SYNTHESIS CHARACTERIZATION AND USE OF ZEOLITE IN REMOVAL OF Pb(II) IONS FROM WASTE WATER

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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.



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