An efficient CuO-γFe2O3 composite activates persulfate for organic pollutants removal: Performance, advantages and mechanism

Towards advanced removal of organics in persulfate solution by heterogeneous iron-based catalyst: A review

Heterogeneous iron-based catalysts have drawn increasing attention in the advanced oxidation of persulfates due to their abundance in nature, the lack of secondary pollution to the environment, and their low cost over the last a few years. In this paper, the latest progress in the research on the activation of persulfate by heterogeneous iron-based catalysts is reviewed from two aspects, in terms of synthesized catalysts (Fe0, Fe2O3, Fe3O4, FeOOH) and natural iron ore catalysts (pyrite, magnetite, hematite, siderite, goethite, ferrohydrite, ilmenite and lepidocrocite) focusing on efforts made to improve the performance of catalysts. The advantages and disadvantages of the synthesized catalysts and natural iron ore were summarized. Particular interests were paid to the activation mechanisms in the catalyst/PS/pollutant system for removal of organic pollutants. Future research challenges in the context of field application were also discussed.

Preparation of chitosan-iron oxide modified sludge-based biochar for effective removal of tetracycline from water: performance and mechanism

The release of antibiotics has attracted wide attention due to their abuse and discharge. How to remove these emerging contaminants is an urgent need to be solved. In the present study, sludge-based biochar combining chitosan and iron oxide was prepared via municipal sewage sludge. The novel biochar modified with chitosan and iron oxide exhibited satisfying performance in eliminating antibiotics from water. The application of modified biochar combined with activated persulfate (PS) showed a remarkable removal efficiency of 96.98% for tetracycline (TC). Analysis of the surface characteristics of the modified biochar showed the presence of structural defects, dispersed iron oxides, abundant functional groups, a porous structure, and a relatively stable crystal structure. These characteristics attributed significant importance to facilitating the degradation of TC. A series of experimental conditions including preparation temperature (600-900 ℃), reaction temperature (15-45 ℃), contaminant concentration (30-180 mg/L), adsorbent usage (0.1-1 g/L), pH (2-10), and persulfate addition concentration (1-5 mmol) were conducted. The results revealed that the highest removal efficiency was achieved at 96.98% under the conditions of TC concentration at 30 mg/L, reaction temperature at 35 ℃, pH of 4, adsorbent addition amount of 0.6 g/L, and PS concentration of 2 mmol, respectively. Three degradation pathways and seven intermediate products of TC were proposed. Therefore, our study provides a promising approach for developing effective removal of antibiotic pollutants.

Bagasse cellulose-based S-type Bi2O3/Zn3In2S6 photocatalyst for efficient and stable degradation of 2,4-dichlorophenol under visible light

The environmental and human health hazards posed by 2,4-dichlorophenol (2,4-DCP) call for effective degradation technologies. This research investigates the design and application of a Bi2O3/Zn3In2S6 heterojunction photocatalyst, a 'S scheme', which was constructed via a simple hydrothermal method. The photocatalyst was then embedded in a sugarcane bagasse cellulose carrier (SBC/BO/ZIS), demonstrating excellent 2,4-DCP degradation capacity. The results show that S-type Bi2O3/Zn3In2S6 promotes the separation of photogenerated carriers. The SBC/BO/ZIS complex, in comparison with Bi2O3 and Zn3In2S6 alone, amplifies specific surface area (91.7880 m2/g) and broadens the light absorption range (570 nm) of materials, showing robust photocatalytic performance. The degradation rate of 50 mg/L 2,4-DCP reached an impressive 97% within 120 min. The encapsulation of BO/ZIS in SBC not only increases the efficiency of material recovery and recycling but also allows for continuous degradation of 2,4-DCP in cyclic manners, maintaining a degradation rate between 90% and 97%. XRD characterization shows that the physical properties of the material are not affected. The degradation of 2,4-DCP was dominantly controlled by active species (·OH and ·O2-) identified by electron paramagnetic resonance analysis and free radical trapping experiments. This innovative design significantly enhances sunlight utilization and effectively curbs charge carrier recombination, while also promoting material recovery and utilization. These attributes establish a foundation for a cost-effective and efficient means of treating actual wastewater containing 2,4-DCP.

Efficient degradation of norfloxacin using a novel biochar-supported CuO/Fe3O4 combined with peroxydisulfate: Insights into enhanced contribution of nonradical pathway

Nonradical persulfate oxidation techniques have evolved as a new contaminated water treatment approach due to its great tolerance to water matrixes. The catalysts of CuO-based composites have received much attention in that aside from SO₄^•-/^•OH radicals, the nonradicals of singlet oxygen (¹O₂) can be also generated during persulfate activation via CuO. However, the issues regarding particles aggregation and metal leaching from the catalysts during the decontamination process remain to be addressed, which could have a remarkable impact on the catalytic degradation of organic pollutants. Accordingly in the present study, a novel biochar-supported bimetallic Fe₃O₄-CuO catalyst (CuFeBC) was facilely developed to activate peroxodisulfate (PDS) for the degradation of norfloxacin (NOR) in aqueous solution. The results showed CuFeBC has a superior stability against metal ions Cu/Fe leaching, and NOR (30 mg L^-1) was degraded at 94.5% within 180 min in the presence of CuFeBC (0.5 g L^-1) and PDS (6 mM) in pH 8.5. The scavenging of reactive oxygen species and electron spin resonance analysis revealed that ¹O₂ dominated the degradation of NOR. Compared with pristine CuO-Fe₃O₄, the interaction between biochar substrate and metal particles could significantly enhance the contribution of the nonradical pathway to NOR degradation from 49.6% to 84.7%. Biochar substrate could efficiently reduce the leaching of metal species from the catalyst, thereby maintaining excellent catalytic activity and lasting reusability of the catalyst. These findings could enlighten new insights into fine-tuning radical/nonradical processes from CuO-based catalysts for the efficient remediation of organic contaminants in polluted water.

Copper oxides activate peroxymonosulfate for degradation of methylene blue via radical and nonradical pathways: surface structure and mechanism

A one-step hydrothermal method for preparation of copper oxides with different valences using ascorbic acid as a reducing reagent was developed for environmental remediation. The results suggested that the notable degradation performance of CuO₀ may be attributable to the abundant active sites, such as Cu or Cu-O, and was not significantly related to the Cu valence state. In contrast to direct degradation of pollutants by traditional superoxide radicals (O₂^•-), O₂^•- played an important role in the reduction of high-valence Cu ions (Cu(III)). In addition, a series of radical quenching, electron paramagnetic resonance (EPR), and electrochemical experiments validated the existence of direct electron transfer between methylene blue (MB) and PMS mediated by CuO₀ and surface-bound radicals. The results suggested that the CuO₀/PMS system may be less susceptible to diverse ions and natural organic matter other than dihydrogen phosphate anions. The mechanism of MB degradation under alkaline conditions was different from that under acidic conditions in that it was not reliant on radicals or charge transfer but direct oxidation by PMS. This study provides new insights into the heterogeneous processes involved in PMS activation by the copper oxides. Furthermore, this paper devotes to providing theoretical basis on pollutant removal via PMS activated by copper oxides and developing low-cost and high-efficiency catalysts.

Low-Temperature Reduction Synthesis of γ-Fe2O3-x@biochar Catalysts and Their Combining with Peroxymonosulfate for Quinclorac Degradation

Biochar loading mixed-phase iron oxide shows great advantages as a promising catalyst owing to its eco-friendliness and low cost. Here, γ-Fe₂O_3-x@biochar (E/Fe-N-BC) composite was successfully prepared by the sol-gel method combined with low-temperature (280 °C) reduction. The Scanning Electron Microscope (SEM) result indicated that γ-Fe₂O_3-x particles with the size of approximately 200 nm were well-dispersed on the surface of biochar. The CO derived from biomass pyrolysis is the main reducing component for the generation of Fe (II). The high content of Fe (II) contributed to the excellent catalytic performance of E/Fe-N-BC for quinclorac (QNC) degradation in the presence of peroxymonosulfate (PMS). The removal efficiency of 10 mg/L of QNC was 100% within 30 min using 0.3 g/L γ-Fe₂O_3-x@biochar catalyst and 0.8 mM PMS. The radical quenching experiments and electron paramagnetic resonance analysis confirmed that •OH and SO₄•^- were the main radicals during the degradation of QNC. The facile and easily mass-production of γ-Fe₂O_3-x@biochar with high catalytic activity make it a promising catalyst to activate PMS for the removal of organic pollutants.

Heterostructured γ-Fe2O3/FeTiO3 magnetic nanocomposite: An efficient visible-light-driven photocatalyst for the degradation of organic dye

The catalyst recovery is the major concern in commercialization of photocatalysts for the industrial effluent treatment process. To overcome this major issue, Fe₂O₃ based magnetic photocatalytic heterostructure ɣ-Fe₂O₃/FeTiO₃ nanocomposite was synthesized by hydrothermal method. Fe₂O₃ is the cheapest visible active magnetic photocatalytic material, but it has the limitation of fast e^-/h + recombination. Titanium (Ti) was loaded on γ-Fe₂O₃ to overcome this issue. The loaded Ti has grown as FeTiO₃ on the surface of ɣ-Fe₂O₃ nanocrystals and emerged as heterostructure ɣ- Fe₂O₃/FeTiO₃ nanocomposites, which was confirmed by XRD and TEM results. The loading concentration of Ti on γ-Fe₂O₃ was optimized to achieve the maximum photocatalytic efficiency without compromising the magnetic property of γ-Fe₂O₃ to facilitate the magnetic separation. DRS-UV spectra revealed the strong visible light response of γ- Fe₂O₃/FeTiO₃ nanocomposite. The photocatalytic efficiencies of the synthesized materials were evaluated using methylene blue (MB) as a model pollutant under sunlight. The built-in electric field between p-n junction between FeTiO₃ and Fe₂O₃ and type II charge transfer mechanism extended the lifetime of the charge carriers at the heterojunction of γ- Fe₂O₃/FeTiO₃, which was confirmed by PL spectra. The vibrating sample magnetometer (VSM) study revealed the decreasing magnetization, coercivity (Hc), and retentivity (Mr) of γ-Fe₂O₃ with increasing concentration of Ti. 92% of the used-up 20 wt% Ti loaded γ-Fe₂O₃/FeTiO₃ magnetic nanocomposite was recovered from the treated wastewater using an electromagnet. Both magnetic properties and efficiency of the nanocomposite increased up to 20 wt% of Ti loading, beyond that decreased due to the increasing composition of antiferromagnetic FeTiO₃ and the increasing number of defect sites as recombination centers. Hence, 20 wt% loading of Ti was concluded as the optimum to enhance the efficiency and to retain the magnetic properties. This work aims the commercialization of magnetic photocatalytic materials for the industrial effluent treatment.

Construction of a C-decorated and Cu-doped (Fe,Cu)S/CuFe2O4 solid solution for photo-Fenton degradation of hydrophobic organic contaminant: Enhanced electron transfer and adsorption capacity

To effectively treat the hydrophobic organic contaminant and utilize an industrial solid waste manganese residue (MR), a novel starch-derived carbon (SC)-decorated and Cu-doped (Fe,Cu)S/CuFe₂O₄ solid solution (CFS/CFO@SC) was prepared from MR via mechanical activation treatment of precursor materials followed by one-step pyrolysis and applied as a photo-Fenton catalyst to treat a hydrophobic organic compound, 17α-ethinylestradiol (EE2). Characterization results showed that the CFS/CFO@SC solid solution with unique crystal and electronic structures exhibited high adsorption capacity and catalytic activity, ascribed to that Cu doping and C decorating enhanced its hydrophobicity and BET surface area. The CFS/CFO@SC showed excellent degradation efficiency with nearly 100% of EE2 removal rate in 40 min (degradation rate constant of 0.112 min^-1), and a high mineralization degree with 95.2% of TOC removal in 180 min. This could be ascribed to that C decorating and the formation of CFS/CFO solid solution promoted the charge transfer in a continuous band, resulting in effective separation of photogenerated holes-electrons (h⁺-e^-). The strong interaction of Fe-Cu collaborating with the photoelectron could effectively accelerate the recycle of Fe³⁺/Fe²⁺ and Cu²⁺/Cu⁺, thus generating more active radicals. Moreover, CFS/CFO@SC showed promising stability and recyclability with the EE2 removal rates all >95% after five cycles. This work brings a valuable approach for the rational design of high-performance Fe-based photo-Fenton catalysts for environmental remediation and the valorization of MR.

The noteworthy chloride ions in reclaimed water: Harmful effects, concentration levels and control strategies

Chloride ions (Cl^-), which are omnipresent in reclaimed water, can cause various problems in water reuse systems, especially during water transmission and at end use sites. Although reverse osmosis (RO) is considered as an effective technology to reduce chloride, its high investment and complex maintenance requirements hinder its application in many water reclamation plants (WRPs). Recently, several technologies bringing new options to better deal with chloride have gained increased attention. This review provides detailed information on the harmful effects, concentration levels, and sources of chloride in reclaimed water and summarizes and discusses various chloride removal technologies, including non-selective methods (e.g., membrane filtration, adsorption and ion exchange, oxidation, and electrochemical methods) and selective methods (e.g. precipitation and specially designed electrochemical methods). Among these, Friedel's salt precipitation and capacitive deionization showed attractive development potential. This review also proposes a holistic framework for chloride control from aspects of "Fit-for-Purpose" planning, technical system development, and whole process optimization, which could facilitate the planning and operation of long-term sustainable water reuse practices.

Decomplexation of Cu(II)-EDTA by synergistic activation of persulfate with alkali and CuO: Kinetics and activation mechanism

Heavy metals usually coexist with a variety of chelating agents to form heavy metal complexes in industrial wastewater. The decomplexation of heavy metal complexes is the crucial step before the removal of heavy metals via alkaline precipitation process. An efficient synergistic activation of persulfate (PS) with alkali and CuO was used for the simultaneous decomplexation of Cu-ethylenediamine tetraacetic acid (Cu(II)-EDTA) (3.14 mM) and the Cu(II) precipitation. The experimental results demonstrated that nearly complete removal of Cu(II) could be achieved by synergistic activation of PS with alkali and CuO at pH 11 after 2 h of decomplexation reaction. However, sole PS could not effectively decomplex Cu(II)-EDTA (13.5%), while the alkaline activation of PS could accomplish 57.0% removal of Cu(II). Radical scavenger tests indicated that reactive oxygen species (ROS) including SO₄^•-, •OH and O₂^•- were responsible for the decomplexation of Cu(II)-EDTA in the synergistic activation of PS with alkali and CuO. As a heterogeneous activator, CuO possessed excellent reusability and long-lasting catalytic activity and the rate constant value (k) of Cu(II) removal showed an increase (from 0.0326 min^-1 in the first cycle to 0.0491 min^-1 in the 24^th cycle) with 24 cycles experiments. Furthermore, the biotoxicity evaluation of treated solution revealed that the biotoxicity of Cu(II)-EDTA contaminated wastewater could be effectively mitigated by the synergistic activation of PS with alkali and CuO because of the efficient precipitation of Cu(II) and oxidative degradation of EDTA organic ligands, which was favorable for the subsequent biochemical treatment.

In Situ-Formed Phenoxyl Radical on the CuO Surface Triggers Efficient Persulfate Activation for Phenol Degradation

Transition-metal oxide (M_xO_y)-based persulfate (PDS) activation processes have demonstrated enormous potential for pollutant degradation in water purification. However, the mechanistic insight of PDS activation by a M_xO_y catalyst concerning the mediate role of the organic substrate remains obscure. Here, we demonstrated that the in situ-formed phenoxyl radical on the CuO surface can trigger efficient persulfate activation for phenol degradation. The formation of the phenoxyl radical was an inner-sphere process, which involved the successive steps of chemisorption through surface hydroxyl group substitution and the subsequent spontaneous electron transfer reaction from adsorbed phenol to CuO. The organic substrate phenol can be oxidized by the PDS molecule and surface-bound SO₄^•- through the nonradical and free-radical pathways, respectively. Such a unique "half-radical" mechanism resulted in an extraordinarily high PDS utilization efficiency of 188.9%. More importantly, a general rule for phenoxyl radical formation was concluded; it can be formed in the cases of organic substrates with a Hammett constant σ⁺ lower than -0.02 and metal ion of a 3d subshell between half-filled and fully filled. This study clarifies the mediate role of the organic substrate for interfacial PDS activation on M_xO_y and also gives new insights into the rational design of a highly efficient M_xO_y catalyst for selective phenolic/aniline pollutant degradation in wastewater.

UV/Vis-Based Persulphate Activation for p-Nitrophenol Degradation

In the present work, the degradation of p-nitrophenol (PNP) and its mineralization by a UV/Vis-based persulphate activation process was investigated. Firstly, a screening of processes as direct photolysis, persulphate alone and persulphate activated by radiation was performed. The incidence of radiation demonstrated to have an important role in the oxidant activation, allowing to achieve the highest PNP and total organic carbon (TOC) removals. The maximum PNP oxidation (100%) and mineralization (61.6%)—both after 2 h of reaction time—were reached when using T = 70 °C, (S2O82−) = 6.4 g/L and I = 500 W/m2. The influence of radiation type (ultraviolet/visible, visible or simulated solar light) was also evaluated, being found that the source with the highest emission of ultraviolet radiation (UV/visible) allowed to achieve the best oxidation efficiency; however, solar radiation also reached very-good performance. According to quenching experiments, the sulphate radical is key in the activated persulphate oxidation process, but the hydroxyl radical also plays an important role.

Application of copper tailings combined with persulfate for better removing methyl orange from wastewater

In this paper, wasted copper tailings (CT) were used to activate persulfate (PS) to degrade azo dye methyl orange (MO). The results show that a large amount of FeS₂ contained in CT can slowly release Fe²⁺ in the aqueous solution to activate PS to generate reactive oxygen species to degrade MO. When the dosage of CT and PS was 2 g/L and 3 mM respectively, the MO degradation efficiency of 20 mg/L in the CT/PS system was 96.52% within 60 min. At the same time, it is found that CT has a certain adsorption capacity for MO, and the intra-particle diffusion model can well describe the adsorption process of MO by CT. The effects of related reaction parameters (CT dosage, PS dosage, initial MO concentration and solution pH) on MO degradation in CT/PS system were investigated. Compared with the direct addition of an equal amount of Fe²⁺ as in the CT/PS system, for homogeneous activated PS to degrade MO (Fe²⁺/PS), the results showed that the degradation efficiency of Fe²⁺/PS system for MO was lower than that of CT/PS system due to excessive Fe²⁺ consumption of SO₄ ^·-. By comparing the Fe²⁺ and Fe³⁺ concentrations in the two systems, it was found that the CT/PS system could maintain a low Fe²⁺ concentration during the reaction process, and the Fe²⁺ released by CT could be used by PS to degrade MO more efficiently. The free radical scavenging experiments showed that the reactive oxygen species in the CT/PS system was mainly SO₄ ^·-. This study not only proposed a new CT utilization approach, but also solved the problem of reduced degradation efficiency of organic pollutants caused by excessive Fe²⁺ in the Fenton-like reaction.

Enhanced catalytic degradation of tetracycline antibiotic by persulfate activated with modified sludge bio-hydrochar

In this research, a one-pot prepared modified sludge bio-hydrochar (IBHC) was prepared to activate persulfate (PS) for the degradation of tetracycline (TC) antibiotic. The obtained IBHC bearing defect structure, dispersed iron and large amounts of surface organic functional groups, acts as an outstanding modified biomass carbonaceous material for catalyzing PS to improve the removal efficiency of TC as high as 99.72%. The IBHC + PS system can remove TC effectively with a relative low IBHC dosage (0.2 g·L^-1), limited PS consumption (5 mmol·L^-1) and wide pH values (2-10). In addition, the degradation of TC still keep in 94.70% after 5 rounds reuse proving that IBHC possesses excellent stability and practicability. During the activation, both and ^•OH were generated and the contribution of each component on the TC degradation in IBHC + PS system was explored. Furthermore, the degradation pathways of TC were proposed based on the results of LC-MS.