CADMIUM ADSORPTION ON MIXED MINERAL ASSEMBLAGES OF KAOLINITE AND HYDROUS FERRIC OXIDE

The bioavailability and mobility of trace metals in near-surface environments, such as soils, sediments, surface and ground waters, are greatly affected by metal binding to minerals via sorption reactions. Because Cd^II is highly toxic, with adverse affects on respiratory and reproductive systems, its presence in aqueous environments at elevated levels due to anthropogenic activities is a concern. Adsorption of Cd^II on kaolinite and HFO, solids abundant in near-surface soils and sediments, and on binary mixtures of the solids, was investigated as a function of pH using batch experiments with varying sorbate/sorbent ratios (10^-4-10^-6 M Cd^II, 0.8-2.8 g/L solid) and ionic strengths (0.001-0.1 M NaNO₃). Each batch containing a specified concentration of Cd^II, NaNO₃ and solid was titrated with NaOH in increments of ~ 0.3 pH. After each stepwise addition of base, and once pH stability criteria were met, aliquots of the slurry were removed from the batch reactor, equilibrated for 24 hrs, centrifuged and the supernatant analyzed for Cd^II via ICP-OES analysis. Cd^II adsorption displayed typical cation adsorption behavior, with negligible adsorption occurring at low pH and 100% adsorption at high pH. Cd^II adsorption on kaolinite was suppressed (i.e. same percent adsorbed at a higher pH) with increased concentrations of either the background electrolyte or Cd^II. Experimental adsorption data for single solid systems were used to obtain thermodynamic stability constants for adsorption of Cd^II on HFO and kaolinite using a double layer surface complexation model. Thermodynamic parameters established for kaolinite could accurately reproduce adsorption with varying ionic strength but represented varying Cd^II concentrations more poorly. Thermodynamic parameters optimized for the single kaolinite and HFO systems were used to predict adsorption for the binary mixtures based on a component additivity model proposed by Davis et al. (1998, ES&T). This approach accurately predicted adsorption of Cd^II on binary systems with a high ratio of kaolinite to HFO (100:1 & 1000:1) but was less effective at predicting adsorption for systems with a greater quantity of HFO (1:1 or 10:1). Ongoing work includes further refining parameters for the HFO system, and investigating Cd^II adsorption on silica and mixtures of silica with kaolinite and HFO.