INTERACTIONS BETWEEN ARSENIC AND IRON OXYHYDROXIDES IN LACUSTRINE SEDIMENTS

Arsenic and iron concentrations were measured in surficial sediments and in interstitial and overlying waters at 22 littoral stations of 16 lakes. The lakes were chosen to cover values of pH between 4.0 and 8.4 and various As concentrations. Depth-distributions of dissolved As and Fe concentrations suggest a close association of both elements in a dissolution-diifusion-precipitation cycle for Fe and adsorbed As. Using a simplified version of the surface complexation model, apparent adsorption constants of As onto natural Fe oxyhydroxides have been calculated from the concentrations of As and Fe determined in leachates of surficial lake sediments and the in situ measurement of dissolved As in their respective overlying waters. These calculations assume, based on thermodynamic considerations and experimental evidence, that only As(V) is associated with the natural Fe oxyhydroxides. The binding intensity values obtained from these lakes are compared to those obtained for the adsorption of As(V) onto various synthetic Fe oxyhydroxides in well-defined media. The binding constants derived from field measurements agree well with those obtained from laboratory experiments performed with amorphous Fe oxyhydroxides. © 1990.