Preparation Of Biochar Materials And Its Removal And Remediation Of Cadmium In Water And Soil

In this paper,walnut shell biochar WSB-300,WSB-400,WSB-500,WSB-600and WSB-700 were prepared from walnut shell at pyrolysis temperatures of 300℃,400℃,500℃,600℃and 700℃,respectively.The characteristics of biochar prepared at different pyrolysis temperatures were tested and analyzed by means of characterization,and it was first used to remove cadmium in water.The kinetic model and isothermal adsorption model were used to fit the test results,and the removal mechanism of cadmium in water by biochar was analyzed by FTIR,XRD and SEM.The effect of walnut shell biochar on soil p H and cadmium morphology was studied through soil culture experiments,and the biochar with optimal repair ability was selected for modification research.The walnut shell biochar prepared at 600℃was selected as the parent material,and the magnetic amino walnut shell biochar（CMWSB）was prepared by loading Fe₃O₄ particles and cross-linking chitosan.XRD,SEM,FTIR,XPS and other analytical techniques proved the successful modification of biochar.The remediation effect of CMWSB on cadmium in water and soil was studied.It provides a theoretical basis for cadmium remediation in water and soil.The main findings of this paper are as follows:（1）All walnut shell biochar have good pore structure.As the temperature increased,the specific surface area increased,the pore diameter became smaller,and the pore volume increased.When the temperature rose to 600℃,the surface wall of biochar became thinner and many small holes appeared on the wall.However,when the temperature rose to 700℃,the surface wall structure of biochar collapsed and broke obviously.The walnut shell biochar mainly contains C,H,O and other elements.With the increased of temperature,H/C,O/C and（O+N）/C decreased accordingly.The number of aliphatic functional groups on the surface of biochar gradually decreased with increasing temperature,and phenolic hydroxyl groups and carbonyl carbons were gradually converted into aromatic bridge carbons.Among them,-OH of phenols and alcohols,C=C of olefins,and C-H of aromatic rings were the main functional groups on walnut shell biochar.Compared with walnut shell biochar prepared at other temperatures,WSB-600 has the largest specific surface area,thinner carbon skeleton,larger pore density and richer functional groups,which may be more suitable for the remediation of cadmium in water and soil.（2）The preparation temperature of walnut shell biochar has a great influence on the Cd（II）adsorption capacity,and WSB-600 has the best adsorption performance on Cd（II）.The adsorption of Cd（II）by WSB-600 presented a phenomenon of"first fast,then slow".The dynamic adsorption and isothermal adsorption processes of Cd（II）by WSB-600 could be well fitted by the PF-second model and Langmuir model respectively.When p H=5.5,dosage=12 g/L and initial ion concentration=60 mg·l^-1,the Langmuir model estimated that the adsorption capacity of WSB-600 for Cd（II）could reach 6.327 mg·g^-1.The adsorption mechanism of WSB for Cd（II）was mainly complexation,cation-πaction,and precipitation;among them,the cation-πaction and precipitation were the main mechanisms of cadmium adsorption by high temperature biochar WSB-600,while complexation was the main mechanism of cadmium removal by low temperature biochar WSB-400.（3）After the walnut shell biochar was applied to the cadmium-polluted soil and cultured for 56 days,the p H value of the soil was effectively increased,among which WSB-600 increased the most;at the same time,it also effectively transformed the extractable and reducible Cd of weak acid into the oxidizable and residual Cd.mong them,WSB-400,WSB-500 and WSB-600 reduced the extractable Cd of weak acid by17.02%,20.20%and 24.53%respectively,while the residue Cd increased by 66.03%,71.43%and 89.21%.（4）The adsorption of CMWSB to Cd（II）could reach equilibrium within 120min,and the dynamic adsorption and isothermal adsorption processes of CMWSB to Cd（II）could be well fitted by the PF-second model and Langmuir model,respectively.When p H=6,dosage=10 g/L,and initial ion concentration=60 mg·l^-1,the Langmuir model estimated that the adsorption capacity of CMWSB to Cd（II）could reach 22.73mg·g^-1,which was much higher than the original walnut shell biochar.With 0.1mol·l^-1 HCl as the desorption agent,the adsorption rate of Cd（II）to CMWSB remained above 90%after 5 times of adsorption-desorption cycles.It indicated that CMWSB has good reusability.（5）After CMWSB was applied to cadmium-contaminated soil and stabilized after 56 days of culture,it effectively increased the soil p H value,among which 10%increased the maximum.At the same time,it also effectively transformed the weak acid extractable Cd and reducible Cd into oxidizable Cd and residue Cd in the soil,among which 3%applied amount had the best repair effect.Compared with the foregoing,the stabilization effect of the modified magnetic amino walnut shell biochar on weakly acid-extractable Cd in soil was increased by up to 53.5%compared with the original biochar.