A Review on the Degradation of Pollutants by Fenton-Like Systems Based on Zero-Valent Iron and Persulfate: Effects of Reduction Potentials, pH, and Anions Occurring in Waste Waters

Catalytic and biocatalytic degradation of microplastics

In recent years, there has been a surge in annual plastic production, which has contributed to growing environmental challenges, particularly in the form of microplastics. Effective management of plastic and microplastic waste has become a critical concern, necessitating innovative strategies to address its impact on ecosystems and human health. In this context, catalytic degradation of microplastics emerges as a pivotal approach that holds significant promise for mitigating the persistent effects of plastic pollution. In this article, we critically explored the current state of catalytic degradation of microplastics and discussed the definition of degradation, characterization methods for degradation products, and the criteria for standard sample preparation. Moreover, the significance and effectiveness of various catalytic entities, including enzymes, transition metal ions (for the Fenton reaction), nanozymes, and microorganisms are summarized. Finally, a few key issues and future perspectives regarding the catalytic degradation of microplastics are proposed.

UV light and Fe2+ catalysed COD removal of AO 8 using NaOCl as oxidant

The present study aims to establish NaOCl as a potential oxidant in the COD removal of Acid Orange 8 using UVC light (λ = 254 nm) and Fe2+ as catalysts. The different systems used in this study are NaOCl, Fe2+/NaOCl, UV/NaOCl, and Fe2+/NaOCl/UV. All these process were found to be operative in acidic, neutral and basic medium. The initial decolorisation and COD removal efficiency (CODeff) for different systems follow the order: Fe2+/NaOCl/UV > UV/NaOCl > Fe2+/NaOCl > NaOCl. Nevertheless, NaOCl can alone be used in the treatment process considering its CODeff to the extent of 95% in 90 min. The change in pH of the solutions after treatment is an important observation - for non-UV systems it remained around 11.0 and 7.0 in other systems. Thus, UV systems are environmental benign. The effect of various anions on CODeff was tested in Fe2+ systems. Presence of F- ions were found to accelerate CODeff in both the systems. However, the effect is more pronounced in Fe2+/ NaOCl/UV, where complete CODeff was observed in the presence of 9.0 gl-1 of F-. The COD removal kinetics for all systems was studied using zero-order, first-order, second-order, and BMG kinetic models. BMG model was found to be more suitable among all and is in good agreement with CODeff of all systems. It is, therefore, established that NaOCl can serve as a powerful oxidant in the advanced oxidation process.

Effective degradation of synthetic micropollutants and real textile wastewater via a visible light-activated persulfate system using novel spinach leaf-derived biochar

A novel biochar (BC), derived from spinach leaves, was utilized as an activator for persulfate (PS) in the degradation of methylene blue (MB) dye under visible light conditions. Thorough analyses were conducted to characterize the physical and chemical properties of the biochar. The (BC + light)/PS system exhibited superior MB degradation efficiency at 83.36%, surpassing the performance of (BC + light)/hydrogen peroxide and (BC + light)/peroxymonosulfate systems. The optimal conditions were ascertained through the implementation of response surface methodology. Moreover, the (BC + light)/PS system demonstrated notable degradation ratios of 90.82%, 81.88%, and 84.82% for bromothymol blue dye, paracetamol, and chlorpyrifos, respectively, under optimal conditions. The predominant reactive species responsible for MB degradation were identified as sulfate radicals. Notably, the proposed system consistently achieved high removal efficiencies of 99.02%, 96.97%, 94.94%, 92%, and 90.35% for MB in five consecutive runs. The applicability of the suggested system was further validated through its effectiveness in treating real textile wastewater, exhibiting a substantial MB removal efficiency of 98.31% and dissolved organic carbon mineralization of 87.49%.

Effective degradation of tetracycline and real pharmaceutical wastewater using novel nanocomposites of biosynthesized ZnO and carbonized toner powder

In this study, novel nanohybrids of biosynthesized zinc oxide (ZnO) and magnetite-nanocarbon (Fe3O4-NC) obtained from the carbonization of toner powder waste were fabricated and investigated for persulfate (PS) activation for the efficient degradation of tetracycline (TCN). The chemical and physical properties of the synthesized catalysts were analyzed using advanced techniques. ZnO/Fe3O4-NC nanohybrid with mass ratio 1:2, respectively in the presence of PS showed the highest TCN removal efficiency compared to the individual components (ZnO and Fe3O4-NC) and other nanohybrids with mass ratios of 1:1 and 2:1. The results indicated that efficient degradation of TCN could be attained at pH 3-7. The optimum operating parameters were TCN concentration of 12.8 mg/L, PS concentration of 7 Mm, and catalyst dose of 0.55 g/L. The high stability of ZnO/Fe3O4-NC (1:2) nanocomposite was assured by the slight drop in TCN degradation percentage from 97.27% to 85.45% after five successive runs under the optimum conditions and the concentrations of leached iron and zinc into the solution were monitored. The quenching experiments explored that the prevailing reactive entities were sulfate radicals. Additionally, the degradation of TCN in various water matrices was investigated, and a degradation pathway was suggested. Further, degradation of real pharmaceutical waste was conducted showing that the removal efficiencies of TCN, total organic carbon (TOC), and chemical oxygen demand (COD) were 89.79, 80.65, and 78.64% after 2 h under the optimum conditions. The effectiveness of the proposed system (ZnO/Fe3O4-NC (1:2) @ PS) for the degradation of real samples compiled from industrial effluents as well as its inexpensiveness and green nature qualify this system for the full-scale application.

Degradation of Diclofenac by Loaded Solid Superbase-Activated Persulfate

Alkali-activated persulfate (PS) is widely used in situ in chemical oxidation processes; however, studies on the innovation of the alkali activation process are very limited. Two supported solid superbases, namely KNO3/γ-Al2O3 (KAl) and KNO3/SBA-15/MgO (KSM), respectively, were prepared and used to activate persulfate to degrade DCF in this work. The results showed that the superbases elevated the solution pH once added and thus could catalyze persulfate to degrade diclofenac efficiently above pH 10.5. The catalytic efficiency of KAl was close to that of sodium hydroxide, and that of KSM was the highest. The mechanism might be that, in addition to raising the solution pH, some potassium existed as K2O2, which had a strong oxidizing effect and was conducive to DCF removal. Hydroxyl, sulfate and superoxide radicals were all found in the reaction system, among which hydroxyl might play the most important role. The material composition ratio, common anion and humic acid all had some influences on the catalytic efficiency. A total of five intermediates were found in the KSM/PS oxidation system, and six oxidation pathways, which were hydroxylation, dehydrogen, dechlorination, dehydration, decarboxylation, and C-N bond breakage, might be involved in the reaction process. Several highly toxic oxidation products that should be paid attention to were also proposed.

An integrated approach to remove endocrine-disrupting chemicals bisphenol and its analogues from the aqueous environment: a review

Bisphenol A (BPA) is a well-known endocrine-disrupting chemical (EDC) used as a plastic enhancer in producing polycarbonate resins to manufacture hard plastics. Due to strict limitations on the manufacturing and utilization of BPA, several bisphenol substitutes, bisphenol F (BPF), bisphenol S (BPS), and bisphenol AF (BPAF), have been developed to replace it in various applications. Because of their widespread use in food containers, infant bottles, and reusable water bottles, bisphenols (BPs) have been identified in different environmental circumstances, including drinking water, seawater, industrial effluent, and endocrine systems such as human blood, urine, and breast milk. However, locating and analyzing them in different conditions has proven to be challenging. Therefore, there is a need to reduce the prevalence of BPs in the environment. The significance of advanced treatment options for treating and eliminating BPA and its alternatives from water bodies are reviewed. Also, the research gaps and future scopes are discussed in this review article. According to the literature survey, adsorption and photocatalytic degradation provide synergistic benefits for environmental challenges because of their substantial adsorption Q5 capacity, high oxidation capability, and low cost compared to alternative individual treatment options.

Comparative study on the treatment of refractory organics in landfill leachate by homogeneous and heterogeneous Fenton advanced oxidation processes

With the advancement of the landfill stabilization process of municipal solid waste, landfill leachate containing a large amount of refractory organic matter is formed. In this study, homogeneous Fenton and heterogeneous Fenton-like (activation by zero valent iron (Fe⁰), pyrite (FeS₂) and magnetite (Fe₃O₄) as solid iron materials) processes have been compared for the removal of refractory organics from landfill leachate. The removal efficiency of organics in the Fenton process was slightly higher than those in the Fenton-like processes. The removal efficiencies based on total organic carbon, UV absorbance at 254 nm (UV₂₅₄) and colour number in the Fenton process were as high as 57.42%, 71.63% and 81.03%, respectively. In the Fenton-like processes, the Fe⁰/H₂O₂ process achieved 35.74%, 66.24% and 86.29% removal efficiencies, respectively. Moreover, the degradation effect on refractory organic substances proved to be better. In the Fenton-like processes, the activation mechanisms with Fe⁰ and FeS₂ involve the homogeneous activation of Fe²⁺ in solution and heterogeneous activation of iron oxides produced during the reaction, respectively. With Fe₃O₄, the activation mechanism is mainly a heterogeneous process involving its intrinsic iron oxide constituents. This study may provide a theoretical basis for the treatment of refractory organics in landfill leachate.

Effective degradation of atrazine by spinach-derived biochar via persulfate activation system: Process optimization, mechanism, degradation pathway and application in real wastewater

Herein, biochar derived from spinach remnants was prepared for the first-time for the utilization in persulfate (PS) activation to effectively degrade atrazine. Characteristics of the prepared biochar were explored using advanced analyses. Control experiments implied the efficient activation of PS in the presence of the synthesized biochar. The highest degradation of atrazine (99.8%) could be attained at atrazine concentration of 7.2 mg/L, PS concentration of 7.7 mM, biochar dose of 1.88 g/L and reaction time of 120 min. The prepared biochar displayed a high recyclability performance attaining degradation ratios of 98.2, 96.53, 96.4, 92.8 and 88% in five sequential cycles under the optimum conditions. The degradation mechanism was explored showing that sulfate radicals were the prime reactive species in the degradation system. The degradation intermediates were specified, and the degradation pathways were propositioned. The highest REs in agrochemical industrial wastewater reached 80.21 and 83.43% of atrazine and TOC after 2 h. NH₃ (348.4 mg/L) was reduced to 168.3 mg/L (RE: 51.7%) while level of NO₃ (94.7 mg/L) was increased by 98.8% (188.3 mg/L) in the treated effluent due to oxidation of NH₃ to nitrite and then nitrate. Extension of reaction time could contribute to achieving full mineralization of the real wastewater due to the residual PS after 120 min. The effectiveness and low-cost of biochar@PS system as well as its high performance in degrading real wastewater support the efficiency of the prepared biochar to be applied on an industrial scale.

Feasibility of a Heterogeneous Nanoscale Zero-Valent Iron Fenton-like Process for the Removal of Glyphosate from Water

Glyphosate is a widely used herbicide, and it is an important environmental pollutant that can have adverse effects on human health. Therefore, remediation and reclamation of contaminated streams and aqueous environments polluted by glyphosate is currently a worldwide priority. Here, we show that the heterogeneous nZVI-Fenton process (nZVI + H₂O₂; nZVI: nanoscale zero-valent iron) can achieve the effective removal of glyphosate under different operational conditions. Removal of glyphosate can also take place in the presence of excess nZVI, without H₂O₂, but the high amount of nZVI needed to remove glyphosate from water matrices on its own would make the process very costly. Glyphosate removal via nZVI--Fenton was investigated in the pH range of 3-6, with different H₂O₂ concentrations and nZVI loadings. We observed significant removal of glyphosate at pH values of 3 and 4; however, due to a loss in efficiency of Fenton systems with increasing pH values, glyphosate removal was no longer effective at pH values of 5 or 6. Glyphosate removal also occurred at pH values of 3 and 4 in tap water, despite the occurrence of several potentially interfering inorganic ions. Relatively low reagent costs, a limited increase in water conductivity (mostly due to pH adjustments before and after treatment), and low iron leaching make nZVI-Fenton treatment at pH 4 a promising technique for eliminating glyphosate from environmental aqueous matrices.

Preparation of hydrochar bio-based catalyst for fenton process in dye-containing wastewater treatment

The use of synthetic dyes in the textile industry pollutes a huge amount of water. Thus, wastewater discharged from many textile companies to the receiving environment without being treated causes serious environmental and human health problems. The development of new techniques has become imperative. In this study, it was aimed to remove anionic dye (RR180) and cationic dye (BR18) by Fenton-like and adsorption process with hydrochars obtained from laurel leaves and watermelon peels. In the comparison of the adsorption and Fenton-like processes used in the dye removal of the produced bio-based materials, the Fenton-like process was selected in order to enhance the highest removal efficiency. The effects of various operating factors such as solution pH, amount of catalysts, hydrogen peroxide (H₂O₂) concentration, and initial dye concentration were evaluated on both dyes removal. The experimental results demonstrated that 99.8% RR180 dye and 98.8% BR18 dye removal efficiency were observed for an initial dye concentration of 100 mg/L with an adsorbent concentration of 1 g/L, H₂O₂ concentration of 15 μL/L, and optimum pH at the end of 60 min of reaction time. It was observed that an increase in initial dye concentration caused to decrease the dye removal efficiency. The optimum pH for the highest RR180 and BR18 dye removal was 4 and 6, respectively. It was observed that the increase in H₂O₂ concentration in the solution also decreased the dye removal efficiency. It turned out that catalysts obtained from hydrochars are an effective process for the high removal performance of cationic and anionic dyes.

Contamination and eco-risk assessment of toxic trace elements in lakebed surface sediments of Lake Yangzong, southwestern China

Toxic trace elements represent an ongoing environmental problem in aquatic ecosystems. However, a lack of quantitative analysis and accurate evaluation has led to unguided control and water management strategies. Lake Yangzong is the main freshwater resource for nearly one million people in Yunnan Province in southwestern China. It has been heavily contaminated in recent years by significant anthropogenic activities including an industrial phosphor-gypsum spill, sewage effluent, and chemical remediation processes. Herein, we combine eco-environmental indices with multiple statistical analyses to determine the ecological risk and degree of contamination of 11 toxic trace elements in the upper sediments of the lakebed. Local geochemical background concentrations were determined using robust regression models developed from sediment core data. Pollution indices (EF/PLI) indicate that severe As contamination was centralized in the southwestern part of the lake. Other toxic trace elements (e.g., Cd, Cu, Pb) are slightly to moderately enriched, and progressively decrease from the northwestern to the southeastern areas of the lake. A more accurate and sensitive index (PCR) was proposed herein, suggesting that contamination was dominated by As and Pb in different lake sections. The northern section of the lake and the southwestern bay exhibited higher contaminant levels than other regions of the lake. Bio-toxic indices (ERF/PERI) indicate that As and Cd pose a high ecological risk, whereas Cu and Pb pose a low risk to biota. Statistical analyses (PCA/PMF) demonstrate that metal contaminants originated from three types of anthropogenic sources: the smelting of metal ores, the leakage of tailings effluent, and coal consumption.

Visible Light-Driven Advanced Oxidation Processes to Remove Emerging Contaminants from Water and Wastewater: a Review

The scientific data review shows that advanced oxidation processes based on the hydroxyl or sulfate radicals are of great interest among the currently conventional water and wastewater treatment methods. Different advanced treatment processes such as photocatalysis, Fenton's reagent, ozonation, and persulfate-based processes were investigated to degrade contaminants of emerging concern (CECs) such as pesticides, personal care products, pharmaceuticals, disinfectants, dyes, and estrogenic substances. This article presents a general overview of visible light-driven advanced oxidation processes for the removal of chlorfenvinphos (organophosphorus insecticide), methylene blue (azo dye), and diclofenac (non-steroidal anti-inflammatory drug). The following visible light-driven treatment methods were reviewed: photocatalysis, sulfate radical oxidation, and photoelectrocatalysis. Visible light, among other sources of energy, is a renewable energy source and an excellent substitute for ultraviolet radiation used in advanced oxidation processes. It creates a high application potential for solar-assisted advanced oxidation processes in water and wastewater technology. Despite numerous publications of advanced oxidation processes (AOPs), more extensive research is needed to investigate the mechanisms of contaminant degradation in the presence of visible light. Therefore, this paper provides an important source of information on the degradation mechanism of emerging contaminants. An important aspect in the work is the analysis of process parameters affecting the degradation process. The initial concentration of CECs, pH, reaction time, and catalyst dosage are discussed and analyzed. Based on a comprehensive survey of previous studies, opportunities for applications of AOPs are presented, highlighting the need for further efforts to address dominant barriers to knowledge acquisition.

Disinfection through Advance Oxidation Processes: Optimization and Application on Real Wastewater Matrices

Disinfection is an essential and significant process for water treatment to protect the environment and human beings from pathogenic infections. In this study, disinfection through the generation of hydroxyl (Fenton process (FP)) and sulfate (Fenton-like process (FLP)) radicals was validated and optimized. The optimization was carried out in synthetic water through an experimental design methodology using the bacteria Escherichia coli as a model microorganism. Different variables were evaluated in both processes: precursor concentration (peroxymonosulfate (PMS) and H₂O₂), catalyst concentration (Fe⁺²), and pH in the Fenton process. After that, the optimized conditions (FP: 132.36 mM H₂O₂, 0.56 mM Fe⁺² and 3.26 pH; FLP: 3.82 mM PMS and 0.40 mM Fe⁺²) were applied to real matrices from wastewater treatment plants. The obtained results suggest that both processes are promising for disinfection due to the high oxidant power of hydroxyl and sulfate radicals.

Utilization of iron waste from steel industries in persulfate activation for effective degradation of dye solutions

The performance of three solid iron wastes (SIW-1, SIW-2 and SIW-3) was evaluated as an activator of persulfate (PS) for the degradation of methylene blue (MB). SIW-3 showed the highest performance among the three catalysts. The morphology, chemical composition and chemical structure of the three SIW were investigated using various analyses. Complete degradation of methylene blue (MB) in neutral pH was achieved after 60 min at PS concentration of 4 mM, initial MB concentration of 10 mg/L and catalyst dose of 1.0 g/100 mL using light. The degradation efficiency of MB decreased from 100% to 34.6% by increasing the initial MB concentration from 10 mg/L to 100 mg/L. The degradation of MB followed the second-order model. Scavenging experiments showed the major role of hydroxyl and sulfate radicals in the MB degradation. The performance of iron waste in the retained form was investigated and the degradation efficiencies were 96%, 91.2%, 91%, 89% and 86% in five succeeding cycles at pH 7, catalyst dose of 1 g/100 mL, initial MB concentration of 10 mg/L and PS concentration of 4 mM. Moreover, the reusability of suspended iron waste was investigated. The degradation efficiencies of methylene blue, methyl red, Congo red and acid blue-25 were 100%, 97%, 96% and 97.3%, respectively after 60 min. The degradation pathways of MB were proposed after the identification of intermediates using liquid chromatography-mass spectroscopy analysis. This study revealed that the iron waste can be efficiently employed for PS activation in the suspended and immobilized modes which reduces the total cost of the Fenton process paving the way for the large-scale application of this technique.

What Are the Oxidizing Intermediates in the Fenton and Fenton-like Reactions? A Perspective

The Fenton and Fenton-like reactions are of major importance due to their role as a source of oxidative stress in all living systems and due to their use in advanced oxidation technologies. For many years, there has been a debate whether the reaction of Fe^II(H₂O)₆²⁺ with H₂O₂ yields OH^• radicals or Fe^IV=O_aq. It is now known that this reaction proceeds via the formation of the intermediate complex (H₂O)₅Fe^II(O₂H)⁺/(H₂O)₅Fe^II(O₂H₂)²⁺ that decomposes to form either OH^• radicals or Fe^IV=O_aq, depending on the pH of the medium. The intermediate complex might also directly oxidize a substrate present in the medium. In the presence of Fe^III_aq, the complex Fe^III(OOH)_aq is formed. This complex reacts via Fe^II(H₂O)₆²⁺ + Fe^III(OOH)_aq → Fe^IV=O_aq + Fe^III_aq. In the presence of ligands, the process often observed is L_n(H₂O)_5−nFe^II(O₂H) → L^•+ + L_n−1Fe^III_aq. Thus, in the presence of small concentrations of HCO₃⁻ i.e., in biological systems and in advanced oxidation processes—the oxidizing radical formed is CO₃^•−. It is evident that, in the presence of other transition metal complexes and/or other ligands, other radicals might be formed. In complexes of the type L_n(H₂O)_5−nM^III/II(O₂H⁻), the peroxide might oxidize the ligand L without oxidizing the central cation M. OH^• radicals are evidently not often formed in Fenton or Fenton-like reactions.

Bicarbonate-enhanced iron-based Prussian blue analogs catalyze the Fenton-like degradation of p-nitrophenol

P-nitrophenol (PNP), a widely used compound, is harmful to the environment and human health. In this study, four iron-based Prussian blue analogs (PBAs) were prepared by coprecipitation (Co-Fe PBA, Mn-Fe PBA, Cu-Fe PBA and Fe-Fe PBA). The Co-Fe PBA exhibited high peroxymonosulfate (PMS) activation performance for PNP degradation, removing over 90% of PNP in 60 min at an optimal pH of 7, temperature at 30 ℃, initial concentration of 20 mg/L, PBA dose of 0.2 g/L and PMS dose of 1 g/L. The physicochemical properties of the Co-Fe PBA were investigated by various characterization methods. The catalytic activity of PBA and the influence of various process parameters and water quality on the catalytic reaction were investigated to elucidate the mechanism of p-nitrophenol degradation by PBA-activated persulfate. Moreover, the mechanism of accelerated degradation of PNP under HCO₃^- conditions and the role of major reactive oxides were determined by EPR measurement methods and free radical trapping experiments. HCO₃^- was found to directly activate PMS to produce reactive oxygen species, and ¹O₂, ∙OH and SO₄^∙- were all greatly increased. This work presents a promising green heterogeneous catalyst for the degradation of emerging contaminants (ECs) in real wastewater with natural organic matter and coexisting anions by PMS activation.

Photosensitized Transformation of Peroxymonosulfate in Dissolved Organic Matter Solutions under Simulated Solar Irradiation

Sulfate radical (SO₄^•-)-mediated advanced oxidation processes via peroxymonosulfate (PMS) activation have been extensively investigated. However, the phototransformation of PMS in sunlit dissolved organic matter (DOM) solution has not been previously examined. For the first time, the photosensitized transformation of PMS in DOM-enriched solutions under simulated solar irradiation was observed. The generation of reactive species, including ¹O₂, SO₄^•-, and ^•OH, was confirmed by electron paramagnetic resonance and quantified by chemical probes. SO₄^•- was the primary reactive species generated via the reaction of excited triplet DOM (³DOM*) with PMS. ³DOM* acted as a reactive reductant and was quickly oxidized by PMS, with an estimated reaction rate constant of (4.09 ± 0.21) × 10⁸ M^-1 s^-1. Compared to ³DOM*, one-electron-reducing DOM (DOM^•-) was a minor contributor to the photosensitized transformation of PMS, and the contribution of DOM^•- relied on the phenolic constituents. In addition, a series of different types of DOM, including terrestrial DOM, autochthonous DOM, and effluent organic matter and its fractions, were employed to examine the photosensitized transformation kinetics of PMS. Overall, the photosensitized transformation of PMS by irradiated DOM could be a useful and economical approach to generate SO₄^•- under environmentally relevant conditions.

Austenitic Stainless Steel as a Catalyst Material for Photo-Fenton Degradation of Organic Dyes

In this paper, a typical austenitic stainless steel was used as a catalyst in the visible photo-Fenton degradation process of two model dyes, methylene blue and methylorange, in the presence of hydrogen peroxide and potassium persulfate as free radical-generating species. The concentration intervals for both peroxide and persulfate were in the range of 333–1667 μg/L. Very high photodecoloration efficiencies have been achieved using peroxide (>93%), while moderate ones have been achieved using persulfate (>75%) at a pH value of 6.5. For methylene blue, the maximum mineralization yield of 74.5% was achieved using 1665 μg/L of hydrogen peroxide, while methylorange was better mineralized using 999 μg/L of persulfate. The photodegradation of the dye occurred in two distinct steps, which were successfully modeled by the Langmuir–Hinshelwood pseudo-first-order kinetic model. Reaction rate constants k between 0.1 and 4.05 h−1 were observed, comparable to those presented in the reference literature at lower pH values and higher concentrations of total iron from the aqueous media.

Key Points of Advanced Oxidation Processes (AOPs) for Wastewater, Organic Pollutants and Pharmaceutical Waste Treatment: A Mini Review

Advanced oxidation procedures (AOPs) refer to a variety of technical procedures that produce OH radicals to sufficiently oxidize wastewater, organic pollutant streams, and toxic effluents from industrial, hospital, pharmaceutical and municipal wastes. Through the implementation of such procedures, the (post) treatment of such waste effluents leads to products that are more susceptible to bioremediation, are less toxic and possess less pollutant load. The basic mechanism produces free OH radicals and other reactive species such as superoxide anions, hydrogen peroxide, etc. A basic classification of AOPs is presented in this short review, analyzing the processes of UV/H2O2, Fenton and photo-Fenton, ozone-based (O3) processes, photocatalysis and sonolysis from chemical and equipment points of view to clarify the nature of the reactive species in each AOP and their advantages. Finally, combined AOP implementations are favored through the literature as an efficient solution in addressing the issue of global environmental waste management.

Efficient Degradation of Iopromide by Using Sulfite Activated with Mackinawite

Iopromide (IOP), an iodinated X-ray contrast medium (ICM), is identified as a precursor to iodide disinfection byproducts that have high genotoxicity and cytotoxicity to mammals. ICM remains persistent through typical wastewater treatment processes and even through some hydroxyl radical-based advanced oxidation processes. The development of new technologies to remove ICMs is needed. In this work, mackinawite (FeS)-activated sulfite autoxidation was employed for the degradation of IOP-containing water. The experiment was performed in a 500 mL self-made temperature-controlled reactor with online monitoring pH and dissolved oxygen in the laboratory. The effects of various parameters, such as initial pH values, sulfite dosages, FeS dosages, dissolved oxygen, and inorganic anions on the performance of the treatment process have been investigated. Eighty percent of IOP could be degraded in 15 min with 1 g L-1 FeS, 400 μmol L-1 sulfite at pH 8, and high efficiency on the removal of total organic carbon (TOC) was achieved, which is 71.8% via a reaction for 1 h. The generated hydroxyl and oxysulfur radicals, which contributed to the oxidation process, were identified through radical quenching experiments. The dissolved oxygen was essential for the degradation of IOP. The presence of Cl- could facilitate IOP degradation, while NO3- and CO32- could inhibit the degradation process. The reaction pathway involving H-abstraction and oxidative decarboxylation was proposed, based on product identification. The current system shows good applicability for the degradation of IOP and may help in developing a new approach for the treatment of ICM-containing water.

Systematic Performance Comparison of Fe3+/Fe0/Peroxymonosulfate and Fe3+/Fe0/Peroxydisulfate Systems for Organics Removal

Activated zero-valent iron (Ac-ZVI) coupled with Fe3+ was employed to activate peroxymonosulfate (PMS) and peroxydisulfate (PDS) for acid orange 7 (AO7) removal. Fe3+ was used to promote Fe2+ liberation from Ac-ZVI as an active species for reactive oxygen species (ROS) generation. The factors affecting AO7 degradation, namely, the Ac-ZVI:Fe3+ ratio, PMS/PDS dosage, and pH, were compared. In both PMS and PDS systems, the AO7 degradation rate increased gradually with increasing Fe3+ concentration at fixed Ac-ZVI loading due to the Fe3+-promoted liberation of Fe2+ from Ac-ZVI. The AO7 degradation rate increased with increasing PMS/PDS dosage due to the greater amount of ROS generated. The degradation rate in the PDS system decreased while the degradation rate in the PMS system increased with increasing pH due to the difference in the PDS and PMS activation mechanisms. On the basis of the radical scavenging study, sulfate radical was identified as the dominant ROS in both systems. The physicochemical properties of pristine and used Ac-ZVI were characterized, indicating that the used Ac-ZVI had an increased BET specific surface area due to the formation of Fe2O3 nanoparticles during PMS/PDS activation. Nevertheless, both systems displayed good reusability and stability for at least three cycles, indicating that the systems are promising for pollutant removal.

Degradation Efficiency and Kinetics Analysis of an Advanced Oxidation Process Utilizing Ozone, Hydrogen Peroxide and Persulfate to Degrade the Dye Rhodamine B

In this study, the effectiveness of a rhodamine B (RhB) dye degradation process at a concentration of 20 mg/L in different advanced oxidation processes—H2O2/UV, O3/UV and PDS/UV—has been studied. The use of UV in a photo-assisted ozonation process (O3/UV) proved to be the most effective method of RhB decolorization (90% after 30 min at dye concentration of 100 mg/L). The addition of sulfate radical precursors (sodium persulfate, PDS) to the reaction environment did not give satisfactory effects (17% after 30 min), compared to the PDS/UV system (70% after 30 min). No rhodamine B decolorization was observed using hydrogen peroxide as a sole reagent, whereas an effect on the degree of RhB degradation was observed when UV rays strike the sample with H2O2 (33% after 30 min). The rhodamine B degradation process followed the pseudo-first-order kinetics model. The combined PDS/O3/UV process has shown 60% color removal after 30 min of reaction time at an initial dye concentration of 100 mg/L. A similar effectiveness was obtained by only applying ozone or UV-activated persulfate, but at a concentration 2–5 times lower (20 mg/L). The results indicated that the combined PDS/O3/UV process is a promising method for high RhB concentrations (50–100 mg/L) comparing to other alternative advanced oxidation processes.