Study On The Mechanism Of Cr(Ⅵ) Removal By Iron-modified Biochar Materials

With the rapid development of industrialization,a large amount of heavy metals remain in water bodies and soil,which seriously affects the ecological safety and people’s health in China.Therefore,it is especially important to explore efficient and inexpensive materials for environmental remediation.In this paper,iron-modified biochar materials were prepared by bioleaching-pyrolysis method using biochar（BC）,a by-product of biomass energy plants,and their effects on the removal of Cr（Ⅵ）from water and soil were explored,respectively.In addition,a synergistic removal test of Cr（Ⅵ）in water by chromium-reducing bacteria（Cr RB）and iron-modified biochar（PMFe BC）was carried out,and the removal mechanism of the material in different environments was elaborated.The main findings of this thesis are as follows.（1）The preparation conditions of bioleaching-pyrolysis modification were explored.The optimum culture temperature of bioleaching for the screened ferrous sulfur oxidation bacterium population was 37℃,and co-cultivated with BC for 72h,followed by secondary pyrolysis at 1000℃in a water vapor atmosphere for 30 min.The prepared PMFe BC showed the best removal of Cr（Ⅵ）from water,and the maximum adsorption capacity reached93.99mg/g much higher than that of the original BC material（16.57mg/g）and bioleaching unpyrolyzed material（7.55mg/g）.The mechanism of Fe modification of PMFe BC is that the bioleaching process loads potassium iron alum（KFe₃（SO₄）₂（OH）₆）onto the biochar,and after high temperature secondary pyrolysis,the potassium iron alum is dehydrated and desulfurized to form Fe₃O₄.Compared with BC,the bioleaching-pyrolysis modified material increases its specific surface area and loads Fe₃O₄on its surface.（2）The process of Cr（Ⅵ）removal from water by PMFe BC is in accordance with quasi-secondary kinetics and Freundlich isothermal adsorption model.Its removal rate of Cr（Ⅵ）increased with the decrease of solution p H,and decreased with the increase of Cr（Ⅵ）concentration and the decrease of dosage,and the removal rate of Cr（Ⅵ）reached 99.81%at p H=2,Cr（Ⅵ）concentration of 100 mg/L and dosage of 1.25 g/L.Combined with FTIR,XPS and other characterization analysis,PMFe BC was modified by secondary pyrolysis to increase the specific surface area to enhance the electrostatic adsorption of Cr（Ⅵ）,while the loaded Fe₃O₄had a certain chemical reduction and co-precipitation effect on Cr（Ⅵ）,which further enhanced the removal efficiency of the material for Cr（Ⅵ）.the synergistic removal of Cr（Ⅵ）in water showed that the synergistic system had the highest removal efficiency of Cr（Ⅵ）when the p H of the medium was 7,and could completely remove 200 mg/L of Cr（Ⅵ）in 3 days.the concentration of Cr（Ⅵ）,the dosage of the material,the p H of the medium and the inoculum of the colony all significantly（p<0.05）affected the removal efficiency of Cr（Ⅵ）.PMFe BC could certain extent to reduce the survival pressure of Cr RB in Cr-stressed environment and promote the bioreduction reaction of Cr RB to Cr（Ⅵ）.（3）The results of soil incubation experiments showed that the solidification efficiency of Cr（Ⅵ）could reach 99.65%at the 60th day with 10‰PMFe BC application.In the leaching toxicity experiment,Cr（Ⅵ）was not easily leached and the acid rain leaching rate was only0.047%,while the PMFe BC application increased the p H value and cation exchange amount of the soil,which was beneficial to the passivation of soil Cr（Ⅵ）.The results of soil chromium morphology distribution showed that PMFe BC-loaded Fe₃O₄could complex with Cr（Ⅵ）,which converted almost all soil exchangeable and carbonate-bound states into biologically unavailable and more stable Fe-Mn oxidation states as well as residue states.