SYNTHESIS AND CHARACTERIZATION OF ZEOLITE AND ITS APPLICATION IN ADSORPTION OF NICKEL FROM AQUEOUS SOLUTION

ABSTRACT

The synthesis and characterization of zeolite and its application in adsorption of nickel from aqueous solution was investigated. Synthesis of zeolite was performed at 90 ^oC for 8 h. The size of the resulting crystals increased with an increase in the water content of the reaction mixture. The synthesized zeolite was characterized by Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD) techniques. Crystal structure of the product was determined as zeolite X by XRD. The concentration of the nickel ion was determined using atomic absorption spectrophotometer (AA 320 – ON). Removal efficiency of nickel ion was dependent on the pH, contact time, adsorbent dosage and temperature. Batch adsorption studies conducted for the removal of nickel(II) ion at 25, 40 and 50 ^oC respectively showed that nickel ion adsorption increased with increase in temperature. The effect of adsorbent dosage of 0.5-4.5 g showed that there was an increase in the adsorption capacity when the adsorbent dose was increased from 2.0-4.0 g. The pH values were adjusted to 3.2, 4.3, 5.3, 6.7, 7.9 and 9.5 and it was evident from the result that at pH 5.3, uptake capacity of nickel(II) ion onto zeolite X was maximum. The effect of contact time at 10, 20, 30, 45, 60, 75 and 90 min was analysed and it was evident that adsorption of nickel was rapid in the first 40 min followed by a gradual increase until equilibrium was attained. Adsorption data was interpreted in terms of Langmuir and Freundlich isotherms. It was observed that the experimental data fitted better to Langmuir model with a correlation factor (R²) value of 0.993 compared to Freundlich with R² value of 0.980. The kinetics rate were modelled using pseudo-first-order and pseudo-second-order models. The pseudo-second-order model explained the adsorption kinetics most effectively. The result showed that zeolite X was effective in the removal of Ni(II) ion from aqueous solution.

CHAPTER ONE

• Introduction

Zeolites are porous crystalline alumino-silicates of regular skeleton structures formed by alternating silicon-oxygen and aluminum-oxygen tetrahedrons. Although only natural zeolites were initially used, synthetic zeolites, due to their well-tailored and highly-reproducible structures, have been used extensively as ion exchangers, adsorbents, separation materials and catalyst¹.The negative charges in aluminum-oxygen tetrahedron, which are not rigidly fixed to the skeleton of zeolites, are compensated with cations, so they are capable of interchanging. Silicon-oxygen and aluminum-oxygen tetrahedrons in the zeolites of the type A, X and Y form a complex structural unit of cubooctahedron. The combination of such units forms the structure of type A, X and Y [fig 7].^. The difference between them consists in the fact that they are interconnected by means of different number of member rings (i.e., eight member rings (A), twelve member rings (X, Y). The chemical difference of zeolite is defined by the ratio of Si/Al. For zeolite A this values is in the range of 0.95-1.05^1-3. Zeolites A, X and Y are the most important ones to be used in pharmaceutical, petrochemical and detergent industries.

Zeolites with different structure are known to be obtained by synthesis ^2-7. They are either synthesized from alumino-silicate hydrogel or by conversion of clay minerals. The hydrogel can be prepared from different sources of silica and alumina, but the types of starting materials and the method of mixing determine the structure of the resulting gel. Moreover, the nature of the gel influences the rate of the subsequent crystallization, which affects the particle size distribution, and the formation of impurities⁸. The general pathway for zeolite synthesis follows a specific temperature gradient at low temperatures (<60 ^oC) where the sources of aluminum, silicon and water are placed in solution and mixed until a gel is formed⁹.   

Figure 1: Structure of zeolite framework      

1.1 Background of Study