| The objective of this study was to investigate the physicochemical properties of biochar at different pyrolysis temperatures and adsorption characteristic mechanism of Zn2+?Cd2+ on biochar.Crayfish shell biochar were prepared at 300?400?500?600 ?by limited oxygen slow pyrolysis(tag LS300?LS400?LS500?LS600).The differences in basic physical and chemical properties of crayfish shell biochar at different pyrolysis temperatures were analyzed,and the lobster shell biochar was characterized by SEM-EDS?FTIR and XRD to explore the differences in the apparent structures of LS300,LS400,LS500 and LS600.The original biochar were characterized by using scanning electron microcopy with enery spectrum analysis(SEM-EDS),fourier trandsfonn infrared spectroscopy(FTIR)and X-ray diffraxtion(XRD).and the differences in the apparent structure and composition of biochar at different pyrolysis temperatures were studied.The adsorption properties of Zn2+?Cd2+ on crayfish biochar prepared at different pyrolysis temperatures were studied by batch adsorption experiments,to analyze the kinetic and isothermal adsorption characteristic of Zn2+and Cd2+,and to explore the factors affecting the adsorption of Zn2+ and Cd2+ by biochhar.The characterization of Zn2+ or Cd2+-loaded biochar was characterized and analyzed to explore the possible adsorption mechanism of crayfish shell biochar on Zn2+?Cd2+.Then,the biochar was demineralized to obtain the demineralized lobster shell biochar,and the contribution ratio of different adsorption mechanisms to the total adsorption was analyzed quantitatively in combination with adsorption experiments of heavy metals by demineralized biochar.The results of this paper can provide theoretical support for the reuse of crayfish shell waste resources and the application of biochar in the treatment of heavy metals in water environment.The main results are as follows:(1)The physical and chemical properties and surface structure of LS300,LS400,LS500 and LS600 were analyzed.The results showed that,in terms of physical and chemical properties,with the increase of pyrolysis temperature,the yield,volatile content and element composition of lobster shell biochar decreased,while ash content and pH value increased.In terms of apparent structure,LS600 is more complete than LS300 in terms of pore structure,pore structure,specific surface area,and more adsorption sites can be provided by LS600.According to FTIR analysis,it was found that the main functional groups of lobster shell biochar were-OH,C-H,C=C,C=O and C-O.XRD analysis showed that the main mineral composition of crayfish shell biochar was CaCO3.Therefore,from the comparison of physical and chemical properties and apparent structure,it can be concluded that the specific surface area,aromatization degree,ash content and adsorption site of LS600 are the most,so it can be speculated that its adsorption effect on heavy metals is the best.(2)The adsorption of Zn2+ by four kinds of lobster shell biochar was studied.The results shows that the adsorption capacity increased with time,solution concentration and pH.LS600 reached adsorption equilibrium at 7 h,and the other biochar reached equilibrium at 24 h.The kinetic fitting equations of the four biochar species are more suitable for pseudo-second-order-kinetic model.The adsorption rate is mainly dominated by the chemical adsorption mechanism,and the adsorption process includes external liquid film diffusion,surface adsorption and particle internal diffusion.The adsorption isotherm of LS600 is more consistent with the Langmuir model.The isotherm of LS300,LS400 and LS500 is more suitable for Freundlich model.The adsorption capacity of LS600 on Zn2+ is the largest,and the adsorption capacity can reach 426.5 mg·g-1.Therefore,in the process of adsorption of Zn2+,the adsorption time,the initial concentration of solution and the initial of pH of lobster shell biochar all affected the adsorption of Zn2+,and the adsorption rate of LS600 on Zn2+ was the fastest and the adsorption effect was the best among the four kinds of biochar.(3)The adsorption of Cd2+ by four kinds of lobster shell biochar was studied.The results shows that the adsorption capacity of Cd2+ by LS300,LS400,LS500,LS600 increase with the increased of time,solution concentration and pH.The adsorption kinetics fitting equations of the four biochar species are more suitable for pseudo-first-order-kinetic model.The adsorption rate is mainly affected by the diffusion step.The adsorption isotherm of LS600 is more consistent with the Langmuir model.The isotherm of LS300,LS400 and LS500 is more suitable for Freundlich model.The adsorption capacity of LS600 on Cd2+ is the largest,and the adsorption capacity can reach 249.54 mg·g-1.Therefore,in the process of adsorption of Cd2+,the adsorption time,the initial concentration of solution and the initial of pH of the four biochar also affected the adsorption of Cd2+.and the adsorption rate of LS300 on Cd2+ was the faster,while the adsorption effect of LS600 on Cd2+ was the best.(4)The biochar loaded Zn2-+ or Cd2+ was characterized respectively.The results showed that the adsorption mechanisms of LS300 and LS600 on heavy metals Zn2+ and Cd2+ were mainly four types:(1)cation exchange effect(2)mineral precipitation effect(3)complexation of oxygen-containing functional groups(4)?electronic coordination.In addition,the adsorption mechanism contribution ratio of LS300 and LS600 to Zn2+ and Cd2+ was quantitatively analyzed through the demorerization of lobster shell biochar.The results showed that the contribution of cation exchange in the adsorption mechanism of Zn2+ by LS300 and LS600 was 73.78%and 58.04%respectively.In the adsorption mechanism of Cd2+ by LS300,the contribution ratio of cation exchange was 41.55%,while the contribution ratio of mineral precipitation in the adsorption mechanism of Cd2+ by LS600 was 38.83%.Therefore,the adsorption of Zn2+ and Cd2+ by LS300 is mainly cationic exchange,while the adsorption of Zn2+ by LS600 is cationic exchange,and the adsorption of Cd2+ is mainly mineral precipitation. |
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