Study On Synthesis Of Zeolites From Bauxite And Adsorptive Removal Of Cadmium Ion From Solution By Synthetic Zeolites

In recent years, the cadmium pollution has become a serious threat to people’s health because of the frequent cadmium pollution accidents. Therefore, exploring a cost-effective material with a high ability to remove cadmium ions from wastewater is an extremely urgent environmental protection task. Synthetic zeolites are recognized to be the most promising materials for the removal of heavy metal ions from wastewater due to their special properties such as high thermal/hydrothermal stability, large surface area, uniform three-dimensional pore structure, high cation exchange capacity, and strong affinity to transition metal cations. The important practical application in wastewater treatment and the high synthesis cost for the synthetic zeolites have incentivized the development of facile methods for synthesizing zeolites from the cost-effective earth-abundant sources.In this study, zeolites are synthesized successfully from the cost-effective low-grade bauxite without any treatment after alkali fusion activation process. The effects on the crystallization of the zeolites were discussed. Various techniques such as X-Ray diffraction, SEM, FTIR, adsorption of nitrogen have been used to characterize the crystal phase, morphology, thermal stability, and specific surface area etc. for the synthesized zeolites. The relationship between the structure properties of zeolites and the adsorption capacity was analyzed. The adsorption isotherms, kinetics, and thermodynamics of adsorption were also studied. All these studies provided some important guidance for the utilization of the low-grade bauxite and the exploiture of low-cost environmental protection material. The main results are summarized as follows:1. Using sodium carbonate as an activator and sodium metaaluminate as an extra aluminum source, phase-pure high-quality zeolite A was synthesized by alkali fusion hydrothermal reaction from the cost-effective low-grade bauxite. All the fused products were used to synthesize zeolite without any futher treatment. The process reduces the costs of synthesis greatly which shows its potential value of industrial applieations. The optimized experimental conditions are:reactant molar ratio1.5Na2O:0.5Al2O3:1SiO2:128H2O, crystallization at90℃for12h. The image of the synthesized zeolite A is a chamfered-edge shape and the mean diameter of the sample is1～2μm. The structure could be retained when the temperature is below700℃. The calcium exchange capacity value is369mg/g which shows its high adsorption capacity. 2. For the first time, Zeolite X and Y were synthesized from the raw low-grade bauxite by a two-step process method:alkali fusion activation with sodium hydroxide and hydrothermal reaction. Sodium silicate solution was added as supplementary silicon source for the synthesis of zeolite X and Y, while directing reagent was introduced into the reaction mixture for the synthesis of zeolite Y. Optimizations of the molar ratios of SiO2/AlO3, Na2O/SiO2, H2O/Na2O, the reaction time, temperature, the composition and amount of directing reagent have been investigated in details.The zeolites of X and Y are octahedral crystals with the average crystal sizes of2to3μm and high thermal stability. The data of the surface area(SBET560m2/g), pore volume(0.441cm3/g), and the calcium exchange capacity value(317mg CaCO3/g zeolite) show the zeolites have the potential value of industrial applications.3. The cadmium ion removal from aqueous solutions using the synthesized zeolite A and X powders were studied. The study of kinetics showed that the pseudo-second-order kinetic adsorption model is suitable for describing the adsorption of Cd2+ion on the zeolite A and X. Both Langmuir and Freundlich models could fit the experimental data well at pH=6for zeolite A, but the Langmuir models fitted better for zeolite X. The maximum adsorption capacitis for zeolite A and X are161.3mg/g and178.6mg/g, respectively. The cadmium ion removal from aqueous solutions increased with the increasing of Cd concentration, pH value, and adsorbent amount used. The studies on thermodynamics show that the adsorption of Cd2+ion on the zeolites is a entropy increment, endothermic, and spontaneous process. Regenerative experiments show that the zeolite could be cyclically used many times. The competitive adsorptions of Zn+, Cd2+and Ni2+ions onto zeolite A were also studied.The isotherms of the Zn+, Cd+and Ni+ions follow the Langmuir model. In single, binary and ternary systems, the adsorption capacity varied in the following order:Cd2+(aq)> Zn2+(aq)> Ni2+(aq), which is accordance with that of the hydration enthalpy and ionic radius of the cations.