THE INFLUENCE OF A SMECTITE CLAY ON THE HYDROLYSIS OF IRON(III)

The hydrolysis and precipitation of Fe(III) in simple inorganic electrolyte solutions and in aqueous dispersions of the clay mineral montmorillonite have been investigated by UV-absorption spectrophotometry, pH-titration, chemical analysis, electron microscopy, electron-probe microanalysis and microelectrophoresis. Experimental evidence indicates that during the neutralization of acid-Fe(III) solutions in perchlorate, nitrate, chloride and sulphate media, Fe(III) speciation is essentially confined to the simple hexa-aquo ion Fe3+, the first hydrolysis product, FeOH2+, and the 1:1 complex species FeCl2+ and FeSO4+. Thermodynamically irreversible precipitation appears to occur in these systems under well-defined solution conditions, yielding compounds which conform to the stoichiometry Fe(OH)xA(3-x)/n where the value of x depends on the identity of the anion species A(n-). The presence of sulphate at SO42-/Fe(III) mole ratios > 1 increases the pH value required for the onset of precipitation owing to formation of the relatively stable complex FeSO4+. Formation function data have been compared with previously proposed hydrolysis/precipitation mechanisms. In the presence of montmorillonite, hydrolysis of Fe(III) is promoted, prior to precipitation, owing to preferential adsorption of FeOH2+ at the clay surface. Whilst the presence of the clay appears to have no significant influence on the conditions required for hydrous oxide precipitation, experimental evidence shows a marked effect of the clay on the stoichiometry and growth of the initially precipitated Fe(III) phase.