Influence of mineral surfaces on Chromium(VI) reduction by Iron(II)

Chromium(VI) is a priority pollutant of some soils and natural waters in in dustrial areas. Iron(ll), an important natural reductant of Cr(VI), is an o ption in remediation of contaminated sites, transforming Cr(VI) to essentia lly nontoxic Cr(III). After kinetics and pathways of this redox reaction ha d been reported to depend strongly on pH and organic ligands, this study in vestigated the influence of mineral surfaces. Kinetic measurements with UV- vis in mineral and sail suspensions at pH 5 showed that all minerals tested , except Al2O3, accelerated the Cr(VI) reduction by Fe(II), in the order of alpha-FeOOH approximate to gamma-FeOOH much greater than montmorillonite > kaolinite approximate to SiO2 much greater than Al2O3. Similar kinetics we re observed with soil from the E and Bw horizons of a Podzol. The reactions appear to be driven by the high reactivity of adsorbed Fe(II). Whereas ads orbed Cr(VI) was reducible by Fe(II), the sparingly soluble BaCrO4 was larg ely protected from reduction. This is of environmental relevance since in m any polluted soils, Cr(VI) is partly present as Pa, Ca, Fe, or Pb salts. Ki netic data and reaction pathways are important in the optimization of Fe(II )/mineral mixtures as reductants of Cr(VI) in technical systems, in the eva luation of insitu remediation of Cr(VI)-contaminated waters and sails by Fe (II), and in qualitative predictions and modeling of Cr(VI) in natural syst ems.