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

Effects of amines on the formation and photodegradation of DCNM under UV/chlorine disinfection

Investigations were conducted to examine the effects of amine type and initial concentration, free chlorine concentration, UV light intensity, pH and tert-butyl alcohol (TBA) on the formation of dichloronitromethane (DCNM) under UV/chlorine. Methylamine (MA), dimethylamine (DMA) and poly-dimethyl diallyl ammonium chloride (PolyDADMAC) were selected as the amine precursors of DCNM. And the reaction products of amines were explored through observing the contents of various nitrogen under UV/chlorine. Experimental results indicated that the higher of the intensity of UV light, the concentration of amines and free chlorine, the greater of the amount of DCNM formation; the amine substance with simple structure is more likely oxidized to form DCNM, so the potential of MA to form DCNM is the largest among three amines; the formation of DCNM decreased with increasing pH from 6.0 to 8.0; due to adding TBA into the reaction solution, halogen and hydroxyl radicals were restrained which resulted the DCNM formation decreased. In the reaction process, the formation of DCNM from amines increased at the beginning, then decreased and almost disappeared due to photodegradation. During the formation and photodegradation of DCNM, the dissolved organic nitrogen could be transformed into the ammonia-nitrogen (NH₃-N) and nitrate-nitrogen (NO₃^--N).

Discovering the Importance of ClO• in a Coupled Electrochemical System for the Simultaneous Removal of Carbon and Nitrogen from Secondary Coking Wastewater Effluent

Inorganic constituents in real wastewater, such as halides and carbonates/bicarbonates, may have negative effects on the performance of electrochemical systems because of their capability of quenching HO^•. However, we discovered that the presence of Cl^- and HCO₃^- in an electrochemical system is conducive to the formation of ClO^•, which plays an important role in promoting the simultaneous elimination of biorefractory organics and nitrogen in secondary coking wastewater effluent. The 6-h operation of the coupled electrochemical system (an undivided electrolytic cell with a PbO₂/Ti anode and a Cu/Zn cathode) at a current density of 37.5 mA cm^-2 allowed the removal of 87.8% of chemical oxygen demand (COD) and 86.5% of total nitrogen. The electron paramagnetic resonance results suggested the formation of ClO^• in the system, and the probe experiments confirmed the predominance of ClO^•, whose steady-state concentrations (8.08 × 10^-13 M) were 16.4, 26.5, and 1609.5 times those of Cl₂^•- (4.92 × 10^-14 M), HO^• (3.05 × 10^-14 M), and Cl^• (5.02 × 10^-16 M), respectively. The rate constant of COD removal and the Faradaic efficiency of anodic oxidation obtained with Cl^- and HCO₃^- was linearly proportional to the natural logarithm of the ClO^• concentration, and the specific energy consumption was inversely correlated to it, demonstrating the crucial role of ClO^• in pollutant removal.

Comparative Study for Interactions of Sulfate Radical and Hydroxyl Radical with Phenol in the Presence of Nitrite

Sulfate radical (SO₄^•-)- and hydroxyl radical (HO^•)-based advanced oxidation processes (AOPs) are effective for the removal of organic pollutants in water treatment. This study compared the interactions of SO₄^•- and HO^• for the transformation of phenol in UV/peroxydisulfate (PDS) and UV/H₂O₂ with the presence of NO₂^-, which is widely present in aquatic environments and transforms SO₄^•- and HO^• to ^•NO₂. By using laser flash photolysis, the products of phenol reacting with SO₄^•- and HO^• were demonstrated to be phenoxy radical and phenol-HO-adduct radical, respectively. This result, along with density functional theory (DFT) calculations, indicate that the predominant reaction mechanisms of phenol with SO₄^•- and HO^• with phenol are electron transfer and addition, respectively. The different mechanisms induced the much higher formation of nitrophenols by SO₄^•- than HO^• in the presence of NO₂^- through the fast combination of phenoxy radicals and ^•NO₂. The conversion yields of phenol to nitrophenols (including 2-nitrophenol and 4-nitrophenol), were 47.5% by SO₄^•- versus 5.3% by HO^• at the experimental conditions. Increasing PDS/H₂O₂ dosages from 0.2 to 1 mM resulted in a 61.9% increase of nitrophenol conversion yield in UV/PDS/NO₂^- but a 35.4% decrease of that in UV/H₂O₂/NO₂^-. In addition, the significant formation of phenoxy radicals by SO₄^•- also induced many nitrated polymers in UV/PDS/NO₂^-, while those induced in UV/H₂O₂/NO₂^- were negligible. The significant formation of nitrophenols and nitrated polymers increased the mutagenicity by 860.5% when the removal rate of phenol was 98% by UV/PDS/NO₂^-. This is the first study to demonstrate the different mechanisms of phenol transformation by SO₄^•- and HO^• in the presence of NO₂^-.

A comparative study of UV/H2O2 and UV/PDS for the degradation of micro-pollutants: kinetics and effect of water matrix

Organic micro-pollutants such as pesticides and endocrine disruptors cause serious harm to human health and aquatic ecosystem. In this study, the potential degradation of atrazine (ATZ), triclosan (TCS), and 2,4,6-trichloroanisole (TCA) by UV-activated peroxydisulfate (UV/PDS) and UV-activated H₂O₂ (UV/H₂O₂) processes were evaluated under different conditions. Results showed that UV/PDS process was more effective than UV/H₂O₂ under the same conditions. Increasing oxidant dosage or decreasing the initial ATZ, TCS, and TCA concentrations promoted the degradation rates of these three compounds. The presence of natural organic matter (NOM) could effectively scavenge sulfate radical (SO₄•^-) and hydroxyl radical (HO•) and reduced the removal rates of target compounds. Degradation rates of ATZ and TCA decreased with pH increasing from 5.0 to 9.0 in UV/PDS process, while in UV/H₂O₂ process, the increase of solution pH had little effect on ATZ and TCA degradation. In the UV/PDS and UV/H₂O₂ oxidation process, when the solution pH increased from 5 to 8, the removal rates of TCS decreased by 19% and 1%, while when the solution pH increased to 9, the degradation rates of TCS increased by 23% and 17%. CO₃^2-/HCO₃^- had a small inhibitory effect on ATZ and TCA degradation by UV/H₂O₂ and UV/PDS processes but promoted the degradation of TCS significantly (> 2 mM). Cl^- had little effect on the degradation of ATZ, TCA, and TCS in UV/H₂O₂ process. Cl^- significant inhibited on the degradation of ATZ and TCS, but the influence of Cl^- on the degradation of TCA was weak in UV/PDS process. Based on these experimental results, the various contributions of those secondary radicals (i.e., carbonate radical, chlorine radical) were discussed. This study can contribute to better understand the reactivities when UV/PDS and UV/H₂O₂ are applied for the treatment of micro-pollutant-containing waters.

Solar photodegradation of the UV filter 4-methylbenzylidene camphor in the presence of free chlorine

UV filters are essential ingredients in sunscreens and many personal care products. The coexposure of UV filters to solar photolysis and free chlorine (solar/free chlorine) is inevitable in outdoor swimming pools and many other aquatic matrices, and this study aims to investigate the degradation mechanism of one specific UV filter, 4-methylbenzylidene camphor (4MBC), under solar/free chlorine system. Under solar irradiation alone, 4MBC only undergoes isomerization from (E)- to (Z)-4MBC; however, in the solar/free chlorine system, 4MBC was significantly degraded, with a pseudo-first-order rate constant of 0.0137 s^-1 (pH = 7). The effects of the initial free chlorine concentration, solution pH and water matrix (presence of dissolved organic matter, HCO₃^- and Cl^-) were studied. The results revealed that reactive chlorine species (RCS) are the dominant species influencing 4MBC degradation via solar/free chlorine, while OH and O₃ played minor roles. These species would likely react with the 4-methylstyrene moiety of 4MBC and subsequently lead to 4MBC degradation through hydroxylation, chlorine substitution, oxidation and demethylation. Nevertheless, the dramatic increase in acute toxicity (Microtox®) during solar/free chlorine degradation of 4MBC highlights the need to further explore the transformation byproducts as well as their associated risks to humans and the environment.

Solar irradiation combined with chlorine can detoxify herbicides

The solar/chlorine process is an energy-efficient advanced oxidation process that can produce reactive species such as hydroxyl radical, reactive chlorine species and ozone. This study investigated the process' ability to detoxify the typical herbicides atrazine and mecoprop (methylchlorophenoxypropionic acid). Both herbicides are resistant to direct solar photolysis or chlorination alone, but they can be degraded by the solar/chlorine process effectively. Atrazine inhibited the development of Arabidopsis thaliana, but such inhibition was negligible after solar/chlorine treatment of an atrazine solution. The transformation of atrazine in the process was shown to be through hydroxylation, hydrogen abstraction and dechlorination but did not involve chlorine substitution or addition. Cl^• reacts with atrazine and mecoprop with rate constants of 6.87 × 10⁹ M^-1s^-1 and 1.08 × 10¹⁰ M^-1s^-1, respectively, while ClO^• reacts with mecoprop with a rate constant of 1.11 × 10⁸ M^-1s^-1. The degradation kinetics of atrazine and mecoprop by solar/chlorine was simulated by modeling, which fitted the experimental results well. Hydroxyl radicals (HO^•) mainly contributed to the degradation of atrazine by solar/chlorine at pH 7 with the contribution of 65%, whereas ClO^• and O₃ were main species responsible for the degradation of mecoprop with the contribution of 72% and 17%, respectively. The pseudo-first-order rate constants (k's) of the two degradations increased substantially (by 28.8% for atrazine and by 198% for mecoprop) when the chlorine dosage was increased from 50 μM to 200 μM. The k's decreased with increasing pH. The presence of natural organic matter inhibited the degradation of both herbicides, while the presence of bromide enhanced their degradation. This work reveals a feasible method for the detoxifying herbicides by combining chlorine with solar radiation.

Effect of DNA sizes and reactive oxygen species on degradation of sulphonamide resistance sul1 genes by combined UV/free chlorine processes

Nowadays, antibiotic resistance genes (ARGs) have been characterized as an emerging environmental contaminant, as the spread of ARGs may increase the difficulty of bacterial infection treatments. This study evaluates the combination of ultraviolet (UV) irradiation and chlorination, the two most commonly applied disinfection methods, on the degradation of sulphonamide resistance sul1 genes. The results revealed that although both of individual UV and chlorination processes were relatively less effective, two of the four combined processes, namely UV followed by chlorination (UV-Cl₂) and simultaneous combination of UV and chlorination (UV/Cl₂), delivered a better removal rate (up to 1.5 logs) with an observation of synergetic effects up to 0.609 log. The mechanisms analysis found that the difference of DNA size affected sul1 genes degradation by UV and chlorination; targeted genes on larger DNA fragments could be more effectively degraded by UV (1.09 logs for large fragments and 0.12 log for small fragments when UV dose reached 432 mJ/cm²), while to degrade ARGs on smaller DNA fragments required less free chlorine dosage (10 mg/L for small fragments and 40 mg/L for large fragments). The sequential combination of UV and chlorination (UV-Cl₂) used the corresponding reactivity of both processes, which could be the reason for the synergetic effect. For UV/Cl₂ process, the formation of reactive oxygen species (ROS) contributed to the synergetic effect. Scavenger analysis showed that the contribution of ROS to the sul1 gene reduction was 0.004 to 0.273 log (up to 45.5 % of the total synergy values), and among the two major reactive species in UV/Cl₂ system, HO was the more important radical, while the contribution of Cl was negligible. Besides, UV/Cl₂ process also used the corresponding reactivity of both processes to generate the remaining synergy values when excluding the contribution by reactive radicals. These findings provide a thorough understanding of the effects of UV and free chlorine on the degradation of ARGs and indicate the potential to utilize the combined processes of UV and free chlorine in water or wastewater treatment practice to control the dissemination of antibiotic resistance.

Influence of pre-ozonation of DOM on micropollutant abatement by UV-based advanced oxidation processes

Dissolved organic matter (DOM), ubiquitously co-present in micropollutant-impaired water, significantly decreases micropollutant removal in UV-based AOPs through consuming radicals and filtering UV light. In this study, pre-ozonation was proposed to alleviate the negative effects of DOM on UV-based AOPs. After ozone treatment of DOM-containing water at ozone dosages of 0.1-1 mg O₃/mg DOC, the degradation rate of benzoic acid (BA) in UV/H₂O₂ process increased by 7-34% mainly due to the enhanced transmission of UV light. The degradation rate of BA in UV/S₂O₈^2- process varied from -11% to 25% at ozone dosages of 0.1-1 mg O₃/mg DOC because of the increased photolysis rate of S₂O₈^2- and the altered reactivity of DOM towards SO₄^-. Pre-ozonation of DOM at ozone dosage of 1 mg O₃/mg DOC enhanced the oxidation rate of BA in UV/chlorine process by 35% due to the increased rate of chlorine photolysis and the decreased reactivity of DOM towards Cl. The influence of pre-ozonation on the formation potential of disinfection by-products (DBPs) depends on the kind of AOPs and the species of DBPs. The energy cost analysis suggests that pre-ozonation of DOM is an economic process for enhancing pollutant abatement by UV-based AOPs.

Photoelectrochemical degradation of perfluorooctanoic acid (PFOA) with GOP25/FTO anodes: Intermediates and reaction pathways

Perfluorooctanoic acid (PFOA) have been widely studied due to their persistence, bioaccumulation and possible toxic effects. In this work, we investigated a photoelectrochemical (PEC) system consisting of a graphene oxide-titanium dioxide (GOP25) anode coated on fluorine-doped tin oxide (FTO) glass for removal of PFOA in an aquatic environment. The GOP25/FTO anode was fabricated and well characterized. Nearly complete decomposition of 0.5 mg/L PFOA was achieved after 4 h of PEC treatment with an initial pH of 5.3 and a current density of 16.7 mA cm^-2. The presence of graphene oxide (GO) on the TiO₂ anode could enhance its electrochemical performance, thereby leading to increased decomposition efficiency. A total of 18 PFOA transformation products, including short-chain perfluoroalkyl acids, are reported in this work, and 13 products were observed for the first time. Four possible routes of PFOA decomposition, namely, decarboxylation followed by oxidation, defluorination, hydroxylation and Cl atom substitution, were determined. The presence of chlorinated byproducts in the system indicated that reactive chlorine species contributed to PFOA degradation.

In Situ Electrochemical Generation of Reactive Chlorine Species for Efficient Ultrafiltration Membrane Self-Cleaning

Reactive membranes based on hydroxyl radical generation are hindered by the need for chemical dosing and complicated module and material design. Herein, we utilize an electrochemical approach featuring in situ generation of reactive (radical) chlorine species (RCS) through anodization of chloride ions for membrane self-cleaning. A hybridized carbon nanotube (CNT)-functionalized ceramic membrane (h-CNT/CM), possessing high hydrophilicity, permeability, and conductivity, was fabricated. Using carbamazepine (CBZ) as a probe, we confirmed the presence of RCS in the electrified h-CNT/CM. The rapid and complete degradation of CBZ in a single-pass ultrafiltration indicates a high localized RCS concentration within the three-dimensional porous CNT interwoven layer. We further demonstrate that the electrogeneration of RCS is a critical prestep for free chlorine (HClO and ClO^-) formation. The self-cleaning efficiency of the membrane after fouling with a model organic foulant (alginate) was assessed using an electrified cross-flow membrane filtration system. The fouled h-CNT/CM exhibits a near complete water flux recovery following a short (1 min) self-cleaning with an applied voltage of 3 or 4 V and feed solutions of 100 or 10 mM sodium chloride, respectively. Considering the superior performance of the RCS-mediated self-cleaning compared to conventional membrane chemical cleaning using sodium hypochlorite, our results exemplify an effective strategy for in situ electrogeneration of RCS to achieve a highly efficient membrane self-cleaning.

UV-LED/chlorine degradation of propranolol in water: Degradation pathway and product toxicity

This study reports on the propranolol (PRO) degradation performance and product toxicity of an ultraviolet light-emitting diode (UV-LED)/chlorine process. The effects of experimental parameters including solution pH, chlorine dosage, and water matrix constituents on PRO removal were evaluated. Up to 94.5% of PRO could be eliminated within 15 min at a PRO-to-chlorine molar ratio of 1:4. The overall removal efficiency of PRO was non-pH dependent in the range of 5-9, while the initial rate was accelerated under alkaline conditions. The presence of Cl^-/HCO₃^- had little influence on the PRO degradation, whereas either humic acid or NO₃^- had an obvious inhibitory effect. Radical scavenger experiments showed that both HO and Cl primarily contributed to the PRO degradation, and electron paramagnetic resonance data demonstrated the generation of ¹O₂. The transformation of PRO during this process led to five detected products, which exhibited a higher acute toxicity than the parent compound according to the bright luminescent bacillus T3 method. It is worth mentioning that under the same ultraviolet illumination intensity, the degradation of PRO under UV-LED/chlorine gave a better performance than UV₂₅₄/chlorine, but the EEO of the former is obviously higher than the latter. So further research is required on improving the electric current to photon conversion efficiency for UV-LED. Additionally, the UV-LED/chlorine system was effective in the degradation of other drugs including sulfamethoxazole, oxytetracycline hydrochloride, and gatifloxacin, suggesting the possible application of the UV-LED/chlorine process for the removal of pharmaceuticals during wastewater treatment.

MBR-UV/Cl2 system in treating polluted surface water with typical PPCP contamination

This study proposed the membrane bioreactor–ultraviolet/chlorine (MBR-UV/Cl₂) process for treating polluted surface water with pharmaceutical personal care product (PPCP) contamination. Results showed that MBR-UV/Cl₂ effectively removed the organic matters and ammonia at approximately 80% and 95%. MBR-UV/Cl₂ was used in the removal of sulfadiazine(SDZ), sulfamethoxazole(SMZ), tetracycline(TC), oxytetracycline(OTC), ciprofloxacin(CIP), ofloxacin(OFX), erythromycin(ERY), roxithromycin(ROX), ibuprofen(IBU) and, naproxen(NAX) at 12.18%, 95.61%, 50.50%, 52.97%, 33.56%, 47.71%, 87.57%, 93.38%, 93.80%, and 71.46% in which their UV/Cl₂ contribution was 12.18%, 95.61%, 29.04%, 38.14%, 25.94%, 7.20%, 80.28%, 33.79%, 73.08%, and 23.05%, respectively. The removal of 10 typical PPCPs using UV/Cl₂ obtained higher contributions than those of the MBR process, except OTC, ROX, and IBU. The UV/Cl₂ process with 3-min hydraulic retention time and chlorine concentration at 3 mg/L effectively removed the trace of PPCPs. MBR-UV/Cl₂ has the potential to be developed as an effective technology in treating polluted surface water with PPCP contamination.

Roles of Bromine Radicals and Hydroxyl Radicals in the Degradation of Micropollutants by the UV/Bromine Process

The inevitable occurrence of Br^- in natural water affects the degradation kinetics of micropollutants in the UV/chlorine process, the radical chemistry of which, however, is largely unclear. As Br^- in the UV/chlorine process first forms free bromine (HOBr/OBr^-), this study investigated the radical chemistry of the UV/bromine process for the degradation of selected micropollutants resistant to bromine, i.e., ibuprofen and benzoate, to focus on the roles of radicals. The actual quantum yields of HOBr and OBr^- by UV photolysis at 254 nm are 0.43 (±0.025) and 0.26 (±0.025) mol Einstein^-1, respectively. Br^• and HO^• are generated first, and then, Br₂^•- is formed, with the signal detectable at 360 nm by laser flash photolysis. Compared with Cl^• in the UV/chlorine system, Br^• exists at higher concentrations (∼10^-12 M) in the UV/bromine system while HO^• exists at similar concentrations. In the UV/bromine process, reactive bromine species (RBS) dominates the degradation of ibuprofen, while HO^• dominates the degradation of benzoate. Br^• and Br₂^•- are reactive toward ibuprofen which second-order rate constants (k) were determined to be 2.2 × 10⁹ and 5.3 × 10⁷ M^-1 s^-1, respectively, by laser flash photolysis. Br^• was the major RBS for ibuprofen degradation by the UV/bromine treatment, whereas Br₂^•- increasingly contributed to ibuprofen degradation with increasing free bromine or Br^- concentrations. Br^• could be scavenged by HCO₃^- and natural organic matter (NOM), and the k with NOM was determined to be 2.6 × 10⁴ (mg/L)^-1 s^-1. Both Br^• and Br₂^•- prefer to react with ibuprofen via electron transfer with activation energy barriers (Δ^‡G⁰_SET) of 1.35 and 7.78 kcal mol^-1, respectively. RBS promoted the formation of hydroxylated products. Then free bromine, rather than RBS, was responsible for the formation of brominated products, increasing the total organic bromine (TOBr) and tribromomethane yields in the UV/bromine system. This study demonstrates for the first time the roles of RBS and HO^• in micropollutant degradation in the UV/bromine process.

Some issues limiting photo(cata)lysis application in water pollutant control: A critical review from chemistry perspectives

For decades, photolysis and photocatalysis have been touted as promising environment-benign and robust technologies to degrade refractory pollutants from water. However, extensive, large-scale engineering applications remain limited now. To facilitate the technology transfer process, earlier reviews have advocated to developing more cost-effective and innocuous materials, maximizing efficiency of photon usage, and optimizing photoreactor systems, mostly from material and reactor improvement perspectives. However, there are also some fundamental yet critical chemistry issues in photo(cata)lysis processes demanding more in-depth understanding and more careful consideration. Hence, this review summarizes some of these challenges. Of them, the first and paramount issue is the interference of coexisting compounds, including dissolved organic matter, anions, cations, and spiked additives. Secondly, considerable concerns are pointed to the formation of undesirable reaction by-products, such as halogenated, nitrogenous, and sulfur-containing compounds, which might increase instead of reduce toxicity of water if inadequate fluence and catalyst/additive are supplied due to time and cost constraints. Lastly, a critical issue lies in the uncertainty of current approaches used for identifying and quantifying radicals, especially when multiple radicals coexist together under changing and interconvertible conditions. The review hence highlights the needs to better understand these fundamental chemistry issues and meanwhile calls for more delicate design of experiments in future studies to overcome these barriers.

Generation and application of reactive chlorine species by electrochemical process combined with UV irradiation: Synergistic mechanism for enhanced degradation performance

Saline wastewater originates from many industries, containing a large amount of salt (NaCl) and other toxic and harmful organic matter, which have a great impact on the soil and groundwater. However, the treatment of saline wastewater is a serious problem because organic contents are hard to degrade with the high salinity by the common water treatment technologies. Herein, an electrochemical process coupled with ultraviolet (UV) irradiation was proposed for the saline wastewater treatment. High efficiency of p-nitrophenol (p-NP) and ammonia degradation were contributed from the in situ electrochemical produced active chlorine and photo-induced chlorine radicals. Under the optimal conditions (0.10 A, 0.05 M NaCl, and pH 6.00), approximately 98.91% p-NP was removed after 60 min with the rate constant of 7.521 × 10^-2 min^-1 in the electrochemical process, and 28.99% mineralization rate was obtained, while with the synergistic effect of UV and electrochemistry, approximately 100% of p-NP removal (k = 9.331 × 10^-2 min^-1) was achieved by 30 min treatment and about 83.70% of p-NP can be mineralized to CO₂ after 60 min. The study on the synergistic mechanism of enhanced degradation performance illustrated that Cl with high oxidation capacity played an important role in the p-NP oxidation. Besides, based on the chlorine radical reactions, this method was also effectively applied to remove ammonia nitrogen (92.00% removal of total nitrogen in 100 min) for nitrogen-containing wastewater. Thus, this study offers a promising approach for the treatment of saline industry wastewater.

Mechanisms of Advanced Oxidation Processes, the Principle of Detailed Balancing, and Specifics of the UV/Chloramine Process

Advanced oxidation processes tend to have very complex reaction mechanisms, and models containing over 150 steps have been developed to describe the chemistry. Without the aid of automation, it is extremely difficult to avoid the development of kinetic mechanisms that violate the principle of detailed balancing. Here, we apply DETBAL, a computer application, to systematically identify many violations of the principle of detailed balancing in a model proposed for the UV/chloramine process. We then show that these violations can also be found in dozens of other proposed models for advanced oxidation processes. Suggested repairs to these violations are provided. These repairs lead to no significant changes in the model predictions because the illegal loops include steps that are unnecessary under the conditions modeled. The model omits certain steps that do have significant effects on the model predictions.

Improved understanding of dissolved organic matter transformation in concentrated leachate induced by hydroxyl radicals and reactive chlorine species

Concentrated leachate (CL) is commonly featured with high salt and dissolved organic matters (DOM). In this study, molecular transformation of DOM was revealed to identify the reactive mechanisms with (non-) radical reactive species in ozonation, electrolysis and E⁺-ozonation processes. Chlorine ions were efficiently activated into non-radical reactive chlorine species (RCS) with 245.7 mg/L, which was more dominant in electrolysis. Compared to ozonation, C_•OH was increased from 2.6 × 10^-4 mg/L into 5.8 × 10^-4 mg/L and the generation of Cl•/ClO• could be concluded according to the decline of non-radical RCS in E⁺-ozonation process. For chromophoric and fluorescent DOM, aromatic compounds and polymerization degree dramatically decreased in E⁺-ozonation. Lipid-like and CRAM/lignin-like compounds were substantially degraded, as •OH and ClO•/Cl• shows an affinity towards oxygen-containing organic compounds via single electron transfer by attracting OH bonds. Especially, carbon/hydrogen/oxygen (CHO-containing) compounds were readily to be degraded with the removal efficiency of 92.5 %, 97.0 % and 98.4 % in electrolysis, ozonation and E⁺-ozonation, respectively. Moreover, nitrogen atoms have a negative effect on DOM degradation, and thus, carbon/hydrogen/nitrogen and carbon/hydrogen/nitrogen/sulfur (CHN- and CHNS-containing) compounds were considered as refractory compounds. This paper is expected to shed light on the synergetic effect in E⁺-ozonation and transformation of refractory DOM in CL treatment.

Nitrogen conversion from ammonia to trichloronitromethane: Potential risk during UV/chlorine process

In this study, the potential formation of trichloronitromethane (TCNM) from model organic compounds in ammonia-containing water treated by UV/chlorine process was evaluated. Monochloramine generated from the reaction of chlorine and ammonia can be photolyzed to produce NO₂^- and reactive nitrogen species (RNS), which play important roles in the formation of TCNM during the subsequent chlorination. The results showed that increase of nitrogen to chlorine molar ratio (from 0 to 1.0) and pH (from 6.5 to 8.0) enhanced the formation of TCNM, mainly due to the increased yield of NO₂^- and RNS from the photolyzed monochloramine. The formation of TCNM was interestingly found to be linearly correlated with Hammett constants of the model precursors, which is theoretically related to the rate constants of RNS with model compounds. Enhanced formation of TCNM was also observed during the treatment of natural organic matter by UV/chlorine process in ammonia-containing water. The toxicity assessment showed that TCNM significantly increased the genotoxicity of formed DBPs. Furthermore, the electrophilic substitution reaction of ^•NO₂ was proved to more likely occur on the ortho and para position of phenol according to the calculation of Gaussian program, and a possible reaction pathway of phenol and ^•NO₂ was proposed based on the calculated results.

Impact of DOM source and character on the degradation of primidone by UV/chlorine: Reaction kinetics and disinfection by-product formation

The presence of Dissolved Organic Matter (DOM) can exert a strong influence on the effectiveness of the UV/chlorine process. This study examined the impact of five DOM isolates with different characteristics on the degradation kinetics of model contaminant primidone (PM) during UV/chlorine treatment. The formation of Disinfection By-Products (DBPs) from DOM after 15-min UV/chlorine treatment followed by 24 h chlorination was investigated and compared with chlorination alone. The use of chemical probes and radical scavengers revealed that ^•OH and ClO• were the main radical species responsible for the loss of PM at acidic and alkaline conditions, respectively. All tested DOM isolates significantly inhibited the decay of PM. A strong negative correlation (>0.93) was observed between the decay rate constants of PM and SUVA of DOM isolates, except for EfOM isolate, which induced the strongest inhibitory effect due to its higher abundance in sulfur-containing functional groups (i.e., sink of ^•OH/Cl^• radicals). Compared with chlorination, the formation of Adsorbable Organic Chlorine (AOCl) and Trichloromethane (TCM) during the UV/Chlorine process was enhanced and hindered for low SUVA isolates and high SUVA DOM, respectively. However, Dichloroacetonitrile (DCAN) formation was generally lower for all isolates except for Ribou Reservoir DOM at pH 8.4 because of its high reactive nitrogenous DBP precursors at caustic conditions. However, when normalized to the chlorine consumed, the UV/Chlorine process always led to a lower DBPs formation compared with chlorination alone.

A direct injection liquid chromatography tandem mass spectrometry method for the kinetic study on iodinated contrast media (ICMs) removal in natural water

Iodinated contrast media (ICMs) are a class of X-ray contrast media worldwide utilized for radiographic procedures. Since they cannot be removed efficiently during water treatment, they can be found in surface and groundwater. In this work, a rapid and sensitive direct injection liquid chromatography-tandem (LC-MS/MS) method for the simultaneous analysis of seven ICMs media (iopamidol, ioxitalamic acid, diatrizoic acid, iothalamic acid, iohexol, iomeprol and iopromide) in complex aqueous matrices has been developed and validated. The MDLs for the analytes ranged from 0.7 to 21 ng L^-1 in ultrapure water, and recoveries ranged from 86 to 100% in drinking water, 85-103% in groundwater and 84-105% in WWTP effluent. A stereo-isomer for iopromide was separated. This analytic method was applied to investigate the removal of target ICMs by low pressure ultra violet light (LPUV) advanced oxidation processes with three oxidants, hydrogen peroxide, free chlorine and monochloramine in groundwater. Results showed that the addition of oxidants did not enhance attenuation of ICMs, since fluence-based decay apparent rate constants were similar (K_UV = 3.2 × 10^-3, K_UV-Cl2 = 3.6 × 10^-3 and K_UV-NH2 = 3.4 × 10^-3 10^-3 cm² mJ^-1). This yielded direct photolysis is the main mechanism to attenuate target ICMs.

Comparative evaluation of metoprolol degradation by UV/chlorine and UV/H2O2 processes

The degradation of metoprolol (MTP), a β-blocker commonly used for cardiovascular diseases, by UV/chlorine and UV/H₂O₂ processes was comparatively evaluated. MTP direct photolysis at 254 nm could be neglected, but remarkable MTP degradation was observed in both the UV/chlorine and UV/H₂O₂ systems. Compared with UV/H₂O₂, UV/chlorine has a more pronounced MTP degradation efficiency. In addition to primary radicals (OH and Cl), secondary radicals (ClO and Cl₂^-) played a pivotal role in degrading MTP by UV/chlorine process. The relative contributions of hydroxyl radicals (OH) and reactive chlorine species (RCS) in the UV/chlorine system varied at different solution pH values (i.e., the contribution of RCS increased from 57.7% to 75.1% as the pH increased from 6 to 8). The degradation rate rose as the oxidant dosage increased in the UV/chlorine and UV/H₂O₂ processes. The presence of Cl^- slightly affected MTP degradation in both processes, while the existence of HCO₃^- and HA inhibited MTP degradation to different extents in both processes. In terms of the overall cost of electrical energy, UV/chlorine is more cost efficient than UV/H₂O₂. The degradation products during the two processes were identified and compared, and the degradation pathways were proposed accordingly. Compared with the direct chlorination of MTP, pre-oxidation with UV/chlorine and UV/H₂O₂ significantly enhanced the formation of commonly known DBPs. Therefore, when using UV/chlorine and UV/H₂O₂ in real waters to remove organic pollutants, the possible risk of enhanced DBP formation resulting from the degradation of certain pollutants during post-chlorination should be carefully considered.

Magnetic Ti3C2Tx (Mxene) for diclofenac degradation via the ultraviolet/chlorine advanced oxidation process

In this study, a magnetic titanium carbide (Ti₃C₂T_x) MXene was synthesized through a one-step chemical co-precipitation method using ammonium bifluoride as a mild etchant and was investigated for photocatalytic degradation of diclofenac (DCF) via the ultraviolet (UV)/chlorine process. The DCF degradation was enhanced by the generation of active radicals such as the hydroxyl radical and reactive chlorine species compared with that resulting from UV and chlorination treatment alone as well as UV/H₂O₂ processes at pH 7. The first-order rate constant of the UV/chlorine process was 0.1025 min^-1, which is 12.7 and 6.8 times higher than those of the only UV and UV/H₂O₂ processes, respectively. Magnetic nanoparticles on the surfaces of Ti₃C₂T_x sheets not only enhanced the adsorption capacity of the synthesized composite but also increased the rate of electron transfer in solution. In addition, the effects of different operating conditions such as magnetic Ti₃C₂T_x dose, pH, and initial chlorine concentration on DCF degradation were investigated. Magnetic Ti₃C₂T_x showed high stability and photodegradation efficiency during seven consecutive degradation reaction cycles. The derivatives of DCF during the photocatalytic degradation process were also investigated based on the observed intermediate products and a degradation pathway was proposed. Thus the synthesized magnetic Ti₃C₂T_x is a simple and affordable photocatalyst, which can significantly enhance DCF degradation in the UV/chlorine advanced oxidation process.

Degradation behaviors of Isopropylphenazone and Aminopyrine and their genetic toxicity variations during UV/chloramine treatment

Combination of ultraviolet and chloramine (i.e., UV/chloramine) treatment has been attracting increasingly attention in recent years due to its high efficiency in removing trace organic contaminants. This study investigated the degradation behaviors of two pyrazolone pharmaceuticals (i.e., Isopropyl phenazone (PRP) and Aminopyrine (AMP)) and their genetic toxicity variations during UV/chloramine treatment. The results showed that chloramine could hardly degrade PRP and AMP, while UV/chloramine greatly increased the observed first-order rate constant (k_obs) of PRP and AMP degradation. The quenching and probe experiments illustrated that the reactive chlorine species (RCS) contributed dominantly to PRP removal, and hydroxyl radical (HO^•) was the major contributor to the degradation of AMP, while the reactive amine radicals (RNS) could hardly degrade them. The overall degradation rates of PRP and AMP decreased as pH increased from 6.5 to 10. The k_obs of PRP and AMP increased along with NH₂Cl dosage increasing and reached a plateau at higher concentrations (0.2-0.5 mM). The present background carbonate (HCO₃^-, 1-10 mM), chloride (Cl^-, 1-10 mM) and natural organic matter (NOM, 5-10 mg-C L^-1) exhibited inhibition impacts on PRP and AMP degradation. In addition, the intermediates/products of PRP and AMP were identified and their general degradation pathways were proposed to be hydroxylation, deacetylation, and dephenylization. Specifically, Cl-substitution was inferred during PRP degradation, while demethylation in tertiary amine group was only observed in AMP degradation. These mechanisms including the main reactive sites of PRP and AMP were further confirmed by the frontier orbitals calculation. Moreover, the results of the genetic toxicity according to the micronucleus test of Viciafaba root tip indicated that UV/chloramine treatment could partially reduce the genetic toxicity of PRP and AMP.

Exhaustive denitrification via chlorine oxide radical reactions for urea based on a novel photoelectrochemical cell

Urea is a major source of nitrogen pollution in domestic sewage and its denitrification is difficult since it is very likely to be converted into ammonia or nitrate instead of expected N₂. Herein, we propose an exhaustive denitrification method for urea via the oxidation of amine/ammonia-N with chlorine oxide radical, which induced from a bi-functional RuO₂//WO₃ anode, and the highly selective reduction of nitrate-N on cathode in photoelectrochemical cell (PEC). Under illumination, the WO₃ photoanode side promotes the quantities hydroxyl and reactive chlorine radical, and these radicals are immediately combined to stronger chlorine oxide radical by RuO₂ side, which obviously enhances the efficiency and speed of the urea oxidation. Synchronously, the over-oxidized nitrate can be selectively reduced by Pd and Au nanoparticles on the surface of cathode. Eventually, exhaustive denitrification is realized by the circulative reaction. Experimental observations and theoretical calculation revealed that chlorine oxide radical promoted significant denitrification of urea with an efficiency of 99.74% in 60 min under the optimum condition. The removal rate constant of the RuO₂//WO₃ anode was 3.08 times than that of single WO₃ anode and 2.64 times than that of single RuO₂ anode, confirming the chlorine oxide radical had stronger ability on denitrification than reactive chlorine radical. Also, the bi-functional anode contributed to best current efficiencies, utilizing the energy availably. This work proposes a promising method of exhaustive denitrification for urea.

Methyl chloride produced during UV254 irradiation of saline water

Ultraviolet (UV) irradiation is widely used for water treatment due to its effectiveness against a wide range of waterborne pathogens with minimal production of regulated disinfection byproducts. However, in this study, the formation of methyl chloride (CH₃Cl) from guaiacol and chloride was observed during UV₂₅₄ irradiation. The results indicated that direct photolysis of guaiacol produced an arenium ion, and the reactive methoxy group was further transformed to CH₃Cl in the presence of chloride. O-quinone was detected as the primary product of the degradation of guaiacol resulting from UV₂₅₄ irradiation. Other organic compounds containing methoxy, ethoxy, or methylamino groups with structures that are similar to guaiacol were also demonstrated to generate halocarbons in aqueous chloride or bromide solution under UV₂₅₄ irradiation. Scavenging experiments and removal of oxygen demonstrated that neither oxygen nor chlorine radicals were involved in CH₃Cl formation. In seawater samples, CH₃Cl was also detected in the presence or absence of added organic matter. These results demonstrate that CH₃Cl can be formed during UV₂₅₄ irradiation in saline water and that attention should be paid to this compound and structurally-related compounds in the application of UV₂₅₄ processes.

Degradation of diiodoacetamide in water by UV/chlorination: Kinetics, efficiency, influence factors and toxicity evaluation

The formation and control of haloacetamides (HAcAms) in drinking water have raised high attention due to their high genotoxicity and cytotoxicity, especially the most cytotoxic one, diiodoacetamide (DIAcAm). In this study, the degradation of DIAcAm by UV/chlorination was investigated in terms of degradation kinetics, efficiency, influencing factors, oxidation products and toxicity evaluation. Results revealed that the degradation of DIAcAm by UV/chlorine process followed pseudo-first-order kinetics, and the rate constant between DIAcAm and OH radicals was determined as 2.8 × 10⁹ M^-1 s^-1. The contribution of Cl to DIAcAm degradation by UV/chlorine oxidation was negligible. Increasing chlorine dosage and decreasing pH significantly promoted the DIAcAm degradation during UV/chlorine oxidation, but the presence of bicarbonate (HCO₃^-) and natural organic matter (NOM) inhibited it. The mass balance analysis of iodine species was also evaluated during UV/chlorine oxidation of DIAcAm. In this process, with DIAcAm decreasing from 16.0 to 0.8 μM-I in 20 min, IO₃^-, I^- and HOI/I₂ increased from 0 to 6.3, 6.1 and 0.5 μM-I, respectively. The increase of CHO cell viability during DIAcAm degradation indicated that the toxicity of DIAcAm could be decreased by chlorination, UV irradiation and UV/chlorine oxidation treatments, in which UV/chlorine oxidation was more effective on toxicity reduction than chlorination and UV irradiation alone.

A comparison of dissolved organic matter transformation in low pressure ultraviolet (LPUV) and ultraviolet light-emitting diode (UV-LED)/chlorine processes

This study compared the degradation of dissolved organic matter (DOM) by UV/chlorine advanced oxidation processes (AOPs) with emerging ultraviolet light-emitting diode (UV-LED, 275 nm) and traditional low pressure UV (LPUV, 254 nm) as UV sources. Excitation emission matrix-parallel factor (EEM-PARAFAC) analysis and two-dimensional (2D) correlation gel permeation chromatograph were applied to explore the evolutions of DOM during oxidation processes. The degradation behaviors of DOM indicated by UV absorbance at 254 nm (UV₂₅₄), dissolved organic carbon (DOC), and fluorophores fitted the pseudo-first-order kinetics well. The removal efficiency of DOM was similar under UV-LED and LPUV irradiation alone. However, UV-LED exhibited much higher degradation rates (increased by 29-160%) than LPUV regardless of the tracking variables during UV/chlorine processes. For three PARAFAC components, humic-like fluorescences were preferentially degraded by UV/chlorine oxidation compared with protein-like fluorescence potentially due to the differences of electronic moieties and molecular weight (MW). The decline in UV₂₅₄, DOC, and fluorophores increased with increasing chlorine dosage; linear correlations between those indicators were observed during the two AOPs. Moreover, UV-LED/chlorine could achieve greater extents of MW change. Our study demonstrated that UV-LED could be a superior alternative for the future selection of UV source in the UV/chlorine process.

Advanced treatment of urban wastewater by UV-C/free chlorine process: Micro-pollutants removal and effect of UV-C radiation on trihalomethanes formation

The effect of the UV-C/free chlorine (FC) process on the removal of contaminants of emerging concern (CECs) from real urban wastewater as well as the effect of UV-C radiation on the formation of trihalomethanes (THMs) compared to FC process alone was investigated. Unlike of FC process, UV-C/FC was really effective in the degradation of the target CECs (carbamazepine (CBZ), diclofenac, sulfamethoxazole and imidacloprid) in real wastewater (87% degradation of total CECs within 60 min, Q_UVC = 1.33 kJ L^-1), being CBZ the most refractory one (49.5%, after 60 min). The UV-C radiation significantly affected the formation of THMs. THMs concentration (mainly chloroform) was lower in UV-C/FC process after 30 min treatment (<1 μgL^-1 = limit of quantification (LOQ)) than in FC process in dark (2.3 μgL^-1). Noteworthy, while in FC treated wastewater chloroform concentration increased after treatment, UV-C/FC process resulted in a significant decrease (residual concentrations below the LOQ), even after 24 h and 48 h post-treatment incubation. The formation of radicals due to UV-C/FC process can reduce THMs compared to chlorination process, because part of FC reacts with UV-C radiation to form radicals and it is no longer available to form THMs. These results are encouraging in terms of possible use of UV-C/FC process as advanced treatment of urban wastewater even for possible effluent reuse.

Chemical behaviors and toxic effects of ametryn during the UV/chlorine process

Ametryn (AMT), one of the most widely used herbicides in agriculture, has been frequently detected as a micropollutant in many aquatic environments. AMT residue not only pollutes water but also acts as a precursor for the production of disinfection by-products (DBPs). This study systematically investigated the fate of AMT during the UV/chlorine process. It was observed that the combination of UV irradiation and chlorination degraded AMT synergistically. The results of the radical quenching experiments suggested that AMT degradation by the UV/chlorine process involved the participation of UV photolysis, hydroxyl radical (OH) reactions, and reactive chlorine species (RCS) reactions, which accounted for 45.4%, 36.4%, and 14.5% of the degradation, respectively. Moreover, we found that Cl- 2 was an important reactive radical for AMT degradation. The chlorine dose, pH, coexisting anions (Cl^- and HCO₃^-), and natural organic matter (NOM) were found to affect AMT degradation during the UV/chlorine process. Nineteen predominant intermediates/products of AMT degradation during UV/chlorine process were identified, including atrazine. Moreover, the corresponding transformation pathways were proposed, including electron transfer, bond cleavage (C-S, C-N), radical (OH, Cl and Cl- 2) reactions, and subsequent hydroxylation. The toxicity tests with Vibrio fischeri on AMT degradation suggested that more DBPs were generated by UV/chlorine-treated AMT, which possessed higher acute toxicity than AMT did. Although the UV/chlorine process evidently promoted the AMT degradation, optimization of process parameters may reduce the DBP production and merits further investigation.

Photoelectrocatalytic degradation of organics and formation of disinfection byproducts in reverse osmosis concentrate

The high content of organics in municipal reverse osmosis concentrate (ROC) requires proper treatment. Here, this study applied the photoelectrocatalysis (PEC) to reduce the concentration of organics in ROC. Meanwhile, the formation of disinfection byproducts (DBPs) was investigated. Participation of primary oxidants in organics removal and DBPs formation was revealed at different anodic potentials and pHs. The results showed that PEC process effectively oxidized the organics in ROC, achieving the highest mineralization rate of 63%. Increasing anodic potential from 0 to 1.0 V enhanced the oxidations of bulk organics (i.e., dissolved organic carbons (DOC), UV₂₅₄, fluorescence, large molecular weight compounds) and trace-level pharmaceuticals. Raising anodic potential to higher than 1.0 V slightly benefited the oxidations of bulk organics, owing to the relatively stable formation of hydroxyl radicals (OH•) and radical reactive chlorine species (r-RCS). The continuously rising concentration of free chlorine (FC) accelerated the decompositions of pharmaceuticals at ≥ 1.0 V. However, the generated FC raised the concentration of DBPs up to 10.36 μmol/L at 3.0 V. Lowering initial pH from 7-9 to 4-6 improved the mineralization rates by around 20% due to the higher formation of OH• at pH 4-6. Further decreasing initial pH from 6 to 4 enhanced the breakdown of large molecular weight compounds as well as the decomposition of pharmaceuticals. This came from the strengthened formation of FC and r-RCS at lower pHs. The intense participation of FC and r-RCS resulted in a higher total DBP concentration at pH 4-6 than that at pH 7-9. However, the individual species of DBPs changed differently toward the pH shift. The results of this study show that PEC could be an alternative for organics oxidation in ROC with proper control of DBPs formation.

Removal of carbamazepine, diclofenac and trimethoprim by solar driven advanced oxidation processes in a compound triangular collector based reactor: A comparison between homogeneous and heterogeneous processes

Contaminants of emerging concern (including pharmaceuticals) are not effectively removed by municipal wastewater treatment plants (WWTPs), so particular concern is related to agricultural wastewater reuse due to their possible uptake in crops irrigated with WWTPs effluents. Advanced oxidation processes (AOPs) and solar AOPs have been demonstrated to effectively remove pharmaceuticals from different aqueous matrices. In this study, an heterogeneous photocatalytic process using powdered nitrogen-doped TiO₂ immobilized on polystyrene spheres (sunlight/N-TiO₂) was compared to the benchmark homogenous AOP sunlight/H₂O₂ in a compound triangular collector reactor, to evaluate the degradation of three pharmaceuticals (carbamazepine (CBZ), diclofenac (DCF), trimethoprim (TMP)) in water. The degradation of the contaminants by sunlight and sunlight-AOPs well fit the pseudo-first order kinetic model (but for TMP under sunlight). High removal efficiency by solar photolysis was observed for DCF (up to 100%, half-life sunlight cumulative energy Q_S,1/2 = 2 kJ L^-1, half-life time t_1/2 = 32 min), while CBZ (32%, Q_S,1/2 = 28 kJ L^-1, t_1/2 = 385 min) and TMP (5% removal after 300 min) removal was poor. The degradation rate of CBZ, TMP and DCF was found to be slower during sunlight/H₂O₂ (Q_S,1/2 = 5 kJ L^-1, t_1/2 = 77 min; Q_S,1/2 = 20 kJ L^-1, t_1/2 = 128 min; Q_S,1/2 = 4 kJ L^-1, t_1/2 = 27 min, respectively) compared to sunlight/N-TiO₂ (Q_S,1/2 = 4 kJ L^-1, t_1/2 = 55 min; Q_S,1/2 = 3 kJ L^-1, t_1/2 = 42 min; Q_S,1/2 = 2 kJ L^-1, t_1/2 = 25 min, respectively). These results are promising in terms of solar technology upscale because the faster degradation kinetics observed for sunlight/N-TiO₂ process would result in smaller treatment volume, thus possibly perspective compensating the cost of the photocatalyst.

Insight into Fe(II)/UV/chlorine pretreatment for reducing ultrafiltration (UF) membrane fouling: Effects of different natural organic fractions and comparison with coagulation

Fe(II)/UV/chlorine was promoted as a pretreatment strategy for UF membrane to mitigate membrane fouling induced from different organic fractions. This treatment could be an emerging alternative prior to UF process attributed to the coupled role of oxidation and coagulation. To obtain a comprehensive understanding of fouling reduction, the influence of Fe(II)/UV/chlorine process on the characteristics of various feed solutions was inspected, including humic acid (HA), bovine serum albumin (BSA), sodium alginate (SA) and their mixture (HSB). The results suggested that Fe(II)/UV/chlorine process exhibited notable performance on membrane fouling control compared to Fe(II) coagulation alone. With the UV exposure of 720 mJ/cm², the certain dose of Fe(II) and chlorine (15 μM and 2 mg/L) effectively prevented the rapid development of fouling caused by the single organic fractions and their mixture. And the increased dosage promoted the performance of membrane fouling mitigation. The reduction of organic loadings and characteristics change of feed water took the main responsibility for the fouling alleviation. The properties of membrane fouling and their correlation with feed water qualities were analyzed. The results and insight analysis were supposed to evaluate and predict the effectiveness of fouling control when the feed solutions were pretreated by Fe(II)/UV/chlorine process according to various compositions and characteristics of the organic fractions.

Comparison of ciprofloxacin degradation in reclaimed water by UV/chlorine and UV/persulfate advanced oxidation processes

This study analyzed the ciprofloxacin (CIP) degradation in real reclaimed water through UV/chlorine and UV/persulfate (UV/PS) advanced oxidation processes. The influence of oxidant dosage, pH, inorganic anions, and humic acid (HA) on the oxidation capacity and performances of various UV-based processes was investigated. The results revealed that the CIP degradation rate constants in the UV/chlorine and UV/PS processes were higher than that in UV/H₂ O₂ , direct-UV, NaClO, and K₂ S₂ O₈ processes. The removal rate peaked at 0.1 mM oxidant dosage for 1 μM CIP, while the rate constant was highest at pH 5 (UV/chlorine) and pH 7 (UV/PS). The presence of Cl^- , HCO₃ ^- , and HA inhibited CIP removal in both processes. The degradation rate observed in reclaimed water was high, but still lower than that in laboratory water by 9.2 (UV/chlorine) and 9 (UV/PS) times. The UV/chlorine and UV/PS processes were found to be more cost-effective and hence more feasible in removing refractory compounds in reclaimed water. PRACTITIONER POINTS: The addition of oxidant and UV irradiation together had a pronounced promotion in the degradation of CIP. Cl^· and SO₄ ^·- had potential importance for enhancing CIP degradation in UV/chlorine and UV/PS process, respectively. UV/chlorine and UV/PS processes exhibited effective removal capability to CIP in real reclaimed water.

Contaminants of emerging concern removal from real wastewater by UV/free chlorine process: A comparison with solar/free chlorine and UV/H2O2 at pilot scale

The removal of contaminants of emerging concern (CECs) from urban wastewater treatment plants (UWTPs) is really important to minimize the risk for human health and environment. In this study, the homogeneous advanced oxidation process (AOP) UV-C/free chlorine (UV-C/FC) was investigated at pilot scale in the degradation of a mixture of four CECs, in different water matrices and compared to a consolidated AOP, namely UV-C/H₂O₂. As matter of fact 90% degradation of CECs was observed after 15 min (Q_UVC = 0.33 kJ L^-1) by UV-C/FC (5 mg L^-1 of FC) and 30 min (0.67 kJ L^-1) by UV-C/H₂O₂ (5 mg L^-1 of H₂O₂) in natural water. However, CECs degradation by UV-C/H₂O₂ and UV-C/FC was comparable (>82%) in wastewater samples, under the investigated conditions (60 min, 1.33 kJ L^-1). The effect of sunlight/FC process on the target CECs was also investigated (in a compound parabolic collector based reactor). Interestingly, a different behaviour was observed between the two light sources. In particular, a total removal of carbamazepine (CBZ) and imidacloprid (IMD) was observed for UV-C/FC process with 0.27 kJ L^-1 and 10 mgL^-1 of FC, while, in the sunlight/FC process (same FC dose), CBZ total removal took place quite fast (0.50 kJ L^-1), but 90% removal of IMD was observed only after 60 min (7.09 kJ L^-1). In conclusion, UV-C/FC process can be an interesting solution for tertiary treatment of urban wastewater for the removal of CECs and sunlight/FC is worthy of further investigation to evaluate its possible application in small UWTPs.

Oxidation of Microcystic-LR via the solar/chlorine process: Radical mechanism, pathways and toxicity assessment

Microcystin-LR (MC-LR) is the most widely distributed and harmful variant toxins released by cyanobacteria, which poses potential threaten to people and aquatic animals when entering natural water. In our research, solar/chlorine process was comprehensively investigated to degrade and detoxify MC-LR. Under the chlorine concentration of 1.0 mg L^-1, MC-LR (1.0 μM) was decreased by 96.7%, 26%, and 9% by solar/chlorine process, chlorination, and solar irradiation respectively. Quenching experiments confirmed that reactive chlorine species (RCS) and hydroxyl radical (HO) were the predominant reactive species in solar/chlorine process at neutral condition, and ozone was generated because of the participation of triplet-state oxygen (O(³P)). The respective contributions of each reactive species were calculated with the order as: RCS, HO, ozone, and solar irradiation. The presence of HCO₃^- and natural organic matter in water inhibited the degradation efficiency of MC-LR. Moreover, the transformation products of MC-LR generated during the solar/chlorine process were identified and a possible pathway was proposed. The hepatotoxicity of MC-LR and its transformation products was compared using protein phosphatase 2A. Our experimental results revealed that the concentration and hepatotoxicity of MC-LR both significantly decreased, and most products were not hepatoxic. Overall, the solar/chlorine process is a promising alternative technology to degrade MC-LR during eutrophication.

Prediction of 1,4-dioxane decomposition during VUV treatment by model simulation taking into account effects of coexisting inorganic ions

1,4-Dioxane is one of the most persistent organic micropollutants and is quite difficult to remove via conventional drinking water treatment consisting of coagulation, sedimentation, and sand filtration. Vacuum ultraviolet (VUV) treatment has recently been found to show promise as a treatment method for 1,4-dioxane removal, but the associated decomposition rate of 1,4-dioxane is known to be very sensitive to water quality characteristics. Some computational models have been proposed to predict the decomposition rate of micropollutants during VUV treatment, but the effects of only bicarbonate and natural organic matter have been considered in the models. In the present study, we attempted to develop a versatile computational model for predicting the behavior of 1,4-dioxane during VUV treatment that took into account the effects of other coexisting inorganic ions commonly found in natural waters. We first conducted 1,4-dioxane decomposition experiments with low-pressure mercury lamps and test waters that had been prepared by adding various inorganic ions to an aqueous phosphate buffer. The apparent decomposition rate of 1,4-dioxane was suppressed when bicarbonate, chloride, and nitrate were added to the test waters. Whereas bicarbonate and chloride directly suppressed the apparent decomposition rate by consuming HO•, nitrate became influential only after being transformed into nitrite by concomitant UV light (λ = 254 nm) irradiation. Cl-related radicals (Cl• and Cl₂•^-) did not react with 1,4-dioxane directly. A computational model consisting of 31 ordinary differential equations with respect to time that had been translated from 84 reactions (10 photochemical and 74 chemical reactions) among 31 chemical species was then developed for predicting the behavior of 1,4-dioxane during VUV treatment. Nine of the parameters in the ordinary differential equations were determined by least squares fitting to an experimental dataset that included different concentrations of bicarbonate, chloride, nitrate, and nitrite. Without further parameter adjustments, the model successfully predicted the behavior of 1,4-dioxane during VUV treatment of three groundwaters naturally contaminated with 1,4-dioxane as well as one dechlorinated tap water sample supplemented with 1,4-dioxane.

Degradation of naproxen in chlorination and UV/chlorine processes: kinetics and degradation products

Naproxen (NAP) is a nonsteroidal anti-inflammatory drug which has been widely used and frequently detected in water environments. This study investigated the NAP degradation in the chlorination and UV/chlorine disinfection processes, which usually acted as the last barriers for water treatment. The results showed that both chlorination and UV/chlorine disinfection could remove NAP effectively. At various chlorine dosages (0.1~0.5 mM), the contributions of chlorination and reactive radicals to the degradation of NAP in the UV/chlorine process were calculated to be 50.5~56.9% and 43.1~49.5%, respectively. However, the reactive radicals dominated in NAP degradation in alkaline solutions, while chlorination dominated in acidic conditions. The HCO₃^- (10~50 mM) slightly inhibited, Cl^- (10~50 mM) gradually promoted, and HA (1~5 mg/L) significantly reduced NAP degradation by UV/chlorine process. The degradation intermediates and products were obtained via high-performance liquid chromatography with QE-MS/MS; NAP was degraded by demethylation, acetylation, and dicarboxylic acid pathways during the chlorination and UV/chlorination processes.

Oxidation byproducts from the degradation of dissolved organic matter by advanced oxidation processes - A critical review

Advanced oxidation processes (AOPs) have been increasingly used for the treatment of source waters and wastewaters. AOPs characteristically produce oxidation byproducts (OBPs) from the partial degradation of dissolved organic matter (DOM) and/or the transformation of inorganic ions (especially, halides) into highly toxic substances including bromate and halogenated organic OBPs (X-OBPs). However, despite the enormous health and environmental risks posed by X-OBPs, an integral understanding of the complex OBP formation mechanisms during AOPs is lacking, which limits the development of safe and effective AOP-based water treatment schemes. The present critical and comprehensive review was intended to fill in this important knowledge gap. The study shows, contrary to the hitherto prevailing opinion, that the direct incorporation of halide atoms (X^•) into DOM makes an insignificant contribution to the formation of organic X-OBPs. The principal halogenating agent is hypohalous acid/hypohalite (HOX/XO^-), whose control is, therefore, critical to the reduction of both organic and inorganic X-OBPs. Significant generation of X-OBPs has been observed during sulfate radical AOPs (SR-AOPs), which arises principally from the oxidizing effects of the unactivated oxidant and/or the applied catalytic activator rather than the sulfate radical as is commonly held. A high organic carbon/X^- molar ratio (>5), an effective non-catalytic activator such as UV or Fe²⁺, a low oxidant concentration, and short treatment time are suggested to limit the accumulation of HOX/XO^- and, thus, the generation of X-OBPs during SR-AOPs. At present, there are no established techniques to prevent the formation of X-OBPs during UV/chlor(am)ine AOPs because the maintenance of substantial amounts of active halogen is essential to these processes. The findings and conclusions reached in this review would advance the research and application of AOPs.

Pilot-scale evaluation of oxidant speciation, 1,4-dioxane degradation and disinfection byproduct formation during UV/hydrogen peroxide, UV/free chlorine and UV/chloramines advanced oxidation process treatment for potable reuse

Advanced oxidation using UV/free chlorine and UV/chloramines are being considered as alternatives to UV/H₂O₂ for treatment of reverse osmosis (RO) permeate in treatment trains for the potable reuse of municipal wastewater. This pilot-scale comparison of the three advanced oxidation processes (AOPs) evaluated three factors important for selecting among these alternatives. First, the study characterized the speciation of oxidants serving as the source of radicals within the AOPs to facilitate process modeling. Kinetic modeling that included consideration of the chloramines occurring in RO permeate accurately predicted oxidant speciation. Modeling of the UV/free chlorine AOP indicated that free chlorine is scavenged by reactions with ammonia and monochloramine in RO permeate, such that oxidant speciation can shift in favor of dichloramine over the short (∼30 s) timescale of AOP treatment. Second, the order of efficacy for degrading the target contaminant, 1,4-dioxane, in terms of minimizing UV fluence was UV/free chlorine > UV/H₂O₂ ≫ UV/chloramines. However, estimates indicated that the UV/chloramines and UV/H₂O₂ AOPs could be similar on a cost-effectiveness basis due to savings in reagent costs by the UV/chloramines AOP, provided the RO permeate featured >3 mg/L as Cl₂ chloramines. Third, the study evaluated whether the use of chlorine-based oxidants within the UV/free chlorine and UV/chloramines AOPs enhanced disinfection byproduct (DBP) formation. Even after AOP treatment and chloramination, total halogenated DBP formation remained low at <15 μg/L for all three AOPs. DBP formation was similar between the AOPs, except that the UV/free chlorine AOP promoted haloacetaldehyde formation, while the UV/H₂O₂ and UV/chloramines AOPs followed by chloramination increased chloropicrin formation. However, total DBP formation on a toxic potency-weighted basis was similar among the AOPs, since haloacetonitriles and haloacetamides were the dominant contributors and did not differ significantly among the AOPs.

Reactive Nitrogen Species Are Also Involved in the Transformation of Micropollutants by the UV/Monochloramine Process

The UV/monochloramine (NH₂Cl) process is an emerging advanced oxidation process (AOP) in water treatment via radicals produced from the UV photolysis of NH₂Cl. This study investigated the degradation of micropollutants by the UV/NH₂Cl AOP, with ibuprofen (IBP) and naproxen (NPX) selected as representative micropollutants. Hydroxyl radical (HO^•) and chlorine atom (Cl^•) were identified in the process, and unexpectedly, we found that reactive nitrogen species (RNS) also played important roles in the transformation of micropollutants. The electron paramagnetic resonance (EPR) analysis proved the production of ^•NO as well as HO^•. The concentrations of HO^•, Cl^•, and ^•NO in UV/NH₂Cl remained constant at pH 6.0-8.6, resulting in the slightly changed UV fluence-based pseudo-first-order rate constants (k') of IBP and NPX, which were about 1.65 × 10^-3 and 2.54 × 10^-3 cm²/mJ, respectively. For IBP, the relative contribution of RNS to k' was 27.8% at pH 7 and 50 μM NH₂Cl, which was higher than that of Cl^• (6.5%) but lower than that of HO^• (58.7%). For NPX, the relative contribution of RNS to k' was 13.6%, which was lower than both Cl^• (23.2%) and HO^• (46.9%). The concentrations of HO^•, Cl^•, and ^•NO increased with the increasing NH₂Cl dosage. Water matrix components of natural organic matter (NOM) and bicarbonate can scavenge HO^•, Cl^•, and RNS. The presence of 5 mg/L NOM decreased the k' of IBP and NPX by 66.9 and 57.6%, respectively, while 2 mM bicarbonate decreased the k' of IBP by 57.4% but increased the k' of NPX by 10.5% due to the contribution of CO₃^•- to NPX degradation. Products containing nitroso-, hydroxyl-, and chlorine-groups were detected during the degradation of IBP and NPX by UV/NH₂Cl, indicating the role of nitrogen oxide radical (^•NO) as well as HO^• and Cl^•. Trichloronitromethane formation was strongly enhanced in the UV/NH₂Cl-treated samples, further indicating the important roles of RNS in this process. This study first demonstrates the involvement of RNS in the transformation of micropollutants in UV/NH₂Cl.

Degradation of imipramine by vacuum ultraviolet (VUV) system: Influencing parameters, mechanisms, and variation of acute toxicity

Degradation of imipramine (IMI) in the VUV system (VUV₁₈₅ + UV₂₅₄) was firstly evaluated in this study. Both HO^• oxidation and UV₂₅₄ direct photolysis accounted for IMI degradation. The quantum yields of UV₂₅₄ direct photolysis of deprotonated and protonated IMI were 1.31×10^-2 and 3.31×10^-3, respectively, resulting in the higher degradation efficiency of IMI at basic condition. Increasing the initial IMI concentration lowered the degradation efficiency of IMI. While elevating reaction temperature significantly improved IMI degradation efficiency through the promotion of both the quantum yields of HO^• and the UV₂₅₄ direct photolysis rate. The apparent activation energy was calculated to be about 26.6 kJ mol^-1. Negative-linear relationships between the k_obs of IMI degradation and the concentrations of HCO₃^-/CO₃^2-, NOM and Cl^- were obtained. The degradation pathways were proposed that cleavage of side chain and hydroxylation of iminodibenzyl and methyl groups were considered as the initial steps for IMI degradation in the VUV system. Although some high toxic intermediate products would be produced, they can be further transformed to other lower toxic products. The good degradation efficiency of IMI under realistic water matrices further suggests that the VUV system would be a good method to degrade IMI in aquatic environment.

Rate Constants and Mechanisms of the Reactions of Cl• and Cl2•- with Trace Organic Contaminants

Cl^• and Cl₂^•- radicals contribute to the degradation of trace organic contaminants (TrOCs) such as pharmaceutical and personal care products and endocrine-disrupting chemicals. However, little is known about their reaction rate constants and mechanisms. In this study, the reaction rate constants of Cl^• and Cl₂^•- with 88 target compounds were determined using laser flash photolysis. Decay kinetics, product buildup kinetics, and competition kinetics were applied to track the changes in their transient spectra. Cl^• exhibited quite high reactivity toward TrOCs with reaction rate constants ranging from 3.10 × 10⁹ to 4.08 × 10¹⁰ M^-1 s^-1. Cl₂^•- was less reactive but more selective, with reaction rate constants varying from <1 × 10⁶ to 2.78 × 10⁹ M^-1 s^-1. Three QSAR models were developed, which were capable of predicting the reaction rate constants of Cl₂^•- with TrOCs bearing phenol, alkoxy benzene, and aniline groups. The detection of Cl^•-adducts of many TrOCs suggested that Cl^• addition was an important reaction mechanism. Single electron transfer (SET) predominated in reactions of Cl^• with TrOCs bearing electron-rich moieties (e.g., sulfonamides), and their cation radicals were observed. Cl^• might also abstract hydrogen atoms from phenolic compounds to generate phenoxyl radicals. Moreover, Cl^• could react with TrOCs through multiple pathways since more than one transient intermediate was detected simultaneously. SET was the major reaction mechanism of Cl₂^•- reactions with TrOCs bearing phenols, alkoxy benzenes, and anilines groups. Cl₂^•- was found to play an important role in TrOC degradation, though it has been often neglected in previous studies. The results improve the understanding of halogen radical-involved chemistry in TrOC degradation.

Degradation of six typical pesticides in water by VUV/UV/chlorine process: Evaluation of the synergistic effect

Vacuum ultraviolet/ultraviolet/chlorine (VUV/UV/chlorine) is considered a novel advanced oxidation process (AOP), but little is known about its kinetics for pollutant degradation in water treatment. This study investigated the degradation of six typical pesticides, namely dimethoate (DMT), atrazine (ATZ), prometon (PMT), propoxur (PPX), bromacil (BRM) and propachlor (PPC), by VUV/UV/chlorine. The results show that all pesticides were rapidly degraded by VUV/UV/chlorine with a high removal efficiency of over 95% after 60 s. The pesticide degradation fitted well with pseudo-first-order reaction kinetics and a significant synergistic effect was observed during the VUV/UV/chlorine process. The synergistic factor (F_V/U/Cl) for DMT, ATZ, PMT, PPX, BRM and PPC were determined to be 1.75, 1.70, 2.06, 1.57, 2.84 and 1.61, respectively, indicating a synergistic improvement of 57%-184% for all pesticides. As hydroxyl radical (HO^•) transformed into reactive chlorine species (RCSs), the contribution ratio of RCSs for the pesticide degradation was much higher than that of HO^• in the VUV/UV/chlorine process, thus causing the synergistic effect. Solution pH ranging from 5.0 to 10.0 had various influence on the pesticide degradation by VUV/UV/chlorine. As initial concentration of free chlorine increased from 0 to 0.25 mM, the apparent rate constants of the pesticides kept on increasing while the F_V/U/Cl first increased and reached the highest value, and decreased afterwards. The formation of nitrite was significantly inhibited during the degradation of all pesticides by VUV/UV/chlorine. It suggests that VUV/UV/chlorine is a promising AOP for the pesticide degradation in water treatment.

Photochemical oxidation of PPCPs using a combination of solar irradiation and free available chlorine

The degradation of pharmaceuticals and personal care products (PPCPs) by using solar photolysis in the presence of free available chlorine (FAC) was investigated in simulated drinking water. The combination of free available chlorine and sunlight irradiation dramatically accelerated the degradation of all the contaminants tested through the generation of hydroxyl radicals, reactive chlorine species (RCS) and ozone. Contaminants containing electron-donating moieties degraded quickly and were preferentially degraded by RCS and/or HO oxidation. Primidone, ibuprofen and atrazine, which contain electron-withdrawing moieties, were mainly degraded by HO. Trace amounts of O₃ contributed greatly to carbamazepine's degradation. Degradation of PPCPs was accelerated in oxygenated solutions. Increasing chlorine concentrations barely enhanced removal of PPCPs bearing electron-withdrawing moieties. Higher pH generally decreased the degradation rate constants along with reduced levels of HO and Cl, but diclofenac, gemfibrozil, caffeine and carbamazepine had peak degradation rate constants at pH 7-8. The cytotoxicity using Chinese hamster ovary (CHO) cell did not show significant enhancement in solar/FAC treated water. Combining chlorination with sunlight may provide a simple and energy-efficient approach for improving the removal of organic contaminants during water treatment.

Enhanced ronidazole degradation by UV-LED/chlorine compared with conventional low-pressure UV/chlorine at neutral and alkaline pH values

Ultraviolet light-emitting diodes (UV-LEDs) are promising alternatives to conventional low-pressure UV (LPUV) lamps, mainly because they contain no toxic mercury and have a potential for less energy consumption and longer lifetime. In this study, UV sources including UV-LEDs (265, 275 and 285 nm) and LPUV (254 nm) were compared in UV/chlorine degradation of an organic contaminant, ronidazole (RNZ). UV-LED/chlorine performed better than LPUV/chlorine at neutral and alkaline pH values for RNZ degradation considering the fluence-based rate constant. However, the wall plug efficiencies of UV-LEDs are relatively low at present and must reach about 20-25% to achieve the same electrical energy per order as the LPUV in UV/chlorine degradation of RNZ at pH 7.5 and 9. Neither the contribution of radical (HO· or Cl·) nor the quantum yield of chlorine could explain the different RNZ degradation rate by UV/chlorine at different wavelengths and pH values, while the chlorine photolysis rate should be the key factor for these phenomena. The effects of common co-existing substances in real water (chloride, bicarbonate and natural organic matter) on UV/chlorine degradation of RNZ were similar at different UV wavelengths. Opposite to other oxidants or reductants, the molar absorption coefficient of chlorine increases when the UV wavelength increases from 254 to 285 nm at neutral and alkaline pH, which makes UV-LED/chlorine one of the best choices for UV-LED-based advanced oxidation/reduction processes.