Solidification/Stabilization of Fe(II)-Treated Cr(VI)-Contaminated Soil

The potential of first pretreating with Fe(II) followed by cement-based solidification/stabilization (S/S) of Cr(VI)-contaminated soil was investigated. The Cr(VI) is highly toxic due to its oxidizing potential and high solubility in water. Contaminated soils were prepared in the laboratory by spiking clayey soil (20% kaolinite clay) with 7,000 mg/kg Cr(VI) (25,000 mg/kg of K₂CrO₄). During the pretreatment process, up to 10% of FeCl₂ was added to the contaminated soil to convert Cr(VI) to Cr(III). Addition of 10% FeCl₂ lowered the Cr(VI) in the contaminated soil to 118 mg/kg (measured by the alkaline extraction), a 98% reduction in Cr(VI). Addition of FeCl₂ to contaminated soil lowered the pH to below 2. Type I Portland cement was then used to solidify and stabilize (S/S treatment) the pretreated contaminated soils at a cement-to-soil (C/S) ratio of 0.2. Based on the TCLP test, an overall treatment efficiency of 99% was achieved despite the fact that the acidic samples affected the cement setting and strength. The X-ray diffractometry (XRD) analysis showed that the chromate was transformed to chromium oxide (Cr₂O₃) when treated with Fe(II), and was converted to Cr(OH)₃ after the S/S treatment.