Study On Adsorption Mechanism Of Cadmium Ion By Shell Powder

Since the beginning of the 20 th century,the production and use of cadmium have increased year by year,and its compounds are widely used in the manufacture of batteries,alloys,pigments and stabilizers for electroplating and plastics.Cadmium is the main heavy metal pollutant in the environment.Excessive cadmium content in water not only seriously endangers the environment,but also poses a great threat to human health.Shell resources in China are abundant in output and cheap in price.If they can be developed into a new kind of clean and environment-friendly adsorption material,they can also solve the problem of disposal of a large number of waste shells piled on the beach,providing valuable theoretical and practical basis for the realization of the resource utilization of shells.In this paper,the mechanism of cadmium adsorption by natural and thermal modified razor clam shell powder was studied.Its main conclusions are as follows:Under the condition of the solution of the initial density must,shell powder on the adsorption of Cd²⁺ rate decreases gradually with the increase of time,and at the beginning of the adsorption phase?0 2 h?,shell powder to a rapid increase in the adsorption of Cd²⁺ as the reaction progresses,in the middle of the adsorption?2 24 h?,shell powder adsorption rate gradually slow down,finally to reach adsorption equilibrium.The number of active adsorption sites on the surface of shell powder is an important factor of its adsorption rate.Under the same adsorption time,the higher the initial concentration of solution,the higher the adsorption capacity of shell powder on Cd²⁺.Adsorption capacity is much higher than K700 K0,when the temperature is 25?,the adsorption of oscillation time in 48 h,the maximum number of equilibrium adsorption K0 is 471.32 mg/g.By comparing the parameters of the two kinds of shell powders' quasi-first-order and quasi-second-order kinetic models for Cd²⁺ adsorption and the inter-particle diffusion model,it can be found that for K0,the quasi-second-order kinetic adsorption fitting model has a higher correlation coefficient,while for K700,the two kinds of kinetic adsorption models can well describe the adsorption process.It can be speculated that the adsorption of Cd²⁺ by two kinds of shell powders is quasi-secondary kinetic adsorption,R2 both reaching 0.99,and the adsorption rate of Cd²⁺ by shell powders may be controlled by chemical adsorption.According to the analysis of the relevant parameters of the particle internal diffusion model,it can be concluded that the adsorption process is relatively complex,and there are several speed control steps in the process in addition to the particle internal diffusion.For K0,the Langmuir isothermal model can better fit the adsorption process,indicating that the adsorption of Cd²⁺ by K0 may be monolayer.For K700,both Langmuir and Freundich isothermal models can well fit the adsorption process,and the adsorption of Cd²⁺ by K700 may be multi-layer adsorption with uneven surface.By Gibbs thermodynamic fitting equation parameter calculation,it can be found that G is negative,which indicates that the adsorption process of shell powder on Cd²⁺ is endothermic and can be spontaneous.The value of H was between 7.7304 and 26.5782,indicating that the adsorption process of shell powder on Cd²⁺ was co-existed between physical adsorption and chemical adsorption.All the S0 values were positive and decreased with the increase of the initial concentration of the solution,indicating that the dispersion degree of the system increased after the adsorption reaction,and the adsorption process of shell powder on Cd²⁺ was the process of the system's degree of freedom increased.High temperature roasting can destroy aragonite crystal structure and greatly reduce the adsorption sites provided by shell powder.At the same time,the destruction of porous structure and cuticle layer in shell powder and the lattice dissolution lead to the decrease of the adsorption performance of K0 on Cd²⁺.Soluble organic matter and high temperature will cause the Ca CO₃ thermal decomposition of insoluble organic matter degradation also gradually,by bonding Ca²⁺ gradually become free,organic matter type aragonite crystal structure gradually transformed into calcite crystal structure,the porosity decreases,and also hindered Cd²⁺ diffusion within the shell powder particles,enough to type exposition stone crystal structure is the key to the shell powder adsorption of Cd²⁺ in water.