Molecular Simulation Of Crystal Configuration Of Kaolinite Minerals And Its Adsorption For Cd²⁺

In view of the increasingly prominent characteristic of heavy metal pollution in water,it is a valuable research subject to find a more economical and rapid method of treatment.Adsorption is the most widely used method at present,and clay minerals can be used as a natural adsorbent because of its unique structure.Kaolinite minerals are widely distributed in the natural world and are typical aluminosilicate layered clay minerals.At present,the studies on the adsorption of heavy metals on kaolinite have been conducted through experimental methods mostly,it has been difficult to reflect the microcosmic adsorption mechanism,however,the method of molecular simulation has gradually gained more agreements due to its unique advantages.Cadmium is a highly toxic and easily carcinogenic heavy metal element and is usually existed in the form of Cd²⁺,it will cause extremely serious damage to the health of organism when it enters the organisms.In order to explore the adsorption mechanism of Cd²⁺on kaolinite minerals in depth,this paper used Materials Studio software and applied quantum mechanics,molecular mechanics,grand canonical Monte Carlo and molecular dynamics simulation methods to do the following work:constructing and discussing the crystal structure of three polytypes of kaolinite minerals—kaolinite,dickite and nacrite;calculating the adsorption of Cd²⁺on?001?surface of three polytypes at different temperatures;in addition,studying the intercalation modification of kaolinite with urea molecules;calculating the adsorption of Cd²⁺on kaolinite-urea intercalation complex;simulating the adsorption and diffusion of the adsorption system in water environment by molecular dynamics method.The results were analyzed by three tools in this paper:particle concentration profile,radial distribution function and mean square displacement?self-diffusion coefficient?.The main conclusions are as follows:?1?The adsorption of Cd²⁺on all three polytypes is physical adsorption,and the difference of adsorption capacity is small,both the average adsorption quantity and the isosteric adsorption heat decrease gradually with the increasing temperature;the adsorption of Cd²⁺on the modified kaolinite-urea intercalation complex is still physical adsorption,both the average adsorption quantity and the isosteric adsorption heatdecrease gradually with the increasing temperature as well,but compared to kaolinite,there is a obvious increase,indicating that intercalation modification can promote the adsorption of Cd²⁺on kaolinite.In addition,with the increase of temperature,the self-diffusion coefficient of Cd²⁺increases gradually at the systems of both kaolinite and the intercalation complex.?2?At the systems where the adsorbents are kaolinite and kaolinite-urea intercalation complex respectively,the distribution positions of Cd²⁺are different:the former is mainly distributed near the two surfaces of kaolinite,while the latter is not only distributed near the two surfaces of kaolinite,but also distributed around the interlaminar urea molecules.?3?In vacuum environment,the hydrogen bonds make the structure of kaolinite-urea intercalation complex very stable,the urea molecules are arranged in the interlayer with a monolayer and biased to the side of the alumina octahedral of kaolinite;in water environment,the water molecules are full of the entire system and the structure of the intercalation complex is destroyed,the interlamellar domain of kaolinite is significantly increased,the distribution of urea molecules is more dispersed and mainly distributed on the side of the silico-oxygen tetrahedron of kaolinite,in addition,the water molecules are arranged on the surface of kaolinite orderly,which makes kaolinite separated from Cd²⁺,resulting in weak adsorption.?4?In different adsorption systems and different environments,the self-diffusion coefficients of the particles are different:in vacuum environment,the self-diffusion coefficient of Cd²⁺at the system ofkaolinite is larger than that of intercalation complex,which is the opposite in water environment,however,the self-diffusion coefficient of Cd²⁺in vacuum environment is larger than that in water environment in the mass;the self-diffusion coefficient of urea molecules in vacuum environment is smaller than that in water environment;the self-diffusion coefficient of water molecules at the system of kaolinite is smaller than that of intercalation complex.