Sorption and precipitation of iron on kaolinite. III. The solubility of iron(III) hydroxides precipated in the presence of kaolinite

Abstract

Iron(111) hydroxides were precipitated upon kaolinite suspended in acidic solutions of either 0.15M NaCl or 0.15M NaClO4. The progress of the reaction was followed for up to 15 weeks by periodic measurements of pH values and total iron(111) concentrations in solution. Total iron(m) concentrations varied from 1 0 - 3 ~ to 2 x 10-7 and pH values from 2.2 to 4.4. In NaCl media, steady state conditions were reached within 4-5 weeks. Allowing for the various iron(III) ionic species present in the solution phase, the solubility product *K++S2 (= [Fe(OH)+2]/aH+) was found to be 4.9 x 10-3 (S.D. = 0.7 x 10-3 log*K++S2 = - 2.31; log Kso = - 38.9). Although the initial precipitates were amorphous, the presence of �-FeOOH in aged suspensions was detected by X-ray analysis, in agreement with electron microscope observations. In NaClO4 media, the solubility of precipitates decreased rapidly during the first 3 weeks and relatively slowly thereafter. Although equilibrium was obviously not established within 5 weeks and possibly not within 15 weeks, values of solubility products, constant over the whole pH range, were derived from results at both reaction times. For suspensions aged for 5 weeks, *K++S2 was 4.85 x 10-3 (S.D. = 0.75 x 10-3 ; logKso = -38.9), whilst for those at 15 weeks, *K++S2 was 2.45 x 10-3 (S.D. = 0.3 x 10-3; log Kso = -39.2). X-ray and infra-red analyses and electron microscope examination indicated the presence of lepidocrocite in the aged products. There was no evidence that either the type or the solubility of products formed on aging was influenced by the presence of kaolinite, although the clay accelerated the rate of crystal growth.