Study On Adsorption Characteristics Of Modified Biochar For Phosphate And Hexavalent Chromium

Excess phosphate and hexavalent chromium in surface water will seriously harm the environment and human health.Adsorption is a common method to remove phosphate and hexavalent chromium,but the adsorption capacity of unmodified biochar is usually very poor.In this paper,Si-PSB and Fe-PSB were prepared by impregnated with sodium silicate and impregnated with ferric nitrate using waste peanut shell as raw materials.To investigate the characteristics and mechanism of phosphate adsorption by Si-PSB.And then,to study the differences of adsorption characteristics and mechanism of Fe-PSB on phosphate and hexavalent chromium,and to further explore the changes of adsorption characteristics and mechanism of Fe-PSB on both in binary system.The adsorption capacity of Si-PSB on phosphate was 3.9 times of that before modification.The introduction of silica in Si-PSB enhanced the reactivity of Ca²⁺in Ca CO₃ on biochar,and it improved the utilization degree of metal calcium ions on biochar its own.With the increase of p H value,the phosphate adsorption capacity of PSB（unmodified biochar）and Si-PSB both decreased first and then increased.In strong acid and alkali environment,Si-PSB had a good adsorption effect on phosphate.When the p H value was 3 and 12,the phosphate adsorption capacity of PSB was 0.9 mg/g and 1.1 mg/g,respectively,while that of Si-PSB was 2.9 mg/g and 3.0 mg/g,respectively.The adsorption of phosphate by PSB and Si-PSB was more consistent with the Pseudo-second order,and the Langmuir model was more suitable for the isothermal adsorption results.Increasing temperature was beneficial to the adsorption of phosphate by PSB and Si-PSB.NH₄⁺in the solution had a slight promoting effect on the adsorption of phosphate by PSB and Si-PSB,while the presence of humic acid could inhibit the adsorption of phosphate.Combined with the adsorption experiments and characterization results,which showed that compared with PSB,the chemical precipitation of phosphate by Si-PSB was more stronger.The optimal pH range of Fe-PSB for phosphate and hexavalent chromium adsorption was2-6 and 2-5,respectively,and the adsorption capacity was maintained at about 5.0 mg/g and 8.2mg/g,respectively.The adsorption of phosphate and hexavalent chromium by Fe-PSB was more consistent with the Pseudo-second order and Langmiur model.Combined with the adsorption experiments and characterization results,it could be seen that the adsorption mechanism of Fe-PSB for phosphate was electrostatic attraction and complexation,and the adsorption mechanism of Fe-PSB for hexavalent chromium was electrostatic attraction,reduction reaction and complexation.In the binary system,Fe-PSB could not simultaneously realize the favorable adsorption of phosphate and hexavalent chromium.The hexavalent chromium had little effect on the adsorption of phosphate,but the phosphate significantly inhibited the adsorption of hexavalent chromium.In this case,the adsorption isotherm of hexavalent chromium was more consistent with the Freundlich model.In this paper,by exploring the modification methods of biochar and studying the changes of biochar adsorption characteristics in different systems,it is expected to provide reference for the modification of biochar and the application of biochar in practice.