Saturday, 15 July 2006

Heavy Metal Sorption by Andic and Non-Andic Soil Horizons Derived from Volcanic Parent Materials.

Hartmut Tanneberg and Reinhold Jahn. Martin-Luther-Univ Halle-Wittenberg, Weidenplan 14, Halle/Saale, Germany

Although vitally important for environmental protection in many regions of the world, information on reaction of metals in soils derived from volcanic parent materials (Andosols) is scarce. For these soils, typically having large contents of organic matter, a large sorption capacity for heavy metals can be assumed. In addition to organic complexes, many trace metals can also form complexes at the surfaces of Si and Al oxides. Similar reactions can be assumed for allophane and imogolite, but few studies have addressed that issue. Due to large contents of organic matter as well as of allophanes, allophane-like phases, and oxides, variable charge sites are predominant in Andosols. Sorption of trace metals, thus, likely is highly pH-depended. In order to predict the fate of heavy metals in Andosols, we carried out batch experiments. In particular, we aimed at estimating the binding and release of metals at different pH values (pH 4, ambient pH, pH 7). Horizons of profiles without vitric or andic properties (Eutric Humi-Tephric Regosol/Italy) and with andic properties (Dystric Fulvi-Silandic Andosol/Italy, Umbric and Acoxic Hydri-Silandic Andosol/The Azores) were selected according to differences in contents of organic matter and short-range ordered minerals. For comparison, the Ap horizon of a Siltic Chernozem (Germany) without short range-ordered minerals was included. We followed the experimental design as outlined in the OECD Guideline 106. Volcanic soils exhibit a high to very high sorption capacity for heavy metals at different pH-values. Generally, we observed large sorption capacities and no clear sorption maximum. The sorption by study samples at ambient pH, ranging from 4.6 to 5.7, was always smaller and less rapid than that of the reference sample (Chernozem Ap, pH 7.5. In particular, the study samples exhibited strong sorption of Pb and Cr at all tested pH values. The rapid sorption kinetics suggests that ion association and ion exchange to govern the binding of heavy metals to all study samples. On average, sorption within the first 10 minutes accounted for 59% Cd, 79% Pb, 87% Ni and 68% Cu of the total amount sorbed over 117 hours. Only for Cr the sorption was distinctly slower for all samples (27%), which is probably due to the formation of inner-sphere surface complexes. Sorption of Cd and Ni was found to be strongly pH-dependent. A clear relation of adsorption and desorption to the content of allophane was not found but with increasing allophane contents, Pb sorption decreased weakly and Cd and Ni sorption decreased strongly in the pH range 4 to 7. At ambient pH and at pH 4, Cd desorbed more completely with increasing allophane content. The high sorption capacity of the studied soils, thus, appears to be caused mainly by organic matter.

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