Effect Of Modified Biochar On Immobilization Remediation Of Weakly Alkaline Cd-contaminated Soil

National Soil Pollution Status Survey Bulletin showed that the overall situation of soil environment in China is relatively severe,and the situation of Cd pollution is the most serious,exceeding the standard point of 7.0%.As a highly aromatic structural material,biochar has a large specific surface area and a large number of phenolic hydroxyl,carboxyl and carbonyl oxygen-containing functional groups,which can effectively adsorb heavy metals in the soil,and reduce its mobility and bioavailability.In the present study,field experiments were conducted to investigate the effects of Ca-,Mn-and Fe-based modified biochar on immobilization remediation of weakly alkaline Cd-contaminated soil,and soil physiochemical properties,aggregate structure,soil environment quality and Cd accumulation in maize.The results above would provide theoretical and technological support for remediation of large-scale weakly alkaline-Cd contaminated soils using modified biochar.The results are as follows:（1）The values of p H,available cation exchange capacity and organic matter were increased after applying Ca-based modified biochar to soil.Compared with the control treatments,the content of DTPA-Cd reduced by 12%to 30.2%,and the acid soluble and reducible states of Cd was converted into residual form in contaminated soil.The application of Ca-based modified biochar reduced the risk of Cd enrichment in maize,and the content of Cd in root,stem,leaf and grain of maize was significantly inhibited.The concentration of Cd in the three maize grain decreased by52.65-72.56%（Zhengdan 958）,37.54%-50.8%（Liyu 86）and 23.6%-51.2%（Sanbei 218）,respectively,in contrast with CK.The addition of Ca-based modified biochar could improve the soil environmental quality,and the activity of catalase,urease and alkaline phosphatase increased gradually with increasing modified biochar.The percentage of 5-8 mm and 2-5 mm particle aggregates was increased after applying Ca-based modified biochar,while the ratio of particle aggregates（≤0.25 mm）was reduced,the amount of geometric mean diameter（GMD）and mean weight diameter（MWD）of soil aggregates increased by 9.57%-33.48%and 4.74%-33.39%,respectively,in contrast with unamended soil.There was an extremely（p<0.01）negatively correlation between the content of Cd in the maize grain（Zhengdan 958）and the extracted content of Cd in soil.Compared with CK,the number of soil bacterial OTUs enhanced,and the Shannon,Chao1,and ACE indexes increased significantly（p<0.05）,indicating that the addition of modified biochar increased the bacterial community richness and diversity.The populations of microorganisms in the soil at phylum level under different modified biochar treatments were similar,and the dominant taxa at phyla level were Proteobacteria,Acidobacteria,Bacteroidetes,Gemmatimonadete and Actinobacteria.At phylum level,the application of Ca-based modified biochar reduced the relative abundance of the Proteobacteria,Bacteroidetes and Gemmatimonadete,while the relative abundance of Actinobacteria increased by from 17.73%to 39.04%.（2）The content of available cation exchange capacity was increased gradually under the treatments of Ca-based modified biochar,and it reached the maximum value under FBC???treatment,which was significantly increased by 17.17%compared with the control treatment（p<0.05）.And the content of soil organic carbon increased by 6.59%to 20.36%after applying Fe-based modified biochar.Compared with the control treatments,the content of DTPA-Cd reduced by 37.74%to41.65%,and the acid soluble and reducible states of Cd was converted into oxidizable and residual form.The addition of Fe-based modified biochar reduced the content of Cd in different parts of maize,among which the concentration of Cd in the three maize grain decreased by 41.31%-76.64%（Zhengdan 958）,38.19%-70.95%（Liyu 86）and 52.30%-59.95%（Sanbei 218）,respectively,in contrast with CK.Fe-based modified biochar optimized the composition structure and stability of soil aggregates,promote the continuous transformation from micro-aggregates to large particle aggregates.Compared with CK,the values of MWD,GMD and R_0.25 of soil aggregates were increased by 10.2%-32.1%,3.7%-32.7%and 0.45%-6.71%,respectively.The activity of catalase,urease and alkaline phosphatase in soil increased gradually with the addition of Fe-based modified biochar.There was an extremely（p<0.01）negatively correlation between the content of Cd in maize grain and the extracted content of Cd in soil.Compared with the control treatments,the number of soil bacterial OTUs enhanced,and the Shannon,Chao1,and ACE indexes increased,indicating the bacterial community richness and diversity increased after applying of Ca-based modified biochar.In general,the populations of microorganisms in the soil at phylum level under different modified biochar treatments were similar,the application of Fe-based modified biochar reduced the relative abundance of the Gemmatimonadete,while it increased the relative abundance of Actinobacteria and Acidobacteria.（3）The content of soil organic carbon increased gradually with adding Mn-based modified biochar,increasing by 3.2%-32%compared with the control group,while the content of DTPA-Cd reduced by 22.34%to 28.42%.The conversion of Cd was from unstable form to stable form,the content of acid soluble and reducible states of Cd decreased,and while it was increased in residual form.The concentration of Cd in the three maize grain decreased by 8.03%-76.97%（Zhengdan 958）,26.40%-66.96%（Liyu 86）,42.22%-83.46%（Sanbei 218）,respectively,in contrast with CK.Mn-based modified biochar improved the composition structure and stability of soil aggregates,and it increased the radio of large aggregates（5-8 mm and 2-5 mm）,while micro-aggregates production was inhibited（≤0.25 mm）.Compared with the control treatment,MWD,GMD and R_0.25 of soil aggregates increased by 15.1%-20.3%,8.1%-22.4%and 0.43-7.6%,respectively.Cd was preferentially enriched on small aggregates,and the mass loading of Cd in soil aggregates increased gradually with decreasing particle size.The distribution factor of Cd showed significant enrichment in the 0.5-1 mm and 0.25-0.5 mm grain sizes,while in the particle size of 5～8 mm and 2～5 mm showed a significant loss.The addition of modified biochar reduced the content of DTPA-Cd in soil aggregates,which reduced by 7.63%-15.14%,15.62%-24.37%,3.66%-13.82%and 11.62%-13.73%,respectively,in the aggregate particle sizes of 5-8,2-5,0.5-1 and 0.25-0.5 mm（p<0.05）,when compared with the control.The number of soil bacterial OTUs and the Shannon indexes increased under the addition of modified biochar,the relative abundance of the Proteobacteria and Gemmatimonadete decreased after applying Fe-based modified biochar to soil,while it was increased for Acidobacteria.