Influence of Mineral Surfaces on Chromium(VI) Reduction by Iron(II)

Abstract

Chromium(VI) is a priority pollutant of some soils and natural waters in industrial areas. Iron(II), an important natural reductant of Cr(VI), is an option in remediation of contaminated sites, transforming Cr(VI) to essentially nontoxic Cr(III). After kinetics and pathways of this redox reaction had been reported to depend strongly on pH and organic ligands, this study investigated the influence of mineral surfaces. Kinetic measurements with UV−vis in mineral and soil suspensions at pH 5 showed that all minerals tested, except Al 2 O 3, accelerated the Cr(VI) reduction by Fe(II), in the order of α-FeOOH ≈ γ-FeOOH ≫ montmorillonite > kaolinite ≈ SiO 2 ≫ Al 2 O 3 . Similar kinetics were observed with soil from the E and B Fe horizons of a Podzol. The reactions appear to be driven by the high reactivity of adsorbed Fe(II). Whereas adsorbed Cr(VI) was reducible by Fe(II), the sparingly soluble BaCrO 4 was largely protected from reduction. This is of environmental relevance since in many polluted soils, Cr(VI) is partly present as Ba, Ca, Fe, or Pb salts. Kinetic data and reaction pathways are important in the optimization of Fe(II)/mineral mixtures as reductants of Cr(VI) in technical systems, in the evaluation of in-situ remediation of Cr(VI)-contaminated waters and soils by Fe(II), and in qualitative predictions and modeling of Cr(VI) in natural systems.