| In our country, groundwater, surface water and soil were severely polluted by chromium residue, and ecological environment and human health were also threatened immensely. Remediation chromium residue contaminated site was one of the desiderated case of our country because the high contamination concentration and the easily diffusion. The difficulty of the remediation technology was that the topsoil of the site is sticky, small particle size and severely polluted. Soil washing can thoroughly remove contaminants from contaminated soils with low cost, and it's easily to control the remedial process, so it is suitable for the remediation topsoil. While, chemical stabilization could overcome the yellowing of soil after washing.The influence of soil characteristics (pH and organic matter content) on the different valence states and different fractionations of chromium were studied to provide fundamental basis for the choice of restoration technique. Based on this, according to the pollution property of topsoil, research of a combined ex-situ remediation technology was conducted in three different chromium-contaminated soils, and the feasibility of the technology was validated. Then, the combination technology of"twice counter-current washing and chemical stabilization"was advanced.The main results of research were shown as follows:(1) The main fractionation of chromium in chromium-contaminated soil was exchangeable and acid-soluble fractionation, and the secondly is oxidizable fractionation. As the pH decreased and the organic matter content increased, the concentration of hexavalent chromium decreased in general, and trivalent chromium inversely increased. Simultaneously, the concentration of exchangeable and acid-soluble fractionation declined opposite to oxidizable fractionation and reducible fractionation with the drop of pH and the rise of organic matter content. The findings indicated that the lower pH, especially under acidic condition, and the higher organic carbon might enhance the deoxidization process of hexavalent chromium into trivalent chromium and the transformation process of exchangeable and acid-soluble fractionation chromium into oxidizable and reducible fractionations chromium.(2) The different washing solutions had no significant difference in the removal rate of total Cr, thay could remove more than 75% of hexavalent chromium, but they had different removal efficiency in different fractionations of chromium. The main fraction of Cr removed by deionized water was exchangeable and weak acid extracted fraction, while EDTA-Na2 could extract Cr with exchangeable and weak acid extracted fraction as well as reducible Cr fraction; besides the first two fractions above, citric acid could also extract oxidizable Cr fraction; however, HCl showed removal ability on all fractions. Due to exchangeable and acid-soluble fractionation was the main fractionation of chromium in chromium-contaminated soil, and in view of economy, the best washing solution was water. The optimal stirring time of water washing was 40 minutes and the suitable ratio of liquid to solid was 9:1.(3) Through comparison of the reduction efficiency of FeSO4, Na2SO3, Na2S, glucose, citric acid and humic acid, it was found that translating exchangeable and acid-soluble fractionation chromium into the other fractionations. FeSO4 had the best reduction efficiency, and the optimal dosage was 20 times of theoretical dose, which was about 1% of soil quality.Topsoil of a chromium residue contaminated site in plateau section of China was treated by a combined ex-situ remediation technology"twice washing and chemical stabilization", the result showed that more then 90% of hexavalent chromium was removed, and the treated soils reached the intended rehabilitation goals. On this basis, the combination technology of"two counter-current washing and chemical stabilization"was advanced.
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