Adsorption Of Cadmium On Rice Straw Biochar

In recent years,cadmium pollution in soil environment has become an environmental problem that can not be ignored.As a porous carbon rich solid material,biochar can be used as a soil conditioner because of its high adsorption capacity and strong chemical stability.It is an environment-friendly new material.In this study,biochar was prepared from rice straw by oxygen limited slow pyrolysis at different temperatures（300℃,400℃,500℃,600℃,700℃）,and the adsorption experiment and characterization were carried out with cadmium ion as the main adsorption target.The adsorption principle of biochar was deeply studied,and the biochar with good adsorption effect was added to the soil in a certain proportion to study the adsorption of cadmium in the solution by the biochar/soil mixed system,The response surface method was used to optimize the adsorption conditions,and the effect of biochar addition on the adsorption of cadmium in biochar/soil mixed system was studied.The main research results are as follows:（1）In the process of preparing biochar,pyrolysis temperature,pyrolysis time and heating rate have obvious effects on the yield and ash content of biochar.With the increase of preparation temperature,the thermal stability of biochar is gradually enhanced,the yield of biochar is gradually reduced,while the ash content and pH value are gradually increased.At the same time,the content of C element of biochar is also gradually increased,and the contents of H,O and N are gradually reduced.The zeta potential of biochar prepared under different conditions gradually decreases with the increase of solution pH.The biochar prepared in the study has a similar tubular structure,and the preparation temperature also has a certain impact on the morphology and pore structure of biochar.With the increase of preparation temperature,the surface of biochar gradually becomes smooth,the specific surface area and pore volume of biochar gradually increase,and the pore size of biochar gradually decreases.FTIR results showed that there were a large number of hydroxyl and carboxyl functional groups on the surface of biochar.XRD results showed that the content of quartz in biochar increased with the increase of preparation temperature.（2）The prepared biochar is used to adsorb heavy metal cadmium ions in aqueous solution.The preparation temperature has a significant impact on the adsorption performance of biochar.The isothermal adsorption results of biochar prepared under five temperatures are well fitted with Langmuir model.The biochar prepared under high temperature conditions（600℃and 700℃）has good adsorption performance.The maximum adsorption capacity of biochar B600 prepared under 600℃is 8.62mg·g^-1,It is 3.73 times that of B300(2.31 mg·g^-1)with the lowest adsorption capacity;The adsorption kinetic results are more in line with the quasi second-order kinetic model,indicating that the adsorption of cadmium by biochar is mainly chemical adsorption.The pH of the solution has an obvious effect on the cadmium adsorption capacity of biochar,and the adsorption capacity gradually increases with the increase of the pH of the solution;The removal rate of cadmium increased with the increase of biochar dosage;The coexisting cations in the solution inhibited the adsorption of cadmium,and the adsorption capacity of biochar decreased with the increase of the concentration of coexisting cations.The characterization results of biochar before and after adsorption show that hydroxyl and carboxyl groups are important factors for the adsorption of cadmium ions on biochar,and the formation of CdCO₃ and Cd（OH）₂precipitation may be the main mechanism of adsorption.（3）B600 biochar with the best adsorption performance was added to the soil in a certain proportion to form a biochar/soil mixed system,which was used to adsorb cadmium in the solution,and the adsorption conditions were optimized by response surface method.Select the biochar addition,initial pH of solution and adsorption time that have a great impact on the adsorption capacity and removal efficiency as the three factors of the response surface experiment,determine the three levels of each factor through the single factor experiment,design the experimental matrix by BBD（box Behnken）method,and construct the regression model of biochar adsorption of Cd（II）by using Design-Expert11 according to the experimental results.Through the analysis of variance results of the model,it can be seen that among the three factors,the biggest influence on the removal rate is the adsorption time,followed by the pH of the solution,and finally the amount of biochar added;The interaction between biochar addition and adsorption time is stronger than other interactions.The optimum adsorption conditions optimized by response surface method were as follows:biochar addition 0.02g（2.1%）,adsorption time 217.56min and initial pH 5.93.Under the optimized conditions,the adsorption capacity of cadmium in the solution is 2.75 mg·g^-1,the removal rate is69.97%,and the deviation from the predicted value of the model is 4.7%（<5%）,indicating that the reliability of the model is high.The dynamic adsorption experiment was carried out under the optimized conditions.The addition of biochar to the soil greatly improved its adsorption performance,and the adsorption capacity increased from 0.74 mg·g^-1（single soil）to 4.15 mg·g^-1（mixed system）,about 5.61 times of the original.Combined with Freundlich model,the theoretical adsorption capacity and adsorption contribution rate of biochar and soil in the mixed system were calculated.It was found that the adsorption contribution rate of biochar gradually increased with the increase of the initial concentration of cadmium solution;Comparing the theoretical adsorption capacity with the actual adsorption results,it is found that there is a large deviation between the actual adsorption capacity of the mixed system and the theoretical adsorption capacity,and the deviation gradually increases with the increase of the initial concentration of the solution.The reason may be that the pore structure of biochar in the adsorption process of the mixed system is blocked by soil particles and the influence of organic matter and minerals in the soil on the adsorption,resulting in the reduction of the adsorption capacity of biochar for cadmium.