Drinking Water

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 (E_HOMO) 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 pK_a 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.

Gravity-driven membrane (GDM) filtration has been investigated for almost 10 years. The technology is characterized not only by relatively lower transmembrane pressures which can be achieved by gravity (extremely low energy consumption), but also by the phenomenon of flux stabilization: A biofilm is allowed to form on the membrane and a stabilization of flux occurs which is related to biological processes within the biofilm layer on the membrane. This enables stable operation during a year or longer without any cleaning or flushing. Initially, the technology was developed mainly for household drinking water treatment, but in the meantime, the research and application has expanded to the treatment of greywater, rainwater, and wastewater as well as the pretreatment of seawater for desalination. This review covers the field from the rather fundamental research on biofilm morphology and microbial community analysis to the impact of feedwater composition, process parameters and organic removal performance. Not only household applications, but also for community-scale treatment and full-scale applications are discussed. In addition, the application potential is highlighted in comparison to conventional ultrafiltration. Finally, an overall assessment is illustrated and the research and development needs are identified.

Cyanobacterial bloom events that produce natural toxins occur in freshwaters across the globe, yet the potential risk of many cyanobacterial metabolites remains mostly unknown. Only microcystins, one class of cyanopeptides, have been studied intensively and the wealth of evidence regarding exposure concentrations and toxicity led to their inclusion in risk management frameworks for water quality. However, cyanobacteria produce an incredible diversity of hundreds of cyanopeptides beyond the class of microcystins. The question arises, whether the other cyanopeptides are in fact of no human and ecological concern or whether these compounds merely received (too) little attention thus far. Current observations suggest that an assessment of their (eco)toxicological risk is indeed relevant: First, other cyanopeptides, including cyanopeptolins and anabaenopeptins, can occur just as frequently and at similar nanomolar concentrations as microcystins in surface waters. Second, cyanopeptolins, anabaenopeptins, aeruginosins and microginins inhibit proteases in the nanomolar range, in contrast to protein phosphatase inhibition by microcystins. Cyanopeptolins, aeruginosins, and aerucyclamide also show toxicity against grazers in the micromolar range comparable to microcystins. The key challenge for a comprehensive risk assessment of cyanopeptides remains their large structural diversity, lack of reference standards, and high analytical requirements for identification and quantification. One way forward would be a prevalence study to identify the priority candidates of tentatively abundant, persistent, and toxic cyanopeptides to make comprehensive risk assessments more manageable.

Chlorine dioxide (ClO₂) is commonly used as an alternative disinfectant tochlorine in drinking water treatment because it produces limited concentrations of halogenatedorganic disinfection byproducts. During drinking water treatment, the primary ClO₂ byproductsare the chlorite (50−70%) and the chlorate ions (0−30%). However, a significant portion of theClO₂ remains unaccounted for. This study demonstrates that when ClO₂ was reacting withphenol, one mole of free available chlorine (FAC) was produced per two moles of consumed ClO₂. The in situ formed FACcompleted the mass balance on Cl for inorganic ClO₂ byproducts (FAC + ClO₂¯+ ClO₃¯). When reacting with organic matterextracts at near neutral conditions (pH 6.5−8.1), ClO₂ also yielded a significant amount of FAC (up to 25%). Up to 27% of thisin situ formed FAC was incorporated in organic matter forming adsorbable organic chlorine, which accounted for up to 7% ofthe initial ClO₂ dose. Only low concentrations of regulated trihalomethanes were produced because of an efficient mitigation oftheir precursors by ClO₂ oxidation. Conversely, dichloroacetonitrile formation from ClO₂-induced generation of FAC washigher than from addition of FAC in absence of ClO₂. Overall, these findings provide important information on the formationof FAC and disinfection byproducts during drinking water treatment with ClO₂.