Some issues limiting photo(cata)lysis application in water pollutant control: A critical review from chemistry perspectives

Macrocyclic porphyrin photocatalysts without metal chelation: A novel pathway for complete degradation of tough halophenols with longwave visible LED light source

Halophenols are toxic and persistent pollutants in water environments which poses harm to various organisms. Due to their high stability and long residence time, ultraviolet radiation, heavy metals and oxidizing agents have been largely adopted on treating these compounds. However, these treatment methods could pose toxicity or hazardous risks to the marine environment and plant operators. In this study, a water-soluble porphyrin photocatalyst was synthesized and introduced for halophenol treatment using UV-free LED white light. The porphyrin catalyst is a macrocyclic ring consisting of pyrroles linked with methine bridges, the highly conjugated ring provided the superior functionality of visible light absorption. Surprisingly, over 99 % degradation of halophenols and over 90 % dehalogenation have been achieved without metal chelation, even higher than those of transition metal porphyrins with inclusion of Fe3+, Zn2+, Cu2+, Co2+, Ni2+, and Mn2+. Ring-opening reactions were confirmed with the formation of carboxylic acids; dicarboxylic acids like acrylic acid, and malonic acid; while fumaric acid was the main product. Total organic carbon results indicated no CO2 produced during the reaction. Triplet absorbance and scavenger studies also indicated that singlet oxygen and conduction band electrons are the main radical species for halophenol degradation. The 100-fold singlet emission quenching over triplet absorption quenching indicated that the excited electrons tend to be transferred via singlet state. This concept brings along new approaches detoxifying halophenol-related wastewater without UV, metals and other additives, which is more environmentally-friendly and sheds light to the conversion of toxic materials into useful chemical precursors.

Challenging established norms: The unanticipated role of alcohols in UV/PDS radical quenching

UV/peroxydisulfate (UV/PDS) process is known to be highly efficient for degrading micropollutants from water by generating sulfate (SO4•-) and hydroxyl radicals (HO•). Reliable analyses of short-lived SO4•- and HO• are therefore critical for understanding reaction mechanisms and optimizing operating conditions. Currently, alcohols are commonly used as quenchers to distinguish radicals based on the assumption that they exclusively react with target radicals without other influences. However, this study for the first time reveals a series of unexpected effects that challenge this conventional wisdom because: 1) adding alcohols altered the decomposition rates of PDS by replacing the reactions between SO4•- and HO• with PDS by the reactions between secondary reactive species and PDS; and 2) SO4•- preferably reacted with alcohols to generate nonnegligible level of hydrogen peroxide (H2O2) under oxygen-rich conditions, which subsequently led to indirect formation of HO•. Additionally, the formation of H2O2 was substantially impacted by the types of alcohols, dosages, dissolved oxygen, and solution pH. Using probe tests as tools, we found that the actual SO4•- levels after dosing alcohols were only slightly different from assumed/expected levels, whereas the actually HO• levels were 43.7, 3364.9, and 12.5 times higher than assumed/expected conditions for samples dosed with methanol, iso-propanol, and tert-butanol, respectively. These unanticipated effects thus suggest that cautions are needed when using alcohols to qualitative and quantitative determine HO• and SO4•- in UV/PDS process.

Vacuum-ultraviolet based advanced oxidation and reduction processes for water treatment

The use of vacuum-ultraviolet (VUV) photolysis in water treatment has been gaining significant interest due to its efficacy in degrading refractory organic contaminants and eliminating oxyanions. In recent years, the reactive species driving pollutant decomposition in VUV-based advanced oxidation and reduction processes (VUV-AOPs and VUV-ARPs) have been identified. This review aims to provide a concise overview of VUV photolysis and its advancements in water treatment. We begin with an introduction to VUV irradiation, followed by a summary of the primary reactive species in both VUV-AOPs and VUV-ARPs. We then explore the factors influencing VUV-photolysis in water treatment, including VUV irradiation dose, catalysts or activators, dissolved gases, water matrix components (e.g., DOM and inorganic anions), and solution pH. In VUV-AOPs, the predominant reactive species are hydroxyl radicals (˙OH), hydrogen peroxide (H2O2), and ozone (O3). Conversely, in VUV-ARPs, the main reactive species are the hydrated electron (eaq-) and hydrogen atom (˙H). It is worth noting that VUV-based advanced oxidation/reduction processes (VUV-AORPs) can transit between VUV-AOPs and VUV-ARPs based on the externally added chemicals and dissolved gases in the solution. Increase of the VUV irradiation dose and the concentration of catalysts/activators enhances the degradation of contaminants, whereas DOM and inorganic anions inhibit the reaction. The pH influences the redox potential of ˙OH, the speciation of contaminants and activators, and thus the overall performance of the VUV-AOPs. Conversely, an alkaline pH is favored in VUV-ARPs because eaq- predominates at higher pH.

Assessing the Chemical-Free Oxidation of Trace Organic Chemicals by VUV/UV as an Alternative to Conventional UV/H2O2

Low-pressure mercury lamps with high-purity quartz can emit both vacuum-UV (VUV, 185 nm) and UV (254 nm) and are commercially available and promising for eliminating recalcitrant organic pollutants. The feasibility of VUV/UV as a chemical-free oxidation process was verified and quantitatively assessed by the concept of H2O2 equivalence (EQH2O2), at which UV/H2O2 showed the same performance as VUV/UV for the degradation of trace organic contaminants (TOrCs). Although VUV showed superior H2O activation and oxidation performance, its performance highly varied as a function of light path length (Lp) in water, while that of UV/H2O2 proportionally decreased with decreasing H2O2 dose regardless of Lp. On increasing Lp from 1.0 to 3.0 cm, the EQH2O2 of VUV/UV decreased from 0.81 to 0.22 mM H2O2. Chloride and nitrate hardly influenced UV/H2O2, but they dramatically inhibited VUV/UV. The competitive absorbance of VUV by chloride and nitrate was verified as the main reason. The inhibitory effect was partially compensated by •OH formation from the propagation reactions of chloride or nitrate VUV photolysis, which was verified by kinetic modeling in Kintecus. In water with an Lp of 2.0 cm, the EQH2O2 of VUV/UV decreased from 0.43 to 0.17 mM (60.8% decrease) on increasing the chloride concentration from 0 to 15 mM and to 0.20 mM (53.5% decrease) at 4 mM nitrate. The results of this study provide a comprehensive understanding of VUV/UV oxidation in comparison to UV/H2O2, which underscores the suitability and efficiency of chemical-free oxidation with VUV/UV.

Progress in environmental monitoring and mitigation strategies for herbicides and insecticides: A comprehensive review

Herbicides and insecticides are pervasively applied in agricultural sector to increase the yield by controlling or eliminating bug vermin and weeds. Although, resistance development occurs, direct and indirect impact on human health and ecosystem is clearly visible. Normally, herbicides and pesticides are water soluble in nature; accordingly, it is hard to decrease their deadliness and to dis-appear them from the environment. They are profoundly specific, and considered as poisonous to various peoples in agricultural and industrial work places. In order to substantially reduce the harmful impacts, it is crucial to thoroughly examine the detection and mitigation measures for these compounds. The primary objective of this paper is to provide an overview of various herbicide and pesticide detection techniques and associated remedial techniques. A short summary on occurrence and harmful effects of herbicides/insecticides on ecosystem has been included to the study. The conventional and advanced, rapid techniques for the detection of insecticides and herbicides were described in detail. A detailed overview on several mitigation strategies including advanced oxidation, adsorption, electrochemical process, and bioremediation as well as the mechanism behind the strategic approaches to reduce the effects of growing pesticide pollution has been emphasized. Regardless of the detection techniques and mitigation strategies, the recent advances employed, obstacles, and perspectives have been discussed in detail.

Comparative study of Fe2+/H2O2 and Fe2+/persulfate systems on the pre-treatment process of real pharmaceutical wastewater

Advanced oxidation technologies based on hydroxyl radical (•OH) and sulfate radical (SO4-•) are two common types of advanced oxidation technologies, but there are not many reports on the application of advanced oxidation methods in actual wastewater pretreatment. This article compares the pre-treatment performance of Fe2+/H2O2 and Fe2+/Persulfate systems in actual pharmaceutical wastewater, and combines EEM, GC-MS, and toxicity testing results to explore the differences in TOC, COD, and NH3-N removal rates, optimal catalyst dosage, applicable pH range, toxicity of effluent after reaction, and pollutant structure between the two systems. The results indicate that the Fe2+/H2O2 system has a higher pollutant removal rate (TOC: 71.9%, COD: 66.9%, NH3-N: 34.1%), but also requires a higher catalyst (Fe2+) concentration (6.0 g/L), and its effluent exhibits characteristic peaks of aromatic proteins. The Fe2+/Persulfate system has a wider pH range (pH ≈ 3-7) and is more advantageous in treating wastewater containing more cyclic organic compounds, but the effluent contains some sulfur-containing compounds. In addition, toxicity tests have shown that the toxicity reduction effect of the Fe2+/Persulfate system is stronger than that of the Fe2+/H2O2 system.

Thermally activated persulfate (TAP)-enhanced tris(2-chloroethyl) phosphate removal in real-world waters based on a response-surface approach as well as toxicological evaluation on its degradation products

As a typical organophosphorus flame retardant, tris(2-chloroethyl) phosphate (TCEP) is refractory in aqueous environment. The application of TAP is a promising method for removing pollutants. Herein, the removal of TCEP using TAP was rigorously investigated, and the effects of some key variables were optimized by the one-factor-at-a-time approach. To further evaluate the interactions among variables, the response surface methodology (RSM) based on central composite design was employed. Under optimized conditions (pH 5, [PS]0: [TCEP]0 = 500:1), the maximum removal efficiency (RE) of TCEP reached up to 90.6%. In real-world waters, the RE of TCEP spanned the range of 56%- 65% in river water, pond water, lake water and sanitary sewage. The low-concentration Cl- (0.1 mM) promoted TCEP degradation, but the contrary case occurred when the high-concentration Cl-, NO3-, CO32-, HCO3-, HPO42-, H2PO4-, NH4+ and humic acid were present owing to their prominently quenching effects on SO4•-. Both EPR and scavenger experiments revealed that the main radicals in the TAP system were SO4•- and •OH, in which SO4•- played the most crucial role in TCEP degradation. GC-MS/MS analysis disclosed that two degradation products appeared, sourcing from the replacement, oxidation, hydroxylation and water-molecule elimination reactions. The other two products were inferred from the comprehensive literature. As for acute toxicity to fish, daphnid and green algae, product A displayed the slightly higher toxicity, whereas other three products exhibited the declining toxicity as compared to their parent molecule. These findings offer a theoretical/practical reference for high-efficiency removal of TCEP and its ecotoxicological risk evaluation.

Photodegradation of halogenated organic disinfection by-products: Decomposition and reformation

In this study, the photodegradation of 33 different DBPs (trihalomethanes, haloacetic acids, haloacetaldehydes, and haloacetonitriles) and TOX with low pressure UV light and the subsequent reformation of DBPs with chlorine and monochloramine were investigated. Results indicated that photodegradation followed the order of TOI > TOBr > TOCl, and treated surface water with low SUVA254 background did not impact the photodegradation of highly UV susceptible DBPs such as triiodomethane (TIM), diiodobromomethane (DIBM), tribromomethane (TBM). The mass balance results of chloride, bromide and iodide showed that the main photodegradation mechanism of TOBr and TOI was dehalogenation supported by halide releases (i.e., Cl-, Br- and/or I- ion). In addition, the photodegradation removal effect was higher, when brominated DBPs formation was high. Although low pressure UV light effectively removed halogenated organic DBPs, subsequent use of disinfectants (Cl2 and NH2Cl) reformed photodegraded DBPs, and the overall DBPs concentrations were increased, which suggested that the released Br- and I- ions will reform DBPs in distribution systems, with oxidants present or added (e.g., booster chlorination) in distribution systems. This study showed that although UV photodegradation will reduce halogenated organic DBPs in distribution systems, especially more toxic iodinated and brominated DBPs, it will be a more effective technology towards the end of the distribution system or a point of entry solution rather than in distribution system with post-disinfection and residence time.

Radical-Mediated Photocatalysis for Lignocellulosic Biomass Conversion into Value-Added Chemicals and Hydrogen: Facts, Opportunities and Challenges

Photocatalytic biomass conversion into high-value chemicals and fuels is considered one of the hottest ongoing research and industrial topics toward sustainable development. In short, this process can cleave Cβ -O/Cα -Cβ bonds in lignin to aromatic platform chemicals, and further conversion of the polysaccharides to other platform chemicals and H2 . From the chemistry point of view, the optimization of the unique cooperative interplay of radical oxidation species (which are activated via molecular oxygen species, ROSs) and substrate-derived radical intermediates by appropriate control of their type and/or yield is key to the selective production of desired products. Technically, several challenges have been raised that face successful real-world applications. This review aims to discuss the recently reported mechanistic pathways toward selective biomass conversion through the optimization of ROSs behavior and materials/system design. On top of that, through a SWOT analysis, we critically discussed this technology from both chemistry and technological viewpoints to help the scientists and engineers bridge the gap between lab-scale and large-scale production.

Mechanistic insight into humic acid-enhanced sonophotocatalytic removal of 17β-estradiol: Formation and contribution of reactive intermediates

In this study, humic acid (HA) enhanced 17β-estradiol (17β-E2) degradation by Er3+-CdS/MoS2 (ECMS) was investigated under ultrasonic and light conditions. The degradation reaction rate of 17β-E2 was increased from (14.414 ± 0.315) × 10-3 min-1 to (122.677 ± 1.729) × 10-3 min-1 within 90 min sonophotocatalytic (SPC) reaction with the addition of HA. The results of quenching coupled with chemical probe experiments indicated that more reactive intermediates (RIs) including reactive oxygen species (ROSs) and triplet-excited states were generated in the HA-enhanced sonophotocatalytic system. The triplet-excited states of humic acid (3HA*), hydroxyl radical (•OH), and superoxide radical (•O2-) were the dominant RIs for 17β-E2 elimination. In addition, the energy- and electron-transfer process via coexisting HA also account for 12.86% and 29.24% contributions, respectively. The quantum yields of RIs in the SPC-ECMS-HA system followed the order of 3HA* > H2O2 > 1O2 > •O2-> •OH. Moreover, the spectral and fluorescence characteristics of HA were further analyzed during the sonophotocatalytic reaction process. The study expanded new insights into the comprehension of the effects of omnipresent coexisting HA and RIs formation for the removal of 17β-E2 during the sonophotocatalytic process.

Degradation efficiency and mechanism of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) by thermally activated persulfate system

Polybrominated diphenyl ethers (PBDEs) as a typical brominated flame retardant (BFR) have attracted worldwide attention due to the high environmental risk and resistance to conventional remediation processes. In this study, thermally activated persulfate (TAP) process was applied to degrade 2,2',4,4'-tetrabromodiphenyl ether (BDE-47), which is the most toxic and representative PBDEs in e-waste dismantling sites. Impact factors such as PDS dosage, heating temperature, and initial pH were evaluated. Results showed that BDE-47 can be 100% degraded within 180 min under the condition of PDS:BDE-47 = 1000:1, 60 °C, and pH = 7. Quenching experiments combined with EPR analysis further proved the important role of SO₄^·- in oxidating BDE-47. According to high-resolution mass spectrometry (HRMS) analysis, only one oxidation product of low toxicity was detected during the oxidation process. Theoretical calculations further revealed that the oxidation process mainly involved radical attack at C-Br bond, cleavage of C-Br bond, and fission of ether bond, and HSO₄· may also play an important role in BDE-47 degradation in TAP system. In addition, TAP system exhibited universality as all selected PBDE congeners can be degraded, and the degradation rate of PBDEs was greatly affected by the number of substituted Br atoms in a negative trend. Overall, these findings indicate that TAP can be applied as an effective method for removal of PBDEs, and we provide a new insight for the practical application of TAP technology in BDE-47 degradation from experimental and theoretical aspects.

Enhanced degradation of oxytetracycline in aqueous solution by DBD plasma-coupled vacuum ultraviolet/ultraviolet (VUV/UVC) system

A systematic investigation of coupling dielectric barrier discharge (DBD) plasma and different ultraviolet bands (UVA, UVB, UVC, and VUV) was constructed for antibiotic-contaminant wastewater treatment. Compared with DBD, UV, or other combined DBD/UV systems, the DBD/VUV/UVC system exhibited excellent degradation and mineralization efficiencies for oxytetracycline (OTC), achieving 93.2% removal rate (reaction rate constant 1.05 min^-1) and higher decarbonization efficiency (mineralization rate 0.47 mg C min^-1) within 2.5 min treatment. The radical quenching tests revealed that HO⋅, [Formula: see text] , and ¹O₂ were all involved in the decomposition of OTC in the DBD/VUV/UVC system, among which [Formula: see text] played a dominant role. Possible degradation pathways of OTC in the DBD/VUV/UVC process were proposed using density functional theory and detected intermediates. Four indexes were used to assess the toxicity of OTC and its degraded intermediates. The inorganic anions and HA slightly reduced the degradation efficiency of the DBD/VUV/UVC system. This research provides new ideas to broaden the application of plasma and alleviate the water environment crisis.

Valorization of lignocellulosic biomass into sustainable materials for adsorption and photocatalytic applications in water and air remediation

An exponential rise in global pollution and industrialization has led to significant economic and environmental problems due to the insufficient application of green technology for the chemical industry and energy production. Nowadays, the scientific and environmental/industrial communities push to apply new sustainable ways and/or materials for energy/environmental applications through the so-called circular (bio)economy. One of today's hottest topics is primarily valorizing available lignocellulosic biomass wastes into valuable materials for energy or environmentally related applications. This review aims to discuss, from both the chemistry and mechanistic points of view, the recent finding reported on the valorization of biomass wastes into valuable carbon materials. The sorption mechanisms using carbon materials prepared from biomass wastes by emphasizing the relationship between the synthesis route or/and surface modification and the retention performance were discussed towards the removal of organic and heavy metal pollutants from water or air (NO_x, CO₂, VOCs, SO₂, and Hg⁰). Photocatalytic nanoparticle-coated biomass-based carbon materials have proved to be successful composites for water remediation. The review discusses and simplifies the most raised interfacial, photonic, and physical mechanisms that might take place on the surface of these composites under light irradiation. Finally, the review examines the economic benefits and circular bioeconomy and the challenges of transferring this technology to more comprehensive applications.

Roles of radical species in vacuum-UV/UV/peroxydisulfate advanced oxidation processes and contributions of the species to contaminant degradation at different water depths

Vacuum-UV (VUV) (wavelength 185 nm)/ UV (wavelength 254 nm) are applied to improve performances of UV-based advanced oxidation processes. However, the improvements were strongly affected by water depth because of poor VUV transmittance in water. In this study, VUV/UV and peroxydisulfate (PDS) were used to degrade carbamazepine. More SO₄^•- oxidation occurred in VUV/UV/PDS than VUV/UV with similar ^•OH oxidation occurring. The additional SO₄^•- oxidation could be caused by VUV/PDS in superficial water or UV/PDS in deeper water. The synergistic factor for VUV/UV/PDS processes relative to VUV/UV and UV/PDS processes was 1.32. VUV/UV/PDS performances were affected by competition for photon absorption by dissolved organic matter (32-58 % inhibition), radical quenching by CO₃^2-/HCO₃^- and NO₃^-, and conversion of ^•OH and SO₄^•- into reactive chlorine species by Cl^-. Radical probe experiments and steady-state kinetic modeling simulations indicated that 34 %, 25 %, and 40 % of carbamazepine degradation occurring in 2-cm-deep bulk solution was due to ^•OH oxidation through VUV/H₂O, SO₄^•- oxidation through VUV/PDS, and SO₄^•- oxidation through UV/PDS, respectively. Contribution of VUV-driven processes decreased with increasing water depth and became equivalent to contribution of 3.5-cm-deep UV-driven processes, which indicated the importance of optimizing water depth in VUV/UV-advanced oxidation process reactors.

Treatment of refractory organics in biologically treated landfill leachate by a zero valent iron enhanced Peroxone process: Degradation efficiency and mechanism study

A zero valent iron (ZVI) enhanced Peroxone process (ZVI/Peroxone) was used to treat biologically treated landfill leachate (BTL). The treatment efficiency of the ZVI/Peroxone process was compared to single (ZVI, O₃ and H₂O₂) and dual (ZVI/H₂O₂, Fe⁰/O₃ and Peroxone) processes. The results showed that ZVI can greatly enhance the treatment capability of the Peroxone process, and the color number (CN), absorbance at 254 nm (UV₂₅₄), and total organic carbon (TOC) removal efficiencies were 98.82, 84.30 and 66.38%, respectively. In the ZVI/Peroxone process, higher O₃ and ZVI dosages improved organics removal, and H₂O₂ could promote organics removal within a certain dosage range. However, too much H₂O₂ decreased treatment efficiency. The best treatment performance by the ZVI/Peroxone process was obtained under acidic conditions. The three-dimensional excitation and emission matrix analysis showed that BTL mainly contained two fluorescent substances, which were fulvic-like substances in the ultraviolet region (Ex/Em = 235-255 nm/410-450 nm) and fulvic-like substances in the visible light region (Ex/Em = 310-360 nm/370-450 nm). Fluorescent substances could be substantially degraded by the ZVI/Peroxone process during the early stages of the reaction. An analysis of ZVI morphology and element valency changes showed that the micro Fe⁰ particles used in this study remained highly reactive during the process. The ZVI enhanced the homogenous Fenton, heterogeneous Fenton, and coagulation-flocculation effects during the Peroxone process.

Dye mineralization under UV/H2O2 promoted by chloride ion at high concentration and the generation of chlorinated byproducts

Chloride ion (Cl^-) may promote or inhibit the oxidation of specific organic compounds treated by hydroxyl radical based advanced oxidation processes (HR-AOPs) depending on the reactivity of chlorine radicals towards the organics. However, the effects of high contents of Cl^- on the removal of total organic compounds (TOC) in high salinity organic wastewater treated by HR-AOPs were unclear. The removal and mineralization of azo dye Orange II (OrgII) by UV/H₂O₂ process with Cl^- at high contents under various pH conditions were investigated. As the pH conditions increased higher than pH 5, TOC removal rates increased slightly possibly related to the increase of O₂^- production and the reduce of futile decomposition of H₂O₂ into O₂. Cl^- at relative high concentration (1000 and 2000 mM) significantly promoted the mineralization of dyes with TOC removal increasing by 10 %-40 % under both acid and alkaline conditions. The proposed mechanism is that the reaction of Cl^- with OH would decline the decomposition of H₂O₂ into O₂ by inhibiting the reaction between OH and H₂O₂, and the generated chlorine species (Cl and Cl₂^-) could further promote the oxidation of dye molecules into intermediates and be helpful for the subsequent mineralization process. In addition, H₂O₂ and Cl^- can slowly react to give HClO and ClO^-, which may partly contribute to the decolorization and mineralization of OrgII. Meanwhile, an appropriate relative proportion between Cl₂^- and OH depending on Cl^- contents and pH conditions is important to enhance the TOC removal. However, the formation of various chlorinated byproducts especially under alkaline condition may increase the risk of environmental pollution accidents. The results demonstrate the promotion of TOC removal by UV/H₂O₂ under certain high contents of Cl^- and provide new insight into the application of HR-AOPs to the pretreatment of high salinity organic wastewater.

VUV/UV oxidation performance for the elimination of recalcitrant aldehydes in water and its variation along the light-path

Vacuum ultraviolet/ultraviolet (VUV/UV) oxidation using a low-pressure mercury lamp emitting dual wavelengths (185 nm (VUV) and 254 nm (UV)) significantly varies in performance along the light-path (l_P), which has not been fully characterized. Therefore, VUV/UV oxidation in solution was investigated at various l_P in terms of the degradation kinetics and mineralization pathway of representative aldehydes with various alkyl-chain lengths. Oxidative degradation of parent aldehydes with shorter alkyl chains was less efficient, specifically the pseudo-zero-order rate constant (k_obs) of formaldehyde was only 51% of that of propionaldehyde (k_obs = 0.078 μM s^-1). In contrast, the mineralization of aldehydes with longer alkyl chains was less efficient because these aldehydes underwent mineralization into more refractory carboxylic byproducts, e.g., oxalic acid. VUV was mainly absorbed by superficial water (l_P < 0.55 cm), which resulted in highly heterogeneous oxidation in homogeneous water. Thus, k_obs of acetaldehyde dramatically decreased from 0.13 to 0.033 μM s^-1 as the total l_P of solution increased from 1.0 to 3.0 cm. On the basis of mineralization pathways proposed above, an iterative kinetic model was developed to characterize the degradation of parent aldehydes and the formation of carboxylic acids along l_P. This model predicted the VUV/UV oxidaton for the first time by considering the fast diffusion of pollutants by limited diffusion of transient radical species. Thus, it realized the prediction of ^•OH concentration at specific water solution and byproduct evolution within specific water solution in turbulent flow regime, wherein ^•OH was predominantly formed in superficial water-layers wherein ^•OH in water-layers of l_P <0.16 cm and <0.81 cm contributed to 50% and 90% of the total oxidation performance, respectively. This result would help to improve the VUV-UV-reactor design in terms of optimizing the thickness of water-layer and turbulence of water-flow.

Degradation of geosmin and 2-methylisoborneol in UV-based AOPs for photoreactors with reflective inner surfaces: Kinetics and transformation products

Geosmin (GSM) and 2-methylisoborneol (2-MIB) are representative musty/earthy odor compounds commonly present in surface water. In present study, the degradation of GSM and 2-MIB subject to different UV-based advanced oxidation processes (AOPs), including UV/H₂O₂, UV/S₂O₈^2-, UV/chlorine, and UV/chloramine, in a phosphate-buffered saline (PBS) was conducted in a photoreactor with reflective inner surfaces and compared with that in an environmental water sample. A dynamic model to predict the degradation of GSM and 2-MIB in the photoreactor with reflective inner surfaces in the four UV-based AOPs was developed applying the second-order rate constants for the GSM and 2-MIB with primary reactive species (i.e., •OH, •Cl, and •SO₄^-) determined in this study. The model was proven to successfully simulate the degradation of GSM and 2-MIB. In addition, 8, 7, 8, and 11 degradation intermediates were detected from UV/H₂O₂, UV/S₂O₈^2-, UV/chlorine, and UV/chloramine in this study, and possible degradation pathways were proposed. This study is the first to report the degradation kinetics and formation products of GSM and 2-MIB in UV/chloramine. Research based on photoreactors with reflective inner surfaces may provide some guidance for eliminating GSM and 2-MIB in UV-based AOPs for full-scale engineering applications.

Detecting trace dissolved organic nitrogen (DON) in freshwater by converting DON rapidly into nitrate with VUV/H2O2 pretreatment

Dissolved organic nitrogen (DON) plays a key role in many biogeochemistry and engineering processes such as forming nitrogenous disinfection byproducts. However, detecting aqueous DON at trace levels is challenging currently because conventional DON conversion methods have very high method detection limits (MDL). In addition, DON is measured indirectly by subtracting dissolved inorganic nitrogen (DIN) from total dissolved nitrogen (TDN), which can propagate analytical errors of each analyte. In order to solve these issues, we isolated DON from DIN with electrodialysis before and herein tested vacuum ultraviolet (VUV) in tandem with several oxidants to convert DON completely into nitrate for subsequent N analysis. Results showed that H₂O₂ was more suitable than chlorine and persulfate because VUV/chlorine or VUV/persulfate is either inefficient to convert DON or subjected to nonnegligible N loss. To verify the efficiency, we evaluated the effects of typical water and operating variables on the conversions of four model DON compounds and their yields of nitrate. Under optimal conditions (pH 10.3 and 500 mg/L H₂O₂), the process converted DON completely into nitrate within just 60 min. Compared to conventional TDN analytical methods, the VUV/H₂O₂ method features not only better analytical precision but also lower MDL because the formed nitrate can be analyzed at very low MDL by ion chromatography (IC). So, this approach moves one step further to achieve a conceptually new DON analytical method by coupling electrodialysis, VUV/H₂O₂, and IC.

One-Step Carbonization Synthesis of Magnetic Biochar with 3D Network Structure and Its Application in Organic Pollutant Control

In this study, a magnetic biochar with a unique 3D network structure was synthesized by using a simple and controllable method. In brief, the microbial filamentous fungus Trichoderma reesei was used as a template, and Fe3+ was added to the culture process, which resulted in uniform recombination through the bio-assembly property of fungal hyphae. Finally, magnetic biochar (BMFH/Fe3O4) was synthesized by controlling different heating conditions in a high temperature process. The adsorption and Fenton-like catalytic performance of BMFH/Fe3O4 were investigated by using the synthetic dye malachite green (MG) and the antibiotic tetracycline hydrochloride (TH) as organic pollutant models. The results showed that the adsorption capacity of BMFH/Fe3O4 for MG and TH was 158.2 and 171.26 mg/g, respectively, which was higher than that of most biochar adsorbents, and the Fenton-like catalytic degradation effect of organic pollutants was also better than that of most catalysts. This study provides a magnetic biochar with excellent performance, but more importantly, the method used can be effective in further improving the performance of biochar for better control of organic pollutants.

Impact of water matrices on oxidation effects and mechanisms of pharmaceuticals by ultraviolet-based advanced oxidation technologies: A review

The binding between water components (dissolved organic matters, anions and cations) and pharmaceuticals influences the migration and transformation of pollutants. Herein, the impact of water matrices on drug degradation, as well as the electrical energy demands during UV, UV/catalysts, UV/O₃, UV/H₂O₂-based, UV/persulfate and UV/chlorine processes were systemically evaluated. The enhancement effects of water constituents are due to the powerful reactive species formation, the recombination reduction of electrons and holes of catalyst and the catalyst regeneration; the inhibition results from the light attenuation, quenching effects of the excited states of target pollutants and reactive species, the stable complexations generation and the catalyst deactivation. The transformation pathways of the same pollutant in various AOPs have high similarities. At the same time, each oxidant also can act as a special nucleophile or electrophile, depending on the functional groups of the target compound. The electrical energy per order (EEO) of drugs degradation may follow the order of EEO_UV > EEO_UV/catalyst > EEO_UV/H2O2 > EEO_UV/PS > EEO_UV/chlorine or EEO_UV/O3. Meanwhile, it is crucial to balance the cost-benefit assessment and toxic by-products formation, and the comparison of the contaminant degradation pathways and productions in the presence of different water matrices is still lacking.

Global occurrence of SARS-CoV-2 in environmental aquatic matrices and its implications for sanitation and vulnerabilities in Brazil and developing countries

This paper includes a systematic review of the SARS-CoV-2 occurrence in environmental aquatic matrices and a critical sanitation analysis. We discussed the interconnection of sanitation services (wastewater, water supply, solid waste, and stormwater drainage) functioning as an important network for controlling the spread of SARS-CoV-2 in waters. We collected 98 studies containing data of the SARS-CoV-2 occurrence in aquatic matrices around the world, of which 40% were from developing countries. Alongside a significant number of people infected by the virus, developing countries face socioeconomic deficiencies and insufficient public investment in infrastructure. Therefore, our study focused on highlighting solutions to provide sanitation in developing countries, considering the virus control in waters by disinfection techniques and sanitary measures, including alternatives for the vulnerable communities. The need for multilateral efforts to improve the universal coverage of sanitation services demands urgent attention in a pandemic scenario.

Treasuring industrial sulfur by-products: A review on add-value to reductive sulfide and sulfite for contaminant removal and hydrogen production

Reductive sulfur-containing by-products (S-BPs) released from industrial process mainly exist in the simple form of sulfide and sulfite. In this study, recent advances to remove and make full use of reductive S-BPs to achieve efficient contaminant removal and hydrogen production are critically reviewed. Sulfide, serves as both reductant and nucleophile, can form intermediates with the catalyst surface functional group through chemical interaction, efficiently promoting the catalytic reduction process to remove contaminants. Sulfite assisted catalytic process could be classified to the advanced reduction processes (ARPs) and advanced oxidation processes (AOPs), mainly depending on the presence of dissolved oxygen (DO) in the solution. During ARPs, sulfite could generate reductive active species including hydrated electron (e_aq^-), hydrogen radical (H·), and sulfite radical (SO₃^•-) under the irradiation of UV light, leading to the efficient reduction removal of a variety of contaminants. During AOPs, sulfite could first produce SO₃^•- under the action of the catalyst or energy, initiating a series of reactions to produce oxysulfur radicals. Various contaminants could be effectively removed under the role of these oxidizing active species. Sulfides and sulfites could also be removed along with promoting hydrogen production via photocatalytic and electrocatalytic processes. Besides, the present limitations and the prospects for future practical applications of the process with these S-BPs are proposed. Overall, this review gives a comprehensive summary and aims to provide new insights and thoughts in promoting contaminant removal and hydrogen production through taking full advantage of reductive S-BPs.

Degradation of chloromethylisothiazolinone antimicrobial by Vacuum-Ultraviolet/Ultraviolet irradiation: Reactive species, degradation pathway and toxicity evaluation

Chloromethylisothiazolinone (CMIT) has been extensively used as antimicrobial in cosmetics, detergents, wall paints, and anti-fouling products. To prevent the potential ecological and health risks, the degradation mechanisms and toxicity changes of CMIT by Vacuum-Ultraviolet/Ultraviolet (VUV/UV) irradiation were investigated in this study. VUV/UV irradiation showed better performance on CMIT degradation compared to sole UV photolysis. The removal efficiency of CMIT with photon fluence of 0.6 μEinstein/cm² was 8% and 100% by UV or VUV/UV irradiation, respectively. Radical quenching experiments indicated that 254 nm photolysis, 185 nm photolysis, and •OH oxidation contributed to CMIT degradation during VUV/UV process, with fluence-based apparent rate constants of 0.16, 0.13, and 4.9 μEinstein^-1cm², respectively. The formation of H₂O₂ during VUV/UV process increased to 0.7 mg/L at 4.5 min, and the concentration of •OH ranged within 1.0-3.8 × 10^-12 M. The degradation of CMIT by VUV/UV irradiation in neutral condition was slightly higher than that in acidic and basic conditions. The removal efficiency of CMIT with reaction time of 2 min decreased from 92.2% to 34.3% when the concentration of HCO₃^-/CO₃^2- increased to 1 mM. The degradation of CMIT by VUV/UV irradiation in secondary effluents was lower than that in ultrapure water because of the •OH scavenging effects, but still 2.9 times higher than that by UV photolysis. Four main degradation mechanisms of CMIT were observed during VUV/UV process, including the oxidation of sulfur, addition of hydroxyl groups on the double-carbon-bond, demethylation on the nitrogen, and substitution of organochlorine atom by hydroxyl group. Based on the quantitative structure activity relationship analysis, most products of CMIT underwent complete detoxification to fish and daphnia. 40% of products still showed acute toxicity to algae, but most of them were less toxic than CMIT.

Ultraviolet light-activated peroxymonosulfate (UV/PMS) system for humic acid mineralization: Effects of ionic matrix and feasible application in seawater reverse osmosis desalination

The use of membrane-based technology has evolved into an important strategy for supplying freshwater from seawater and wastewater to overcome the problems of water scarcity around the world. However, the presence of natural organic matter (NOM), including humic substances affects the performance of the process. Here, we present a systematic report on the mineralization of humic acid (HA), as a model for NOM, in high concentration of salts using the ultraviolet light-activated peroxymonosulfate (UV/PMS) system as a potential alternative for HA elimination during membrane-based seawater desalination and water treatment processes. Effects of various parameters such as PMS concentration, solution type, pH, anions, and anion-cation matrix on HA mineralization were assessed. The results show that 100%, 78% and 58% of HA (2 mg/L TOC) were mineralized with rate constants of 0.085 min^-1, 0.0073 min^-1, and 0.0041 min^-1 after 180 min reaction time at pH 7 when 0.5 mM PMS was used in deionized water, sodium chloride solution (35,000 ppm) and synthetic seawater, respectively. The reduced efficiency under saline conditions was attributed to the presence of anions in the system that acted as sulfate and hydroxyl radicals' scavengers. Furthermore, the safety of the treated synthetic seawater was evaluated by analyzing the residual transformed products. Overall, pretreatment with the UV/PMS system mitigated fouling on the RO membrane.

Mineralization, characteristics variation, and removal mechanism of algal extracellular organic matter during vacuum ultraviolet/ozone process

Extracellular organic matter (EOM) produced by algal blooms in source water is detrimental to drinking water treatment processes and supplied water quality. Ozonation has been used to treat algal EOM, but it could not mineralize EOM effectively. In this study, mineralization and characteristics variation of EOM by vacuum ultraviolet/ozone (VUV/O₃) and its sub-processes were comprehensively investigated. Results showed that EOM removal in different processes followed the order of VUV/O₃ > UV/O₃ > O₃ > VUV > UV. For VUV/O₃ process, removal efficiencies of dissolved organic carbon (DOC), UV₂₅₄, protein, and polysaccharide at 50 min were 75.6%, 80.8%, 80.1%, and 78.0%, respectively, and fluorescence components received very high removal rates (≥92.8%, at 10 min). The yield of trichloromethane dropped from 102.0 to 30.1 μg/L after treating for 50 min by VUV/O₃. Besides, effects of O₃ dosage, initial pH, and water matrices on EOM removal in VUV/O₃ process were investigated. Moreover, fluorescent molecular probe experiments confirmed that hydroxyl radical and superoxide radical were the main reactive oxygen species (ROS) in VUV/O₃ process, and the transformation of ROS was proposed. The mechanism of EOM removal by VUV/O₃ included VUV photolysis, direct O₃ oxidation, and ROS oxidation. Furthermore, the removal of EOM in filtered water by VUV/O₃ was satisfactory. All results indicated that VUV/O₃ process had great application potential in treating EOM-rich filtered water.

Total organic fluorine (TOF) analysis by completely converting TOF into fluoride with vacuum ultraviolet

Quantifying total organic fluorine (TOF) in water is vital in monitoring the occurrence and persistence of all fluorine-containing organic compounds in the environment, while currently most studies focus on analyzing individual fluorine-containing organic compounds. To fill the technology gap, we herein proposed to convert TOF completely into fluoride with vacuum ultraviolet (VUV) photolysis, followed by analysis of fluoride with ion chromatography. Results showed that the tailored VUV photoreactor achieved satisfying recoveries of fluorine from ten model TOF compounds not only in ultrapure water (83.9 ± 2.0% to 109.4 ± 0.8%) but also in real water samples (92.1 ± 1.0%-106.2 ± 15.7%). Unlike other ultraviolet-based processes that favor alkaline conditions, this VUV process preferred either neutral or acidic conditions to defluorinate selected compounds. While the mechanisms remain to be explored in the future, it has been evidenced that the photo-degradation and photo-defluorination rates of these TOF compounds varied significantly among compounds and operation conditions. The method obtained a method detection limit (MDL) of 0.15 μg-F/L, which is lower than the MDLs of many other TOF analytical methods, along with excellent calibration curves for concentrations ranging from 0.01 to 10.0 mg-F/L. Notably, minimizing fluoride in sample prior to photoconversion was necessary to avoid subtraction-induced errors for TOF measurement, especially when the fluoride/TOF ratio was high. The robust VUV is also green for sample pretreatment due to its unreliance of chemicals or additives.

Coexisting oxidation and reduction of chloroacetaldehydes in water by UV/VUV irradiation

Haloacetaldehydes (HALs) are the third largest disinfection by-product (DBP) ubiquitously detected in finished drinking water and have relatively higher toxicity than currently regulated DBPs. To efficiently alleviate them, this study investigated a green, chemical-free technology by using ultraviolet/vacuum ultraviolet (UV/VUV) on degrading three refractory chlorinated HALs (Cl-HALs). The results indicate that the rates of Cl-HALs decomposition in tap water irradiated by UV/VUV were 23-70 times higher than those irradiated by UV, proving that VUV instead of UV played the key role in degrading Cl-HALs. Increasing Cl-HALs dosage, pH, and dissolved oxygen (DO) all decreased the Cl-HALs degradations significantly, and the rates in tap water were apparently lower than those in ultrapure water. Unlike previous studies, this study proved that both oxidation and reduction were present during the VUV process. Photooxidation via oxidative radicals like •OH mineralized Cl-HALs, leading to substantial drops of total organic carbon; photoreduction via reductive radicals like •H dehalogenated Cl-HALs, resulting in formation of considerable intermediate organics (e.g., formic acid and acetic acid). No matter what pathway, the mass balances of chlorine were always maintained, meaning that dehalogenation occurred instantaneously rather than sequentially. Although the overall photodegradation rates dropped with rising pH and DO, photoreduction was increased with rising pH while photooxidation was elevated with rising DO. The results hence provide insights to better understand the VUV technology in controlling micropollutants in water.

Insight into integration of photocatalytic and microbial wastewater treatment technologies for recalcitrant organic pollutants: From sequential to simultaneous reactions

The more and more stringent environmental standards for recalcitrant organic pollutants pushed forward the development of integration of photocatalytic and microbial wastewater treatment technologies. The past studies proposed mainly two typical integration ways: a) Independent sequence of photocatalysis and biodegradation (ISPB) conducting the sequential reactions; b) Intimate coupling of photocatalysis and biodegradation (ICPB) conducting the simultaneous reactions. Although ICPB has received more attraction recently due to its novelty, ISPB gives an edge in certain cases. The article reviews the state-of-the-art ISPB and ICPB studies to comprehensively compare the two systems. The strengths and weaknesses of ISPB and ICPB regarding the treatment efficiency, cost, toxicity endurance and flexibility are contradistinguished. The reactor set-ups, photocatalysts, microbial characteristics of ISPB and ICPB are summarized. The applications for different kinds of recalcitrant compounds are elaborated to give a holistic view of the removal efficiencies and transformation pathways by the two technologies. Currently, in-depth understandings about the interference among mixed pollutants, co-existing components and key parameters in realistic wastewater are urgently needed. The long-term and large-scale application cases of the integration technologies are still rare. Overall, we conclude that both ISPB and ICPB technologies are reaching maturity while challenges still exist for two systems especially regarding the reliability, economy and generalization for realistic wastewater treatment plants. Future research should not only manage to reduce the cost and energy consumption by upgrading reactors and developing novel catalysts, but also attach importance to the cocktail effects of wastewater during the sequential or simultaneous photocatalysis and biodegradation.

Promotive effects of vacuum-UV/UV (185/254 nm) light on elimination of recalcitrant trace organic contaminants by UV-AOPs during wastewater treatment and reclamation: A review

The use of vacuum-UV/UV (185/254 nm) for trace organic contaminants (TOrCs) elimination during wastewater treatments has attracted much attention. Advanced oxidation processes which combine VUV/UV and additional oxidants (vacuum-UV/UV-based advanced oxidation processes, VUV/UV-AOPs) provide a promising method for eliminating recalcitrant and toxic TOrCs for wastewater reclamation. Researches in this area are increasing but the promoting effects, mechanisms, and influencing factors have not been well summarized. A comprehensive discussion of the limitations of this technique and future research directions is needed. VUV/UV-AOPs have considerable synergistic effects by increasing usage of VUV/UV photons and the oxidant, which increases radical generation. In terms of elimination kinetics, VUV/UV-AOPs outperform conventional UV-AOPs and VUV/UV processes in most cases; a 1.2-87.7-fold increase of the fluence-based kinetic constant is achieved. In terms of energy efficiency per order (EE/O) of TOrCs elimination, the EE/O of VUV/UV-AOPs only accounts for 4% of UV-AOPs and 63% of VUV/UV. However, VUV/UV-AOPs still need to be further investigated. Firstly, although VUV and UV processes have similar radical formation pathways, limited information is available on the quantum yields of photolysis and radical formation of oxidants under VUV irradiation. Secondly, optimization of VUV/UV-AOPs operating conditions, especially oxidant dosage and water-flow patterns, is needed. Thirdly, VUV/UV-AOPs are significantly inhibited by organic and inorganic matters, but the mechanisms of inhibition on VUV/UV scattering, radical quenching, and radical conversion are not well understood. Such inhibition suggests that the use of VUV/UV-AOPs would be limited to relatively clear water treatment, e.g., reverse osmosis effluent for potable water reuse and ultrapure water production. Related research is needed to establish a clearer scheme for VUV/UV-AOPs in terms of the spatial distribution of radical species in the VUV/UV irradiation system and the relevant optimization method for promoting oxidation performance.

Overlooked Formation of H2O2 during the Hydroxyl Radical-Scavenging Process When Using Alcohols as Scavengers

Hydroxyl radical (•OH) is an active species widely reported in studies across many scientific fields, and hence, its reliable analysis is vitally important. Currently, alcohols are commonly used as scavengers for •OH determination. However, the impacts of alcohols on the reliability of •OH detection remain unknown. In this study, we found that adding different types and different amounts of alcohols in water samples treated with ultraviolet irradiation undesirably produced substantial amounts of hydrogen peroxide (H₂O₂), which is a known •OH precursor. This means that the conventional •OH determination method using alcohols is likely unreliable or even misleading. Through careful investigation, we revealed an overlooked reaction pathway during H₂O₂ and •OH transformations. Varying oxygen concentrations, pHs, alcohol dosages, and types altered H₂O₂ formation, which can affect •OH determination accuracy. Among alcohols, n-butanol is the best scavenger because it quenches •OH rapidly but re-forms little H₂O₂.

Degradation of tris(2-chloroethyl) phosphate (TCEP) by thermally activated persulfate: Combination of experimental and theoretical study

Organophosphorus esters (OPEs), one kind of the emerging contaminants with high frequency of detection, is rather refractory in natural environment, thus posing great threat to human health. This study investigated the feasibility and mechanism of tris(2-chloroethyl) phosphate (TCEP) degradation in thermally activated persulfate (TAP) system. Influence of impact factors, such as PDS dosage, temperature, initial pH, and presence of natural water matrix (Cl^-, NO₃^-, H₂PO₄^-, NH₄⁺, humic acid), were evaluated. Results showed that 100% degradation of TCEP can be achieved in TAP system in 40 min at 60 °C. SO₄^·- as the dominant oxidant for TCEP degradation was proved by quenching experiment and verified by EPR analysis. Alkaline condition exerted great inhibitory effect by affecting the constituents of oxidative radicals. It is suggested that Cl^- and H₂PO₄^- at lower dosages promoted the degradation by stimulating ·OH production and forming oxidative radicals with better selectivity. Intermediates identified by high resolution mass spectrometer was suggested less toxic than TCEP by ECOSAR program. Meanwhile, the illustrated oxidation mechanism mainly involved radical attack at CCl bond and cleavage of CO bond, as further confirmed by frontier electron density calculation and wavefunction analysis. Moreover, cyclic degradation of TCEP indicated the constant release of SO₄^·- in 450 min, suggesting high efficiency and stability of PDS in TAP system. Four selected OPEs achieved complete removal in TAP system and their degradation discrepancy was further discussed based on the distinctive structures. Altogether, TAP technology can be used as an efficient method in TCEP removal with great potential for application.

Di-functional Cu2+-doped BiOCl photocatalyst for degradation of organic pollutant and inhibition of cyanobacterial growth

Photocatalytic oxidation of contaminants in water has recently gained extensive attentions. In this study, Cu²⁺-doped BiOCl microsphere photocatalysts were prepared using solvothermal method. The effects of Cu²⁺ doping ratio on the morphological structures and photoelectric and photocatalytic properties of BiOCl were studied in detail. Results showed that Cu²⁺ doping affected the particle size of BiOCl microspheres. The introduction of Cu²⁺ ions gradually increased the light absorption range and decreased the electron recombination rate of photocatalysts as shown by ultraviolet-visible diffuse reflection and photoluminescence spectra. The best doping ratio was 0.25 Cu²⁺-BiOCl, showing the highest photocatalytic activity for rhodamine B (14.25 time higher than BiOCl) and a good inhibition of algal growth. The main reactants in the photocatalytic system were·OH and h⁺ (electron holes). Density functional theory (DFT) calculations further demonstrated that the doping of Cu²⁺ ions made the photogenerated carriers in BiOCl easier to generate and ensured the charge was transferred more rapidly. In conclusion, a novel high-efficiency multifunctional photocatalyst is proposed for the efficient organic pollutants removal and algae growth inhibition from water.

Efficient decontamination of ciprofloxacin at neutral pH via visible light assisted Fenton-like process mediated by Fe(III)-GLDA complexation

This work demonstrated a novel N,N-bis(carboxymethyl)glutamic acid (GLDA) modified visible (vis) light assisted Fenton-like system which could effectively removal ciprofloxacin (CIP) at neutral pH. The removal rate of CIP in the GLDA/Fe(III)/H₂O₂/vis system was 97.9% within 120 min and was approximately twice as high as that in the Fe(III)/H₂O₂/vis system. GLDA could significantly accelerate the Fe(III)/Fe(II) cycle under visible light irradiation. Radical scavenging experiments demonstrated that 74.2% of the Fe(II) in the GLDA/Fe(III)/H₂O₂/vis system originated from the ligand-to-metal charge transfer reaction between Fe(III) and GLDA. The hydroxyl radical was the dominant species for CIP degradation. H₂O₂ utilization kinetic modeling exhibited that 90.1% of H₂O₂ was used for CIP mineralization. The effects of experimental parameters and coexisting substances on the removal of CIP in the Fe(III)/GLDA/H₂O₂/vis system were investigated in detail. The intermediate products of CIP were explored via the high-performance liquid chromatography-mass spectrometry.

Synergistic effects of ozone/peroxymonosulfate for isothiazolinone biocides degradation: Kinetics, synergistic performance and influencing factors

Synergistic effects of ozone (O₃) and peroxymonosulfate (PMS, HSO₅^-) for isothiazolinone biocides degradation was studied. The synergistic ozonation process (O₃/PMS) increased the efficiency of methyl-isothiazolinone (MIT) and chloro-methyl-isothiazolinone (CMIT) degradation to 91.0% and 81.8%, respectively, within 90 s at pH 7.0. This is 30.6% and 62.5% higher than the corresponding ozonation efficiency, respectively. Total radical formation value (R_ct,R) for the O₃/PMS process was 24.6 times that of ozonation alone. Calculated second-order rate constants for the reactions between isothiazolinone biocides and (k_SO4-,MIT and k_SO4-,CMIT) were 8.15 × 10⁹ and 4.49 × 10⁹ M^-1 s^-1, respectively. Relative contributions of O₃, hydroxyl radical (OH) and oxidation to MIT and CMIT removal were estimated, which were 15%, 45%, and 40% for O₃, OH and oxidation to MIT, and 1%, 67%, and 32% for O₃, OH and oxidation to CMIT at pH 7.0, respectively. Factors influencing the O₃/PMS process, namely the solution pH, chloride ions (Cl^-), and bicarbonate (HCO₃^-), were evaluated. Increasing the solution pH markedly accelerated O₃ decay and OH and formation, thus weakening the relative contribution of O₃ oxidation while enhancing that of OH and . Cl^- had a negligible effect on MIT and CMIT degradation. Under the dual effect of bicarbonate (HCO₃^-) as inhibitor and promoter, low concentrations (1-2 mM) of bicarbonate weakly promoted MIT and CMIT degradation, while high concentrations (10-20 mM) induced strong inhibition. Lastly, oxidation performance of O₃ and O₃/PMS processes for MIT and CMIT degradation in different water matrices was compared.

Current status of hypochlorite technology on the wastewater treatment and sludge disposal: Performance, principals and prospects

As cost-effective and high-efficient oxidants, the hypochlorite chemicals have been widely utilized for bleaching and disinfection. However, its potential applications in wastewater treatment and sludge disposal were less concerned. This paper mainly summarized the state-of-the-art applications of hypochlorite technology in wastewater and sludge treatment based on the main influencing factors and potential mechanisms of hypochlorite treatment. The results indicated that the hypochlorite approaches were not only effective in pollutants removal and membrane fouling mitigation for wastewater treatment, but also contributed to sludge dewatering and resource recovery for sludge disposal. The ClO^- and large generated free active radicals (i.e., reactive chlorine species and reactive oxygen species), which possessed strong oxidative ability, were the primary contributors to the pollutants decomposition, and colloids/microbes flocs disintegration during the hypochlorite treatment process. The performance of hypochlorite treatment was highly associated with various factors (i.e., pH, temperature, hypochlorite types and dosage). In combination with the reasonable activators (i.e., Fe²⁺ and ultraviolet), auxiliary agents, and innovative processes (i.e., hydrothermal and electro-oxidation), the operational performance of hypochlorite technology could be further enhanced. Finally, the feasibility and benefits of hypochlorite application for wastewater and sludge treatment were analyzed, and the existing challenges and future research efforts that need to be made have also prospected. The review can hopefully provide a theoretical basis and technical guidance to extend the application of hypochlorite technology for wastewater treatment and sludge disposal on large scale.

The optimal dose of oxidants in UV-based advanced oxidation processes with respect to primary radical concentrations

UV-based advanced oxidation processes (AOPs) via photolysis of precursor chemical oxidants have been of interest to numerous researchers over the past several decades due to their capacity to generate highly active radical species and interesting radical chemistry. However, applications of UV-based AOPs have been commonly optimized case by case, due to the lack of theoretical investigations on process optimization, especially on oxidant doses. In this study, a simple equation for UV/H₂O₂ (•OH as the sole primary reactive species (PRS)) to obtain the theoretical optimal concentration (C_{opt-theoretical}) for H₂O₂ was derived (C_{opt-theoretical}=A_b·S_cε·k). The equation was then validated for its accuracy in the calculation of C_{opt-theoretical} for H₂O₂ in the UV/H₂O₂ AOP using a well-established comprehensive kinetic model. A competition kinetics method for the measurement of scavenging capacity (S_c, the unknown parameter for the simple equation) was designed, for which nitrobenzene was employed as the probe compound and tert‑butyl alcohol was introduced as the standard compound. Based on this simple equation, we calculated the C_{opt-theoretical} of 77 environmental water samples and introduced the concept of a practical optimal oxidants dose for the UV/H₂O₂ AOP, while minimizing the operation costs in engineering applications. Moreover, this study mathematically proved that the simple equation obtained from UV/H₂O₂ could be successfully extended to other UV-based AOPs, including UV/chlorine, UV/NH₂Cl, UV/S₂O₈^2-, and UV/peracetic acid. The simple equation of C_{opt-theoretical} derived in this study may not only help to provide instructions for engineering applications, but also point out the ultimate treatment capability of each UV-based AOPs.

Novel VUV/g-C3N4 system with high adaptability to varied environmental conditions and outstanding degradation capacity for chlorophenols

Given the limitations of conventional vacuum ultraviolet (VUV) systems, a novel vacuum ultraviolet/graphite carbon nitride (VUV/g-C₃N₄) system with high adaptability to varying environmental conditions was developed. Compared with conventional VUV and UV/g-C₃N₄ systems, the VUV/g-C₃N₄ system demonstrates a much higher ability for the efficient degradation of chlorophenols (CPs). In particular, the VUV/g-C₃N₄ system exhibits outstanding performance even at low pH and high concentrations of humic acid and SO₄^2-. Alkaline conditions and the presence of HCO₃^- can further promote CP removal. In addition, the feasibility of the VUV/g-C₃N₄ system was verified by its stable operation in both river water and tap water. Unlike conventional photochemical systems relying on •OH, the dominant reactive species for CP degradation by the VUV/g-C₃N₄ system was identified to be •O₂^-. This study conclusively provided a novel system for the efficient photocatalytic treatment of pollutants.

Oil field-produced water treatment: characterization, photochemical systems, and combined processes

Produced water, a mixture of inorganic and organic components, comprises the largest effluent stream from oil and gas activities. The removal of contaminants from this wastewater is receiving special attention of the researchers since most of them are persistent and difficult to remove with simple techniques. Several technologies from conventional to advanced oxidation processes have been employed to treat produced water. However, the achievement of greater efficiency may be conditioned to a combination of different wastewater treatment techniques. Hereupon, the present paper discusses three important aspects regarding produced water treatment: analytical methods used for characterization, relevant aspects regarding photochemical systems used for advanced oxidation processes, and combined techniques for treating oil field wastewaters. Analytical methods employed for the quantification of the main species contained in produced water are presented for a proper characterization. Photochemical aspects of the reaction systems such as operating conditions, types of irradiation sources, and technical details of reactors are also addressed. Finally, research papers concerning combined treatment techniques are discussed focusing on the essential contributions. Thus, this manuscript aims to assist in the development of novel techniques and the improvement of produced water treatment to obtain a high-quality treated effluent and reduce environmental impacts.

The promotions on radical formation and micropollutant degradation by the synergies between ozone and chemical reagents (synergistic ozonation): A review

The combination of ozone (O₃) and chemical reagents (such as H₂O₂) shows synergies on the radical formation and micropollutant degradation. The promoting performance was associated with various parameters including chemical reagents, micropollutants, solution pH, and the water matrix. In this review, we summarized existing knowledge on radical formation pathways, radical yields, and radical oxidation for different synergistic ozonation processes in various water matrices (such as groundwater, surface water, and wastewater). The increase of radical yields by synergistic ozonation processes was positively related to the increase of O₃-decay, with the increase being 1.1-4.4 folds than ozonation alone (0.2). Thus, synergistic ozonation can promote the degradation rate and efficiency of O₃-resistant micropollutants (second order rate constant, k_P,O3 < 200 M^-1 s^-1), but only slightly affects or even minorly inhibits the degradation of O₃-reactive micropollutants (k_P,O3 > 200 M^-1 s^-1). The water matrices^, such as the dissolved organic matters, negatively suppressed the degradation of micropollutant by quenching O₃-oxidation and radical oxidation (i.e. maximum promoting was decreased by 1.3 times), but may positively extend the promoting effects of synergistic ozonation to micropollutants that are more reactive to O₃ (i.e. k_P,O3 was extended from <200 to <2000 M^-1 s^-1). The formation of bromate would be increased through increasing radical oxidation by synergistic ozonation, but can be depressed by relative higher H₂O₂ as the reducing agent of HOBr/OBr^- intermediate. The increase in bromate formation by O₃/permononsulfate is a considerable concern due to permononsulfate cannot reduce the HOBr/OBr^- intermediate.

Enhanced Radical Generation in an Ultraviolet/Chlorine System through the Addition of TiO2

Ultraviolet (UV)/chlorine draws increasing attention for the abatement of recalcitrant organic pollutants. Herein, it was found that TiO₂ would significantly promote the degradation of dimethyl phthalate (DMP) in the UV/chlorine system (from 19 to 84%). Hydroxyl radicals (HO^•) and chlorine radicals (Cl^•) were the dominant reactive species for DMP degradation in the UV/chlorine/TiO₂ system. Chlorine decayed much faster in UV/chlorine/TiO₂ compared with UV/chlorine, which is possibly because photogenerated electrons (e_cb^-) and superoxide radicals (O₂^•-) have high reactivity with chlorine. As a result, the recombination of photogenerated holes (h_vb⁺) and e_cb^- was inhibited and the accumulation of HO^• and Cl^• was facilitated. A kinetic model was established to simulate the reaction process, and it was found that the concentrations of HO^• and Cl^• were several times to dozens of times higher in UV/chlorine/TiO₂ than that in UV/chlorine. The contributions of HO^• and Cl^• to DMP degradation were 70.3 and 29.7% by model simulation, respectively, and were close to the probe experiment result. In the UV/chlorine/TiO₂ system, the degradation of DMP did not follow pseudo-first-order kinetics but the degradation of benzoate fitted well with pseudo-first-order kinetics. This phenomenon was elucidated by the structure of the pollutant and TiO₂ and further tested by calculating the adsorption energy (E_ads)/binding energy (E_b) with density functional theory. Due to faster decay of chlorine, lower amounts of disinfection byproducts formed in UV/chlorine/TiO₂ compared with UV/chlorine. Adding TiO₂ into the UV/chlorine system can promote the degradation of recalcitrant organic pollutants in an aqueous environment.

Roles and Knowledge Gaps of Point-of-Use Technologies for Mitigating Health Risks from Disinfection Byproducts in Tap Water: A Critical Review

Due to rising concerns about water pollution and affordability, there is a rapidly-growing public acceptance and global market for a variety of point-of-use (POU) devices for domestic uses. However, the efficiencies and mechanisms of POU technologies for removing regulated and emerging disinfection byproducts (DBPs) are still not systematically known. To facilitate the development of this field, we summarized performance trends of four common technologies (i.e., boiling, adsorption, membrane filtration, and advanced oxidation) on mitigating preformed DBPs and identified knowledge gaps. The following highest priority knowledge gaps include: 1) data on DBP levels at the tap or cup in domestic applications; 2) certainty regarding the controls of DBPs by heating processes as DBPs may form and transform simultaneously; 3) standards to evaluate the performance of carbon-based materials on varying types of DBPs; 4) long-term information on the membrane performance in removing DBPs; 5) knowledge of DBPs' susceptibility toward advanced redox processes; 6) tools to monitor/predict the toxicity and diversity of DBPs formed in waters with varying precursors and when implementing different treatment technologies; and 7) social acceptance and regulatory frameworks of incorporating POU as a potential supplement to current centralized-treatment focused DBP control strategies. We conclude by identifying research needs necessary to assure POU systems protect the public against regulated and emerging DBPs.

Key structural features that determine the selectivity of UV/acetylacetone for the degradation of aromatic pollutants when compared to UV/H2O2

Acetylacetone (AA) has proven to be a potent photo-activator for the decolorization of dyes. However, there is very limited information on the quantitative structure-activity relationship (QSAR) and the mechanisms of dye degradation by UV/AA. Herein, the photolysis of 65 aromatic compounds (dyes and dye precursors) was investigated at three pH values (4.0, 6.0, 9.0) by UV/AA and UV/H₂O₂. The obtained pseudo-first-order photodegradation rate constants (k₁) were processed using statistical analysis. The correlation between the k₁ values and the number of photons absorbed by AA, together with the observed pH effect, suggested that the protonated enol structure of AA plays a crucial role in the photodecolorization of dyes. According to quantum chemical computation, photo-induced direct electron transfer between the excited state of AA and the dye was the main mechanism in the UV/AA process. QSAR models demonstrated that the molecular size and stability were the key factors that determined the efficiency of UV/H₂O₂ for dye degradation. Statistically, the UV/AA process was target-selective and suffered less from the inner filter effect, which made it more effective than the UV/H₂O₂ process for dye degradation. The selectivity of the UV/AA process was mainly embodied in the substituent effects: dyes with hydroxyl groups in conjugated systems decomposed faster than those with nitro-substitution or ortho-substituted sulfonate groups. The results can be used for the selection of appropriate photochemical approaches for the treatment of dye-contaminated water.