Photolytic dehalogenation of disinfection byproducts in water by natural sunlight irradiation

Evaluating the photochemical reactivity of disinfection byproducts formed during seawater desalination processes

Reverse osmosis (RO) is widely used for seawater desalination but pre-chlorination of intake water produces halogenated disinfection byproducts (DBPs). The fate and environmental impacts associated with the discharge of DBP-containing RO brine wastewater are unknown. Therefore, to evaluate if photochemistry plays a role in DBP degradation in seawater, we collected samples at a desalination plant, which were desalted and concentrated using two-inline solid phase extraction (SPE) techniques combining reverse-phase polymeric (PPL) and weak anion exchange (WAX) resins. Both filtered water samples and SPE samples (extracts reconstituted in open ocean seawater) were exposed to simulated sunlight in a custom-built flow-through system. Optical property analysis during irradiation experiments did not provide distinguishing features between intake water and RO reject water (brine). Extractable organic bromine (organoBr) concentrations were low in intake water samples (7.8 μg Br L-1) and did not change significantly due to irradiation. OrganoBr concentrations in laboratory-chlorinated raw water were much higher (135 μg Br L-1) and on average decreased by 42 % after 24 h irradiation. However, while organoBr concentrations were highest in RO reject water (473 μg Br L-1), changes in organoBr concentrations in PPL SPE samples after 24 h irradiation were variable, ranging from a 1-46 % loss. Furthermore, most bromine-containing molecular ions identified by high resolution mass spectrometry that were present in RO reject water before irradiation were also found after both 24 h and 50 h exposures. Although only one RO reject water sample was tested in this study, results highlight that hundreds of yet to be identified brominated DBPs in RO reject water could be resistant to photodegradation or phototransform into existing DBPs in the environment. Future work examining the biolability of DBPs in RO reject water, as well as the interplay between photochemical and biological DBP cycling, is warranted.

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.

Freezing-Enhanced Photoreduction of Iodate by Fulvic Acid

Iodate is a stable form of iodine species in the natural environment. This work found that the abiotic photosensitized reduction of iodate by fulvic acid (FA) is highly enhanced in frozen solution compared to that in aqueous solution. The freezing-induced removal of iodate by FA at an initial pH of 3.0 in 24 h was lower than 10% in the dark but enhanced under UV (77.7%) or visible light (31.6%) irradiation. This process was accompanied by the production of iodide, reactive iodine (RI), and organoiodine compounds (OICs). The photoreduction of iodate in ice increased with lowering pH (pH 3-7 range) or increasing FA concentration (1-10 mg/L range). It was also observed that coexisting iodide or chloride ions enhanced the photoreduction of iodate in ice. Fourier transform ion cyclotron resonance mass spectrometric analysis showed that 129 and 403 species of OICs (mainly highly unsaturated and phenolic compounds) were newly produced in frozen UV/iodate/FA and UV/iodate/FA/Cl- solution, respectively. In the frozen UV/iodate/FA/Cl- solution, approximately 97% of generated organochlorine compounds (98 species) were identified as typical chlorinated disinfection byproducts. These results call for further studies of the fate of iodate, especially in the presence of chloride, which may be overlooked in frozen environments.

Potential and mechanism of disinfection by-products removal in drinking water by bubbling corona discharge

Disinfection by-products (DBPs) with significant teratogenic and carcinogenic properties have become a growing concern among the public. As an efficient and environmentally friendly technology, non-thermal plasma offers potential for removing emerging micro-pollutants. In this study, the degradation performance of bubbling corona discharge was evaluated on 24 halogenated alicyclic and aliphatic DBPs present in drinking water at concentrations ranging from ng/L to μg/L. The degradation of DBPs followed pseudo-first-order kinetics with rate constants (kobs) in the descending order of halonitromethanes (HNMs), halogenated benzoquinones (HBQs), haloacetonitriles, trihalomethanes (THMs), haloketones, halogenated aldehydes, and haloacetic acids (HAAs). THMs, HNMs, and HBQs were effectively removed within 5 min under a discharge power of 28 W. Degradation rates achieved by plasma treatment surpass those of other conventional treatment technologies. The required energy consumption was in the range of 5-30 kW·h/m3/order. Furthermore, the study investigated the effects of discharge power, initial concentration, and economic analysis on the degradation of four selected DBPs as representatives of mono-, di- and multi-carbon-containing DBPs, namely chloroform (TCM) and bromoform (TBM), tribromoacetic acid (TBAA), and 2,3,5,6-tetrachloro-1,4-benzoquinone (TetraC-BQ). Reactive radicals in the plasma system were investigated using electron paramagnetic resonance, optical emission spectroscopy, fluorimetry, and radical scavengers. Hydrated electrons and hydroxyl radicals played an important role in the removal of DBPs. The intermediates generated during the degradation of TCM, TBM, TBAA, and TetraC-BQ were identified, and the possible degradation pathways for mono- and binary C-DBPs and HBQs were deduced. The breakdown of HBQs did not produce secondary contamination with aliphatic DBPs. The carbon in DBPs was primarily converted to formic acid, acetic acid, and oxalic acid, and the halogens were mainly converted to halogen ions. Additionally, luminescent bacteria toxicity testing confirmed that plasma treatment could reduce the acute toxicity of water samples. These findings demonstrate the potential of plasma treatment as a post-treatment device at the household level.

The impact of inorganic ions on the solar photolysis of chlorinated dissolved organic matter from different sources: Spectral characteristics, disinfection byproducts, and biotoxicities

Dissolved organic matter (DOM) from wastewater treatment plant (WWTP) effluent is chlorinated and then discharged into natural waters, where it is subject to solar irradiation. However, the impacts of inorganic ions in natural waters on the photochemical transformations of the chlorinated DOM (DOM-Cl) have not been studied comprehensively. In this study, variations in the spectral characteristics, disinfection byproducts (DBPs), and biotoxicities of DOM-Cl under solar irradiation at different pH values and in the presence of NO3- and HCO3- were revealed. Three sources of DOM, including DOM from a WWTP effluent, natural organic matter from the Suwannee River, and DOM from plant leaf leachate, were investigated. Solar irradiation resulted in the oxidation of the highly reactive aromatic structures and then reduced the amounts of chromophoric and fluorescent DOM, especially under alkaline conditions. Moreover, alkaline conditions significantly promoted the detected DBPs degradation and the biotoxicities attenuation, while NO3- and HCO3- generally impeded them (or did not work). Dehalogenation of the unknown halogenated DBPs and photolysis of the nonhalogenated organics were the main mechanisms for the DOM-Cl biotoxicity reductions. Hence, improving the ecological safety of WWTP effluents could be achieved through solar irradiation by removing the DBPs formed.

Degradation of natural organic matter and disinfection byproducts formation by solar photolysis of free available chlorine

Environment disinfection effectively curbs transmission of the Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2). However, elevated concentration of free available chlorine (FAC) in disinfectants can be discharged into surface water, generating toxic disinfection byproducts (DBPs). The impact of solar photolysis of FAC on natural organic matter (NOM) to form DBPs has not been well studied. In this work, solar photolysis of FAC was found to result in higher formation of DBPs, DBPs formation potential (DBPsFP), total organic chlorine (TOCl) and lower specific ultraviolet absorbance at 254 nm (SUVA₂₅₄), compared to dark chlorination. In solar photolysis of FAC, formation of total DBPs was promoted by pH=8, but hindered by the addition of HCO₃^-, radical scavenger or deoxygenation, while addition of NO₃^-and NH₄⁺both enhanced the formation of nitrogenous DBPs. Differences in the formation of DBPs in solar photolysis of FAC under various conditions were influenced by reactive species. The formation of trichloromethane (TCM) and haloacetic acids (HAAs) in solar photolysis of FAC positively correlated with the steady-state concentrations of ClO^• and O₃. The steady-state concentrations of ^•NO and ^•NH₂ positively correlated with the formation of halonitromethanes (HNMs). HAAs and haloacetonitriles (HANs) mainly contributed to calculated cytotoxicity of DBPs. This study demonstrates that solar photolysis of FAC may significantly impact the formation of DBPs in surface water due to extensive use of disinfectants containing FAC during SARS-CoV-2 pandemic.

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.

Solar photocatalytic degradation of total organic halogen in water using TiO2 catalyst

Disinfection byproducts (DBPs) in treated wastewater effluents pose environmental and health risks during water reuse. Solar-TiO₂ photocatalysis is a promising technology for degrading organic pollutants in treated effluents. In this study, total organic halogen (TOX) was used as an analytical tool to determine the efficiency of solar-TiO₂ photocatalytic process for the dehalogenation of DBPs in water. Natural solar photocatalytic experiments using TiO₂ particles were conducted to evaluate dehalogenation kinetics of different TOX groups formed by fulvic acid including total organic chlorine (TOCl), bromine (TOBr) and iodine (TOI). The results showed that the mixed phase TiO₂ (Aeroxide P25) was much more effective at TOX removal than the anatase (Hombikat UV-100) and rutile (TiOxide) TiO₂ particles. The TOX photocatalytic degradation rates of different halogen substituents ranked as TOI > TOCl (NH₂Cl) > TOBr > TOCl (Cl₂). The TOX removal followed first-order kinetics with half-lives of 42.8, 11.0, 5.0 and 2.7 min for TOCl (Cl₂), TOBr, TOCl (NH₂Cl), and TOI, respectively, at the 100 mg L^-1 TiO₂ dose. The TOX dehalogenation was enhanced at pH 9 compared to pH 5, and the addition of hydrogen peroxide had limited improvement in the TOX removal. Hydrophobic and molecular weight (MW) > 1 kDa fractions of TOCl (Cl₂) were more susceptible to the solar photocatalytic process than the hydrophilic and MW < 1 kDa fractions. The solar-TiO₂ photocatalytic process also effectively removed TOX in chlorinated and chloraminated wastewater samples. The results of this study suggest that the solar-TiO₂ photocatalysis is an effective treatment technology for TOX removal in water reuse.

Occurrence of brominated disinfection by-products in thermal spas

Thermal spas are gaining more and more popularity among the population because they are used for recreational purposes. Disinfecting these baths without losing the health benefits poses a challenge for swimming pool operators. Previous studies have mainly focused on regulated chlorinated DBPs in freshwater pools with no bromide or seawater pools with very high bromide content. Thermal water pools have a low bromide content and in combination with chlorine can lead to chlorinated, brominated and mixed halogenated DBP species. The occurrence of brominated and mixed halogenated DBPs in these types of pools is largely unexplored, with very few or limited studies published on regulated DBPs and even fewer on emerging DBP classes. In the field of swimming pool water disinfection, apart from extensive studies in the field of drinking water disinfection, only a few studies are known in which >39 halogenated and 16 non-halogenated disinfection by-products, including regulated trihalomethanes (THM) and haloacetic acids (HAA), were investigated in swimming pool water. Calculated bromine incorporation factor (BIF) demonstrated that even small amounts of bromide in swimming pool water can lead to a large shift in DBP species towards brominated and mixed halogenated DBPs. Dihaloacetonitriles (DHANs) accounted for >50% of the calculated cytotoxicity and genotoxicity on average. Comparison of the target analysis with the TOX showed that a major part of the measured TOX (69% on average) could be explained by the regulated classes THMs, HAAs, and the unregulated class of HANs. This study aims to help operators of swimming pools with bromide-containing water to gain a better understanding of DBP formation in future monitoring and to fill the knowledge gap that has existed so far on the occurrence of DBPs in thermal water pools.

Ecotoxicological effects of DBPs on freshwater phytoplankton communities in co-culture systems

Intensive disinfection of wastewater during the COVID-19 pandemic might elevate the generation of toxic disinfection byproducts (DBPs), which has triggered global concerns about their ecological risks to natural aquatic ecosystems. In this study, the toxicity of 17 DBPs typically present in wastewater effluents on three representative microalgae, including Scenedesmus sp. (Chlorophyta), Microcystis aeruginosa (Cyanophyta), and Cyclotella sp. (Bacillariophyta) was investigated. The sensitivities of the three microalgae to DBPs varied greatly from species to species, indicating that DBPs may change the structure of phytoplankton communities. Later, co-cultures of these phytoplankton groups as a proxy of ecological freshwater scenario were conducted to explore the impacts of DBPs on phytoplankton community succession. M. aeruginosa became surprisingly dominant in co-cultures, representing over 50% after dosing with monochloroacetic acid (MCAA, 0.1-10 mg/L). The highest proportion of M. aeruginosa was 70.3% when exposed to 2 mg/L MCAA. Although Scenedesmus sp. dominated in monochloroacetonitrile (MCAN) exposure, M. aeruginosa accounted for no less than 30% even at 40 mg/L MCAN. In this study, DBPs disrupted the original inter-algal relationship in favor of M. aeruginosa, suggesting that DBPs may contribute to the outbreak of cyanobacterial blooms in aquatic ecosystems.

An overview on non-spherical semiconductors for heterogeneous photocatalytic degradation of organic water contaminants

Because of their carcinogenicity and mutagenicity, the elimination of organic contaminants from surface and subsurface water is a subject of environmental significance. Conventional water decontamination approaches such as membrane separation, ultrafiltration, adsorption, reverse osmosis, coagulation, etc., have relatively higher operating costs and can generate highly toxic secondary contaminants. On the other hand, heterogeneous photocatalysis, an advanced oxidation process (AOP), is considered a clean and cost-effective process for organic pollutants degradation. Owing to their distinctive structure and physicochemical properties non-spherical semiconductors have gained considerable limelight in the photocatalytic degradation of organic contaminants. The current review briefly introduces a wide range of organic water contaminants. Recent advances in non-spherical semiconductor assembly and their photocatalytic degradation applications are highlighted. The underlying mechanism, fundamentals of photocatalytic reactions, and the factors affecting the degradation performance are also alluded including the current challenges and future research perspectives.

Treating disinfection byproducts with UV or solar irradiation and in UV advanced oxidation processes: A review

This review focuses on the degradation kinetics and mechanisms of disinfection byproducts (DBPs) under UV and solar irradiation and in UV-based advanced oxidation processes (AOPs). A total of 59 such compounds are discussed. The processes evaluated are low pressure, medium pressure and vacuum UV irradiation, solar irradiation together with UV/hydrogen peroxide, UV/persulfate and UV/chlorine AOPs. Under UV and solar irradiation, the photodegradation rates of N-nitrosamines are much higher than those of halogenated DBPs. Among halogenated DBPs, those containing iodine are photodegraded more rapidly than those containing bromine or chlorine. This is due to differences in their bond energies (E_N-N < E_C-I < E_C-Br < E_C-Cl). Molar absorption coefficients at 254 nm and energy gaps can be used to predict the photodegradation rates of DBPs under low pressure UV irradiation. But many DBPs of interest cannot be degraded to half their original concentration with less than a 500 mJ cm^-2 dose of low pressure UV light. HO^• generally contributes to less than 30% of the degradation of DBPs except iodo-DBPs in UV/H₂O₂ AOPs. Reaction mechanisms under UV irradiation and in HO^•-mediated oxidation are also summarized. N-N bond cleavage initiates their direct UV photolysis of N-nitrosamines as C-X cleavage does among halogenated compounds. HO^• generally initiates degradation via single electron transfer, addition and hydrogen abstraction pathways. Information on the reaction rate constants of SO₄^•- and halogen radicals with DBPs is rather limited, and little information is available about their reaction pathways. Overall, this review provides improved understanding of UV, solar and AOPs.

Making Swimming Pools Safer: Does Copper-Silver Ionization with Chlorine Lower the Toxicity and Disinfection Byproduct Formation?

Swimming pools are commonly treated with chlorine, which reacts with the natural organic matter and organic matter introduced by swimmers and form disinfection byproducts (DBPs) that are associated with respiratory-related issues, including asthma, in avid swimmers. We investigated a complementary disinfectant to chlorine, copper-silver ionization (CSI), with the aim of lowering the amount of chlorine used in pools and limiting health risks from DBPs. We sampled an indoor and outdoor pool treated with CSI-chlorine during the swimming season in 2017-2018 and measured 71 DBPs, speciated total organic halogen, in vitro mammalian cell cytotoxicity, and N-acetyl-l-cysteine (NAC) thiol reactivity as a cytotoxicity predictor. Controlled, simulated swimming pools were also investigated. Emerging DBP concentrations decreased by as much as 80% and cytotoxicity decreased as much as 70% in the indoor pool when a lower chlorine residual (1.0 mg/L) and CSI was used. Some DBPs were quantified for the first time in pools, including chloroacetaldehyde (up to 10.6 μg/L), the most cytotoxic haloacetaldehyde studied to date and a major driver of the measured cytotoxicity in this study. Three highly toxic iodinated haloacetic acids (iodoacetic acid, bromoiodoacetic acid, and chloroiodoacetic acid) were also quantified in pools for the first time. We also found that the NAC thiol reactivity was significantly correlated to cytotoxicity, which could be useful for predicting the cytotoxicity of swimming pool waters in future studies.

UV/ peroxymonosulfate process for degradation of chloral hydrate: Pathway and the role of radicals

In this study, kinetics, influencing factors and potential mechanisms involved in the degradation of chloral hydrate (CH) by UV/peroxymonosulfate (PMS) process were demonstrated. The degradation rate of CH could reach 89.6% by UV₂₅₄/PMS process, significantly exceeding UV₃₀₀/PMS (0.7%), UV₃₅₀/PMS (6.3%), UV₂₅₄ direct photolysis (9.0%) and PMS alone (0.0%) processes. CH degradation in UV₂₅₄/PMS system followed pseudo first-order degradation kinetics with an apparent rate constant of 0.186 min^-1, which was suppressed by Cl^- and HCO₃^-. The optimal pH for CH degradation was around 5.0. Direct mineralization accounted for the CH degradation in UV/PMS system. Interestingly, the addition of PMS at the neutral condition before UV irradiation transferred CH into trichloroacetic acid (TCAA). The transformation efficiency of CH into TCAA at 10 min was enhanced from 2.17%-40.38% with the elevation of initial pH from 7.0-8.0. The subsequent exposure of UV lamps ceased the transformation of CH into TCAA and facilitated the direct mineralization of CH, but it did not work in the refractory TCAA degradation. Finally, it was revealed that HO predominantly participated CH degradation in UV/PMS process, while O₂^- was responsible for the transformation of CH into TCAA by addition of PMS before UV irradiation.

Photolysis and photocatalysis of haloacetic acids in water: A review of kinetics, influencing factors, products, pathways, and mechanisms

Haloacetic acids (HAAs) are a group of pollutants ubiquitous in natural environment and anthropogenic systems, and therefore in need of control. Photolysis and photocatalysis techniques via ultraviolet (UV)-based technologies have held promise for decades in degrading organic molecules in water, but their capacities in removing HAAs remain to be explored. To better understand the trends in the existing literature and to identify the knowledge gaps that may merit further exploration, this review compares the HAAs photodegradation kinetics, influencing factors, reaction products, pathways, and mechanisms for a variety of UV technologies. The selected UV processes are classified into three types: UV-only photolysis, photooxidation, and photoreduction. Overall, although trends vary significantly depending upon many factors, the photo-susceptibility of HAAs always increases with rising molecular weight of substituted halogen atom(s), with those chlorinated HAAs being the most refractory species. Notably, while many processes proved hydroxyl radical (OH) as the forcing driver, the patterns of kinetics among HAAs were not consistent among processes, suggesting that OH was not the only driver. Compared to earlier studies focusing on specific technologies to treat numerous contaminants through a material perspective, this review commits to understanding the commonalities and differences among multiple UV-based technologies in treating only one group of compound mainly via a chemistry viewpoint.

In Situ-Formed PdFe Nanoalloy and Carbon Defects in Cathode for Synergic Reduction-Oxidation of Chlorinated Pollutants in Electro-Fenton Process

Complete dechlorination and mineralization of chlorophenols via the reduction-oxidation-mediated electro-Fenton process with a composite bulk cathode is first proposed. The in situ formation of a PdFe nanoalloy and carbon defects as key active sites is mutually induced during the formation of a carbon aerogel-based electrode. Specifically, the PdFe nanoalloy promotes the generation of [H]_ads as reduction sites and improves the electron transfer via an electrical circuit, while the carbon defects selectively favor the 2e^- oxygen reduction pathway. Notably, this work implies a novel electrocatalytic model for the formation of ·OH via (2 + 1)e^- oxygen reduction by a consecutive reaction with carbon defects and a PdFe nanoalloy. Complete total organic carbon removal and dechlorination of 3-chlorophenol were performed after 6 h. The kinetic rate constant for removing haloacetamides (HAMs) in drinking water was 0.21-0.41 h^-1, and the degradation efficiency was self-enhanced after electrolysis for 2 h because of the increased concentration of [H⁺]. The specific energy consumption was ∼0.55 W·h·g^-1 at 100% removal of some HAMs, corresponding to a power consumption of 0.6-1.1 kW·h for complete dehalogenation per ton of drinking water in waterworks. Moreover, the PdFe alloy/CA exhibited extreme mechanical and electrochemical stability with limited iron (∼0.07 ppm) and palladium (0.02 ppm) leaching during the actual application.

Phototransformation of halophenolic disinfection byproducts in receiving seawater: Kinetics, products, and toxicity

Flushing toilet with seawater is an effective method for preserving freshwater resources, but it introduces iodide and bromide ions into domestic wastewater. During chlorine disinfection, iodide and bromide ions in the saline wastewater effluent lead to the formation of iodinated and brominated aromatic disinfection byproducts (DBPs). Examples of aromatic DBPs include iodophenolic, bromophenolic and chlorophenolic compounds, which generally display substantially higher toxicity than haloaliphatic DBPs. This paper presented for the first time the rates of phototransformation of 21 newly identified halophenolic DBPs in seawater, the receiving waterbody of the wastewater effluent. The phototransformation rate constants (k) were in the range from 7.75 × 10^-4 to 4.62 × 10^-1 h^-1, which gave half-lives of 1.5-895 h. A quantitative structure-activity relationship was established for the phototransformation of halophenolic DBPs as logk=-0.0100×ΔG_f⁰+5.7528×logMW+0.3686×pKa-19.1607, where ΔG_f⁰ is standard Gibbs formation energy, MW is molecular weight, and pK_a is dissociation constant. This model well predicted the k values of halophenolic DBPs. Among the tested DBPs, 2,4,6-triiodophenol and 2,6-diiodo-4-nitrophenol were found to exhibit relatively high risks on marine organisms, based on toxicity indices and half-lives. In seawater, the two DBPs underwent photonucleophilic substitutions by bromide, chloride and hydroxide ions, resulting in the conversion to their bromophenolic and chlorophenolic counterparts (which are less toxic than the parent iodophenolic DBPs) and to their hydroxyphenolic counterparts (iodo(hydro)quinones, which are more toxic than the parent iodophenolic DBPs). The formed iodo(hydro)quinones further transformed to hydroxyl-iodo(hydro)quinones, which have lower toxicity than the parent compounds.

Photolysis of chloral hydrate in water with 254 nm ultraviolet: Kinetics, influencing factors, mechanisms, and products

Chloral hydrate (CH) is a common disinfection by-product found in treated water, and its effective control is important to human health. This study evaluated the effects of some environmental factors (e.g., pH, CH dosage, typical ions) and operational variables (e.g., lamp power, irradiation time) on CH photolysis efficiency via low-pressure mercury lamp-induced ultraviolet (LPUV) at 254 nm. The results demonstrated that the photolysis rate increased significantly with increasing pH from 7.0 to 10.5 and lamp power from 6 to 12 W. Meanwhile, the presence of nitrate, iodide, or free chlorine facilitated CH photolysis, whereas the existence of natural organic matter hindered the process. Together, these factors may help explain varying CH photolysis in different types of waters: seawater > ultrapure water > tap water > lake water. In addition, the initial CH dosage also played an important role, with higher CH being degraded more slowly. Mechanistically, although no catalyst or oxidant was added, CH photolysis was to some extent inhibited by a hydroxyl radical quencher, tert-butyl alcohol, suggesting that indirect photolysis was also responsible for CH loss. In terms of reaction products, the CH photolysis yielded primarily chloride ions and carbon dioxide, thus supporting mineralization as the key pathway. The results may help better understand the control of CH in water using UV.

Ultraviolet absorption redshift induced direct photodegradation of halogenated parabens under simulated sunlight

As disinfection by-products of parabens, halogenated parabens are frequently detected in aquatic environments and exhibit higher persistence and toxicity than parabens themselves. An interesting phenomenon was found that UV absorption redshift (∼45 nm) occurs after halogenation of parabens at circumneutral pH, leading to overlap with the spectrum of terrestrial sunlight. This work presents the first evidence on the direct photodegradation of seven chlorinated and brominated parabens under simulated sunlight. These halogenated parabens underwent rapid direct photodegradation, distinguished from the negligible degradation of the parent compounds. The photodegradation rate depended on their forms and substituents. The deprotonation of halogenated parabens facilitated the direct photodegradation. Brominated parabens exhibited higher degradation efficiency than chlorinated parabens, and mono-halogenated parabens had higher degradation than di-halogenated parabens. The pseudo-first-order rate constants (k_obs) for brominated parabens (0.075-0.120 min^-1) were approximately 7-fold higher than those of chlorinated parabens (0.011-0.017 min^-1). A quantitative structure-activity relationship (QSAR) model suggested that the photodegradation was linearly correlated with the C-X bond energies, electronic and steric effects of halogen substituents. The photodegradation products were identified using QTOF-MS analyses and a degradation pathway was proposed. The yeast two-hybrid estrogenicity assay revealed that the estrogenic activities of the photoproducts were negligible. These findings are important for the removal of halogenated parabens and predictions of their fate and potential impacts in surface waters.

Simultaneous determination of iodinated haloacetic acids and aromatic iodinated disinfection byproducts in waters with a new SPE-HPLC-MS/MS method

Iodinated disinfection byproducts (DBPs) are an emerging category of halogenated DBPs in concern due to their high toxicity. Among them, polar iodinated DBPs, mainly including iodinated haloacetic acids (HAAs) and aromatic iodinated DBPs, were reported to be especially toxic. Thus, simultaneous determination of these polar iodinated DBPs in disinfected waters is of great significance for DBP studies. In this study, it was found that traditional liquid-liquid extraction, which was adopted for the determination of polar iodinated DBPs, was actually not suitable for the determination of monoiodoacetic acid (MIAA) and diiodoacetic acid (DIAA) due to the low recoveries, and thus a new SPE-HPLC-MS/MS method was developed for the simultaneous determination of iodinated HAAs and aromatic iodinated DBPs. The parameters for SPE pretreatment were optimized, including SPE cartridge, eluent volume, formic acid content in eluent, and sample pH before SPE. The new method was demonstrated to be sensitive and accurate with detection limits of 0.15, 0.04, 0.03, 0.02, 0.06, and 0.06 ng/L, quantitation limits of 0.48, 0.13, 0.10, 0.06, 0.19, and 0.19 ng/L, and precision of 8.3%, 6.0%, 12.3%, 8.8%, 11.4%, and 15.6% for MIAA, DIAA, 3,5-diiodo-4-hydroxybenzaldehyde, 3,5-diiodosalicylic acid, 2,6-diiodo-4-nitrophenol and 2,4,6-triiodophenol, respectively. The recoveries of these six polar iodinated DBPs were all in the range of 70-110%. The new method was applied to the determination of iodinated HAAs and aromatic iodinated DBPs in nine tap water samples, and they were detected with concentrations ranging from 0.03 to 3.97 ng/L, among which MIAA was detected in all the samples with the highest concentrations.

Solar light irradiation significantly reduced cytotoxicity and disinfection byproducts in chlorinated reclaimed water

Chlorinated reclaimed water is widely used for landscaping and recreational purposes, resulting in human exposure to toxic disinfection byproducts. Although the quality of chlorinated reclaimed water might be affected by sunlight during storage, the effects of solar light irradiation on the toxicity remain unknown. This study investigated the changes in cytotoxicity and total organic halogen (TOX) of chlorinated reclaimed water exposed to solar light. Irradiation with solar light for 12 h was found to significantly reduce the cytotoxicity of chlorinated reclaimed water by about 75%, with ultraviolet light being responsible for the majority of this reduction. Chlorine residual in reclaimed water tended to increase the cytotoxicity, and the synergy between solar light and free chlorine could not enhance the reduction of cytotoxicity. Adding hydroxyl radical scavengers revealed that the contribution of hydroxyl radical to cytotoxicity reduction was limited. Solar light irradiation concurrently reduced TOX. The low molecular weight (<1 kDa) fraction was the major contributor of cytotoxicity and TOX in chlorinated reclaimed water. Detoxification of the low molecular weight fraction by light irradiation was mainly a result of TOX dehalogenation, while detoxification of the high molecular weight (>1 kDa) fraction was probably caused by photoconversion from high toxic TOX to low toxic TOX.

Distribution and relevance of iodinated X-ray contrast media and iodinated trihalomethanes in an aquatic environment

Distribution and relevance of iodinated X-ray contrast media (ICM) and iodinated disinfection byproducts (I-DBPs) in a real aquatic environment have been rarely documented. In this paper, some ICM were proven to be strongly correlated with I-DBPs through investigation of five ICM and five iodinated trihalomethanes (I-THMs) in surface water and two drinking water treatment plants (DWTPs) of the Yangtze River Delta, China. The total ICM concentrations in Taihu Lake and the Huangpu River ranged from 88.7 to 131 ng L^-1 and 102-252 ng L^-1, respectively. While the total I-THM concentrations ranged from 128 to 967 ng L^-1 in Taihu Lake and 267-680 ng L^-1 in the Huangpu River. Iohexol, the dominant ICM, showed significant positive correlation (p < 0.01) with CHClI₂ in Taihu Lake. Iopamidol and iomeprol correlated positively (p < 0.01) with some I-THMs in the Huangpu River. The observed pronounced correlations between ICM and I-THMs indicated that ICM play an important role in the formation of I-THMs in a real aquatic environment. Characteristics of the I-THM species distributions indicated that I-THMs may be transformed by natural conditions. Both DWTPs showed negligible removal efficiencies for total ICM (<20%). Strikingly high concentrations of total I-THMs were observed in the finished water (2848 ng L^-1 in conventional DWTP and 356 ng L^-1 in advanced DWTP). Obvious transformation of ICM to I-THMs was observed during the chlorination and ozonization processes in DWTPs. We suggest that ICM is an important source for I-DBP formation in the real aquatic environment.

Transformation among Aromatic Iodinated Disinfection Byproducts in the Presence of Monochloramine: From Monoiodophenol to Triiodophenol and Diiodonitrophenol

Aromatic iodinated disinfection byproducts (DBPs) are a newly identified category of highly toxic DBPs. Among the identified aromatic iodinated DBPs, 2,4,6-triiodophenol and 2,6-diiodo-4-nitrophenol have shown relatively widespread occurrence and high toxicity. In this study, we found that 4-iodophenol underwent transformation to form 2,4,6-triiodophenol and 2,6-diiodo-4-nitrophenol in the presence of monochloramine. The transformation pathways were investigated, the decomposition kinetics of 4-iodophenol and the formation of 2,4,6-triiodophenol and 2,6-diiodo-4-nitrophenol were studied, the factors affecting the transformation were examined, the toxicity change during the transformation was evaluated, and the occurrence of the proposed transformation pathways during chloramination of source water was verified. The results revealed that 2,4,6-triiodophenol and 2,6-diiodo-4-nitrophenol, which could account for 71.0% of iodine in the transformed 4-iodophenol, were important iodinated transformation products of 4-iodophenol in the presence of monochloramine. The transformation pathways of 4-iodophenol in the presence of monochloramine were proposed and verified. The decomposition of 4-iodophenol in the presence of monochloramine followed a pseudo-second-order decay. Various factors including monochloramine dose, pH, temperature, nitrite concentration, and free chlorine contact time (before chloramination) affected the transformation. The cytotoxicity of the chloraminated 4-iodophenol samples increased continuously with contact time. The proposed transformation pathways occurred during chloramination of source water.

Insight into the risk of replenishing urban landscape ponds with reclaimed wastewater

Increasing use of reclaimed wastewater (RW) for replenishing urban landscape ponds has aroused public concern about the water quality. Three ponds replenished with RW in three cities in China were chosen to investigate 22 indexes of water quality in five categories. This was achieved by comparing three pairs of ponds in the three different cities, where one pond in each pair was replenished with RW and the other with surface water (SW). The nutrients condition, heavy metal concentration and ecotoxicity did not differ significantly between RW- and SW-replenished ponds. By contrast, significant differences were observed in algal growth and pathogen risk. RW ponds presented a Cyanophyta-Chlorophyta-Bacillariophyta type with high algal diversity while SW ponds presented a Cyanophyta type with low diversity. Regrowth of bacterial pathogens and especially survival of viral pathogens in RW, was the main driver behind the higher risk for RW ponds compared with SW ones. The duration of RW replenishment was proved to have a marked impact on the algal growth and pathogen risk. With continued RW replenishment, non-dominant algal species subjected to decrease while dominant species were enhanced resulting in the biomass increasing but diversity declining, and the risk posed by viral pathogens might become greater.

Photoconversion of Chlorinated Saline Wastewater DBPs in Receiving Seawater is Overall a Detoxification Process

Chlorine disinfection of wastewater effluents rich in bromide and iodide ions results in the formation of relatively toxic bromo- and iodo-disinfection byproducts (DBPs), especially highly toxic bromophenolic and iodophenolic DBPs, which could harm the marine ecosystem when they are discharged into receiving seawater along with the wastewater effluents. In this study, we investigated the conversion of three individual halophenolic DBPs (5-bromosalicylic acid, 2,5-dibromohydroquinone, and 2,4,6-triiodophenol) and two chlorinated saline wastewater DBP mixtures in seawater. The conversion products were analyzed with ultra performance liquid chromatography/electrospray ionization-triple quadrupole mass spectrometry, and the conversion of overall halo-DBPs in the wastewater DBP mixtures was monitored by measuring total organic halogen. The photoconversion-induced variations in the toxicity were evaluated using the embryos of a marine polychaete. Halophenolic DBPs were found to undergo photoconversion in seawater. The conversion was triggered by photonucleophilic substitution: bromophenolic and iodophenolic DBPs were converted to their chlorophenolic or hydroxyphenolic analogues, via substituting the bromine and iodine atoms with chloride or hydroxide ions in seawater; chlorophenolic DBPs were converted to their hydroxyphenolic analogues, via substituting the chlorine atoms with hydroxide ions in seawater. The hydroxyphenolic analogues thus formed further decomposed and finally cleaved to aliphatic compounds. The photoconversion of chlorinated saline wastewater DBPs in receiving seawater was overall a dehalogenation and detoxification process.