Nitrate-induced mobilization of trace elements in reduced groundwater environments.

Groundwater is an essential source for drinking water production. Nitrate infiltration into groundwater due to over-fertilization can cause a potential risk for groundwater quality. Pyrite and other geogenic minerals can be oxidized and trace metals consequently released into water, e.g., nickel and uranium. To achieve a better understanding of the nitrate-induced mobilization of metals, this study investigated the release of antimony, arsenic, chromium, cobalt, molybdenum, uranium, and vanadium from three different reduced sediments after nitrate addition. The experiments were conducted as batch and soil column tests under oxygen-free conditions. In addition to the ORP, the pH value was a relevant driver for the metal mobilization due to pH dependent adsorption and ion exchange processes. Uranium concentrations in the water increased with increasing redox potential. Also, antimony and, to a lesser extent, molybdenum showed higher mobilization at higher ORP as well as at higher pH values. On the contrary, arsenic and cobalt was immobilized with increasing redox potential. Pourbaix diagrams demonstrated very complex species distributions even in synthetic water. The mobilization of trace metals is expected to be also influenced by the type of surrounding rocks and water quality parameters such as dissolved organic carbon.

Pool water disinfection by ozone-bromine treatment: Assessing the disinfectant efficacy and the occurrence and in vitro toxicity of brominated disinfection by-products.

Pool water is continuously circulated and reused after an extensive treatment including disinfection by chlorination, ozonation or UV treatment. In Germany, these methods are regulated by DIN standard 19643. Recently, the DIN standard has been extended by a new disinfection method using hypobromous acid as disinfectant formed by introducing ozone into water with naturally or artificially high bromide content during water treatment. In this study, we tested the disinfection efficacy of the ozone-bromine treatment in comparison to hypochlorous acid in a flow-through test rig using the bacterial indicator strains Escherichia coli, Enterococcus faecium, Pseudomonas aeruginosa, and Staphylococcus aureus and the viral indicators phage MS2 and phage PRD1. Furthermore, the formation of disinfection by-products and their potential toxic effects were investigated in eight pool water samples using different disinfection methods including the ozone-bromine treatment. Our results show that the efficacy of hypobromous acid, depending on its concentration and the tested organism, is comparable to that of hypochlorous acid. Hypobromous acid was effective against five of six tested indicator organisms. However, using Pseudomonas aeruginosa and drinking water as test water, both tested disinfectants (0.6 mg L-1 as Cl2 hypobromous acid as well as 0.3 mg L-1 as Cl2 hypochlorous acid) did not achieve a reduction of four log10 levels within 30 s, as required by DIN 19643. The formation of brominated disinfection by-products depends primarily on the bromide concentration of the filling water, with the treatment method having a smaller effect. The eight pool water samples did not show critical values in vitro for acute cytotoxicity or genotoxicity in the applied assays. In real pool water samples, the acute toxicological potential was not higher than for conventional disinfection methods. However, for a final assessment of toxicity, all single substance toxicities of known DBPs present in pool water treated by the ozone-bromine treatment have to be analyzed additionally.

Deiodination in the presence of Dehalococcoides mccartyi strain CBDB1: comparison of the native enzyme and co-factor vitamin B12.

Triiodinated benzoic acid derivatives are widely used as contrast media for medical examinations and are found at high concentrations in urban aquatic environments. During bank filtration, deiodination of iodinated contrast media has been observed under anoxic/anaerobic conditions. While several bacterial strains capable of dechlorination and debromination have been isolated and characterized, deiodination has not yet been shown for an isolated strain. Here, we investigate dehalogenation of iodinated contrast media (ICM), triiodobenzoic acids (TIBA), and analogous chlorinated compounds by Dehalococcoides mccartyi strain CBDB1 and its corrinoid co-factor vitamin B12. No cell growth of CBDB1 was observed using iodinated compounds as electron acceptor. Only negligible deiodination occurred for ICM, whereas 2,3,5-TIBA was nearly completely deiodinated by CBDB1 without showing cell growth. Furthermore, TIBA inhibited growth with hexachlorobenzene which is usually a well-suited electron acceptor for strain CBDB1, indicating that TIBA is toxic for CBDB1. The involvement of CBDB1 enzymes in the deiodination of TIBA was verified by the absence of deiodination activity after heat inactivation. Adding iodopropane also inhibited the deiodination of TIBA by CBDB1 cells, indicating the involvement of a corrinoid-enzyme in the reductive TIBA deiodination. The results further suggest that the involved electron transport is decoupled from proton translocation and therefore growth. Graphical abstract.

Reaction kinetics of corrinoid-mediated deiodination of iodinated X-ray contrast media and other iodinated organic compounds.

Iodinated contrast media (ICM) are found at considerably higher concentrations than any other pharmaceutical in waste water, surface water and bank filtrate. While the compounds are persistent to deiodination in aerobic environments, field data from bank filtration transects have demonstrated a partial deiodination in reducing soil-water environments. In a previous lab study, we have shown that this reductive deiodination is abiotically catalyzed by (free) corrinoids. To achieve a better understanding of the incomplete deiodination in the environment, we now investigated the reaction kinetics based on the decrease of the iodinated compound, the formation of deiodinated transformation products and the iodide release. The deiodination follows first-order kinetics and consists of three partial reactions for the release of three iodine atoms. The deiodination rate decreased with decreasing iodination degree with the deiodination rate constants k1 > k2 > k3. In contrast to the ICM, 2,4,6- and 2,3,5-triiodobenzoic acid, 5-amino-2,4,6-triiodoisophthalic acid and monoiodobenzoic acids did not show a complete deiodination under the same test conditions. Our results show that the deiodination strongly depends on the substitution pattern of the bound iodine atoms as well as on adjacent functional groups. Iodine atoms in ortho-position to another iodine atom or a carboxyl group were released more easily while an amino group in ortho-position inhibited the deiodination. Tests in tap water in the presence of B12 showed a much slower deiodination than in ultrapure water, most likely caused by competitive electron acceptors in the water matrix.

Abiotic reductive deiodination of iodinated organic compounds and X-ray contrast media catalyzed by free corrinoids.

Iodinated X-ray contrast media are known for their stability concerning deiodination in the aquatic environment under aerobic conditions. In this study, we demonstrate the abiotic reductive deiodination of the iodinated contrast media iopromide, iopamidol and diatrizoate in the presence of corrinoids. In addition, triiodinated benzoic acid derivatives with iodine atoms bound at different positions were investigated. Corrinoids like cyanocobalamin (vitamin B12) and dicyanocobinamide served as electron shuttles and as catalysts between the reducing agent (e.g., titanium (III) citrate) and the electron accepting iodinated compound. The concentration decrease of the iodinated compounds followed first-order kinetics with rate constant kobs depending on the iodinated compound. A linear correlation between the rate of iodide release and the corrinoid concentration was observed, with deiodination rates for dicyanocobinamide twice as high as for vitamin B12. Reducing agents with a less negative standard redox potential like dithiothreitol or cysteine caused slower deiodination as the cobalt center was only reduced to its CoII oxidation state. With a temperature increase from 11 to 23 °C, the concentrations of released iodide doubled. A complete deiodination was only observed for the iodinated contrast media but not for structurally similar iodinated benzoic acid derivatives.