Biochar Supported Nanoscale Zero Valent Iron Synthesized Using Green Tea Extractes Remove Hexavalent Chromium From Groundwater

If the groundwater pollution problem is not effectively contained and resolved,it will pose a great threat to human's future development.Chromium was widely used in manufacturing industries.It migrated to the groundwater with waste water and waste slag and caused groundwater pollution.The in-situ remediation method and the traditional extraction method were common methods of remediation for hexavalent chromium-contaminated groundwater.The nanoscale zero valent iron had many advantages,such as strong adsorption and strong reducibility.It can effectively remove hexavalent chromium from groundwater.There were many disadvantages to the conventional method for preparing nanoscale zero valent iron,but the green synthesis method had advantages,such as no secondary pollution.Further,crop straw-biochar was used to loaded green synthetic nanoscale zero valent iron,the removal of hexavalent chromium in groundwater can effectively increase.In this study,FTIR,SEM,XRD and XPS were used to characterize green synthesis biochar supported nanoscale zero valent iron?GT-nZVI/BC?.The composition,surface structure,functional groups and material changes of GT-nZVI/BC were analyzed.A series of experimental materials were studied to study the effect of preparation conditions on the removal rate of hexavalent chromium in groundwater,including the calcination temperature of biochar and the Fe/BC load ratio.The effect of the environmental conditions of the reaction,including the pH,temperature and initial hexavalent chromium concentration were studied.Finally,a variety of kinetic models and thermodynamic model were used to study the mechanism of GT-n ZVI/BC removal hexavalent chromium.The main conclusions were as follows:?1?The green tea extract can reduce ferric ions to Fe⁰ nanoparticles with diameter of20-50 nm,and GT-n ZVI evenly attach to the surface of flake-like biochar to form GT-n ZVI/BC materials.The organic substances in the green tea extract,such as flavonoids and polysaccharides,were attached to the surface of GT-nZVI/BC,which enhanced the stability and reactivity of GT-nZVI/BC removal contaminants.After reaction of GT-n ZVI/BC with Cr?VI?,Fe₂O₃ and FeOOH were mainly generated.?2?The optimal preparation conditions of GT-nZVI/BC:calcination of biochar at 400°C and 2:1 Fe/BC loading ratio.When the calcination temperature was low,the adsorption capacity of BC was not strong enough.If the temperature was high,the acidic functional groups on the surface of BC reduced and the removal of Cr?VI?reduced.When the Fe/BC load was low,the nZVI on the surface of BC was not enough,resulting in a low adsorption and reduction of Cr?VI?in the groundwater.The biochar prepared at a calcining temperature of 400°C,the initial pH of the solution was 5,the initial Cr?VI?concentration of the solution was 5 mg/L,and the reaction temperature was 303 K.GT-nZVI/BC can remove 100%Cr?VI?from groundwater within 120 minutes.?3?Reducing the pH of the groundwater solution and raising the reaction temperature within a certain range was conducive to the removal of Cr?VI?in groundwater by GT-n ZVI/BC.When the initial pH was 5,7 and 9,the removal rates of Cr?VI?were 100%,73.5%and 42.3%within 120 minutes,respectively.When the reaction temperature was 293 K,303 K and 313 K,Cr?VI?removal efficiency was 46.1%,73.5%and 86.1%within 120minutes,respectively.When Cr?VI?initial concentrations was 5 mg/L,10 mg/L and 15 mg/L,the Cr?VI?removal rate was 100%,67.2%,and 54.5%within 120 minutes,respectively.With the increase of the initial concentration of Cr?VI?,the removal rate decreased continuously.The adsorption of Cr?VI?was 4.98 mg/g,6.72 mg/g and 8.18 mg/g,respectively.?4?Pseudo second-order kinetic model can better fit the process of removing Cr?VI?in groundwater by GT-nZVI/BC.Whe the material addition was 1 g/L,the biochar calcination temperature was 400°C,the initial pH value of the solution was 5,the initial Cr?VI?concentration of the solution was 5 mg/L and the reaction temperature was 303 K,the adsorption rate constant was 0.017 64 g·?mg·min?^-1.?5?The removal effect of Cr?VI?by GT-nZVI/BC was obviously greater than that of GT-n ZVI and BC.The removal of Cr?VI?by BC was mainly through physical adsorption,and the removal of Cr?VI?by GT-nZVI had both adsorption and redox effect,but its surface needs to both adsorb and reduce Cr?VI?.The limited Cr?VI?removal effect of GT-nZVI surface was normal.GT-nZVI/BC removed Cr?VI?by combining the advantages of GT-nZVI and BC.The respective advantages of nZVI and BC made the performance of GT-nZVI/BC greatly enhanced.?6?The Langmuir adsorption isotherm model can well describe the process of Cr?VI?removal by GT-n ZVI/BC.The Cr?VI?adsorption process was dominated by monolayer adsorption,and the effective sites of GT-n ZVI/BC adsorbing Cr?VI?were evenly distributed.?7?The Bangham pore diffusion model can better describe the process of removing Cr?VI?by GT-nZVI/BC at higher temperature.The molecular motion was stronger at higher temperature and the redox removal rate of Cr?VI?was faster.The Cr?VI?removal was mainly affected by a single redox reaction.The Weber-Morris model cannot fit the removal process of Cr?VI?by GT-n ZVI/BC.The rate determination step was not single,including particle uutward diffusion,particles inward diffusion and redox,the increase in temperature caused a gradual decrease in rate-determining steps.?8?Thermodynamic analysis showed that Cr?VI?removal by GT-nZVI/BC was a spontaneous,endothermic and entropy increasing reaction.