Carbonate adsorption on goethite under closed and open CO2 conditions

Measurements of carbonate sorption to goethite in equilibrium with aqueous carbonate concentrations ranging from 10 muM to 5 mM were performed as a fu nction of pH and ionic strength. Adsorbed carbonate was determined indirect ly from knowledge of total carbonate added and headspace P-CO2. Adsorption was investigated under both closed (fixed total CO2) and open (fixed P-CO2) conditions. In closed systems maximum adsorption occurred between pH 6 and 7 (up to 0.5 mu mol/m(2), for 133 muM [CO2](tot)). Open systems around atm ospheric P-CO2 (240-410 matm) and at 5.5 mu atm CO2 showed continuously inc reasing sorption with pH, where maximum surface coverage was measured aroun d 1.2-1.4 mu mol/m(2), corresponding to equilibrium [CO3(aq)] of 1.5 mM and 5.5 mM, respectively. Ionic strength affected sorption considerably. Below pH 7 increasing ionic strength decreased adsorption, but above it no ionic strength effect was observed, and possibly an enhancement of carbonate ads orption. Carbonate uptake enhanced proton sorption, by direct adsorption or ligand exchange of surface >OH. An average of 1.5 protons sorbed per carbo nate moiety adsorbed was determined and remained approximately constant wit h pH for all systems studied. The conditions investigated are representativ e of natural aqueous settings and suggest the potential for coverage of nea rly half the total number of surface sites, or more, in soils and sediments . This has importance in the potential repercussions for enhancement of tra ce element mobility and dispersion through subsurface environments. Copyrig ht (C) 2000 Elsevier Science Ltd.