Thesis 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.

Thesis Overview

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:

Mx/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

The term ‘zeolite’ was first mentioned by A. F. Cronstedt a Swedish mineralogist in 1756 as a name of an aluminosilicate mineral (stilbite) that seemed to boil when heated5. Cronstedt gave these minerals an aptly descriptive name: “zeolite”, a term that etymologically comes from classic Greek “ξειv” (zeo), which means to boil, and “λιϑoς” (lithos) which means stone. Therefore, these materials are literarily called “boiling stones”6.

There are about 40 natural zeolites which have been identified during the past 200 years and more than 150 zeolites have been synthesized. The most common of natural zeolite are analcime, chabazite, clinoptilolite, erionite, mordenite and phillipsite7 while as for synthetic zeolites; the most common are zeolites A, X, Y, L and ZSM-5. Both natural and synthetic zeolites are used commercially because of their unique adsorption, ion-exchange, molecular sieve and catalytic properties.

The naturally occurring zeolites are formed as a result of the chemical reaction between volcanic glass and saline water. The temperature favoring the natural reaction ranges between 27- 55°C with pH between 9 and 10. However, nature requires 50 to 50,000 years to complete the reaction with rarely phase-pure state of zeolite. These types of zeolites are contaminated to varying degrees by other minerals such as Fe+2, quartz, SO4–, other zeolites and amorphous glass8.

Synthetic zeolites on the other hand, hold some advantages over their natural analogs. The synthetics can be manufactured in a uniform phase-pure state. It is also possible to manufacture desirable structure which does not appear in nature such as zeolite A9. Since the principal raw materials used to manufacture zeolite are silica and alumina, which are among the abundant mineral components on earth, the potential to supply zeolite is virtually unlimited.

Recently, through the advancement of modern science and technology, zeolites can contribute to a cleaner, safer environment in a great numbers of ways: in powder detergents, zeolites have replaced the harmful phosphate builder, which have been banned in many countries due to the water pollution risks, in petroleum and hydrocarbon industry, the chemical process can be more efficient with zeolite acting as the catalyst, thus saving the energy and indirectly reduces pollution10. The processes can be carried out in a fewer step and minimizing unnecessary waste and by-products11. Furthermore, zeolites can also act as solid acids which could reduce the need for corrosive liquid acids and as redox catalyst and sorbents where they can remove atmospheric pollutants such as engine exhaust gases and ozone depleting CFCs 12. Zeolite can also be used to separate harmful organics from water 13 and remove heavy metal ions including those produced by nuclear fission from water 14.

1.2   STATEMENT OFTHE PROBLEM

The need for cleaner fuels and industrial processes that would minimize the consumption of energy, production of waste or the use of corrosive, explosive, volatile and non-biodegradable materials has been met by a range of nanoporous materials15. Synthetic zeolites has advantages over the use of natural zeolites in heavy metal adsorption because natural zeolites are contaminated to varying degrees by other minerals, other zeolites16 and amorphous glass; also synthetic zeolites can be prepared in uniform phase-pure state17. It is with this interest that prompted the synthesis of zeolites and its application in the removal of heavy metals such as chromium from aqueous solutions18.

1.3       OBJECTIVES OF THE STUDY

1. To synthesize zeolites of varying particle sizes.
2. To characterize the synthesized zeolites particles using Scanning Electron Microscopy and X-Ray Diffraction.

• To find out the effect of adsorbent dosage and initial concentration on adsorption rate of chromium onto zeolite.

1. To determine the amount of heavy metal (chromium) that can be adsorbed by these different particle sized zeolites.

1.4       JUSTIFICATION OF THE STUDY

The International Agency for Research on Cancer (IARC) has determined that chromium compounds are carcinogenic to humans20. The most common health challenge of exposure to chromium involves the respiratory track; these health effects include irritation of the lining of the nose, runny nose, asthma, cough, wheezing, sperm damage and damage to the male reproductive system.   The Environmental Protection Agency (EPA) has established a maximum contaminant level of 0.1mg/l for total chromium in drinking water21. The wide range of applications of zeolites especially in heavy metal removal and the need to know the parameters that enhances the rate of adsorption of these zeolites prompted this research work.