Study On Preparation Of ZnS-Biochar And Its Adsorption Properties For Mercury

As a highly toxic heavy metal,mercury could cause great harm to the environment and human beings.The problem of mercury pollution caused the attention of the scholars from all over the world.Biochar has a large specific surface area,developed pores and rich functional groups,so it has a good adsorption effect on pollutants.Because of its simple preparation,low price and environmental friendliness,it was widely used in contaminated soil remediation.In this paper,biochar was preduced by pyrolyzing ZnCl2 and S modified corn stalks in oxygen-free environment.It was used to repair mercury-contaminated soil,adsorb Hg2+ in water and remove Hg0 from flue gas.The property of biochar was characterized,and the adsorption properties of biochar on Hg2+ in water and Hg0 in flue gas was studied.Biochar was used to repair the mercury-contaminated soil for a long time.We studied the effects of biochar on the main physical and chemical properties,the mercury leaching concentration and the mercury species in soil.We discussed the mechanism of biochar for remediation of mercurycontaminated soil.The main conclissions are as follows:(1)ZnCl2 and S co-modified biochar could effectively reduce the environmental risk of mercury in soil.After adding it to the soil for 14 days,the mercury leaching concentration decreased from 2.7 mg·L-1 to 0.014 mg·L-1,the removal rate was 99.5%,and the exchangeable mercury content reduced to 0%.The optimal preparation conditions of biochar were as follows:activated corn stover with 30%ZnCl2 and 5%S,and carbonized at 350℃ for 1 h.The comodification of ZnCl2 and S improved the adsorption capacity of biochar for Hg2+,and its saturated adsorption capacity for Hg2+ was 278.2 mg·g-1.Hg2+ was stabilized by the formation of HgS,HgO and HgSO4 on ZnS-biochar.ZnS-biochar could effectively remove Hg0 from flue gas with Hg0 concentration of 50～1000 μg·m-3.(2)The characterization of biochar showed that after S modification,the pore wall of biochar became thinner,the pore size and specific surface area became larger,and the content of S=O bond increased.The modification of ZnCl2 caused the collapse of biochar pores,decreased the specific surface area,and increased the content of C-Cl and C=Cl bonds in biochar.The co-modification of ZnCl2 and S caused some pore collapse,increased the specific surface area,increased the content of S=O,C-Cl and C=Cl bonds,and formed ZnS in biochar.(3)The best remediation conditions for mercury-contaminated soil were as follows:adding 2%ZnS-biochar,adding 1%organic matter and maintaining the soil water content at 20～25%.ZnS-biochar increased soil CEC,reduced soil Eh and increased soil pH.After one day of remediation with ZnS-biochar,the mercury leaching concentration decreased from 2.7 mg·L-1 to below the 0.1 mg· L-1.And after 30 days,it decreased to 0.63 μg·L-1 and the stabilization rate reached 99.99%.After 7 days of remediation,the exchangeable mercury content decreased to 0%and the stable organic bound mercury and residual mercury contents significantly increased in the soil,and the remediation effect was stable for 90 days.(4)Elements such as O,S and Cl of ZnS-biochar may play an important role in adsorbing mercury.ZnS-biochar stabilized mercury in the soil by increasing soil pH,increasing soil CEC and reducing soil Eh.ZnS-biochar and mercury formed stable HgS,HgSO4 and HgCl2 compounds in the soil.