Optimizing Potable Water Reuse Systems: Chloramines or Hydrogen Peroxide for UV-Based Advanced Oxidation Process?

Built-in electric field of Ag2Se thermoelectric effect activated persulfate for humic acid decomposition in water: Molecular transformation mechanism

Water temperature fluctuations directly impact pollutant decomposition processes in wastewater. Thermoelectric effect is considered an alternative to utilize these temperature variations for pollution control. In this study, a system for persulfate (PS) activation by Ag2Se thermoelectric catalyst under water temperature gradients (Ag₂Se/ΔT/PS) was developed for humic acid (HA) degradation in water. The experimental results showed that the Ag2Se/ΔT/PS system achieved a 90.7 % HA removal efficiency, outperforming both PS/ΔT (PS with temperature gradients) and Ag2Se/ΔT systems. Thermoelectric simulations indicated that Ag2Se generated an electric field under temperature variations, with higher current density at surface pores where polarized charges efficiently activated PS. Density functional theory calculations revealed that the thermoelectric effect of Ag2Se lowered the energy barriers for PS activation and ·SO4- generation. Different from ·OH-led decomposition of HA in the Ag₂Se/ΔT system, ·SO4- and ·OH dominated HA decomposition in the Ag₂Se/ΔT/PS system, and ¹O₂ also contributed this process. Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) revealed that oxidation, decarboxylation, and sulfidation were the primary pathways driving HA degradation, leading to decreases in CHO-containing compounds and formation of S-rich byproducts. These findings highlighted the potential of thermoelectric catalysts in advancing water treatment technologies.

Advances in Two-Electron Water Oxidation Reaction for Hydrogen Peroxide Production: Catalyst Design and Interface Engineering

Hydrogen peroxide (H2O2) is a versatile and zero-emission material that is widely used in the industrial, domestic, and healthcare sectors. It is clear that it plays a critical role in advancing environmental sustainability, acting as a green energy source, and protecting human health. Conventional production techniques focused on anthraquinone oxidation, however, electrocatalytic synthesis has arisen as a means of utilizing renewable energy sources in conjunction with available resources like oxygen and water. These strides represent a substantial change toward more environmentally and energy-friendly H2O2 manufacturing techniques that are in line with current environmental and energy goals. This work reviews recent advances in two-electron water oxidation reaction (2e-WOR) electrocatalysts, including design principles and reaction mechanisms, examines catalyst design alternatives and experimental characterization techniques, proposes standardized assessment criteria, investigates the impact of the interfacial milieu on the reaction, and discusses the value of in situ characterization and molecular dynamics simulations as a supplement to traditional experimental techniques and theoretical simulations. The review also emphasizes the importance of device design, interface, and surface engineering in improving the production of H2O2. Through adjustments to the chemical microenvironment, catalysts can demonstrate improved performance, opening the door for commercial applications that are scalable through tandem cell development.

Nanoparticles in drinking water: Assessing health risks and regulatory challenges

Nanoparticles (NPs) pose a significant concern in drinking water due to their potential health risks and environmental impact. This review provides a comprehensive analysis of the current understanding of NP sources and contamination in drinking water, focusing on health concerns, mitigation strategies, regulatory frameworks, and future perspectives. This review highlights the importance of nano-specific pathways, fate processes, health risks & toxicity, and the need for realistic toxicity assessments. Different NPs like titanium dioxide, silver, nanoplastics, nanoscale liquid crystal monomers, copper oxide, and others pose potential health risks through ingestion, inhalation, or dermal exposure, impacting organs and potentially leading to oxidative stress, inflammatory responses, DNA damage, cytotoxicity, disrupt intracellular energetic mechanisms, reactive oxygen species generation, respiratory and immune toxicity, and genotoxicity in humans. Utilizing case studies and literature reviews, we investigate the health risks associated with NPs in freshwater environments, emphasizing their relevance to drinking water quality. Various mitigation and treatment strategies, including filtration systems (e.g., reverse osmosis, and ultra/nano-filtration), adsorption processes, coagulation/flocculation, electrocoagulation, advanced oxidation processes, membrane distillation, and ultraviolet treatment, all of which demonstrate high removal efficiencies for NPs from drinking water. Regulatory frameworks and challenges for the production, applications, and disposal of NPs at both national and international levels are discussed, emphasizing the need for tailored regulations to address NP contamination and standardize safety testing and risk assessment practices. Looking ahead, this review underscores the necessity of advancing detection methods and nanomaterial-based treatment technologies while stressing the pivotal role of public awareness and tailored regulatory guidelines in upholding drinking water quality standards. This review emphasizes the urgency of addressing NP contamination in drinking water and provides insights into potential solutions and future research directions. Lastly, this review worth concluded with future recommendations on advanced analytical techniques and sensitive sensors for NP detection for safeguarding public health and policy implementations.

Persulfate photolysis and limited irrigation of recycled wastewater for turfgrass growth: Accumulation of pharmaceutical and personal care products and physiological responses

Recycled wastewater effluent irrigation and implementing limited irrigation rates are two promising strategies for water conservation in agriculture. However, one major challenge is the accumulation and translocation of Pharmaceutical and Personal Care Products (PPCPs) from recycled water to crops. This study investigated the effects of UV persulfate (UV/PS) treatment of recycled water and limited irrigation rate on PPCPs accumulation and physiological responses of St. Augustine turfgrass via a 14-week field trial. Carbamazepine (CBZ), sulfamethoxazole (SMX), triclosan (TCS), fluoxetine (FLX) and diclofenac (DCF) were spiked at 0.1-1.5 µg/L into recycled water and two limited irrigation rates corresponding to 60 % and 80 % of reference Evapotranspiration (ETo) were applied. Results showed that UV/PS removed 60 % of CBZ and > 99 % of other PPCPs from recycled water. Irrigation with UV/PS treated recycled water resulted in approximately a 60 % reduction in CBZ accumulation and complete removal of SMX, DCF, FLX and TCS in both turfgrass leaves and roots. A more limited irrigation rate at 60 % ETo resulted in a higher accumulation of CBZ accumulation compared to 80 % ETo. Similarly, the canopy temperature increased under 60 % ETo irrigation rate compared to 80 % ETo, suggesting that turfgrass under 60 % ETo was more prone to water stress. Applying a 60 % ETo irrigation rate was not sufficient to maintain the turfgrass quality in the acceptable range. A negative correlation between the visual quality and cumulative mass of PPCPs in turfgrass leaves at different irrigation rates was observed, yet irrigation rate was the major driver of turfgrass overall quality and health. Insights from this study will help to integrate recycled water with treatment and limited irrigation, thereby enhancing agricultural water reuse practices.

Dichloramine Hydrolysis in Membrane Desalination Permeate: Mechanistic Insights and Implications for Oxidative Capacity in Potable Reuse Applications

Dichloramine (NHCl2) naturally exists in reverse osmosis (RO) permeate due to its application as an antifouling chemical in membrane-based potable reuse treatment. This study investigated mechanisms of background NHCl2 hydrolysis associated with the generation of oxidative radical species in RO permeate, established a kinetic model to predict the oxidative capacity, and examined its removal efficiency on trace organic contaminants in potable reuse. Results showed that NHCl2 hydrolysis generated transient peroxynitrite (ONOO-) and subsequently dissociated into hydroxyl radical (HO•). The maximal HO• exposure was observed at an RO permeate pH of 8.4, higher than that from typical ultraviolet (UV)-based advanced oxidation processes. The HO• exposure during NHCl2 hydrolysis also peaked at a NH2Cl-to-NHCl2 molar ratio of 1:1. The oxidative capacity rapidly degraded 1,4-dioxane, carbamazepine, atenolol, and sulfamethoxazole in RO permeate. Furthermore, background elevated carbonate in fresh RO permeate can convert HO• to carbonate radical (CO3•-). Aeration of the RO permeate removed total carbonate, significantly increased HO• exposure, and enhanced the degradation kinetics of trace organic contaminants. The kinetic model of NHCl2 hydrolysis predicted well the degradation of contaminants in RO permeate. This study provides new mechanistic insights into NHCl2 hydrolysis that contributes to the oxidative degradation of trace organic contaminants in potable reuse systems.

UV/TiO2 photocatalysis as post-treatment of anaerobic membrane bioreactor effluent for reuse

Advanced oxidation processes have been widely applied as a post-treatment solution to remove residual organic compounds in water reuse schemes. However, UV/TiO2 photocatalysis, which provides a sustainable option with no continuous chemical addition, has very rarely been studied to treat anaerobically treated effluents. In the current study, the removal of organics and nutrients from an anaerobic membrane bioreactor (AnMBR) effluent is evaluated during adsorption and photocatalysis processes under various conditions of TiO2 dose and UV intensity and compared to the effluent from an aerobic membrane bioreactor (AeMBR). The sequence for preferential adsorption on TiO2 was found to be phosphorus, inorganic carbon and then ammonia/organic carbon were found. The competing effect between the organics and nutrients, along with the low UV transmission efficiency caused by the need for high doses of TiO2, ultimately compromise the organic removal efficiency in the AnMBR permeate. TiO2 dosage was found to have a greater impact than UV intensity on improving the overall removal performance as nutrients are competing for the adsorption site but are not photodegraded. Under the same operational condition, the UV/TiO2 photocatalysis displayed a higher removal efficiency of organic matter and phosphorus in the AeMBR effluent due to a lower initial organics concentration and absence of ammonia as compared to the AnMBR effluent.

A multi-criteria decision framework for circular wastewater systems in emerging megacities of the Global South

Lima faces increasing water stress due to demographic growth, climate change and outdated water management infrastructure. Moreover, its highly centralized wastewater management system is currently unable to recover water or other resources. Hence, the primary aim of this study is to identify suitable wastewater treatment alternatives for both eutrophication mitigation and indirect potable reuse (IPR). For eutrophication mitigation, we examined MLE, Bardenpho, Step-feed, HF-MBR, and FS-MBR. For IPR, we considered secondary treatment+UF + RO + AOP or MBR + RO + AOP. These alternatives form part of a WWTP network at a district level, aiding Lima's pursuit of a circular economy approach. This perspective allows reducing environmental impacts through resource recovery, making the system more resilient to disasters and future water shortages. The methods used to assess these scenarios were Life Cycle Assessment for the environmental dimension; Life Cycle Costing for the economic perspective; and Multi-Criteria Decision Analysis to integrate both the quantitative tools aforementioned and qualitative criteria for social and techno-operational dimensions, which combined, strengthen the decision-making process. The decision-making steered towards Bardenpho for eutrophication abatement when environmental and economic criteria were prioritized or when the four criteria were equally weighted, while HF-MBR was the preferred option when techno-operational and social aspects were emphasized. In this scenario, global warming (GW) impacts ranged from 0.23 to 0.27 kg CO2eq, eutrophication mitigation varied from 6.44 to 7.29 g PO4- equivalent, and costs ranged between 0.12 and 0.17 €/m3. Conversely, HF-MBR + RO + AOP showed the best performance when IPR was sought from the outset. In the IPR scenario, GW impacts were significantly higher, at 0.46-0.51 kg CO2eq, eutrophication abatement was above 98 % and costs increased to ca. 0.44 €/m3.

The role of reactive chlorine and nitrogen species in micropollutant degradation in UV/monochloramine

Monochloramine (NH2Cl) is applied upstream of reverse osmosis (RO) membranes for biofouling control. Residual NH2Cl can undergo UV photolysis downstream, generating reactive species for an AOP to occur. At the bench-scale, NH2Cl is typically generated from combining sodium hypochlorite and ammonium chloride or sulfate. This study investigated the degradation of four compounds of interest - acetaminophen, caffeine, sucralose and 1,4-dioxane - in UV/NH2Cl at the bench scale to study their reactivity with reactive chlorine species (RCS) and reactive nitrogen species (RNS). With methanol acting as a scavenger of •OH radicals, the performance of UV/NH2Cl was compared to UV/H2O2 and UV/HOCl. In UV/H2O2, dioxane was severely inhibited at 1-2 mg/L H2O2 and comparable at 5 mg/L to UV/NH2Cl. When ammonium sulfate ((NH4)2SO4) was used as the ammonia source over ammonium chloride (NH4Cl), the overall degradation of micropollutants was higher and caffeine was exclusively degraded. At 1-2 mg/L NH2Cl, dioxane degraded by 16.2-17.8% and 2.92-5.29% from (NH4)2SO4 and NH4Cl respectively while caffeine degraded by 7.45-9.61% with NH2Cl ((NH4)2SO4), but not degrade with NH2Cl (NH4Cl). The higher concentration of chloride ions from NH4Cl significantly influenced the speciation of generated radicals and impacted micropollutant degradation. This suggests that the reactivity of more selective RCS (Cl2•-, •ClO, ClOH•-) and RNS (•NH2, •NO, •NO2, etc.) varies with micropollutants of interest. The presence of higher chloride concentration from the ammonia source inhibited the generation of •OH radicals with •OH consumed by RNS to form NO3- (μg/L levels), showing the impact of the choice of ammonia source and the water matrix on UV/NH2Cl performance.

Ozone- and Hydroxyl Radical-Induced Degradation of Micropollutants in a Novel UVA-LED-Activated Periodate Advanced Oxidation Process

In this study, novel light emitting diode (LED)-activated periodate (PI) advanced oxidation process (AOP) at an irradiation wavelength in the ultraviolet A range (UVA, UVA-LED/PI AOP) was developed and investigated using naproxen (NPX) as a model micropollutant. The UVA-LED/PI AOP remarkably enhanced the degradation of NPX and seven other selected micropollutants with the observed pseudo-first-order rate constants ranging from 0.069 ± 0.001 to 4.50 ± 0.145 min-1 at pH 7.0, demonstrating a broad-spectrum micropollutant degradation ability. Lines of evidence from experimental analysis and kinetic modeling confirmed that hydroxyl radical (•OH) and ozone (O3) were the dominant species generated in UVA-LED/PI AOP, and they contributed evenly to NPX degradation. Increasing the pH and irradiation wavelength negatively affected NPX degradation, and this could be well explained by the decreased quantum yield (ΦPI) of PI. The degradation kinetics of NPX by the UVA-LED/PI AOP in the presence of water matrices (i.e., chloride, bicarbonate, and humic acid) and in real waters were examined, and the underlying mechanisms were illustrated. A total of nine transformation products were identified from NPX oxidation by the UVA-LED/PI AOP, mainly via hydroxylation, dealkylation, and oxidation pathways. The UVA-LED/PI AOP proposed might be a promising technology for the treatment of micropollutants in aqueous solutions. The pivotal role of ΦPI during light photolysis of PI may guide the future design of light-assisted PI AOPs.

Earth Abundant Oxidation Catalysts for Removal of Contaminants of Emerging Concern from Wastewater: Homogeneous Catalytic Screening of Monomeric Complexes

Twenty novel Mn, Fe, and Cu complexes of ethylene cross-bridged tetraazamacrocycles with potentially copolymerizable allyl and benzyl pendant arms were synthesized and characterized. Multiple X-ray crystal structures demonstrate the cis-folded pseudo-octahedral geometry forced by the rigidifying ethylene cross-bridge and show that two cis coordination cites are available for interaction with substrate and oxidant. The Cu complexes were used to determine kinetic stability under harsh acidic and high-temperature conditions, which revealed that the cyclam-based ligands provide superior stabilization with half-lives of many minutes or even hours in 5 M HCl at 50-90 °C. Cyclic voltammetry studies of the Fe and Mn complexes reveal reversible redox processes indicating stabilization of Fe2+/Fe3+ and Mn2+/Mn3+/Mn4+ oxidation states, indicating the likelihood of catalytic oxidation for these complexes. Finally, dye-bleaching experiments with methylene blue, methyl orange, and rhodamine B demonstrate efficient catalytic decolorization and allow selection of the most successful monomeric catalysts for copolymerization to produce future heterogeneous water purification materials.

Photolysis of chloramines in vacuum-UV and vacuum-UV/chlorine advanced oxidation processes for removal of 1,4-dioxane: Effect of water matrix, kinetic modeling, and implications for potable reuse

Advanced oxidation processes (AOPs) are a key step in eliminating persistent micropollutants in potable reuse trains. Under such conditions, chloramines are an inevitable component in the AOP feed water given their application as an antifouling agent for the upstream membrane processes. In cases when other oxidants, such as free chlorine, are to be used in the AOP treatment, the effect of background chloramines and any potential interplays between the oxidants should be considered. In this study, vacuum-UV (VUV) and VUV/Cl₂ have been proposed as promising AOP alternatives for potable reuse and the effect of chloramine photolysis has been considered on the removal of 1,4-dioxane. Results indicated that while presence of chloramine reduces the treatment efficiency in the VUV AOP, coexistence of free chlorine and chloramine oxidants significantly improves 1,4-dioxane degradation rates. Experimental data and kinetic modeling both confirmed the roles of OH^• and Cl₂^•- in 1,4-dioxane removal with 62.5% and 32.5% contribution in the VUV/Cl₂/chloramines, respectively. Among the other water matrix conditions, Cl^- was shown to improve the degradation rates while HCO₃^- suppressed the reactions by scavenging radical species. Overall, the findings of this research are informative for the design and development of VUV AOPs at small scale potable reuse facilities.

Energy and material refineries of future: Wastewater treatment plants

There have been many important milestones on humanity's long journey towards achieving environmental sanitation. In particular, the development of the activated sludge system can be claimed to be one of the most groundbreaking advances in the protection of both public health and the wider ecosystem. The first wastewater treatment plants (WWTPs) were developed over a century ago and were soon configured for use with activated sludge. However, despite their long history and service, conventional activated sludge (CAS) plants have become an unsustainable method of wastewater treatment. In addition, conventional WWTPs are intensive energy-consumers and at best allow only very limited material recovery. A paradigm shift to convert existing WWTPs into more sustainable facilities must therefore be considered necessary and to this end the wastewater biorefinery (WWBR) concept may be considered a solution that maximizes both energy and material recovery, in line with the circular economy approach.

Mechanisms of oxidative removal of 1,4-dioxane via free chlorine rapidly mixing into monochloramine: Implications on water treatment and reuse

Free chlorine (HOCl) and monochloramine (NH₂Cl) are two of the most commonly used water disinfectants in water treatment; however, the capability of rapid mixing of HOCl into NH₂Cl to induce oxidative reactions for efficient removal of contaminants remains largely unknown. In this study, 1,4-dioxane (1,4-D) removal was quantified during the rapid mixing of HOCl into NH₂Cl, to evaluate the effects of solution pH and HOCl-to-NH₂Cl ratio, and to identify mechanisms by which reactive species are generated in the system. Results showed that the highest 1,4-D removal was observed at the near-neutral pH of 6 with the HOCl-to-NH₂Cl molar ratio of 1. Hydroxyl radical (HO^•) contributed to 60-70 % of 1,4-D degradation and its generation was initiated by the hydrolytic decay of NH₂Cl and NHCl₂ upon HOCl addition to NH₂Cl with rapid mixing, and subsequent transformation of peroxynitrite (ONOO^-) and peroxynitrous acid (ONOOH). The results also confirmed that the presence of dissolved oxygen was required to form ONOO^-/ONOOH, and ONOO^- was a crucial precursor for reactive radical generation. These findings provide insight into the reaction mechanism associated with the system of rapidly mixed HOCl into NH₂Cl with the potential optimization and application for efficient trace organics removal in water treatment and reuse.

Antibiotics degradation by UV/chlor(am)ine advanced oxidation processes: A comprehensive review

Antibiotics are emerging contaminants in aquatic environments which pose serious risks to the ecological environment and human health. Advanced oxidation processes (AOPs) based on ultraviolet (UV) light have good application prospects for antibiotic degradation. As new and developing UV-AOPs, UV/chlorine and derived UV/chloramine processes have attracted increasing attention due to the production of highly reactive radicals (e.g., hydroxyl radical, reactive chlorine species, and reactive nitrogen species) and also because they can provide long-lasting disinfection. In this review, the main reaction pathways of radicals formed during the UV/chlor (am)ine process are proposed. The degradation efficiency, influencing factors, generation of disinfection by-products (DBPs), and changes in toxicity that occur during antibiotic degradation by UV/chlor (am)ine are reviewed. Based on the statistics and analysis of published results, the effects caused by energy consumption, defined as electrical energy per order (EE/O), increase in the following order: UV/chlorine < UV/peroxydisulfate (PDS)< UV/H₂O₂ < UV/persulfate (PS) < 265 nm and 285 nm UV-LED/chlorine (EE/O). Some inherent problems that affect the UV/chlor (am)ine processes and prospects for future research are proposed. The use of UV/chlor (am)ine AOPs is a rich field of research and has promising future applications, and this review provides a theoretical basis for that.

Peracetic Acid Enhances Micropollutant Degradation by Ferrate(VI) through Promotion of Electron Transfer Efficiency

Ferrate(VI) and peracetic acid (PAA) are two oxidants of growing importance in water treatment. Recently, our group found that simultaneous application of ferrate(VI) and PAA led to much faster degradation of micropollutants compared to that by a single oxidant, and this paper systematically evaluated the underlying mechanisms. First, we used benzoic acid and methyl phenyl sulfoxide as probe compounds and concluded that Fe(IV)/Fe(V) was the main reactive species, while organic radicals [CH₃C(O)O^•/CH₃C(O)OO^•] had negligible contribution. Second, we removed the coexistent hydrogen peroxide (H₂O₂) in PAA stock solution with free chlorine and, to our surprise, found the second-order reaction rate constant between ferrate(VI) and PAA to be only about 1.44 ± 0.12 M^-1s^-1 while that of H₂O₂ was as high as (2.01 ± 0.12) × 10¹ M^-1s^-1 at pH 9.0. Finally, further experiments on ferrate(VI)-bisulfite and ferrate(VI)-2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic)acid systems confirmed that PAA was not an activator for ferrate(VI). Rather, PAA could enhance the oxidation capacity of Fe(IV)/Fe(V), making their oxidation outcompete self-decay. This study, for the first time, reveals the ability of PAA to promote electron transfer efficiency between high-valent metals and organic contaminants and confirms the benefits of co-application of ferrate(VI) and PAA for alkaline wastewater treatment.

Rate constants of chlorine atom reactions with organic molecules in aqueous solutions, an overview

Rate constants of chlorine atom (Cl^•) reactions (k_Cl•) determined using a large variation of experimental methods, including transient measurements, steady-state and computation techniques, were collected from the literature and were discussed together with the reaction mechanisms. The k_Cl• values are generally in the 10⁸-10⁹ mol^-1 dm³ s^-1 range when the basic reaction between the Cl^• and the target molecule is H-atom abstraction. When Cl^• addition to double bonds dominates the interaction, the k_Cl• values are in the 1 × 10⁹-2 × 10¹⁰ mol^-1 dm³ s^-1 range. In the k_Cl• = 1 × 10¹⁰-4 × 10¹⁰ mol^-1 dm³ s^-1 range, single-electron-transfer reactions may also contribute to the mechanism. The Cl^• reactions with organic molecules in many respects are similar to those of ^•OH, albeit Cl^• seems to be less selective as ^•OH. However, there is an important difference, as opposed to Cl^• in the case of ^•OH single-electron-transfer reactions have minor importance. The uncertainty of Cl^• rate constant determinations is much higher than those of ^•OH. Since Cl^• reactions play very important role in the emerging UV/chlorine water purification technology, some standardization of the rate constant measuring techniques and more k_Cl• measurements are recommended.

UV/chlorinated cyanurates as an emerging advanced oxidation process for drinking water and potable reuse treatments

Chlorinated cyanurates, prepared by application of hypochlorite to cyanuric acid at different ratios, have been commonly employed for disinfection. Combining UV with chlorinated cyanurates (UV/Cl-cyanurates) can be a novel and effective advanced oxidation process (AOP) because (1) Cl-cyanurates structurally resemble chlorinated amides that feature low reactivity with radicals, and (2) Cl-cyanurates, which bear multiple -Cl, may exhibit high molar absorptivity at 254 nm due to red-shifting absorption. Those chemiphysical properties of Cl-cyanurates may facilitate oxidant photolysis rate and lower radical scavenging rates in an AOP, thereby increasing steady-state concentrations of radicals. In this study, UV spectra measured for Cl-cyanurates highlighted molar absorptivities at 254 nm (∼200 M^-1cm^-1) much higher than free chlorine or H₂O₂, while k_•OH determined using competition kinetics suggests low •OH reactivity (<1.95 × 10⁷ M^-1s^-1) for Cl-cyanurates. Photolysis of Cl-cyanurates forms •Cl (i.e., Cl-N cleavage), and •Cl converts to •OH; formation of •OH during a UV/Cl-cyanurates AOP was evaluated using terephthalate as a probe compound. Experiments systematically investigated the effects of pH, Cl₂ dosage, and cyanuric concentration (three key factors affecting the equilibrium concentrations of chlorinated-cynaurate species) on the efficacy of removing three indicator contaminants by UV/Cl-cyanurates AOP. UV/Cl-cyanurates AOP conducted in phosphate buffers or authentic surface waters highlighted efficiencies up to 170% higher than UV/Cl₂ AOP at neutral pH when the same dosage of oxidants was employed, and the presence of certain levels of background ammonia or chloramines further enhanced its performance. Transformation of cyanuric acid or Cl-cyanurates by reacting with radicals during a UV/Cl-cyanurates AOP treatment was minimum. Toxicity assay indicated that UV/Cl-cyanurates AOP treated water was comparable or less toxicity than UV/H₂O₂ or UV/Cl₂ AOP treated water, and the initial cost estimate indicates UV/Cl-cyanurates AOP is potentially a cost-effective alternative AOP.

Micropollutant abatement by the UV/chloramine process in potable water reuse: A review

The need in using reclaimed water increased significantly to address the water shortage and its continuing quality deterioration in sustaining societal development. Degrading micropollutants in wastewater treatment plant effluents is one of the most important tasks in supplying safe drinking water, which is often achieved by full advanced treatment technologies (FATs), including reverse osmosis (RO) and the UV-based advanced oxidation process (AOP). As an emerging AOP, UV/chloramine process shows many noteworthy advantages in the scenario of potable water reuse, including membrane biological fouling control by chloramine, producing highly reactive radicals (e.g., Cl^•, HO^•, Cl₂^•-, and reactive nitrogen-containing species) to degrade the RO permeated pollutants, and acting as long-lasting disinfectant in the potable water distribution system. In addition, chloramine is often designedly produced by taking advantage of the ammonia in source. Thus, UV/chloramine processes gather much attention from researcher and published papers on UV/chloramine process have drastically increased since 2016, which were thoroughly reviewed in this paper. The fundamentals of chloramine photolysis, including the photolysis kinetics, the quantum yield, the generation and transformation of radicals and the final products, were scrutinized. Further, the impacts of reaction conditions such as pH, chloramine dosage and water matrix on the degradation of micropollutants by the UV/chloramine process are discussed. Moreover, the formation potential of disinfection by-products is debated. The opportunity of application of the UV/chloramine process in real-world practice is also presented, emphasizing the need for extensive efforts to remove currently prevalent knowledge roadblocks.

Photolysis of Chlorine Dioxide under UVA Irradiation: Radical Formation, Application in Treating Micropollutants, Formation of Disinfection Byproducts, and Toxicity under Scenarios Relevant to Potable Reuse and Drinking Water

Conversion of potable reuse water utilities and drinking water utilities from a low-pressure UV/H₂O₂ (LPUV/H₂O₂) advanced oxidation process (AOP) to alternative AOPs in which oxidants can effectively absorb photons and rapidly generate radicals has attracted great interest. Herein, we propose a novel UVA/ClO₂ AOP for different water treatment scenarios because of reduced photon absorption by the background matrix and high molar absorptivity for ClO₂ at UVA wavelengths. While the photolysis of ClO₂ produces ^•Cl + O₂ or ^•ClO + O(³P) via distinct product channels, we determined the parameters needed to accurately model the loss of oxidants and the formation of byproducts and combined a kinetic model with experimental data to determine quantum yields (Φ). Modeling incorporating the optimized Φ simultaneously predicted oxidant loss and the formation of major products -HOCl, Cl^-, and ClO₃^-. We also systematically investigated the removal of three contaminants exhibiting different radical reactivities, the formation of 35 regulated and unregulated halogenated disinfection byproducts (DBPs), DBP-associated toxicity, and N-acetylcysteine thiol reactivity in synthetic or authentic RO permeates/surface waters treated by different AOPs. The kinetic model developed in this study was used to optimize operating conditions to control undesired products and improve contaminant removal efficiency. The results indicate that UVA/ClO₂ can outperform LPUV/H₂O₂ in terms of electrical energy per order of contaminant degradation, disinfection byproduct formation, and toxicity indices.

Evaluation of advanced oxidation processes for β-blockers degradation: a review

This study summarizes the results of scientific investigations on the removal of the three most often used β-blockers (atenolol, metoprolol and propranolol) by various advanced oxidation processes (AOP). The free radical chemistry, rate constants, degradation mechanism and elimination effectiveness of these compounds are discussed together with the technical details of experiments. In most AOP the degradation is predominantly initiated by hydroxyl radicals. In sulfate radical anion-based oxidation processes (SROP) both hydroxyl radicals and sulfate radical anions greatly contribute to the degradation. The rate constants of reactions with these two radicals are in the 10⁹-10¹⁰ M^-1 s^-1 range. The degradation products reflect ipso attack, hydroxylation on the aromatic ring and/or the amino moiety and cleavage of the side chain. Among AOP, photocatalysis and SROP are the most effective for degradation of the three β-blockers. The operating parameters have to be optimized to the most suitable effectiveness.

UV-based advanced oxidation of dissolved organic matter in reverse osmosis concentrate from a potable water reuse facility: A Parallel-Factor (PARAFAC) analysis approach

Disposal of reverse osmosis concentrate (ROC) from advanced water purification facilities is a challenge associated with the implementation of reverse osmosis-based treatment of municipal wastewater effluent for potable reuse. In particular, the dissolved organic matter (DOM) present in ROC diminishes the quality of the receiving water upon environmental disposal and affects the toxicity, fate, and transport of organic contaminants. This study investigates UV-based advanced oxidation processes (UV-AOPs) for treating DOM in ROC using a Parallel Factor Analysis (PARAFAC) approach. DOM composition and degradation were tested in UV-only and three UV-AOPs using hydrogen peroxide (H₂O₂), free chlorine (Cl₂), and persulfate (S₂O₈^2-). The four-component PARAFAC model consisted of two terrestrial humic-like components (C_UVH and C_VisH), a wastewater/nutrient tracer component (C_NuTr), and a protein-like (tyrosine-like) component (C_PrTy). Based on the observed loss in the maximum fluorescence intensity of the components, DOM degradation was determined to be dependent on UV fluence, oxidant dose, and dilution factor of the ROC (i.e., bulk DOM concentration). C_VisH was most the photolabile component in the UV-only system, followed by C_NuTr, C_PrTy, and C_UVH, respectively. Furthermore, UV-H₂O₂ and UV-S₂O₈^2- displayed faster overall reaction kinetics compared to UV-Cl₂. The degradation trends suggested that C_NuTr and C_PrTy consisted of chemical moieties that were susceptible to reactive oxygen species (HO^•) but not reactive chlorine species; whereas, C_VisH was sensitive to all reactive species generated in the three UV-AOPs. Compared to other components, C_PrTy was recalcitrant in all treatment scenarios tested. Calculations using chemical probe-based analysis also confirmed these trends in the reactivity of DOM components. The outcomes of this study form a foundation for characterizing ROC reactivity in UV-AOP treatment technologies, to ultimately improve the sustainability of water reuse systems.

Rate constants of dichloride radical anion reactions with molecules of environmental interest in aqueous solution: a review

Natural waters, water droplets in the air at coastal regions and wastewaters usually contain chloride ions (Cl-) in relatively high concentrations in the milimolar range. In the reactions of highly oxidizing radicals (e.g., •OH, •NO3, or SO4•-) in the nature or during wastewater treatment in advanced oxidation processes the chloride ions easily transform to chlorine containing radicals, such as Cl•, Cl2•-, and ClO•. This transformation basically affects the degradation of organic molecules. In this review about 400 rate constants of the dichloride radical anion (Cl2•-) with about 300 organic molecules is discussed together with the reaction mechanisms. The reactions with phenols, anilines, sulfur compounds (with sulfur atom in lower oxidation state), and molecules with conjugated electron systems are suggested to take place with electron transfer mechanism. The rate constant is high (107-109 M-1 s-1) when the reduction potential the one-electron oxidized species/molecule couple is well below that of the Cl2•-/2Cl- couple.

Graphitic carbon nitride photocatalysis: the hydroperoxyl radical role revealed by kinetic modelling

The behaviour of graphitic carbon nitride photocatalysis for phenol removal and H2O2 evolution was fully analysed by kinetic modelling, rediscovering the contribution of oxygen, reactive oxygen species, photogenerated holes and intermediate products.