Study On The Removal Of Chromium From Soil By Extraction Agent

Soil is the place where all organisms live,but currently,soil chromium pollution has begun to bring a certain impact on food security,ecological balance and human health.So it is necessary to remove the chromium from soil.Chemical extraction is an effective method for the removal of heavy metals from soil and the selection of effective extracting agents is the key to this method.In this paper,bio-surfactants Sapindus saponin,rhamnolipids and chelating agent GLDA were used as extraction agents,and a artificial polluted soil was served as the test material.The investigation was carried out through the oscillation extraction experiments,studying the effects of the concentration of extraction agents,pH value,contact time,extraction times and the combination types of different extraction agents on the removal of chromium from soil.Finally,results were obtained and listed below by analyzing the variations of chromium in the extracted soil.(1)Both Sapindus saponin and GLDA can effectively remove the chromium from soil.The removal rate of chromium from soil can be enhanced by increasing the concentration of extraction agents,prolonging the contact time and increasing the extraction times.When the Sapindus saponin concentration was 21 g.L-1 and the contact time was 24 h,the removal effect was the best,32.05%.When the GLDA concentration was 0.70%and the contact time was 24 h,the removal rate was 67.59%.However,the optimum conditions for the extraction of the two extraction agents are not the same.The extracted effect of Sapindus saponin is the best under acidic condition;while GLDA under alkaline condition gains the best results,followed by acidic condition,and the worst neutral condition,which is associated with the nature of the two extraction agents themselves.(2)BCR method was applied to study the variation of chromium before and after extraction.Results show that Sapindus saponin could partially remove the acid-extractable fraction and reducible fraction of chromium from soil,and the removal rates were 59.25%and 40.09%respectively.However,the oxidizable fraction and the residual fraction of chromium were almost not removed.GLDA could effectively remove acid-extractable fraction and reducible fraction of chromium from soil and partially remove oxidizable fraction of chromium from soil,with the removal rates of 100%,87.59%and 33.13%,respectively.But it was still not obvious for removal of the residual fraction of chromium.Acid-extractable fraction and reducible fraction of chromium are two active forms in the soil,both of which can be obviously reduced after extracted by the two extraction agents,thus reducing the environmental risk.(3)Two kinds of composite extraction agents were obtained by combining the bio-surfactants and the chelating agent,and their effects on the removal of chromium from soil were studied.The extraction effect of Sapindus saponin was significantly improved after combined with GLDA.When the GLDA concentration was lower than 0.50%,GLDA and Sapindus saponin had a synergistic effect in the removal of chromium,while the antagonistic effect was dominant when the GLDA concentration was higher than 0.50%.The extraction effect of rhamnolipid was also significantly improved after combined with GLDA.Both prolonging contact time and reducing Soil/liquid ratio can improve the removal rate of chromium.Different combination types show different extraction effect,but the extraction effect of combining two kinds of extraction agents is usually better than that of using one alone.For the two kinds of composite extraction agents,their optimum acidic and alkaline condition during extraction are opposite.The Sapindus saponin combined with GLDA compound is suitable for extraction under acidic condition,while the combination of rhamnolipid and GLDA is suitable for extraction under alkaline condition.The optimum pH range was determined by the narrower scope of the one between the two extraction agents.