Dissolution, Adsorption, and Redox Reaction in Ternary Mixtures of Goethite, Aluminum Oxides, and Hydroquinone

To better understand the oxidative reactivity of iron oxides in the fate of contaminants in acidic environments, we examined the reactivity of goethite in binary mixtures with Al2O3 by carrying out oxidation experiments of hydroquinone (HQ) in the presence of goethite and/or Al2O3 at pH 3. Kinetic results revealed inhibiting effects of 0.2-20 g/L of three different types of Al2O3 on the oxidative reactivity of goethite. Surprisingly, soluble Al ions of 0.18-18 mM had a negligible impact on the reactivity. It turned out that the Fe3+ dissolved from goethite partly contributed to the observed HQ oxidation and the Al2O3 adsorbed the Fe3+ to lead to the slower HQ oxidation. The observed pseudo-first-order rate constants in HQ oxidation had a strong linear correlation with Fe3+ concentration in various goethite and Al2O3 mixtures. Separate experiments confirmed the reactivity of Fe3+ toward HQand the linear correlation between [Fe3+] and HQ oxidation reactivity. Finally, sedimentation experiments showed negligible heteroaggregation between goethite and AluC Al2O3- or nAl(2)O(3) but intensive heteroaggregation between goethite and Alu 65-Al2O3, which explained the observed highest inhibition effect of Alu 65. Overall, oxide mixtures are very complex whose reactivity is determined by many factors such as oxide dissolution.