Single And Competitive Adsorption Of Lead And Cadmium By Modified Walnut Shell And Study On Cadmium Repair In Soil

The main purpose of this study was to prepare adsorbents containing sulfur-containing functional groups to effectively remove lead and cadmium in single and composite solutions,and to effectively apply to the repair of soil cadmium.The modified walnut shell was prepared by walnut shell.The adsorption properties of lead and cadmium were studied,and the competitive adsorption relationship between lead and cadmium on adsorbents was explored.The effects of pH,adsorbent dosage,contact time,initial concentration and temperature on adsorption were explored.The adsorption properties of walnut shell before and after modification were compared,and the adsorption thermodynamics,adsorption kinetics and adsorption isotherm model was investigated to evaluate adsorption process.The adsorption mechanism of modified walnut shells was analyzed by scanning electron microscopy(SEM),specific surface area and pore analysis(BET),Fourier transform infrared spectroscopy(FTIR)and X-ray photoelectron spectroscopy(XPS).The modified walnut shell was applied to the repair of cadmium-contaminated soil,and its repairing characteristics to cadmiumcontaminated soil were discussed.The main contents and results of this research were as follows:(1)The environmentally friendly and low-cost walnut shell was used as a precursor.The sulfur-containing functional group was grafted to the walnut shell with xanthate to prepare the magnesium-sulfur-containing functional group grafted walnut shell(SWM).The material was well-developed pore structure with large specific surface area and rich functional groups.The results showed that the adsorption performance of the modified walnut shell was significantly better than that of the unmodified walnut shell due to the complexation and ion exchange of the sulfur-containing functional group.In the single metal solution system,SWM has high adsorption efficiency for Pb(II)and Cd(II).Besides,SWM has a wide range of pH and initial solution concentration for cadmium.The adsorption kinetics and isothermal adsorption model are established.The adsorption behavior of the adsorbent on Pb(II)and Cd(II)accorded with the quasi-secondary reaction kinetics and the Langmuir isotherm adsorption model.That is,the adsorption of Pb(II)and Cd(II)by SWM occurs in a homogeneous manner,and the adsorption was chemical adsorption on a single layer interface.In addition,the adsorption process was spontaneous and exothermic.(2)The optimal adsorption conditions of SWM for Pb(II): 298 K,when the pH was 5.0,the dosage was 8 g/L,and the initial concentration of the solution was 100 mg/L,the mixture was shaken at 180 rpm for 2 h.The equilibrium adsorption capacity was 12.453 mg/g,the maximum adsorption capacity was 25.51 mg/g,and the adsorption rate was up to 99.71%.The optimal adsorption condition of SWM for Cd(II): 298 K,when the pH was 7.0,the dosage was When 15 g/L,and the initial concentration was 150 mg/L,the mixture was shaken at 180 rpm for 2 h.The equilibrium adsorption amount was 9.93mg/g,the maximum adsorption amount was 17.79mg/g,and the adsorption rate was up to 98.77%.(3)In the mixed solution of dual system(cadmium and lead),the adsorption process was more conducive to the adsorption of lead.This may be due to the fact that lead and cadmium was removed by complexing sulfur-containing groups for soft ligand groups.According to the soft and hard acid-base theory,the absolute hardness parameter of Pb(II)is 8.5,and the absolute hardness parameter of Cd(II)is 10.3.Therefore,the sulfur-containing functional group has a stronger affinity with lead and was more likely to form a stable complex with lead.(4)After applying SWM in cadmium-contaminated soil,the content of exchangeable cadmium in the soil decreased significantly,and the residual cadmium content increased significantly.It indicated that SWM reduced the migration and bioavailability of soil cadmium,resulting in reduced toxicity.For low cadmium contaminated soil,the passivation effect was best when the SWM addition amount was 1%;for the high cadmium contaminated soil,the passivation effect was best when the SWM addition amount is 10%.Among all contaminated soils,soils with a pollution degree of 10 mg/kg had the best passivation effect.