts for the activation of potassium monopersulfate triple salt (PMS) for the degradation of levofloxacin (LFX). Linear sweep voltammograms and quenching experiments were used to identify the mechanisms of PMS activation and LFX oxidation by B/N-C@Fe, where SO₄^- as well as HO were proved to be the main radicals for the reaction processes. This study also discussed how the fluvic acid and inorganic anions in the aqueous solutions affected the degradation of LFX and use this method to simulate the degradation in the real wastewater. The synthesized materials showed a high efficiency (85.5% of LFX was degraded), outstanding stability, and excellent reusability (77.7% of LFX was degraded in the 5th run) in the Fenton-like reaction of LFX. In view of these advantages, B/N-C@Fe have great potentials as novel strategic materials for environmental catalysis.

Insights into the mechanism of nonradical reactions of persulfate activated by carbon nanotubes: Activation performance and structure-function relationship

This study aimed to elucidate the intrinsic mechanisms of PS activation by carbon nanotubes (CNTs). Singlet oxygen generation (¹O₂) and direct CNTs-mediated electron transfer were hypothesized to be two major pathways of the oxidation of 2,4-dichlorophenol (2,4-DCP) by PS in the presence of both unmodified and modified CNTs. For the first time, roles of CNT active sites responsible for PS activation were determined using CNT derivatization and structural characterization. By selectively deactivating the carbonyl, hydroxyl or carboxylic groups on CNTs surface and linear sweep voltammetry (LSV) analysis, CO groups were determined to be the main active sites contributing to the direct electron transfer oxidation, while singlet oxygen was generated at CNTs defects. Subsequent UV irradiation was shown to cause the recovery of surface defects with I_D/I_G of CNTs increasing by 21%. This resulted in the regeneration of the performance for the coupled system and allowed for multi-cycle activation of PS by CNTs. These results suggest that CNTs/PS system combined with regeneration based on UV irradiation can be used as an effective alternative process for continuous degradation of recalcitrant aqueous contaminants through the non-radical mechanism.

Cotransformation of Carbon Dots and Contaminant under Light in Aqueous Solutions: A Mechanistic Study

In this study, the photochemistry of carbon dots (CDs) and their effects on pollutant transformation were systematically examined. Diethyl phthalate (DEP) degradation was strongly enhanced by CDs under UV light, with the observed reaction rate constant ( k_obs) increased by 2.4-15.1-fold by CDs at a concentration of 0.5-10 mg/L. Electron paramagnetic resonance (EPR) spectrometry combined with free radical quenching experiments with various chemical probes indicated the production of reactive oxygen species (ROS), including hydroxyl radicals (^•OH), singlet oxygen (¹O₂), and superoxide radical anions (O₂^•-), and these contributed to the enhanced DEP degradation. Meanwhile, CDs were also degraded to low-molecular-weight species and partially mineralized to CO₂ by ROS, as evidenced by Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) and total organic carbon (TOC) analysis, and transformation of CDs was accelerated by DEP. Furthermore, CDs were degraded rapidly under natural sunlight, accompanied by the formation of ^•OH and ¹O₂. Anions such as CO₃^2-, NO₃^-, and Cl^- had limited effects on transformation of CDs, while humic substances greatly inhibited this process. Our results indicate that photoreactions of CDs play an important role in influencing the transformation of pollutants and CDs themselves in the natural aquatic environment. The findings provide invaluable information for evaluating risks associated with the release of CDs into the natural environment.

Enhanced Perfluorooctanoic Acid Degradation by Electrochemical Activation of Sulfate Solution on B/N Codoped Diamond

Electrochemical oxidation based on SO₄^•- and ^•OH generated from sulfate electrolyte is a cost-effective method for degradation of persistent organic pollutants (POPs). However, sulfate activation remains a great challenge due to lack of active and robust electrodes. Herein, a B/N codoped diamond (BND) electrode is designed for electrochemical degradation of POPs via sulfate activation. It is efficient and stable for perfluorooctanoic acid (PFOA) oxidation with first-order kinetic constants of 2.4 h^-1 and total organic carbon removal efficiency of 77.4% (3 h) at relatively low current density of 4 mA cm^-2. The good activity of BND mainly originates from a B and N codoping effect. The PFOA oxidation rate at sulfate electrolyte is significantly enhanced (2.3-3.4 times) compared with those at nitrate and perchlorate electrolytes. At sulfate, PFOA oxidation rate decreases slightly in the presence of ^•OH quencher while it declines significantly with SO₄^•- and ^•OH quenchers, indicate both SO₄^•- and ^•OH contribute to PFOA oxidation but SO₄^•- contribution is more significant. On the basis of intermediates analysis, a proposed mechanism for PFOA degradation is that PFOA is oxidized to shorter chain perfluorocarboxylic acids gradually by SO₄^•- and ^•OH until it is mineralized.

Geoenvironmental characteristics of bisphenol A contaminated soil after persulfate treatment with different activation/enhancement methods

Persulfate (PSF) is a strong oxidant that has been used extensively in the In-Situ Chemical Oxidation (ISCO) technology. The geoenvironmental impact of PSF treatment is barely investigated. This situation should be carefully considered as it may affect the reutilization of contaminated soil as engineering materials. This paper studied the removal of bisphenol A (BPA) by PSF with Nano Zero-Valent Iron (nZVI) and percarbonate (SPC) activated/enhanced and their subsequent impacts on the engineering properties of soil. The physicochemical and geotechnical properties of soils before and after treatment were evaluated using batch experiments. The results indicate that the introduced pristine PSF can be activated by some naturally occurring matters and subsequently lead to the mineralization of BPA. Both non-activated PSF and activated/enhanced PSF treatment led to the soil improvement in the undrained shear strength at different degrees. The primary mechanism of soil improvement is ascribed to the heterogeneous sulfate and/or carbonate precipitation. Meanwhile, Ca²⁺ in the pore fluid played a significant role in the enhancement of the soil strength. A conclusion was drawn that the treatment of both non-activated PSF, nZVI- and SPC-activated PSF treatment can achieve removal of BPA and soil improvement in the short-term simultaneously. This study can improve the PSF-involved remediation of brownfields and dredged sediments for a sustainable and low-carbon society.

Oxidation Mechanisms of the UV/Free Chlorine Process: Kinetic Modeling and Quantitative Structure Activity Relationships

Recently, the UV/free chlorine process has gained attention as a promising technology for destroying refractory organic contaminants in the aqueous phase. We have developed a kinetic model based on first-principles to describe the kinetics and mechanisms of the oxidation of organic contaminants in the UV/free chlorine process. Substituted benzoic acid compounds (SBACs) were chosen as the target parent contaminants. We determined the second-order rate constants between SBACs and reactive chlorine species (RCS; including [Formula: see text], [Formula: see text] and [Formula: see text]) by fitting our model to the experimental results. We then predicted the concentration profiles of SBACs under various operational conditions. We analyzed the kinetic data and predicted concentration profiles of reactive radicals ([Formula: see text] and RCS), we found that [Formula: see text] was the dominant radicals for SBACs destruction. In addition, we established quantitative structure activity relationships (QSARs) that can help predict the second-order rate constants for SBACs destruction by each type of reactive radicals using SBACs Hammett constants. Our first-principles-based kinetic model has been verified using experimental data. Our model can facilitate a design for the most cost-effective application of the UV/free chlorine process. For example, our model can determine the optimum chlorine dosage and UV light intensity that result in the lowest energy consumption.

A novel method of ultraviolet/NaClO2-NH4OH for NO removal: Mechanism and kinetics

The key step for nitric oxide (NO) removal using oxidation method is to efficiently oxidize NO. This study developed a novel advanced oxidation process (AOP) of ultraviolet light (UV) catalysis of chlorite (NaClO₂) to oxidize NO. The production of nitric dioxide (NO₂) and photo-production of chlorine dioxide (ClO₂) were suppressed by adding ammonium hydroxide (NH₄OH). The NO conversion efficiency was 98.1% using UV/NaClO₂-NH₄OH. Electron spin resonance (ESR) tests confirmed the roles of hydroxyl radical (HO) and oxychloride radical (ClO/Cl₂O₂) in the oxidation of NO. Kinetics analyses showed that NO flux was significantly enhanced by radical-induced (HO/ClO) oxidation of NO. In the presence of UV, the overall reaction rates (k_ov1*) were 3-8 times higher than those without UV. The Hatta number, namely the enhanced factor, was calculated in the range of 229-403 and 730-780 corresponding to without and with UV light, suggesting that NO oxidation belonged to fast and/or instantaneous reaction. Thus, the gas-film mass transfer resistance was the rate-determining step. N-containing product was determined as NH₄⁺ and NO₃^- according to X-ray photoelectron spectroscopy (XPS).