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

Abstract

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.

Keywords: aqueous oxidation; chemical oxidants; micropollutants; prediction; radical species; reaction kinetics; reaction mechanisms; transformation products.

Figure 1

Experimental and theoretical tools to understand and predict the oxidative transformation of organic compounds including micropollutants. The following abbreviations are used: LC: liquid chromatography; MS: mass spectrometry; MS/MS: tandem mass spectrometry; GC: gas chromatography; IRMS: isotope ratio mass spectrometry; UHR: ultrahigh-resolution; IC: ion chromatography; SFC: supercritical fluid chromatography; NMR: nuclear magnetic resonance; and QSAR: quantitative structure activity relationship.

Figure 2

Apparent second-order rate constants, k_app (M^–1 s^–1) at pH 7, for the reactions of selected functional groups with dosed chemical oxidants and in situ formed radicals.^,,− Note that the compounds in the different classes are not always the same for all oxidants, which may lead to differences in reactivity ranges.

Figure 3

Major reaction mechanisms induced by ozone, free available chlorine (FAC), chlorine dioxide and the in situ formed radicals (^•OH, SO₄^•–, Cl^•, Cl₂^•–, Br^•, Br₂^•–, NO^•, NO₂^•, CO₃^•–).