Study On Removal Of Soil Cr (?) By Mineral-Biochar Immobilized Microorganisms

Cr ???enters the soil environment due to mineral mining,metal smelting,chemical fuel combustion,pesticides and fertilizer use.It is highly toxic and easy to accumulate in organisms,which seriously harms human and animal life.As a kindly environment-friendly technology,microbial remediation has attracted extensive attention because it will not damage the soil environment required for plant growth,will not produce secondary pollution and simple operation.However,the addition of free exogenous microorganisms to the contaminated soil will be difficult to survive due to the competitive effect of indigenous microorganisms,the toxic effect of pollutants and other factors,and the remedidation effect will not as expected.And the immobilized microorganism technology can fix the microorganism on the carrier,protect the bacteria and improve the viability of the microorganism,and thus achieve the repair effect.Biochar is an ideal material for immobilized carrier due to its large specific surface area,porosity and good biocompatibility.However,biochar is not completely inert.The aging of biochar will cause its surface structure to change,which will reduce the stability of biochar in soil and affect its efficiency in repairing soil pollution.Therefore,it is necessary to improve the stability of biochar.In this study,an agar dilution method was used to screen a dominant strain with Cr ???resistance from four PAHs-degrading bacteria,and their ability to react to Cr ???solutions at different concentrations were investigated.Secondly,montmorillonite and different iron sources?ferric chloride,magnetite and ferric acetylacetone?were used as mineral additives to prepare biochar under the condition of high temperature and hypoxia,and the effects of iron and montmorillonite on the stability of biochar were evaluated.The adsorption effect and mechanism of Cr ???in solution by original biochar and mineral-biochar were studied and prepared mineral-biochar as the immobilized carrier,the immobilized microbe pellets were prepared by adsorption embedding method to study the remediation effect of mineral-biochar immobilized microbe on water and soil polluted by Cr ???.The main research results are as follows:?1?A strain of Cr ???resistant strain F14 was screened from the existing strain in the laboratory.The strain was a fusion strain constructed by protoplast fusion technology,which not only had the ability to degrade PAHs,but also had the ability to remove the heavy metal Cr ???.Cr ???in the solution could be effectively removed by F14 fusion strain,in which the removal rate of Cr ???solution at 25 mg/L could reach 92.41%.Using scanning electron microscopy?SEM?and fourier infrared spectrometer?FTIR?to characterize the bacteria before and after the reaction,it was speculated that F14 could achieve the adsorption of Cr ???by binding the carboxyl,alkylamide group and hydroxyl group on the cell surface.?2?The addition of montmorillonite and iron can improve the stability of biochar.The results showed that C-O-Fe metal organic complexes were formed on the surface of biochar with iron source and montmorillonite added.Compared with the original biochar?BC?,the relative atomic contents of C=O and COOH of Fe-montmorillonite biochar increased from 0and 3.7%to 6.5-8.4%and 5.5-6.3%,respectively,indicating that iron and montmorillonite improve the oxidation resistance of biochar.The antioxidant index R₅₀,_biochar can evaluate the stability of biochar,thermogravimetric analysis results show that the addition of iron and montmorillonite improves the stability of biochar to varying degrees,among which magnetite has the best effect on the stability improvement of biochar,R₅₀ increases by 10%.?3?The adsorption effect and mechanism of Cr ???in solution by BC and montmorillonite magnetite biochar?MB-Mag?were studied.Meanwhile,MB-Mag biochar immobilized carrier was used to prepare immobilized microbe pellets and applied to the study of Cr ???removal.The results showed that MB-Mag could effectively improve the removal efficiency of Cr ???,and the Fe₃O₄ crystals covered by MB-Mag could form H-Fe-O-Mⁿ⁺species with heavy metal ions in the liquid phase to improve the adsorption capacity,while its abundant surface functional groups?-OH/-COOH/Fe-O?could reduction Cr ???to Cr???.The Langmuir isotherm adsorption model shows that the adsorption of Cr ???by MB-Mag biochar was monolayer adsorption.Therefore,the mechanism of Cr ???removal by MB-Mag biochar is mainly chemisorption and reduction.By comparing the removal effect of Cr ???between free bacteria and immobilized microbial pellets,it can be concluded that the removal effect of Cr ???by immobilized biological pellets is better than that of free bacteria.Among them,the removal effect of mineral-biochar immobilized microbial pellets?F14-MB-SA?was the best,indicating that the immobilized strain F14 and the immobilized carrier biochar directly promote the removal of Cr ???.The removal pathway of Cr ???in the pellet is mainly reduction and biosorption.?4?The feasibility of remediation of heavy metal Cr ???contaminated soil by immobilized microorganism technology was investigated,and the results showed that:in the soil remediation experiment containing Cr ???,the removal rates of Cr ???in the soil of 25,50 and 100 mg/kg were 88.37%,74.13%and 48.85%after 30 days by mineral-biochar immobilized microspheres,which were 26.06%³2.31%higher than the free bacteria.After 30days,the number of viable bacteria in mineral-biochar immobilized pellets was significantly higher than that without mineral-biochar immobilized pellets,indicating that mineral-biochar was beneficial to the growth of embedded microorganisms.