Kinetics of the formation of goethite in the presence of sulfates and chlorides of monovalent cations

The synthesis of goethite by oxidation of Fe2+ in presence of metallic iron was undertaken in an aqueous medium containing indifferent salts such as Na2SO4, (NH4)(2)SO4, NaCl, and NH4Cl. Temperature and bubbling air rate were maintained, respectively, at 70 degrees C and 1 L/min. The influence of anions and cations on the kinetics of each step of the process has been followed distinctly, the iron dissolution rate has been determined by the variation of the medium acidity, and the precipitation of goethite has been determined by gravimetric measurements. With respect to Cl-, the SO42- anion decreases the rate of the two reactions. NH4+ acts as an inhibitor when it is present at low concentrations and as an accelerator for higher concentrations; the limit corresponding to the change of NH4+ behavior depends on the nature of the counter ion. The reaction product is composed of pure goethite in the presence of sulfate salts, whereas a mixture of goethite and lepidocrocite, respectively, 60-70 and 40-30%, was observed in the presence of chloride salts. (C) 1997 Academic Press.