Immobilization Effect Of Fe-Mn Modified Biochar On Mercury And Cadmium Contaminated Paddy Soil And Joint Regulation Remediation

Mercury（Hg）and cadmium（Cd）are two heavy metal elements that are toxic to organisms due to their high bioaccumulation and persistent toxicity.The National Soil Pollution Status Survey Bulletin revealed that the total exceedance rate of cultivated land reached 16.1%,of which the exceedance rates of Hg and Cd were 1.6%and 7.0%,respectively.In situ immobilization remediation technology has been widely used in the remediation of heavy metal contamination farmland soil due to its fast remediation rate,excellent stability,and simple operation.However,highly efficient and environmentally friendly materials for the remediation of Hg and Cd co-contaminated paddy soils were still lacking,and the effects of different agronomic management measures on the immobilization remediation effect need to be investigated in depth.In this study,Fe-Mn modified biochar（FMBC）was prepared as remediation material,and the adsorption performance and binding mechanism of functional material for Hg（II）and Cd（II）in aqueous solutions were revealed through batch adsorption experiments,characterization and density functional theory（DFT）calculations.The immobilization effects of FMBC on Hg and Cd co-contaminated soil and its inhibition mechanisms for the uptake and accumulation of Hg and Cd in rice in the soil-rice system were clarified,as well as different agronomic measures（water and selenium fertilizer regulation）on the remediation effect of Hg and Cd co-contaminated paddy soil.In order to provide important technical support for the safe utilization of Hg and Cd co-contaminated farmland remediation.The main results are as follows:（1）Both batch adsorption experiments,characterization and density functional theory（DFT）calculations results revealed that the loading of Fe-Mn oxides can significantly improve the adsorption capacities of biochar for Hg（II）and Cd（II）.The difference in electronegativity between carbon and Fe/Mn atoms can effectively modify the charge distribution near the doping site,leading to charge transfer between the carbon and metal atoms.The batch adsorption experiments results demonstrated that the adsorption of Hg（II）and Cd（II）onto FMBC was p H-dependent,and the adsorption process was in accorded with pseudo-second-order and Freundlich models.The maximum adsorption capacities of FMBC for Hg（II）and Cd（II）reached 86.82 and 131.03 mg·g^-1,which were 3.7 and 11.2 times that of the original biochar,respectively.The main adsorption mechanism of Hg（II）and Cd（II）was the chemical complexation reaction with oxygen-containing functional groups and metal-oxygen groups（Fe O_xand Mn O_x）in FMBC to form monodentate or multidentate inner-sphere complexes（C-O-Cd/Hg,COO-Hg/Cd and Fe/Mn-O-Hg/Cd）,Cd（II）can co-precipitate with carbonate in FMBC.In addition,electrostatic attraction,ion exchange and metal-πinteraction might be involved in the adsorption mechanism.（2）The pot experiment results of Fe-Mn oxide modified biochar（FMBC）on Hg and Cd co-contaminated paddy soil showed that the addition of BC and FMBC significantly reduced the concentration of soil Na₂S₂O₃-Hg and DTPA-Cd,and inhibited the transition of Hg and Cd from the soil to roots by enhancing the content of iron-manganese plaques.Moreover,FMBC treatments can more effectively inhibit the enrichment of Hg and Cd in rice grains than BC treatments at the same dosage,and the Hg and Cd concentrations in rice grains after FMBC incorporation were reduced by 18.32%-71.16%and 59.52%-72.11%compared with the control group,respectively.In terms of soil environmental effects,the increase of soil total organic carbon（TOC）,labile organic C fractions（DOC,MBC,EOC）and C-pool management index（CPMI）indicated that FMBC can enhance soil C storage and nutrient cycling function.FMBC altered the composition and metabolism function of soil bacterial communities,especially increasing the abundance of keystone phylum.Partial least square path modeling（PLSPM）revealed that the contents of Na₂S₂O₃-Hg,DTPA-Cd and iron-manganese plaques were the key indicators affecting Hg and Cd accumulation in rice grains.In conclusion,FMBC can be employed as an efficient material to remediate Hg and Cd co-contaminated paddy soils and restrict their uptake in rice grains,hence alleviating food security issues.（3）By exploring the effects of different water management measures（long-term flooding irrigation and aerobic irrigation）combined with FMBC on rice growth,heavy metal accumulation in rice tissues and microbial community structure,the results showed that long-term flooding irrigation treatments significantly increased crop yield as compared to aerobic irrigation treatments,and the combined regulation of FMBC and flooding irrigation further promoted rice growth and plant tillering.Under different water management measures,there were differences in the absorption and accumulation characteristics of Hg and Cd in rice tissues.The application of FMBC combined with flooding irrigation treatments significantly reduced the soil available Cd contents and the Cd enrichment ability of rice grains,while increased THg and Me Hg contents in rice grains.However,the application of FMBC combined with aerobic irrigation effectively reduced the concentration of THg and Me Hg in rice grains and its target hazard quotient（THQ）.The concentration of THg(0.02 mg·kg^-1)and Cd content(0.05mg·kg^-1)were below the national standard limit under AFM and FFM treatments,respectively.In addition,compared with aerobic irrigation,the flooding irrigation treatments decreased the diversity of bacterial communities,and enhanced the relative abundance of methylators containing hgc A gene sequence to a large extent,which were related to the increased risks of soil Hg methylation.（4）The addition of FMBC and Na₂Se O₃reduced the contents of Na₂S₂O₃-Hg and DTPA-Cd in the soil,and the passivation efficiency in the compound treatments was better than the single Na₂Se O₃or FMBC treatment.The concentration of Na₂S₂O₃-Hg and DTPA-Cd in the compound treatments（FMSe1and FMSe2）decreased by 20.42%-26.78%and 6.04%-8.84%,respectively.The compound treatments of FMBC and Na₂Se O₃significantly reduced the enrichment ability of rice roots,stems,leaves and grains for Hg and Cd,and inhibited the upward transportation of Hg and Cd from root to internode 3,node 2 to internode 2,node 1 to internode 1 and panicle to rice grains,effectively prevented the accumulation and enrichment of Hg and Cd in rice grains,and reduced the health exposure risks of Hg and Cd.Compared with the control group,the Hg and Cd contents in rice grains were significantly reduced by 72.14%-75.87%and 51.71%-63.38%under the compound treatments of FMBC and Na₂Se O₃,respectively.In addition,the addition of FMBC and Na₂Se O₃changed the redistribution ability of rice roots,nodes and panicles to Hg and Cd,and rice roots and nodes played an important role in intercepting the upward transport of Hg and Cd.