CARBONATE ADSORPTION ON GOETHITE UNDER CLOSED AND OPEN CO2 CONDITIONS

Abstract

Measurements of carbonate sorption to goethite in equilibrium with aqueous carbonate concentrations ranging from 10 μM to 5 mM were performed as a function of pH and ionic strength. Adsorbed carbonate was determined indirectly from knowledge of total carbonate added and headspace PCO2. Adsorption was investigated under both closed (fixed total CO2) and open (fixed PCO2) conditions. In closed systems maximum adsorption occurred between pH 6 and 7 (up to 0.5 μmol/m2, for 133 μM [CO2]tot). Open systems around atmospheric PCO2 (240-410 matm) and at 5.5 μatm CO2 showed continuously increasing sorption with pH, where maximum surface coverage was measured around 1.2-1.4 μmol/m2, 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 adsorption. Carbonate uptake enhanced proton sorption, by direct adsorption or ligand exchange of surface >OH. An average of 1.5 protons sorbed per carbonate moiety adsorbed was determined and remained approximately constant with pH for all systems studied. The conditions investigated are representative of natural aqueous settings and suggest the potential for coverage of nearly half the total number of surface sites, or more, in soils and sediments. This has importance in the potential repercussions for enhancement of trace element mobility and dispersion through subsurface environments.