Research Speciation of Dissolved Iron(II) and Manganese(II) in a Groundwater Pollution Plume DORTHE L. JENSEN,* JENS K. BODDUM, SABINE REDEMANN, AND THOMAS H. CHRISTENSEN Department of Environmental Science and Engineering/ Groundwater Research Centre, Building 115, Technical University of Denmark, DK-2800 Lyngby, Denmark
Groundwater samples with high concentrations of Fe(II) and Mn(II), obtained from an anaerobic pollution plume, were speciated under anaerobic conditions in terms of colloidal (screen filtration and cross-flow ultrafiltration), complexed (ion-exchange resins), and free divalent species of Fe(II) and Mn(II). Most of the metal content in a nonfiltered sample passed through a 0.001 µm membrane and was considered truly dissolved [Fe(II), 78-100%, average of 90%; Mn(II), 87-100%, average of 91%]. Although free metal ions were the dominant species in the dissolved fraction, speciation modeling using MINTEQA2 suggested an average of 19% Fe(II) and 20% Mn(II) were present as bicarbonate complexes, which are here operationally defined as labile. Organic complexes were not significant, although the groundwater contained up to 67 mg of C L-1. Calculations with MINTEQA2 indicated that sample solutions representative of the truly dissolved fraction (
