FABRICATION AND CHARACTERIZATION OF ZEOLITES AND ITS APPLICATION IN HEAVY METAL CAPTURE

                                                            ABSTRACT

Zeolite crystals were synthesized by mixing sodium silicate and sodium aluminate to obtain aluminosilicate gel which was further treated hydrothermally to obtain the final product. The zeolite crystals were characterized by x-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD result identified the synthesized crystals as zeolite z; additional evidence was provided by the SEM images which showed that the zeolite crystals were disc-shaped and the particle sizes ranged between 13.4 – 53.6 µm. batch adsorption studies using the synthetic zeolite showed efficient removal of chromium (III) ion from aqueous solution. The atomic absorption spectroscopy (AAS) result which gave the final metal ion concentration indicated that the rate of adsorption increased with increase in the mass of adsorbent (zeolite) and decrease in particle size of the zeolite crystals. In the course of treatment, different quantities of zeolite ranging from 0.5 – 2.5 g were used; also different concentrations of Cr (III) ion (10, 15, 20, 25, 30 ppm) were used to determine the extent of adsorption. The percentage adsorption increased from 55.75 – 96.6 % and decreased from 98 -88.3 % respectively in each case. As the pH values were adjusted between 1 – 11, percentage adsorption increased from 55 – 97 % with a sharp increase at pH 7. While in the case of the zeolite samples with different particle sizes, the percentage adsorption reduced from 98.9 -70 % as particle sizes increased from 13.4 – 53.6 µm. The high percentage adsorption of the zeolite samples suggests that zeolites are good adsorbents for the removal of Cr from aqueous solutions. Also the batch experiment conducted showed that the adsorption pattern followed the Langmuir and Freundlich isotherm models with correlation factors (R²) values of 0.997 and 0.963 respectively.

CHAPTER ONE

INTRODUCTION

Nanoporous materials consist of a regular organic or inorganic framework supporting a regular, porous structure. The pore size regime for nanoporous materials ranges from 1nm region to 1000 nm. Most nanoporous materials can be classified as bulk materials or membranes. Activated carbon and zeolites are two examples of bulk nanoporous materials while cell membranes can be thought of as nanoporous membranes¹.  According to IUPAC, pore sizes can be classified thus:

• Microporous materials: 0-2 nm pores
• Mesoporous materials: 2-50 nm pores
• Macroporous materials: 50 nm pores and above².

Zeolites are crystalline hydrated aluminosilicates containing pores and cavities of molecular dimensions, their structures are formed by regular and uniform channels and cavities creating a nanoscale framework. Zeolites can also be defined as crystalline hydrated tectoaluminosilicates of alkali and alkali-earth cations with fully cross-linked open-framework structures made up by sharing TO₄ tetrahedral, (where T = Si or Al). Basically, zeolite frameworks consist of silicon and aluminium atoms and oxygen in the crystal lattice. The chemical formula of aluminosilicates, zeolites with cations, is:

M_x/n [(AlO₂) _x (SiO₂) _y] wH₂O.

The formula in parentheses represents the framework composition.

M is the non-framework cation of valence n.

w is the number of water molecules present in a unit cell and

x the number of Al atoms per unit cell, usually 1≤ y/x ≤5.

The value of the variables x and y depends on the structure.

The total number of tetrahedra in a unit cell is the sum (x+y).

The exact Si/Al ratio depends on the crystallite size and the porosity³. Zeolites possess unique surface, structural and bulk properties that make them important in various fields such as ion exchange, separation, purification, catalysis e.t.c. This has resulted to their widespread applications as dehydrating agents, selective adsorbents, catalysts and in selectivity of a huge number of different reactions. Zeolites can also be used for drying refrigerants, removal of atmospheric pollutants such as SO₂, separation of paraffin hydrocarbons, recovery of radioactive ions from waste solutions, catalysis of hydrocarbon reactions and curing of plastics and rubber⁴.

1.1  BACKGROUND OF THE STUDY