Formation and release of cobalt(II) sorption and precipitation products inaging 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 to tal cobalt concentration (Co-T) and near-neutral pH (7.5-7.8) conditions. D issolved cobalt, aluminum, and silicon concentrations were monitored by ICP MS, and cobalt-containing products were identified by EXAFS spectroscopy. I n each uptake experiment, cobalt sorbed to kaolinite as a mixture of surfac e-adsorbed monomers or polymers and hydrotalcite-like precipitates of the a pproximate composition CoxAl(OH)(2x+2)(A(n-))(1/n), where 2 less than or eq ual to x less than or equal to 4 and A(n-) is nitrate or silicate anion, Pr ecipitate stoichiometry varied with experimental conditions, with the highe st Co:Al ratio in the high Co-T/high pH experiment. Cobalt surface adsorpti on occurred within seconds, whereas precipitation was slower and continued for the duration of the experiments. Consequently, the proportion of precip itate in the sorbed mixture increased with time in all experiments. The mos t rapid precipitation occurred in the high Co-T/high pH experiment, where s olutions were most supersaturated with respect to cobalt hydrotalcite. Prec ipitates incorporated some previously adsorbed cobalt, as well as cobalt fr om solution. Cobalt release from the solid phase was effected by lowering s olution pH to 7.0. Release experiments initiated after shorter sorption tim es returned a larger fraction of cobalt to solution than those initiated af ter longer sorption times, for a fixed duration of release. In other words, sorption product stability increased with sorption time. Specifically, und er the conditions of the release experiments, the hydrotalcite-like precipi tates are more stable than smaller adsorbates, and precipitates that formed over longer time periods are more stable than those that formed rapidly. T he latter result suggests that precipitates ripened or modified their struc ture or composition to become more stable over the course of the several-th ousand-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.