Dr Zhaohui Wang

Purpose: Soil organic matter is an important part of the earth¿s carbon cycle, of which stability is related to its own obstinacy, and environmental disturbance also plays an important role. Soil minerals can combine with organic matter, preventing organic matter decomposition. Understanding the impact of environmental change and the mechanism of mineral stabilization of soil organic matter is important for evaluating global carbon storage and recycling. Materials and methods: We review the input, transformation on the soil organic matter composition, and the influence of environmental factors on soil organic matter stabilization and discuss the mechanism of mineral stabilization on soil organic matter, especially the role of iron. Results and discussion: At present, there are many studies on the stabilization mechanism and stable medium of soil organic matter. Environmental changes such as pH, moisture, temperature, and fertilization have a significant impact on the stabilization of soil organic matter, which affect the adsorption, co-precipitation of soil organic matter, or damage the soil structure, resulting in the re-exposure of organic matter. The stabilizing effect of minerals on soil organic matter is discussed, especially iron. Conclusions: This review gives an overview of the effect of environmental disturbance on the stabilization of soil organic matter and the protection of minerals to soil organic matter. The current research focuses on the adsorption of some organic matter or organic matter functional groups by minerals. However, this does not represent the real situation in soil. The composition and stabilization mechanisms of soil organic matter are complicated and ambiguous. It is critical to develop in situ analysis techniques to further study the stabilization mechanism of soil organic matter. Meanwhile, establishing mathematical models to simulate the changes of environment is conducive to forming a better understanding of the stability of organic matter. These studies can not only solve the problem of the ¿black box¿ of soil organic matter, but also promote further understanding of the impact of soil organic matter on the global environment and carbon cycle.

A majority of clay minerals contain Fe, and the redox cycling of Fe(III)/Fe(II) in clay minerals has been extensively studied as it may fuel the biogeochemical cycles of nutrients and govern the mobility, toxicity and bioavailability of a number of environmental contaminants. There are three types of Fe in clay minerals, including structural Fe sandwiched in the lattice of clays, Fe species in interlayer space and adsorbed on the external surface of clays. They exhibit distinct reactivity towards contaminants due to their differences in redox properties and accessibility to contaminant species. In natural environments, microbially driven Fe(III)/Fe(II) redox cycling in clay minerals is thought to be important, whereas reductants (e.g., dithionite and Fe(II)) or oxidants (e.g., peroxygens) are capable of enhancing the rates and extents of redox dynamics in engineered systems. Fe(III)-containing clay minerals can directly react with oxidizable pollutants (e.g., phenols and polycyclic aromatic hydrocarbons (PAHs)), whereas structural Fe(II) is able to react with reducible pollutants, such as nitrate, nitroaromatic compounds, chlorinated aliphatic compounds. Also structural Fe(II) can transfer electrons to oxygen (O2), peroxymonosulfate (PMS), or hydrogen peroxide (H2O2), yielding reactive radicals that can promote the oxidative transformation of contaminants. This review summarizes the recent discoveries on redox reactivity of Fe in clay minerals and its links to fates of environmental contaminants. The biological and chemical reduction mechanisms of Fe(III)-clay minerals, as well as the interaction mechanism between Fe(III) or Fe(II)-containing clay minerals and contaminants are elaborated. Some knowledge gaps are identified for better understanding and modelling of clay-associated contaminant behavior and effective design of remediation solutions.

The neutralization treatment of acid mine drainage involves the oxidation of Fe(II), but little is known about the effects of co-existing minerals on the oxidation and hydrolysis of Fe(II) to iron oxides. Here we investigated the transformation of fresh and heated Fe(II) oxidation coprecipitates, which were synthesized in the presence and the absence of five co-existing minerals (montmorillonite, kaolin, quartz (SiO2), aluminium oxide (Al2O3) and calcium carbonate (CaCO3)). In the FeSO4 system with montmorillonite or kaolin, the formation of lepidocrocite was inhibited with the increase of clay mineral contents. In the same system, heated coprecipitates of montmorillonite were mainly comprised of amorphous ferrihydrite and its transformation was retarded by the excess montmorillonite. In the FeCl2 system with SiO2, Al2O3 or CaCO3, akaganeite formation was inhibited with the increase in the corresponding mineral contents. In the same system, goethite formation was blocked by either CaCO3 or Al2O3 and the growth of lepidocrocite was inhibited by CaCO3 or SiO2. However, magnetite formation was enhanced by addition of CaCO3. These findings are important for predicting products of abiotic Fe(II) oxidation during the neutralization of acid mine drainage and for better understanding the transformation of amorphous iron oxides in the complicated environmental matrix.

Available online Iodinated X-ray contrast media (ICMs) are clinical drugs used to enhance the imaging effect. Triiodobenzene ring structures of ICMs lead to its extremely high chemical stability, biological inertness, which makes it difficult to be completely removed by traditional water treatment processes. Hence, considerable concentration of ICMs can be frequently detected in aquatic environment. Relying on the strong oxidation capacity of HO¿ or SO4¿¿, various advanced oxidation processes (AOPs) have demonstrated substantial removal efficiency for ICMs. It is evident that ICMs can be decomposed mainly through (1) deiodination, (2) dehydration, (3) decarboxylation, (4) H-abstraction, (5) hydroxyl addition, (6) hydroxyl substitution, (7) oxidation of alcohol groups, (8) cleavage of amide bond, and (9) amino oxidation. However, during the ICMs removal process, the C-I bonds of ICMs molecules are broken, giving rise to the formation of cytotoxic iodination disinfection by-products (I-DBPs) that are potentially more harmful to the ecosystem and human health than their parent compounds. To better understand the technology gaps, this review elaborates the major AOPs which are effective for ICMs removal and emphasizes on the main degradation routes of ICMs in different oxidation system. Some prevailing concerns and challenges are discussed for optimizing the ICMs treatment process.

Sulfate radical AOPs (SR-AOP) were successfully utilized in degradation of antibiotics in water and wastewater treatment. The review discusses details on SR-AOPs mechanisms and applications for antibiotics degradation. The progress in this field was discussed, highlighting the most promising developments and remaining challenges. The applicability of SR-AOPs was summarized revealing the most susceptible and persistent to oxidation groups of pharmaceuticals. Highest effectiveness was reported for degradation of pharmaceuticals on ppb level. Systems revealed a scavenging effect in case of oxidant dose 0.7 mM of the PS and 2 mM of PMS. Future development demands simple persulfates activation systems for real matrix treatment.

For a large amount of spent lithium battery electrode materials (SLBEMs), direct recycling by traditional hydrometallurgy or pyrometallurgy technologies suffers from high cost and low efficiency and even serious secondary pollution. Therefore, aiming to maximize the benefits of both environmental protection and e-waste resource recovery, the applications of SLBEM containing redox-active transition metals (e.g., Ni, Co, Mn, and Fe) for catalytic decontamination before disposal and recycling has attracted extensive attention. More importantly, the positive effects of innate structural advantages (defects, oxygen vacancies, and metal vacancies) in SLBEMs on catalytic decontamination have gradually been unveiled. This review summarizes the pretreatment and utilization methods to achieve excellent catalytic performance of SLBEMs, the key factors (pH, reaction temperature, coexisting anions, and catalyst dosage) affecting the catalytic activity of SLBEM, the potential application and the outstanding characteristics (detection, reinforcement approaches, and effects of innate structural advantages) of SLBEMs in pollution treatment, and possible reaction mechanisms. In addition, this review proposes the possible problems of SLBEMs in practical decontamination and the future outlook, which can help to provide a broader reference for researchers to better promote the implementation of ¿treating waste to waste¿ strategy.

Degradation of bisphenol S was studied using ozone activated by sodium percarbonate and the effectiveness of optimized process was compared with the peroxone process. The influence of several factors including sodium percarbonate concentration, ozone dose, pH, and water matrix were investigated. A synergetic coefficient of 3.84 was achieved for the combination of sodium percarbonate and ozone, confirming the effectiveness of this hybrid process. Scavenging tests revealed, that carbonate radicals, hydroxyl radicals, superoxide radicals, and singlet oxygen contributed to the degradation of bisphenol S. At the same operating condition, degradation effectiveness values of 99% and 81% were obtained by ozone combined with sodium percarbonate and hydrogen peroxide, respectively, demonstrating the superiority of sodium percarbonate over hydrogen peroxide in combination with ozone for the degradation of bisphenol S. Low concentration of inorganic anions had a negligible effect on the degradation, while carbonate ions increased the first-order degradation rate constant by 56%.

Memantine (MEM) is an important and widely used pharmaceutical for the treatment of Alzheimer's disease. However, due to MEM structural features, it is considered fairly persistent and bioaccumulative chemical and is therefore potentially harmful to the environment. Here, the degradation of MEM by UV-C/H2O2 and UV-A/TiO2 processes under optimized conditions was investigated. Complete degradation of MEM (0.1 mM) was achieved in < 3.5 min by UV-C/H2O2 process (pH 5.1, [H2O2]=15.21 mM) and in ~90 min by UV-A/TiO2 process (pH 4, ¿(TiO2)= 0.955 gL-1). Under the same conditions, 90% mineralization was achieved in ~90 and ~330 min, respectively. A second-order rate constant for the reaction of MEM with hydroxyl radical (HO·) (kHO¿/MEM) was determined based on the competitive kinetic experiments, amounting 6.16 × 109 M-1s-1. The relative contribution of reactive oxygen species in the photocatalytic treatment was determined by selective scavenging, assigning a major contribution to HO·, while superoxide radicals played a minor role in the degradation of MEM. A detailed survey of the intermediates of MEM degradation was performed using accurate mass spectrometry, focusing on the correlation of 25 identified intermediates with changes in biodegradability and toxicity to Daphnia magna. It was found that MEM and its degradation intermediates are non-biodegradable and relatively toxic, requiring extensive mineralization, even above 50%, to improve these ecotoxicity parameters.

Halide ions are common in wastewater but their roles in advanced oxidation processes (AOPs) and the degradation of pollutants are reportedly highly variable. This review seeks to reconcile conflicting data on the degradation of pollutants in the presence of halides by peroxygens-based AOPs reported in the literatures: peroxymonosulfate (PMS)-, peroxydisulfate (PDS)-, and hydrogen peroxide (H2O2)-based AOPs, and electrochemical advanced oxidation processes (EAOPs). Seven common substituent groups on the aromatic ring (i.e., hydroxy groups (¿OH), amino groups (¿NH2), alkyl groups (¿R), olefinic compounds, carboxyl groups(¿COOH), nitro groups (¿NO2), halogens (¿X)) are considered, and halide ions were found to affect contaminants¿ degradation in all cases, either negatively or positively. The key influencing factors such as substrate properties and operating parameters (e.g., halides dosage, ionic strength, pH, and activation method) are examined. Each individual variable may affect degradation rates, but many of these effects are a combination of these variables in experimental or natural systems. Thus, the research work on the effect of halides on the simultaneous removal of more organics is needed. This present study details the unresolved challenges to provide a path for engineering challenges on AOPs. According to the factors affecting the performance of halides during decontamination processes, better AOP technologies should be selected to diminish adverse effects and take advantage of positive effects to deal with target pollutants in the saline environment, so as to achieve the purpose of reducing the operating costs and emission reduction.

High-performance water treatment systems based on cavitational processes have received an increasing interest of scientific community in the past few decades. Numerous studies indicated the advantageous application of hydrodynamic cavitation as an alternative, reagent-free treatment method of various pollutants in water. Both approaches were proved as an effective method to achieve mineralization of many organic contaminants as well as a disinfection method, which is able to eliminate pathogenic microorganisms. This makes cavitation-based methods a promising candidate implemented in a post-treatment stage of water treatment facilities. Nowadays, hybrid methods based on combination of cavitation with advanced oxidation processes (AOPs), possessing enhanced oxidation capacity were proposed. Compared to the individual utilization of cavitation and AOPs (e.g., O3, H2O2, Fenton's process), hybrid processes are capable to degrade even highly persistent contaminants and shorten the operation time reducing the overall consumption of energy and oxidants. The improved performance of hybrid methods is attributed to the synergistic effect occurring between integrated technologies, which is expressed by the synergistic index. In this paper, recent reports focusing on coupling of cavitation and AOPs were reviewed to reveal major principles and mechanisms governing the synergistic effect. The review discusses the effect of process parameters (oxidant type, pH, hydraulic and ultrasonic properties, Kow) on the oxidation effectiveness. Comparative analysis was provided in order to highlight the advantages and limits laying behind the discussed methods. The analysis of the economic feasibility was performed to assess the potential applicability of hybrid techniques in large-scale wastewater treatment.

Over the past years, persulfate (PS) is widely applied due to their high versatility and efficacy in decontamination and sterilization. While treatment of organic chemicals, remediation of soil and groundwater, sludge treatment, disinfection on pathogen microorganisms have been covered by most published reviews, there are no comprehensive and specific reviews on its application to address diverse sustainability challenges, including solid waste treatment, resources recovery and regeneration of ecomaterials. PS applications mainly rely on direct oxidation by PS itself or the reactive sulfate radical (SO4¿-) or hydroxyl radical (¿OH) from the activation of peroxodisulfate (PDS, S2O82-) or peroxymonosulfate (PMS, HSO5-) in SO4¿--based advanced oxidation processes (SO4¿--AOPs). From a broader perspective of environmental cleanup and sustainability, this review summarizes the various applications of PS except pollutant decontamination and elaborates the possible reaction mechanisms. Additionally, the differences between PS treatment and conventional technologies are highlighted. Challenges, research needs and future prospect are thus discussed to promote the development of the applications of PS-based oxidation processes in niche environmental fields. In all, this review is a call to pay more attention to the possibilities of PS application in practical resource reutilization and environmental protection except widely reported pollutant degradation.

The present review emphasizes the role of hydrodynamic cavitation (HC) and acoustic cavitation in clean and green technologies for selected fuels (of hydrocarbon origins such as gasoline, naphtha, diesel, heavy oil, and crude oil) processing applications including biodiesel production. Herein, the role of cavitation reactors, their geometrical parameters, physicochemical properties of liquid media, liquid oxidants, catalyst loading, reactive oxygen species, and different types of emulsification and formation of radicals, formation as well as extraction of formed by-products are systematically reviewed. Among all types of HC reactors, vortex diode and single hole orifices revealed more than 95 % desulfurization yield and a 20 % viscosity reduction in heavy oil upgrading, while multi-hole orifice (100 holes) and slit Venturi allowed obtaining the best biodiesel production processes in terms of high (%) yield, low cost of treatment, and short processing time (5 min; 99 % biodiesel; 4.80 USD/m3). On the other hand, the acoustic cavitation devices are likely to be the most effective in biodiesel production based on ultrasonic bath (90 min; 95 %; 6.7 $/m3) and desulfurization treatment based on ultrasonic transducers (15 min; 98.3 % desulfurization; 10.8 $/m3). The implementation of HC-based processes reveals to be the most cost-effective method over acoustic cavitation-based devices. Finally, by reviewing the ongoing applications and development works, the limitations and challenges for further research are addressed emphasizing the cleaner production and guidelines for future scientists to assure obtaining comprehensive data useful for the research community.

Simultaneous conversion of most harmful As(III) and Cr(VI) to their less toxic counterparts is environmentally desirable and cost-effective. It has been confirmed that simultaneous oxidation of As(III) to As(V) and reduction of Cr(VI) to Cr(III) can occur via free radical or mediated electron transfer processes. While Cr(VI) is reduced by reacting with H¿, eaq-, photoelectron directly or undergoing ligand exchange with H2O2 and SO32-, As(III) is oxidized by HO¿, SO4¿-, O2¿-, and holes (h+) in free radical process. The ability to concentrate Cr and As species on heterogeneous interface and conductivity determining the co-conversion efficiency in mediated electron transfer process. Acidity has positive effect on these co-conversion, while mediated electron transfer process is not much affected by dissolved oxygen (O2). Organic compounds (e.g., oxalate, citrate and phenol) commonly favor Cr(VI) reduction and inhibit As(III) oxidation. To better understand the trends in the existing data and to identify the knowledge gaps, this review elaborates the complicated mechanisms for co-conversion of As(III) and Cr(VI) by various methods. Some challenges and prospects in this active field are also briefly discussed.

Recently, the degradation of organic compounds in saline dye wastewater by sulfate radicals (SO4[rad]-)-based advanced oxidation processes (AOPs) have attracted much attention. However, previous studies on these systems have selected non-chlorinated dyes as model compounds, and little is known about the transformation of chlorinated dyes in such systems. In this study, acid yellow 17 (AY-17) was selected as a model of chlorinated contaminants, and the degradation kinetics and evolution of oxidation byproducts were investigated in the UV/PDS system. AY-17 can be efficiently degraded (over 98% decolorization) under 90 min irradiation at pH 2.0¿3.0, and the reaction follows pseudo-first order kinetics. Cl¿ accelerated the degradation of AY-17, but simultaneously led to an undesirable increase of absorbable organic halogen (AOX). Several chlorinated byproducts were identified by liquid chromatography-mass spectrometry (LC¿MS/MS) in the UV/PDS system. It indicates that endogenic chlorine and exogenic Cl¿ reacted with SO4[rad]- to form chloride radicals, which are involved in the dechlorination and rechlorination of AY-17 and intermediates. The possible degradation mechanisms of AY-17 photooxidative degradation are proposed. This work provides valuable information for further studies on the role of exogenic chloride in the degradation of chlorinated azo dyes and the kinetic parameters in the PDS-based oxidation process.

Pharmaceuticals are one of the most reported categories of anthropogenic micropollutants, which often require a specific remediation type to be eliminated from the environment. This study aimed to address the potential of degrading the pharmaceutical pollutant paracetamol (PCT) in the aqueous environment under ultrasound (US) assisted electro-Fenton by Fe2O3 (hematite) nanoparticles (HNPs) as a catalyst. The synthesized sample was characterized by various techniques including XRD, FESEM, EDS, X-ray dot-mapping, and FTIR. The performance of the electro-Fenton (EF) and US processes was evaluated separately and in combination under optimum conditions. The results showed that the sonoelectro-Fenton (SEF) process under optimum conditions, including pH of 5, HNPs dosage 0.15 g/L, applied current 230 mA, initial PCT concentration 20 mg/L, resulted in a PCT degradation of 98.9% within 60 min of electrolysis time. PCT degradation was well-fitted to the pseudo-first-order kinetic model. Meanwhile, scavenging experiments indicated the vital role of [rad]OH in the decomposition of PCT compared to the negligible role of O2·-. The nitrate ions had a strong inhibitory effect, whereas chloride anions affected PCT elimination slightly. The reusability test of HNPs revealed that almost a 14% drop occurred at the end of the fourth cycle. The HNPs showed high catalytic activity for degradation of PCT compared to other conventional homogeneous transition metals. Besides, SEF-HNPs can successfully detoxify the PCT solution based on the bioassay test. The by-products of PCT degradation by SEF-HNPs were determined and degradation pathway was also proposed. Conclusively, the SEF-HNPs process could be an appropriate system for the removal of various contaminants from aqueous solutions.

Persulfate (peroxymonosulfate or peroxydisulfate) ions, as strong oxidants, have a broad application in the field of advanced oxidation processes (AOPs) for oxidative degradation of organic pollutants in soil and contaminated water. Persulfate concentration is an important operational parameter in persulfate degradation of organic pollutants based on SO4¿--AOPs. At present, the analytical methods used for quantification of persulfate mainly include traditional titration, polarography, electrochemiluminescence, electrochemical methods, spectrometry and chromatography. These methods vary in their limit of detection (LOD), execution time, operational difficulty, accuracy and sensitivity. The traditional methods are cheap and easy to operate, but the poor sensitivity and the high LOD limit its application. Electrochemiluminescence methods are highly sensitive with low detection limit, but requires rather specialized instruments. Electrochemical methods are repaid and have high sensitivity, low LOD and wide detection range, but relatively large reagent volume is needed in the determination process. Spectrophotometry has high accuracy with low consumption rate of sample and reagents, but it is susceptible to colored substances. Ion chromatography has the advantages of high sensitivity, less reagent consumption and simultaneous detection of multiple ions, but no advantage in detection time and LOD. This review presents the principles, merits and demerits of various existing analytical methods for persulfate determination, and discusses the limitations and applicability of various methods. This review is expected to provide some references for the establishment of novel detection methods for persulfate in the future.

Halogenated organic compounds, also termed organohalogens, were initially regarded to be of almost exclusively anthropogenic origin. However, recent research has demonstrated that photochemical reactions are important abiotic sources of organohalogen compounds in sunlit surface waters. Halide ions (X-, X represents Cl, Br and I) are common anions in natural waters and might be oxidized by reactive species originated from photochemistry of dissolved organic matter (DOM) or inorganic photoactive species. The resulting reactive halogen species may react with organic substances with diverse bimolecular reaction rate constants, depending on the complexity and structure of organic substances. Therefore, the chemical mechanism of halogenation remains challenging to be fully elucidated. To better understand the trends in the existing data and to identify the knowledge gaps that may merit further investigation, this review gives an integrative summary on the sources of reactive oxygen species (ROS) and halogen radicals (X[rad]/X2[rad]-). Photochemical halogenation of phenolic compounds and formation of methyl halide and brominated organic pollutants are highlighted. By evaluating existing literature and identifying some uncertainties, this review emphasizes the environmental significance of sunlight-driven halogenation and proposes further research directions on mechanistic investigation and rational experimental design close to natural systems.

The identification of reactive radical species using quenching and electron paramagnetic resonance (EPR) tests has attracted extensive attention, but some mistakes or misinterpretations are often present in recent literature. This review aims to clarify the corresponding issues through surveying literature, including the uncertainty about the identity of radicals in the bulk solution or adsorbed on the catalyst surface in quenching tests, selection of proper scavengers, data explanation for incomplete inhibition, the inconsistent results between quenching and EPR tests (e.g., SO4¿- is predominant in quenching test while the signal of ¿OH predominates in EPR test), and the incorrect identification of EPR signals (e.g., SO4¿- is identified by indiscernible or incorrect signals). In addition, this review outlines the transformation of radicals for better tracing the origin of radicals. It is anticipated that this review can help in avoiding mistakes while investigating catalytic oxidative mechanism with quenching and EPR tests.

Bimetallic Fe-Mn oxide (BFMO) has been regarded as a promising activator of peroxysulfate (PS), the sustained activity and durability of BFMO for long-term activation of PS in situ, however, is unclear for groundwater remediation. A BFMO (i.e., Mn1.5FeO6.35) was prepared and explored for PS-based in situ chemical oxidation (ISCO) of trichloroethylene (TCE) in sand columns with simulated/actual groundwater (SGW/AGW). The sustained activity of BFMO, oxidant utilization efficiency, and postreaction characterization were particularly investigated. Electron spin resonance (ESR) and radical scavenging tests implied that sulfate radicals (SO4â¢-) and hydroxyl radicals (HOâ¢) played major roles in degrading TCE, whereas singlet oxygen (1O2) contributed less to TCE degradation by BFMO-activated Oxone. Fast degradation and almost complete dechlorination of TCE in AGW were obtained, with reaction stoichiometry efficiencies (RSE) of Î"TCE/Î"Oxone at 3-5%, much higher than those reported RSE values in H2O2-based ISCO (=0.28%). HCO3- did not show detrimental effect on TCE degradation, and effects of natural organic matters (NOM) were negligible at high Oxone dosage. Postreaction characterizations displayed that the BFMO was remarkably stable with sustained activity for Oxone activation after 115 days of continuous-flow test, which therefore can be promising catalyst for Oxone-based ISCO for TCE-contaminated groundwater remediation.

A facile and scalable one-pot synthesis method was designed to fabricate N-doped biochar from wood residue for peroxymonosulfate (PMS) activation. The urea:wood residue w/w ratio was varied from 0:1 to 100:1, and the intrinsic (surface chemistry) and extrinsic (morphology, specific surface area, etc.) properties of the N-doped biochar were systematically investigated. Notably, the urea:wood residue w/w ratio played a significant role in tuning the N species composition in the N-doped biochar. A higher urea:wood residue w/w ratio resulted in greater at.% of graphitic N and higher specific surface area. The results show that the catalyst prepared with urea:wood residue w/w ratio of 100:1 (NC800-20) manifested the greatest catalytic activity as PMS activator for organics removal due to its highly favorable combination of intrinsic (highest at.% graphitic N) and extrinsic (excellent specific surface area) properties. The catalyst loading, PMS dosage and pH were found to be operationally crucial for PMS activation. Under the best condition, the NC800-20 can be used to degrade various organic pollutants. The recyclability of NC800-20 was studied and while the catalytic activity gradually deteriorated over successive cycles, it can be completely restored by simple calcination at 500 °C. The used catalyst was extensively characterized, revealing insights into the mechanism of PMS activation. The prevailing mechanism for PMS activation was the nonradical pathway. Overall, this research provides insights into preparing N-doped biochar with favorable extrinsic and intrinsic properties from biowaste for sustainable pollutant removal via PMS activation.

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[rad]-. The common water constituents showed inhibitory effects on IOP decomposition to various extent. An insufficient supply of SO4[rad]- 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.

The interaction of reductive metal ions and peroxymonosulfate (PMS) is necessary for the generation of sulfate radials (SO4[rad]-), 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.

Thermally activated persulfate oxidation process was applied to degrade sodium diatrizoate in the aqueous solution. The impact factors, such as temperature, the initial concentrations of K2S2O8 and substrate, the initial solution pH and water matrix were investigated. The results show that diatrizoate degradation by thermally activated persulfate oxidation was fitted well with pseudo-first-order mode and Arrhenius equation. The apparent activation energy for this reaction was estimated as 105.57 kJ¿mol-1. Increasing initial persulfate concentration or temperature and reducing initial substrate concentration significantly increased the apparent rate constant (k) of diatrizoate degradation (k=0.0062×[K2S2O8]0, R2=0.98, [K2S2O8]0=2~8 mmol¿L-1; lnk=-12698×1/T+34.91, R2=0.98, T=325~345 K; k=-1.54×[diatrizoate]0+ 0.064, R2=0.98), while k was less affected by the initial pH of the solution. The presence of Cl-, HCO3- and HA in the solution inhibited the degradation of diatrizoate. The low TOC removal rate and formation of organic by-products indicate that the degradation of diatrizoate was incomplete and only its side chain was destructed. The transformation pathways of diatrizoate in the heat-activated persulfate system mainly included amide bond cleavage, amino oxidation and decarboxylation-hydroxylation.

© 2017 Elsevier B.V.The efficiency and, accordingly, the success of the advanced oxidation processes (AOPs) has generally been evaluated on the basis of degradation kinetics. In practice, chloride in saline wastewater is often found to inhibit degradation processes. Therefore its highly desirable to develop more effective processes which are not affected by chloride. In this study, no significant interference of chloride with monochlorophenols (MCPs, e.g. 2-CP, 3-CP and 4-CP) degradation by the UV photo-activated persulfate (UV/PS) process has been observed. This indicated the ¿illusion¿ that the UV/PS process might have been an appropriate technology working under saline conditions. To further evaluate its applicability, the generation of reaction intermediates, of adsorbable organic halogen (AOX) accumulation and of acute toxicity of MCPs in the UV/PS system were examined. In reality, several aromatic chlorinated compounds (number of chlorine atoms¿=2), such as dichlorophenols and 2,3,5,3',5'-pentachloro-biphenyl, were identified and quantified. An accumulation and relative increase of AOX with reaction time was observed in the UV/PS/Cl system. The acute toxicity tests with Photobacterium phosphoreum indicated that the inhibition effect of UV/PS reactions increased with reaction time regardless of the presence of chloride or not. The results of this study might be helpful for assessing the PS-based technologies for saline wastewater treatment.

Trace bromide (Br-) released from industrial effluents or brominated compounds is able to directly react with peroxymonosulfate (PMS) to generate a series of reactive oxidants which can oxidize and also halogenate organics. We report the identification and evolution of by-products during 2,4,6-trichlorophenol (TCP) degradation in the presence of PMS and trace Br-. The influencing factors, including Br- concentration and pH, were investigated. The depletion of TCP was accelerated with increasing trace Br- concentration (0¿0.2¿mM) and was affected by the initial pH (3.0¿7.0). The chlorinated and brominated compounds were identified in simulated wastewater during treatment with PMS. Notably, the potential formation of chlorobromoaromatic by-products was demonstrated for the first time in the presence of PMS and trace Br-. The possible reaction pathways of TCP and its derivatives are discussed. These findings have important implications for the future applications of PMS-based oxidation processes.

A considerable effort has been devoted to elucidating the roles of chloride in oxidative degradation and chlorination of dyes. However, few investigations are available on kinetic analysis and transformation pathways of secondary degradation byproducts of dyes in saline wastewater treatment. Here the impact of chlorine on the degradation rate of phthalic acid, a typical dye degradation intermediate, by the Co2+/peroxymonosulfate (PMS) process was examined. Degradation efficiency, intermediate products, AOX (adsorbable organic halogen) formation and mineralization were considered. An overall negative impact was observed within the concentration of Cl- up to 100 mM, differing from the dual effect of chloride on dye degradation process as previously observed. The presence of high levels of Cl- led to a low production of AOX and a reduction of the formation of chlorinated by-products. The mineralization was also restrained when the Cl- concentration was increased. Degradation pathways for these processes are proposed. These findings provide valuable information about the degradation pathways of dyes and about the formation mechanism of chlorinated by-products in industrial saline wastewater treatment.

Most of the current advanced oxidation processes (AOPs) are vulnerable to the presence of chloride in saline wastewater treatment because chloride not only affects the degradation kinetics but also probably leads to absorbable organic halogen (AOX) formation. Here we report an UV/Ce(iv) process can efficiently oxidize organic pollutants such as Acid Orange 7, even in the presence of chloride. Fluorescent probe technology suggests hydroxyl radicals were generated in UV/Ce(iv) process, but not in UV/Ce(iv)/Cl- system. In the presence of chloride, Ce(iv)-chloride complex was formed, which can directly oxidize dyes or generate reactive oxygen species by chlorine activation. Although degradation and mineralization rates of dyes were still inhibited to some extents by large amounts of chloride, but negligible AOX was generated. Therefore, UV/Ce(iv) process can be recommended as an alternative AOPs when treating acidic saline wastewater.

Reduction of Cr(VI) is often deemed necessary to detoxify chromium contaminants; however, few investigations utilized this reaction for the purpose of treating other industrial wastewaters. Here a widely used Cr(VI)-sulfite reaction system was upgraded to simultaneously transform multiple pollutants, namely, the reduction of Cr(VI) and oxidation of sulfite and other organic/inorganic pollutants in an acidic solution. As(III) was selected as a probe pollutant to examine the oxidation capacity of a Cr(VI)-sulfite system. Both ¿OH and SO4¿- were considered as the primary oxidants for As(III) oxidation, based on the results of electron spin resonance, fluorescence spectroscopy, and specific radicals quenching. As(III)-scavenging, oxidative radicals greatly accelerated Cr(VI) reduction and simultaneously consumed less sulfite. In comparison with a Cr(VI)-H2O2 system with 50 µM Cr(VI), Cr(VI), the sulfite system had excellent performance for both As(III) oxidation and Cr(VI) reduction at pH 3.5. Moreover, in this escalated process, less sulfite was required to reduce Cr(VI) than the traditional Cr(VI) reduction by sulfite process. This effectively improves the environmental compatibility of this Cr(VI) detoxification process, alleviating the potential for SO2 release and sulfate ion production in water. Generally, this study provides an excellent example of a "waste control by waste" strategy for the detoxification of multiple industrial pollutants.

2-line ferrihydrite, a ubiquitous iron oxy-hydroxide found in natural and engineered systems, is an efficient sink for the toxic metalloids such as arsenic. While much is known of the excellent capacity of ferrihydrite to coprecipitate arsenate, there is little information concerning the long-term stability of arsenate-accumulated ferrihydrite. By thermal treatment methodology, the expedited transformation of ferrihydrite in the presence of coprecipitated arsenate was studied at varying As/Fe ratios (0-0.5) and different heating temperature (40, 300, 450, 600. °C). Pure and transformed minerals were characterized by thermogravimetry (TG), X-ray diffraction (XRD), Electron Spin Resonance (ESR), Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy (SEM-EDX) and Fourier Transform Infrared Spectroscopy (FTIR). Arsenate was found to retard the thermal transformation of ferrihydrite. The extents of ferrihydrite transformation to hematite decreased with increasing As/Fe ratios, but increased at a higher heating temperature. It is predicted that the coprecipitated arsenate can stabilize the amorphous iron oxides against the transformation to more crystalline solids. Arsenate concentration appears to play an important role in this predicted long-term stability.

Dechlorination and denitration are known to occur during the oxidative degradation of chloronitroaromatic compounds, but the possibility of re-chlorination and re-nitration of chloro and nitro groups is not assessed despite of its importance in evaluating the applicability of advanced oxidation processes (AOPs). In this study, transformation of chloro and nitro groups in degradation of 4-chloro-2-nitrophenol (4C2NP) by sulfate radical generated via Co-mediated peroxymonosulfate activation was investigated. Both chloride and nitrate ions were found as the main inorganic products of chloro and nitro groups in 4C2NP, but their levels were much lower than that of degraded parent 4C2NP. A typical dual effect of chloride on the 4C2NP degradation kinetics was observed, whereas no measurable influence was found for addition of low level nitrate. Re-chlorination took place, but re-nitration was not verified because several polychlorophenols but none of polynitrophenols were detected. The specific degradation mechanism involved in the transformation of nitro group and chloro group was proposed.

In this study, the potential ecological risks and eco-toxicity of heavy metals (Cu, Pb, Zn, Ni, Cd, Cr, As and Hg) in sewage sludge were quantitatively evaluated. Sewage sludge samples were collected from three wastewater treatment plants in Wuxi city during five years from 2009 to 2013. The levels of the eight metals temporally varied. The contents of Zn and Cu in the tested sewage sludge were the highest, followed by Cr, Ni, Pb and As, and the contents of Cd and Hg were the least. The Community Bureau of Reference (BCR) sequential extraction results showed that Zn, Ni and Cd had the highest mobilisation potential, while Cu, Pb, Cr and Hg were not stabilised and would release to the environment under oxidising conditions. The removal efficiency of Cu and Cr was higher than 70%, while that of As, Cd and Hg was less than other heavy metals. The overall potential ecological risk index (RI) of heavy metals in sewage sludge was 1376.34, revealing very high risk. However, the environmental risk values based on the index of geo-accumulation (Igeo) and risk assessment code (RAC) were both low. Cd contamination is the major concern while the treated sewage sludge is used for agricultural purposes.

In this study, the applicability of glow discharge plasma for simultaneous redox transformations of Cr(VI) and As(III) was evaluated in aqueous solution. The results showed that there was a beneficially synergistic effect between Cr(VI) reduction and As(III) oxidation. The presence of Cr(VI) can significantly enhance As(III) oxidation, whereas a slight increase for Cr(VI) reduction mediated by As(III) was observed. The added ethanol or dicarboxylic acids can increase the Cr(VI) reduction with different mechanisms, but significantly retard As(III) oxidation. The conversions of both As(III) and Cr(VI) can also be effectively increased from 96% to 100% and 53% to 77%, respectively, by increasing the voltage input from 530 to 600V. Cr(VI) reduction proceeded more rapidly, approximately 96% at pH 2.0, while As(III) oxidation was depressed under acidic conditions, only 70% at pH 2.0. And the optimal pH value for As(V) formation facilitated by Cr(VI) is 7.0. H2O2 generated in-situ in glow discharge plasma can reduce Cr(VI) and produce highly oxidizing OH that was responsible for As(III) oxidation. Reaction between As(III) and OH in turn partially avoided the re-oxidation of Cr(III) by OH to Cr(VI), thus facilitating the net conversion of Cr(VI). Our study demonstrated the feasibility of GDP for the coconversion of As(III) and Cr(VI), holding a great promise for real-world wastewater remediation.

Formation of toxic chlorinated organic byproducts is of great concern when selecting electrochemical oxidation (EO) as decontamination technology for saline dye wastewater, but still not verified. To test the applicability of EO, methyl orange (MO) was used as a model dye for anodic contact glow discharge electrolysis (CGDE) and conventional electrolysis (CE) in the presence of chloride. The degradation kinetics and organic intermediates were analyzed. In the presence of chloride, the rates of dye degradation were significantly increased as CGDE and CE were applied. CE resulted in higher mineralization efficiency than CGDE which needs much energy input. Several refractory chlorinated aromatic and even aliphatic compounds were identified during MO degradation, as well as the other anthraquinone dye, alizarin red S (AR). Therefore, the issues of toxic chlorinated byproducts and energy cost should be preferentially evaluated prior to the selection of EO technologies.

Oxidation of arsenite (As(III)) is a critical yet often weak link in many current technologies for remediating contaminated groundwater. We report a novel, efficient oxidation reaction for As(III) conversion to As(V) using commercial available peroxymonosulfate (PMS). As(III) is rapidly oxidized by PMS with a utilization efficiency larger than 90%. Increasing PMS concentrations and pH accelerate oxidation of As(III), independent to the availability of dissolved oxygen the addition of PMS enables As(III) to oxidize completely to As(V) within 24 h, even in the presence of high concentrations of radical scavengers. On the basis of these observations and theoretical calculations, a two-electron transfer (i.e., oxygen atom transfer) reaction pathway is proposed. Direct oxidation of As(III) by PMS avoids the formation of nonselective reactive radicals, thus minimizing the adverse impact of coexisting organic matter and maximizing the utilization efficiency of PMS therefore, this simple approach is considered a cost-effective water treatment method for the oxidation of As(III) to As(V). © 2014 American Chemical Society.

The simultaneous reduction of Chromium(VI) (Cr(VI)) and oxidation of cationic dyes in dispersions of Montmorillonite K10 (MK10) were examined under visible irradiation (¿ 420 nm). The iron species (i.e. iron oxides, structural iron and exchangeable interlayer iron) in layered clays are active for catalytically reducing Cr(VI) by using Malachite green (MG) and Rhodamine B (RhB) as the electron donors. Molecular oxygen does not have a significant effect on clay-catalyzed Cr(VI) reduction, but is important for oxidative degradation of dye pollutants. MK10 catalysts are stable and reusable, and are therefore considered as a promising naturally-abundant material for decontamination of dye and heavy metals. © 2013 Elsevier Ltd.

The dicarboxylic acids oxalate (Oxal) and malonate (Mal) are frequently detected as the final low-molecular-weight organic acids during oxidative degradation of aromatic compounds. Here a distinct effect of Oxal versus Mal on iron-based photocatalytic technologies was reported by testing the degradation efficiency of the dye rhodamine B. The rates of dye degradation in irradiated Fe(III) solutions depended on Fe(III/II) speciation, photoreactivities of Fe complexes and reactivities of Fe(II) complexes with H2O2. Photolysis of the Fe(III)-oxalato complex was favorable due to the formation of O2-, HO2 and OH for oxidizing the dye; however, an excess of H2O2 could quench the excited state of ferrioxalate, decreasing the degradation efficiency. In contrast, activities of UV/Fe(III) in the presence of Mal were significantly diminished because Fe(III)-Mal complexes, with much lower quantum yield of Fe(II) from photoreduction, dominated Fe(III) speciation. The results provide data for an understanding of the mechanism of iron redox (photo)chemistry mediated by diacids, which will aid in selecting appropriate Fe ligands, screening photo-Fenton conditions and designing UV/Fe(III) treatability. © 2013 Elsevier Ltd.

The effect of Cl- on the oxidative degradation of Acid Orange 7 (AO7) was investigated in UV/S2O82- system to elucidate the chlorination pathways in saline wastewaters. Lower amount of Cl- as well as Br- enhanced the decoloration of AO7, but such promotion effect reduced gradually with the increasing halide ion dosage. The dye mineralization was found to be inhibited by Cl-, especially under acidic conditions. Results of kinetics modeling demonstrated that the fraction of different oxidizing radicals largely depended on the content of Cl-. At the initial pH of 6.5, Cl2- was much more abundant than SO4-. The significance of Cl2- for AO7 degradation increased with the increasing Cl- concentration and overwhelmed that of SO4- at [Cl-]>1mM. Without Cl-, SO4- was the predominant radical for AO7 degradation under acidic conditions, while OH prevailed gradually at higher pH. Under high salinity conditions, more OH can be formed and contributed to the dye degradation especially in alkaline medium, leading to higher destruction efficiency of AO7. Several chlorinated byproducts were detected in the presence of chloride ions, and SO4-/Cl2--based degradation pathways of AO7 were proposed. This work provides further understanding of the complex reaction mechanisms for SO4--based advanced oxidation processes in chloride-rich environments. © 2014 Elsevier Ltd.

Activation of peroxygens is a critical method to generate oxidative species, but often consumes additional chemical reagents and/or energy. Here we report a novel and efficient activation reaction for peroxymonosulfate (PMS) by phosphate anions (PBS). The PBS/PMS coupled system, at neutral pH, is able to decompose efficiently even mineralize a variety of organic pollutants, such as Acid Orange 7, Rhodamine B and 2,4,6-trichlorophenol. In contrast, no measurable degradation was observed when the PMS was replaced by other peroxygens (i.e. hydrogen peroxide and peroxydisulfate). Both PMS and PBS are indispensable for the oxidative degradation of pollutants. Increasing pH and concentrations of PMS and PBS significantly accelerate the degradation of organics. It is proposed that OH would be the major radical for contamination degradation at pH 7.0 through the radical quenching experiments. This work provides a new way of PMS activation for decontamination at neutral pH, in particular for phosphate-rich wastewater treatment.

Semiconductor-mediated photocatalysis has received tremendous attention as it holds great promise to address the worldwide energy and environmental issues. To overcome the serious drawbacks of fast charge recombination and the limited visible-light absorption of semiconductor photocatalysts, many strategies have been developed in the past few decades and the most widely used one is to develop photocatalytic heterojunctions. This review attempts to summarize the recent progress in the rational design and fabrication of heterojunction photocatalysts, such as the semiconductor-semiconductor heterojunction, the semiconductor-metal heterojunction, the semiconductor-carbon heterojunction and the multicomponent heterojunction. The photocatalytic properties of the four junction systems are also discussed in relation to the environmental and energy applications, such as degradation of pollutants, hydrogen generation and photocatalytic disinfection. This tutorial review ends with a summary and some perspectives on the challenges and new directions in this exciting and still emerging area of research. This journal is © the Partner Organisations 2014.

Photooxidation of Fe(II) at acidic pH occurs in photocatalytic processes of Fe(III) species, but its reaction mechanism is not well understood. The kinetics of Fe(II) oxidation in irradiated aqueous solutions at pH 3.0 have been investigated in terms of kinetic modeling approach, rate constant estimation and the significance of various oxidation pathways. Fe(II) oxidation kinetics strongly rely on the availability of UV light, Fe(III) ions and oxygen. The presence of a portion of Fe(III) in Fe(II)-containing solutions favors the rapid oxidation of Fe(II). At high concentration of Fe(II), excitation of Fe(III) species may be quenched by Fe(II) in deaerated systems or may sensitize Fe(II) oxygenation under oxic conditions. By incorporation of this photosensitization pathway, the established model in this study is shown to be able to adequately describe the oxidation of Fe(II) at pH 3.0. Sensitivity analysis indicates that photolysis of Fe(III) species is critically important for overall Fe(II) oxidation kinetics. Fe(III)-catalyzed oxygenation of Fe(II), and oxidation of Fe(II) by HO2 and OH also exerts a marked impact on the oxidation of Fe(II). Therefore, Fe(III)-catalyzed oxygen activation and their secondary reactive oxygen species (ROS) account for the oxidation kinetics of Fe(II) at acidic pH. © 2013 Elsevier B.V.

A kind of Fe mineral-biogenic jarosite was prepared by biooxidation of Fe2+ using Acidithiobacillus ferrooxidans. Decolorization of reactive dyes, Reactive Blue 19 (RB) and Reactive Orange 1 (RO), were conducted using asprepared jarosite as a catalyst in the presence of H2O2 via a Fenton-like reaction process. The experimental results indicated that jarosite can serve as an efficient Fe catalyst to initiate Fenton-like reaction in the process of dye decolorization. 98% of decolorization efficiency of RB was achieved within 105 min of reaction time by using 5 mM H2O 2 and 1 g/L jarosite catalyst at pH 5.0. The current investigation may provide a practical and costseffective way using biogenic Fe minerals for remediation of wastewater contaminated with industrial colorants. © 2014 Science &Technology Network, Inc.

Tetrabromobisphenol A (TBBPA), one of the most frequently used brominated flame retardants (BFRs), is only soluble in strongly alkaline solutions where most advanced oxidation processes (AOPs) are inefficient or in organic solvents. It is highly desirable to develop an environmentally friendly cleanup technology for TBBPA which operates at alkaline pH without the need of organic solvents to enhance its solubility in water. In this study a favorable UV/base/persulfate (PS) system for rapid degradation of TBBPA in aqueous solution is reported. Increases of initial pH and PS concentrations have positive effects on the degradation of TBBPA and a high initial TBBPA concentration has a negative influence. The addition of Br- has a negligible effect on the degradation of TBBPA. The low TOC removal and degradation intermediates of TBBPA identified by GC-MS suggested that TBBPA was degraded via the cleavage between the isopropyl group and one of the benzene rings rather than complete debromination and degradation. The present study provides a simple and green approach to detoxify TBBPA in water rather than using toxic organic solvents. © 2014 Elsevier B.V.

The conventional photo-Fenton reaction often suffers from the constraints of operation pH, low iron loading, ultraviolet availability in solar light and instability of iron-based catalysts. Here we report a novel heterogeneous Fenton reaction which works with a dye-photosensitized structural Fe(iii)/Fe(ii) redox cycling mechanism. The synthesized nontronite catalyst (NAU) was characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectra (FTIR), X-ray photoelectron spectroscopy (XPS) analysis, and thermal gravimetric analysis (TG). NAU exhibited excellent catalytic activity over a wide pH range (3.0-8.0) for highly efficient degradation of Rhodamine B by hydrogen peroxide (H2O2) under visible light irradiation (¿ > 420 nm). The excited dye molecule donates electrons to structural iron sandwiched in NAU which further catalyzes H2O2 to generate highly reactive OH radicals. This iron-rich clay mineral (total Fe, 24.4 wt%) is chemically and mechanically stable. There are no measurable iron leaching, nor any noticeable loss of activity and damage to the clay structure observed after 6 recycles. Therefore, NAU clay has outstanding merits for the practical treatment of organic dye pollutants at large scale. This journal is © the Partner Organisations 2014.

We demonstrated the importance of basal planes and crystal edge for electron transfer within montmorillonite (MK10) and nontronite (NAu-2) by a facile dye-sensitized photoreduction method. It was found that not all structural Fe in the clay matrix was redox-active. The results are vital to the utilization of naturally abundant clays in environmental redox chemistry. This journal is © the Partner Organisations 2014.

The diverse redox processes of the photo/ferrioxalate system (PFS) were investigated by varying the concentrations of Fe(iii), oxalate and oxygen. Photoreactivity of PFS is determined by the prevalence of the most photolabile Fe(iii) and abundance of Fe(iii) and oxalate, which is critical for the operation optimization of PFS in wastewater treatment.

In this study, Ni/Fe bimetallic nanoparticles were prepared by a liquid-phase chemical reduction method and characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS) with image mapping, transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The as-prepared Ni/Fe material was applied to remove Cr(vi) via a coupled adsorption/reduction process. It was found that Cr(vi) removal followed pseudo-second-order reaction kinetics. Acidic pH favored the efficient removal of Cr(vi) due to the abundance of reactive H species that were mediated by the Ni catalyst. XPS studies demonstrated that Cr(vi) removal on the surface of the bimetallic nanoparticles was a synergistic adsorption and reduction process. The introduction of nickel to nZVI not only controls iron passivation but also facilitates the efficient flow of electron transfer between iron and Cr(vi), and thus the efficient reduction of Cr(vi) to Cr(iii). Hydroxylated Cr(OH)3 and co-precipitation of CrxFe1-x(OH)3 were the final products of Cr(vi) removal by the Ni/Fe material. This journal is

Arsenic and chromium are often abundant constituents of acid mine drainage (AMD) and are most harmful as arsenite (As(III)) and hexavalent (Cr(VI)). To simultaneously change their oxidation state from As(III) to As(V), and Cr(VI) to Cr(III), is a potentially effective and attractive strategy for environmental remediation. The coabundance of As(III) and Cr(VI) in natural environments indicates their negligible direct interaction. The addition of H 2O2 enables and greatly accelerates the simultaneous oxidation of As(III) and reduction of Cr(VI). These reactions are further enhanced at acidic pH and higher concentrations of Cr(VI). However, the presence of ligands (i.e., oxalate, citrate, pyrophosphate) greatly retards the oxidation of As(III), even though it enhances the reduction of Cr(VI). To explain these results we propose a reaction mechanism where Cr(VI) is primarily reduced to Cr(III) by H2O2, via the intermediate tetraperoxochromate Cr(V). Cr(V) is then involved in the formation of ¿OH radicals. In the presence of ligands, the capacity of Cr(V) to form ¿OH radicals, which are primarily responsible for As(III) oxidation, is practically inhibited. Our findings demonstrate the feasibility for the coconversion of As(III) and Cr(VI) in AMD and real-world constraints to this strategy for environmental remediation. © 2013 American Chemical Society.

An efficient and green advanced oxidation process (i.e., photo-sulfite reaction) for the simultaneous oxidation of sulfite and organic pollutants in water is reported. The photo-sulfite system (UV-Fe(III)-sulfite) is based on the Fe-catalyzed sulfite oxidation and photochemistry of Fe(III) species. SO 4¿- and ¿OH radicals were identified in the photo-sulfite system with radical scavenging experiments using specific alcohols. This novel technology was consistently proven to be more favorable than the alternative Fe(III)-sulfite systems for the degradation of 2,4,6-trichlorophenol (2,4,6-TCP) and other organic pollutants at all conditions tested. The reactivity of photo-sulfite system was sustained due to the spontaneous switch of photoactive species from Fe(III)-sulfito to Fe(III)-hydroxo complexes with the depletion of sulfite and the decrease in pH. In contrast, in the absence of light the performance of the Fe(III)-sulfite system was greatly diminished after the consumption of sulfite. The formation of the Fe(III)-sulfito complex is a necessary step for initiating the photo-sulfite reaction. Inhibition of the oxidation of 2,4,6-TCP and methyl orange (MO) was observed in the presence of ligands that can stabilize one or more of the reactants: Fe(III), Fe(II), or sulfite. Our study provides a new facile route for the generation of SO4¿- and simultaneous removal of organic and inorganic pollutants. © 2013 American Chemical Society.

Iron-catalyzed oxidation of As(III) to As(V) can be highly effective for toxic arsenic removal via Fenton reaction and Fe(II) oxygenation. However, the contribution of ubiquitous organic ligands is poorly understood, despite its significant role in redox chemistry of arsenic in natural and engineered systems. In this work, selected naturally occurring organic ligands and synthetic ligands in co-oxidation of Fe(II) and As(III) were examined as a function of pH, Fe(II), H2O2, and radical scavengers (methanol and 2-propanol) concentration. As(III) was not measurably oxidised in the presence of excess ethylenediaminetetraacetic acid (EDTA) (i.e. Fe(II):EDTA<1:1), contrasting with the rapid oxidation of Fe(II) by O2 and H2O2 at neutral pH under the same conditions. However, partial oxidation of As(III) was observed at a 2:1 ratio of Fe(II):EDTA. Rapid Fe(II) oxidation in the presence of organic ligands did not necessarily result in the coupled As(III) oxidation. Organic ligands act as both iron speciation regulators and radicals scavengers. Further quenching experiments suggested both hydroxyl radicals and high-valent Fe species contributed to As(III) oxidation. The present findings are significant for the better understanding of aquatic redox chemistry of iron and arsenic in the environment and for optimization of iron-catalyzed arsenic remediation technology. © 2013 Elsevier Ltd.

Transition-metal is known to catalyze peroxymonosulfate (PMS) decomposition to produce sulfate radicals. Here we report reactions between PMS and chloride, without a need of transition metals, also can be used to degrade organic dye pollutant (Rhodamine B, (RhB)). Some important operating parameters, such as dosages of PMS and Cl-, pH of solution, temperature, ionic strength, and several common cations, were systematically investigated. Almost complete decoloration of RhB was achieved within 5 min ([PMS] = 0.5 mM, [Cl-] = 120 mM, and pH 3.0), and RhB bleaching rate increased with the increased dosages of both PMS and chloride ion, following the pseudo-first-order kinetic model. However, the total organic carbon (TOC) removal results demonstrated that the decoloration of RhB was due to the destruction of chromophore rather than complete degradation. RhB decoloration could be significantly accelerated due to the high ionic strength. Increasing of the reaction temperature from 273 K to 333 K was beneficial to the RhB degradation, and the activation energy was determined to be 32.996 kJ/mol. Bleaching rate of RhB with the examined cations increased with the order of NH4+ < Na+ < K+ < Al3+ < Ca2+ < Mg2+. Some major degradation products of RhB were identified by GC-MS. The present study may have active technical implications for the treatment of dyestuff wastewater in practice. © 2013 Springer-Verlag Berlin Heidelberg.

The wastewater source of 4# tailing pond in Dexing copper mine consists of alkaline flotation pulp and acid mine drainage (AMD) from the nearby opencast mine. Therefore, the heavy metals in tailing ore are very likely to be released due to acidification from AMD. The leaching behaviors of Zn, Cu, Fe and Mn in mine tailings from Dexing copper mine were investigated by a series of laboratory batch experiments. The effectcs of pH, temperature, particle size and contact time on the leachability of such heavy metals were examined. It was evident that Zn, Cu, Fe and Mn were major heavy metals in the tailings while gangue minerals like quartz were major constituents in examined tailings. The tailing dissolution reaction was controlled by the acid, whose kinetics could be expressed according to the heterogeneous reaction models and explained by a shrinking core model with the surface chemical reaction as the rate-controlling step. The leachability of all metals examined depended on pH and contact time. The batch studies indicated that the maximum leaching ratios of Zn, Cu, Fe and Mn at pH 2.0 were 5.4%, 5.8%, 11.1% and 34.1%, respectively. The dissolubility of all metals examined was positively correlated to the temperatures. The particle size would not change dissolution tendency of those heavy metals, but decrease the concentrations of leached heavy metals. © 2013 The Nonferrous Metals Society of China.

The degradation of 2,4,6-trichlorophenol (TCP) by sulfate radical generated via Co(II)-mediated activation of peroxymonosulfate (PMS) was examined. The influencing factors, such as substrate concentration and pH were investigated. The initial pH and its adjustment orders significantly affected the TCP degradation and mineralization. Several chlorinated products were detected, as well as some carboxylic acids, such as glycolic acid and oxalic acid. Many polychlorinated (chlorine atom number =3) aromatics (e.g. 2,4,5-trichlorophenol, 2,3,4,6-tetrachlorophenol, 2,3,5,6-tetrachloro-1,4-benzenediol) and even their ring-opening products (e.g. 2,4-dichloro-5-oxo-2-hexenedioic acid, 1,1,3,3-tetrachloro-2-propanone) were identified, indicating a de novo formation mechanism of organohalogens may be involved in TCP degradation. The released chlorine atoms from TCP and/or dichloride radicals activated by sulfate radicals played an important role. This finding may have significant scientific and technical implications for utilizing Co/PMS reagent to detoxify chlorinated pollutants. © 2012 Elsevier B.V.

CBr bond cleavage is considered as a key step to reduce their toxicities and increase degradation rates for most brominated organic pollutants. Here a sequential reduction/oxidation strategy (i.e. debromination followed by photocatalytic oxidation) for photocatalytic degradation of tetrabromobisphenol A (TBBPA), one of the most frequently used brominated flame retardants, was proposed on the basis of kinetic analysis and intermediates identification. The results demonstrated that the rates of debromination and even photodegradation of TBBPA strongly depended on the atmospheres, initial TBBPA concentrations, pH of the reaction solution, hydrogen donors, and electron acceptors. These kinetic data and byproducts identification obtained by GC-MS measurement indicated that reductive debromination reaction by photo-induced electrons dominated under N2-saturated condition, while oxidation reaction by photoexcited holes or hydroxyl radicals played a leading role when air was saturated. It also suggested that the reaction might be further optimized for pretreatment of TBBPA-contaminated wastewater by a two-stage reductive debromination/subsequent oxidative decomposition process in the UV-TiO2 system by changing the reaction atmospheres. © 2012 Elsevier B.V.

The influencing factors in adsorption such as adsorption time, pulp concentration, bacterial concentration, pH as well as ionic strength were investigated to explore the relationship among them and bacterial adsorption. The adsorption was a rapid process for bacterial adhesion to chalcopyrite. The extent of adsorption increased with increasing initial bacterial concentration and pulp concentration. The optimal pH for Acidithiobacillus ferrooxidans adsorption onto chalcopyrite surfaces was in the range of pH 1-3. The increase of ionic strength led to decrease in bacterial adsorption, which can be well explained by electric double layer theory. The adsorption behavior appeared to be controlled by both hydrophobic and electrostatic interactions at the interface of bacteria and mineral. © 2012 The Nonferrous Metals Society of China.

Iron is one of the most abundant metals in the continental crust, while most of dissolved iron is complexed with organic ligands. The irradiated iron complexes in the environment undergo direct photolysis and secondary (photo)chemical reactions, generating Fe(II), organic radicals and some reactive oxygen species (ROS). Environmental photochemistry of iron complexes can greatly affect ROS dynamics, organics degradation and redox cycling of other elements. Therefore, it is becoming a hot topic in the international field of environmental sciences research. This review firstly summarizes three types of iron complexes including inorganic Fe complexes, simple organic Fe complexes and macrocyclic organic Fe complexes, and photoreduction mechanisms of iron complexes. Secondly, the potential oxidants of Fe(II) in acidic aquatic environment are introduced. The oxidation kinetics, possible reaction mechanism and influencing factors (such as dissolved oxygen concentration, pH, ionic strength, temperature and natural organic matters concentrations) of (photo)chemical oxidation of Fe(II) are elucidated. This review also highlights recent findings in the study of environmental processes involving iron photochemistry. Finally, the future prospects in this field are discussed based on the current status.

Chloride ion as one of the major salt components in dyestuff wastewaters can greatly affect the homogenous OH radical-based advanced oxidation processes (AOPs), but little information is available now for heterogeneous AOPs like UV/TiO2 under similar conditions. Here the effects of chloride on kinetics and reaction intermediates of dye degradation in heterogeneous photocatalysis were examined. The dye degradation efficiency in UV/TiO2 process was investigated as a function of a wide range of salinity and pH. The chloride ion was found to have a dual effect on both the dye decoloration and mineralization in UV/TiO2 system due to different mechanisms involved. Higher Cl- concentration would inhibit dye degradation, especially in acidic medium. AOX (halogenated organic compounds adsorbable on activated carbon) generated increased with the increasing content of chloride ion present in the solution. Several toxic chlorinated byproducts were firstly identified during the dye degradation in UV/TiO2 process using GC-MS method and possible decomposition pathways were proposed. These findings may have significant technical implications for optimizing the photochemical technologies in salt-rich wastewater treatment. © 2012 Elsevier B.V.

The photoreactions between Fe(III)/Fe(II) and low-molecular-weight organic matter (LMWOM) under solar irradiation have significant implications for many biogeochemical cycles on the Earth and for the fates of environmental pollutants. In this Perspective, we focus on several fundamental aspects of the photochemical processes that couple the redox cycling of iron species and transformation of organic substrates. The primary photoprocesses (e.g., intramolecular electron transfer or photodissociation) are first highlighted by introducing the recently disputed observations on the photolysis of ferrioxalate complexes. The effects of LMWOM and its daughter radicals on the photochemical redox cycling of iron species are discussed with special attention given to the example of Fe-malonate complexes. These processes and mechanisms would provide us some refreshed understanding of environmental photochemistry of LMWOM and the iron species and would be helpful for our assessment of photochemical decontamination of organic pollutants. © 2012 American Chemical Society.

Orange II (Org II), one of the most common used azo dyes, was taken as a model to investigate the effects of chloride ion on dye decoloration in cobalt/peroxymonosulfate (Co/PMS) system. A significant decrease in the rate of Org II decoloration was observed upon addition of Cl- (0.05-10mM), but further addition of Cl- (>50mM) apparently accelerated dyes degradation. This dual effect of chloride on dyes bleaching was also observed as other halide ions (e.g. Br-, I-) or other azo dyes were present in Co/PMS system. In the Co-free PMS solutions, the observed first-order rate constant always exponentially increased with the chloride content. The reactive chlorine species generated from chloride oxidation by PMS should be responsible for this non-radical mechanism for dye decoloration, however, these rapid decoloration of Org II as chloride ion was present, did not readily lead to much mineralization. Therefore, this finding may have significant technical implications for utilizing Co/PMS regent to detoxify chloride-rich azo dyes wastewater. © 2011 Elsevier B.V.

Sodium chloride is a common salt used during textile wet processes. Here a dual effect of chloride (i.e. inhibitory and accelerating effect) on azo dye (Acid Orange 7, AO7) degradation in an emerging cobalt/peroxymonosulfate (Co/PMS) advanced oxidation process (AOP) was reported. Compared to OH-based AOPs, high concentrations of chloride (>5mM) can significantly enhance dye decoloration independent of the presence of the Co 2+ catalyst, but did greatly inhibit dye mineralization to an extent which was closely dependent upon the chloride content. Both UV-vis absorbance spectra and AOX determination indicated the formation of some refractory byproducts. Some chlorinated aromatic compounds, including 3-chloroisocoumain, 2-chloro-7-hydroxynaphthalene, 1,3,5-trichloro-2-nitrobenzene and tetrachlorohydroquione, were identified by GC-MS measurement in both Co/PMS/Cl - and PMS/Cl - reaction systems. Based on those experimental results, two possible branched (SO 4- radical-based and non-radical) reaction pathways are proposed. This is one of the very few studies dealing with chlorinated organic intermediates formed via chlorine radical/active chlorine species (HOCl/Cl 2) attack on dye compounds. Therefore, this finding may have significant technical implications for utilizing Co/PMS regent to detoxify chloride-rich azo dyes wastewater. © 2011 Elsevier B.V.

The surface charge density, space charge density, surface potential (f0) and Donnan potential (fDon) on the surface of Acidithiobacillus ferrooxidans cultivated with elemental sulfur (S-A.ferrooxidans) were calculated by numerical simulation using the ion-impenetrable and ion-penetrable models. Zeta potential measurement show that the isoelectric point (IEP) of S-A.ferrooxidans is higher than that of bacterium cultured with Fe2+. It is concluded from fitting data by ionizable surface group model that S-A.ferrooxidans surface probably consists of much proteins, the ionization of amino group of which causes higher IEP (> 2). The ion-penetrable model reveals that f0 and fDon decrease rapidly at pH < 5 but hardly change when pH is 6-7. The thickness of electrical double layer of S-A.ferrooxidans is estimated as 5 nm. The electrical double layer of bacterial cell surface may have significant implications for ions transfer and nutrient transport, but their interaction is unfavorable for bacterial aggration.

Many paramagnetic species such as active surface sites, reactive oxygen species (ROS) and organic radicals derived from the photodegradation of organics were involved in the heterogeneous photocatalysis. Identification and characterization of these paramagnetic species are essentially conducted using electron spin resonance (ESR) techniques, which can provide a detailed understanding of the radical composition and structure on the basis of the unique feature of g, hyperfine, and superhyperfine tensors for each kind of radicals of interest. In the present review, the basic theoretical principles of the ESR technique and its application in the field of TiO2-based heterogeneous photocatalysis are generally summarized. Two experimental approaches typical of ESR-direct monitoring of paramagnetic species or sites and spin-trapping technique have been described with particular attention to the choices of spin traps. The general features of the ESR spectra of photoinduced charges, inorganic and organic radicals in photocatalytic events are presented and discussed. In addition, some remarks will be made about the use of ESR in revealing the origin of enhanced photocatalytic activities in the mixed phase titania (P25) and visible-light-response N-doped TiO2. © 2010 Elsevier B.V.

Adsorption mechanism of l-cysteine on pyrite was investigated by thermogravimetric and electrochemical techniques. TG curves provided the direct evidence for chemisorption of cysteine on pyrite surface. Once cysteine adsorbed to pyrite surface, Ecorr (corrosion potential) sharply lowered whereas Icorr (corrosion current) increased rapidly. Pyrite became more susceptible to be oxidized even at lower potential as cysteine was added. However, the mechanism for pyrite oxidation does not fundamentally change, although cysteine can obviously accelerate oxidation rate of pyrite. These findings have important implications for understanding the mechanism of bacterial adhesion to pyrite and even metal sulfide bioleaching. © 2009 Elsevier B.V. All rights reserved.

The interaction of glucose with pyrite has been investigated by a series of surface analyses, such as amounts adsorbed, TG, FTIR and XRD measurements. The adsorption experiment reveals that glucose would rapidly adsorb on the pyrite surface within 60 min. However, physical adhesion characterized as multi-molecular layer adsorption may occur based on adsorption isotherm type. All results obtained by TG and FTIR suggest that no adsorption took place after several times of washing. XRD results indicate that crystal structure of pyrite was not obviously destructed after glucose was added. The interplane distances of (111), (400) after interaction between glucose and pyrite were widened, which showed the similar manner with that of before interaction. All these convincing data imply that physical adsorption predominantly governs the interaction of glucose on pyrite. © 2010 Elsevier B.V. All rights reserved.

The roles of chromium species on photochemical cycling of iron and mineralization of polycarboxylates are examined in the presence of Cr(VI) or Cr(III) at pH 2.2-4.0. Under UV irradiation, Cr(III) altered the redox equilibrium of iron species, leading to the shift of the photosteady state toward Fe(II). After a longer time of illumination, total organic carbon (TOC) approached a steady state in the presence of Cr(III) or Cr(VI), whereas oxalate was thoroughly mineralized in the absence of Cr species. The TOC of steady state was closely related to the kind of polycarboxylates, Cr species dosages, pH and O2 atmosphere, but hardly affected by more addition of Fe(III). ESI-MS data indicates that several Cr-oxalate complexes formed in the photochemical reactions, which are responsible for protecting oxalate against further oxidation. A mechanism is proposed for the inhibitory effect of Cr species on oxidation of oxalate and Fe(II). The present study may provide a new insight into the dual environmental effects induced by Cr contaminants especially at heavily chromium-contaminated and dissolved organic matter (DOM)-rich sites. © 2008 American Chemical Society.

Treatment of waste water raised from powder blue production was investigated in this paper. The dosage of alkali used to adjust pH was given, and the influence of NH3 on solution chemistry was studied. The scheme of sulphur-precipitation coagulation was carried by using the waste water raised from noble metal production. The results show that when the mix is below 10:1 between waste water raised from powder blue production and from noble metal production, it's no need to add Na2S to make waste water in fit with national drainage standards; but it needs to add Na2S if the mix is above 10:1. The orthogonal experiment results show that the residual heavy metal is affected by dosage of Na2S, dosage of PFS, and pH, respectively in a descent order, and when Na2S dosage is 0.146 g/L and PFS dosage is 1.596 g/L at pH 7.0, the content of heavy metal in waste water can fit with that of national drainage standards.