| This study investigated the adsorption of Ni(Ⅱ)and Cr(Ⅵ)heavy metal ions by PBNC,a material of agricultural and forestry waste walnut shell treated and magnetically modified,in solution,and analyzed its physical and chemical properties and adsorption mechanism of heavy metal ions.At the same time,scanning electron microscopy(SEM),X-ray diffraction(XRD),Fourier infrared spectroscopy(FTIR)Specific surface area and pore analysis(BET)and X-ray photoelectron spectroscopy(XPS)were used to analyze the basic properties of adsorption materials before and after adsorption;The relationship between the competitive adsorption of Ni(Ⅱ)and Cr(VI)by PBNC was discussed,the affinity and adsorption capacity of adsorption materials for different kinds of metals were explored,and the adsorption mechanism of PBNC for different metal ions was explored using characterization methods.The main research contents and results are as follows:(1)The walnut shell,a by-product of agriculture,was used as raw material to pyrolyze into biochar at high temperature,and then grafted with chitosan(CTS)and modified with zero-valent iron nanoparticles to prepare modified biochar composite(PBNC).The characterization results showed that the modified biochar material PBNC was rich in magnetism and functional groups.The crystals of CTS and Fe were successfully expressed in the peak pattern of PBNC,with high crystallinity and obvious peak expression,reflecting the excellent chemical stability of PBNC.Through the analysis of the adsorption results of PBNC and BN,it is concluded that the removal rates of Ni(Ⅱ)and Cr(Ⅵ)by PBNC are higher than that by BN,indicating that the modification is beneficial to adsorption;(2)The optimal adsorption conditions for Ni(Ⅱ)by PBNC were as follows:at 298 K,the p H of the mixed solution was 5.0,the amount of adsorption material added was 0.4 g/L,and the initial concentration of metal ions was 50 mg/L,the equilibrium adsorption amount was110.86 mg/g,and the adsorption rate can reach 93.05%;The optimal adsorption conditions for Cr(Ⅵ)are as follows:At a temperature of 298 K,a p H of 2.0 for the mixed solution,an amount of 1.2 g/L of adsorption material,and an initial concentration of 50 mg/L of metal ions for 2.5h of shock adsorption,the equilibrium adsorption amount is 106.25 mg/g,and the adsorption rate can reach 90.03%;(3)The adsorption process of Ni(Ⅱ)and Cr(Ⅵ)by PBNC conforms to the quasi-second-order reaction kinetics and Langmuir isotherm adsorption model,that was the adsorption of Ni(Ⅱ)and Cr(Ⅵ)by PBNC occurs in a monolayer adsorption on a uniform surface and chemical adsorption;(4)In the competitive adsorption process of two kinds of Ni(Ⅱ)and Cr(Ⅵ)coexisting,PBNC showed strong selective adsorption,with the order of Cr(Ⅵ)>Ni(Ⅱ).The main reason is that it was related to the characteristics of HMs itself,such as flexibility,stability constant of associated metal hydroxide,hydration half-diameter,hydrolysis constant and electronegativity.The K_d value of PBNC for two metal ions Ni(Ⅱ)and Cr(Ⅵ)(5.43×10~4 and 2.04×10~5m L/g,respectively),indicating that PBNC has higher adsorption selectivity for Cr(Ⅵ),indicating that PBNC had high affinity and selectivity for metal ions with strong oxidation.(5)The main adsorption mechanism of PBNC for the coexistence of heavy metal ions with different properties was preliminarily explored.In the solution,the heavy metal ions first changed into the particulate compound state through the chemical reaction of the strong reducing nanometer zero-valent iron,and then carried out surface complexation with the inner spherical complexes of hydroxyl,carboxyl and amino groups.More importantly,the adsorption mechanism of Cr(Ⅵ)by PBNC was found through analysis,which was to reduce Cr(Ⅵ)through reduction reaction,and generate part of Cr(Ⅲ)to react with functional groups and part of surface precipitation.Therefore,the adsorption mode of Cr(Ⅵ)by PBNC material can provide reference for the subsequent research on the degradation of Cr(Ⅵ).(6)The desorption performance of PBNC for heavy metal ions was discussed.The results show that PBNC can be used as a potential adsorption material to realize the adsorption and recovery of heavy metal ions in industrial wastewater conveniently and efficiently. |
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