The different reaction mechanisms by which chlorine and chlorine dioxide react with polycyclic aromatic hydrocarbons (PAH) in water

The removal of PAH from surface water by disinfectants like chlorine or chlorine dioxide is important where contamination by these compounds is concerned and no other water treatment processes are available. Our particular interest in these reactions arise from the fact that PAH can be used as an excellent model for the investigation of the different mechanisms by which the two oxidants react with aquatic organics. The vast differences between the rates of Cl₂ and ClO₂ reactions with various PAH, as well as the physical and chemical factors influencing those reactions indicate that chlorine reacts with PAH by several possible mechanisms, e.g. addition, substitution and oxidation. Chlorine dioxide on the other hand reacts mainly as a pure oxidant and a one-electron acceptor. As a consequence, chlorine dioxide reacts much more specifically with those PAH that undergo facile oxidation. Therefore, some PAH that react quite easily with Cl₂, do not react at all with ClO₂, while other PAH react with ClO₂ much more rapidly than with Cl₂. The widespread and highly carcinogenic benzo(a)pyrene and benzo(a)anthracene for example react with ClO₂ much faster than with Cl₂.     

Reactions of chlorine and chlorine dioxide with resorcinol in aqueous solution and adsorbed on granular activated carbon

Resorcinol reacted with chlorine and chlorine dioxide at pH 7 in dilute aqueous solution and in the presence of granular activated carbon (GAC) to produce numerous chlorinated compounds. Major products included chlororesorcinols and various chlorinated cyclopentenediones. Few reaction products were found in common to both Cl₂ and ClO₂ when reacted with resorcinol in the absence of GAC. However, both oxidants produced the same major recoverable chlorinated product, a dichlorocyclopentenedione, when reacted with resorcinol preadsorbed on GAC columns. No products were found when aqueous resorcinol was applied to GAC columns following contact of the carbon with Cl₂ or ClO₂ solutions.     

Aqueous chlorination of carbamazepine: Kinetic study and transformation product identification

Carbamazepine reactivity and fate during chlorination was investigated in this study. From a kinetic standpoint, a third-order reaction (first-order relative to the CBZ concentration and second-order relative to the free chlorine concentration) was observed at neutral and slightly acidic pH, whereas a second-order reaction (first order relative to the CBZ concentration and first order relative to the free chlorine concentration) was noted under alkaline conditions. In order to gain insight into the observed pH-dependence of the reaction order, elementary reactions (i.e. reactions of Cl₂, Cl₂O, HOCl with CBZ and of ClO⁻ with CBZ or of HOCl with the ionized form of CBZ) were highlighted and second order rate constants of each of them were calculated. Close correlations between the experimental and modeled values were obtained under these conditions. Cl₂ and Cl₂O were the main chlorination agents at neutral and acidic pH. These results indicate that, for a 1 mg/L free chlorine concentration and 1–10 mg/L chloride concentration at pH 7, halflives about 52–69 days can be expected. A low reactivity of chlorine with CBZ could thus occur under the chlorination steps used during water treatment. From a mechanistic viewpoint, several transformation products were observed during carbamazepine chlorination. As previously described for the chlorination of polynuclear aromatic or unsaturated compounds, we proposed monohydroxylated, epoxide, diols or chlorinated alcohol derivatives of CBZ for the chemical structures of these degradation products. Most of these compounds seem to accumulate in solution in the presence of excess chlorine.     

A qualitative analysis of atmospheric reactions between SO2 and NH3

The reactions between SO₂ and NH₃ are of great significance in the understanding of the formation of the atmospheric aerosols, the environmental sulfur cycle and the fate of atmospheric sulfur dioxide. The majority of the studies of this reaction system, so far, have been on the catalytic role of the ammonium ion in the oxidation of sulfur dioxide in solution and on the formation of ammonium sulfate in solution through a weak acid-base reaction. Thus, a number of very significant side reactions have been largely ignored. The qualitative analysis of the products of reactions between very dilute concentrations of SO₂ and NH₃ both in the presence and absence of water suggests a wide range of possible parallel reactions to sulfate formation with a number of both stable, metastable and unstable sulfur compounds, as well as elemental sulfur.Very dilute, slightly SO₂ rich SO₂/NH₃ reaction system in a dry atmosphere leads to eight distinct reactions. In the presence of water, the number of reactions exceeds nineteen. In the laboratory, it was possible to confirm most of these reactions directly from the presence of the primary reaction products; the remaining reactions were inferred through the presence of secondary products.     

Photodecomposition du bioxyde de chlore et des ions chlorite par irradiation u.v. en milieu aqueux-partie II. Etude cinetique

The decomposition of ClO₂ and ClO₂⁻ by u.v. radiation leads to the production of chlorate, chloride and oxygen as end-products via complex reactions which are initiated by the products generated by the primary reactions of photolysis (Buxton and Subhani, 1972a; Mialocq et al., 1973; Karpel Vel Leitner et al., 1992). As far as the rate of decomposition is concerned, Bowen and Cheung (1932) and Zika et al. (1985) have shown that the quantum yield of photodecomposition of chlorine dioxide (overall reaction) increases when the wavelength decreases [Zika et al. (1985): φ = 0.46 at 366 nm and 1.4 at 296.7 nm]. However, the values of the quantum yield of photodecomposition of ClO₂ and ClO₂⁻ at 253.7 nm as well as the quantum yields for the primary reactions of photolysis of ClO₂ and ClO₂⁻ at different wavelengths are not given in the literature.The aim of this work was to study the kinetics of photodecomposition of chlorine dioxide and of chlorite by u.v. irradiation.     

Review of photochemical reaction constants of organic micropollutants required for UV advanced oxidation processes in water

Emerging organic contaminants (pharmaceutical compounds, personal care products, pesticides, hormones, surfactants, fire retardants, fuel additives etc.) are increasingly found in water sources and therefore need to be controlled by water treatment technology. UV advanced oxidation technologies are often used as an effective barrier against organic contaminants. The combined operation of direct photolysis and reaction with hydroxyl radicals ensures good results for a wide range of contaminants. In this review, an overview is provided of the photochemical reaction parameters (quantum yield, molar absorption, OH radical reaction rate constant) of more than 100 organic micropollutants. These parameters allow for a prediction of organic contaminant removal by UV advanced oxidation systems. An example of contaminant degradation is elaborated for a simplified UV/H₂O₂ system.     

Reactions of tetracycline antibiotics with chlorine dioxide and free chlorine

Tetracyclines (TCs) are a group of widely used antibiotics that have been frequently found in the aquatic environment. The potential reactions of TCs with common water disinfection oxidants such as chlorine dioxide (ClO₂) and free available chlorine (FAC) have not been studied in depth and are the focus of this study. The oxidation kinetics of tetracycline, oxytetracycline, chlorotetracycline and iso-chlorotetracycline by ClO₂ and FAC are very rapid (with large apparent second-order rate constants k_app = 2.24 × 10⁵-1.26 × 10⁶ M⁻¹ s⁻¹ with ClO₂ and k_app = 1.12 × 10⁴-1.78 × 10⁶ M⁻¹ s⁻¹ with FAC at pH 7.0) and highly dependent on pH. Species-specific rate constants are obtained by kinetic modeling that incorporates pH-speciation of TCs and the oxidants (for FAC), and reveal that TCs primarily react with ClO₂ and FAC by their unprotonated dimethylamino group and deprotonated phenolic-diketone group. The modest difference in reactivity among the four TCs toward the oxidants is consistent with expectation and can be explained by structural influences on the two reactive moieties. Product evaluation shows that oxidation of TCs by ClO₂ leads to (hydr)oxylation and breakage of TC molecules, while oxidation of TCs by FAC leads to chlorinated and (hydr)oxylated products without any substantial ring breakage. Results of this study indicate that rapid transformation of TCs by oxidants such as ClO₂ and FAC under water and wastewater treatment conditions can be expected.     

Mode d'action du bioxyde de chlore sur les composes organiques en milieu aqueux: Consommations de bioxyde de chlore et reactions sur les composes phenoliques

The aim of our work was to study the reaction between chlorine dioxide (ClO₂) and organic substances. In the first part of our survey, chlorine dioxide demands were measured in diluted aqueous solutions of various kinds of organic compounds (5 × 10⁻⁵ −10 M) at pH 7. In the second part, the study of the action of chlorine dioxide on phenols (phenol, di and triphenols) was undertaken by observing the change of the organic substance through global parameter controls (COT, u.v. spectrum) and by trying to identify a certain number of oxidation products by means of chromatographic analysis (HPLC and GC), of mass spectrometry and of Nuclear Magnetic Resonance spectrometry (NMR).     

The influence of disinfection on aquatic biodegradable organic carbon formation

The aim of this paper was to assess the extent of biodegradable dissolved organic carbon formation upon disinfection of water with chlorine dioxide. Wide diversity of natural waters has been subjected to reactions with various amounts of ClO₂. For comparison examined waters have also been treated with ozone and chlorine. The application of chlorine dioxide and ozone significantly changed the molecular weight distribution of aquatic organic matter. As a result significant amounts of biodegradable carboxylic acids and aldehydes were generated. The formic, acetic, oxalic and ketomalonic acids as well as formaldehyde, acetaldehyde, glyoxal, methylglyoxal were identified. The productivity of aldehydes calculated for all examined waters and disinfectants amounted 12.7-47.7 μg mg⁻¹ DOC in the case of ozonation, 1.3-8.1 μg mg⁻¹ DOC after chlorination and 1.7-9.4 μg mg⁻¹ DOC for ClO₂ treatment. The highest total concentration of carboxylic acids was determined after the ozonation processes. In this case the organic acids' formation potential was in the range 10.8-62.8 μg mg⁻¹ DOC. Relatively high formation potential (5.3-17.9 μg mg⁻¹ DOC) was determined after the oxidation with ClO₂ as well. In the case of chlorination, the productivity of organic acids was low and did not exceed 3.4 μg mg⁻¹ DOC. The relatively high correlation between BDOC formation and carboxylic acids' formation potential was observed. Thus, carboxylic acids' formation potential may be used as a measure of water potential to form BDOC.     

Rate constants of reactions of ozone with organic and inorganic compounds in water—I: Non-dissociating organic compounds

Rate constants of reactions of ozone with non-ionized solutes, such as aliphatic alcohols, olefins, chlorosubstituted ethylenes, substituted benzenes and carbohydrates, have been determined from the absolute rates with which ozone reacts in the presence of various concentrations of these compounds in water. They have been tested by comparison with the relative rates by which pairs of these solutes are transformed by ozone. Different experimental methods have been developed to determine such rate constants in the range from 10^?2 to 10⁵ M^?1 s^?1. Interferences between the direct reactions of ozone and reactions due to its preliminary decomposition to secondary oxidants could be eliminated. The kinetics of all the reactions studied are first order with respect to ozone and solute concentration. The rate constants of many types of organic compounds in water are of the same order of magnitude as in organic solvents. Substituted benzenes, however, react in water about 100 times faster. They obey a linear free energy relationship with p = ?3.1 when based on δ_p⁺ values. Comparisons of rate constants with chemical structures of the reacting groups show that all reactions of ozone are highly selective and electrophilic. The kinetic data allow explanation of the chemical effects of ozone observed in water treatment practice.     

Trihalomethanes formation in water treated with chlorine dioxide

Formation of trihalomethanes (THMs) was investigated in water treated with chlorine dioxide (ClO₂) and/or chlorine (Cl₂) where humic acid (HA) was used as THMs precursors. When ClO₂ was used as the only disinfectant, no THMs were detected in bromide-free water; while only CHBr₃ was formed in water containing bromide ion because ClO₂ could oxide bromide to form hydrobromous acid which subsequently reacted with HA, and the yield CHBr₃ increased with bromide concentration and ClO₂ dosing. When water was treated with ClO₂ combined with Cl₂, only CHCl₃ was formed in the absence of bromide, however, all four species of THMs were formed in the presence of bromide; the THMs formation potential decreased gradually with an increase in the ratio of ClO₂ to Cl₂ because ClO₂ reacted with HA to render them unreactive or unavailable for THMs production. When water (with or without bromide ion) was irradiated by light, the yield of THMs was increased as a function of irradiation time to a maximum, and thereafter decreased markedly; the possible mechanism is that irradiation could activate the THMs precursors in HA, and at the same time destroy the reactivity of ClO₂ or Cl₂. The same results could be collected from natural water treated by ClO₂ with or without irradiation.     

Oxidation of fluoroquinolone antibiotics and structurally related amines by chlorine dioxide: Reaction kinetics, product and pathway evaluation

Fluoroquinolones (FQs) are a group of widely prescribed antibiotics and have been frequently detected in the aquatic environment. The reaction kinetics and transformation of seven FQs (ciprofloxacin (CIP), enrofloxacin (ENR), norfloxacin (NOR), ofloxacin (OFL), lomefloxacin (LOM), pipemidic acid (PIP) and flumequine (FLU)) and three structurally related amines (1-phenylpiperazine (PP), N-phenylmorpholine (PM) and 4-phenylpiperidine (PD)) toward chlorine dioxide (ClO₂) were investigated to elucidate the behavior of FQs during ClO₂ disinfection processes. The reaction kinetics are highly pH-dependent, can be well described by a second-order kinetic model incorporating speciation of FQs, and follow the trend of OFL > ENR > CIP ∼ NOR ∼ LOM > > PIP in reactivity. Comparison among FQs and related amines and product characterization indicate that FQs’ piperazine ring is the primary reactive center toward ClO₂. ClO₂ likely attacks FQ’s piperazinyl N4 atom followed by concerted fragmentation involving piperazinyl N1 atom, leading to dealkylation, hydroxylation and intramolecular ring closure at the piperazine moiety. While FQs with tertiary N4 react faster with ClO₂ than FQs with secondary N4, the overall reactivity of the piperazine moiety also depends strongly on the quinolone ring through electronic effects. The reaction rate constants obtained in clean water matrix can be used to model the decay of CIP by ClO₂ in surface water samples, but overestimate the decay in wastewater samples. Overall, transformation of FQs, particularly for those with tertiary N4 amines, could be expected under typical ClO₂ disinfection conditions. However, the transformation may not eliminate antibacterial activity because of little destruction at the quinolone ring.     

Rate constants of reactions of ozone with organic and inorganic compounds in water—II: Dissociating organic compounds

Rate constants of reactions of ozone with non-ionized solutes, such as aliphatic alcohols, olefins, chlorosubstituted ethylenes, substituted benzenes and carbohydrates, have been determined from the absolute rates with which ozone reacts in the presence of various concentrations of these compounds in water. They have been tested by comparison with the relative rates by which pairs of these solutes are transformed by ozone. Different experimental methods have been developed to determine such rate constants in the range from 10⁻² to 10⁵ M⁻¹ s⁻¹. Interferences between the direct reactions of ozone and reactions due to its preliminary decomposition to secondary oxidants could be eliminated. The kinetics of all the reactions studied are first order with respect to ozone and solute concentration. The rate constants of many types of organic compounds in water are of the same order of magnitude as in organic solvents. Substituted benzenes, however, react in water about 100 times faster. They obey a linear free energy relationship with p = −3.1 when based on δ_p⁺ values. Comparisons of rate constants with chemical structures of the reacting groups show that all reactions of ozone are highly selective and electrophilic. The kinetic data allow explanation of the chemical effects of ozone observed in water treatment practice.     

Contribution of human‐related sources to indoor volatile organic compounds in a university classroom

Although significant progress has been made in understanding the sources and chemistry of indoor volatile organic compounds (VOCs) during the past decades, much is unknown about the role of humans in indoor air chemistry. In the spring of 2014, we conducted continuous measurements of VOCs using a proton transfer reaction mass spectrometer (PTR‐MS) in a university classroom. Positive matrix factorization (PMF) of the measured VOCs revealed a ‘human influence’ component, which likely represented VOCs produced from human breath and ozonolysis of human skin lipids. The concentration of the human influence component increased with the number of occupants and decreased with ventilation rate in a similar way to CO₂, with an average contribution of 40% to the measured daytime VOC concentration. In addition, the human skin lipid ozonolysis products were observed to correlate with CO₂ and anticorrelate with O₃, suggesting that reactions on human surfaces may be important sources of indoor VOCs and sinks for indoor O₃. Our study suggests that humans can substantially affect VOC composition and oxidative capacity in indoor environments.     

Leeches as in situ biomonitors of chlorinated phenolic compounds. Part 2: Pulp mill investigations

Leeches (Nephelopsis obscura Verrill) were evaluated as in situ biomonitors of chlorinated phenolic compounds in the Fraser River at Prince George, BC, downstream from three bleached kraft pulp mills practising chlorine dioxide (ClO₂) substitution. Five biomonitoring periods, of 7 days duration, were selected to cover a full range of seasonal river conditions.Leeches bioconcentrated tri- and tetrachlorinated phenolics (bioconcentration factors: 465–6000) in proportions similar to those present in both mill effluent and river water. Leeches showed 3,4,5-trichloroguaiacol (3,4,5-TCG) to be the most consistent tracer of bleached kraft mill discharge. Leeches provided direct evidence that increasing ClO₂ substitution reduces amounts of chlorinated phenolics accumulated by aquatic organisms, with sharp decreases observed at ClO₂ levels greater than 90%. This study suggests that leeches could be applied as routine biomonitors for environmental impact monitoring.     

Contribution à l'étude de la détermination des conditions de formation des haloformes

In recent years the presence of haloforms has been observed by some researchers in waters of various origins.The aim of this study is to determine the conditions in which chloroform is formed by chlorination of aqueous solutions of some organic compounds.The first part of this work was carried out by chlorination of synthetic basic solutions of acetone and thus enabled us to determine the reagent concentrations needed to obtain the maximum quantity of chloroform.We studied next the effect of chlorination on organic material solutions (humic acids and phenols), in the course of an oxidation treatment; the results obtained by chlorination of aqueous solutions of humic substances show that the quantity of chloroform produced passes through a maximum value which depends on the time of ozonisation. Similar results were obtained for the chlorination of phenol solutions during oxidation by the u.v. + H₂O₂ system. These results may be explained by the partial degradation of the molecules which leads in the formation of reactional intermediate precursors of the haloform reaction. We undertook at the same time the study of a rapid test allowing us to detect the presence of haloform reaction precursors in a certain sample of water.     

An investigation of the formation of chlorate and perchlorate during electrolysis using Pt/Ti electrodes: The effects of pH and reactive oxygen species and the results of kinetic studies

The characteristics of chlorate (ClO₃⁻) and perchlorate (ClO₄⁻) formation were studied during the electrolysis of water containing chloride ions (Cl⁻). The experiments were performed using an undivided Pt/Ti plate electrode under different pH conditions (pH 3.6, 5.5, 7.2, 8.0 and 9.0). ClO₃⁻ and ClO₄⁻ were formed during electrolysis in proportion to the Cl⁻ concentration. The generation rates of ClO₃⁻ and ClO₄⁻ under acidic conditions (pH 3.6 and 5.5) were lower than in basic pH conditions (pH 7.2, 8.0 and 9.0). However, the pH of the solution did not influence the conversion of ClO₃⁻ to ClO₄⁻. The effects of intermediately formed oxidants on the production of ClO₃⁻ and ClO₄⁻ were observed using sodium thiosulfate (Na₂S₂O₃) as the active chlorine scavenger and tertiary butyl alcohol (t-BuOH) as the hydroxyl radical (OH) scavenger. The results revealed that electrolysis reactions that involved active chlorine contributed dominantly to ClO₃⁻ production. The direct oxidation reaction rate of Cl⁻ to ClO₃⁻ was 13%. The OH species that were intermediately formed during electrolysis were also found to significantly affect ClO₃⁻ and ClO₄⁻ production. The key formation pathways of ClO₃⁻ and ClO₄⁻ were studied using kinetic model development.     

Assessing the efficacy of pre-harvest,chlorine-based sanitizers against human pathogen indicator microorganisms and Phytophthora capsici in non-recycled surface irrigation water

Many factors must be considered in order to develop and implement treatment systems to improve the microbial quality of surface water and prevent the accidental introduction of plant and human pathogens into vegetable crops. The efficacy of chlorine gas (Cl_2(g)) and chlorine dioxide (ClO₂) injection systems in combination with rapid sand filtration (RSF) was evaluated in killing fecal indicator microorganisms in irrigation water in a vegetable-intensive production area. The efficacy of ClO₂ and Cl_2(g) was variable throughout the distribution systems and coliform bacteria never dropped below levels required by the United States Environmental Protection Agency for recreational waters. Sampling date and sampling point had a significant effect on the abundance of coliforms in Cl_2(g)- and ClO₂-treated water. Sampling date and sampling point also had a significant effect on the abundance of generic Escherichia coli in Cl_2(g) treated water but only sampling point was significant in ClO₂ treated water. Although the waterborne plant pathogen Phytophthora capsici was detected in five different sources of surface irrigation water using baiting and P. capsici-specific PCR, in vitro studies indicated that ClO₂ at concentrations similar to those used to treat irrigation water did not reduce mycelial growth or direct germination of P. capsici sporangia and reduced zoospore populations by less than 50%. This study concludes that injection of ClO₂ and Cl_2(g) into surface water prior to rapid sand filtration is inadequate in reducing fecal indicator microorganism populations and ClO₂ ineffectively kills infectious propagules of P. capsici. Additional research is needed to design a system that effectively targets and significantly reduces both plant and human pathogens that are present in surface irrigation water. A model for a multiple barrier approach to treating surface water for irrigation is proposed.     

Removal of charged micropollutants from water by ion-exchange polymers - Effects of competing electrolytes

A wide variety of environmental compounds of concern, e.g. pharmaceuticals or illicit drugs, are acids or bases that may predominantly be present as charged species in drinking water sources. These charged micropollutants may prove difficult to remove by currently used water treatment steps (e.g. UV/H₂O₂, activated carbon (AC) or membranes). We studied the sorption affinity of some ionic organic compounds to both AC and different charged polymeric materials. Ion-exchange polymers may be effective as additional extraction phases in water treatment, because sorption of all charged compounds to oppositely charged polymers was stronger than to AC, especially for the double-charged cation metformin. Tested below 1% of the polymer ion-exchange capacity, the sorption affinity of charged micropollutants is nonlinear and depends on the composition of the aqueous medium. Whereas oppositely charged electrolytes do not impact sorption of organic ions, equally charged electrolytes do influence sorption indicating ion-exchange (IE) to be the main sorption mechanism. For the tested polymers, a tenfold increased salt concentration lowered the IE-sorption affinity by a factor two. Different electrolytes affect IE with organic ions in a similar way as inorganic ions on IE-resins, and no clear differences in this trend were observed between the sulphonated and the carboxylated cation-exchanger. Sorption of organic cations is five fold less in Ca²⁺ solutions compared to similar concentrations of Na⁺, while that of anionic compounds is three fold weaker in SO₄^2- solutions compared to equal concentrations of Cl⁻.     

Trichloramine in swimming pools--formation and mass transfer

Trichloramine is a volatile, irritant compound of penetrating odor, which is found as a disinfection by-product in the air of chlorinated indoor swimming pools from reactions of nitrogenous compounds with chlorine. Acid amides, especially urea, ammonium ions and α-amino acids have been found as most efficient trichloramine precursors at acidic and neutral pH. For urea a relative NCl₃ formation of 96% at pH 2.5 and 76% at pH 7.1 was determined. Even under sub-stoichiometric molar ratios of Cl/N the formation of NCl₃ is favored over mono and dichlorinated products. However, the reaction kinetics of urea with chlorine is slow under conditions relevant for swimming pools. Also the mass transfer of NCl₃ from water to the gas phase which was calculated by the Deacon’s boundary layer model could be shown as a relatively slow process. Mass transfer would take 20 h or 5.8 d for a rough or a quiescent surface of the water, respectively. This is much more than a typical turnover rate of 6-8 h of a treatment cycle of a 25 m swimming pool. Therefore processes to remove NCl₃ and its precursors can help to minimize the exposure of bathers.