Study On Earthworm-mediated Nano-zerovalent Iron（nZVI） Remediation Of Cadmium-contaminated Soil And Its Mechanism

Cadmium is one of the most polluted heavy metal elements in our country,and it has a significant negative impact on soil quality,crop yields and human health.In order to solve the problem of heavy metal pollution in the soil,the combined effect of n ZVI and other chemical materials has been extensively studied.However,studies on the effects of n ZVI on soil organisms and soil properties,as well as animal-mediated restoration of heavy metal pollution by n ZVI in soils and soil ecological environmental response during the restoration process are extremely rare.In this study,we took Eisenia foetida（5pieces/barrel）and n ZVI（mass ratio is 0,0.05%,0.25%and 0.5%）as the research objects.Through indoor simulation experiments,we explored the toxic effects of n ZVI on earthworms,the effect of the combined effects of earthworms and n ZVI on soil properties and the remediation of cadmium-contaminated soil by n ZVI mediated by earthworms.It is expected to provide a theoretical basis for the composite restoration of heavy metal pollution in farmland by n ZVI and soil animals.The results of the study are as follows:1.The toxicity test of n ZVI on earthworms has shown that:in the initial period of exposure（15d）,n ZVI had a certain negative effect on earthworms.However,with the passage of exposure time,the relevant indexes of earthworms in different concentrations of n ZVI system were not significantly different after 45 days of exposure.Compared with exposure for 15 days,the survival rate of earthworms and MDA content in the system of5-0.5%（5 represents the number of earthworms,0.5%represents the mass ratio of n ZVI to the total amount of soil）exposure for 45 days decreased by 29.63%and 14.29%.However,the reduction in earthworm biomass and CAT activity increased by 1.2 times and 2.62 times,respectively.However,the reduction in earthworm biomass and CAT activity increased by 1.2 times and 2.62 times,respectively.2.The study on the effect of the combined action of earthworms and nano-zero-valent iron（n ZVI）on soil properties has shown:（1）The combined action of earthworms and n ZVI could significantly improve the stability of the soil aggregate structure（P<0.05）,increase the proportion of large aggregates（d>250μm）and the average weight diameter of the aggregates in soil.Compared with the blank treatment（0-0 system）,the proportion of large aggregates（d>250μm）in the soil and the average weight diameter of aggregates in the 5-0.5%system increased by 15.69%and 12.59%,respectively.（2）The combined effect of n ZVI and earthworms significantly affected the total nitrogen（P=0.005）and available phosphorus content（P=0.001）in the soil,and there was no significant effect on the soil organic matter content,alkaline hydrolysis content and total phosphorus（P>0.05）.The content of available phosphorus in the soil of the 5-0.5%system increased by 21.2%compared with that of the 0-0 system.The content of available phosphorus in the soil of the 5-0.5%system increased by 15.14%compared to the 0-0.5%system.（3）0.5%n ZVI had no significant effect on the relative abundance and diversity index of different phyla or different genus microorganisms at the phylum level or genus level in the soil（P>0.05）.But the combined action of earthworms activity and n ZVI earthworm increased the relative abundance of indicator species in the soil.3.Experiments on the remediation of cadmium-contaminated soil by n ZVI mediated by earthworms has shown that:（1）The combined action of earthworms and n ZVI significantly reduced the toxic effect of Cd²⁺on earthworms.Treatment for 45 d compared to 15 d,the SOD activity and CAT activity in earthworms in 5-0.5%system were reduced by 19.38%and 38.45%,respectively,and the MDA content increased slightly,but it was significantly lower than other repair systems treated for 45 d.（2）The combined action of earthworms and n ZVI significantly reduced the available cadmium content in the soil（P<0.001）.The available cadmium content in the soil of the 5-0.25%system is the lowest(7.40 mg kg^-1),and the restoration efficiency was as high as 75.33%,which was higher than a single earthworm or n ZVI restoration system.Compared with the 0-0.25%repair system and 5-0.5%repair system,5-0.25%repair efficiency increased by 21.3%and9.62%,respectively.Earthworms and n ZVI not only significantly reduced the effectiveness of Cadmium,but also inhibited the loss of organic matter,total phosphorus,total nitrogen,available phosphorus and alkali hydrolyzable nitrogen in the soil,and improve the stability of aggregate structure and soil p H.（3）The combined effect of earthworms and n ZVI significantly increased the relative abundance and diversity of microorganisms and the relative abundance of indicator species during the process of cadmium-polluted soil.This study found that n ZVI had no significant toxic effects on earthworms and microbial systems in the soil,and the activities of earthworms in the soil had a significant effect on the soil properties.At the same time,the combined action of earthworms and n ZVI had a good remediation effect on heavy metal pollution in the soil and significantly slowed down the negative effects of heavy metal ions on soil organisms,soil structure and soil properties.Therefore,from the perspective of material environmental safety and remediation effect,the combined action of earthworms and n ZVI has the potential to be used to remediate heavy metal（colloid）contaminated soil.This study can improve the scientific theoretical basis.