Remediation of a Cr(VI) contaminated soil: Soil washing followed by Cr(VI) reduction using a sorbent prepared from rice husk

A Cr (VI) contaminated soil with concentrations up to 16,000 ppm was washed with 0.1M citric, 0.1M acetic, 0.1M HCl, 0.1M sulphuric acid, 0.1M NaOH and hot distilled water. Soil characterisation including pH, metal content and cation exchange capacity was carried out. Since the soil was highly basic (pH 11.6), hot water was found to be most effective for soil washing. Rice husk was used to prepare a carbonaceous sorbent using sulphuric acid treatment. The removal kinetics of Cr (VI) from aqueous solutions was investigated at different temperature, pH, metal ion concentration and sorbent status (wet and dry). The rate of Cr (VI) reduction at low pH values followed a first order rate expression approaching equilibrium within 50 hours for wet sorbent and 70 hours for the one which was previously dried. The soil leachate was acidified to pH ≤ 2 and reduced to Cr (III) using the carbonaceous sorbent. The carbonaceous sorbent seems cheap and efficient for Cr (VI) removal.