Formation and Stability of Hydroxy Aluminum‐Iron‐Montmorillonite Complexes: Influence of Ferrous Iron

Abstract

AbstractSynthetic hydroxy Al‐Fe gels were prepared at pH 6.0 by mixing different amounts of Fe(ClO4)2 to a solution of Al(ClO4)3 (5 mol Al kg‐1 clay) to result in an initial Fe(II)/Al molar ratio (R) of 0, 0.1, 0.3, 0.5, 1.0, and ∞, both in the absence and presence of Na‐saturated montmorillonite. The suspensions were aged up to 100 d at 25°C or 30 d at 70°C. Gibbsite was formed in all the samples (R = 0–1.0) after 30 d of aging at 25°C or 1 d at 70°C, except in the sample formed at R = 1.0, where gibbsite was detected only after >2 mo of aging at 25°C or after aging for 6 d at 70°C. At R = ∞, lepidocrocite, goethite, and ferrihydrite were the Fe oxides/oxyhydroxides formed in the absence of montmorillonite. No crystalline Fe oxides were observed in the presence of montmorillonite. After aging, the degree of interlayering in the montmorillonite was observed to be always greater in the samples where Al and Fe were coprecipitated than in the samples formed at R = 0 or ∞. After prolonged aging up to 100 d at 25°C or 30 d at 70°C, the interlayering was stabilized in the samples formed at R = 0.1 to 1.0, with d‐spacings ranging from 1.48 to 1.85 nm or from 1.55 to 1.74 nm, respectively. All the samples aged for 100 d at 25°C or 30 d at 70°C were treated using acid ammonium oxalate. The oxalate‐extractable Fe/Al molar ratios of the acid ammonium oxalate extractable mixed hydroxy Al‐Fe complexes were much higher than the initial solution Fe/Al molar ratios. The stabilized short‐range ordered materials present in the interlayers of montmorillonite in the samples that formed at R values ranging from 0.1 to 1.0 were apparently Al‐substituted Fe oxides.