Saturday, 15 July 2006
139-13

Modelling the Competition from Fe(III) and Al(III) on Trace Metal Binding to a Spodosol Mor Layer.

Jon Petter Gustafsson1, Ingmar Persson2, Dan Berggren Kleja3, and Joris W.J. Van Schaik2. (1) KTH (Royal Inst. of Technology), Dept of Land & Water Resources Engineering, Brinellvägen 28, Stockholm, SE-10044, Sweden, (2) Swedish Univ of Agricultural Sciences (SLU), Dept of Chemistry, Box 7015, Uppsala, SE-750 07, Sweden, (3) Swedish Univ of Agricultural Sciences, Dept of Soil Sciences, Box 7014, Uppsala, 750 07, Sweden

The binding of trace metals to organic soils in the presence of common competing ions such as Al3+ and Fe3+ is still not well understood. In this study we studied the pH-dependent binding of Cd2+, Co2+, Ni2+ and VO2+ to a Swedish mor layer (Risbergshöjden Oe) in the absence or presence of different background concentrations of Al3+ and Fe3+ as nitrate salts. At pH 4, the order of binding for the four selected trace metals was: VO2+ >> Ni2+ > Cd2+ » Co2+. Additions of Al3+ and Fe3+ to the mor layer suspensions caused competition effects, although they were relatively modest in the case of VO2+. We used a geochemical model, Visual MINTEQ with the Stockholm Humic Model (SHM), to derive complexation constants for the trace metals in this system. The modelling relied mainly on complexation reactions that have been used in previous work with the SHM and WHAM models. However, because the complexation mode for Fe3+ in organic soils is poorly known, we conducted Fe K-edge EXAFS spectroscopy on the mor layer to constrain the Fe3+ complexation reaction in the model. The resulting model was capable of correctly accounting for the pH-dependent competition effects of Fe3+ and Al3+. These results will help to simulate trace metal binding in polluted organic soils.

 


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