The Study Of Adsorption Of Cadmium In Aqueous Solution By Phosphate-Modified Bamboo Biochar

Cadmium had been officially identified as the primary contaminant of the soil in China.The method of using biochar as a chemical passivation agent of heavy metal could reduce the heavy metal content of the absorbable state and the exchangeable state to make soil in situ reparation,had great economic and environmental advantages.In this paper,bamboo was used as a raw material,and the phosphate and phosphoric acid were used to produce phosphorus-enriched samples.The modified biochar was prepared on a self-made fixed-bed pyrolysis system.The modified biochar was characterized with elemental analysis,specific surface area and pore structure analysis,Fourier transform infrared spectroscopy（FTIR）,X-ray diffraction（XRD）,and environmental scanning electron microscopy and energy dispersive X-ray spectroscopy（ESEM-EDX）tests.The modified biochar prepared under different conditions was used to conduct cadmium adsorption experiments.According to the adsorptive capacity of heavy metals,the modified biochar with the best adsorption effect was selected and then to study the adsorption the adsorption characteristics.The mechanism of immobilization of heavy metal ions by biochar modified with phosphorus was also investigated through the biochar yield,pore structure and the surface chemistry of the biochar.The main findings are as follows:（1）Three kinds of phosphates（potassium phosphate,potassium dihydrogen phosphate and diammonium phosphate）and phosphoric acid were used to modify bamboo biomass and produce different biochars with different impregnant types,different impregnation ratios and different pyrolysis temperatures.The pore structure,pH values,char yield and adsorption capacity of modified biochar and the raw bamboo were studied.The results showed that,after modification,pore structure of the four kinds of impregnants modified biochar were all obviously improved.And the potassium phosphate modified biochar has the highest pH value.And the lower the final pyrolysis temperature and the impregnation ratio,the higher the yield of biochar was.In addition,the results of adsorption experiments of biochar on Cd²⁺showed that the adsorption capacity of potassium phosphate-modified biochar has been significantly improved,compared with the other three kinds of impregnants modified biochar and raw bamboo biochar.For the potassium phosphate-modified biochar,when preparing with the impregnation ratio of 2:1 and the pyrolysis temperature of 550°C（1-TKP-BC550）,has the largest adsorption capacity of 238.71 mg·g^-1,which is 10 times that of the raw bamboo and4 times that of the commercial activated carbon(58.18 mg·g^-1).（2）On the research of the Cd²⁺adsorption characteristics of potassium phosphate modified biochar,based on the adsorption dynamics research results,it’s best to use first order kinetics model fitting for the modified potassium phosphate biochar and its adsorption process on the adsorption of heavy metals achieved apparent balance after 5h;The isothermal adsorption result shows that the Langmuir model fitting results was best,the maximum number of Cd²⁺adsorption of 2-TKP-BC550(252.73 mg·g^-1)and 2-TKP-BC750(249.79mg·g^-1)was 10 times more than the bamboo raw bamboo biochar’s(25.67 mg·g^-1);experimental results of the effect of different pH on heavy metals Cd²⁺adsorption of potassium phosphate modified carbon showed that when pH value was 4,the adsorbance of potassium phosphate modified biochar 2-TKP-BC750 maximize,that is 271.77 mg·g^–1.Under the conditions of the pH value of 4 to 6,the removal rate of potassium phosphate modified biochar was above 70%.The FTIR and XRD results of different modified biochar surface before and after the adsorption of heavy metal analysis shows that phosphate foundation group kept on the surface of modified biochar after involved in heavy metal adsorption process.And combined with K⁺,Cl^-,PO₄^3-,CO₃^2-in the solution,Cd²⁺formed insoluble complex,which attributed to chemical adsorption of heavy metals of modified biochar compounds containing phosphorus.The ESEM results of different modified biochar before and after the adsorption of cadmium showed that after adsorption of heavy metals,there is obvious precipitation appeared on the surface of modified biochar,mainly containing C,O,Si,P,Cd,K.So the excellent heavy metal adsorption ability of potassium phosphate modified carbon caused by that potassium rich phosphorous groups introduced by modify of phosphate phosphorus was involved in the process of chemical adsorption of heavy metals and combined with heavy metals to form the insoluble phosphate.