Study On The Extraction Of Chromium From Contaminated Soil In Serious Pollution Events

Chromium and its chemical compounds are common pollutants in soil, mainly including the waste water, gas and residue containing chromium generated in the process of production, storage, transportation, management, usage and treatment of industries such as chromic salts production, metallurgy, electroplating, leatherworking, painting, printing and dyeing, wood preservation and so on. Chromium mainly exists in soil in forms of Cr (â…¢) and Cr (â…¥). Cr (â…¢) is a necessary microelement of human body. It is non-toxic and exists in soil as indissolvable hydroxide. However, Cr (â…¥) has strong toxicity and solubility, so Cr (â…¥) should be emphasized in remediation of soil. Accompany with the development of chromium industry, serious environmental pollution accidents caused by leakage, fire and explosion of chemicals containing chromium happen frequently, thus it is necessary to take measures to contral and remedy the chromium contaminated fields.This study had taken high concentration chromium contaminated soil as the research object and carried out speciation analysis of chromium and soil washing/extraction experiments on two different kinds of soil which were sampled from Haidian District (HD) and Daxing District (DX) respectively. The following conclusions were reached:(1)Chromium exists as acid-extractable in both of the two kinds of soil while reducible, oxidizable and residual have relatively lower content.(2)Soil pH has impact on chromium species. Acid-extractable chromium may transform to oxidizable and reducible chromium under acid conditions.(3)Organic matter content had obvious impact on both valence state and species. Along with the increase of content of added humic acid, Cr (â…¥) content had obviously decreased and Cr (â…¢) content had increased. Simultaneously acid-extractable chromium had decreased continuously, transforming to oxidizable and reducible chromium, however residual chromium content remained little and hardly changed.(4)The maximum extraction yield of HD soil with DI water, citric acid, CaCl₂, EDTA was 69.71%, 66.31%, 66.38% and 85.77%. The maximum extraction yield of DX soil with DI water, citric acid, CaCl₂, EDTA was 81.10%, 77.08%, 81.05%, 96.87%. The maximum extraction yield (96.87%) of all experiments generated when EDTA was selected as the extractant, in condition that concentration of EDTA was 0.01mol/L, soil/liquid ratio was 1/25, pH of the extractant was 7, and reaction time was 5min. At this time, the total chromium concentration of leachate was 9.08mol/L, and the hexavalent chromium of leachate was 4.17mol/L, which is lower than the limit value in Identification standards for hazardous wastes-Identification for extraction toxicity (GB5085.3-2007), so the extracted soils are not hazardous wastes.(5)Every one of the four extractants has good remove results of acid-extractable chromium. Citric acid has particular good remove results of oxidizable chromium; CaCl₂ has good remove results of both oxidizable and reducible chromium; EDTA can remove reducible chromium successfully.(6)The extraction technology in this study is suitable for sandy soil which has better coefficient of permeability, but it is not so ideal for clay soil which can't meet the?standards for hazardous wastes after extraction.