A Study On Immobilization Of Selenium/Chromium In Polluted Soil By Biochar Composite Materials

In recent years,the situation of soil heavy metal pollution has become increasingly severe,which has caused serious threats to biological health and the ecological environment.Because soil pollution by heavy metals has the characteristics of concealment and stagnancy,it brings great difficulties and challenges to the contamination remediation work.Biochar has a large specific surface area,rich oxygen-containing functional groups,high chemical stability,superior adsorption performance,and environmental friendliness.It is regarded as a long-acting soil improver and contamination remediation agent,and has received intense attention from researchers.In this study,nano-zero valent iron and polysulfide were used to modify biochar to prepare a new type of biochar composite material.Then,the artificial selenium-contaminated farmland soil and the actual chromium-contaminated soil near the chromium slag field were used as research objects.Three kinds of biochar materials were used:BC,n ZVI@BC,and PS-n ZVI@BC to stabilize/immobilize the target soils.The effects of contaminants leaching concentration,morphological distribution,XPS scanning functional groups and other indicators were studied,and the effect of biochar materials on heavy metal contaminated soil was evaluated.The possible remediation mechanism was discussed through the correlation analysis of various parameters.The details are as follows:?1?PS-n ZVI@BC was prepared by pyrolysis of pre-zero valence iron method,and the biochar materials were characterized by SEM,FTIR,XRD,TGA and XPS.The results showed that the type and number of functional groups on the surface of the modified biochar material were greatly enriched,and polysulfide and zero-valent iron were successfully supported on the surface of the biochar with uniform dispersion.PS-n ZVI@BC increases soil p H,reduces the release of free iron ions,increases the contents of organic matter,and has better soil improvement effect than the other two biochars.?2?The farmland soil contaminated by exogenous selenium(measured selenium concentration is 125±10 mg kg^-1)was used as the research object,and the target soil was stabilized/immobilized with three kinds of biochar materials for 30 days at room temperature and dark condition.After 30 days of remediation,the available selenium content of each treatment decreased to varying degrees.The Se immobilization efficiency of BC,n ZVI@BC,and PS-n ZVI@BC was 56.11%,60.61%,and 77.29%,respectively.It was found that the selenium forms of the treatment changed from the more accessible?water-soluble and exchangeable form?to the less-available?acid-soluble and organic matter-bound?,especially almost all of them were converted to organic matter bound state?63.45%?in PS-n ZVI@BC treatment.According to the XPS spectrum,the oxygen-containing functional groups were combined with selenium,and Se?VI?and Fe?II?undergo redox reactions,and the active sulfides were converted into selenide.The immobilization mechanism of PS-n ZVI@BC on selenium mainly includes the adsorption of surface pores and oxygen-containing functional groups,the reduction of Fe S and Fe S₂,the complexation of Fe₂O₃,and the precipitation of Fe Se and Fe-Se_x.Pot experiments showed that pure biochar promoted the growth of corn,while n ZVI@BC and PS-n ZVI@BC inhibited.In addition,three biochars can effectively reduce the bioavailability of selenium and reduce the accumulation of selenium by corn.?3?The actual chromium-contaminated soil near the chromium slag field(measured chromium concentration is 734.83±5.56 mg kg^-1)was used as the research object,the target soil was stabilized with three types of biochars for 30 days at room temperature and dark.After repairing Cr-contaminated soil for 30 days,compared with the control group,the Cr?VI?concentrations in the TCLP extract and Ca Cl₂ extract of the PS-n ZVI@BC treatment decreased by 92.5%and 92.0%,respectively,this is much higher than other treatments,which can effectively reducing the dissolution of hexavalent chromium.At the same time,the Cr_total concentration also decreased significantly,and the proportion of Cr?III?increased.PS-n ZVI@BC can effectively promote the conversion of Cr?VI?to Cr?III?with low toxicity,thus reducing the mobility and bioavailability of Cr?VI?in soil.The Cr forms after PS-n ZVI@BC treatment were transformed from the more accessible Cr?exchangeable and carbonate-bound form?to the less-available Cr?iron-manganese oxide-bound form and organic matter form?.According to the XPS spectrum,the functional groups and the elements Fe,S in PS-n ZVI@BC undergo redox reactions with Cr,and most of Cr?VI?was reduced to Cr?III?.The immobilization mechanism of PS-n ZVI@BC on Cr mainly includes surface pores and adsorption of oxygen-containing functional groups,reduction of Fe₂O₃,Fe S and Fe S₂,surface complexation and precipitation of Cr₂O₃/Cr?OH?₃.Pot experiments showed that pure biochar promoted the growth of corn,while n ZVI@BC and PS-n ZVI@BC inhibited.In addition,three biochars can effectively reduce corn's accumulation of chromium.PS-n ZVI@BC treatment can significantly reduce the bioavailability and migration of chromium,improve the stability of chromium,reduce the potential harmful effects on the environment.