Suppression of photoreactivation of E. coli by excimer far-UV light (222 nm) via damage to multiple targets.

Low-pressure mercury lamps emitting at 254 nm (UV254) are used widely for disinfection. However, subsequent exposure to visible light results in photoreactivation of treated bacteria. This study employed a krypton chloride excimer lamp emitting at 222 nm (UV222) to inactivate E. coli. UV222 and UV254 treatment had similar E. coli-inactivation kinetics. Upon subsequent irradiation with visible light, E. coli inactivated by UV254 was reactivated from 2.71-log to 4.75-log, whereas E. coli inactivated by UV222 showed negligible photoreactivation. UV222 treatment irreversibly broke DNA strands in the bacterium, whereas UV254 treatment primarily formed nucleobase dimers. Additionally, UV222 treatment caused cell membrane damage, resulting in wizened, pitted cells and permeability changes. The damage to the cell membrane was mainly due to the photolysis of proteins and lipids by UV222. Furthermore, the photolysis of proteins by UV222 destroyed enzymes, which blocked photoreactivation and dark repair. The multiple damages can be further evidenced by 4.0-61.1 times higher quantum yield in the photolysis of nucleobases and amino acids for UV222 than UV254. This study demonstrates that UV222 treatment damages multiple sites in bacteria, leading to their inactivation. Employing UV222 treatment as an alternative to UV254 could be viable for inhibiting microorganism photoreactivation in water and wastewater.

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.

Ferrate(VI) assists in reducing cytotoxicity and genotoxicity to mammalian cells and organic bromine formation in ozonated wastewater.

Ozonation of wastewater containing bromide (Br-) forms highly toxic organic bromine. The effectiveness of ozonation in mitigating wastewater toxicity is minimal. Simultaneous application of ozone (O3) (5 mg/L) and ferrate(VI) (Fe(VI)) (10 mg-Fe/L) reduced cytotoxicity and genotoxicity towards mammalian cells by 39.8% and 71.1% (pH 7.0), respectively, when the wastewater has low levels of Br-. This enhanced reduction in toxicity can be attributed to increased production of reactive iron species Fe(IV)/Fe(V) and reactive oxygen species (•OH) that possess higher oxidizing ability. When wastewater contains 2 mg/L Br-, ozonation increased cytotoxicity and genotoxicity by 168%-180% and 150%-155%, respectively, primarily due to the formation of organic bromine. However, O3/Fe(VI) significantly (p < 0.05) suppressed both total organic bromine (TOBr), BrO3-, as well as their associated toxicity. Electron donating capacity (EDC) measurement and precursor inference using Orbitrap ultra-high resolution mass spectrometry found that Fe(IV)/Fe(V) and •OH enhanced EDC removal from precursors present in wastewater, inhibiting electrophilic substitution and electrophilic addition reactions that lead to organic bromine formation. Additionally, HOBr quenched by self-decomposition-produced H2O2 from Fe(VI) also inhibits TOBr formation along with its associated toxicity. The adsorption of Fe(III) flocs resulting from Fe(VI) decomposition contributes only minimally to reducing toxicity. Compared to ozonation alone, integration of Fe(VI) with O3 offers improved safety for treating wastewater with varying concentrations of Br-.

Ozone, hydrogen peroxide, and peroxymonosulfate disinfection of MS2 coliphage in water.

The control of viruses in water is critical to preventing the spread of infectious viral diseases. Many oxidants can inactivate viruses, and this study aims to systematically compare the disinfection effects of ozone (O3), peroxymonosulfate (PMS), and hydrogen peroxide (H2O2) on MS2 coliphage. The effects of oxidant dose and contact time on disinfection were explored, as were the disinfection effects of three oxidizing agents in secondary effluent. The 4-log inactivation of MS2 coliphage required 0.05 mM O3, 0.5 mM PMS, or 25 mM H2O2 with a contact time of 30 min. All three oxidants achieved at least 4-log disinfection within 30 min, and O3 required only 0.5 min. In secondary effluent, all three oxidants also achieved 4-log inactivation of MS2 coliphage. Excitation-emission matrix (EEM) results indicate that all three oxidants removed dissolved organic matter synchronously and O3 oxidized dissolved organic matter more thoroughly while maintaining disinfection efficacy. Considering the criteria of oxidant dose, contact time, and disinfection efficacy in secondary effluent, O3 is the best choice for MS2 coliphage disinfection among the three oxidants.

Molecular insight of dissolved organic matter and chlorinated disinfection by-products in reclaimed water during chlorination with permanganate preoxidation.

Permanganate is a common preoxidant applied in water treatment to remove organic pollutants and to reduce the formation of disinfection by-products. However, the effect of permanganate preoxidation on the transformation of dissolved effluent organic matter (dEfOM) and on the formation of unknown chlorinated disinfection by-products (Cl-DBPs) during chlorination remains unknown at molecular level. In this work, the molecular changes of dEfOM during permanganate preoxidation and subsequent chlorination were characterized using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Permanganate preoxidation was found to decrease the DBE (double bond equivalent) and AImod (modified aromaticity index) of the dEfOM. The identity and fate of over 400 unknown Cl-DBPs during KMnO4-chlorine treatment were investigated. Most Cl-DBPs and the precursors were found to be highly unsaturated aliphatic and phenolic compounds. The Cl-DBPs precursors with lower H/C and lower O/C were preferentially removed by permanganate preoxidation. Additionally, permanganate preoxidation decreased the number of unknown Cl-DBPs by 30% and intensity of unknown Cl-DBPs by 25%. One-chlorine-containing DBPs were the major Cl-DBPs and had more CH2 groups and higher DBEw than Cl-DBPs containing two and three chlorine atoms. 60% of the Cl-DBPs formation was attributed to substitution reactions (i.e., +Cl-H, +2Cl-2H, +3Cl-3H, +ClO-H, +Cl2O3-2H). This work provides detailed molecular level information on the efficacy of permanganate preoxidation on the control of overall Cl-DBPs formation during chlorination.

Single-atom Ag-loaded carbon nitride photocatalysts for efficient degradation of acetaminophen: The role of Ag-atom and O2.

Carbon nitride has been extensively used as a visible-light photocatalyst, but it has the disadvantages of a low specific surface area, rapid electron-hole recombination, and relatively low light absorbance. In this study, single-atom Ag was successfully anchored on ultrathin carbon nitride (UTCN) via thermal polymerization, the catalyst obtained is called AgUTCN. The Ag hardly changed the carbon nitride's layered and porous physical structure. AgUTCN exhibited efficient visible-light photocatalytic performances in the degradation of various recalcitrant pollutants, eliminations of 85% were achieved by visible-light irradiation for 1 hr. Doping with Ag improved the photocatalytic performance of UTCN by narrowing the forbidden band gap from 2.49 to 2.36 eV and suppressing electron-hole pair recombination. In addition, Ag doping facilitated O2 adsorption on UTCN by decreasing the adsorption energy from -0.2 to -2.22 eV and favored the formation of O2·-. Electron spin resonance and radical-quenching experiments showed that O2·- was the major reactive species in the degradation of Acetaminophen (paracetamol, APAP).

Phototransformation of Brominated Disinfection Byproducts and Toxicity Elimination in Sunlit-Ozonated Reclaimed Water.

Ozonation is universally used during water treatment but can form hazardous brominated disinfection byproducts (Br-DBPs). While sunlight exposure is advised to reduce the risk of Br-DBPs, their phototransformation pathways remain insufficiently understood. Here, sunlight irradiation was found to reduce adsorbable organic bromine by 63%. Applying high-resolution mass spectrometry, the study investigated transformations of dissolved organic matter in sunlit-ozonated reclaimed water, revealing the number and abundance of assigned formulas decreased after irradiation. The Br-DBPs with O/C < 0.6 and MW > 400 Da were decreased or removed after irradiation, with the majority being CHOBr compounds. The peak intensity reduction ratio of CHOBr compounds correlated positively with double bound equivalent minus oxygen ratios but negatively with O/C, suggesting that photo-susceptible CHOBr compounds were highly unsaturated. Mass difference analysis revealed that the photodegradation pathways were mainly oxidation aligned with debromination. Three typical CHOBr molecular structures were resolved, and their photoproducts were proposed. Toxicity estimates indicated decreased toxicity in these photoproducts compared to their parent compounds, in line with experimentally determined values. Our proposed phototransformation pathways for Br-DBPs enhance our comprehension of their degradation and irradiation-induced toxicity reduction in reclaimed water, further illuminating their transformation under sunlight in widespread environmental scenarios.

Vacuum ultraviolet irradiation for reduction of the toxicity of wastewater towards mammalian cells: Removal mechanism, changes in organic compounds, and toxicity alternatives.

Vacuum ultraviolet (VUV, 185 + 254 nm) irradiation performs well for oxidation of model pollutants. However, oxidation of pollutants does not necessarily lead to a reduction in toxicity. Currently, a comprehensive understanding of the effect of VUV irradiation on the toxicity of real wastewater is still lacking. In this study, the influence of VUV irradiation on the toxicity of secondary effluents to Chinese hamster ovary (CHO) cells was investigated. The induction units of endogenous reactive oxygen species (ROS) and 8-hydroxyguanosine (8-OHdG) in cells continuously decreased with prolonged irradiation time. After 36 min of irradiation, the cytotoxicity and the genotoxicity of the secondary effluents were reduced by 57%-63% and 56%-61%, respectively. The UV (254 nm), •OH, and other substances generated during the VUV irradiation directly drive toxicity changes of wastewater. The contribution of •OH generated during VUV irradiation to the reductions in cytotoxicity and genotoxicity of the secondary effluents reached 72%-78% and 77%-84%, respectively. Hydroxyl radicals generated during VUV irradiation played an important role in the detoxification. The relative signal intensity of dissolved organic carbon (DOC) > 500 Da was partially removed, whereas that of DOC < 500 Da was small changed. Since the content of DOC > 500 Da in the samples was much lower than that of DOC < 500 Da, the removal of total DOC was only 15.8%-20.0% after 36 min of irradiation. The UV254 values and the fluorescence intensity values for different molecular weights (MWs) were all reduced effectively by VUV irradiation. Electron-rich organic compounds of all MWs were all sensitive to VUV irradiation. There were mono-linear relationships between changes in chemical indexes and changes in cytotoxicity or genotoxicity. The total fluorescence intensity (Ex: 220-420 nm, Em: 280-560 nm) was identified as the best indicator of the reduction in toxicity.

Efficient removal of electroneutral carbonyls by combined vacuum-UV oxidation and anion-exchange resin adsorption: mechanism, model simulation, and optimization.

Electroneutral carbonyls (ENCs) with low molecular weights (e.g., aldehydes and ketones) are recalcitrant to single water treatment process to achieve ultralow concentration. Residual ENCs are present in reverse osmosis permeate and pose risks to human health during potable use or industrial application in manufacturing processes. Herein, a combined vacuum-UV (VUV) oxidation and anion-exchange resin (AER) adsorption method was developed to treat the ENCs and reduce total organic carbon (TOC) to an ultralow concentration (< 5 µg/L) with high efficiency and at low cost. VUV-AER was 2.1-2.4 times more efficient than VUV alone for the removal of TOC. VUV oxidized the ENCs to electronegative carboxylic acids, which were adsorbed by the AER through electrostatic interactions and hydrogen bonding. When the VUV fluence was lower than 643 mJ cm-2, the AER could not achieve ultralow TOC removal of ENCs. The treat capacity of 1500-2900 valid bed volume (BVs) was achieved after increasing the VUV fluence to 1929 mJ cm-2. The AER could more efficiently adsorb carboxylic acids that contained more carboxylic groups or shorter carbon chain. Acetate was identified as the primary breakthrough product at relatively low VUV fluence, and oxalate was the main byproduct at relatively high VUV fluence. A mathematical model to predict TOC breakthrough was developed considering the VUV-oxidation kinetics and the AER breakthrough curve. The model was used to optimize the method to maximize TOC removal and minimize energy consumption. These results imply that VUV-AER is technically feasible and economically applicable to eliminate recalcitrant ENCs to ultralow concentration for the production of water requires high quality (e.g., potable water or electronic-grade ultrapure water).

Reduction of byproduct formation and cytotoxicity to mammalian cells during post-chlorination by the combined pretreatment of ferrate(VI) and biochar.

Ferrate [Fe(VI)] can efficiently degrade various pollutants in wastewater. Biochar application can reduce resource use and waste emission. This study investigated the performance of Fe(VI)/biochar pretreatment to reduce disinfection byproducts (DBPs) and cytotoxicity to mammalian cells of wastewater during post-chlorination. Fe(VI)/biochar was more effective at inhibiting the cytotoxicity formation than Fe(VI) alone, reducing the cytotoxicity from 12.7 to 7.6 mg-phenol/L. The concentrations of total organic chlorine and total organic bromine decreased from 277 to 130 µg/L and from 51 to 39 µg/L, compared to the samples without pretreatment. Orbitrap ultra-high resolution mass spectrometry revealed that the number of molecules of DBPs decreased substantially from 517 to 229 by Fe(VI)/biochar, with the greatest reduction for phenols and highly unsaturated aliphatic compounds. In combination with the substantial reduction of 1Cl-DBPs and 2Cl-DBPs, 1Br-DBPs and 2Br-DBPs were also reduced. Fluorescence excitation-emission matrix coupled with parallel factor analysis suggested that fulvic acid-like substances and aromatic amino acid was obviously reduce likely due to the enhanced oxidation of Fe(IV)/Fe(V) produced by Fe(VI)/biochar and adsorption of biochar. Furthermore, the DBPs generated by electrophilic addition and electrophilic substitution of precursors were reduced. This study shows that Fe(VI)/biochar pretreatment can effectively reduce cytotoxicity formation during post-chlorination by transforming DBPs and their precursors.

Oxygen doping of cobalt-single-atom coordination enhances peroxymonosulfate activation and high-valent cobalt-oxo species formation.

The high-valent cobalt-oxo species (Co(IV)=O) is being increasingly investigated for water purification because of its high redox potential, long half-life, and antiinterference properties. However, generation of Co(IV)=O is inefficient and unsustainable. Here, a cobalt-single-atom catalyst with N/O dual coordination was synthesized by O-doping engineering. The O-doped catalyst (Co-OCN) greatly activated peroxymonosulfate (PMS) and achieved a pollutant degradation kinetic constant of 73.12 min-1 g-2, which was 4.9 times higher than that of Co-CN (catalyst without O-doping) and higher than those of most reported single-atom catalytic PMS systems. Co-OCN/PMS realized Co(IV)=O dominant oxidation of pollutants by increasing the steady-state concentration of Co(IV)=O (1.03 × 10-10 M) by 5.9 times compared with Co-CN/PMS. A competitive kinetics calculation showed that the oxidation contribution of Co(IV)=O to micropollutant degradation was 97.5% during the Co-OCN/PMS process. Density functional theory calculations showed that O-doping influenced the charge density (increased the Bader charge transfer from 0.68 to 0.85 e), optimized the electron distribution of the Co center (increased the d-band center from -1.14 to -1.06 eV), enhanced the PMS adsorption energy from -2.46 to -3.03 eV, and lowered the energy barrier for generation of the key reaction intermediate (*O*H2O) during Co(IV)=O formation from 1.12 to 0.98 eV. The Co-OCN catalyst was fabricated on carbon felt for a flow-through device, which achieved continuous and efficient removal of micropollutants (degradation efficiency of >85% after 36 h operation). This study provides a new protocol for PMS activation and pollutant elimination through single-atom catalyst heteroatom-doping and high-valent metal-oxo formation during water purification.

Ozone micro-bubble aeration using the ceramic ultrafiltration membrane with superior oxidation performance for 2, 4-D elimination.

Micro-bubble aeration is an efficient way to promote ozonation performance, but the technology is challenged by extensive energy cost. Here, a ceramic ultrafiltration membrane was used to achieve ozone micro-bubble (0-80 µm) aeration in a simple way at gaseous pressures of 0.14-0.19 MPa. Compared with milli-bubble aeration, micro-bubble aeration increased the equilibrium aquatic O3 concentrations by 1.53-3.25 times and apparent O3 transfer rates by 3.12-3.35 times at pH 5.0-8.0. Consequently, the •OH yield was 2.67-3.54 times via faster O3 transfer to the aquatic solution followed by decomposition rather than interfacial reaction. Ozone micro-bubble aeration outperformed milli-bubble aeration, with the degradation kinetics of 2,4-D being 3.08-4.36 times higher. Both O3-oxidation and •OH oxidation were important to the promotion with the contributions being 35.8%-45.9% and 54.1%-64.2%, respectively. The operational and water matric conditions influenced the oxidation performance via both O3 oxidation and •OH oxidation, which is reported for the first time. In general, the ceramic membrane offered a low-energy approach of ozone micro-bubble aeration for efficient pollutant degradation. The O3 oxidation and •OH oxidation were proportionally promoted by ozone micro-bubble due to O3 transfer enhancement. Thus, the promotive mechanism can be interpreted as the synchronous enchantment on ozone exposure and •OH exposure for the first time.

PPCP degradation by ammonia/chlorine: Efficiency, radical species, and byproducts formation.

Pharmaceutical and personal care products (PPCPs) are frequently detected in water bodies and have potential risks to human health and the ecosystem. The degradation of eight structurally diverse PPCPs by ammonia/chlorine was systematically investigated in this study. Compared with chlorination, ammonia/chlorine markedly enhanced PPCP degradation, and the degradation efficiencies of most PPCPs were greater than 70%. Tert-butanol strongly suppressed PPCP degradation, while bicarbonate suppressed it moderately, suggesting the importance of ClO⋅and ⋅CO3- in PPCP degradation. In neutral conditions, PPCP degradation was mainly attributed to ⋅OH, with its contribution ranging from 74% to 100% at a Cl2/N molar ratio of 1.6. Regarding the effect of natural organic matter, atrazine and primidone were inhibited the most, while carbamazepine (CBZ), metoprolol (MTP), and atenolol (ATN) were affected the least. PPCP degradation was suppressed in reclaimed water; the degradation of CBZ, MTP, and ATN was suppressed the least, with degradation efficiencies of 77.1%-85.4%, 75.1%-77.1%, and 64.6%-68.8%, respectively. Furthermore, compared with chlorination, fewer volatile halogenated byproducts were formed in reclaimed water when using the ammonia/chlorine process, and the concentration of each byproduct formed by ammonia/chlorine was less than 10 µg/L. This study suggests the feasibility of using ammonia/chlorine oxidation to degrade PPCPs in reclaimed water.

Overlooked Cytotoxicity and Genotoxicity to Mammalian Cells Caused by the Oxidant Peroxymonosulfate during Wastewater Treatment Compared with the Sulfate Radical-Based Ultraviolet/Peroxymonosulfate Process.

Byproduct formation (chlorate, bromate, organic halogen, etc.) during sulfate radical (SO4•-)-based processes like ultraviolet/peroxymonosulfate (UV/PMS) has aroused widespread concern. However, hypohalous acid (HOCl and HOBr) can form via two-electron transfer directly from PMS, thus leading to the formation of organic halogenated byproducts as well. This study found both PMS alone and UV/PMS can increase the toxicity to mammalian cells of wastewater, while the UV/H2O2 decreased the toxicity. Cytotoxicity of two wastewater samples increased from 5.6-8.3 to 15.7-29.9 mg-phenol/L, and genotoxicity increased from 2.8-3.1 to 5.8-12.8 µg 4-NQO/L after PMS treatment because of organic halogen formation. Organic halogen formation from bromide rather than chloride was found to dominate the toxicity increase. The SO4•--based process UV/PMS led to the formation of both organic halogen and inorganic bromate and chlorate. However, because of the very low concentration (<20 µg/L) and relatively low toxicity of bromate and chlorate, contributions of inorganic byproducts to toxicity increase were negligible. PMS would not form chlorate and bromate, but it generated a higher concentration of total organic halogen, thus leading to a more toxic treated wastewater than UV/PMS. UV/PMS formed less organic halogen and toxicity because of the destruction of byproducts by UV irradiation and the removal of byproduct precursors. Currently, many studies focused on the byproducts bromate and chlorate during SO4•--based oxidation processes. This work revealed that the oxidant PMS even needs more attention because it caused higher toxicity due to more organic halogen formation.

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 CO32-/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.

Changes in toxicity and adsorbable organic bromine concentrations in ozonated reclaimed water irradiated with sunlight.

Disinfecting reclaimed water for safe reuse can produce toxic disinfection by-products such as adsorbable organic bromine (AOBr). Irradiating stored reclaimed water with sunlight is a "green" and free method for eliminating some toxic disinfection by-products, but the effects of irradiation with sunlight on ozonated reclaimed water containing bromide are not well understood. In this study, AOBr was found at concentrations of 171-180 (µg Br)/L in ozonated reclaimed water containing bromide at a concentration of 2 (mg Br)/L and dissolved organic carbon at a concentration of ∼5 (mg C)/L. Irradiation with sunlight degraded 53-74% of the AOBr in two reclaimed water samples in 8 h, and the pseudo-first-order rate constants (k) were 0.09-0.17 h-1. The concentration of tribromomethane, a typical Br-containing disinfection by-product, was decreased by >96% by irradiation for 8 h (k = 0.42-0.47 h-1). Irradiation with sunlight decreased the toxicity of ozonated reclaimed water to Chinese hamster ovary cells. Irradiation with sunlight decreased the degree of intracellular oxidative stress and oxidative damage caused by ozonated reclaimed water. Irradiation with sunlight for 8 h decreased cytotoxicity of the ozonated reclaimed water samples by 79% and 65%. The change in AOBr concentration correlated with the change in toxicity (R2=0.69, p<0.05). The relationships between sunlight wavelength and decreases in the AOBr concentration and toxicity were assessed, and it was found that UV in sunlight was predominantly responsible for decreasing the AOBr concentration and toxicity by reclaimed water. During irradiation for 8 h, UV was responsible for 65%-66% of the decrease in the AOBr concentration and 65-79% of the decrease in reclaimed water induced cytotoxicity. Irradiation with sunlight is a promising method for degrading AOBr and detoxifying ozonated reclaimed water during storage to allow the water to be reused.

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 (lP), which has not been fully characterized. Therefore, VUV/UV oxidation in solution was investigated at various lP 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 (kobs) of formaldehyde was only 51% of that of propionaldehyde (kobs = 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 (lP < 0.55 cm), which resulted in highly heterogeneous oxidation in homogeneous water. Thus, kobs of acetaldehyde dramatically decreased from 0.13 to 0.033 µM s-1 as the total lP 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 lP. 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 lP <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.

Electrochemical Synthesis of Hydrogen Peroxide Catalyzed by Carbon Nanotubes with Surface Co-NX Sites and Encapsulated Co Nanoparticles.

The exploitation of M-NX sites, where M is a transition metal atom, is widely regarded as an effective catalytic strategy to promote the oxygen reduction reaction (ORR). In addition, some studies have shown that transition metal nanoparticles (M-NPs) coated with carbon layers can improve the reactivity of ORR and ameliorate the overpotential of the reaction. In this study, we synthesized carbon nanotubes with single-atom Co-NX active sites, Co-NPs outside the tube, and Co-NPs wrapped in the tube by the complexation-pyrolysis synthesis method and explored the role of the particles and Co-NX sites through different pickling steps. The catalyst synthesized with the new stratagem in this study shows outstanding selectivity and also ORR activity. Furthermore, a natural air diffusion electrode fabricated using this material can produce H2O2 at 323 mg L-1 h-1 under neutral conditions without oxygen aeration.

Chlorination of isothiazolinone biocides: kinetics, reactive species, pathway, and toxicity evolution.

Due to the Covid-19 pandemic, the worldwide biocides application has been increased, which will eventually result in enhanced residuals in treated wastewater. At the same time, chlorine disinfection of secondary effluents and hospital wastewaters has been intensified. With respect to predicted elevated exposure in wastewater, the chlorination kinetics, transformation pathways and toxicity evolution were investigated in this study for two typical isothiazolinone biocides, methyl-isothiazolinone (MIT) and chloro-methyl-isothiazolinone (CMIT). Second-order rate constants of 0.13 M-1·s-1, 1.95 × 105 M-1·s-1 and 5.14 × 105 M-1·s-1 were determined for the reaction of MIT with HOCl, Cl2O and Cl2, respectively, while reactivity of CMIT was around 1-2 orders of magnitude lower. While chlorination of isothiazolinone biocides at pH 7.1 was dominated by Cl2O-oxidation, acidic pH and elevated Cl- concentration favored free active chlorine (FAC) speciation into Cl2 and increased overall isothiazolinone removal. Regardless of the dominant FAC species, the elimination of MIT and CMIT resulted in an immediate loss of acute toxicity under all experimental conditions, which was attributed to a preferential attack at the S-atom resulting in subsequent formation of sulfoxides and sulfones and eventually an S-elimination. However, chlorination of isothiazolinone biocides in secondary effluent only achieved <10% elimination at typical disinfection chlorine exposure 200 mg·L-1·min, but was predicted to be remarkably increased by acidizing solution to pH 5.5. Alternative measures might be needed to minimize the discharge of these toxic chemicals into the aquatic environment.

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-/CO32- 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.