Study On The Removal Of Heavy Metal Chromium In Contaminated Soil By Biosurfactant

The contamination of Chromium (Cr) has a huge potential threat to the food safety, ecosystem and human health, which has been a vital environmental problem widely focused. Soil washing technology is an effective and widely used method for soil remediation, and effective washing reagent is the key of this technology. Biosurfactant, environmental friendly, has become a new type of soil washing reagent, to wash the heavy metals (such as Cu, Zn, Pb, Ni, Cd) but rarely involve the Cr in contaminated soil.In this article, the feasibility of removing chromium from a single artificial chromium contaminated soil using two kinds of biosurfactant (the rhamnolipid and saponin) was studied by the oscillation extraction experiments. The effects of reagent concentration, pH value, oscillating time, soil water ratio, extraction times and the combination of different extracting agent on the removal rate of chromium were investigated. Moreover, the variation of Cr fractions and Cr valence state in contaminated soil before and after the treatment of different extraction agents were studied. The conclusions are as follows:(1) Rhamnolipid and saponin can effectively remove the chromium in the contaminated soil. Increasing the biosurfactant concentration and extraction times as well as extending oscillation time were benefit to improve the removal rate of chromium. Alkaline condition was favorable to chromium removal when using rhamnolipid as washing reagent, while in acidic medium saponin had a good chromium removal effect. The effect of reducing soil-water ratio on chromium removal efficiency was limited. When the concentration of rhamnolipid was 10 g.L-1, pH value was 10, soil water ratio was 1:20, extraction time for 24 h, extraction times of 5, the removal rate of chromium reached 96.08%. Under the conditions of saponin 10 g.L-1, pH 5.5, soil-water ratio 1:20, extraction for 24 h, extraction times of 5, the removal rate of chromium was 79.96%.(2) The results, obtained in the experiments of two-step extraction with the combination of a variety of extracting agents, showed that EDTA and rhamnolipid mixed system used for chromium removing in contaminated soil was feasible. The accumulation removal rates of chromium removed from contaminated soil by two-step extraction using EDTA-Rhamnolipid and Rhamnolipid-EDTA were 85.70% and 83.85%, respectively. The extraction order of citric acid and saponin combination system would affect the accumulation removal rate of chromium:the total chromium removal rate of Citric acid-Saponin combination was 80.70%, while Saponin-Citric acid combination with an accumulation chromium removal rate of 65.44%.(3) Rhamnolipid could effectively remove acid-extractable fraction and reducible fraction of chromium, also could remove a part of oxidizable fraction and a small amount of residual fraction, the removal rate were 94.47%,84.17%, 25.82% and 7.64%, respectively. Saponin mainly removed acid-extractable fraction and reducible fraction of chromium in the soil, the data of removal were 43.98% and 51.84%, respectively. The removal efficiency of oxidizable fraction and residual fraction were poorer. The removal rate of hexavalent chromium when using rhamnolipid was 66.59%, while the proportion of hexavalent chromium in total chromium for the treated soil was reduced by 9.54%. The removal rate of hexavalent chromium when using saponin was 58.02%, while the proportion of hexavalent chromium in total chromium for the treated soil was reduced by 18.66%. Rhamnolipid and saponin could reduce the content and proportion of hexavalent chromium, which alleviated the hurt of hexavalent chromium to environment.