| Coal is the ballast of national energy security and stable supply,and its main energy status is difficult to change in the short term.In China,the desulfurization sludge produced by coal-fired power plants through the desulfurization system is a hazardous solid waste.Most of the trace heavy metal ions in the wet desulfurization wastewater will enter into the desulfurization sludge,which is less stable in the environment and has high leaching toxicity,and will react with acidic water bodies and so on,thus causing pollution to water bodies,which will cause harm to the environment if not treated.Combined with the mainstream disposal method of landfill in China,it is necessary to evaluate and control the hazards of desulfurization sludge.As there is no common method for environmental risk assessment of desulphurisation sludge from coal-fired power plants,in order to make an objective assessment of the hazards of trace elements in sludge from multiple perspectives and in a comprehensive manner,this study analyses the chemical forms of ten hazardous trace elements(Hg,As,Se,Pb,Cd,Cr,Ni,Cu,Zn and Sb)in desulphurisation sludge by BCR sequential extraction experiment.The RAC method with the Nemerow Integrated Pollution Index method was chosen to carry out the environmental risk assessment.Specifically.the hazardous trace elements in the desulphurisation sludge from coal-fired power plants ranged from 1 5.0 to 591.1 μg/g for the other eight elements,except for Hg and Cd.which were slightly lower.And the chemical morphological analysis showed that all but a few elements showed a strong environmental transport capacity.Based on the results of the RAC method assessment,most of the trace elements in the sludge pose a threat to the environment,with Cd,Hg and Zn reaching medium to high risk levels.According to the Nemerow Integrated Pollution Index analysis,the combined pollution levels of desulphurisation sludge from coal-fired power plants reach "Slightly polluted" to "Seriously polluted" levels.In order to solidify harmful trace elements in sludge,biochar,a by-product of pyrolysis of agricultural and forestry wastes,was used as raw material,and the sulfurmodified wheat straw biochar was selected to prepare sulfur-modified wheat straw biochar for the solidification and stabilization of harmful trace elements in desulfurization sludge of coal-fired power plants by combining the characteristics of sulfur’s strong affinity for heavy metal elements.TCLP experiments showed that the leaching concentration of trace elements gradually decreased with the increase of the addition ratio of modified biochar,and the best effect was achieved with the addition of 5%modified biochar.The curing effect of all elements,except Asreached more than 70%.The BCR sequential extraction experiment demonstrated that hydrogen sulphide modified biochar can transform hazardous trace elements from an easily leachable chemical form to a more stable chemical form during the curing process,thus reducing the risk of environmental leaching.By the RAC method,the risk level of each element is reduced to low or no risk after curing,except for Zn.By the Nemerow Integrated Pollution Index method,the overall pollution level of the FGD sludge was also changed from "Seriously polluted" to "Slightly polluted",which significantly controlled the environmental risk.In order to investigate in depth the mechanism of solidification of hazardous trace elements by hydrogen sulphide modified biochar,the adsorption process of hazardous trace elements by typical pores was simulated by molecular dynamics using element Hg as an example.It was found that both microporous and graded pores undergo a sequence of adsorption from external free to internal adsorption and then to surface enrichment.The whole adsorption process includes physical and chemical adsorption,and the graded pore model has better adsorption effect than the micropore model. |
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