Study On In Situ Remediation Of Cadmium Contaminated Paddy Soil By Magnetic Nanophosphate Functional Material Fe₃O₄@Ca（H₂PO₄）₂

In this study,magnetic nano phosphate functionalized materials（MFC）were prepared by coprecipitation method with magnetic nano Fe₃O₄and Ca（H₂PO₄）₂as raw materials.Taking a heavy metal cadmium contaminated paddy field in Bishan District of Chongqing City as the research object,the in-situ remediation of heavy metal cadmium contaminated paddy field was carried out by using the technology of‘farmland soil flooding–rotary tillage–adding MFC remediation material–stirring evenly–loading heavy metal cadmium in soil by MFC–magnetic separation recovery remediation material’,remediation effect of magnetic nanophosphate functionalized atemrials on cadmium contaminated soil;the restoration effect of local low-enriched rice（Shen9you 28,S）and conventional rice varieties（Yixiangyou 2115,Y）were selected for cultivation experiments in the restored farmland to study the effects of MFC materials on soil physical and chemical properties,microbial composition,soil enzyme activity and plant enzyme activity at different growth stages of rice after soil remediation,and to explore the practical feasibility of MFC materials used in the remediation of cadmium-contaminated farmland to lay a theoretical and practical foundation,which is of great practical significance.The main research contents are as follows:（1）The preparation of MFC,the remediation of contaminated soil and the study on the enhanced remediation effect of organic acids:Using n Fe₃O₄,Ca（H₂PO₄）₂and montmorillonite as raw materials,magnetic nano-phosphate functionalized materials were prepared by co-precipitation method MFC,the remediation effect of magnetic nano phosphate materials on low cadmium contaminated soil was studied through greenhouse tests.Three organic acids,citric acid,oxalic acid and tartaric acid,were combined with MFC to study the enhanced remediation of contaminated soil.The results showed that after applying 1.0%MFC material alone,the elimination rate of total cadmium in soil was 22.75%,and the curve of decline of vailable cadmium was 17.66%.The removal of total cadmium and effective Cd in soil was enhanced by applying low molecular organic acid+MFC material under three concentration gradients of 1.0,5.0,and 10.0 mmol/L for enhanced remediation,and with the increase of organic acid concentration,the removal effect The better;from the overall point of view,the relationship between the activation of the three organic acids is:citric acid>oxalic acid>tartaric acid.（2）In situ remediation of MFC cadmium-contaminated farmland on farmland soil p H,CEC,organic matter,N,P,K:In-situ remediation of cadmium contaminated farmland with a total cadmium content of 0.411 mg/kg was carried out by’flooding-rotary tillage-adding MFC（0.5%,1.0%）-mixing uniform-MFC loaded heavy metal cadmium-magnetic separation recovery repair material technology’.After material recovery,the soil p H value of the treatment group with 0.5%restoration material increased by 0.46 units,and the organic matter,CEC and total phosphorus increased by 5.9%,7.6%and 103%;and the soil in the treatment group added with1.0%remediation material,the p H value increased by 0.56 p H units,and the organic matter,CEC,and total phosphorus increased by 12.3%,25.96%,and 163%.At the same time,the p H value,organic matter,CEC and total phosphorus content of’Shen9you 28’and’Yixiangyou 2115’were higher than the control group at jointing stage,heading stage and mature stage.In addition,the application of MFC materials had little effect on total nitrogen and total potassium in soil.（3）Effects of in-situ remediation of cadmium contaminated farmland by MFC on total Cd and available Cd in farmland soil:After material recovery,the total Cd removal rate of 0.5%remediation material treatment group was 21.65%（P<0.05）,and the effective Cd reduction rate was 2.49%;the total Cd removal rate was 24.03%（P<0.05）and the effective Cd reduction rate was 12.3%（P<0.05）in the treatment group with 1.0%repair material.At the same time,the total Cd and available Cd contents in each treatment group were lower than those in the control group at jointing stage,heading stage and maturity stage.（4）Effects of in-situ remediation of Cd contaminated farmland by MFC on Cd content and yield of rice rhizomes,leaves and brown rice:The remediation of MFC materials can effectively reduce the total Cd content in roots,stems,leaves and brown rice of rice plants.From the holistic point of view,Application of 1%repair agent treatment group（1.0%repair agent magnetic dressing soil after planting Shen9you 28,referred to as S-1.0%;1.0%repair agent magnetic dressing soil after planting Yixiangyou 2115,referred to as Y-1.0%）,and the Cd reduction effect of rice was better than that of 0.5%remediation agent treatment group（0.5%repair agent magnetic dressing soil after planting Shen9you 28,referred to as S-0.5%;0.5%repair agent magnetic dressing soil after planting Yixiangyou 2115,referred to as Y-0.5%）,and the relationship between cadmium content in rice tissues was root>stem and leaf>grain.The Cd content in brown rice of low enrichment rice variety Shen9you28was lower than that of conventional rice variety Yixiangyou2115.The total Cd content in brown rice of S-0.5%and S-1.0%treatment groups decreased by 30.95%and61.90%,while the rice yield increased by 5.7%and 8.0%.The Cd content in brown rice of Y-0.5%and Y-1.0%treatment groups of‘Yixiangyou 2115’decreased by35.29%and 41.17%,and the rice yields increased by 3.2%and 2.9%.（5）Effects of in-situ remediation of farmland contaminated by MFC cadmium on soil microbial community composition:after the application of MFC magnetic separation to repair soil cadmium,the diversity and richness of soil bacterial community were promoted to a certain extent.The dominant bacterial phyla of the two rice varieties in the three periods were Chloroflex,Acidobacteriota and Actinobacteriota,accounting for about 64.45%–75.04%of the total bacterial community.The repair of soil by MFC materials increased the relative abundance of proteus in the treatment group at maturity,accounting for 8%more than that in the control group.The dominant fungal phyla of the two rice varieties in the three periods were Ascomycota,Rozellomycota and unclassified-k-Fungi,with the relative abundance of 81.39%–95.07%.MFC materials generally increased the abundance of ascomycota after remediation and recovery of cadmium contaminated soil.（6）Effects of in situ remediation of MFC cadmium-contaminated farmland on soil enzyme activity and plant enzyme activity:From an overall point of view,after MFC material remediation and recycling of cadmium-contaminated soil,the urease activity of each treatment group increased,and the mature stage The urease activity of the Y-0.5%group had the best improvement effect,reaching 55.4%,which was significantly different from that of the other groups;and the dehydrogenase activity of each treatment group also increased,at the mature stage S-0.5%,S-1.0%and The soil dehydrogenase activities in the Y-0.5%group increased by 112.24%,44.89%and 37.21%,respectively,with significant differences（P<0.05）;while the activities of catalase in each treatment group decreased.After remediation and recovery of cadmium-contaminated soil,MFC material regulated the antioxidant enzyme activities of the two rice varieties,increased CAT enzyme activity in roots,decreased CAT enzyme activity in stems,and increased POD enzyme activities in stems and leaves compared with the control group.,decreased POD enzyme activity in roots;increased SOD enzyme activity in roots and leaves decreased SOD enzyme activity in stems.