Study Of Using Biochar Produced From Malt Root To Remediate Mercury-contaminated Soil

Mercury is a highly toxic heavy metal with strong toxic effect on nervous system.Mercury presented in the soil can be absorbed by plants and migrate to the human body through the food chain.Presently,technnologies of remediating mercury-contaminated soils mainly include thermal desorption,chemical leaching,electrokinetic remediation,bioremediation,and immobilization technologies.As a material of stabilization,biochar is promising to be used in remediating heavy metal contaminated soil with the characterizarion of simply preduced,inexpensive,environment-friendly and being able to improve soil properties.In this paper,biochar was preduced by pyrolyzing malt root,the by-product of beer production,in oxygen-free environment.The basic properties of biochar were tested and the effects of biochar on the main physical and chemical properties of soil and on mercury content in soils and plants were studied.By analyzing the changes of soil properties,functional groups in biochar and soil,and soil microorganisms,we discussed the mechanism of biochar acting on mercury in soil.The main conclussions are as follows:(1)Different kinds of biochar were prepared by pyrolyzing malt root at 300 ?,400 ?,500 ?,650 ? and 750 ? respectively,and the property of biochar was characterized.Characterization results showed that biochar has good tubular porous structure.The pore density of biochar slightly increased,and the carbon skeleton wall became thinner with increase of pyrolysis temperature.When the pyrolysis temperature reach up to 750 ?,obvious pore collapse occurred in biochar.Biochars produced in different temperature had different surface properties.The specific surface area,pore volume and pore size of biochars were 22.75?323.34m2/g,0?0.086mL/g and 0.55?0.94nm respectively.With the increase of pyrolysis temperature,the specific surface area and pore volume of biochar became larger,the pore size became smaller.The main components of biochar prepared by malt root are amorphous carbon,with few inorganic minerals.There were-OH,-CH2,-COOH,aromatic ring,C-Cl,C-Br and other functional groups on biochar suface.When the pyrolysis temperature increase,the content of-CH2,aromatic ring and C-Cl reduced until disappear.Compared with the biochar prepared in other temperature conditions,BC650 has higer pore density,smaller pore diameter,and larger specific surface area;it may be more suitable for the remediation of mercury-contaminated soil.(2)Biochar can enhance the soil water holding ability and increase the content of organic matter,decrease soil CEC,but have little effect on soil pH.When biochar is used for the remediation of mercury-contaminated soil,the main physicochemical properties of soil are affected,and the soil has been improved with water-holding capacity and fertilizerholding capacity increased.(3)After 62 days of remediation with biochar,the content of exchangable Hg in soil decreased by 60.78-82.34%,the content of Hg combained with carbonate was nearly zero,the content of Hg combained with Fe-Mn oxides decreased by 47.61-74.59%,while the content of Hg combained with organic matter and residual Hg increased by 14.61%?20.68%and 14.61?114.40%,respectively.Biochar could effectively reduce the amount of mercury absorbed by plants(decreased by 0.03?44.21%),with BC650 reduced the mercury absorbed by cucumber plants most effectively by 44.21%.Biochar can effectively reduce the bioavailability of mercury in soil.(4)Biochar has good adsorption properties of mercury in solution.Biochars prepared at different temperatures had different mercury adsorption capacity,of which BC650 was best for adsorpting mercury,with adsorption capacity of 17.24mg/kg.The adsorption process between biochar and mercury was Langmuir-type adsorption process,controlled by external diffusion.The contents of exchangable Hg,Hg combained with carbonate and Hg combained with Fe-Mn oxides in soil were significantly negatively correlated with soil organic matter and soil CEC.Biochar can stabilize mercury in soil by increasing soil organic matter content and increasing soil CEC.Biochar reduces the-COO content of soil by reducing organic functional groups in the soil,reducing the ability of soil to hold mercury and fixing more mercury to the surface of biochar.Biochar can increase soil microbial diversity and have great impact on microbial community structure.At the same time,biochar can increase the abundance of Geobacter and Bacillus,which could increase microbial methylation of mercury,and promote the volatilization and stabilization of mercury,thereby remediate mercury-contaminated soil.