Preparation Of Thiol-Modified Montmorillonite And Mechanism Study Of Cadmium(â…¡) Adsorption/Desorption

Cadmium is a very rare and widely distributed element, the background value of cadmium in common soil is generally less than0.2μg/g. Ionic state of cadmium enters human body through the food chain, it poses a great threat to ecological environment and human when the concentration exceeds base standard. At present, in many areas the cadmium content seriously exceeded the environmental quality standard value, and it affected the normal growth of crops. For example, in Yangtze River Delta and Dongting Lake region in Hunan Province the cadmium content in soil and rice grain exceed standard, it is harmful to human health. Our country is rich in montmorillonite. Because of good ion exchange performance and large surface area, montmorillonite is used extensively in cadmium pollution control. But natural montmorillonite has some certain limitations in the practical application. In order to improve the adsorption capacity of heavy metals on montmorillonite, now there were many studies on modification treatment of montmorillonite. Thiol group has good adhesion ability, it can immobilize heavy metals, and the heavy metals were difficult to be eluted. At present, some studies reported on modification of montmorillonite by thiol group, but these modification methods were cumbersome. In this study, the thiol-modifided montmorillonite was prepared by solution method with3-mercaptopropyltrimethoxysilane. The adsorption conditions of cadmium on thiol-modified montmorillonite were studied, and the adsorption mechanism was discussed. The thiol-modified montmorillonite was applied to cadmium contaminated soil, and obtained a quite good result. The main research results were shown below:1. The thiol-modifided montmorillonite was prepared by simple solution method with3-mercaptopropyltrimethoxysilane. The characteristic results of FT-IR, XRD and thermal analysis indicated the successful tethering of thiol group to montmorillonite. The optimum preparation conditions of thiol-modified montmorillonite were:the temperature was room temperature, reaction time was6h, the solid-liquid ratio was1:100, and the mass ratio of modifier to acid montmorillonite was1:1. The relationship among the solid-liquid ratio, the amount of acid montmorillonite and modifier had a great effect on preparation of thiol-modified montmorillonite.2. The experimental results of cadmium adsorption on thiol-modified montmorillonite and desorption by simulant acid rain showed that the equilibrium conditions of cadmium adsorption were:the pH was6, the concentration of KNO3was0.1mol/L, shaking time was60min, and the adsorption capacity of cadmium achieved maximum when solid-liquid ratio was1:200; the saturated adsorption capacity of cadmium on thiol-modifid montmorillonite was42.65mg/g under equilibrium conditions, it was39times higher than natural montmorillonite. The equilibrium conditions of cadmium desorption by simulant acid rain were:the pH of simulant acid rain was3.5, ionic strength was0.3mmol/L, shaking time was60min, the desorption capacity of cadmium by simulant acid rain was far less than natural montmorillonite.3. This paper studied kinetics and thermodynamics of cadmium adsorption on thiol-modified montmorillonite and desorption by simulant acid rain under equilibrium condition. The experimental results showed that kinetics of cadmium adsorption and desorption were fitted well with Lagergren-second-order kinetic model, it indicated that process of adsorption and desorption were chemical reactions. Thermodynamics of cadmium adsorption was fitted to Langmuir adsorption isotherm. The thermodynamic parameters (AH, AG and AS) showed that the adsorption of cadmium on thiol-modified montmorillonite was a spontaneous endothermic process.4. The experimental results of cadmium adsorption on thiol-modified montmorillonite and cadmium desorption by simulant rain, kinetics and thermodynamics on thiol-modified montmorillonite were indicated that the adsorption of cadmium on thiol-modified montmorillonite was a chemical reaction process, the adsorption process was the result of a combination of electrostatic adsorption, ion-exchange, and coordination reaction (hydroxy and thiol coordination), the forces of four reaction increased in the order of electrostatic adsorption, ion-exchange, hydroxyl coordination and thiol coordination.5. Natural montmorillonite and thiol-modified montmorillonite were used to the experiments of soil pollution remediation respectively after soil was cultivated with cadmium and lead. The concentrations of available cadmium increased in the order of using thiol-modified montmorillonite, using natural montmorillonite, and without montmorillonite. The experimental results indicated that thiol-modified montmorillonite had a good adsorption effect on cadmium, and it might be used to immobilization and remediation of cadmium pollution.The results of these studies provided the treatment of soil cadmium pollution with the immobilization and remediation materials, and they could be used as reference to cadmium pollution control in water and air, so they had important environmental significance and social values.