Dark Chlorination Research Articles

Formation of Targeted and Novel Disinfection Byproducts during Chlorine Photolysis in the Presence of Bromide.

Chlorine photolysis is an advanced oxidation process that relies on the combination of direct chlorination by free available chlorine, direct photolysis, and reactive oxidants to transform contaminants. In waters that contain bromide, free available bromine and reactive bromine species can also form. However, little is known about the underlying mechanisms or formation potential of disinfection byproducts (DBPs) under these conditions. We investigated reactive oxidant generation and DBP formation under dark conditions, chlorine photolysis, and radical-quenched chorine photolysis with variable chlorine (0-10 mg-Cl2/L) and bromide (0-2,000 μg/L) concentrations, as well as with free available bromine. Probe loss rates and ozone concentrations increase with chlorine concentration and are minimally impacted by bromide. Radical-mediated processes partially contribute to the formation targeted DBPs (i.e., trihalomethanes, haloacetic acids, haloacetonitriles, chlorate, and bromate), which increase with increasing chlorine concentration. Chlorinated novel DBPs detected by high-resolution mass spectrometry are attributable to a combination of dark chlorination, direct halogenation by reactive chlorine species, and transformation of precursors, whereas novel brominated DBPs are primarily attributable to dark bromination of electron-rich formulas. The formation of targeted and novel DBPs during chlorine photolysis in waters with elevated bromide may limit treatment applications.

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.

Kinetic and mechanism insights into the degradation of venlafaxine by UV/chlorine process: A modelling study

• The combination of UV irradiation and chlorine significantly facilitated VEN degradation. • Kinecus software could simulated the VEN degradation well. • HO· was the dominant radical species responsible for the loss of VEN at acidic pH. • Transformation of VEN in UV/chlorine process can occur via four dominant reactions. • The toxicity of the products are generally lower than that of VEN. The presence of serotonin-norepinephrine reuptake inhibitors (SNRIs) such as venlafaxine (VEN) in surface waters has caused some concerns due to their harmful impacts on the human health and environment security. In the study, ultraviolet (UV) coupled with chlorine was developed for degrading VEN. Among the experiments, the UV/chlorine process demonstrated the highest performance in eliminating VEN. UV irradiation alone played a negligible role in VEN degradation. 39.56% of VEN was degraded by dark chlorination in 30 min, while 76.02% of VEN was decayed by UV/chlorine treatment within 30 min. Adding chlorine dosage and raising solution pH both facilitated the VEN removal. HCO 3 − , Cl − and HA inhibited VEN degradation during UV/chlorine treatment in accordance with predicted data. Furthermore, Kintecus software was applied to simulate the process of VEN degradation. Under UV/chlorine co-exposure, hydroxyl radical (HO·), chlorine and reactive chlorine species (RCS) were all proved to provide significant contribution to VEN oxidation. Note that both experimental and predicted contributions of HO· decreased as solution pH increased from 5.0 to 8.0. Four transformation pathways of VEN during UV/chlorine process were elucidated on basis of the DFT calculation and LC/MS analysis. Moreover, ECOSAR model program showed that UV/chlorine process reduced ecological toxicity of VEN obviously. Considering the influence of various factors, the most economical experimental conditions consist of chlorine (2.0–4.0 mg L −1 ) and pH (7.0–8.0).

Treatment of synthetic dye containing textile raw wastewater effluent using UV/Chlorine/Br photolysis process followed by activated carbon adsorption.

This study investigated the efficiency and feasibility of ultraviolet (UV)-assisted photolysis of synthetic dye containing textile raw wastewater effluent. For a said purpose, in-house developed UV/Chlorine/Br process was followed in the presence of activated carbon (AC) which additionally facilitate the dye adsorption. In UV/Chlorine process Cl•, Cl2•-, and HO• are generated in the solution and destroyed compounds that cannot be oxidized by the conventional oxidant. In this process, free bromine is formed and photolyzed by UV radiation and generate Br• and Br2•- that can enhance the rate of pollutant degradation. In the present study, the dye removal efficiency was contributed by dark bromide (7.18%), UV irradiation (26.8%), dark chlorination (78.67%), and UV/Chlorine/Br (87.01%), respectively. With increasing pH from 3.0 to 8.30, the dye removal efficiency was enhanced but decreased by further increasing pH values. In addition, magnetized activated carbon from pomegranate husk using dual-stage chemical activation was used for post-adsorption of the residual dye and its degradation byproducts. The adsorption of the dye residues by AC followed the second-order kinetics with the rate constant of 1.7 × 10-3. The phytotoxicity of the treated textile wastewater by UV irradiation, dark chlorination, and UV/Chlorine/Br was assessed by seed germination of Lepidium sativum seeds. The highest inhibition effect on seed germination was related to treated wastewater by UV irradiation (more than 90% inhibition) that alleviated to less than 10% when this effluent diluted to 5% v/v. The highest germination was observed when the seeds were irrigated by the effluent of the UV/Chlorine/Br process. The significant reduction in the toxicity of the treated wastewater revealed that the UV/Chlorine/Br process has a considerable potential to effectively detoxify textile wastewater. Graphical abstract.

Decolorization Kinetics of Ponceau S Dye by Chemical Chlorination: A Comparison with Sunlight/ Chlorine and UV/Chlorine Processes

Aqueous chlorination of Ponceau S dye (PS) was kinetically investigated through a classical method and under solar or UV radiations. For the classical method, the kinetics of PS chlorination was established by varying concentration ratios R = [Free chlorine (Cl(+I))]/[PS], pH, temperature, and bromide ions concentration. The decolorization reactions exhibited pseudo-first-order kinetics with respect to Ponceau S dye and their apparent rate constants (kapp ) increased when R increased, in fact, kapp reaches 0.205 min-1 when R=10. Important removal levels were obtained when pH= 10, 100% of decolorization obtained at only 5 minutes which means that the hypochlorite ion (ClO-) was more reactive than hypochlorous acid (HClO) for decolorizing PS dye. Otherwise, the apparent rate constant of PS disappearance increased with increasing medium temperature and led to the activation energy of 39.64 Kj/mol. When bromide ions were present in the medium, the decolorization of the PS solutions was more important than the simple chlorination when R=2.5 and at pH 7.20 following an oxidation result of bromide ions by sodium hypochlorite to hypobromite and/or hypobromous acid, the addition to the N=N double bond of dye is easier in the case of Br(+I) than Cl(+I). Under UV or solar radiations, and when R=1 and at pH 4.5, the decolorization rates of PS solutions were clearly increased in comparison with dark chlorination. Thus, the comparative order obtained in terms of apparent rate constant was kapp (Cl(+I)/UV)> kapp (Cl(+I)/Sunlight)> kapp (Cl(+I)). The disappearance efficiencies were 47.6%, 52.4% and 99.2% respectively when using dark chlorination, Cl(+I)/Sunlight and Cl(+I)/UV processes. Thus, it was clear that the advanced oxidation processes have proven to be more effective in terms of water treatment by chlorination. Thus, compared with dark chlorination, the enhanced decolorization of PS could be attributed to the formation of reactive radicals that adopt essentially radical mechanisms that are faster compared to molecular attack.

UV/chlorine process for degradation of benzothiazole and benzotriazole in water: Efficiency, mechanism and toxicity evaluation

Benzothiazole (BZA) and benzotriazole (BTZ) as emerging contaminants were found persistent in aquatic environments and toxic to aquatic organisms. The degradation of BZA and BTZ by UV/chlorine was systematically investigated in this study, and the results showed that BZA and BTZ can be remarkably removed by UV/chlorine compared with UV alone and dark chlorination. The radical quenching tests showed that degradation of BZA and BTZ by UV/chlorine involved the participation of reactive chlorine species (RCS), hydroxyl radical (HO·), and UV photolysis. HO· dominated BZA degradation at neutral and alkalinity, while RCS dominated BTZ degradation. The second-rate order constants for ClO· and BZA and BTZ were 2.22 × 108 M−1 s−1, and 2.40 × 108 M−1 s−1, respectively. Besides, the second-order rate constants for HO· and BZA and BTZ were also determined at pH 5.0, 7.0, and 9.0, respectively. The degradation efficiency of BZA by UV/chlorine was substantially promoted at acidic conditions, while the degradation efficiency of BTZ was promoted at both acidic and specific alkaline range mainly due to the reactivity of radical species and deprotonated form. The influence of Cl− was negligible, but the suppression effect of humic acid was slight during the BZA and BZT degradation by UV/chlorine. The transformation products were detected and the possible pathways were proposed. Seven disinfection by-products (DBPs) were identified both in BZA and BTZ degradation and trichloromethane was the main DBP. The toxicity assessment performed by luminescent bacteria and ECOSAR analysis indicated that the detoxification of BZA could be achieved by UV/chlorine, whereas the toxicity of BTZ was increased mainly due to the formation of intermediates. The findings from this study demonstrated UV/chlorine is likewise efficient for BZA and BTZ removal but the toxicity should be considered in the BTZ degradation.

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

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

Roles of bromine radicals, HOBr and Br2 in the transformation of flumequine by the UV/chlorine process in the presence of bromide

In the presence of bromide (Br−), new reactive species, such as bromine and reactive bromine species (RBS), are formed by the UV/chlorine process. In this study, the effects of Br− on the kinetics, products, and pathways of flumequine (FLU) transformation by the UV/chlorine process were investigated. The pseudo first-order rate constant (k′) for FLU degradation in the UV/chlorine process was enhanced by 2.5 times by 50 μM Br− at pH 7, as HOBr, Br2, RBS and HO• were formed, and they contributed 29.2%, 7.8%, 57.4% and 4.8% to k′, respectively. The second-order rate constants (k) of FLU reacting with HOBr, OBr−, Br2 and Br• were determined to be 115.6 M−1 s−1, negligible, 1.5 × 106 M−1 s−1 and 2.4 × 108 M−1 s−1, respectively. The contribution of Br2 was enhanced at a higher Br− level and a lower pH, which accounted for 48.2% of k′ at pH 6. The overall k′ value decreased by 97.9% from pH 6 to 9, during which k′ by HOBr, Br2 and RBS decreased significantly. The formation of brominated products was mainly attributed to HOBr and Br2, while RBS and HO• primarily promoted hydroxylation and ketonization during UV/chlorine treatment with Br−. The formation of total organic bromine (TOBr) by the UV/chlorine process with Br− was comparable to that during dark chlorination with Br−, while the yield of brominated disinfection byproducts in the former was much higher, further indicating that RBS and HO• promoted the formation of small-molecule products. In addition, the acute toxicity of FLU was significantly reduced during the UV/chlorine process with Br−. This is the first study to reveal the significant roles of Br2 and Br• in the degradation of micropollutants by the UV/chlorine process in Br−-containing water.

Mechanism insight of acetaminophen degradation by the UV/chlorine process: kinetics, intermediates, and toxicity assessment.

The removal of acetaminophen (AAP) in aqueous solution by the UV/chlorine process was evaluated. The effect of chlorine dose, the initial AAP concentration, pH value, and UV intensity on the reaction were also investigated. The degradation mechanism and the ecological risk were further discussed. The results indicated that AAP degradation fitted pseudo-first-order kinetics. Compared with UV alone or dark chlorination, the combination of UV and chlorine significantly accelerated the degradation process. The AAP degradation was positively affected by chlorine dose and UV intensity, while negatively affected by the initial AAP concentration and ammonia nitrogen concentration during the UV/chlorine process. The frontier orbital theory analysis shows that the C5 position in the benzene ring of AAP is likely to be the first site attacked by HO• and Cl• radical to form the products. Twelve intermediates were identified by Q-TOF and GC-MS. The possible degradation pathways were also proposed. Luminescent bacteria experiment and ECOSAR prediction both revealed that acute toxicity of AAP degradation could only be partially reduced. Ecological risks during the UV/chlorine process need to be further evaluated.

PPCP degradation and DBP formation in the solar/free chlorine system: Effects of pH and dissolved oxygen

The solar/chlorine process produces multiple reactive species by solar photolysis of chlorine, which can be used as an energy-efficient technology for water treatment. This study investigated the effects of pH and dissolved oxygen (DO) on the degradation of pharmaceuticals and personal care products (PPCPs) and on the formation of disinfection byproducts (DBPs) in the solar/chlorine system. The degradation of 24 structurally diverse PPCPs was enhanced appreciably in the solar/chlorine system compared to solar irradiation and dark chlorination. The reactive species in the solar/chlorine system were identified to be hydroxyl radicals (HO), reactive chlorine species (RCS, i.e., Cl and ClO) and ozone. With increasing pH from 6 to 8, the steady-state concentrations of HO and Cl decreased from 1.23 × 10−14 M to 4.79 × 10−15 M and from 9.80 × 10−16 M to 4.31 × 10−16 M, respectively, whereas that of ClO increased from 5.30 × 10−14 M to 2.68 × 10−13 M and the exposure of ozone increased from 0.44 μM min to 1.01 μM min in 90 min. Accordingly, the removal efficiencies of 6 PPCPs decreased and 11 PPCPs increased. The decreased removal of PPCPs with increasing pH was due to the decrease in HO and Cl, while the increased removal was attributed to the increased ClO and ozone. The presence of DO enhanced the degradation of most PPCPs, indicating the role of ozone on the degradation. The formation of total organic chlorine (TOCl) and known DBPs was enhanced by 60.7% and 159.4%, respectively, in the solar/chlorine system compared to chlorination in a simulated drinking water containing 2.5 mg L−1 natural organic matter (NOM). As the pH rose from 6 to 8, TOCl formation decreased by 16.2%, while that of known DBPs increased by 58.6% in solar/chlorine. The absence of DO slightly suppressed the formation of TOCl and known DBPs. This study illustrated the significant role of RCS in the solar/chlorine system, which enhanced the degradation of micropollutants but increased the formation of chlorinated DBPs.

Ciprofloxacin degradation in UV/chlorine advanced oxidation process: Influencing factors, mechanisms and degradation pathways

Ciprofloxacin (CIP) is a widely used third generation fluoroquinolone antibiotics, and has been often detected in wastewater treatment plants. Finding an effective way to remove them from wastewater is of great concern. Ultraviolet (UV)/chlorine advanced oxidation process (AOP) has many advantages in micropollutant removal. In this study, CIP degradation in UV/chlorine process was investigated. Only 41.2% of CIP was degraded by UV photolysis and 30.5% by dark chlorination in 30 min, while 98.5% of CIP was degraded by UV/chlorine process in 9 min. HCO3− had markedly inhibition, NO3− and SO42- had slight inhibition, and Cl− had a marginal inhibition on CIP degradation in UV/chlorine system. The degradation of CIP in UV/chlorine process was mainly attributed to the attack of reactive species. The relative contributions of hydrated electrons (eaq), hydroxyl radicals (HO), chlorine atoms (Cl), and UV photolysis were investigated. Under neutral condition in aqueous solution, CIP degradation had highest pseudo first-order reaction rate constant, in which eaq had the highest contribution, followed by Cl, HO, and UV photolysis. The intermediates and byproducts were identified and the degradation pathway was proposed. The total organic chlorine (TOCl) and biotoxicity were further assessed. CIP and natural organic matters (NOMs) were removed efficiently in real water. UV/chlorine showed the potential for the wastewater treatment containing CIP.

Elimination kinetics and detoxification mechanisms of microcystin-LR during UV/Chlorine process

Microcystin-LR (MC-LR), a toxin produced by cyanobacteria, is very toxic and poses a threat to public health when entering water treatment works. In this study, UV/chlorine process, as an advanced oxidation process (AOP), has been demonstrated for effective elimination of MC-LR levels and associated toxicity. At a chlorine dose of 3.0 mg L−1 and UV fluence of 125 mJ cm−2, MC-LR (initial concentration 1.0 μM) was reduced by 92.5%, which was much higher than 20.3% removal under UV irradiation alone and 65.1% removal during dark chlorination. Enhanced degradation was attributed by hydroxyl radicals (HO) and reactive chlorine species (RCS), mainly Cl2- and ClO. Increasing chlorine doses or lowering pH favored MC-LR removal. Increased bicarbonate and natural organic matter concentrations inhibited MC-LR removal, but bromide ions enhanced MC-LR removal instead. MC-LR elimination rates in natural waters were roughly two times smaller than those in ultrapure water. The reactive radicals promoted hydroxylation of both diene of Adda moiety and double bond of Mdha moiety in MC-LR. UV exposure enhanced the dechlorination of chloro-MC-LR via the cleavage of CCl bond. The toxicity was evaluated by a protein phosphatase (PP2A) inhibition assay. At a chlorine dose of 3.0 mg L−1 and UV fluence of 125 mJ cm−2, the toxicity of the treated water was reduced by 75.0%, which was also higher than 25.7% and 46.7% removal under UV irradiation alone and during dark chlorination, respectively. These results highlight UV/chlorine is an efficient AOP for MC-LR degradation and detoxification.

Degradation of clofibric acid in UV/chlorine disinfection process: kinetics, reactive species contribution and pathways.

As a potential endocrine disruptor, clofibric acid (CA) was investigated in this study for its degradation kinetics and pathways in UV/chlorine process. The results showed that CA in both UV photolysis and UV/chlorine processes could be degraded via pseudo-first-order kinetics, while it almost could not be degraded in the dark chlorination process. The observed rate constant (kobs) in UV photolysis was 0.0078 min−1, and increased to 0.0107 min−1 combining with 0.1 mM chlorine. The kobs increased to 0.0447 min−1 with further increasing the chlorine dosage from 0.1 to 1.0 mM, and reached a plateau at higher dosage (greater than 1.0 mM). The higher kobs was obtained at acid solution rather than basic solution. Moreover, the calculated contributions of radical species to kobs indicated that the HO• contributed significantly to CA degradation in acidic conditions, while the reactive chlorine species and UV direct photolysis dominated in neutral and basic solution. The degradation of CA was slightly inhibited in the presence of (1 ∼ 50 mM), barely affected by the presence of Cl− (1 ∼ 200 mM) and greatly suppressed by humic acid (0 ∼ 5 mg l−1). Thirteen main degradation intermediates and three degradation pathways of CA were identified during UV/chlorine process.

Degradation of phenacetin by the UV/chlorine advanced oxidation process: Kinetics, pathways, and toxicity evaluation

The degradation of phenacetin (PNT) by the combination of low-pressure mercury lamp and chlorine (UV/chlorine), an advanced oxidation process (AOP) of recent interest, was systematically investigated in terms of degradation kinetics, effects of chlorine dosage and water parameters, oxidation products as well as toxicity evaluation. The degradation of PNT followed pseudo first-order kinetics. The first-order rate constant (kobs) in the UV/chlorine AOP was 4.3, 8.4, and 11.1 times that of dark chlorination, UV/H2O2, and UV/PS, respectively, with the same molar dosage of oxidant at pH 7.2. Radical quenching tests suggested that chlorination, OH and reactive chlorine species were responsible for the UV/chlorine oxidation of PNT with contributions of 26.33%, 14.6% and 59.07%, at pH 7.2. As chlorine dosage gradually increased from 100 to 500 μM, the corresponding kobs monotonically increased from 0.0229 to 0.216 min−1. kobs was not apparently affected by the pH and coexisting chloride, but decreased by 56.5% and 75.4% in the presence of 10 mM HCO3− and 10 mg/L NOM. Slight decreases (around 15%) of kobs occurred in the raw water and tap water tests, while little effect was observed in filtered water samples compared with ultrapure water tests. Six typical disinfection byproducts including trichloromethane (TCM), chloral hydrate, dichloropropanone, trichloropropanone, trichloronitromethane, and dichloroacetonitrile were detected. The TCM yields increased to 159.95 μg/L within 20 min reaction of UV/chlorine, comprising 13.3% of the degraded PNT (per mole of carbon). The acute toxicity to luminescent bacterium Q67 by UV/chlorine was lower than chlorination under similar reaction conditions.

Roles of reactive chlorine species in trimethoprim degradation in the UV/chlorine process: Kinetics and transformation pathways

The UV/chlorine process, which forms several reactive species including hydroxyl radicals (HO) and reactive chlorine species (RCS) to degrade contaminants, is being considered to be an advanced oxidation process. This study investigated the kinetics and mechanism of the degradation of trimethoprim (TMP) by the UV/chlorine process. The degradation of TMP was much faster by UV/chlorine compared to UV/H2O2. The degradation followed pseudo first-order kinetics, and the rate constant (k′) increased linearly as the chlorine dosage increased from 20 μM to 200 μM and decreased as pH rose from 6.1 to 8.8. k′ was not affected by chloride and bicarbonate but decreased by 50% in the presence of 1-mg/L NOM. The contribution of RCS, including Cl, Cl2− and ClO, to the degradation removal rate was much higher than that of HO and increased from 67% to 87% with increasing pH from 6.1 to 8.8 under the experimental condition. The increasing contribution of RCS to the degradation with increasing pH was attributable to the increase in the ClO concentration. Kinetic modeling and radical scavenging tests verified that ClO mainly attacked the trimethoxybenzyl moiety of TMP. RCS reacted with TMP much faster than HOCl/OCl− to form chlorinated products (i.e., m/z 325) and chlorinated disinfection byproducts such as chloroform, chloral hydrate, dichloroacetonitrile and trichloronitromethane. The hydroxylation and demethylation of m/z 325 driven by HO generated m/z 327 and m/z 341. Meanwhile, reactions of m/z 325 with HO and RCS/HOCl/OCl− generated dichlorinated and hydroxylated products (i.e., m/z 377). All the chlorinated products could be further depleted to produce products with less degree of halogenation in the UV/chlorine process, compared to dark chlorination. The acute toxicity to Vibrio fischeri by UV/chlorine was lower than chlorination at the same removal rate of TMP. This study demonstrated the importance of RCS, in particular, ClO, in the degradation of micropollutants in the UV/chlorine process.

Formation of halogenated organic byproducts during medium-pressure UV and chlorine coexposure of model compounds, NOM and bromide

When chlorine is applied before or during UV disinfection of bromide-containing water, interactions between chlorine, bromide and UV light are inevitable. Formation of halogenated organic byproducts was studied during medium-pressure UV (MPUV) and chlorine coexposure of phenol, nitrobenzene and benzoic acid and maleic acid, chosen to represent electron-donating aromatics, electron-withdrawing aromatics, and aliphatic structures in natural organic matter (NOM), respectively. All were evaluated in the presence and absence of bromide. MPUV and chlorine coexposure of phenol produced less total organic halogen (TOX, a collective parameter for halogenated organic byproducts) than chlorination in the dark, and more haloacetic acids instead of halophenols. Increases in TOX were found in the coexposure of nitrobenzene and benzoic acid, but maleic acid was rather inert during coexposure. The presence of bromide increased the formation of brominated TOX but did not significantly affect total TOX formation, in spite of the fact that it reduced hydroxyl radical levels. MPUV and chlorine coexposure of NOM gave a higher differential UV absorbance of NOM and a larger shift to lower molecular weight compounds than chlorination in the dark. However, TOX formation with NOM remained similar to that observed from dark chlorination.

Bromate ion formation in dark chlorination and ultraviolet/chlorination processes for bromide-containing water

Bormate (BrO 3 −) is a carcinogenic chemical produced in ozonation or chlorination of bromide-containing water. Although its formation in seawater with or without sunlight has been previously investigated, the formation of bromate in dilute solutions, particularly raw water for water treatment plant, is unknown. In this article, the results of bench scale tests to measure the formation rates of bromate formation in dilute solutions, including de-ionized water and raw water from Yangtze River, were presented in dark chlorination and ultraviolet (UV)/chlorination processes. And the effects of initial pH, initial concentration of NaOCl, and UV light intensity on bromate formation in UV/chlorination of the diluted solutions were investigated. Detectable bromate was formed in dark chlorination of the two water samples with a relatively slow production rate. Under routine disinfecting conditions, the amount of formed bromate is not likely to exceed the national standards (10 μg/L). UV irradiation enhanced the decay of free chlorine, and, simultaneously, 6.6%–32% of Br − was oxidized to BrO 3 −. And the formation of bromate exhibited three stages: rapid stage, slow stage and plateau. Under the experimental conditions (pH = 4.41–11.07, C Cl 2= 1.23–4.50 mg/L), low pH and high chlorine concentration favored the generation of bromate. High light intensity promoted the production rate of bromate, but decreased its total generation amount due to acceleration of chlorine decomposition.

Sunlight photochlorination and dark chlorination of monoterpenes

The aqueous chlorination of the monoterpenes camphene, limonene, α-pinene and β-pinene produced complex polychlorinated product mixtures. The extent of chlorination was primarily dependent on pH and light conditions. At pH2 and exposure to sunlight, product mixtures were obtained that had striking similarities to Toxaphene. At higher pH or in the dark, less extensive but still substantial chlorination took place; these lower chlorinated compounds could be mistaken for biologically degraded Toxaphene in environmental samples.