Formation and release of cobalt(II) sorption and precipitation products in aging kaolinite-water slurries

The uptake and release behavior of cobalt(II) was studied over thousands of hours in CO2-free aqueous suspensions of kaolinite under three pairs of total cobalt concentration (Co-T) and near-neutral pH (7.5-7.8) conditions. Dissolved cobalt, aluminum, and silicon concentrations were monitored by ICPMS, and cobalt-containing products were identified by EXAFS spectroscopy. In each uptake experiment, cobalt sorbed to kaolinite as a mixture of surface-adsorbed monomers or polymers and hydrotalcite-like precipitates of the approximate composition CoxAl(OH)(2x+2)(A(n-))(1/n), where 2 less than or equal to x less than or equal to 4 and A(n-) is nitrate or silicate anion, Precipitate stoichiometry varied with experimental conditions, with the highest Co:Al ratio in the high Co-T/high pH experiment. Cobalt surface adsorption occurred within seconds, whereas precipitation was slower and continued for the duration of the experiments. Consequently, the proportion of precipitate in the sorbed mixture increased with time in all experiments. The most rapid precipitation occurred in the high Co-T/high pH experiment, where solutions were most supersaturated with respect to cobalt hydrotalcite. Precipitates incorporated some previously adsorbed cobalt, as well as cobalt from solution. Cobalt release from the solid phase was effected by lowering solution pH to 7.0. Release experiments initiated after shorter sorption times returned a larger fraction of cobalt to solution than those initiated after longer sorption times, for a fixed duration of release. In other words, sorption product stability increased with sorption time. Specifically, under the conditions of the release experiments, the hydrotalcite-like precipitates are more stable than smaller adsorbates, and precipitates that formed over longer time periods are more stable than those that formed rapidly. The latter result suggests that precipitates ripened or modified their structure or composition to become more stable over the course of the several-thousand-hour sorption experiments. Precipitates that formed over hundreds of hours or longer did not dissolve over thousands of hours at the lower pH. (C) 2000 Academic Press.