Chromate Adsorption on Iron Oxyhydroxides with Different Crystal Forms in the Presence of Soil Materials

Abstract

Adsorption experiments with chromate ions were carried out employing iron oxyhydroxides (α-, β-, and γ-FeOOHs) in the presence of a weathered granite soil (Masatsuchi) and a clay. The Cr(VI) uptakes on FeOOHs increased with a decrease in pH and attained a plateau value under acidic conditions (pH = 3–5). The Cr(VI) uptakes by α- and γ-FeOOH increased in the presence of the clay, but not significantly in the presence of the Masatsuchi. The Cr(VI) ions were reduced to Cr(III) ions on reaction with organic materials leached from the soil materials to form Cr(III) hydroxides and to precipitate with the FeOOHs. On the other hand, the Cr(VI) uptake by β-FeOOH decreased in the presence of the soil materials; this is presumably due to the competitive interaction with the PO₄^3– ion leached from soil materials. Oxyanions such as SO₄^2– and PO₄^3– and reducing reagents such as SO₃^2– and Fe²⁺ result in the reduction of the Cr(VI) anions to afford Cr(III) cations, thus interfering with the Cr(VI) uptake by FeOOHs.