Surface complexation modeling and FTIR study of carbonate adsorption to goethite

Experimental data for carbonate adsorption onto synthetic goethite, spannin g 3 orders of magnitude in carbonate concentrations, were simulated using t he triple-layer surface complexation model (TLM). A single set of TLM param eters successfully described the adsorption behavior versus pH over the con centration range obtained from closed and open CO2 conditions. An optimizat ion analysis was performed for all possible interfacial charge configuratio ns using FITEQL3.2. The results yielded an optimum charge allocation of 0 a nd -1 in the 0- and beta -planes, respectively, which suggests a monodentat e complex most probably in an inner-sphere configuration (SOCOO-(beta)). Fo urier transform infrared (FTIR) spectroscopic measurements on open systems at atmospheric P-CO2 confirmed this result by showing a clear peak split (1 55 cm(-1)) of the nu (3) C-O asymmetric stretching frequency of surface-bou nd carbonate, consistent with that reported for monodentate Co(III)-carbona to inner-sphere solution complexes. An additional Naf-ternary complex (SOCO ONa) was invoked in the TLM construct to improve simulations of the enhance d carbonate adsorption occurring at high ionic strength and high pH. The mo del was successful in predicting carbonate adsorption behavior under difffe rent conditions than it was calibrated for. Projections for equilibration a t higher P-CO2's (1-10%) than those used in this work show the potential fo r carbonate sorption densities of up to 2.5-3 mu mol/m(2). (C) 2001 Academi c Press.