Long-Term Effects Of Different Amendments On The Immobilization Of Heavy Metals In Contaminated Soils

Soil heavy metals(HMs)contamination has emerged a global environmental issue,which need to be addressed on priority basis.This study concentrated on the long-term stability(immobilization)of the remediation agents,choosing four kinds of normal chemical materials,including lime(CaO),fly ash,sodium sulphide(Na2S)and sulfur(S).Contaminated soil samples were collected from three different locations,namely Tongguan,Shanxi(Province),Yichang,Hubei(Province)and Jinhua,Zhejiang(Province),hereafter denoted as SX,HB and ZJ,respectively.Long-term lab incubation method was used to test the effect of soil amendments on heavy metals immobilization during one year.The effect of remediation on soil quality was assessed by growing Chinese cabbage(Botanical name)in pots after incubation of soil.Finally according to different soil types and the concentrations of heavy metals in soils,dosages of different kinds of chemical additives were recommended,which provided reference for practical engineering remediation.The main results were as follows:(1)Lime can significantly increase the pH and EC of soils.At the end of incubation,the pH value of SX,HB and ZJ soil increased 1.32-4.45,1.35-4.52 and 1.30-5.66 units,respectively.Application of 10% lime content increased the EC value of SX,HB and ZJ soil by 16 times,43 times and 31 times compared to the initial soil value,respectively.The effects of immobilization on Cu,Zn and Pb in soils were significant and keeping stable or strength over time.Application of 10% lime content got the highest efficiency of HMs stabilization.Lime treatment had no immobilization effect on Cd in HB and ZJ soils.Lime inhibited the growth of Chinese cabbage in these three soils when the content was more than 2.5%.Lime can inhibit the accumulation of HMs in plant shoot and root significantly.The enrichment coefficient and transport coefficient of HMs decreased at the same time.The recommended usage of lime for SX,HB and ZJ soil was 2.5%,2.5% and 1%,respectively.(2)The values of soil p H increased after the treatment of fly ash.The values of HB and ZJ soil EC had significantly increased.Fly ash had impressive impacts in reducing Cu,Zn and Pb contents in soils but the effect on Pb was instability over time.Fly ash had no stabilization effect on Cd in ZJ soil.The yield of Chinese cabbage increased significantly after fly ash treatment in HB and ZJ soil.The treatment of fly ash decreased the contents of Cu,Zn and Pb in roots.The recommended usage of fly ash for SX,HB and ZJ soil was 2.5%,1% and 10%,respectively.(3)Application of 0.5% and 1% Na2 S content led serious alkalization and salinization in HB and ZJ soil: the p H values more than 12 and EC values more than 1000 ?S cm-1 at the end of incubation.Na2 S had obvious immobilization effect on Cu in soils,while the effects were not stable,which showed an increasing trend with the extended time period,especially for Pb,which indicated that the growth of plant weakened the stabilization effect on HMs of sodium sulfide.The values of Zn and Cd in plant shoots increased after Na2 S processing and the ability of stem leaf transporting HMs was enhanced.The recommended usage of Na2 S for SX,HB and ZJ soil was 0.1%,0.5% and 0.5%,respectively.(4)The pH values of soils reduced significantly after adding sulphur.At the end of incubation,the pH of SX and HB soil decreased in the range of 1~2 units,and there was serious acidification in ZJ soil,decreasing in the range of 2~3 units.The final pH of ZJ soil was 2-3.The EC values of soils significantly increased after sulfur treatment.The concentration of DTPA-HMs increased with the extension of incubation time,in addition to the content of Cu in ZJ soil.Agreed with the results of incubation,the contents of DTPA-HMs increased after plant harvest compared with control soil,especially for Zn and Cd.The biomass of plant increased significantly with the ratio of 0.25% and 0.5% sulphur treatment.Under the ratio of 0.25%-2% of sulfur,the biomass of plant in HB soil increased significantly.The biomass of Chinese cabbage biomass was zero in ZJ soil under sulfur treatment.The concentrations of HMs in shoot and root increased and also the enrichment coefficient of HMs for shoot and root,especially for Cd.Sulfur treatment was not suggested for ZJ soil,and the recommended usage of S for SX,and HB soil was 0.5% and 2%,respectively.