Remediation Of Cd From Contaminated Lou Soil By Washing With Environmental Friedgly Eluants

Facing increasingly serious heavy metal-contaminated soil all over the world,a variety of treatment measures have been used for remediation of heavy metal from contaminated soil in the different regions and good effect has been received.Soil washing is an efficient and complete technology to remove heavy metal from contaminated soil,and it has been widely used for these advantages.Selecting effective eluant is the key of all the factors affecting the soil washing efficiency.As the environmental friendly biosurfactant has become the focus of environmental science in recent years,and then saponin,rhamnolipid and some small molecule organic acid such as malic acid,citric acid and chelating agent EDTA were choosen to representative in the test to investigate the washing remediation effect of Cd in contaminated Lou soil.In this paper,batch shaking test and soil column washing methods were performed to explore the optimal test parameters(washing time,pH value,concentration of different elutant,washing times,concentration of background electrolyte and different concentration ratio of different washing elutant),the washing effect of Cd were researched and morphological variation of Cd was compared before and after washing using diffenent eluant(single and combined),then helped providing a scientific basis for pollution control of heavy metal of calcareous contaminated Lou soil.The results were shown below:(1)The kinetics of washing Cd from contaminated Lou soil by different single eluant showed the first fast and then slow two stages,the maximum washing value of Cd using saponin,rhamnolipid,citric acid,malic acid reached washing equilibrium in a short time(30min)respectively(except EDTA),in order to improve washing effect of Cd,240 min was selected as optimal washing time in the test.(2)The optimal conditions of washing Cd from contaminated Lou soil with no background electrolyte after single washing with rhamnolipid and saponin respectively was as follow: 240 min,pH 7.00 and 6.00,concentration of two biosurfactant was 0.2 mol·L-1and0.06 mol·L-1,the max washing percentage of Cd was 14.64% and 16.79%;The same optimal conditions with citric acid,malic acid,EDTA respectively was as in turn: 240 min,pH 5.00,5.00 and 6.00,concentration of three eluant was 0.15 mol·L-1,0.5 mol·L-1and 0.015 mol·L-1;the max washing percentage of Cd was 39.45%,45.18% and 38.97%;adding different concentration of Na+ in different eluant respectively,the washing percentage of Cd increased significantly from contaminated Lou soil by promoting transform of Cd from contaminated Lou soil to liquid phase of soil.(3)The cumulative washing results of Cd after repeated 3 times with different eluant respectively indicated the washing percentage of Cd increased significantly by increasing washing times from contaminated Lou soil under optimal condition,the cumulative washing percentage of Cd were 22.83%、25.50%、41.39%、60.61%、54.72% using rhamnolipid,saponin,citric acid,malic acid,EDTA respectively.(4)The fractionation results of Cd by Sposito morphological classification method from contaminated Lou soil before and after repeated washing showed as follow: the contents of adsorbed,carbonate,sulfide residual,residual fraction of Cd was effectively removed with citric acid,malic acid and EDTA washing respectively,the contents of exchangeable and organic fraction of Cd were kept a certain value by redistribution of irons of Cd;the contents of each fraction of Cd was removed effectively with saponin and rhamnolipid at some extent.(5)A series of shaking washing test has different result with different combined eluant.It has antagonistic effect for removal of Cd from contaminated Lou soil with rhamnolipid(0.06mol·L-1),saponin(0.2 mol·L-1)combined different concentration of EDTA;the effect was due to complexation was stopped effectively between EDTA and heavy metals by molecule peptization of biosurfactant micelle to EDTA molecule.It has antagonistic effect for removal of Cd with lower concentration of citric acid(0-0.08mol·L-1)combined rhamnolipid(0.06mol·L-1);and then the stimulative effect for removal of Cd when rhamnolipid(0.06 mol·L-1)with higher concentration of citric acid(above 0.08 mol·L-1)was showed.The stronger antagonistic effect of Cd,the higher ratio of rhamnolipid in the combined eluant(rhamnolipid and malic acid).The washing percentage of Cd with combined eluant was much less than rhamnolipid and malic acid single as rhamnolipid showed antagonistic effect for removal of Cd when it combined malic acid.It then has stimulative effect when lower concentration of malic acid with rhamnolipid,and the washing percentage of Cd with combined eluant was much more than rhamnolipid single.It has stimulative effect for removal of Cd from contaminated Lou soil when lower concentration of citric acid,malic acid with optimal concentration of saponin(0.2 mol·L-1).The stronger stimulative effect for removal of Cd when the mole ratio of saponin with citric acid and malic acid respectively in combined eluant,and the highest percentage of Cd was 35.33%,30.79%;on the contrary it showed antagonistic effect.(6)The stimulative effect for removal of Cd when saponin,rhamnolipid combined some organic acid indicated acidic complexation was playing an important role in removal heavy metal from contaminated Lou soil.The antagonistic effect for removal of Cd when saponin,rhamnolipid combined some organic acid indicated may be that the biosurfactant micelle bound acid organic molecules,and the combination between the same stracture of hydrophilic group in biosurfactant and soil particle could not making dissociation of Cd from soil exist as fraction of canbonate and residual.Therefore the combination ratio of biosurfactant and organic acid should pay attention in the application of biosurfactant remediation of heavy metal-organic contaminated soil.(7)The batching repeatedly washing content of Cd showed obvious difference between the lower concentration of different eluant during soil column test.The washing content of Cd showed an increasing trend with rhamnolipid,the washing content trend of Cd had little difference between lower concentration of saponin and malic acid,the highest washing content of Cd was showed in the first column washing with citric acid and EDTA,and then decreased slowly.the test results showed the mass percentage of direct bioavailbility of Cd in contaminated soil increased with each elutant by different ways owing to activation state of Cd was promoted effectively,and therefore rhamnolipid,saponin,citric acid and malic acid could be used in phytoremediation of heavy metal contaminated soil as it could promote plant purification of heavy metal contaminated soil effectively.