| The purpose of this study was to elucidate reactions involving carbonate at the goethite-aqueous interface, under conditions applicable to those found in natural settings. Increased understanding of the goethite-carbonate interactions will provide insight to the range of effects on adsorption of trace elements.; The goals of this investigation were: (i) to study the effects of the following chemical variables on the quantitative sorption of aqueous carbonate onto goethite: a pH interval between 3 and 9, ionic strengths of 0.1 M and 0.01 M NaCl and NaNO3, and equilibrium aqueous carbonate concentrations between 10 muM and 5 mM; (ii) to gain insight into the bonding mechanisms of aqueous carbonate species to goethite using a triple layer surface complexation modeling approach (TLM), as well as Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR); and (iii) to establish predictive capabilities of its interaction and effects on the adsorption of chromium(VI) and lead(II) onto goethite, using the TLM.; It was found that carbonate adsorption is strongly affected by pH and ionic strength. A thorough modeling, as well as ATR-FTIR spectroscopic, analyses, revealed that complexation to the surface occurs most likely in an inner-sphere monodentate fashion. One single complex on the goethite surface was found to explain most of the quantitative carbonate sorption occurring in the range of conditions investigated. The concept of "inner-sphere complex", in the realm of triple-layer surface complexation modeling, was expanded from the conventional notions. Allocating charge beyond the surface (0-) plane for inner-sphere complexes, allowed a complexation behavior that is more strongly influenced by electrostatic conditions at the solid-water interface.; Projections for situations under natural conditions yielded a potential for coverage by carbonate of more than half the total number of sites on goethite-like particles. Available data on the effects of natural concentration levels of carbonate on the adsorption of two representative trace-metal systems, were adequately predicted by the TLM. Reduction of anionic Cr(VI) sorption to goethite was successfully simulated, through competitive and electrostatic effects. Formation of a ternary Pb(II) carbonato surface complex on goethite, was proposed to explain adsorption behavior in the mixed aqueous Pb(II)-carbonate system. |
