ABSTRACT
The removal of Pb(II) ions from aqueous model solution using zeolite has been investigated under different operational parameters like heavy metal ion concentration, adsorbent amount and particle size. The zeolite used was synthesized and characterized using SEM and XRD analysis. The equilibrium adsorption capacity of zeolite used for lead removal were measured and the experimental data analyzed by means of Freundlich and Langmuir isotherm
models. The adsorption efficiency of Zeolite in removing Pb2+ ions at room
temperature and 60 minute agitation time at pH<10 was 98%. The results also show that the adsorbent with the lowest particle size of 53.6µm had the highest adsorption efficiency(98.33%) The concentration of metal ions were measured by Atomic Absorption Spectroscopy (AAS). Overall, the results showed that synthetic zeolite could be considered as a potential adsorbent for lead removal from aqueous solutions.
CHAPTER ONE INTRODUCTION
In developing countries, rapid growth of urbanization and industrialization has generated large volume of waste containing toxic heavy metals. Heavy metal contamination exists in aqueous waste water streams of many industries such as metal plating facilities, mining operations, tanneries etc1. Environmental pollution due to these toxic metals have been of major concern to environmental engineers; the ions from these heavy metals cause damage to humans e.g. cadmium poisoning causes acute chronic disorders such as renal damage and hypertension, problem in Haemoglobin synthesis, kidney, gastrointestinal tract, joints and reproductive disorders. Acute or chronic dosage results in damage of the nervous system2. Within the body, lead is absorbed and stored in the bones, blood, and tissues. It does not stay there permanently, rather it is stored there as a source of continual internal exposure 3. As time goes by, the bones demineralize and the internal exposures may increase as a result of larger releases of lead from the bone tissue. There is also concern that lead may mobilize from the bone among women undergoing menopause4. Post menopausal women have been found to have higher
blood lead levels than pre-menopausal women5.
Lead poisoning occurs if a person is exposed to very high levels of lead over a short period of time. When this happens, a person may feel abdominal pain,
constipated, tired, headachy, irritable, loss of appetite, memory loss, pain or tingling in the hands and/or feet and weak.
Generally, lend affects children more that it does adults. Children tend to show signs of sever lead toxicity at lower levels than adults. Neurological effects and mental retardation have also occurred in children whose parents may have job- related lead exposure6. The health effects from prolonged exposure to lead included abdominal pain, depression, forgetfulness among others. Also, the Department of Health and Human Services (DHHS), Environmental Protection Agency (EPA), and the International Agency for Research on cancer (IARC) have determined that lead is probably cancer-causing in human7.
Exposure to chromium results in asthma, chronic bronchitis, chronic
irritation, chronic pharyngitis, chronic rhinitis, congestion and hyperemia, polyps of the upper respiratory tract, tracheobronchitis, and ulceration of the nasal mucosa with possible septal perforation though zinc is considered to be relatively nontoxic, particularly if taken only. However, manifestations of overt toxicity symptoms (nausea, vomiting, epigastric pain, lethargy and fatique) will occur with extremely high intakes 8.
Arsenic and mercury are other heavy metals that are highly toxic even on
minimal exposure. Arsenic is classified as a metalloid usually found combined with oxygen, chlorine, and sulphur. Exposure to arsenic include sore throat and
irritated lungs as much as skin effects. Longer exposure at lower concentrations can lead to circulatory and peripheral nervous disorders as well as high risk of lung cancer 9. Health effect of mercury include hydrargyria or mercurialism. Elemental mercury does cause damage by blocking blood vessels, damage to the brain, kidneys and lungs 10. Mercury poisoning can result in several diseases, including
acrodynia (pink disease)11, Hunter–Russell syndrome and minamata disease
chronic exposure to excessive manganese levels can lead to variety of psychiatric and motor disturbances, termed manganism. Generally, exposure to ambient manganese air concentrations in excess of 5 micrograms Mn/m3 can lead to Mn- induced symptoms 12.
Adsorption of these metal ions from industrial effluent before discharged into the environment is of great importance so as to control the risk and endangerment they cause. To achieve this i.e. elimination or adsorption of heavy metals from industrial effluents, adsorbents such as zeolites are employed for effective adsorption of heavy metals from waste water or industrial effluents so as to free the effluents of the heavy metal ions such as Pb ions, Cd ions, Cr ions etc before they are discharged or released into the environment13.
1.0 BACKGROUND OF STUDY
1.1 HEAVY METAL TOXICITY
Heavy metal is a metal with a fairly high relative atomic mass, and specific gravity greater than 5.0 especially those that are significantly toxic (e.g., lead, cadmium, mercury).They persist in the environment and can accumulate in plant and animal tissues. Mining and industrial wastes and sewage sludge are potential sources of heavy metal pollution16.
With the rapid development of industries such as metal plating facilities,
mining operations, fertilizer industries, tanneries, batteries, paper industries and pesticides etc, heavy metal wastewaters are directly or indirectly discharged into the environment increasingly, especially in developing countries such as Nigeria. Unlike organic contaminants, heavy metals are not biodegradable and tend to accumulate in living organisms and many heavy metal ions are known to be toxic or carcinogenic. Toxic heavy metals of particular concern in the treatment of industrial waste waters include zinc, copper, nickel, mercury, cadmium, lead and chromium.
Now-a-days heavy metals are the environmental priority pollutants and are becoming one of the most serious environmental problems. So these toxic heavy metals should be removed from industrial waste water or effluents to protect the people and the environment.
1.2 METHODS OF HEAVY METAL REMOVAL
Many methods that are been used to remove heavy metal ions include chemical precipitation17, sulfide precipitation18 investigated pyrite and synthetic iron sulphide for removal of lead and copper. Ion-exchange processes have been widely employed to remove heavy metals from effluents due to their many advantages, such as high treatment capacity, high removal efficiency and fast kinetics19-21.
Adsorption additives22, tannic acids23, magnesium24, surfactants25 and
activated carbon composite could be effective adsorbents for heavy metals. Agricultural waste materials as potential adsorbent for sequestering heavy metal ions from aqueous solutions26, membrane filtration27, nanofiltration (NF) used for
nickel28,29 performed a new working system of investigate the removal of
hexavalent chromium ions using electrolysis electrochemical treatment technologies, etc30 studied the performance of an electrochemical treatment technologies system with aluminum electrodes for removal of metal ions from water.
1.3 TYPES OF HEAVY METAL ADSORBENTS
Various types of adsorbent used in heavy metal removal are activated carbon31, clay minerals32,33, biomaterials34, zeolites35,36, and some industrial solid
wastes37,38 have been widely used as adsorbents for adsorption of ions and organics in waste water treatment.
Adsorption is now recognized as an effective and economic method for heavy metal removal in waste water treatment. The adsorption process offer flexibility in design and operation and in many cases will produce high-quality treated effluent. In addition, because adsorption is sometimes reversible, adsorbents can be regenerated by suitable desorption process. In this work, zeolite will be used as the adsorbent in removal of lead ions from simulated waste water.
1.3.1 Zeolites
Zeolites are microporous, alumino silicate minerals commonly used as commercial adsorbents39. Some zeolites occur naturally while others are synthetic. Zeolite has a three-dimensional structure with pores.
The zeolite history began with the discovery of stilbite by Crönstedt, a Swedish mineralogist in year 1756. Upon heating the zeolite released occluded water, which gave the materials their general name, zeolite, after the Greek words, “ξειv” (zeo) , to boil, and “λιϑoς” (lithos), stone. A representative empirical formula of a zeolite is
M2/nO . Al2O3 . xSiO2. yH2O
where M represents the exchangeable cation of valence n. M is generally a Group I
or II ion, although other metal, non-metal and organic cations may also balance the
negative charge created by the presence of Al in the structure. The framework may contain cages and channels of discrete size, which are normally occupied by water. It consists of silicon, aluminium and oxygen ions. The silicon ions are neutrally charged in the crystal structure. Aluminium ions create negative places. To keep the cargo in balance, a counter ion (Na+, K+) or a proton (H+) is present in the pores. One type of zeolite have just as large pores through the entire crystal structures. All natural zeolites contain aluminium and are hydrophilic in nature40.
Zeolites are widely used in industries for water purification, as catalysts, for
the preparation of advanced materials and in nuclear processing. Their biggest use is in the production of laundry detergents. Zeolites are also used in medicine and in agriculture.
Zeolites have a porous structure that can accommodate a wide variety of cations such as Na+, K+, Ca2+, Mg2+ and others. These positive ions are rather loosely held and can readily be exchanged for others in a contact solution. Some of the more common mineral Zeolites are Analcine, Chabazite, Clinoptiloite, Heulandites, Natrolite, Phillipsite and Stilbite. An example mineral formula is Na2Al2Si3O10.2H2O, the formula for natrolite.
Natural zeolites form where volcanic rocks and ash layers react with alkaline
ground water. Zeolites also crystallize in post-depositional environments over periods ranging from thousands to millions of years in shallow marine basins.
Naturally occurring zeolites are rarely pure and are contaminated to varying degrees by other minerals, metals, quartz, or other zeolites. For this reason, naturally occurring zeolites are excluded from many important commercial applications where uniformity and purity are essential.
Zeolites are the aluminosilicate members of the family of microporous solids known as “molecular sieves”. The term molecular sieve refers to a particular property of these materials, i.e. the ability to selectively sort molecules based primarily on a size exclusion process. This is due to a very regular pore structure of molecular dimensions. The maximum size of the molecular or ionic species that can enter the pores of a zeolite is controlled by the dimensions of the channels. These are conventionally defined by the ring size of the aperture, where, for example, the term “8-ring” refers to a closed loop that is built from 8-tetrahedrally coordinated silicon (or aluminium) atoms and 8 oxygen atoms. These rings are not always perfectly symmetrical due to a variety of effects, including strain induced by the bonding between units that are needed to produce the overall structure, or coordination.
1.3.2 Use of Synthetic Zeolite for Wastewater Treatment
The use of synthetic zeolite for the environmental protection is stimulated by its good physico-chemical properties e.g. selective sorption,its non-toxic nature and availability. A great deal of research on zeolite has focused on a wide range of
applications including waste water treatment or purification with emphasis on the ammonia and heavy metal removal41, removal of radioactive 137Cs and 90Sr from low-level waste streams of nuclear installations42, and recently also for the removal of organic pollutants, like hydrochloroflourocarbons (HCFCs) from petroleum products from water43. They can be used as barriers to contaminant migration or as binders in waste solidification systems.
There are increasing demands for healthier environment, with the emphasis on high-quality drinking water and on the removal of contaminants from industrial, agricultural and municipal waste waters. Most technologies using zeolites for water and soil purification are based on the unique cation-exchange behaviour of zeolites through which dissolved cations are removed from water or soil by exchanging with cations in zeolites exchange sites. The most common cation in waters
affecting human and animal health is NH4+. It can be replaced with biologically
accepted cations, like Na+, K+ or Ca2+ in the zeolite. Ammonia removal is very important to prevent oxygen depletion and algae bloom and due to its extreme toxicity to most fish species44. Additionally, it has detrimental effects on disinfection of water supplies and corrosive action on certain metals and construction materials. Nitric oxides, nitrates and ammonia/ammonium are very soluble in water and can quickly end up in ground and drinking water. Some naturally occurring zeolite such as chabazite and clinoptilolite showed the best
results for ammonia removal. Heavy metals are well known for their toxicity and their disposal is a significant industrial waste problem. Pb2+, Cu2+, Fe3+, Cd2+ and Cr3+ are especially common metals in industrial wastes that tend to accumulate in organisms, causing numerous diseases and disorders45.
1.3.3 Mechanisms of Heavy Metal Removal from Industrial Waste Water
The contamination with heavy metals exists in aqueous waste streams of many industries such as metal plating industries, dyes and textile industries, mining operations etc. The amount of heavy metal waste is increasing on yearly basis; they tend to accumulate in living organisms. Treatment processes for the removal of heavy metals from waste water include coagulation, carbon adsorption, ion exchange, reverse Osmosis etc46. The sorption processes are the most attractive since their application is simple, and they require mild operating conditions. The limiting factor could be the regeneration of the sorbing materials.
The sorption of heavy metals by zeolites is a complex process because of the inner and outer charged surfaces, imperfections on the surfaces, mineralogical heterogeneity among others that can also contribute to the overall sorption capacity. The extensive research of adsorption isotherms revealed that ion exchange or chemisorptions on zeolites governs the immobilization of metal cations especially in natural zeolites tuffs47.
Following the ion exchange mechanism, ions present in the pores of zeolite crystalline lattices, like Na+, K+, Ca2+ etc are substituted by metal ions from the solution. The chemisorption always results in the formation of stable inner-sphere or outer-sphere complexes, where functional groups on the zeolite framework (mainly OH-) form strong chemical bonds with the metal ions. In clinoptilolite and the majority of zeolites, ion-exchange processes generally dominate over chemisorption. The sorption of heavy metal by the zeolite is directly related to the charge of the zeolite framework, i.e the quantity of aluminium present in the zeolite framework, the nature and concentration of the cationic species, the size and distribution of zeolite tuff particles, the solvent and the temperature.48
1.4 ADSORPTION
Adsorption refers to the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface 49. This process creates a film of the adsorbate on the surface of the adsorbent. This process differs from absorption in which a fluid (the absorbate) permeates or is dissolved by a liquid or solid (the absorbent)50. Adsorption is a surface-based process while absorption involves the whole volume of the material. The term sorption encompasses both processes, while desorption is the reverse of it. Adsorption is a surface phenomenon. Adsorption is present in many natural, physical, biological and chemical systems, and is widely applied in industrial processes such as activated charcoal, capturing and using waste heat to
provide cold water for air conditioning and other process requirements (adsorption chillers), synthetic resins and water purification. The word “adsorption” was coined in 1881 by German physicist Heinrich Kayser 51.
1.4.1 Adsorption Isotherms
Adsorption is usually described through isotherm, that is, the amount of adsorbate on the adsorbent as a function of its pressure (if gas) or concentration (if liquid) at constant temperatures. The quantity adsorbed is nearly always normalized by the mass of the adsorbent to allow comparison of different materials.
The equilibrium distribution of metal ions between the sorbent and the solution is important in determining the maximum sorption capacity. Several isotherm models are available to describe the equilibrium sorption distribution in which two models are used to fit the experimental data: Langmuir and Freundlich models. The linear form of Langmuir model52 is given as:
= 1 . +
Where qe is metal concentration on the zeolite at equilibrium (mg of metal ion/g of zeolite), Qmax (mg/g) and KL (1/mg) are Langmuir constants related to the maximum adsorption capacity corresponding to complete coverage of available adsorption sites and a measure of adsorption energy (equilibrium adsorption
constant) respectively. These constants are found from the slope and intercept of
Ce/qe Vs Ce linear plot so that Qmax = 1/slope and KL = slope/intercept. The linear form of the Freundlich model53 is given as:
lnqe = In KF + (1/n) In Ce
Where KF and n are Freundlich constants determined from the slope and intercept of plotting In qe vs In Ce.
Amount of metal ion adsorbed on zeolite is calculated at the difference between initial and final concentrations at equilibrium.
qe = (Ci-Ce)/S
Where qe is the ion concentration adsorbed on the zeolite at equilibrium (mg of ion/g of zeolite). Ci is the initial concentration of ions in the solution (mg/L). The slurry concentration, S, is expressed by :
S = m/v
Where v is the initial volume of ions solution used (L) and m is the mass of zeolite used (g). The percent adsorption (%) is calculated using the equation.
% adsorption = (Ci -Ce/Ci) x 100%
1.5 STATEMENT OF PROBLEMS
With the rapid development of industries such as metal plating facilities, mining operations, fertilizer industries, tanneries, batteries, paper and pesticide industries, heavy metal wastewaters are directly or indirectly discharged into the environment increasingly, especially in developing countries such as Nigeria.
Unlike organic contaminants, heavy metals are not biodegradable and tend to accumulate in living organisms and many heavy metal ions are known to be toxic or carcinogenic.
Despite the very useful collection of verified synthesis of zeolite materials, recipes for zeolite synthesis are often difficult to follow.
1.6 OBJECTIVE OF THE STUDY
The aim of the research is to study the adsorption capacity of synthetic zeolite synthesized from aluminosilicate solutions and gels. To achieve this, a study was carried out with the following objectives:
i. To synthesize zeolite X from sodium aluminosilicate solution via hydrothermal sol gel process.
ii. To characterize the synthesized zeolite via Scanning Electron Microscopy and X-ray diffraction.
iii. To evaluate the potential of the produced synthetic zeolite on its capacity as an adsorbent for adsorption of Pb ions from waste water
1.7 JUSTIFICATION OF THE STUDY
The hydrothermal approach used to synthesize the zeolitic adsorbent not only proffered acheaper route and lower reaction time for synthesis, it also gave high yield of the product with high purity hence its efficacy in removal of metallic lead ions in waste water.
This material content is developed to serve as a GUIDE for students to conduct academic research
SYNTHESIS CHARACTERIZATION AND USE OF ZEOLITE IN REMOVAL OF Pb(II) IONS FROM WASTE WATER
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