Study On Optimization Of Preparation Technology For Modified Biochar And Its Adsorption Properties For Cd(Ⅱ) And Pb(Ⅱ)

While the rapid economic growth is driven by industrial development,the corresponding environmental problems are gradually exposed.Heavy metal pollution in water bodies is an extremely hazardous environment problem.Such as Pb（Ⅱ）and Cd（Ⅱ）,they are not only not degraded,but even converted into methyl compounds,which cause greater toxicity.And they can also accumulate in the organism through the food chain.Rice straw biochar（RBC）,as an amendment,was prepared from straw waste under anaerobic condition.The advantages of the abundance of functional groups and the wide source of raw materials make it become a research hotspot.However,there are limitations such as poor dispersion and limited adsorption capacity of the original biochar in water remediation.So,modification is generally necessary to strengthen the function.In this study,the RBC was modified with Calcium thioglycolate trihydrate solution（CTT）by a combination of agitation and ultrasound to obtain new modified materials.The major purposes were to determine the optimum preparation conditions,simplify the modification process,reduce the preparation cost and energy consumption,increase the material surface groups and improve the adsorption ability of materials for heavy metals in wastewater.The main research is as follows.The adsorption capacities of Cd（II）by the modified biochar,prepared under different conditions,were compared to clarify the optimal preparation conditions.The changes in the physicochemical properties of RBC before and after modification were explored by employing phase characterization.Effects of environmental factors on adsorption capacity of modified biochar were analyzed through batch adsorption test.The adsorption mechanism of SRBC on Cd（II）and Pb（II）in single and binary systems were investigate by model fitting and phase characterization.The main results were as follows:（1）The effects of the CTT concentration,agitation and ultrasonic treatment time on the adsorption capacity of the materials were analyzed.With the increase of the CTT concentrations,the adsorption performance of the modified material for Cd（II）followed 30>20>40 mg·L^-1.The extension of stirring time could ensure sufficient contact between RBC and CTT,the adsorption performance of the modified material for Cd（II）at different stirring time followed 2 h≈3 h>1 h.The structure of the material would change when the ultrasonic time increased,the adsorption performance of the modified material for Cd（II）at different ultrasonic time followed 1 h>0.5h≈1.5 h.It was summarized that the optimum conditions for the modified material were:the CTT concentration was 30 mg·L^-1,stirring time was 2 h and ultrasonic time was 1h.（2）The surface morphology and elemental composition were explored by scanning electron microscopy-energy dispersive X-ray spectroscopy（SEM-EDS）.The changes of mineral compositions and functional groups of the materials were analyzed by X-ray diffractometer（XRD）,X-ray photoelectron spectroscopy（XPS）and Fourier infrared spectrometer（FTIR）techniques.The specific surface area was investigated by automatic specific surface area analyzer（BET）.The analysis indicated that after modification,the surface of SRBC became rougher,with both S and Ca elements appearing.And Ca C₂O₄·H₂O and Ca CO₃ peak in biochar appeared,and a new crystal structure formed.-SH,C-S and RSO₃^-might be the structure of S,Ca C₂O₄·H₂O,Ca CO₃and X-Ca might be the structure of Ca.（3）The adsorption capacity of SRBC for Cd（II）and Pb（II）in single and competitive systems was researched.In single system,the adsorption capacity of SRBC for Pb（II）and Cd（II）were 125.43 and 67.07 mg·g^-1,respectively,under the conditions that the initial solution concentration of 100 mg·L^-1 and p H of 5,the SRBC dosage of 0.8 g·L^-1,and the reaction time of 120 min.The change of ionic strength and co-existing cations would affect the adsorption performance of SRBC.In binary system,the adsorption capacity of SRBC for Cd（II）and Pb（II）reduced to 39.44 and 121.34 mg·g^-1at the same test conditions.The adsorption performance of SRBC to HMs was reduced with different coexisting ions.The SRBC had better stability and regeneration ability by the desorption experiments,and the SRBC after saturation adsorption was desorbed with 0.05 M HNO₃ for 4 times.（4）In single system,the quasi-second-order kinetic equation could better describe the adsorption process of Cd（II）by SRBC.Both quasi-second-order and quasi-first-order equations could better describe the adsorption behavior of SRBC to Pb（II）.The Freundlich isotherm model has the best fit to the adsorption process of Cd（II）,and the Langmuir model could perfectly reflect the adsorption behavior of Pb（II）.The adsorption of SRBC for Cd（II）was a spontaneous endothermic process.In binary system,the adsorption kinetic process of Cd（II）and Pb（II）were described with quasi-second-order kinetic equation.The competitive adsorption behavior could be explained by the Langmuir competitive adsorption isotherm.The adsorption mechanisms included complexation reaction and ion exchange.In addition,the adsorption mechanisms of Cd（II）by SRBC in the binary system also includedπelectron coordination and the formation of cadmium-sulfur compounds.In conclusion,the modified biochar material is a convenient,economical,green and efficient new adsorbent prepared by stirring combined with ultrasonic method.The new material has excellent adsorption performance for Cd（II）and Pb（II）from wastewater.This work enriches the modification technology of biochar materials and provides a new direction for biochar on the treatment of heavy metal pollution in wastewater.