Advanced Oxidation Process System Research Articles

An insight into the role of experimental parameters in advanced oxidation process applied for pharmaceutical degradation.

The advanced oxidation process (AOP) is an efficient method to treat recalcitrance pollutants such as pharmaceutical compounds. The essential physicochemical factors in AOP experiments significantly influence the efficiency, speed, cost, and safety of byproducts of the treatment process. In this review, we collected recent articles that investigated the elimination of pharmaceutical compounds by various AOP systems in a water medium, and then we provide an overview of AOP systems, the formation mechanisms of active radicals or reactive oxygen species (ROS), and their detection methods. Then, we discussed the role of the main physicochemical parameters (pH, chemical interference, temperature, catalyst, pollutant concentration, and oxidant concentration) in a critical way. We gained insight into the most frequent scenarios for the proper and improper physicochemical parameters for the degradation of pharmaceutical compounds. Also, we mentioned the main factors that restrict the application of AOP systems in a commercial way. We demonstrated that a proper adjustment of AOP experimental parameters resulted in promoting the treatment performance, decreasing the treatment cost and the treatment operation time, increasing the safeness of the system products, and improving the reaction stoichiometric efficiency. The outcomes of this review will be beneficial for future AOP applicants to improve the pharmaceutical compound treatment by providing a deeper understanding of the role of the parameters. In addition, the proper application of physicochemical parameters in AOP systems acts to track the sustainable development goals (SDGs).

Unraveling Different Reaction Characteristics of Alkoxy Radicals in a Co(II)-Activated Peracetic Acid System Based on Dynamic Analysis of Electron Distribution.

Peracetic acid (PAA)-based advanced oxidation processes (AOPs) have shown broad application prospects in organic wastewater treatment. Alkoxy radicals including CH3COO• and CH3COOO• are primary reactive species in PAA-AOP systems; however, their reaction mechanism on attacking organic pollutants still remains controversial. In this study, a Co(II)/PAA homogeneous AOP system at neutral pH was constructed to generate these two alkoxy radicals, and their different reaction mechanisms with a typical emerging contaminant (sulfacetamide) were explored. Dynamic electron distribution analysis was applied to deeply reveal the radical-meditated reaction mechanism based on molecular orbital analysis. Results indicate that hydrogen atom abstraction is the most favorable route for both CH3COO• and CH3COOO• attacking sulfacetamide. However, both radicals cannot react with sulfacetamide via the radical adduct formation route. Interestingly, the single-electron transfer reaction is only favorable for CH3COO• due to its lower ESUMO. In comparison, CH3COOO• can react with sulfacetamide via a similar radical self-sacrificing bimolecular nucleophilic substitution (SN2) route owing to its high ESOMO and easy escape of unpaired electrons from n orbitals of O atoms in the peroxy bond. These findings can significantly improve the knowledge of reactivity of CH3COO• and CH3COOO• on attacking organic pollutants at the molecular orbital level.

Nanoconfined catalytic membrane assembled by nitrogen-doped carbon encapsulating Fe-based nanoparticles for rapid removal of 2,4-dichlorophenol in wastewater by peroxymonosulfate activation

Surface-dependent non-radical oxidation of carbon materials-based persulfate systems show a better application prospect in the removal of pollutants in complex wastewater. However, their potential is severely limited by the restricted liquid-to-solid mass transfer efficiency of conventional suspension systems. In this paper, a nitrogen-doped carbon encapsulating iron-based nanoparticles (Fe@NC) was prepared, and loaded onto a polyvinylidene fluoride (PVDF) membrane to construct a novel catalytic membrane Fe@NC/PVDF. The Fe@NC/PVDF/PMS system could achieve 99.74% of 2,4-dicholophenol (2,4-DCP) removal within a retention time of 0.867 s, the kinetic constant is 840 times higher than that of Fe@NC/PMS system, and 2–5 orders of magnitude higher than that of various reported advanced oxidation processes systems. The system exhibits strong anti-interference to various water matrices, long-time operational stability at high flux (306 L·m−2·h−1), universality to pollutants that do not contain strong electron-withdrawing groups and mitigation of membrane fouling. Mechanism studies indicate that the electron transfer pathway dominates the 2,4-DCP removal, and singlet oxygen (1O2) plays an auxiliary role. The higher mass transfer efficiency of the filtration mode releases the full potential of the non-radical pathway. This paper provides theoretical and technical support for the development and efficient utilization of carbon-based materials with excellent persulfate catalytic properties.

Analysis of scientific and technological trends in the incorporation of activated carbon in advanced oxidation processes-a bibliometric study.

There is high interest in the development of water pollution remediation technologies. Advanced oxidation processes (AOPs) are a promising alternative for the degradation of organic compounds; however, these technologies have been limited mainly by high operating costs and, in some cases, by forming byproducts, which can be more hazardous than the original pollutants. Activated carbon (AC) is a porous material that can be combined with AOP systems in various ways, given its adsorbent and catalytic characteristics. In addition, AC is a flexible, adaptable, and low-cost material. This article presents a bibliometric analysis of AOPs incorporating CA in scientific research and patents; the Scopus database was used to obtain patents and Orbit Express for patents. The most investigated AOPs incorporating AC are photocatalysis processes, Fenton processes, persulfate-based AOP, electrochemical processes, and ozonation. However, it is the persulfate-based AOP that has seen the greatest growth in scientific publications in recent years; this great interest can be related to the synergy that the process has with AC, allowing the degradation of contaminants via radical and non-radical. According to the maturity analysis of scientific publications, photocatalysis, Fenton, electrochemistry, ozonation, and persulfate technologies are in a growth stage and will reach maturity in 2034, 2042, 2040, 2034, and 2035, respectively; these technologies coupled with AC are expected to generate a greater number of patents when they reach maturity.

Highly graphitized biochar as nonmetallic catalyst to activate peroxymonosulfate for persistent quinclorac removal in soil through both free and non-free radical pathways

To explore highly effective and nonmetallic advanced oxidation process system to alleviate the persistent herbicides contamination in agricultural soils is of great importance. In this study, a significantly graphitized biochar with abundant C = O structure is prepared as non-metallic catalyst (WCGBC800). Different from other traditional amorphous pristine biochar, WCGBC800 can accelerate the electron transfer in the system and then effectively activate peroxymonosulfate (PMS) to produce free (OH, O2–, SO4−, etc.) and non-free radicals (1O2, electron transfer, etc.). Consequently, WCGBC800 can degrade 100 % the persistent organic herbicide quinclorac (QNC, with a content of 50 mg·kg−1) in soil within 20 min after PMS addition. The T.E.S.T-QSAR software simulation evidence that the toxicity of degradation intermediates is much lower than QNC. The stress of QNC on rice seedlings was directly described through rice cultivation experiment. The stress of rice seedlings was relieved after WCGBC800 + PMS treatment. The growth experiment of rice seedlings also shows that with an original QNC concentration of 7 mg·kg−1 in soil, the root length, root surface area, leaf length, leaf area, and leaves perimeter of rice increase of 255.30 %, 148.38 %, 207.64 %, 204.19 %, and 63.69 % respectively after detoxification by WCGBC800 + PMS system. This functional material and the WCGBC800 + PMS system provide new solutions for solving the problem of herbicide residues in soil with promising safety.

Intensified degradation of reactive blue 222 (RB222) textile dye by a hybrid AOP system of hydrodynamic cavitation coupled with inline UV and PMS oxidant

Treatment of textile wastewater poses a significant problem due to the recalcitrant nature of dyes. The present work aims to evaluate a hybrid advanced oxidation process (AOP) comprising hydrodynamic cavitation (HC), ultraviolet irradiation (UV) and different oxidants. Synthetic wastewater (SW) was prepared by dissolving 50 ppm of reactive blue 222 dye, salt, sodium carbonate and detergent. A venturi, a single-hole orifice plate and multiple-hole orifice plates were compared as cavitating devices for HC. Multiple-hole orifice plate 3 proved to be the most efficient cavitation device. Three different oxidants, namely Fenton, potassium peroxymonosulfate (PMS), and potassium persulfate (KPS), were compared in combination with HC + UV. PMS turned out to be the most effective oxidant at pH 6.5, and HC + UV + PMS was the most effective treatment process as compared to standalone methods. NaCl (salt) in wastewater promotes the degradation of RB222 dye, and sodium carbonate suppresses the degradation of RB222 by scavenging radicals. After 120 min, the TOC reduction for the HC-only, UV-only, HC + UV, UV + PMS (2 g/L), and HC + UV + PMS (2 g/L) process was 3.23 %, 4.17 %, 9.64 %, 32.48 %, 38.51 %, and 75.96 %, respectively. After 180 min of treatment with HC + UV + PMS (2 g/L), the maximum 85.66 % TOC reduction of SW was reached, and the complex intermediates formed are also reported.

FeS and Fe3O4 Co-modified biochar to build a highly resistant advanced oxidation process system for quinclorac degradation in irrigation water

Advanced oxidation processes (AOPs), based on sulfate radical (SO4·-) produced by peroxymonosulfate (PMS), can effectively mineralize refractory organic pollutants. However, the coexistence of anions and natural organic matters in actual wastewater prevents the application of AOPs. A simple one-step method was used to prepare FeS/Fe3O4 co-modified biochar materials (FFB) that could activate PMS to degrade quinclorac (QNC) with a removal rate of 100%, even exhibiting optimum degradation of QNC reached 99.31% in irrigation water, demonstrating excellent anti-interference performance for co-existing anions and natural organic matter. Meanwhile, ecotoxicity analysis showed that the toxicity of degradation intermediates was lower than that of QNC. Characterization results demonstrated the even distribution of FeS and Fe3O4 onto biochar, supplying abundant Fe2+ to activate PMS producing reactive oxygen species (ROS), while the generated Fe3+ after reactive continue to be reduced with sulfur species to promote the cycle of Fe2+/Fe3+. The coexistence of ·OH, SO4·-, 1O2, and O2·- in the FFB/PMS-QNC system suggest the possession of two pathway with free radical and non-free radical pathways to degrade QNC. The density functional theory (DFT) was used to analyze the adsorption sites and adsorption energy of PMS, as well as the differential charge density, which further proved the generation of SO4·-, O2·- and 1O2. In addition, the electrochemical test results showed that electron transfer also played an important role in the degradation of QNC. This study provides a feasible approach for the removal of organic pollutants in actual water.

Reduced TiO2 nanotube arrays as environmental catalysts that enable advanced oxidation processes: A mini review

Advanced oxidation processes (AOPs) that utilize heterogeneous catalysts are an engaging and compelling technology for treating emerging contaminants in water and wastewater. Substantial efforts have been devoted to developing cost-effective and environmentally benign materials for application in AOPs. Reduced TiO2 nanotube arrays (r-TNAs) have recently emerged as promising environmental materials for efficiently mitigating persistent organic pollutants (POPs) resistant to conventional biological treatment. r-TNAs possess distinct electrical, optical, and catalytic features, including superior conductivity and solar light absorption due to structural defects involving oxygen vacancies and Ti3+ states. Moreover, their well-organized open channel structures provide a substantial surface area, facilitating the efficient mass transfer of reactants and products. Notably, r-TNAs can be grown directly on the substrate without using organic adhesives or binders, which improves their mechanical and chemical durability and facilitates their repeated use. This article represents the pioneering comprehensive review of r-TNAs in AOP applications. We introduce an electrochemical anodization technique to fabricate TNAs and summarizes the reduction methods employed in r-TNA synthesis. We also elucidate the appealing attributes of r-TNAs and explore their applications in emerging AOPs, including persulfate activation, electrocatalysis, and photo(electro)catalysis. Finally, we outline critical challenges and provide insights into future prospects for the development of AOP systems based on r-TNAs.

Site Engineering of Covalent Organic Frameworks for Regulating Peroxymonosulfate Activation to Generate Singlet Oxygen with 100 % Selectivity

AbstractSinglet oxygen (1O2) is an excellent reactive oxygen species (ROSs) for the selective conversion of organic matter, especially in advanced oxidation processes (AOPs). However, due to the huge dilemma in synthesizing single‐site type catalysts, the control and regulation of 1O2 generation in AOPs is still challenging and the underlying mechanism remains largely obscure. Here, taking advantage of the well‐defined and flexibly tunable sites of covalent organic frameworks (COFs), we report the first achievement in precisely regulating ROSs generation in peroxymonosulfate (PMS)‐based AOPs by site engineering of COFs. Remarkably, COFs with bipyridine units (BPY‐COFs) facilitate PMS activation via a nonradical pathway with 100 % 1O2, whereas biphenyl‐based COFs (BPD‐COFs) with almost identical structures activate PMS to produce radicals (⋅OH and SO4⋅−). The BPY‐COFs/PMS system delivers boosted performance for selective degradation of target pollutants from water, which is ca. 9.4 times that of its BPD‐COFs counterpart, surpassing most reported PMS‐based AOPs systems. Mechanism analysis indicated that highly electronegative pyridine‐N atoms on BPY‐COFs provide extra sites to adsorb the terminal H atoms of PMS, resulting in simultaneous adsorption of O and H atoms of PMS on one pyridine ring, which facilitates the cleavage of its S−O bond to generate 1O2.

Prussian blue analogues derived magnetic FeCo@GC material as high-performance metallic peroxymonosulfate activators to degrade tetrabromobisphenol A over a wide pH range.

Metal-organic frame (MOF) materials can effectively degrade organic pollutants, whereas the MOF is rapidly hydrolysed in water and has poor stability and low reusability. However, in the current advanced oxidation process (AOP) system, the removal effect of pollutants under alkaline condition is not ideal. In this study, a magnetic composite material derived from MOF was synthesised and used as a new catalyst for rapid degradation of tetrabromobisphenol A (TBBPA). Compared to precarbonisation, FeCo@GC formed a conductive graphite carbon skeleton, retained the complete rhombododecahedron structure, had a larger specific surface area and provided more active sites for peroxymonosulfate (PMS) activation. The target pollutant TBBPA (20 mg/L) was completely degraded within 30 min, and the mineralisation rate reached 40.98% in the FeCo@GC (150 mg/L) and PMS (1 mM) systems, owing to the synergistic interaction between Fe, Co and graphite carbon. The reactive oxygen species (ROS) involved in the reaction were determined to be SO4•-, ·OH, 1O2 and O2•- by electron paramagnetic resonance and free radical scavenging experiments, and the 1O2 played a dominant role. Based on the results of LC-MS analysis results, the main degradation pathways of TBBPA involve three mechanisms: the debromination reaction, hydroxylation and cleavage of the benzene ring. In addition, compared with previous AOP systems, FeCo@GC/PMS overcomes the disadvantage of poor degradation effect of TBPPA under alkaline conditions, has a wide range pH (3-11) application and has the best effect on TBBPA degradation under alkaline conditions. FeCo@GC has an excellent cycle performance, with a removal rate of re-calcined material of 88.52% after five cycles. Therefore, FeCo@GC can be used as a promising and efficient catalyst for removing environmental organic pollutants.

Site Engineering of Covalent Organic Frameworks for Regulating Peroxymonosulfate Activation to Generate Singlet Oxygen with 100% Selectivity.

Singlet oxygen (1O2) is an excellent reactive oxygen species (ROSs) for the selective conversion of organic matter, especially in advanced oxidation processes (AOPs). However, due to the huge dilemma in synthesizing single-site type catalysts, the control and regulation of 1O2 generation in AOPs is still challenging and the underlying mechanism remains largely obscure. Here, taking advantage of the well-defined and flexibly tunable sites of covalent organic frameworks (COFs), we report the first achievement in precisely regulating ROSs generation in peroxymonosulfate (PMS)-based AOPs by site engineering of COFs. Remarkably, COFs with bipyridine units (BPY-COFs) facilitate PMS activation via a nonradical pathway with 100% 1O2, whereas biphenyl-based COFs (BPD-COFs) with almost identical structures activate PMS to produce radicals (•OH and SO4•-). The BPY-COFs/PMS system delivers boosted performance for selective degradation of target pollutants from water, which is ca. 9.4 times that of its BPD-COFs counterpart, surpassing most reported PMS-based AOPs systems. Mechanism analysis indicated that highly electronegative pyridine-N atoms on BPY-COFs provide extra sites to adsorb the terminal H atom of PMS, resulting in simultaneous adsorption of O and H atoms of PMS on one pyridine ring, which facilitates the cleavage of its S-O bond to generate 1O2.

Synergy between UV light and trichloroisocyanuric acid on methylisothiazolinone degradation: Performance, kinetics and degradation pathway.

Methylisothiazolinone (MIT) is widely used in daily chemicals, fungicides, and other fields and its toxicity has posed a threat to water system and human health. In this study, ultraviolet (UV)/trichloroisocyanuric acid (TCCA), which belongs to advanced oxidation processes (AOP), was adopted to degrade MIT. Total chlorine attenuation detection proved that TCCA has medium UV absorption and a strong quantum yield (0.49 mol E−1). At a pH of 7.0, 93.5% of MIT had been decontaminated after 60 min in UV/TCCA system (kobs = 4.4 × 10−2 min−1, R2 = 0.978), which was much higher than that in the UV alone system and TCCA alone system, at 65% (1.7 × 10−2 min−1, R2 = 0.995) and 10% (1.8 × 10−3 s−1, R2 = 0.915), respectively. This system also behaved well in degrading other five kinds of contaminants. Tert-butanol (TBA) and carbonate (CO32−) were separately used in quenching experiments, and the degradation efficiency of MIT decreased by 39.5% and 46.5% respectively, which confirmed that HO• and reactive chlorine species (RCS) were dominant oxidants in UV/TCCA system. With TCCA dosage increasing in a relatively low concentration range (0.02–0.2 mM) and pH decreasing, the effectiveness of this AOP system would be strengthened. The influences of coexisting substances (Cl−, SO42−, CO32−, NO2− and NO3−) were explored. MIT degradation pathways were proposed and sulfur atom oxidation and carboxylation were considered as the dominant removal mechanisms of MIT. Frontier orbital theory and Fukui indexes of MIT were employed to further explore the degradation mechanism.

Kinetic study on the degradation of organic pollutants in UV/persulfate and in other advanced oxidation processes: Role of radicals and improvement of the degradation rates

Kinetic degradation of five typical pollutants including clothianidin (CLO), levofloxacin (LEV), p-nitrosodimethylaniline (RNO), paracetamol (PCM), and basic fuchsin (BF) by UV/persulfate (UV/PS) and other advanced oxidation processes (AOPs) (i.e., UV/H2O2, Fe2+/H2O2, electrochemical process, UV/PS/heat, UV/PS/Fe2+, UV/PS/Fe2O3, and UV/PS/rGO) were studied. The result showed that all pollutants were effectively degraded by UV/PS with different degradation rates. The oxidizing factors contributing to the pollutant degradation in UV/PS were studied, showing that UV is the predominant contributor to CLO degradation, while its contribution is negligible in the degradation of other compounds. •OH and SO4•- were the main radicals contributing to the degradation of all pollutants in UV/PS, except CLO. The second-order rate constant of •OH and SO4•- towards pollutants was determined according to the competition kinetic model, to be in the range of 109-1010 M−1 s−1. The degradation rate of pollutants in different AOPs were varied, because the oxidizing factors and their role in AOP systems directly affect the degradation efficiency of pollutants. In all AOPs, •OH radical played a major role in their degradation. The presence of water matrices has different influences on the degradation rates of pollutants. For example, 10 mM NO2- inhibited the degradation of pollutants to 99.1%, whereas Br-, SO42-, and F- had a lower inhibitory effect on their degradation. HPO42- can improve the degradation rate of BF, but it also significantly inhibited the degradation of PCM. The results also revealed that UV/PS/Fe2+ and UV/PS/rGO can remarkably increase the degradation efficiency of all pollutants, while using α-Fe2O3 and heat were less efficient. In addition, other parameters including COD removal, energy consumption, and toxicity assessment were also investigated to evaluate the potential application of UV/PS in practice.

Unraveling Janus-like behavior of copper phosphide for selective production of reactive oxygen species: Singlet oxygen versus hydroxyl radical

The selective control of reactive oxygen species (ROS) and its mechanistic understanding are crucial to effectively abate aquatic organic pollutants in advanced oxidation processes (AOPs). The ROS are generated by single or multiple reaction steps and mainly depend on an electron and/or energy transfer achieved either by an external energy input (e.g., heating, applying electricity, light irradiation, etc.) or a spontaneous process stimulated under certain experimental conditions (△Go < 0). In this study, we demonstrated the Janus-like behavior of copper phosphide (CuxP) for the selective production of ROS under different conditions. The self-dissolution of CuxP at pH 3 in the dark resulted in the formation of hydroxyl radical (∙OH) via a sequential reaction step (i.e., dissolved oxygen (O2) → hydrogen peroxide (H2O2) → ∙OH), while visible light irradiation (λ ≥ 420 nm) at pH 7 led to the production of singlet oxygen (1O2) via an energy transfer from the photo-excited CuxP. CuxP underwent gradual loss of the oxidation activity with the dissolution of Cu and P species during the repetitive reactions at pH 3 in the dark, whereas it maintained the activity without the dissolution at pH 7 under visible light irradiation. The Janus-like behavior of CuxP enables its application not only as a solid oxidative reagent under acidic and dark conditions, but also as a photosensitizer under neutral and light conditions for the treatment of specific organic pollutants vulnerable to the radical and non-radical attacks. The photo-switching control for a selective ROS formation provides a new insight on water treatment by these species and paves the way for the development of highly efficient AOP systems.

Utilization of ferrosilicon from waste carbide slag in enhancing light-driven peroxymonosulfate activation for rapid and clean dyes decolorization

The model of efficient, clean, and cheap collaborative governance has received considerable attention in the treatment of pollution such as wastewater and solid waste. The “waste control by waste” strategy is proposed and put into practice in this work, that is, the ferrosilicon from waste carbide slag is utilized for treating dyes wastewater via the advanced oxidation process (AOP) of ferrosilicon/PMS/light. The main component of ferrosilicon is FeSi and Fe5Si3, which are riched in Fe element and suitable for inducing the AOP reaction. When the ferrosilicon is used in the AOP system, 20 mg L−1 MB can be completely decolorized within 45 min. The excellent performance of the ferrosilicon/PMS/light system is also observed in various wastewater conditions such as solution pH (3–9) and dye types (MB, MO, MG, and CR), which is superior to common PMS/light and other reported system. In addition, the ferrosilicon catalyst can keep the MB removal at nearly 100% in five cycles, and the proportion of its leached iron is always less than 1%, which effectively solves the performance degradation and leaching loss of commonly used zero-valent iron (ZVI) catalyst. In the ferrosilicon/PMS/light system, the SO4●ˉ and 1O2 radicals act as the main active species and their production is due to the redox of iron elements and the self-decomposition of PMS, respectively. Overall, this research progress provides a valuable reference for clean and cheap environmental governance.

Highly efficient production of hydroxyl radicals from oxygen reduction over Ni-Fe dual atom electrocatalysts for removing emerging contaminants in wastewater

Advanced oxidation process (AOP) based on hydroxyl radicals (•OH) produced from electrocatalytic oxygen reduction reaction (ORR) is regarded as a promising technique for treating industrial wastewater. However, the poor efficiency of •OH production considerably limits the •OH-based AOP efficiency. In this study, a high-efficiency production of •OH from oxygen reduction is realized over Ni-Fe dual atom supported on nitrogen-doped carbon electrocatalysts (Ni0.5–Fe0.5–NC), where the O2 is preferentially adsorbed onto Ni sites. With the aid of neighboring electron-rich Fe atoms, the 2-electron intermediate *H2O2 over Ni sites is inclined to gain one electron to produce •OH (ΔG = -2.69 eV) rather than desorb to form H2O2 (ΔG = -1.05 eV). Even when hydrogen peroxide is produced by desorption, it is quickly activated to generate •OH by neighboring electron-rich Fe atoms. The in-situ produced •OH on Ni0.5–Fe0.5–NC reaches up to 4.61 mmol L−1 min−1 gcat−1, presenting the state-of-the-art AOP. Some emerging refractory contaminants (florfenicol, thiamphenicol, bisphenol A, phenol, sulfamethoxazole, and tetracycline hydrochloride) in wastewater can be fast and deep removed in the proposed AOP system based on ORR (C0 = 20 mgL−1, pH 7, −0.5 V vs SCE, working electrode: 6 cm2 carbon paper loading 6.0 mg catalyst). This study provides an ORR catalyst design inspiration for energetic and green generation of •OH, which would supply a reliable method for the development of wastewater treatment technology.

Metal-Free Activation of Sulfite by Benzoquinone-Derived Carbon for Efficient Organic Contaminant Degradation: Identification and Regulation of Active Sites

Sulfite [S(IV)]-based advanced oxidation processes (AOPs) driven by carbon materials provide an environmentally friendly, economical route for environmental remediation. However, the active sites and involved radical mechanisms are still controversial due to the complexity of the carbon structure, thus greatly hampering the development of highly efficient systems. Herein, a benzoquinone-derived carbon (BQC)/S(IV) system was developed for organic pollutant degradation in an acidic medium for the first time. The quinone groups and conductivity were confirmed as the key factors for S(IV) activation through cyclic voltammetry, antitheses, specific site masking, and quantitative structure–activity relationship methods. Benefiting from the high content of quinone groups and excellent conductivity, the BQC catalyst exhibited much more effective in activating Na2SO3 for organic pollutant degradation compared to most traditional carbon or even metal catalysts. The chronopotentiometry and in situ Raman results revealed that the formation of C–S(IV)* complexes was the primary step to produce SO3·–, followed by the production of SO4·– via a chain reaction, in which the SO4·– radical was responsible for pollutant degradation. The findings may provide novel insights into reaction mechanisms in S(IV)-AOP systems and pave the way toward highly efficient carbon catalysts to activate S(IV) for the elimination of organic contaminants.

Construction of an underwater plasma and Fenton hybrid system for the rapid oxidation of organic dyes and antibiotics

In this study, the decomposition efficiency and oxidation rate of organic pollutants were improved using a combination of a dielectric barrier discharge (DBD) plasma system and Fe3O4 Fenton catalyst, a representative advanced oxidation process (AOP). The combined plasma Fenton system showed >99 % degradation of 200 mg L−1 dyes within 35 min, while other AOP systems achieved lesser degradation of the same concentration of dyes in 40 min: no degradation in the Fe3O4 Fenton system and <25 % in the Ozone(O3) system. Additionally, >92 % of tetracycline and ciprofloxacin (TC, CIP: 10 mg L−1) was effectively degraded in the plasma Fenton system within 5 min, in contrast to the Fe3O4 Fenton system. The rapid degradation of organic compounds was attributed to various oxidant species, including O3, H2O2, and OH, from the reaction between the plasma electrode and Fenton catalyst. By analyzing chemically active species and interfacial reactions of the multiple-hole DBD underwater plasma Fenton system, we improved the accessibility of AOP element technology and proposed a research foundation for the decomposition reaction, thereby providing a theoretical basis for developing this technology.

A novel oxygen vacancies enriched Co[sbnd]Ni LDO catalyst activated peroxymonosulfate for the efficient degradation of tetracycline

Flower-like CoNi layered double oxide (LDO) was successfully synthesized and applied to activate peroxymonosulfate (PMS) to degrade tetracycline (TC). CoNi LDO was rich in oxygen vacancies (OV) and had a high specific surface area. 30 mg/L TC was 100 % degraded in 60 min under the condition of 0.1 g/L CoNi LDO, 1.6 mM PMS, and unadjusted pH (5.45). A reaction mechanism dominated by OV was proposed in the TC removal system. Superoxide radical and singlet oxygen played major role in the reaction system, while sulfate radical and hydroxyl radical played minor role. The anti-interference ability of the CoNi LDO/PMS system to anions was relatively strong. At the same time, CoNi LDO showed high recovery efficiency. The degradation efficiency of TC was still higher than 92 % after five cycles. Through the detection of degradation intermediates, a possible degradation path was proposed. This study provided an efficient approach for TC degradation and provided a new clue for improving catalytic performance by introducing OV into the catalysts in PMS-based advanced oxidation processes systems.

A dual-oxidant advanced oxidation process system containing CaO2 and peroxymonosulfate for organic pollutant degradation: High adaptability and synergistic effect

In this study, a dual-oxidant advanced oxidation process (AOP) system containing calcium peroxide (CaO2) and peroxymonosulfate (PMS) was developed for the first time to efficiently degrade refractory organic pollutants. It was found that the system with 0.1 g/L CaO2 and 0.3 g/L PMS could degrade more than 91% of 50 mg/L tetracycline (TC) within 4 min at room temperature, and the degradation process much followed the pseudo second-order kinetic model. The presented AOP system exhibited strong anti-interference capabilities towards coexisting inorganic anions (Cl−, SO42–, NO3–, HCO3– and HPO42–) and humic acid (HA), even at high concentrations. Moreover, the system could efficiently work in a wide pH (3−12) range, suitable for natural waterbodies and various organic pollutants. The results of quenching experiments and electron paramagnetic resonance (EPR) tests demonstrated that singlet oxygen (1O2) was the dominate reactive oxygen species (ROS) responsible for organics degradation. A synergistic mechanism of the dual-oxidant system was confirmed by a series of comparative experiments, where CaO2 continuously provided OH− and H2O2, and then combined with PMS to produce predominant 1O2 and inferior O2•–, respectively. Further, the possible pathways of TC degradation were deduced based on analyses of the intermediates.