Drinking Water

Geogenic groundwater arsenic (As) contamination is pervasive in many aquifers in south and southeast Asia. It is feared that recent increases in groundwater abstractions could induce the migration of high-As groundwaters into previously As-safe aquifers. Here we study an As-contaminated aquifer in Van Phuc, Vietnam, located ~10 km southeast of Hanoi on the banks of the Red River, which is affected by large-scale groundwater abstraction. We used numerical model simulations to integrate the groundwater flow and biogeochemical reaction processes at the aquifer scale, constrained by detailed hydraulic, environmental tracer, hydrochemical and mineralogical data. Our simulations provide a mechanistic reconstruction of the anthropogenically induced spatiotemporal variations in groundwater flow and biogeochemical dynamics and determine the evolution of the migration rate and mass balance of As over several decades. We found that the riverbed–aquifer interface constitutes a biogeochemical reaction hotspot that acts as the main source of elevated As concentrations. We show that a sustained As release relies on regular replenishment of river muds rich in labile organic matter and reactive iron oxides and that pumping-induced groundwater flow may facilitate As migration over distances of several kilometres into adjacent aquifers.

Enteric viruses, such as enterovirus, norovirus, and rotavirus are among the leading causes of disease outbreaks due to contaminated drinking and recreational water. Viruses are difficult to remove from water through filtration based on physical size exclusion - for example by gravity driven filters - due to their nanoscale size. To understand virus removal in drinking water treatments systems, the colloidal nanostructure of a model virus, the MS2 bacteriophage, has been investigated in relation to the effect of pH and natural organic matter in water. Dynamic light scattering, small angle X-ray scattering and cryogenic transmission electron microscopy demonstrated that the water pH has a major influence on the colloidal structure of the virus: The bacteriophage MS2’s structure in water in the range pH = 7.0 to 9.0 was found to be spherical with core-shell type structure with a total diameter of 27 nm and a core radius of around 8 nm. Reversible transformations from 27 nm particles at pH = 7.0 to micrometer sized aggregate at pH = 3.0 were observed. In addition, the presence of natural organic matter that simulates the organic components present in surface water was found to enhance repulsion between virus particles, reduce size of aggregates and promote disaggregation upon pH increase. These findings allow a better understanding of virus interactions in water and have implications for water treatment using filtration processes and coagulation. The results will further guide the comprehensive design of advanced virus filter materials.

The generation of hydroxyl radicals (^•OH) during the chlorination of air saturated solutions of different hydroxyphenols (hydroquinone, resorcinol, catechol, gallic and tannic acids) at pH 7 has been determined by the formation of phenol (in presence of benzene in excess) or 2-hydroxyterephthalic acid (in presence of terephthalic acid). Formation of ^•OH was only detected during the chlorination of o- or p-hydroxyphenols, compounds that react with chlorine by electron transfer forming the corresponding semiquinones/quinones. In aerated solutions, oxygen is reduced by the semiquinone to the superoxide radical, O₂^•-, which reacts with HOCl to ^•OH. Compared to the studied o-hydroxyphenols, the lower reactivity of hydroquinone towards chlorine favours the reaction between chlorine and O₂^•-, and its ^•OH formation potential is ∼50 times higher. The extent of ^•OH generated increased with the concentration of the hydroxyphenol and chlorine, but the ^•OH yield (moles formed per mole of hydroxyphenol eliminated), decreased due to the formation of the quinone, that acts as O₂^•- scavenger. The yield was almost not affected by the pH (6 ≤ pH ≤ 7.5), whereas a strong impact of dissolved O₂ was observed. The ^•OH production was null in absence of O₂ and 2.5-3 times higher at oxygen saturated conditions compared to air-saturated. Contrary to chlorination, during bromination of hydroquinone ^•OH was not formed, which can be attributable to a much faster consumption of the oxidant, with no chance for O₂^•- to react with bromine.
Formation of ^•OH during the chlorination of different NOM extracts (SRHA, SRFA, PLFA and Nordic Lake NOM) and water from Lake Greifensee (Switzerland) was also studied using terephthalic acid as ^•OH scavenger. For SRHA, SRFA and Nordic Lake NOM (all of allochthonous origin and presenting high electron-donating capacity, EDC), ^•OH yields expressed as moles formed per mole of DOC₀ (%), were between 1.1 and 2.0, similar to that of hydroquinone (∼1.5). For PLFA and Lake Greifensee water (autochthonous, lower EDC) much lower ^•OH yields were observed (0.1-0.3). Both chlorination rate and EDC, the later favouring the formation/stabilization of O₂^•-, seem to be key factors involved in ^•OH generation during the chlorination of NOM. A mechanism for these findings is proposed based on kinetic simulations of hydroquinone chlorination at pH 7.

Oxidative treatment of iodide-containing waters can lead to a formation of potentially toxic iodinated disinfection byproducts (I-DBPs). Iodide (I^-) is easily oxidized to HOI by various oxidation processes and its reaction with dissolved organic matter (DOM) can produce I-DBPs. Hydrogen peroxide (H₂O₂) plays a key role in minimizing the formation of I-DBPs by reduction of HOI during H₂O₂-based advanced oxidation processes or water treatment based on peracetic acid or ferrate(VI). To assess the importance of these reactions, second order rate constants for the reaction of HOI with H₂O₂ were determined in the pH range of 4.0-12.0. H₂O₂ showed considerable reactivity with HOI near neutral pH (k_app = 9.8 × 10³ and 6.3 × 10⁴ M^-1s^-1 at pH 7.1 and 8.0, respectively). The species-specific second order rate constants for the reactions of H₂O₂ with HOI, HO₂^- with HOI, and HO_2^- with OI^- were determined as k_H2O2+HOI = 29 ± 5.2 M^-1s^-1, k_HO2^-_+HOI = (3.1 ± 0.3) × 10⁸ M^-1s^-1, and k_HO2^-_+OI^- = (6.4 ± 1.4) × 10⁷ M^-1s^-1, respectively. The activation energy for the reaction between HOI and H₂O₂ was determined to be E_a = 34 kJ mol^-1. The effect of buffer types (phosphate, acetate, and borate) and their concentrations was also investigated. Phosphate and acetate buffers significantly increased the rate of the H₂O₂-HOI reaction at pH 7.3 and 4.7, respectively, whereas the effect of borate was moderate. It could be demonstrated, that the formation of iodophenols from phenol as a model for I-DBPs formation was significantly reduced by the addition of H₂O₂ to HOI- and phenol-containing solutions. During water treatment with the O₃/H₂O₂ process or peracetic acid in the presence of I^-, O₃ and peracetic acid will be consumed by a catalytic oxidation of I^- due to the fast reduction of HOI by H₂O₂. The O₃ deposition on the ocean surface may also be influenced by the presence of H₂O₂, which leads to a catalytic consumption of O₃ by I^-.

Dissolution of Fe(III) phases is a key process in making iron available to biota and in the mobilization of associated trace elements. Recently, we have demonstrated that submicromolar concentrations of Fe(II) significantly accelerate rates of ligand-controlled dissolution of Fe(III) (hydr)oxides at circumneutral pH. Here, we extend this work by studying isotope exchange and dissolution with lepidocrocite (Lp) and goethite (Gt) in the presence of 20 or 50 μM desferrioxamine-B (DFOB). Experiments with Lp at pH 7.0 were conducted in carbonate-buffered suspensions to mimic environmental conditions. We applied a simple empirical model to determine dissolution rates and a more complex kinetic model that accounts for the observed isotope exchange and catalytic effect of Fe(II). The fate of added tracer ⁵⁷Fe(II) was strongly dependent on the order of addition of ⁵⁷Fe(II) and ligand. When DFOB was added first, tracer ⁵⁷Fe remained in solution. When ⁵⁷Fe(II) was added first, isotope exchange between surface and solution could be observed at pH 6.0 but not at pH 7.0 and 8.5 where ⁵⁷Fe(II) was almost completely adsorbed. During dissolution of Lp with DFOB, ratios of released ⁵⁶Fe and ⁵⁷Fe were largely independent of DFOB concentrations. In the absence of DFOB, addition of phenanthroline 30 min after tracer ⁵⁷Fe desorbed predominantly ⁵⁶Fe(II), indicating that electron transfer from adsorbed ⁵⁷Fe to ⁵⁶Fe of the Lp surface occurs on a time scale of minutes to hours. In contrast, comparable experiments with Gt desorbed predominantly ⁵⁷Fe(II), suggesting a longer time scale for electron transfer on the Gt surface. Our results show that addition of 1–5 μM Fe(II) leads to dynamic charge transfer between dissolved and adsorbed species and to isotope exchange at the surface, with the dissolution of Lp by ligands accelerated by up to 60-fold.