COPPER(II) ADSORPTION ON HYDROUS FERRIC OXIDE AND KAOLINITE—A SURFACE COMPLEXATION APPROACH TO MODELING ADSORPTION IN NATURAL SYSTEMS

In order to better understand and quantify copper bioavailability and transport, copper adsorption has been extensively studied. Cu adsorption on single, pure adsorbents has been described using thermodynamic surface complexation models (SCMs, e.g. Dzombak and Morel, 1990, Hydrous Ferric Oxide). However, SCMs have only been used to quantify Cu adsorption on a small suite of minerals (e.g. Maqueda et al., 2002, Soil Science, Cu/goethite; Dubbin et al., 2000, Soil Science, Cu/gibbsite; Dzombak and Morel, 1990, Cu/HFO). In this study, Cu(II) adsorption on synthetic hydrous ferric oxide (HFO) and natural kaolinite was measured as a function of pH (2-10), ionic strength (0.001-0.1M NaNO3) and Cu concentration (1·10-6-1·10-4M Cu, 2g/L solid). In each experiment, a slurry of dissolved Cu and solid was titrated with HNO3 and NaOH. Aliquots were periodically removed, equilibrated for 24hrs on a shaker, and centrifuged to separate solids from supernatants. Acidified supernatants were analyzed for remaining dissolved Cu using ICP-MS. The measured pH and ionic strength dependence of Cu adsorption on HFO is in good agreement with SCM predictions using published stability constants from Dzombak and Morel (1990). Experimental data demonstrate a weaker affinity of Cu for kaolinite, with very slight dependence on ionic strength and Cu concentration. Derivation of SCM stability constants for Cu adsorption on kaolinite is in progress. Few, if any, SCM studies quantify Cu adsorption on binary or ternary combinations of colloids, which should better represent natural systems than single pure minerals. Derived stability constants for Cu adsorption on kaolinite will be used to predict Cu adsorption for binary adsorbent systems (kaolinite + HFO) using the "component additivity" approach proposed by Davis et al. (1998, ES&T). Experiments will be conducted to corroborate these SCM predictions and evaluate the component additivity approach.