Reactivity of Cyanobacteria Metabolites with Ozone: Multicompound Competition Kinetics.

Cyanobacterial blooms occur at increasing frequency and intensity, notably in freshwater. This leads to the introduction of complex mixtures of their products, i.e., cyano-metabolites, to drinking water treatment plants. To assess the fate of cyano-metabolite mixtures during ozonation, a novel multicompound ozone (O3) competition kinetics method was developed. Sixteen competitors with known second-order rate constants for their reaction with O3 ranging between 1 and 108 M-1 s-1 were applied to cover a wide range of the O3 reactivity. The apparent second-order rate constants (kapp,O3) at pH 7 were simultaneously determined for 31 cyano-metabolites. kapp,O3 for olefin- and phenol-containing cyano-metabolites were consistent with their expected reactivity (0.4-1.7 × 106 M-1 s-1) while kapp,O3 for tryptophan- and thioether-containing cyano-metabolites were significantly higher than expected (3.4-7.3 × 107 M-1 s-1). Cyano-metabolites containing these moieties are predicted to be well abated during ozonation. For cyano-metabolites containing heterocycles, kapp,O3 varied from <102 to 5.0 × 103 M-1 s-1, giving first insights into the O3 reactivity of this class of compounds. Due to lower O3 reactivities, heterocycle- and aliphatic amine-containing cyano-metabolites may be only partially degraded by a direct O3 reaction near circumneutral pH. Hydroxyl radicals, which are formed during ozonation, may be more important for their abatement. This novel multicompound kinetic method allows a high-throughput screening of ozonation kinetics.

Role of Carbonyl Compounds for N-Nitrosamine Formation during Nitrosation: Kinetics and Mechanisms.

N-Nitrosamines are potential human carcinogens frequently detected in natural and engineered aquatic systems. This study sheds light on the role of carbonyl compounds in the formation of N-nitrosamines by nitrosation of five secondary amines via different pathways. The results showed that compared to a control system, the presence of formaldehyde enhances the formation of N-nitrosamines by a factor of 5-152 at pH 7, depending on the structure of the secondary amines. Acetaldehyde showed a slight enhancement effect on N-nitrosamine formation, while acetone and benzaldehyde did not promote nitrosation reactions. For neutral and basic conditions, the iminium ion was the dominant intermediate for N-nitrosamine formation, while carbinolamine became the major contributor under acidic conditions. Negative free energy changes (<-19 kcal mol-1) and relatively low activation energies (<18 kcal mol-1) of the reactions of secondary amines with N2O3, iminium ions with nitrite and carbinolamines with N2O3 from quantum chemical computations further support the proposed reaction pathways. This highlights the roles of the iminium ion and carbinolamine in the formation of N-nitrosamines during nitrosation in the presence of carbonyl compounds, especially in the context of industrial wastewater.

Predicting Transformation Products during Aqueous Oxidation Processes: Current State and Outlook.

Water quality and its impacts on human and ecosystem health presents tremendous global challenges. While oxidative water treatment can solve many of these problems related to hygiene and micropollutants, identifying and predicting transformation products from a large variety of micropollutants induced by dosed chemical oxidants and in situ formed radicals is still a major challenge. To this end, a better understanding of the formed transformation products and their potential toxicity is needed. Currently, no theoretical tools alone can predict oxidatively induced transformation products in aqueous systems. Coupling experimental and theoretical studies has advanced the understanding of reaction kinetics and mechanisms significantly. This perspective article highlights the key progress made concerning experimental and computational approaches to predict transformation products. Knowledge gaps are identified, and the research required to advance the predictive capability is discussed.

Reactivity of Bromine Radical with Dissolved Organic Matter Moieties and Monochloramine: Effect on Bromate Formation during Ozonation.

Bromine radical (Br•) has been hypothesized to be a key intermediate of bromate formation during ozonation. Once formed, Br• further reacts with ozone to eventually form bromate. However, this reaction competes with the reaction of Br• with dissolved organic matter (DOM), of which reactivity and reaction mechanisms are less studied to date. To fill this gap, this study determined the second-order rate constant (k) of the reactions of selected organic model compounds, a DOM isolate, and monochloramine (NH2Cl) with Br• using γ-radiolysis. The kBr• of all model compounds were high (kBr• > 108 M-1 s-1) and well correlated with quantum-chemically computed free energies of activation, indicating a selectivity of Br• toward electron-rich compounds, governed by electron transfer. The reaction of phenol (a representative DOM moiety) with Br• yielded p-benzoquinone as a major product with a yield of 59% per consumed phenol, suggesting an electron transfer mechanism. Finally, the potential of NH2Cl to quench Br• was tested based on the fast reaction (kBr•, NH2Cl = 4.4 × 109 M-1 s-1, this study), resulting in reduced bromate formation of up to 77% during ozonation of bromide-containing lake water. Overall, our study demonstrated that Br• quenching by NH2Cl can substantially suppress bromate formation, especially in waters containing low DOC concentrations (1-2 mgC/L).

Critical Review on Bromate Formation during Ozonation and Control Options for Its Minimization.

Ozone is a commonly applied disinfectant and oxidant in drinking water and has more recently been implemented for enhanced municipal wastewater treatment for potable reuse and ecosystem protection. One drawback is the potential formation of bromate, a possible human carcinogen with a strict drinking water standard of 10 µg/L. The formation of bromate from bromide during ozonation is complex and involves reactions with both ozone and secondary oxidants formed from ozone decomposition, i.e., hydroxyl radical. The underlying mechanism has been elucidated over the past several decades, and the extent of many parallel reactions occurring with either ozone or hydroxyl radicals depends strongly on the concentration, type of dissolved organic matter (DOM), and carbonate. On the basis of mechanistic considerations, several approaches minimizing bromate formation during ozonation can be applied. Removal of bromate after ozonation is less feasible. We recommend that bromate control strategies be prioritized in the following order: (1) control bromide discharge at the source and ensure optimal ozone mass-transfer design to minimize bromate formation, (2) minimize bromate formation during ozonation by chemical control strategies, such as ammonium with or without chlorine addition or hydrogen peroxide addition, which interfere with specific bromate formation steps and/or mask bromide, (3) implement a pretreatment strategy to reduce bromide and/or DOM prior to ozonation, and (4) assess the suitability of ozonation altogether or utilize a downstream treatment process that may already be in place, such as reverse osmosis, for post-ozone bromate abatement. A one-size-fits-all approach to bromate control does not exist, and treatment objectives, such as disinfection and micropollutant abatement, must also be considered.

Enhanced Treatment of Municipal Wastewater Effluents by Fe-TAML/H2O2: Efficiency of Micropollutant Abatement.

Combining iron with a tetraamido-macrocyclic ligand (Fe-TAML) as a catalyst and with hydrogen peroxide (H2O2) as the bulk oxidant is a process that has been suggested for the oxidative abatement of micropollutants during water treatment. In this study, the reactivity of the Fe-TAML/H2O2 system was evaluated by investigating the degradation of a group of electron-rich organic model compounds with different functional groups in a secondary wastewater effluent. Phenolic compounds and a polyaromatic ether are quickly and substantially abated by Fe-TAML/H2O2 in a wastewater effluent. For tertiary amines, a moderate rate of abatement was observed. Primary and secondary amines, aromatic ethers, aromatic aldehydes, and olefins are oxidized too slowly in the investigated Fe-TAML/H2O2 systems to be significantly abated in a secondary wastewater effluent. Trichlorophenol is readily oxidized to chloromaleic acid and chlorofumaric acid, which support a one-electron transfer reaction as the initial step of the reaction between Fe-TAML/H2O2 and the target compound. Fe-TAML/H2O2 does not oxidize bromide to hypobromous acid; however, iodide is oxidized to hypoiodous acid, and as a consequence, the H2O2 consumption is accelerated by a catalytic reaction in iodide-containing water. Overall, Fe-TAML/H2O2 is a rather selective oxidant, which makes it an interesting system for the abatement of electron-rich phenolic-type pollutants.

Reaction of DMS and HOBr as a Sink for Marine DMS and an Inhibitor of Bromoform Formation.

Recently, we suggested that hypobromous acid (HOBr) is a sink for the marine volatile organic sulfur compound dimethyl sulfide (DMS). However, HOBr is also known to react with reactive moieties of dissolved organic matter (DOM) such as phenolic compounds to form bromoform (CHBr3) and other brominated compounds. The reaction between HOBr and DMS may thus compete with the reaction between HOBr and DOM. To study this potential competition, kinetic batch and diffusion-reactor experiments with DMS, HOBr, and DOM were performed. Based on the reaction kinetics, we modeled concentrations of DMS, HOBr, and CHBr3 during typical algal bloom fluxes of DMS and HOBr (10-13 to 10-9 M s-1). For an intermediate to high HOBr flux (≥10-11 M s-1) and a DMS flux ≤10-11 M s-1, the model shows that the DMS degradation by HOBr was higher than for photochemical oxidation, biological consumption, and sea-air gas exchange combined. For HOBr fluxes ≤10-11 M s-1 and a DMS flux of 10-11 M s-1, our model shows that CHBr3 decreases by 86% compared to a lower DMS flux of 10-12 M s-1. Therefore, the reaction between HOBr and DMS likely not only presents a sink for DMS but also may lead to suppressed CHBr3 formation.

Reactions of α,ß-Unsaturated Carbonyls with Free Chlorine, Free Bromine, and Combined Chlorine.

Chemical disinfectants employed in water and wastewater treatment can produce a variety of transformation products, including carbonyl compounds (e.g., saturated and unsaturated aldehydes and ketones). Experiments conducted under conditions relevant to chlorination at drinking water treatment plants and residual chlorine application in distribution systems indicate that α,ß-unsaturated carbonyl compounds readily react with free chlorine and free bromine over a wide pH range but react slowly with combined chlorine (i.e., NH2Cl). For nearly all of the 11 α,ß-unsaturated carbonyl compounds studied, the apparent second-order rate constants for the reaction with free chlorine increased in a linear manner with hypochlorite (OCl-) concentrations, yielding species-specific second-order rate constants for the reaction with OCl- ranging from 0.21 to 12 M-1 s-1. Predictions based on the second-order rate constants indicate that a substantial fraction (i.e., >60%) of several of the more prominent α,ß-unsaturated carbonyls (e.g., acrolein, crotonaldehyde) will be transformed to an appreciable extent in distribution systems by free chlorine. Products from the reaction of chlorine with acrolein, crotonaldehyde, and methyl vinyl ketone were tentatively identified using nuclear magnetic resonance (NMR) and gas chromatography coupled to high-resolution time-of-flight mass spectrometry (GC-HRT-MS). These products lacked unsaturated carbons and, in some cases, contained multiple halogens.

Micropollutant Oxidation Studied by Quantum Chemical Computations: Methodology and Applications to Thermodynamics, Kinetics, and Reaction Mechanisms.

The abatement of organic micropollutants during oxidation processes has become an emerging issue for various urban water systems such as drinking water, wastewater, and water reuse. Reaction kinetics and mechanisms play an important role in terms of efficiency of these processes and the formation of transformation products, which are controlled by functional groups in the micropollutants and the applied oxidants. So far, the kinetic and mechanistic information on the underlying reactions was obtained by experimental studies; additionally, predictive quantitative structure-activity relationships (QSARs) were applied to determine reaction kinetics for the oxidation of emerging compounds. Since this experimental approach is very laborious and there are tens of thousands potential contaminants, alternative strategies need to be developed to predict the fate of micropollutants during oxidative water treatment. Due to significant developments in quantum chemical (QC) computations in recent years and increased computational capacity, QC-based methods have become an alternative or a supplement to the current experimental approach. This Account provides a critical assessment of the current state-of-the-art of QC-based methods for the assessment of oxidation of micropollutants. Starting from a given input structure, QC computations need to locate energetic minima on the potential energy surface (PES). Then, useful thermodynamic and kinetic information can be estimated by different approaches: Experimentally determined reaction mechanisms can be validated by identification of transition structures on the PES, which can be obtained for addition reactions, heavy atom transfer (Cl+, Br+, O·) and H atom transfer (simultaneous proton and electron transfer) reactions. However, transition structures in the PES cannot be obtained for e--transfer reactions. Second-order rate constants k for the reactions of micropollutants with chemical oxidants can be obtained by ab initio calculations or by QSARs with various QC descriptors. It has been demonstrated that second-order rate constants from ab initio calculations are within factors 3-750 of the measured values, whereas QSAR-based methods can achieve factors 2-4 compared to the experimental data. The orbital eigenvalue of the highest occupied molecular orbital ( EHOMO) is the most commonly used descriptor for QSAR-based computations of k-values. In combination with results from experimental studies, QC computations can also be applied to investigate reaction mechanisms for verification/understanding of oxidative mechanisms, calculation of branching ratios or regioselectivity, evaluation of the experimental product distribution and assessment of substitution effects. Furthermore, other important physical-chemical constants such as unknown equilibria for species, which are not measurable due to low concentrations, or p Ka values of reactive transient species can be estimated. With further development of QC-based methods, it will become possible to implement kinetic and mechanistic information from such computations in in silico models to predict oxidative transformation of micropollutants. Such predictions can then be complemented by tailored experimental studies to confirm/falsify the computations.

Molecular-Level Transformation of Dissolved Organic Matter during Oxidation by Ozone and Hydroxyl Radical.

Ozonation of drinking and wastewater relies on ozone (O3) and hydroxyl radical (•OH) as oxidants. Both oxidants react with dissolved organic matter (DOM) and alter its composition, but the selectivity of the two oxidants and mechanisms of reactivity with DOM moieties are largely unknown. The reactions of O3 and •OH with two DOM isolates were studied by varying specific ozone doses (0.1-1.3 mg-O3/mg-C) at pH 7. Additionally, conditions that favor O3 (i.e., addition of an •OH scavenger) or •OH (i.e., pH 11) were investigated. Ozonation decreases aromaticity, apparent molecular weight, and electron donating capacity (EDC) of DOM with large changes observed when O3 is the main oxidant (e.g., EDC decreases 63-77% for 1.3 mg-O3/mg-C). Both O3 and •OH react with highly aromatic, reduced formulas detected using high-resolution mass spectrometry (O:C = 0.48 ± 0.12; H:C = 1.06 ± 0.23), while •OH also oxidizes more saturated formulas (H:C = 1.64 ± 0.26). Established reactions between model compounds and O3 (e.g., addition of one to two oxygen atoms) or •OH (e.g., addition of one oxygen atom and decarboxylation) are observed and produce highly oxidized DOM (O:C > 1.0). This study provides molecular-level evidence for the selectivity of O3 as an oxidant within DOM.

Quenching of an Aniline Radical Cation by Dissolved Organic Matter and Phenols: A Laser Flash Photolysis Study.

Aromatic amines are relevant aquatic organic contaminants whose photochemical transformation is affected by dissolved organic matter (DOM). The goal of this study is to elucidate the underlying mechanism of the inhibitory effect of DOM on such reactions. The selected model aromatic amine, 4-(dimethylamino)benzonitrile (DMABN), was subjected to laser flash photolysis in the presence and absence of various model photosensitizers. The produced radical cation (DMABN•+) was observed to react with several phenols and different types of DOM on a time scale of ∼100 µs. The determined second-order rate constants for the quenching of DMABN•+ by phenols were in the range of (1.4-26) × 108 M-1 s-1 and increased with increasing electron donor character of the aromatic ring substituent. For DOM, quenching rate constants increased with the phenolic content of the DOM. These results indicate the reduction of DMABN•+ to re-form its parent compound as the basic reaction governing the inhibitory effect. In addition, the photosensitized oxidation of the sulfonamide antibiotic sulfadiazine (SDZ) was studied. The observed radical intermediate of SDZ was quenched by 4-methoxyphenol less effectively than DMABN•+, which was attributed to the lower reduction potential of the SDZ-derived radical compared to DMABN•+.

Persulfate-Based Advanced Oxidation: Critical Assessment of Opportunities and Roadblocks.

Reports that promote persulfate-based advanced oxidation process (AOP) as a viable alternative to hydrogen peroxide-based processes have been rapidly accumulating in recent water treatment literature. Various strategies to activate peroxide bonds in persulfate precursors have been proposed and the capacity to degrade a wide range of organic pollutants has been demonstrated. Compared to traditional AOPs in which hydroxyl radical serves as the main oxidant, persulfate-based AOPs have been claimed to involve different in situ generated oxidants such as sulfate radical and singlet oxygen as well as nonradical oxidation pathways. However, there exist controversial observations and interpretations around some of these claims, challenging robust scientific progress of this technology toward practical use. This Critical Review comparatively examines the activation mechanisms of peroxymonosulfate and peroxydisulfate and the formation pathways of oxidizing species. Properties of the main oxidizing species are scrutinized and the role of singlet oxygen is debated. In addition, the impacts of water parameters and constituents such as pH, background organic matter, halide, phosphate, and carbonate on persulfate-driven chemistry are discussed. The opportunity for niche applications is also presented, emphasizing the need for parallel efforts to remove currently prevalent knowledge roadblocks.

Chlorination of Phenols Revisited: Unexpected Formation of α,ß-Unsaturated C4-Dicarbonyl Ring Cleavage Products.

Despite decades of research on the fate of phenolic compounds when water is disinfected with hypochlorous acid (HOCl), there is still considerable uncertainty regarding the formation mechanisms and identity of ring cleavage products, especially at higher chlorine doses. This study focuses on the formation of electrophilic ring cleavage products-a class of compounds that poses potential health risks at relatively low concentrations-from the reactions of phenols with chlorine. By monitoring the formation of products of reactions between ring cleavage products and the model nucleophile N-α-acetyl-lysine, we identified the α,ß-unsaturated dialdehyde 2-butene-1,4-dial (BDA) and its chlorinated analogue, chloro-2-butene-1,4-dial (Cl-BDA), after the chlorination of phenol, para- and ortho-substituted chlorophenols (2-Cl, 4-Cl, 2,4-diCl-, 2,6-diCl, and 2,4,6-triCl-phenol), and 3,5-di-Cl-catechol. Maximum yields of BDA were observed when chlorine was present in large excess (HOCl/phenol ratios of 30:1 to 50:1), with yields ranging from 18% for phenol to 46% for 3,5-diCl-catechol. BDA and Cl-BDA formation was also observed during the chlorination of brominated phenols. For methyl-substituted phenols, the presence of methyl substituents in both positions ortho to the hydroxy group inhibited BDA and Cl-BDA formation, but the chlorination of cresols and 2,3-dimethylphenol yielded methyl- and dimethyl-BDA species. This study provides new insights into the formation of reactive and toxic electrophiles during chlorine disinfection. It also provides evidence for the importance of phenoxy radicals produced by one-electron transfer reactions initiated by chlorine in the production of dicarbonyl ring cleavage products.

Differences in Viral Disinfection Mechanisms as Revealed by Quantitative Transfection of Echovirus 11 Genomes.

Virus inactivation mechanisms can be elucidated by methods that measure the loss of specific virus functionality (e.g., host attachment, genome internalization, and genome replication). Genome functionality is frequently assessed by PCR-based methods, which are indirect and potentially inaccurate; genome damage that affects detection by high-fidelity PCR enzymes may not adversely affect the ability of actual cellular enzymes to produce functional virus. Therefore, we developed here a transfection-based assay to quantitatively determine viral genome functionality by inserting viral RNA into host cells directly to measure their ability to produce new functional viruses from damaged viral genomes. Echovirus 11 was treated with ozone, free chlorine (FC), UV light at 254 nm (UV254), or heat, and then the reductions in genome functionality and infectivity were compared. Ozone reduced genome functionality proportionally to infectivity, indicating that genome damage is the main mechanism of virus inactivation. In contrast, FC caused little or no loss of genome functionality compared to infectivity, indicating a larger role for protein damage. For UV254, genome functionality loss accounted for approximately 60% of virus inactivation, with the remainder presumably due to protein damage. Heat treatment resulted in no reduction in genome functionality, in agreement with the understanding that heat inactivation results from capsid damage. Our results indicate that there is a fundamental difference between genome integrity reductions measured by PCR enzymes in previous studies and actual genome functionality (whether the genome can produce virus) after disinfection. Compared to PCR, quantitative transfection assays provide a more realistic picture of actual viral genome functionality and overall inactivation mechanisms during disinfection.IMPORTANCE This study provides a new tool for assessing virus inactivation mechanisms by directly measuring a viral genome's ability to produce new viruses after disinfection. In addition, we identify a potential pitfall of PCR for determining virus genome damage, which does not reflect whether a genome is truly functional. The results presented here using quantitative transfection corroborate previously suggested virus inactivation mechanisms for some virus inactivation methods (heat) while bringing additional insights for others (ozone, FC, and UV254). The developed transfection method provides a more mechanistic approach for the assessment of actual virus inactivation by common water disinfectants.

Laser flash photolysis study of the photoinduced oxidation of 4-(dimethylamino)benzonitrile (DMABN).

Aromatic amines are aquatic contaminants for which phototransformation in surface waters can be induced by excited triplet states of dissolved organic matter (3DOM*). The first reaction step is assumed to consist of a one-electron oxidation process of the amine to produce its radical cation. In this paper, we present laser flash photolysis investigations aimed at characterizing the photoinduced, aqueous phase one-electron oxidation of 4-(dimethylamino)benzonitrile (DMABN) as a representative of this contaminant class. The production of the radical cation of DMABN (DMABN˙+) after direct photoexcitation of DMABN at 266 nm was confirmed in accord with previous experimental results. Moreover, DMABN˙+ was shown to be produced from the reactions of several excited triplet photosensitizers (carbonyl compounds) with DMABN. Second-order rate constants for the quenching of the excited triplet states by DMABN were determined to fall in the range of 3 × 107-5 × 109 M-1 s-1, and their variation was interpreted in terms of electron transfer theory using a Rehm-Weller relationship. The decay kinetics of DMABN˙+ in the presence of oxygen was dominated by a second-order component attributed to its reaction with the superoxide radical anion (O2˙-). The first-order rate constant for the transformation of DMABN˙+ leading to photodegradation of DMABN was estimated not to exceed ≈5 × 103 s-1.

Hypobromous Acid as an Unaccounted Sink for Marine Dimethyl Sulfide?

Marine emissions of dimethyl sulfide (DMS) to the atmosphere play a fundamental role in the global sulfur (S) cycle and have important consequences for the Earth's radiative balance. In the ocean, DMS is mainly produced by marine algae and bacteria via cleavage of the precursor compound dimethylsulfoniopropionate (DMSP). Here, we studied the reaction between DMS and the strong oxidant hypobromous acid (HOBr), which is also produced by marine algae. Further, reactions between DMS oxidation products and HOBr were studied. The second-order rate constants were determined in competition kinetic experiments using sulfite as a competitor. In addition, we developed a new HPLC-ICP-MS/MS method to identify and quantify the oxidation products of DMS and related compounds. We found that HOBr reacts very fast with DMS to dimethyl sulfoxide (DMSO), with a second-order rate constant of 1.6 × 109 M-1 s-1, while the subsequent oxidation of DMSO to dimethyl sulfone (DMSO2) is much slower (0.4 M-1 s-1). Concentrations of DMSP, DMSO2, and methanesulfonic acid (MSA) did not decrease when exposed to excess concentrations of HOBr, implying that these S-containing compounds are not or only slightly reactive toward HOBr. A quantitative comparison of known DMS sinks shows that HOBr may be an important, hitherto neglected sink for marine DMS that needs to be considered in ocean-atmosphere chemistry models.

Effects of Ozone on the Photochemical and Photophysical Properties of Dissolved Organic Matter.

This study focused on the effects of ozonation on the photochemical and photophysical properties of dissolved organic matter (DOM). Upon ozonation, a decrease in DOM absorbance was observed in parallel with an increase in singlet oxygen (1O2) and fluorescence quantum yields (Φ1O2 and ΦF). The increase in Φ1O2 was attributed to the formation of quinone-like moieties during ozonation of the phenolic moieties of DOM, while the increase in ΦF can be explained by a significant decrease in the internal conversion rate of the first excited singlet state of the DOM (1DOM*). It is a consequence of an increase in the average energy of the first electronic transition (S1 → S0) that was assessed using the wavelength of maximum fluorescence emission (λF,max). Furthermore, ozonation did not affect the ratio of the apparent steady-state concentrations of excited triplet DOM (3DOM*) and 1O2, indicating that ozonation does not affect the efficiency of 1O2 production from 3DOM*. The consequences of these changes for the phototransformation rates of micropollutants in surface waters were examined using photochemical model calculations. The decrease in DOM absorbance caused by ozonation leads to an enhancement of direct photolysis rates due to the increased transparency of the water. Rates of indirect photooxidation induced by 1O2 and 3DOM* slightly decrease after ozonation.

Oxidation Processes in Water Treatment: Are We on Track?

Chemical oxidants have been applied in water treatment for more than a century, first as disinfectants and later to abate inorganic and organic contaminants. The challenge of oxidative abatement of organic micropollutants is the formation of transformation products with unknown (eco)toxicological consequences. Four aspects need to be considered for oxidative micropollutant abatement: (i) Reaction kinetics, controlling the efficiency of the process, (ii) mechanisms of transformation product formation, (iii) extent of formation of disinfection byproducts from the matrix, (iv) oxidation induced biological effects, resulting from transformation products and/or disinfection byproducts. It is impossible to test all the thousands of organic micropollutants in the urban water cycle experimentally to assess potential adverse outcomes of an oxidation. Rather, we need multidisciplinary and automated knowledge-based systems, which couple predictions of kinetics, transformation and disinfection byproducts and their toxicological consequences to assess the overall benefits of oxidation processes. A wide range of oxidation processes has been developed in the last decades with a recent focus on novel electricity-driven oxidation processes. To evaluate these processes, they have to be compared to established benchmark ozone- and UV-based oxidation processes by considering the energy demands, economics, the feasibilty, and the integration into future water treatment systems.

Kinetics of Inactivation of Waterborne Enteric Viruses by Ozone.

Ozone is an effective disinfectant against all types of waterborne pathogens. However, accurate and quantitative kinetic data regarding virus inactivation by ozone are scarce, because of the experimental challenges associated with the high reactivity of ozone toward viruses. Here, we established an experimental batch system that allows tailoring and quantifying of very low ozone exposures and simultaneously measuring virus inactivation. Second-order ozone inactivation rate constants (kO3-virus) of five enteric viruses [laboratory and two environmental strains of coxsackievirus B5 (CVF, CVEnv1, and CVEnv2), human adenovirus (HAdV), and echovirus 11 (EV)] and four bacteriophages (MS2, Qß, T4, and Φ174) were measured in buffered solutions. The kO3-virus values of all tested viruses ranged from 4.5 × 105 to 3.3 × 106 M-1 s-1. For MS2, kO3-MS2 depended only weakly on temperature (2-22 °C; Ea = 22.2 kJ mol-1) and pH (6.5-8.5), with an increase in kO3-MS2 with increasing pH. The susceptibility of the selected viruses toward ozone decreases in the following order: Qß > CVEnv2 > EV ≈ MS2 > Φ174 ≈ T4 > HAdV > CVF ≈ CVEnv1. On the basis of the measured kO3-Virus and typical ozone exposures applied in water and wastewater treatment, we conclude that ozone is a highly effective disinfectant for virus control.

In Situ Formation of Free Chlorine During ClO2 Treatment: Implications on the Formation of Disinfection Byproducts.

Chlorine dioxide (ClO2) is commonly used as an alternative disinfectant to chlorine in drinking water treatment because it produces limited concentrations of halogenated organic disinfection byproducts. During drinking water treatment, the primary ClO2 byproducts are the chlorite (50-70%) and the chlorate ions (0-30%). However, a significant portion of the ClO2 remains unaccounted for. This study demonstrates that when ClO2 was reacting with phenol, one mole of free available chlorine (FAC) was produced per two moles of consumed ClO2. The in situ formed FAC completed the mass balance on Cl for inorganic ClO2 byproducts (FAC + ClO2- + ClO3-). When reacting with organic matter extracts at near neutral conditions (pH 6.5-8.1), ClO2 also yielded a significant amount of FAC (up to 25%). Up to 27% of this in situ formed FAC was incorporated in organic matter forming adsorbable organic chlorine, which accounted for up to 7% of the initial ClO2 dose. Only low concentrations of regulated trihalomethanes were produced because of an efficient mitigation of their precursors by ClO2 oxidation. Conversely, dichloroacetonitrile formation from ClO2-induced generation of FAC was higher than from addition of FAC in absence of ClO2. Overall, these findings provide important information on the formation of FAC and disinfection byproducts during drinking water treatment with ClO2.