Experimental data for carbonate adsorption onto synthetic goethite, spanning 3 orders of magnitude in carbonate concentrations, were simulated using the triple-layer surface complexation model (TLM). A single set of TLM parameters successfully described the adsorption behavior versus pH over the concentration range obtained from closed and open CO(2) conditions. An optimization analysis was performed for all possible interfacial charge configurations using FITEQL3.2. The results yielded an optimum charge allocation of 0 and -1 in the 0- and beta-planes, respectively, which suggests a monodentate complex most probably in an inner-sphere configuration (SOCOO(-beta)). Fourier transform infrared (FTIR) spectroscopic measurements on open systems at atmospheric P(CO(2)) confirmed this result by showing a clear peak split (155 cm(-1)) of the nu(3) C-O asymmetric stretching frequency of surface-bound carbonate, consistent with that reported for monodentate Co(III)-carbonato inner-sphere solution complexes. An additional Na(+)-ternary complex (SOCOONa) was invoked in the TLM construct to improve simulations of the enhanced carbonate adsorption occurring at high ionic strength and high pH. The model was successful in predicting carbonate adsorption behavior under diffferent conditions than it was calibrated for. Projections for equilibration at higher P(CO(2))'s (1-10%) than those used in this work show the potential for carbonate sorption densities of up to 2.5-3 µmol/m(2). Copyright 2001 Academic Press.