Washing Technology Fot The Remediation Of Chromium Residue-Contaminated Soil

Chromium residue easily resulted in soil chromium contamination, so, quick and effective remediation technology is urgently required for remediation of such contaminated soil. Washing technology had a potential in soil remediation.In this study, we examined the effects of different washing reagents on the removal of Cr from Cr contaminated soils which was caused by surrounding chromium residue discarded from a chemical plant. The effects of reagent concentration, reaction time, washing time, soil and liquid ratio, and washing numbers as well as the complex reagents on the efficiency were investigated. By soil column washing experiment, we studied the removal efficiency of Cr treated by citric acid, and the variation of Cr fractionations before and after treated. Meanwhile, treatment of the washing liquid by photocatalytic reduction was studied, as affected by different factors.The results showed that, compared with other washing reagents, citric acid gave the highest removal as82.8%under optimal conditions, i.e., soil and liquid ratio was1:20, reaction time was24hour, and the soils were washed twice by citric acid solution. The removal rate of Cr, treated by the combination of SDBS,EDTA and citric acid in order, did not change; while the removal rate of Cr after simply treated by SDBS, EDTA and citric acid decreased. When soil was washed by cirtic acid at higher concentration, both of the remained Cr(VI) and Cr(III) were low.During the soil column washing, both the accumulated removal amount of total Cr and Cr(VI) increased with the washing amount. Soil washing can effectively remove the major pollutant Cr(VI). When the washing amount reached five pore volumes, the highest accumulated removal rate of Cr(VI) was obtained. The highest removal rate was85%. Meanwhile, soil pH after treated with citric acid decreased from11.9to8.8. The amount of Cr(VI) increased with the column depth because of Cr(VI) moved from the top to the bottom during the washing process. After washed by citric acid, the percentage of acid extractable and residual fractionations decreased, and the percentage of reducible and oxidisable fractionations increased.Photocatalytic reduction could effectively remove both Cr(VI) and total Cr from wastewater. The removal rate increased with the amount of TiO2and the intensity of irradiation, and decreased with solution pH. Citric acid and Fe(III) improved the photocatalytic reduction of Cr(VI). This technology also reduced Cr(VI) in the real washing solution. In the absent of citric acid, the removal rate of Cr(VI) was very low; however, the removal rate of Cr(VI) increased with citric acid. When the concentration of citric acid was0.5mol·L-1, solution Cr(VI) was totally reducted after an hour.