Understanding the chemical speciation of metals in solution is necessary for evaluating their toxicity and mobility in soils. Soil samples from the Powder River Basin, Wyoming were extracted with distilled deionised H2O. Soil water extracts were subjected to chemical speciation to determine the relative distribution and chemical forms of copper (Cu), zinc (Zn) and lead (Pb) in acidic environments. As pyrite oxidised, the pH decreased from 6.6 to 2.4, concentration of dissolved sulfate (ST) increased from 259 to 4,388 mg L-1 and concentration of dissolved organic carbon (DOC) decreased from 56.9 to 14.4 mg L-1. Dissolved Cu concentrations ranged from 0.06 to 0.42 mg L-1 and dissolved Zn concentrations ranged from 0.084 to 4.60 mg L-1. Dissolved concentrations of Pb were found to be 0.003 to 0.046 mg L-1. Chemical speciation indicated that at near neutral pH, dissolved metal concentration in soil water extracts was dominated by DOC- metal complexes. At low pH, dissolved metal concentration in soil water extracts was dominated by free ionic forms (e.g. Cu2+, Zn2+, Pb2+) followed by ion pairs (e.g. CuSO40, ZnSOinf4sup0, PbSOin4sup0). Results obtained in this study suggest that as soil pH decreased, the availability and mobility of metal ions increased due to the chemical form in which these metal ions are present in soil solutions.
availability of metals chemical speciation inorganic complexes organic complexes