•Review covers the progress in photo-activated PS-based AOPs for water treatment.•The various underlying photo-activation mechanisms are comprehensively discussed.•The roles of light to improve ...PS-based AOPs for water treatment are summarized.
Persulfate-based advanced oxidation processes (PS-based AOPs) under UV, visible, or solar irradiation are being intensively investigated for water treatment. Tremendous advances have been made for enhancing the performance towards the destruction of target pollutants, but a deeper understanding of the role of light in different photo-activated PS-based AOPs is still needed as a basis for improving the efficiency. This paper intends to provide an in-depth review of the underlying photo-activation mechanisms and recent progress in various common photo-activated PS-based AOPs reported over the last decade. Based on a comprehensive survey of previous studies, we categorize these processes according to their reaction mechanisms, including activation by direct UV radiation, processes based on dye-photosensitization, activation through ligand-to-metal charge transfer (LMCT), and photocatalytic processes. Moreover, the improvement in performance of contaminant degradation in these processes compared with those in the absence of light are summarized. Finally, we conclude this review by proposing critical challenges and future perspectives for developing efficient photo-activated PS-based AOPs toward improvement in water treatment and remediation.
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•The recent advances in ZVI are reviewed.•A review of contaminants removed by ZVI from groundwater and wastewater is made.•Excellent removal efficiencies of contaminants by ZVI are reported.•Reaction ...mechanisms of ZVI with contaminants are discussed.•The review suggests research needs for future work.
Recent industrial and urban activities have led to elevated concentrations of a wide range of contaminants in groundwater and wastewater, which affect the health of millions of people worldwide. In recent years, the use of zero-valent iron (ZVI) for the treatment of toxic contaminants in groundwater and wastewater has received wide attention and encouraging treatment efficiencies have been documented. This paper gives an overview of the recent advances of ZVI and progress obtained during the groundwater remediation and wastewater treatment utilizing ZVI (including nanoscale zero-valent iron (nZVI)) for the removal of: (a) chlorinated organic compounds, (b) nitroaromatic compounds, (c) arsenic, (d) heavy metals, (e) nitrate, (f) dyes, and (g) phenol. Reaction mechanisms and removal efficiencies were studied and evaluated. It was found that ZVI materials with wide availability have appreciable removal efficiency for several types of contaminants. Concerning ZVI for future research, some suggestions are proposed and conclusions have been drawn.
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•Step-scheme SnFe2O4/ZnFe2O4 heterojunctions were synthesized by solvothermal method.•SnFe2O4/ZnFe2O4 exhibits visible-light response, nontoxicity and strong magnetism.•5% ...SnFe2O4/ZnFe2O4 shows the highest visible-light photocatalytic degradation activity.•Toxicity of tetracycline was effectively eliminated by photocatalysis of SnFe2O4/ZnFe2O4.•Step-scheme SnFe2O4/ZnFe2O4 is promising in efficacious wastewater treatment.
An efficient and environmentally-friendly photocatalytic degradation process by nontoxic and easily-recovered photocatalysts is desirable for wastewater treatment. Step-scheme (S-scheme) SnFe2O4/ZnFe2O4 nano-heterojunctions with nontoxicity, strong magnetism, and high visible-light photocatalytic performance were synthesized by one-step solvothermal strategy, and SnFe2O4 content was optimized. 5% SnFe2O4/ZnFe2O4 heterojunctions exhibited optimal visible-light photocatalytic performance in degrading tetracycline with 93.2% removal efficiency, and the COD of actual pharmaceutical wastewater decreased by 77.5% due to the efficient production of active species. Moreover, 5% SnFe2O4/ZnFe2O4 heterojunctions is very convenient to be recycled from solution using external magnetic field, and the photocatalytic activity remains almost unchanged in repeated use due to its strong magnetism and high stability. More significantly, the toxicity evaluation confirmed nontoxicity of SnFe2O4/ZnFe2O4 and rapid toxicity elimination process of tetracycline by photocatalysis, indicating that environment-friendly and magnetically recoverable S-scheme SnFe2O4/ZnFe2O4 heterojunction is a promising photocatalyst for wastewater treatment.
Heterogeneous Fenton catalysts are emerging as excellent materials for applications related to water purification. In this review, recent trends in the synthesis and application of heterogeneous ...Fenton catalysts for the abatement of organic pollutants and disinfection of microorganisms are discussed. It is noted that as the complexity of cell wall increases, the resistance level towards various disinfectants increases and it requires either harsh conditions or longer exposure time for the complete disinfection. In case of viruses, enveloped viruses (e.g. SARS-CoV-2) are found to be more susceptible to disinfectants than the non-enveloped viruses. The introduction of plasmonic materials with the Fenton catalysts broadens the visible light absorption efficiency of the hybrid material, and incorporation of semiconductor material improves the rate of regeneration of Fe(II) from Fe(III). A special emphasis is given to the use of Fenton catalysts for antibacterial applications. Composite materials of magnetite and ferrites remain a champion in this area because of their easy separation and reuse, owing to their magnetic properties. Iron minerals supported on clay materials, perovskites, carbon materials, zeolites and metal-organic frameworks (MOFs) dramatically increase the catalytic degradation rate of contaminants by providing high surface area, good mechanical stability, and improved electron transfer. Moreover, insights to the zero-valent iron and its capacity to remove a wide range of organic pollutants, heavy metals and bacterial contamination are also discussed. Real world applications and the role of natural organic matter are summarised. Parameter optimisation (e.g. light source, dosage of catalyst, concentration of H2O2 etc.), sustainable models for the reusability or recyclability of the catalyst and the theoretical understanding and mechanistic aspects of the photo-Fenton process are also explained. Additionally, this review summarises the opportunities and future directions of research in the heterogeneous Fenton catalysis.
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•Generalised mechanisms of electron transfer in Fenton catalysts.•Heterogeneous photo-Fenton for the removal of bacteria and viruses.•Advanced hybrid catalysts of MOFs, perovskites, g-C3N4 etc.•ZVI for removing heavy metal ions and halogenated organic compounds.•Future research directions for improving heterogeneous Fenton catalysis.
This study investigated the effects of metals (Fe3+, Cu2+, Ni2+, and Zn2+) and phenolic compounds (PCs: hydroquinone, catechol, and phenol) loaded on biomass on the formation of persistent free ...radicals (PFRs) in biochar. It was found that metal and phenolic compound treatments not only increased the concentrations of PFRs in biochar but also changed the types of PFRs formed, which indicated that manipulating the amount of metals and PCs in biomass may be an efficient method to regulate PFRs in biochar. These results provided direct evidence to elucidate the mechanism of PFR formation in biochar. Furthermore, the catalytic ability of biochar toward persulfate activation for the degradation of contaminants was evaluated. The results indicated that biochar activates persulfate to produce sulfate radicals (SO4 •–) and degraded polychlorinated biphenyls (PCBs) efficiently. It was found that both the concentration and type of PFRs were the dominant factors controlling the activation of persulfate by biochar and that superoxide radical anions account for 20–30% of sulfate radical generation in biochar/persulfate. This conclusion was supported by linear correlations between the concentration of PFRs consumed and the formation of SO4 •– and between λ (λ = formed sulfate radicals/consumed PFRs) and g-factors. The findings of this study provide new methods to manipulate PFR concentration in biochar for the transformation of contaminants and development of new alternative activators for persulfate-based remediation of contaminated soils.
Photocatalytic mechanism of 2.5% Ag/AgIn5S8 photocatalyst.
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•2.5% Ag/AgIn5S8 was fabricated via solvothermal method and further photo-reduction approach.•2.5% Ag/AgIn5S8 heterojunction ...shows the highest visible-light photocatalytic activity.•The degradation pathways of TCHCl and photocatalytic mechanism were proposed.•2.5% Ag/AgIn5S8 can treat real pharmaceutical industry wastewater effectively.
The deposition of Ag nanoparticles (NPs) on semiconductors has been demonstrated to be an efficient route to improve the separation of photogenerated electrons and holes due to plasma resonance effect, but the photocatalytic efficiency of the available Ag-based photocatalysts is still low and far from practical application. In this study, a novel photocatalyst with Ag NPs deposited on the surface of AgIn5S8 (Ag/AgIn5S8) was fabricated via solvothermal method and further photo-reduction approach. The amount of deposited Ag nanoparticles has an obvious effect on the charge separation and visible-light photocatalytic activity of Ag/AgIn5S8, and 2.5% Ag/AgIn5S8 nanocomposites exhibit the highest visible-light photocatalytic activity with 95.3% degradation efficiency of tetracycline hydrochloride (TCHCl) compared with that of the other samples due to the surface plasmon resonance of Ag NPs, proper bandgap of AgIn5S8 and the synergistic effect between them. The main reactive species in TCHCl degradation are OH and O2−. The possible degradation pathway of TCHCl and photocatalytic mechanism of Ag/AgIn5S8 were proposed according to high performance liquid chromatography-tandem mass spectrometry (HPLC-MS) analysis, main reactive species, and conduction band and valence band of AgIn5S8. Moreover, Ag/AgIn5S8 nanohybrids were applied to treat real pharmaceutical industry wastewater, and it was found that the mineralization efficiency and COD removal of real pharmaceutical industry wastewater can reach 56.3% and 77.6%, respectively. The above results indicate Ag/AgIn5S8 photocatalysts have a promising prospect in the treatment of real pharmaceutical industry wastewater.
Microplastics were demonstrated to be an environmental sink for hydrophobic organic pollutants, while they can also serve as a potential source of such pollutants. In this study, the sorption and ...release of bisphenol A in marine water were investigated through laboratory experiments. Sorption and desorption isotherms were developed, and the results reveal that sorption and desorption depend on the crystallinity, elasticity, and hydrophobicity of the polymer concerned. The adsorption and partition of bisphenol A can be quantified using a dual-mode model of the sorption mechanisms. Polyamide and polyurethane were found to exhibit the highest sorption capacity for bisphenol A, and it was almost irreversible, probably due to hydrogen bonding. Polyethylenes and polypropylene exhibited high and reversible sorption without noticeable desorption hysteresis. Glassy polystyrene, poly(vinyl chloride), poly(methyl methacrylate), and poly(ethylene terephthalate) exhibited low sorption capacity and only partial reversibility. Low-density polyethylene and polycarbonate microplastic particles were for the first time proved to be a persistent source releasing bisphenol A into aquatic environments. Salinity, pH, coexisting estrogens, and water chemistry influence the sorption/desorption behaviors to different degrees. Plastic particles can serve as transportation vectors for bisphenol A, which may constitute an ecological risk.
The combination of peroxymonosulfate (PMS) and electrolysis with an activated carbon fiber (ACF) as cathode (E-ACF-PMS) was systematically investigated. A synergistic effect was observed in the ...E-ACF-PMS process. Compared with the E-ACF-PDS process, the E-ACF-PMS process spent one-third as much energy for elimination of carbamazepine (CBZ). Increased PMS concentration, current density, and pH value significantly enhanced CBZ elimination. It was also noted that the presence of phosphate (PO43−), bicarbonate (HCO3−), and humic acid (HA) inhibited CBZ removal, while the presence of chloride ion (Cl−) accelerated it. According to radical scavenging experiments and the estimation of relative contribution, reactive oxygen species oxidation (including OH, SO4•−, and 1O2) played an important role in CBZ degradation, accounting for 75.67%. We systematically explored the production mechanism for 1O2 and the results demonstrated that 1O2 was mainly generated on the cathode, rather than generated by O2•− or O2 reported by other researchers. Possible degradation pathways for CBZ in E-ACF-PMS process were also proposed. Finally, the potential for practical applications was explored and compared with E-ACF-PDS. The results of SEM images, BET, and nitrogen adsorption isotherm before and after ACF reuse for 50 times suggested that ACF could maintain its adsorption capacity and catalytic ability in the E-ACF-PMS process. Testing also suggested that the protection of ACF in electrochemical oxidation was based on its relatively high current intensity and removal efficiency. The removal efficiencies of other organic pollutants, including nitrobenzene (NB), sulfamethoxazole (SMX), diclofenac (DC), and tetracycline (TC) were also evaluated. In addition, experiments were conducted to study the effects of different water matrices and toxicology implications and results demonstrated that substituting PMS for PDS in an E-ACF system could create a more efficient, sustainable, and with less secondary toxicity process for wastewater treatment.
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•E-ACF-PMS exhibited excellent removal efficiency for refractory organic pollutants.•Reactive oxygen species oxidation played the dominant role in E-ACF-PMS process.•Hydroxyl radical could react with PMS to generate singlet oxygen on cathode.•ACF could maintain adsorption and catalytic ability after 50 times of reuse.•E-ACF-PMS process exhibited energy-saving, sustainability and hypotoxicity
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