CHROMATE AND OXALATE ADSORPTION ON GOETHITE .2. SURFACE COMPLEXATION MODELING OF COMPETITIVE ADSORPTION

Adsorption of binary mixtures of chromate and oxalate onto alpha-FeOOH was quantified as a function of pH for a wide range of adsorbate concentrations. Oxalate diminished the adsorption of chromate most effectively at low pH and when adsorbate concentrations were near surface-saturation levels. Chromate significantly inhibited oxalate adsorption over the entire pH range, reflecting the higher affinity of chromate for the alpha-FeOOH surface. The results indicate the adsorption of organic acids can enhance the mobility of chromate in acidic environments, while competitive adsorption of inorganic oxy anions may sharply diminish organic acid adsorption, thereby exerting an important control on mineral weathering rates. The ability of two surface complexation models to quantitatively predict the binary-solute data was evaluated using model constants that successfully described goethite surface hydrolysis and single-solute adsorption. Diffuse layer and triple-layer model simulations were highly similar and quantitatively accounted for binary-solute adsorption as a function of pH when surface concentrations of both coadsorbates were high. However, these models significantly underpredicted adsorption of minor species. Model predictions were particularly poor when the concentration of only one adsorbate was very low.