Dynamic interactions of dissolution, surface adsorption, and precipitationin an aging cobalt(II)-clay-water system

We have studied the dynamic interactions of mineral dissolution, cation sur face adsorption, and precipitation in aqueous slurries initially containing kaolinite and millimolar quantities of cobalt(II) in 0.1 molar NaNO3 solut ion. Batch slurries spent an initial 24 hours at pH 4.2, then NaOH was adde d to increase pH to 7.8, where pH was maintained for several months. Dissol ved Co, Al, and Si concentrations were monitored by ICP-MS; solid-phase pro ducts were characterized using EXAFS and TEM. At low pH, kaolinite dissolve d, releasing Al and Si to solution. Aluminum hydroxide and aluminosilicate phases precipitated rapidly as pH increased. Cobalt uptake from solution be gan as pH increased and continued throughout the remainder of the experimen t. Rapid surface adsorption and precipitation accounted for early Co uptake ; much slower precipitation caused continued Co uptake. Cobalt adsorbed on kaolinite in the form of hydroxy-bridged polymers or multimers and precipit ated as a hydrotalcite-like solid that contained aluminum ions derived from kaolinite and aluminum hydroxide dissolution. The cobalt hydrotalcite prec ipitate had the approximate stoichiometry [Co6Al2(OH)(16)(A(n-))(2/n)], wit h nitrate or silicate anions occupying A sites. Precipitate particles were several nanometers in diameter and some probably precipitated homogeneously . Some of the Co originally adsorbed was later incorporated into precipitat e. Slow kaolinite dissolution near neutral pH limited the rate of Co uptake by precipitation, thereby accounting for the slow second stage of Co uptak e. Continued changes in solution composition indicate that even this relati vely simple system did not reach equilibrium under fixed solution condition s after several months. For this set of conditions, cobalt hydrotalcite app ears to be the stable Go-containing phase, and its precipitation can reduce dissolved metal ion concentrations below levels achievable by simple adsor ption on kaolinite. Copyright (C) 1999 Elsevier Science Ltd.