KINETICS OF FERRIC OXYHYDROXIDE PRECIPITATION

Precipitation of ferric oxyhydroxide is defined as loss of ferric iron from solution and includes any or all of the following processes: formation of small precursor polymers, growth of these polymers to colloidal particles and subsequent condensation to a settleable precipitate. The precipitation kinetics is reported for simple ferric chloride solutions. The kinetics are described by an initial slow nucleation rate followed by a more rapid crystallite growth rate. Both rates are first order in the calculated concentration of aqueous Fe(OH)30 and are consistent with a kinetic model in which precipitation of the ferric solid from Fe(OH)30 is the rate-controlling step. Crystallite growth rates are retarded by the presence of Fe3+ (and to a lesser extent, FeOH2+). This effect is due to the adsorption of Fe3+ onto active surface sites or by the incorporation of Fe3+ into the structure of the precipitating solid resulting in a solid solution. This mechanism is proposed as a possible cause of the mixed stoichiometry and poorly crystalline form of fresh ferric oxyhydroxide precipitates observed in nature.