Department Water Resources and Drinking Water

We studied arid desert soils from Namibia (Rosh Pinah) that were contaminated with up to 7 mg kg⁻¹ of thallium (Tl) via dust emitted from a local flotation tailing dam. Chemical extractions of waste and soil materials indicated that most of the Tl is strongly bound, in accordance with X-ray diffraction and X-ray absorption spectroscopy data that point to the predominant association of Tl with metal sulfides and phyllosilicates. The isotope fractionation factor ε²⁰⁵Tl of the soil samples (from −0.4 to +3.8) shows a positive linear relationship (R² = 0.62) with 1/Tl, indicative for the mixing of two major Tl pools, presumably anthropogenic Tl and geogenic Tl. The ε²⁰⁵Tl value for the topmost soil samples (∼+3) closely matches the ε²⁰⁵Tl value for post-flotation waste particles with a diameter of <0.05 mm, whereas the bulk flotation waste exhibits a significantly larger ε²⁰⁵Tl value (∼+6). These variations are in accordance with predominant atmospheric transfer of Tl from the tailings to the adjacent soils via fine (dust) particles. The identified minimal Tl alteration in soils indicates that only a small part of the Tl could be potentially released and passively enter the vegetation, local population and/or food chain in the long term. From this viewpoint, Tl does not represent such an important environmental concern as other (abundant) contaminants at the locality. Furthermore, there could be a relevance for other alkaline desert soils, including those where Tl pollution plays a major role.

Chemical oxidants have been applied in water treatment formore than a century, first as disinfectants and later to abate inorganic andorganic contaminants. The challenge of oxidative abatement of organicmicropollutants is the formation of transformation products with unknown(eco)toxicological consequences. Four aspects need to be considered foroxidative micropollutant abatement: (i) Reaction kinetics, controlling theefficiency 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 productsand/or disinfection byproducts. It is impossible to test all the thousands oforganic micropollutants in the urban water cycle experimentally to assesspotential adverse outcomes of an oxidation. Rather, we need multidisciplinaryand automated knowledge-based systems, which couple predictions of kinetics,transformation and disinfection byproducts and their toxicological consequencesto assess the overall benefits of oxidation processes. A wide range of oxidation processes has been developed inthe last decades with a recent focus on novel electricity-driven oxidation processes. To evaluate these processes, they have to becompared 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.

Phenolic moieties are common functional groups in organic micropollutants and in dissolved organic matter, and are exposed to ozone during drinking water and wastewater ozonation. Although unsubstituted phenol is known to yield potentially genotoxic p-benzoquinone during ozonation, little is known about the effects of substitution of the phenol ring on transformation product formation. With batch experiments employing differing ozone/target compound ratios, it is shown that para-substituted phenols (p-alkyl, p-halo, p-cyano, p-methoxy, p-formyl, p-carboxy) yield p-benzoquinones, p-substituted catechols, and 4-hydroxy-4-alkyl-cyclohexadien-1-ones as common ozonation products. Only in a few cases did para-substitution prevent the formation of these potentially harmful products. Quantum chemical calculations showed that different reaction mechanisms lead to p-benzoquinone, and that cyclohexadienone can be expected to form if no such pathway is possible. These products can thus be expected from most phenolic moieties. Kinetic considerations showed that substitution of the phenolic ring results in rather small changes of the apparent second order rate constants for phenol–ozone reactions at pH 7. Thus, in mixtures, most phenolic structures can be expected to react with ozone. However, redox cross-reactions between different transformation products, as well as hydrolysis, can be expected to further alter product distributions under realistic treatment scenarios.