Dissolved Organic Matter-Mediated Photosensitized Activation of Monochloramine for Micropollutant Abatement in Wastewater Effluent.

Utilizing solar light and water matrix components in situ to reduce the chemical and energy demands would make treatment technologies more sustainable for micropollutant abatement in wastewater effluents. We herein propose a new strategy for micropollutant abatement through dissolved organic matter (DOM)-mediated photosensitized activation of monochloramine (NH2Cl). Exposing the chlorinated wastewater effluent with residual NH2Cl to solar irradiation (solar/DOM/NH2Cl process) degrades six structurally diverse micropollutants at rate constants 1.26-34.2 times of those by the solar photolysis of the dechlorinated effluent (solar/DOM process). Notably, among the six micropollutants, the degradation rate constants of estradiol, acetaminophen, bisphenol A, and atenolol by the solar/DOM/NH2Cl process are 1.13-4.32 times the summation of those by the solar/DOM and solar/NH2Cl processes. The synergism in micropollutant degradation is attributed to the generation of reactive nitrogen species (RNS) and hydroxyl radicals (HO·) from the photosensitized activation of NH2Cl. Triplet state-excited DOM (3DOM*) dominates the activation of NH2Cl, leading to the generation of RNS, while HO· is produced from the interactions between RNS and other photochemically produced reactive intermediates (e.g., O2·- and DOM·+/·-). The findings advance the knowledge of DOM-mediated photosensitization and offer a sustainable method for micropollutant abatement in wastewater effluents containing residual NH2Cl.

Far-UVC Photolysis of Peroxydisulfate for Micropollutant Degradation in Water.

Increasing radical yields to reduce UV fluence requirement for achieving targeted removal of micropollutants in water would make UV-based advanced oxidation processes (AOPs) less energy demanding in the context of United Nations' Sustainable Development Goals and carbon neutrality. We herein demonstrate that, by switching the UV radiation source from conventional low-pressure UV at 254 nm (UV254) to emerging Far-UVC at 222 nm (UV222), the fluence-based concentration of HO• in the UV/peroxydisulfate (UV/PDS) AOP increases by 6.40, 2.89, and 6.00 times in deionized water, tap water, and surface water, respectively, with increases in the fluence-based concentration of SO4•- also by 5.06, 5.81, and 55.47 times, respectively. The enhancement to radical generation is confirmed using a kinetic model. The pseudo-first-order degradation rate constants of 16 micropollutants by the UV222/PDS AOP in surface water are predicted to be 1.94-13.71 times higher than those by the UV254/PDS AOP. Among the tested water matrix components, chloride and nitrate decrease SO4•- but increase HO• concentration in the UV222/PDS AOP. Compared to the UV254/PDS AOP, the UV222/PDS AOP decreases the formation potentials of carbonaceous disinfection byproducts (DBPs) but increases the formation potentials of nitrogenous DBPs.

A High-Radical-Yield Advanced Oxidation Process Coupling Far-UVC Radiation with Chlorinated Cyanurates for Micropollutant Degradation in Water.

Increasing the radical yield and reducing energy consumption would enhance the sustainability and competitiveness of advanced oxidation processes (AOPs) for micropollutant degradation in water. We herein report a novel AOP coupling far-UVC radiation at 222 nm with chlorinated cyanurates (termed the UV222/Cl-cyanurates AOP) for radical generation and micropollutant abatement in water. We experimentally determined the concentrations of HO•, Cl•, and ClO• in the UV222/Cl-cyanurates AOP in deionized water and swimming pool water. The radical concentrations are 10-27 times and 4-13 times, respectively, higher than those in the UV254/Cl-cyanurates AOP and the well-documented UV254/chlorine AOP under comparable conditions (e.g., same UV fluence and oxidant dosing). We determined the molar absorption coefficients and innate quantum yields of two chlorine species and two Cl-cyanurates at 222 nm and incorporated these parameters into a kinetic model. The model enables accurate prediction of oxidant photodecay rates as well as the pH impact on radical generation in the UV222/Cl-cyanurates AOP. We predicted the pseudo-first-order degradation rate constants of 25 micropollutants in the UV222/Cl-cyanurates AOP and demonstrated that many micropollutants can be degraded by >80% with a low UV fluence of 25 mJ cm-2. This work advances the fundamental photochemistry of chlorine and Cl-cyanurates at 222 nm and offers a highly effective engineering tool in combating micropollutants in water where Cl-cyanurates are suitable to use.

Dominant Dissolved Oxygen-Independent Pathway to Form Hydroxyl Radicals and the Generation of Reactive Chlorine and Nitrogen Species in Breakpoint Chlorination.

Due to the complexities of the interactions between ammonia, chlor(am)ine, and intermediate species such as ONOOH, the radical formation in breakpoint chlorination and the consequential removal of micropollutants remain largely unexplored. In this study, the dominant generation pathway of HO•, as a primary radical in breakpoint chlorination, was examined, and the generations of HO•, reactive chlorine species (RCS), and reactive nitrogen species (RNS) were quantitatively evaluated. A dissolved oxygen (DO)-independent pathway was verified by 18O labeling and contributed over 90% to HO• generation. The commonly believed pathway, the decomposition of ONOOH involving DO, contributed only 7% to HO• formation in breakpoint chlorination. The chlorine to nitrogen (Cl/N) ratio and pH greatly affected the generations and speciations of the reactive species. An optimum Cl/N mass ratio for HO•, Cl2•-, and RNS generations occurred at the breakpoint (i.e., Cl/N mass ratio = 9), whereas excessive free chlorine shifted the radical speciation toward ClO• at Cl/N mass ratios above the breakpoint. Basic conditions inhibited the generations of HO• and RNS but significantly promoted that of ClO•. These findings improved the fundamental understanding of the radical chemistry of breakpoint chlorination, which can be extended to estimate the degradations of micropollutants of known rate constants toward the reactive species with influences from the Cl/N ratio and pH in real-world applications.

Generation of Reactive Nitrogen Species in UV Photolysis of Dichloramine and Their Incorporation into Nitrogenous Byproducts.

Dichloramine (NHCl2) often coexists with monochloramine (NH2Cl) in reverse osmosis (RO) permeate in potable reuse scenarios when NH2Cl is added upstream of RO for membrane fouling control such that UV photolysis of NHCl2 occurs during the downstream UV/chloramine process. However, the formation of reactive nitrogen species (RNS) and their incorporation into byproducts during the UV/NHCl2 process are largely unknown. This study quantitatively evaluated the generation of RNS in the UV/NHCl2 process and investigated the role of RNS in micropollutant transformation. UV photolysis of NHCl2 produced comparable RNS concentration to that of NH2Cl at the same oxidant dosage (100 µM) at pH 5.5. Under the experimental conditions, the RNS contributed greatly (40.6%) to N,N-diethyl-3-methylbenzamide (DEET) degradation. By using 15N-labeling and mass spectrometry methods, seven nitrogenous byproducts of DEET degradation with the incorporation of nitrogen originating from the RNS were detected. Among these seven byproducts, six were identified to contain a nitro group (-NO2). While the UV/NHCl2 process formed comparable intensities of -NO-containing products to those in the UV/NH2Cl process, the later process formed 3-91% higher intensities of -NO2-containing products. These findings are essential in furthering our understanding of the contribution of the UV/NHCl2 process in potable reuse scenarios.

A Novel UVA/ClO2 Advanced Oxidation Process for the Degradation of Micropollutants in Water.

Ultraviolet (UV)-based advanced oxidation processes (AOPs) are increasingly used for the degradation of micropollutants in water and wastewater. This study reports a novel UVA/chlorine dioxide (ClO2) AOP based on the photolysis of ClO2 using energy-efficient UV radiation sources in the UVA range (e.g., UVA-LEDs). At a ClO2 dosage of 74 µM (5.0 mg L-1 as ClO2) and a UV fluence at 47.5 mJ cm-2, the UVA365/ClO2 AOP generated a spectrum of reactive species, including chlorine oxide radicals (ClO•), chlorine atoms (Cl•), hydroxyl radicals (HO•), and ozone at a concentration of ∼10-13, ∼10-15, ∼10-14, and ∼10-7 M, respectively. A kinetic model to simulate the reactive species generation in the UVA365/ClO2 AOP was established, validated against the experimental results, and used to predict the pseudo-first-order rate constants and relative contributions of different reactive species to the degradation of 19 micropollutants in the UVA365/ClO2 AOP. Compared to the well-documented UVC254/chlorine AOP, the UVA365/ClO2 AOP produced similar levels of reactive species at similar oxidant dosages but was much less pH-dependent and required much lower energy input, with much lower formation of chloro-organic byproducts and marginal formation of chlorite and chlorate.

Dosing low-level ferrous iron in coagulation enhances the removal of micropollutants, chlorite and chlorate during advanced water treatment.

Drinking water utilities are interested in upgrading their treatment facilities to enhance micropollutant removal and byproduct control. Pre-oxidation by chlorine dioxide (ClO2) followed by coagulation-flocculation-sedimentation and advanced oxidation processes (AOPs) is one of the promising solutions. However, the chlorite (ClO2-) formed from the ClO2 pre-oxidation stage cannot be removed by the conventional coagulation process using aluminum sulfate. ClO2- negatively affects the post-UV/chlorine process due to its strong radical scavenging effect, and it also enhances the formation of chlorate (ClO3-). In this study, dosing micromolar-level ferrous iron (Fe(II)) into aluminum-based coagulants was proposed to eliminate the ClO2- generated from ClO2 pre-oxidation and benefit the post-UV/chlorine process in radical production and ClO3- reduction. Results showed that the addition of 52.1-µmol/L FeSO4 effectively eliminated the ClO2- generated from the pre-oxidation using 1.0 mg/L (14.8 µmol/L) of ClO2. Reduction of ClO2- increased the degradation rate constant of a model micropollutant (carbamazepine) by 55.0% in the post-UV/chlorine process. The enhanced degradation was verified to be attributed to the increased steady-state concentrations of HO· and ClO· by Fe(II) addition. Moreover, Fe(II) addition also decreased the ClO3- formation by 53.8% in the UV/chlorine process and its impact on the formation of chloro-organic byproducts was rather minor. The findings demonstrated a promising strategy to improve the drinking water quality and safety by adding low-level Fe(II) in coagulation in an advanced drinking water treatment train.

Molecular characterization of transformation and halogenation of natural organic matter during the UV/chlorine AOP using FT-ICR mass spectrometry.

UV/chlorine process, as an emerging advanced oxidation process (AOP), was effective for removing micro-pollutants via various reactive radicals, but it also led to the changes of natural organic matter (NOM) and formation of disinfection byproducts (DBPs). By using negative ion electrospray ionization coupled with Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS), the transformation of Suwannee River NOM (SRNOM) and the formation of chlorinated DBPs (Cl-DBPs) in the UV/chlorine AOP and subsequent post-chlorination were tracked and compared with dark chlorination. In comparison to dark chlorination, the involvement of ClO•, Cl•, and HO• in the UV/chlorine AOP promoted the transformation of NOM by removing the compounds owning higher aromaticity (AImod) value and DBE (double-bond equivalence)/C ratio and causing the decrease in the proportion of aromatic compounds. Meanwhile, more compounds which contained only C, H, O, N atoms (CHON) were observed after the UV/chlorine AOP compared with dark chlorination via photolysis of organic chloramines or radical reactions. A total of 833 compounds contained C, H, O, Cl atoms (CHOCl) were observed after the UV/chlorine AOP, higher than 789 CHOCl compounds in dark chlorination, and one-chlorine-containing components were the dominant species. The different products from chlorine substitution reactions (SR) and addition reactions (AR) suggested that SR often occurred in the precursors owning higher H/C ratio and AR often occurred in the precursors owning higher aromaticity. Post-chlorination further caused the cleavages of NOM structures into small molecular weight compounds, removed CHON compounds and enhanced the formation of Cl-DBPs. The results provide information about NOM transformation and Cl-DBPs formation at molecular levels in the UV/chlorine AOP.

UV Photolysis of Mono- and Dichloramine Using UV-LEDs as Radiation Sources: Photodecay Rates and Radical Concentrations.

UV-LEDs with four characteristic wavelengths (255, 265, 285, and 300 nm) were used to investigate the wavelength-dependence of the photolysis of two inorganic chloramines (NH2Cl and NHCl2) and their subsequent radical formation. The fluence-based photodecay rates of NH2Cl decreased with increasing wavelength from 255 to 300 nm, while NHCl2 photodecay rates exhibited the opposite wavelength-dependence. The fluence-based photodecay rate of NH2Cl was comparable to that of NHCl2 at 255 nm, but was lower than NHCl2 at other tested wavelengths. The wavelength-dependence was more influenced by the molar absorption coefficient than the apparent/innate quantum yield and the lower photosensitivity was mainly attributed to the higher bond (N-Cl) dissociation energy (BDE) of NH2Cl than NHCl2. The steady-state concentrations of HO• and reactive chlorine species (e.g., Cl2•-, ClO•, and Cl•) that were generated from the photolysis of NH2Cl and NHCl2 at different wavelengths were determined experimentally and compared with the simulated results by a kinetic model. UV photolysis of NHCl2 at 265, 285, and 300 nm generated higher concentrations of radicals (e.g., HO•, ClO•, Cl•, and Cl2-•) than NH2Cl, while UV photolysis of NH2Cl at 255 nm generated higher concentrations of HO•, ClO•, and Cl• but not Cl2-• than NHCl2. The findings of this study provide fundamental information to be used in selecting specific wavelengths of UV radiation for enhancing/optimizing NH2Cl/NHCl2 photodecay in swimming pools and radical generation for micropollutant abatement in drinking water treatment or potable water reuse.

Influence of the UV/H2O2 Advanced Oxidation Process on Dissolved Organic Matter and the Connection between Elemental Composition and Disinfection Byproduct Formation.

The UV/H2O2 process is a promising advanced oxidation process (AOP) for micropollutant abatement in drinking water treatment and water reuse plants. However, during micropollutant degradation by the AOP, dissolved organic matter (DOM) and the disinfection byproduct (DBP) formation potential may also be altered. This study investigated the influence of the UV/H2O2 AOP on the elemental composition and DBP formation potential of two DOM isolates by using ultrahigh-resolution mass spectrometry (UHRMS). After the AOP, 629 new chemical formulas with an increased degree of oxidation and decreased aromaticity were obtained. Such alterations led to the formation of 226 unknown DBPs with decreased aromaticity indices (AImod) in the subsequent 3-day chlorination. Links between the unknown DBPs and the corresponding precursors in DOM were visualized by network computational analysis. The analysis gave three zones in the van Krevelen diagram based on the possibility of the C7-22HnOm formulas located in each zone to link to the corresponding DBPs. A further investigation with two model compounds reconfirmed the hydroxylation and ring cleavage of DOM by HO· attack during the AOP and the influence on DBP formation. These results obtained from UHRMS build the connection between the elemental composition of DOM and the formation potential of DBPs.

Novel Visible Light-Driven Photocatalytic Chlorine Activation Process for Carbamazepine Degradation in Drinking Water.

Photolysis of free chlorine (HOCl/ClO-) is an advanced oxidation process (AOP) to produce hydroxyl (HO•) and other radicals for refractory micropollutant degradation. However, HOCl/ClO- is only conducive to activation and production of radicals by ultraviolet (UV) light. For the first time, we show the use of visible light (>400 nm) to produce HO• and ClO•, through use of graphitic carbon nitride (g-C3N4) and photogenerated hvb+, ecb-, and O2•- in the presence of HOCl/ClO-, which was termed visible light g-C3N4-enabled chlorine AOP (VgC-AOP). The VgC-AOP increased the pseudo first-order degradation rate constant of a model micropollutant, carbamazepine, by 16 and 7 times higher than that without g-C3N4 and HOCl/ClO-, respectively, and remained active over multiple use cycles. Effects of water quality [pH, alkalinity, Cu(II), and natural organic matter (NOM)] and the operational conditions (g-C3N4 and HOCl/ClO- concentrations, irradiation wavelength, and dose) were investigated. Of particular significance is its superior performance in the presence of NOM, which absorbs less light at visible light wavelengths and scavenges less surface-bonded reactive species, compared against UV/TiO2 or UV/chlorine AOPs. The VgC-AOP is practically relevant, feasible, and easily implementable and it expands the potential types of light sources (e.g., LEDs and solar light).

The Multiple Role of Bromide Ion in PPCPs Degradation under UV/Chlorine Treatment.

This study investigated the role of bromide ions in the degradation of nine pharmaceuticals and personal care products (PPCPs) during the UV/chlorine treatment of simulated drinking water containing 2.5 mgC L-1 natural organic matter (NOM). The kinetics of contributions from UV irradiation and from oxidation by free chlorine, free bromine, hydroxyl radical and reactive halogen species were evaluated. The observed loss rate constants of PPCPs in the presence of 10 µM bromide were 1.6-23 times of those observed in the absence of bromide (except for iopromide and ibuprofen). Bromide was shown to play multiple roles in PPCP degradation. It reacts rapidly with free chlorine to produce a trace amount of free bromine, which then contributes to up to 55% of the degradation of some PPCPs during 15 min of UV/chlorine treatment. Bromide was also shown to reduce the level of HO• and to change the reactive chlorine species to bromine-containing species, which resulted in decreases in ibuprofen degradation and enhancement in carbamazepine and caffeine degradation, respectively. Reactive halogen species contributed to between 37 and 96% of the degradation of the studied PPCPs except ibuprofen in the presence of 10 µM bromide ion. The effect of bromide is non-negligible during the UV/chlorine treatment.

Radical Chemistry and Structural Relationships of PPCP Degradation by UV/Chlorine Treatment in Simulated Drinking Water.

The UV/chlorine process is an emerging advanced oxidation process (AOP) used for the degradation of micropollutants. However, the radical chemistry of this AOP is largely unknown for the degradation of numerous structurally diverse micropollutants in water matrices of varying quality. These issues were addressed by grouping 34 pharmaceuticals and personal care products (PPCPs) according to the radical chemistry of their degradation in the UV/chlorine process at practical PPCP concentrations (1 µg L-1) and in different water matrices. The contributions of HO• and reactive chlorine species (RCS), including Cl•, Cl2•-, and ClO•, to the degradation of different PPCPs were compound specific. RCS showed considerable reactivity with olefins and benzene derivatives, such as phenols, anilines, and alkyl-/alkoxybenzenes. A good linear relationship was found between the RCS reactivity and negative values of the Hammett ∑σp+ constant for aromatic PPCPs, indicating that electron-donating groups promote the attack of benzene derivatives by RCS. The contribution of HO•, but not necessarily RCS, to PPCP removal decreased with increasing pH. ClO• showed high reactivity with some PPCPs, such as carbamazepine, caffeine, and gemfibrozil, with second-order rate constants of 9.2 × 107, 1.03 × 108, and 4.16 × 108 M-1 s-1, respectively, which contributed to their degradation. Natural organic matter (NOM) induced significant scavenging of ClO• and greatly decreased the degradation of PPCPs that was attributable to ClO•, with a second-order rate constant of 4.5 × 104 (mg L-1)-1 s-1. Alkalinity inhibited the degradation of PPCPs that was primarily attacked by HO• and Cl• but had negligible effects on the degradation of PPCPs by ClO•. This is the first study on the reactivity of RCS, particularly ClO•, with structurally diverse PPCPs under simulated drinking water condition.

Coupling Light Emitting Diodes with Photocatalyst-Coated Optical Fibers Improves Quantum Yield of Pollutant Oxidation.

A photocatalyst-coated optical fiber was coupled with a 318 nm ultraviolet-A light emitting diode, which activated the photocatalysts by interfacial photon-electron excitation while minimizing photonic energy losses due to conventional photocatalytic barriers. The light delivery mechanism was explored via modeling of evanescent wave energy produced upon total internal reflection and photon refraction into the TiO2 surface coating. This work explores aqueous phase LED-irradiated optical fibers for treating organic pollutants and for the first time proposes a dual-mechanistic approach to light delivery and photocatalytic performance. Degradation of a probe organic pollutant was evaluated as a function of optical fiber coating thickness, fiber length, and photocatalyst attachment method and compared against the performance of an equivalent catalyst mass in a completely mixed slurry reactor. Measured and simulated photon fluence through the optical fibers decreased as a function of fiber length, coating thickness, or TiO2 mass externally coated on the fiber. Thinner TiO2 coatings achieved faster pollutant removal rates from solution, and dip coating performed better than sol-gel attachment methods. TiO2 attached to optical fibers achieved a 5-fold higher quantum yield compared against an equivalent mass of TiO2 suspended in a slurry solution.

DBP formation from degradation of DEET and ibuprofen by UV/chlorine process and subsequent post-chlorination.

The formation of disinfection by-products (DBPs) from the degradation of N,N-diethyl-3-methyl benzoyl amide (DEET) and ibuprofen (IBP) by the ultraviolet irradiation (UV)/chlorine process and subsequent post-chlorination was investigated and compared with the UV/H2O2 process. The pseudo first-order rate constants of the degradation of DEET and IBP by the UV/chlorine process were 2 and 3.1 times higher than those by the UV/H2O2 process, respectively, under the tested conditions. This was due to the significant contributions of both reactive chlorine species (RCS) and hydroxyl radicals (HO) in the UV/chlorine process. Trichloromethane, 1,1,1-trichloro-2-propanone and dichloroacetic acid were the major known DBPs formed after 90% of both DEET and IBP that were degraded by the UV/chlorine process. Their yields increased by over 50% after subsequent 1-day post-chlorination. The detected DBPs after the degradation of DEET and IBP comprised 13.5% and 19.8% of total organic chlorine (TOCl), respectively, and the proportions increased to 19.8% and 33.9% after subsequent chlorination, respectively. In comparison to the UV/H2O2 process accompanied with post-chlorination, the formation of DBPs and TOCl in the UV/chlorine process together with post-chlorination was 5%-63% higher, likely due to the generation of more DBP precursors from the attack of RCS, in addition to HO.

The roles of reactive species in micropollutant degradation in the UV/free chlorine system.

The UV/free chlorine process forms reactive species such as hydroxyl radicals (HO(•)), chlorine atoms (Cl(•)), Cl2(•-), and O(•-). The specific roles of these reactive species in aqueous micropollutant degradation in the UV/chlorine process under different conditions were investigated using a steady-state kinetic model. Benzoic acid (BA) was chosen as the model micropollutant. The steady-state kinetic model developed fitted the experimental data well. The results showed that HO(•) and Cl(•) contributed substantially to BA degradation, while the roles of the other reactive species such as Cl2(•-) and O(•-) were negligible. The overall degradation rate of BA decreased as the pH increased from 6 to 9. In particular, the relative contributions of HO(•) and Cl(•) to the degradation changed from 34.7% and 65.3% respectively at pH 6 to 37.9% and 62% respectively at pH 9 under the conditions evaluated. Their relative contributions also changed slightly with variations in chlorine dosage, BA concentration and chloride concentration. The scavenging effect of natural organic matter (NOM) on Cl(•) was relatively small compared to that on HO(•), while bicarbonate preferentially reduced the contribution of Cl(•). This study is the first to demonstrate the contributions of different reactive species to the micropollutant degradation in the UV/chlorine system under environmentally relevant conditions.

Electrospray ionization-tandem mass spectrometry method for differentiating chlorine substitution in disinfection byproduct formation.

An electrospray ionization-tandem mass spectrometry (ESI-tqMS) method was developed to identify the location of chlorine substitution during the chlorination of model organic compounds. The chlorine substitution in the aliphatic part and that in the benzene ring of an organic molecule can be differentiated by their corresponding ranges of optimum collision energies, 5-7 eV and over 15 eV, respectively, in the precursor ion scan of m/z 35. The method was applied to predict the structures of intermediates and reveal the transformation pathways during the chlorination of 4-amino-2-chlorobenzoic acid and phenylalanine as a function of reaction time and the chlorine-to-precursor ratio. In the case of phenylalanine, chlorine was found to replace one hydrogen atom attached to the aliphatic nitrogen; in the case of 4-amino-2-chlorobenzoic acid, chlorine was found to replace the hydrogen atoms attached to the aromatic rings.

Protecting against micropollutants in water storage tanks using in-situ TiO2 coated quartz optical fibers.

Photocatalyst-coated optical fibers (P-OFs) using UV-A LEDs offer a highly promising solution for the degradation of micropollutants within municipal, reuse, industrial or home distribution systems, by integrating P-OFs into water storage tanks. P-OFs have photocatalysts attached to bundles of optical fibers, enabling their direct deployment within tanks. This eliminates the necessity for photocatalyst slurries, which would require additional membrane or separation systems. However, a current limitation of P-OFs is light management, specifically light oversaturation of the coated photocatalysts and short light transmission distances along fibers. This study overcomes this limitation and reveals strategies to improve the light dissipation uniformity along P-OFs, and demonstrates the performance of P-OFs on degrading a model micropollutant, carbamazepine (CBZ). Key tunable variables of fibers and light emission conditions, including photocatalyst coating patchiness (p), minimum light incident angles (θm), radiant flux launched to fibers (Φi), and fiber diameters (D), were modeled to establish their relationships with the light dissipation uniformity in TiO2-coated quartz optical fibers (TiO2-QOFs). We then validated modeling insights by conducting experiments to examine how these variables influence the generation of evanescent waves which are localized energy on fiber surfaces, leading to either photocatalyst activation or the recapture of unused light back into fibers. We observed substantial enhancements in evanescent waves generation by decreasing p and increasing θm, resulting in uniform light dissipation which reduces light oversaturation and improves light transmission distances. Moreover, these optimizations led to a remarkable three-fold improvement in CBZ degradation rates and a 65% reduction in energy consumption. Such improvement substantially reduces the capital and operational cost and enhances practicality of energy-efficient photocatalysis without additional chemical oxidants for micropollutant degradation in water storage tanks.

Advanced oxidation of recalcitrant chromophores in full-scale MBR effluent for non-potable reuse of leachate co-treated municipal wastewater.

Wastewater non-potable reuse involves further processing of secondary effluent to a quality level acceptable for reuse and is a promising solution to combating water scarcity. Recalcitrant chromophores in landfill leachate challenge the water quality for non-potable reuse when leachate is co-treated with municipal wastewater. In this study, we first use multivariate statistical analysis to reveal that leachate is an important source (with a Pearson's coefficient of 0.82) of recalcitrant chromophores in the full-scale membrane bioreactor (MBR) effluent. We then evaluate the removal efficacies of chromophores by chlorination, breakpoint chlorination, and the chlorination-UV/chlorine advanced oxidation treatment. Conventional chlorination and breakpoint chlorination only partially remove chromophores, leaving a colour level exceeding the standards for non-potable reuse (>20 Hazen units). We demonstrate that pre-chlorination (with an initial chlorine dosing of 20 mg/L as Cl2) followed by UV radiation (with a UV fluence of 500 mJ/cm2) effectively degraded recalcitrant chromophores (>90%). By quantifying the electron donating capacity (EDC) and radical scavenging capacity (RSC) of the reclaimed water, we demonstrate that pre-chlorination reduces EDC and RSC by up to 64%, increases UV transmittance by 32%, and increases radical yields from UV photolysis of chlorine by 1.7-2.2 times. The findings advance fundamental understanding of the alteration of dissolved coloured substances by (photo)chlorination treatment and provide implications for applying advanced oxidation processes in treating wastewater effluents towards sustainable non-potable reuse.

The impacts of climate change on groundwater quality: A review.

Groundwater has been known as the second largest freshwater storage in the world, following surface water. Over the years, groundwater has already been under overwhelming pressure to satisfy human needs for artificial activities around the world. Meanwhile, the most noticeable footprint of human activities is the impact of climate change. Climate change has the potential to change the physical and chemical properties of groundwater, thereby affecting its ecological functions. This study summarizes existing research affiliated with the possible effects of a changing climate on the quality of groundwater, including changes in water availability, increased salinity and pollution from extreme weather events, and the potentiality of seawater intrusion into coastal aquifers. Previous works dealing with groundwater-induced responses to the climate system and climate impacts on groundwater quality through natural and anthropogenic processes have been reviewed. The climate-induced changes in groundwater quality including pH, dissolved oxygen level, salinity, and concentrations of organic and inorganic compounds were assessed. Some future research directions are proposed, including exploring the potential changes in the occurrences and fate of micropollutants in groundwater, examining the relationship between the increase of microcystin in groundwater and climate change, studying the changes in the stability of metals and metal complexation, and completing studies across different regional climate regions.