Dynamics of Cu, Zn, Cd, and Hg release from sediments at surface conditions


Authors: Komárek M, Zeman J

Published in: Bulletin of Geosciences, volume 79, issue 2; pages: 99 - 106; Received 17 April 2003; Accepted in revised form 4 July 2003;

Keywords: flow-through reactors, sequential analysis, system sediment-water, interactions, geochemical modeling, dynamics of heavy metal release, calcite dissolution,

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Sediments and soils have a high capacity for binding heavy metals from contaminated waters. However, these sediments become a potential source of water contamination by subsequently releasing the metals into the water when environmental conditions change.
Samples of typical sediments from the Vienna Basin were collected for the experimental study of Cu, Cd, Zn, and Hg release from sediments to water. These sediments were characterized using chemical analysis, XRD analysis, the measurement of sediment sorption parameters, grain and pore analysis, and sequential analysis.
Initially, a set of sediment samples was placed into batch reactors, each containing 0.1Msolution of CuSO4, CdSO4, ZnSO4, or HgCl2, and kept there for two months. These artificially contaminated sediments were subsequently leached by distilled water in flow-through reactors.
Described experiments show that a significant proportion of the metal remains strongly bound to the sediment after the leaching process, suggesting that the reactions by which they are bound to the sediment are not completely reversed. The rate of metal release is significantly affected by sediment composition and bond type. Geochemical modeling and sequential analysis have confirmed that the metals are not only bound to sediments by sorption, but also by the precipitation of secondary minerals such as brochantite, tenorite (Cu2+), hydrozincite (Zn2+), and otavite (Cd2+). Significant quantities of the metals are releasable from the sediments only in extreme conditions (Cd, Zn, and Cu in strongly acidic reducing conditions, and Hg in acidic oxidizing conditions). The presence of calcite in the sediment decreases the rate of Hg, Zn and Cu release through pH buffering by calcite dissolution (in solutions with pH values ranging from 7 to 7.5). Under these conditions the metals are not released, and can remain bound to the sediment.