Cr-removal Efficiency as Affected by the Cr-bonding Fractionation in Soil Treated with Trivalent and Hexavalent Chromium

Under natural environmental conditions, chromium is often present in trivalent Cr(III) or hexavalent Cr(VI) states. Cr(III) and Cr(VI) have different mobility and toxicity. Furthermore, trivalent chromium is present in cationic state, e.g. CrCl3 and Cr(NO3)3, and hexavalent chromium as oxyanions, e.g. K2Cr2O7 and K2CrO4. This study was to compare the adsorption and distribution of Cr(III) and Cr(VI) in contaminated soils and the influence of citric acid and pH on soil washing efficiency. CrCl3 and K2Cr2O7 were the selected as the representative cationic (Cr+3) and anionic (Cr2O7 -2) species for study. Two synthetic Cr-contaminated soils were prepared by immersing the soil column in the Cr solutions and the Cr bonding fractions in soils was studied. Furthermore, citric acid solutions with various pH values were used to wash the contaminated soils. The washing efficiency of various chromium bonding fractions as affected by pH and washing time was studied. The results showed that there was no significant difference in the distribution of Cr bonding fraction between the cationic Cr+3 and anionic Cr2O7 -2 treated soils. For Cr+3 and Cr2O7 -2 contaminated soils, the Fe-Mn oxide bonding fraction was the most dominant, followed by the organic bonding fraction. For the CrCl3 contaminated soils, when the washing time was adequate (48 h), citric acid with lower pH had the highest Cr removal efficiency. For the K2Cr2O7 contaminated soil, the citric acid with pH 6 had the best removal efficiency.