Preparation Of Walnut Charcoal And Its Adsorption Properties For Metal Ions In Wastewater

The adsorption method can remove metal ions effectively in the wastewater with simple operation.Adsorbents are the core of adsorption technology.In this paper,biomass activated carbon was prepared from walnut peel by pyrolysis and carbonization.Ammonium oxalate was introduced as a pore-forming agentand N source to increase the specific surface area and the nitrogen content for activated carbon.The material structure and surface properties of as-made carbons were determined by BET,SEM,TEM and XPS analysis.The static and dynamic adsorption properties of the carbon materials for wastewater containing Zn²⁺,Co²⁺and Ni²⁺were studied.Three kinds of walnut skin charcoal with different final temperatures were prepared by using walnut green skin as carbon source.The results show that the as-made biomass activated carbons have an irregular block structure without correlation between the blocks and the blocks.The surface of the carbon block particles is relatively regular withvery small specific surface area because of no pore and hollow structure.When the pyrolysis temperature was 530℃,the content of C,O and N element of HTP₅₃₀30 was 78.96%,16.05%,and 4.12%,respectively.The content of C=O in the O element is 9.64%,which is 1.96 times than that of commercial activated carbon CAC;the content of-OH is 6.38%,which is 4.59 times than that of CAC;the content of N element is 45 times than that of CAC.The adsorption of metal ions showed that the adsorption capacity of Zn²⁺,Co²⁺and Ni²⁺is high due to the presence of a large number of oxygen-containing and nitrogen-containing functional groups.The saturated adsorption capacity for Zn²⁺,Co²⁺and Ni²⁺was 76.33 mg/g,32.28 mg/g,and 28.68 mg/g,respectively.The adsorption capacity of three metal ions on commercial activated carbon is less than 2mg/g.The kinetic experiments show that the fitting parameter R² of the pseudo-second-kinetic model is higher,and the pseudo-second parameter R² of HTP₅₃₀30 for Zn²⁺reaches 0.9974.The adsorption capacity calculated by the pseudo-secondary model is also closer to the experimental value,so the adsorption process is consistent with the characteristics of chemical adsorption.The adsorption isotherm experiment shows that Langmuir has a higher correlation degree than that of the Freundlich model,and the adsorption of metal ions on the carbon is monolayer.（2）Hollow walnut charcoal was prepared from walnut peel as carbon source by adding ammonium oxalate as pore-forming agent and nitrogen source.The results show that the carbon material is stacked with gaps between the carbon and the carbon.The carbon has a large size hollow structure with medium and large pores.The biomass carbon has a significant improvement compared to the carbon prepared in the previous chapter.Compared with the commercial carbon,the hollow walnut skin carbon has a high content of O and N elements,while C content is low.Among them,the O and N element of HTP₇₃3 is as high as 29.61%,and 5.77%,respectively,while the C element is only 64.61%.These O-and N-containing elements provide a large number of chemical functional groups to improve their adsorption properties for metal ions greatly.Most of the O elements of hollow activated carbon exist in the form of C=O and C-OH without COOH,so the adsorption of metal ions can be improved due to the alkaline characterization.HTP₇₃3 shows the largest adsorption capacity for metal ions,and the saturated adsorption amounts for Zn²⁺,Co²⁺and Ni²⁺were 120.17 mg/g,71.84 mg/g and 72.29 mg/g,respectively.The adsorption isotherms were fitted by Langmuir and Freundlich isotherm adsorption models.The results show that the Langmuir adsorption isotherm model has a better fitting effect with R² value of 0.994,indicating that the process is monolayer adsorption.The adsorption kinetics fitting results show that the adsorption of metal ions on the hollow carbon can be described by pseudo-second-kinetic model with the R² values of 0.998,indicating that the adsorption process is chemical adsorption.（3）The breakthrough curve experiment was carried out by using HTP₅₃₀30 and HTP₇₃3 as the packed column materials.The effects of initial concentration,flow rate,and adsorbent species on the breakthrough curve were studied.The results show that the lower the initial concentration of metal ions,the longer the penetration time of the adsorption column,the smoother the penetration curve,the more the breakthrough point of the adsorption column moves backward,and the saturation rate of the adsorption column slows down.With the flow rate of the solution increase,the breakthrough and depletion point time of the packed column decrease.For the adsorption of Zn²⁺,Co²⁺and Ni²⁺,the adsorption capacity of HTP₇₃3 carbon material is better than that of HTP₅₃₀.In general,according to the calculation method of the average value for breakthrough and exhaustion time,it is concluded that theadsorption capacity of HTP₇₃3 on Zn²⁺is 41.55%higher than that of HTP₅₃₀.For Co²⁺,the adsorption capacity of HTP₇₃3 was increased by 37.94%compared with HTP₅₃₀.For Ni²⁺,the adsorption effect of HTP₇₃3 is 52.93%higher than that of HTP₅₃₀.