Effect Of Pyrite-modified Biochar On The Passivation Of Hg-Cd In Soil And Its Influence On Microbial Communit

With the development of mineral mining and smelting industry,the problem of heavy metal pollution in the soil around mining areas is becoming increasingly serious.Due to the association of cadmium（Cd）in mercury mines,there is a phenomenon of Hg-Cd compound pollution in the soil of mining areas.Hg and Cd have higher toxicity and latency,and are prone to progressive bioaccumulation that can harm human health through the food chain.As a new type of passivator,biochar has great potential in immobilization of heavy metals in soil.However,the immobilization mechanism of Hg and Cd in compound contaminated soil and the response mechanism of microbial communities are still unclear.To explore the immobilization mechanism of biochar on Hg and Cd in soil and the response mechanism of soil microbial community,based on the sulfur affinity of Hg and Cd,this study used pyrite and three types of biomasses（distillers grains,pig manure and rape straw）to prepare pyrite-modified biochar.This study mainly conducted static adsorption experiments,soil column leaching experiments,incubation experiments,and pot experiments（Brassica chinensis L.）to explore the mobility and bioavailability of pyrite-modified biochar on Hg and Cd in soil.Zeta potential,SEM-EDS,XRD,FTIR,and XPS were used to analyze the possible mechanisms of Hg²⁺and Cd²⁺adsorption by pyrite-modified biochar,and the effect of pyrite-modified biochar on soil microbial community was explored by Illumina high-throughput sequencing.The main findings are as follows:（1）The physicochemical characterization results of biochar showed that the surface of biochar modified by pyrite contained rich functional groups such as-OH,C=O,S=S,and-SH;The relative contents of carboxyl,lactone,and phenolic hydroxyl groups in biochar decreased with increasing pyrolysis temperature;The total amount of functional groups in pristine pig manure biochar（PBC）and pyrite-modified pig manure biochar（PPBC）prepared at the same temperature was greater than that in distillers grains biochar（DBC）and rape straw biochar（RBC）;In addition,the Zeta potential of pyrite-modified biochar decreased with the increase in solution p H.（2）The results of static adsorption experiments showed that the preparation conditions with a pyrolysis temperature of 600℃,a mass ratio of pyrite to biomass of 1:3,and an adsorption condition with a dosage of 0.625 g/L and p H=7.0 were more conducive to the adsorption of Hg²⁺and Cd²⁺by pyrite-modified biochar（the maximum removal efficiencies were 99.03%and 15.16%,respectively）;Coexisting monovalent cations（K⁺and Na⁺）in water had no significant effect on the adsorption capacity of Hg²⁺and Cd²⁺by pyrite-modified biochar,while divalent cations(Ca²⁺and Mg²⁺)inhibited the adsorption of Hg²⁺and Cd²⁺by pyrite-modified biochar;Humic acids promoted the adsorption of Hg²⁺and Cd²⁺by pyrite-modified biochar.（3）The results of adsorption kinetics and adsorption isotherm experiments indicated that the adsorption process of pyrite-modified biochar for Hg²⁺was dominated by homogeneous chemical adsorption,while the adsorption process for Cd²⁺was composed of homogeneous physicochemical adsorption processes.The adsorption of Hg²⁺and Cd²⁺by pyrite-modified biochar conformed to the Langmuir model（R²=0.9896～0.9971）,and the maximum adsorption capacities for Hg²⁺and Cd²⁺were 410.32 mg/g and 15.10 mg/g,respectively.According to the characterization results of pyrite-modified biochar before and after adsorption,the removal of Hg²⁺by pyrite-modified biochar was mainly through surface precipitation,reduction,functional group complexation,while the removal of Cd²⁺was mainly dominated by pore filling,ion exchange,surface complexation,and electrostatic attraction.（4）The results of soil column leaching experiments showed that the addition of PPBC could improve soil p H,electrical conductivity（EC）,and organic matter content,and effectively immobilize Hg and Cd in soil（up to 81.16%of Hg and 54.09%of Cd）.Therefore,the leaching amounts of Hg and Cd in the soil column leachate treated with PPBC gradually decreased with the increase of leaching time.（5）Soil incubation experiments showed that PPBC significantly increased the abundance of Proteobacteria,followed by Acidobacter,Actinobaciota,Chloroflexi,Nitrospira,and Firmicutes.The results of redundancy analysis showed that the content of soil organic matter and the content of soil available Hg/Cd are the main environmental factors that affect the composition of microbial communities.（6）Pot experiment results showed that the application of PPBC significantly increased the fresh weight（17.65%～568.18%）,plant height（35.03%～118.11%）,root length（33.15%～252.81%）,and chlorophyll（7.71%～43.69%）content of Chinese cabbage.The order of concentration of Hg and Cd in various organs of Chinese cabbage was:root>stem>leaf.With the increase in the application rate of PPBC,the content of available Hg and Cd in soil gradually decreased,Hg and Cd changed from unstable species to more stable specie,which was more conducive to the growth of Chinese cabbage.