Study On Passivation Remediation Of Hexavalent Chromium Contaminated Soil By Modified Biochar

With the rapid development of industry,agriculture and economy,soil heavy metal pollution at home and abroad is an unavoidable problem.Because heavy metals are difficult to degrade,easy to accumulate,and toxic,they have caused serious harm to the ecological environment,food safety,people ’s health and sustainable agricultural development.Based on this,the remediation technology of soil heavy metal pollution in China has developed rapidly in the past ten years.However,soil heavy metal pollution has the characteristics of complex causes,regional characteristics,and differences in pollution levels.It is necessary to select suitable remediation technologies for local soil heavy metal pollution according to local conditions.In this study,aiming at the problem of heavy metal hexavalent chromium pollution in calcareous soil in northern China,referring to the application of previous remediation technologies,it was found that biochar,phosphate materials and iron salt materials can be used as heavy metal stabilizers.However,due to the unsatisfactory adsorption effect of original biochar,the low solubility and poor dispersibility of chloroapatite,the direct use of ferric chloride will cause soil acidification and other problems,which cannot be widely used in the remediation of heavy metal contaminated sediments.At present,the adsorption of hexavalent chromium by biochar mainly shows good adsorption effect under strong acidic conditions,while the adsorption effect is poor under weak acidic or neutral conditions.Therefore,previous studies have focused on strong acidic conditions.In view of the characteristics of northern soil,in this study,sesame straw was used as a precursor to prepare biochar by pyrolysis,and it was modified with chloroapatite and ferric chloride to improve the adsorption effect of biochar on hexavalent chromium under weak acidic conditions.Scanning electron microscopy,specific surface area and pore structure analysis,Fourier transform infrared spectroscopy,X-ray photoelectron spectroscopy,X-ray diffraction analysis and other technical means were used to characterize and analyze the adsorption mechanism of modified biochar on hexavalent chromium.The passivation and remediation ability of modified biochar to hexavalent chromium in soil was discussed through soil culture test and heavy metal form risk assessment.The main research contents and conclusions are as follows :(1)Iron-modified biochar(FBC)and iron-doped chloroapatite-modified biochar(FBCP)are hierarchical porous materials with mesoporous structure,which have good stability in the environment.Biochar as a skeleton can give full play to the advantages of chloroapatite and ferric chloride,and the modified materials are well dispersed on the surface and pores of biochar.Compared with the original biochar(BC),the specific surface area,total porosity and acidity of FBC and FBCP are larger,which contributes to the enhancement of adsorption capacity.(2)The results of static adsorption test showed that when p H=5 and biochar dosage was150 mg,the maximum removal rate of Cr(VI)by FBC was 85.88 %,and the maximum removal rate of Cr(VI)by FBCP was 94.21 %.The fitting model of adsorption results showed that the adsorption of Cr(VI)by FBC and FBCP was a spontaneous endothermic reaction,which was consistent with the pseudo-second-order kinetic model and Langmuir model.The adsorption was dominated by monolayer chemical adsorption,and the intraparticle diffusion and chemical reaction between Cr(VI)and active groups were the main rate-controlling steps for the adsorption of Cr(VI)by modified biochar(FBC,FBCP).The fitting results of Langmuir model showed that the maximum saturated adsorption capacity of FBC for Cr(VI)was 7.907 mg/g,and the maximum saturated adsorption capacity of FBCP for Cr(VI)was 12.625 mg/g,which were 4.06 and 6.48 times of the maximum saturated adsorption capacity of original biochar(BC),respectively.(3)The adsorption mechanism of Cr(VI)on FBC and FBCP was reduction,physical adsorption,ion exchange and functional group complexation.Compared with FBC,phosphate ions in FBCP can also co-precipitate with Cr(III),which further improves the removal efficiency of pollutants.(4)After FBC and FBCP were added to Cr(VI)contaminated soil,the physical and chemical properties of soil were changed,the cation exchange capacity(CEC)and available phosphorus content of soil were increased to a certain extent,and the p H value of soil showed a downward trend,but the overall change range was not large.After 90 days of remediation of Cr(VI)contaminated soil by FBC and FBCP,compared with CK,the contents of available Cr(VI),weak acid extractable state and reduced state decreased significantly,and the contents of oxidizable state and residual state increased significantly.In terms of the amount of modified biochar applied,the passivation effect is 5% > 3% > 1%.In terms of passivation effect of modified biochar,FBCP is more suitable for long-term application of Cr(VI)contaminated soil passivation remediation than FBC.(5)After 90 days of incubation with different proportions of biochar,the direct toxicity and environmental risk level of hexavalent chromium in soil were weakened,and the weakening effect of FBCP-5 % was the most obvious.After application,the soil was close to pollutionfree state,and the environmental risk level was reduced to low risk.