DYNAMIC INTERACTIONS OF DISSOLUTION, SURFACE ADSORPTION, AND PRECIPITATION IN AN AGING COBALT(II)-CLAY-WATER SYSTEM - EVIDENCE FOR A TWO-STEP MECHANISM

Abstract

We have studied the dynamic interactions of mineral dissolution, cation surface adsorption, and precipitation in aqueous slurries initially containing kaolinite and millimolar quantities of cobalt(II) in 0.1 molar NaNO3 solution. Batch slurries spent an initial 24 hours at pH 4.2, then NaOH was added to increase pH to 7.8, where pH was maintained for several months. Dissolved Co, Al, and Si concentrations were monitored by ICP-MS; solid-phase products were characterized using EXAFS and TEM. At low pH, kaolinite dissolved, releasing Al and Si to solution. Aluminum hydroxide and aluminosilicate phases precipitated rapidly as pH increased. Cobalt uptake from solution began as pH increased and continued throughout the remainder of the experiment. 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 precipitated as a hydrotalcite-like solid that contained aluminum ions derived from kaolinite and aluminum hydroxide dissolution. The cobalt hydrotalcite precipitate had the approximate stoichiometry [Co6Al2(OH)16(An-)2/n], with 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 precipitate. Slow kaolinite dissolution near neutral pH limited the rate of Co uptake by precipitation, thereby accounting for the slow second stage of Co uptake. Continued changes in solution composition indicate that even this relatively simple system did not reach equilibrium under fixed solution conditions after several months. For this set of conditions, cobalt hydrotalcite appears to be the stable Co-containing phase, and its precipitation can reduce dissolved metal ion concentrations below levels achievable by simple adsorption on kaolinite.