Study On The Effects Of Exogenous Additives On The Bioremediation Of Cd Contaminated Soil By Earthworm

Soil contamination with potentially toxic elements has become a global issue concerning the sustainable development of human society.Cadmium(Cd),as one of the main heavy metal pollution elements in soil,has become the focus of remediation of soil heavy metal pollution due to its high toxicity,strong migration and great harm.Bioremediation is considered as an economic and environmental protection method of soil remediation,which mainly includes phytoremediation,animal remediation and microbial remediation.At present,the research focus is mainly on phytoremediation technology.However,the efficiency of phytoremediation is limited by the small biomass and slow growth of plant.As the "natural engineer" of the soil ecosystem,earthworms can improve the soil environment effectively.Meanwhile,it has been proved that earthworms have a certain enrichment effect on Cd in the soil.Therefore,the animal bioremediation based on earthworm for heavy metal contaminated soil is a technology worthy of further research.In this study,earthworms(Eisenia fetida)are combined with four additives including Ethylene Diamine Tetraacetic Acid(EDTA),bean dregs,Bacillus megatherium and biochar to remediate the low-level Cd contaminated soil(2.5mg·kg-1).The effects of exogenous additives on earthworm tolerance and soil properties in Cd contaminated soil were studied.At the same time,the remediation effect of exogenous additives combined with earthworm on Cd contaminated soil and the bioremediation mechanism of earthworm were studied.In addition,pot experiment was used to verify the feasibility of earthworm combined with exogenous additives in remediation of Cd contaminated soil.The major research results are as follows:(1)The remediation of Cd contaminated soil using earthworm alone or combine with EDTA and bean dregs was studied.The results showed that Cd can be removed by earthworms from soils,and the remediation process was accelerated by both EDTA and bean dregs.With time goes by,Cd gradually accumulated in earthworm from soil.The content of Cd in earthworms was 8.11 mg·kg-1,12.8 mg·kg-1 and 9.26 mg·kg-1,respectively,and the content of Cd in soil is reduced by 28.95%,36.53% and 30.8%,respectively,in earthworms treated alone,combined with EDTA and combined with soybean residue.Additionally,the concentrations of water/DTPA extractable Cd were also reduced,indicating the low Cd mobility after amendment.In general,EDTA and Bean dregs improved the remediation efficiency of earthworm for Cd contaminated soil in the following order: earthworm + EDTA>earthworm+ bean dregs>earthworms single.Finally,the environmental quality of soil was improved after remediation by earthworm.The soil p H,EC,NO3--N,available P,and K contents were increased,soil SOM,DOC,and NH4+-N contents are decreased,soil enzyme activities(soil enzyme activities(phosphatase activity,?-glucosidase activity,and urease activity)increased,and soil microbial community was enhanced.Therefore,we concluded that earthworms,alone or combined with EDTA and bean dregs,were feasible for the remediation of Cd-contaminated soil.(2)The remediation of Cd contaminated soil using earthworm alone or combine with Bacillus megatherium and biochar was studied.The results showed that earthworms could uptake Cd in soil,and biochar and Bacillus megatherium could effectively enhance the remediation efficiency of earthworms on Cd contaminated soil.Compared with the treatment of earthworms single,the removal efficiency of Cd contamination in soil by adding biochar and Bacillus megatherium increased by 12.79% and 17.05%,respectively.Moreover,the content of the available Cd in the soil with earthworm was significantly reduced.With the increase of remediation time,the accumulation of Cd in earthworms gradually increased,and the accumulation of Cd in earthworms was followed by earthworms + biochar>earthworm +Bacillus megatherium>earthworm single.The bioaccumulation factor(BCF)in the treatment with Bacillus megatherium and biochar were greater than 1,reaching 9.94 and 8.45,respectively.After remediation by earthworm,soil microbial community was enhanced,soil environmental quality was improved,and soil enzyme activities(phosphatase activity,?-glucosidase activity,and urease activity)were increased.(3)The Brassica chinensis L.was planted in the remediated soil to test the effects of earthworm-based remediation in Cd contaminated soil.The results showed that the Chinese cabbage had a certain tolerance to low-level Cd contamination.After the improvement of soil environment by earthworm and exogenous additives,the biomass of shoots and roots of cabbage increased.After the remediation by earthworm,the proportion of DTPA-Cd content in the soil to total Cd increased by 8.7-46.5%,but the total amount of DTPA-Cd in the soil decreased.The dry weight of cabbage increased by 26.0-78.3% and 20.2-110.6%,respectively,and the total Cd content decreased by 3.3-20.8% after the remediated by earthworm combined with exogenous additives.For the moderate and mild Cd contaminated soil,the remediation method using Eisenia fetida as the main body and EDTA as the activator is a potential bioremediation technology of heavy metal pollution in soil.This study provides a new idea for the remediation of moderate and mild Cd contaminated soil.