The Adsorption Mechanism Of Chromium By Biochar And Its Effect On The Migration And Transformation Of Chromium In Soil

Heavy metal chromium pollution has been seriously harmful to human health,the remediation of soil heavy metal chromium pollution has become an urgent work at present.As an efficient adsorbent,biochar has a good adsorption capacity for heavy metals in soil and can effectively control their migration.In this study,biochar prepared from corn stalks was used as raw carbon（RC）and fractionated to prepare inorganic carbon（IC）,organic carbon（OC）,blocking hydroxyl carbon（BHC）and blocking carboxyl carbon（BCC）components,with heavy metal chromium（Cr（Ⅲ）and Cr（Ⅵ））of the two kind of existence form as the research object.Through the characterization of biochar and its different components before and after the adsorption of Cr（Ⅲ）and Cr（Ⅵ）,it is clear that its effect the adsorption mechanism;The adsorption properties of biochar and its different components for Cr（Ⅲ）and Cr（Ⅵ）were investigated by adsorption kinetics and adsorption thermodynamics experiments;The effects of the input of biochar and its different components on the migration and transformation of Cr（Ⅲ）and Cr（Ⅵ）in soil was studied through the soil column experiment of dynamic simulation of acid rain.This study has positive significance and long-term application prospect for remediation of soil heavy metal chromium pollution.The main conclusions are as follows:（1）According to the contribution rate of different components of biochar to Cr（Ⅲ）and Cr（Ⅵ）,the adsorption contribution rate of OC to Cr（Ⅵ）was 96.13%,which was much greater than IC.The reduction effect played a major role in the adsorption process,accounting for 53.04%.The adsorption capacity of IC for Cr（Ⅲ）was the highest（15.15mg/g）,and the ion exchange between Cr（Ⅲ）and metal cations such as K⁺and Ca²⁺was the main factor,accounting for 64.55%.The reduction effect of-πelectrons in BHC and BCC is stronger,which can reduce Cr（Ⅵ）to Cr（Ⅲ）,and the reduction effect accounts for 65.62%and 72.76%,respectively.（2）According to the analysis of adsorption kinetics and adsorption thermodynamics,the adsorption process of Cr（Ⅲ）and Cr（Ⅵ）by biochar and its different components is spontaneous,and the temperature rise is conducive to the adsorption.The adsorption processes all conform to Freundlich adsorption isotherm and qseudo-second-order adsorption kinetics model,and intra-particle diffusion is not the only factor to control the adsorption process.The adsorption process is a synergistic action of multiple adsorption mechanisms,and chemisorption is the main one.（3）According to the correlation between p H and Cr（Ⅲ）and Cr（Ⅵ）in soil in the soil column simulation experiment,the inputs of RC,IC,BHC and BCC increased the p H of the soil.For the soil column of Cr（Ⅲ）,the p H value after the input of biochar was inversely proportional to the depth of soil column and the total amount of Cr（Ⅲ）in soil at different depths,and had a certain correlation with the four fraction of Cr（Ⅲ）in soil at each layer.For Cr（Ⅵ）,the p H value after the input of biochar was directly proportional to the depth of soil column and the total amount of Cr（Ⅵ）in soil at different depths,and had no significant correlation with the forms of Cr（Ⅵ）in each layer of soil.（4）In the soil column simulation experiment,the content distribution of Cr（Ⅲ）and Cr（Ⅵ）was as follows:the residual state>organic bound state>weak acid soluble state>Fe-Mn oxidation bound state;Cr（Ⅲ）mainly exists in the form of residual state,while Cr（Ⅵ）mainly exists in the form of residue state and organic combined state.The contents and total amount of Cr（Ⅲ）in soil all decreased with the increase of soil depth,while Cr（Ⅵ）first decreased and then increased.Among them,the migration of Cr（Ⅲ）in the soil was low,and there was a significant hysteresis in the migration,while the migration of Cr（Ⅵ）in the soil was strong.Biochar and its components had a certain stabilizing effect on the Cr（Ⅲ）in soil,but promote the migration and transformation of Cr（Ⅵ）.（5）The adsorption mechanism of biochar and its different components on Cr（Ⅲ）and Cr（Ⅵ）in the soil is relatively complicated,and there are synergistic effects among various adsorption mechanisms.The mechanisms of biochar and its different components on the migration and transformation of Cr（Ⅲ）and Cr（Ⅵ）in the soil include:electrostatic interaction and pore adsorption on the surface of biochar,reduction of Cr（Ⅵ）by different components of biochar,the complexation of Cr（Ⅲ）by carboxyl functional group（-COOH）,the ion exchange between Cr（Ⅲ）and metal cations and co-precipitation between basic substances and Cr（Ⅲ）.