CHROMATE AND OXALATE ADSORPTION ON GOETHITE .1. CALIBRATION OF SURFACE COMPLEXATION MODELS

Adsorption of chromate an oxalate on alpha-FeOOH was quantified for single-adsorbate systems as a function of pH, ionic strength, and adsorbate and adsorbent concentrations. The comprehensive data base was used to calibrate and compare two surface complexation models. Both anions exhibited Langmuir-type adsorption with maximum adsorption densities (GAMMA(max)) of 2.2 (oxalate) and 2.4 mumol/m2 (chromate). Increasing ionic strength diminished the adsorption of both anions. However, the background electrolyte affected oxalate adsorption more than chromate adsorption, reflecting a higher intrinsic affinity of chromate for the goethite surface. Measured proton/anion adsorption stoichiometry ratios (r(H)) ranged from 0.3 to 1.1, increased with pH, but showed little dependence on surface concentration of adsorbates. The diffuse layer model (DLM) and triple-layer model (TLM) reproduced all major adsorption features with a single set of constants for each model, but more surface species were required for the DLM. Another difference between models arose in the computation of r(H): DLM-calculated values were closer to the experimental values. Comparison of DLM-based constants for goethite and those obtained elsewhere for amorphous hydrous ferric oxide indicates that the intrinsic affinity of these anions for both iron oxides is very similar.