Water reuse has been proposed and implemented as a promising solution to address the water scarcity issue. Water reuse treatment typically contains monochloramine (NH2Cl) and dichloramine (NHCl2). Chloramines bypass the RO membrane and photolyze as photo-oxidants in the ultraviolet light-based advanced oxidation processes (UV/AOP). Small and neutral contaminants easily diffuse through membrane-based treatments, and UV/AOP oxidizes organic micropollutants. However, the treatment requires high energy consumption. The goal of this dissertation is to study the mechanistic insights of chloramine hydrolysis and its potential implications for rapid trace organic removal in water reuse scenarios. First, the formation and the subsequent hydrolysis of NHCl2 were investigated in the mixed system of HOCl and NH2Cl. It was observed that the highest 1,4-D removal was at pH 6 with the HOCl-to-NH2Cl molar ratio of 1. These conditions favored the formation and the hydrolytic decay of NHCl2. It was also found that the crucial intermediate, peroxynitrite (ONOO-), was experimentally measured in the mixed system of HOCl/NH2Cl. Further, it was determined that 60-70% of 1,4-D removal was due to the generated HO• in the mixed HOCl and NH2Cl system. Second, the mechanisms that govern the rapid hydrolytic decay of NHCl2 for HO• generation were uncovered. Also, the oxidative capacity of the NHCl2-initiated system was examined in water reuse-related conditions. It was determined that the highest HO• exposure was at pH 8.4 with the NH2Cl-to-NHCl2 molar ratio of 1 in the NHCl2-initiated system. It was validated that the decay of NHCl2 and the intermediate ONOO- decided the eventual HO• exposure. It was also demonstrated that the NHCl2-initiated oxidative system is comparable to the newly proposed UV/AOPs. Third, the NHCl2 hydrolysis system could be exploited for organic contaminant removal when the pH is alkaline with UV irradiation. This process was shown to produce HO•, demonstrating that ONOO- was generated during the process. Lastly, the oxidative capacity of the UV/ONOO- at alkaline pHs was confirmed to be comparable to other existing UV/AOPs. The outcomes of the aforementioned projects allow water reuse facilities to design better treatment processes and optimize the operational parameters for potential implications of the novel techniques.

Current reductive water treatment processes typically transform soluble Cr(VI), a toxic carcinogen to particulate Cr(III), a micronutrient. These processes do not completely and effectively remove residual Cr(III) particles. Furthermore, it is assumed that Cr(III) entering a drinking water distribution remains chemically inert. However, this is not the case. Nationwide surveys have observed an increase of Cr(VI) levels from the entry point to the point of distribution. Cr can accumulate in corrosion scales and act as an unrecognized in situ source for the inadvertent generation of Cr(VI), especially in the presence of a residual disinfectant.

The goal of this dissertation is to elucidate the transformation of Cr(III) solids to Cr(VI) in drinking water conditions. First, extensive data mining was done on the data reported as part of the U.S. EPA’s Unregulated Contaminants Monitoring Rule. It was observed that Cr(VI) levels in drinking water averaged from non-detectable to 97 µg/L at the entry point to drinking water distribution systems with the majority of sample detections ranging between 0.03 to 5 µg/L. Additionally, it was calculated that 20% of Cr(III) solids can be potentially oxidized to Cr(VI), especially in the presence of a residual disinfectant. Second, the kinetics of Cr(III) solids oxidation and subsequent formation of Cr(VI) by chlorine were investigated. Batch experiments were carried out with chromium hydroxide, chromium oxide, copper chromite, and iron chromium hydroxide as model Cr(III) sources. It was found that Cr(VI) was produced with the consumption of chlorine, suggesting that Cr(III) was oxidized to toxic Cr(VI) at environmentally relevant time scales. Third, the impact of pH (6-8.5), bromide concentration (0.1-5 mg/L), alkalinity (1-5 mM) and iron content (0.84-8.4 mM). The results highlighted that the presence of low level of bromide catalyzed the formation rates of Cr(VI) by one order of magnitude whereas increasing iron decreases Cr(VI) formation rates. Fourth, a lack of stoichiometric conversion between the molar amount of chlorine consumed and Cr(VI) generated was observed, suggesting the formation of Cr intermediates. The outcome of this study aimed to inform Cr(III) control strategies to minimize Cr(VI) formation in drinking water distribution systems.

Contamination of hexavalent chromium Cr(VI), pentavalent vanadium(V) and nitrate is threatening the availability of drinking water resources. Their reductive transformation into environmentally benign products is a viable and sustainable strategy. The goal of this dissertation is to investigate the reaction kinetics and mechanism of reductive transformation of Cr(VI), vanadium(V), and nitrate.

First, highly reductive TiO2 nanocrystals were synthesized for photocatalytic Cr(VI) reduction in drinking water matrices. The synthesized catalyst with surface-bonded diethylene glycol exhibited a high electron-releasing capacity. Under UV irradiation, Cr(VI) was efficiently reduced with a fast precipitation of trivalent chromium(III) hydroxide Cr(OH)3(s). The synthesized catalyst was reused for multiple cycles and exhibited excellent performance in Cr(VI)-contaminated groundwater.

Second, a novel denitrification process was developed for nitrate removal in drinking water matrices. Reactive radicals generated from nitrate photolysis oxidized formate into formate radical (CO2·-). Kinetic modelling and principal component analyses showed that synergistic nitrate photolysis with CO2·--induced reduction of nitrogen intermediates enabled synchronous removal of dissolved nitrogen and organic carbon. pH and dissolved organic matter at levels similar to those in groundwater had a negligible impact on the dentification performance. The proposed process exhibited strong application potential in treating nitrate-contaminated groundwater.

Third, the redox chemistry of vanadium(V) species and their transformation products were investigated with state-of-the-art rotating ring-disk electrode techniques. VO2+ and NaxHyV10O28(6-x-y)- predominantly exhibited faster intrinsic reduction kinetics than HxV4O12+x(4+x)-, and HVO42- The reduction of vanadium(V) underwent a one-electron transfer process except for that of NaxHyV10O28(6-x-y)- with a two-electron transfer. Ring electrode current showed that the reduction product of NaxHyV10O28(6-x-y)- was stable, while the other three vanadium(V) species had half-lives from milliseconds to seconds. Phosphate complexation favored the reduction of vanadium(V) and inhibited the re-oxidation of its reduction product in water treatment.

Finally, radical-induced photochemical reduction of Cr(VI) and nitrate in the spent ion-exchange regenerant brine was investigated to increase the sustainability of ion exchange processes. CO2·- has been demonstrated to be more effective than alcohol-derived carbon-centered radicals to remove Cr(VI) in the spent brine. The reduction of Cr(VI) was favored at a high dosage of formate and low pHs and was not affected by ionic strength. Chloride transformed NO2· into less reactive NO· and Cl2·- and inhibited Cr(VI) reduction. Co-removal of Cr(VI) and nitrate was achieved with an extended reaction time.

In summary, novel redox-based treatment processes have been developed to remove Cr(VI) and nitrate from drinking water and wastewater matrices. Fundamental redox chemistry of vanadium(V) species and their reduction products have been unveiled electrochemically by rotating ring-disk electrode. The developed treatment processes can be integrated into decentralized water treatment and reuse facilities to remove Cr(VI) and nitrate from local water resources. The redox chemistry of vanadium could help design effective treatment processes for vanadium(V) removal. Future work will be conducted to design treatment modules to remove Cr(VI) and nitrate in actual water matrices and extend the application of rotating ring-disk electrode for other aquatic contaminants.

Brackish water desalination has become increasingly important in arid inland regions for sustainable water supplies, but the management of desalination brine waste is costly. In particular, the presence of oversaturated calcium as scale-forming compounds in the brine is the most important factor limiting the recovery of water. Removal of these antiscalants from the brine can induce the precipitation of oversaturated scale-forming substances, enable additional water recovery from RO concentrate, and reduce the risk of eutrophication after brine disposal. This study investigated the impact of photochemical degradation of three widely used antiscalants, i.e., nitrilotri-methylenephosphonic acid (NTMP), ethylenediaminetetra-methylenephosphonic acid (EDTMP) and diethylenetriaminepentakis-methylphosphonic acid (DTPMP) with hydrogen peroxide and persulfate as the photo-oxidants and on inducing the precipitation of calcium sulfate. Results showed that the antiscalants could be effectively degraded at the optimized conditions. DTPMP degrades much slower compared to NTMP and EDTMP with all the oxidants in this study. DTPMP also has much higher inhibition capacity in delaying the precipitation of gypsum. The degradation byproduct generated by the oxidation of DTPMP are likely responsible for the higher inhibition capacity in delaying the precipitation. Among the oxidants tested, persulfate shows the highest degradation rate of all the antiscalants tested. It was also observed that induction time depends on the extent of UV exposure. This study shows an effective way to increase the water recovery in desalination concentrate with the help of a UV/oxidant-assisted removal of TDS and thereby employing a secondary RO.

The occurrence of hexavalent chromium Cr(VI) as a carcinogen in drinking water is widely reported. Cr can accumulate in corrosion scales in drinking water distribution systems (DWDSs) and act as the potential source for Cr(VI) release. The goal of this dissertation is to elucidate the mechanisms of Cr(VI) formation through the oxidation of Cr solids by disinfectant free chlorine, and to provide control strategies for Cr(VI) formation in DWDSs. First, X-Ray Absorption Spectroscopy (XAS) analysis discovered that zerovalent Cr(0) coexisted with trivalent Cr(III) in the corrosion scales. The oxidation of the corrosion scales by residual disinfectant chlorine released Cr(VI) and exhibited a three-phase kinetics behavior: an initial 2-hour fast reaction phase, a subsequent 2-to-12-hour transitional phase and a final 7-day slow reaction phase approximately two orders of magnitude slower than the initial phase. Because Cr(0)(s) exhibited a much higher reactivity than Cr(III)(s), the oxidation of Cr(0)(s) in the iron corrosion scales was the dominant reaction for the Cr(VI) formation in drinking water. Second, the corrosion control strategies through adjustments of chemical water parameters were investigated. The results showed an increase in pH, silicate, alkalinity, and calcium suppressed Cr(VI) formation that was mainly attributed to in situ surface precipitation of new Cr(III) solids on the surface of Cr(0)(s), including Cr(OH)3(s), Cr2(SiO3)3(s), CrPO4(s), Cr2(CO3)3(s) and Cr10Ca(CO3)16(s). The Cr(III) surface precipitates were much less reactive with chlorine than Cr(0)(s) and suppressed the Cr redox reactivity. Adding phosphate either promoted or inhibited the Cr(VI) formation, depending on the concentration. Third, it is investigated that the effects of bromide and manganese on Cr(VI) formation through the oxidation of Cr(0)(s) by free chlorine. The present of Br- and Mn2+ significantly enhanced the Cr (VI) formation. Br- and Mn2+ can react with free chlorine to form free bromine and manganese dioxide. Free bromine had faster reaction kinetics with Cr(0)(s) than free chlorine. Although manganese dioxide had slower reaction kinetics with Cr(0)(s) than free chlorine, it can accelerate the reaction between Cr(0)(s) and free chlorine. The outcome of this study aimed to provide control strategies to minimize Cr(VI) formation by inhibiting Cr(0) reactivity in drinking water distribution systems.

Climate change and population growth pose increasing challenges to the availability of fresh water resources. Remediation of contaminated groundwater and recycling of wastewater are two promising solutions to increase fresh water supplies. Persulfate (S2O82-)-based advanced oxidation processes (AOPs) have gained increasing attention in recent years due to the generation of highly reactive and selective sulfate radicals (SO4•-). S2O82--based AOPs have been widely used in in situ chemical oxidation (ISCO) for groundwater remediation. UV/S2O82- is also an alternative to UV/H2O2 that can be used to treat recycled wastewater into high quality drinking water.

In this dissertation work, we examined the application of S2O82--based AOPs in ISCO and UV/AOPs. First, the effects of important groundwater chemical parameters, i.e., alkalinity, pH and chloride on benzene degradation via heterogeneous S2O82- activation by three Fe(III)- and Mn(IV)-containing aquifer minerals (e.g. ferrihydrite, goethite and pyrolusite) were examined. A comprehensive kinetic model was established to elucidate the mechanisms of radical generation and mineral surface complexation. Results showed

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that an increase of alkalinity up to 10 meq/L decreased the rates of persulfate decomposition and benzene degradation, which was associated with the formation of unreactive surface carbonato complexes. An increase in pH generally accelerated persulfate decomposition due to enhanced formation of reactive surface hydroxo complexation. A change in the chloride level up to 5 mM had a negligibly effect on the reaction kinetics. Kinetics modeling also suggested that SO4•- was transformed into hydroxyl radicals (HO•) and carbonate radicals (CO3•-) at higher pHs.

Secondly, we investigated 1,4-dioxane degradation by UV/S2O82- in the presence of monochloramine (NH2Cl), a membrane anti-fouling reagent. The mixing effects of NH2Cl and S2O82- on 1,4-dioxane degradation were examined at various oxidant doses, chloride concentrations, solution pHs and dissolved O2 levels. Results showed that a NH2Cl-to-S2O82- molar ratio of 0.1 was optimal, beyond which the scavenging effects of NH2Cl on HO•, SO4•-, and Cl2•- radicals decreased 1,4-dioxane degradation rate. At the optimal ratio, the degradation rate of 1,4-dioxane increased linearly with the total oxidant dose up to 6 mM. The simultaneous photolysis of NH2Cl and S2O82- was sensitive to the solution’s pH, due to a disproportionation of NH2Cl into less photo-reactive dichloramine (NHCl2) and radical scavenger NH4+ at pH lower than 6. An increase of chloride concentration transformed reactive HO• and SO4•- to less reactive Cl2•-, while the presence of dissolved O2 promoted 1,4-dioxane degradation and gaseous nitrogen production by participating in the 1,4-dioxane and NH4+ oxidation steps.

Third, based on the experimental investigations, a kinetic model was developed and

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the fundamental mechanisms of contaminant degradation by three UV/AOPs, i.e., UV/hydrogen peroxide (H2O2), UV/S2O82- and UV/chlorine (HOCl) were simulated. Results showed that the treatment efficiency of contaminants generally followed the order of UV/S2O82- > UV/H2O2 > UV/HOCl under chemical conditions relevant to reverse osmosis (RO) permeate. The generation of HO� was important in UV/H2O2, whereas both SO4•- and HO� were important in UV/S2O82-. CO3•- predominated in UV/HOCl. Among the three UV/AOPs, the treatment efficiency of UV/S2O82- was most sensitive to pH, chloride and inorganic carbon. The results provided guidance on the design and optimization of UV/AOP systems for water reuse under diverse chemical conditions.

Finally, the oxidation byproducts (OBPs) of benzene and 1,4-dioxane by S2O82- based AOPs were investigated, and the cytotoxicity and genotoxicity levels of the parent compound and oxidation byproducts were compared. Results showed that phenol and aldehyde are the major products from benzene degradation. 1,4-dioxane was transformed in to six major intermediates after UV/AOP treatments including ethylene glycol diformate, formaldehyde, glycolaldehyde, glycolic acid, formic acid, and methoxyacetic acid. Toxicity assays showed that the aldehyde and phenol were extremely genotoxic and cytotoxic compare to benzene and 1,4-dioxane. Results suggested that evaluation of the efficiency of any AOPs applied to water treatment must not only include a study of the disappearance of the parent compound, but also needs a study of the toxicity of the subsequential oxidation byproducts.

Global water scarcity poses a major challenge to agricultural productivity. This dissertation investigates the use of recycled water for irrigation, focusing on the occurrence of pharmaceutical and personal care products (PPCPs), their accumulation in edible crops, and the impact of irrigation water quantity on this accumulation. Analysis reveals that PPCPs, including sulfamethoxazole, are present in recycled water at concentrations ranging from 130-1400 ng/L in secondary effluent and 25-400 ng/L in tertiary effluent. The study shows that PPCP uptake and accumulation vary between leafy and fruity vegetables, with diclofenac and fluoxetine being most prevalent in each, respectively. Key factors affecting PPCP accumulation include transpiration rate and osmotic adjustments under limited water availability. The research explores two strategies: recycled wastewater effluent irrigation and limited irrigation rates, aimed at mitigating PPCPs accumulation and conserving irrigation water. A 14-week field trial on St. Augustine turfgrass assesses the effects of UV persulfate (UV/PS) treatment and limited irrigation rates on PPCPs accumulation and plant health. Results indicate that UV/PS treatment effectively removes 60% of carbamazepine and over 99% of other PPCPs from recycled water, significantly reducing PPCP levels in turfgrass leaves and roots. Limited irrigation at 60% ETo increases carbamazepine accumulation and canopy temperature, suggesting higher water stress compared to 80% ETo. Additionally, greenhouse experiments with lettuce, carrot, and tomato, using PPCP-spiked recycled water, UV/PS treated recycled water, and tap water at 60%, 80%, and 100% crop evapotranspiration rates (ETc), show that UV/PS treatment reduces PPCP accumulation by over 99%. Lettuce benefits from reduced irrigation, while carrot and tomato show increased accumulation due to osmotic adjustment. Combining UV/PS treatment with deficit irrigation conserves water, maintains crop yield, and minimizes PPCP accumulation. The findings offer valuable insights for developing strategies to safely and effectively reuse recycled water in agriculture, supporting sustainable practices and improving food safety.