ABSTRACT
This project is concerned with the development and evaluation of an atom-trapping technique for determination of heavy metals in environmental samples. Modified atom-trapping technique using fabricated nickel tube/sample holder (crucible) was employed to increase the sensitivity of a flame atomic absorption spectrometry. Basic performance data for different parameters, such as burner height and the height of the support, were studied. Different designs and ways of placing the atom trap over the burner were evaluated to optimize the experimental conditions. The height of the tube’s support was varied, as well as, its design. The optimal position, for maximum light path through the tube, was obtained experimentally. Results of this attempt using nickel atom trap method on a number of standards of lead, cadmium, copper, zinc, cobalt, manganese and nickel concentrations, showed high sensitivity for the technique (1.2 – 4.1 fold) when compared to the conventional flame atomic absorption spectrometry. The improvement factors on the absorbance signals for the various metals were as follows: Cd =3.8 – 4.1; Pb = 1.8 – 2.2; Cu = 1.9 – 2.3; Mn = 1.2 – 1.7; Zn 2.3 – 2.5; Co = 1.2 – 1.8 and Ni = 1.6 – 2.1. These data indicated that a significant improvement in sensitivity has been achieved, using the nickel tube atom trap method. The results from this study suggested that the method, as an analytical procedure, is more sensitive than the conventional method for the determination of heavy metals in environmental samples. Thus a study was carried out to analyse effluents discharged from small and medium scale industries. These are
xix
Naraguta leather tanning effluents, Jos; Majema leather tanning effluents, Kano; a small-scale industry located at Sharada Industrial Estate Phase III leather tanning effluents, Kano; NASCO Group of Companies, Jos and Jos International Brewery (JIB), Jos. The results showed some pollution in these effluents for the various elements above the Interim Effluent Limitation Guidelines in Nigeria for all Categories of Industries. The Naraguta leather tanning had high value of zinc. The Majema leather tanning effluent is polluted with lead, copper, and zinc, while the Sharada leather tanning effluent had high zinc, with a very close value to the limit of copper. NASCO is polluted with copper, and zinc, as well as, Jos International Brewery (JIB).An attempt was made to increase sample volume from 0.2 ml to 2 ml by redesigning the nickel tube, accommodating more analyte reaching the flame. Analytical performance, such as sensitivity, precision, detection limit, and accuracy of the method, was carried out and the results compared with those from literature. One advantage of the developed nickel tube atom trap method is the very small sample solution requirement for analysis. Further more; its simplicity and low cost would be an advantage to many laboratories with limited resources. The interference study of NaCl on the absorbance of lead was carried out. The result showed that NaCl exhibited a significant level of suppression at 5mg/L of Pb (i.e. 1.17% at 10% NaCl to 1.28% at 30% NaCl reduction in absorbance signal).
TABLE OF CONTENTS
TITLE PAGE……………………………………… .……….…………… i
CERTIFICATION………………………………………………………… ii
DECLARATION…………………………………………………………. iii
ACKNOWLEDGEMENT………………………………………………… iv
DEDICATION……………………………………………………………. vi
TABLE OF CONTENTS…………………………………………………. vii
LIST OF TABLES……………………………………………………..…. xi
LIST OF FIGURES………………………………………………………… xiii
LIST OF PLATES………………………………………………………….. xiv
ABSTRACT……………………………………………………….………… xv
CHAPTER ONE
INTRODUCTION
1.1. BACKGROUND TO THE STUDY………………………..…… 1
1.1.1 Environmental Pollution…………………………………………….. 1
1.1.2 Sources of Pollution…………………………………………………. 4
1.1.2.1 Land Pollution……………………………………………………… 4
1.1.2.2 Water Pollution………………………………………………….. 5
1.1.2.3 Air Pollution………………………………………………………… 5
viii
1.1.2.4 Noise Pollution……………………………………………………… 6
1.1.3 Heavy Metals in Environmental Pollution………………………. 6
1.1.4 Effects of Heavy Metals on Human………………………………… 8
1.1.5 Industrial Effluent Discharges……………………………………… 15
1.1.6 Regulations and Monitoring of Pollutants………………………………… 16
1.1.7 Atomic Absorption Spectrometry ……………………………………. 20
1.1.7.1 Problems Encountered in the Conventional Flame Atomic
Absorption Spectrometry………………………………………………… 30
1.1.7.2 Methods for Improving the Sensitivity of Flame Atomic
Absorption Spectrometry…………………………………………………. 30
1.2. CONCEPT…………………………………………………………………. 32
1.3. AIMS/OBJECTIVES OF THE STUDY……………………………….…… 32
1.4. SCOPE AND LIMITATIONS OF THE STUDY. …………………….….. 33
1.5. SIGNIFICANCE OF THE STUDY…………………………………………… 33
CHAPTER TWO
LITERATURE REVIEW
2.1. APPROACHES FOR THE IMPROVEMENT OF SENSITIVITY OF FLAME
ATOMIC ABSORPTION SPECTROMETRY…..……………………………… 36
ix
2.2. SLOTTED TUBE ATOM TRAP……………………………………………… 41
2.3. WATER- COOLED SILICA TUBE…………………………………………… 49
CHAPTER THREE
MATERIALS AND METHODS
3.1. MATERIALS……………………………………………………………….…. 57
3.1.1 Atomic Absorption Spectrometer……………………………………………… 57
3.1.2 SP9 Pye-Unicam Model Atomic Absorption Spectrometer………………..…. 57
3.1.3 Hollow Cathode Lamp Sources……………………………………………… 63
3.1.4 Reagents and Stock Solutions………………….…………………………..…. 66
3.1.5 Sampling Site and Sample Collection………………………………………. 66
3.2. METHODS ………………………………………………………………… 67
3.2.1 Sample Handling.…………………………………………………………….. 67
3.2.2 Evaluation of Techniques……………………………………………………. 73
3.2.3 Sensitivity………………………………………………..………………….. 73
3.2.4 Detection Limits…………………………………………………..………… 73
3.2.5 Precision and Accuracy………………………………………………..……. 73
3.2.6 The Development of Modified Atom Trapping
Collecting Tubes………………………………………………………….… 74
x
3.2.7 Optimization of Experimental Conditions………………………………….. 76
3.2.8 Procedure…………………………………………………………………… 79
CHAPTER FOUR
RESULTS
4.1. INITIAL EVALUATION OF TUBE DESIGNS……………….………….. 83
4.2. OPTIMIZATION STUDY………………………………………………….. 86
4.3. THE NICKEL TUBE AND ITS ASSEMBLIES…………………………… 86
4.4. COMPARISONS OF Pb AND Cd SIGNALS OBTAINED WITH THE
USE OF CONVENTIONAL FLAME AAS AND THE DEVELOPED
ATOM-TRAP METHOD…………………………………………………… 88
4.5. DETERMINATION OF THE EASILY VOLATILE ELEMENTS IN
EFFLUENTS………………………………………………………..………… 94
4.6. EFFECT OF THE NICKEL TUBE ON THE ABSORBANCE SIGNALS
OF THEEASILY VOLATILE ELEMENTS IN ENVIRONMENTAL
SAMPLES………………………………..………………………………… 107
4.6.1 Lead………………………………………………………………………… 107
4.6.2 Cadmium……………………………………………………………………. 107
4.6.3 Copper……………………………………………………………………….. 108
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4.6.4 Zinc………………………………………………………………………….. 108
4.6.5 Manganese, Cobalt and Nickel……………………………………………… 109
4.7. ENVIRONMENTAL ASSESSMENT……………………………………… 109
4.7.1 Lead………………………………………………………………………… 109
4.7.2 Copper………………………………………………………………………. 110
4.7.3 Manganese……………………………………………………………….….. 111
4.7.4 Cadmium…………………………………………………………………..… 111
4.7.5 Zinc…………………………………………………………………….……. 112
4.7.6 Cobalt ……………………………………………………………………… 112
4.7.7 Nickel …………………………………………………….………….…….. 112
4.7.8 Chromium………………………………………………………..……… … 113
4.8. INTERFERENCE EFFECTS………………………………….………….. 113
4.9. NARAGUTA EFFLUENT SAMPLES……………………………..……… 116
4.10. THE INFLUENCE OF INCREASED VOLUME ON THE
ABSORBANCE SIGNALS… ……………………………………………… 116
4.11. IMPROVEMENT ON THE MODIFIED NICKEL TUBE…………………. 121
4.12. ANALYTICAL PERFORMANCE……………………..…………………… 121
4.12.1 Precision……………………………………………………………………… 121
4.12.2 Sensitivity…………………………………………………………..……….. . 123
4.12.3 Detection Limit……………………………………………………..……….. 123
4.12.4 Accuracy…………………………………………………………………..…. 123
CHAPTER FIVE
DISCUSSION AND CONCLUSION
5.1. DISCUSSION………………………………………………………………… 126
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5.2. CONCLUSION……………………………………………………….…….. 128
5.3. RECOMMENDATIONS …………………………………………………… 128
5.4. SUMMARY OF RESULTS………………………………………………… 129
5.5 CONTRIBUTION TO KNOWLEDGE……………………………………… 130
REFERENCES………………………………………..……………………………. 131
APPENDIX I………………………………………………..…………………….. 148
APPENDIX II……………………………………………………………………….. 156
LIST OF TABLES
1. The Maximum Tolerable Levels for Trace Elements Proposed
by World Health Organization (1984)………………………………………….. 9
2. Interim Guidelines and Standards for Water Quality for Various Uses
…………..………………………………………………………………. 19
3. List of Hollow Cathode Lamps and their Operating Currents
(for SP9 Pye Unicam Spectrometer)……………………………………….…. 64
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4.. Operating Condition of the SP9 Pye-Unicam with 10cm
Burner…………………………………………………………………….….. 65
5. Stainless (Non-Magnetic) Atom Trap Tube
Specifications………………………………………………………………… 84
6. Nickel Tube/Crucible Joined…………………………………………..…… 85
7. Absorbance Signals Obtained Using the Conventional
Flame AAS and Atom Trap Method for Lead in Leather
Tanning Effluents (Naraguta, Jos) ……………………………………… 92
8. Absorbance Signals Obtained Using the Conventional Flame
AAS and the Developed Nickel Atom Trap Method for Cadmium
in Leather Tanning Effluents (Naraguta, Jos) ……………………………… 93
9. Absorbance Signals of the Easily Volatile Elements Using
the Conventional Flame AAS and the Developed Nickel Atom
Trap Method in Leather Tanning Effluents (Majema area, Kano) …………. 100
10. Concentrations of the Easily Volatile Elements in Leather
Tanning Effluents (Majema area, Kano)……………………………..…….. 101
11. Absorbance Signals of the Easily Volatile Elements Using
the Conventional Flame AAS and the Developed Nickel Atom
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Trap Method in Leather Tanning Effluents (Leather Processing
Factory, Kano) ……………………………………………………………… 102
12. Concentration Values for the Easily Volatile Elements in
Leather Effluent Samples (Leather Processing Factory, Kano) ………….… 103
13. Absorbance Signals of the Easily Volatile Elements Using
the Conventional Flame AAS and the Developed Nickel Atom
Trap Method in NASCO and JIB Effluents……………………………….… 104
14. Concentration Values for the Easily Volatile Elements in
NASCO and JIB Effluent Samples (Jos) …………………………………… 105
15. Interim Effluent Limitation Guidelines in Nigeria for all
Categories of Industries……………………………………………………… 106
16. Absorbance Signals Obtained by the Conventional FAAS and
the Atom Trap of Naraguta Samples…………………………….…………. 117
17. Concentration Values for Naraguta Effluent Samples……………… ……… 118
18. Concentrations in mg/L of the Heavy Metals in Various Effluents… ……… 119
19. Absorbance Signals Obtained by Increasing Sample
Volume for Various Concentrations of Cadmium………………………….. 120
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20. Statistical Data for the 2 ml Modified Nickel Tube…………………….…… 122
21. Sensitivity Increase for the Developed Atom Trap………………………… 124
22. Results of Analysed Certified Reference Materials………………………… 125
CHAPTER ONE
INTRODUCTION
1.1. BACKGROUND TO THE STUDY
Pollution is contamination by a chemical or any agent that renders part of the environment unfit for man or other living organisms. Pollution damages the land, water and air. It results in contamination of the earth’s environment with materials that interfere with human health, the quality of life and the natural functioning of ecosystems. Pollution is usually caused by human actions but can also be the consequence of natural disasters (Rao, 2006). A pollutant is defined as a substance that occurs in the environment as a result of human activities and which has a deleterious effect on the environment (Moriarity, 1990). Domestic, industrial and agricultural processes and other sources produce large quantities of waste products that cause rapid changes to the environment. Exposure to the pollutant at sufficiently high concentrations can cause a variety of health effects (Dara, 2002).
1.1.1 Environmental Pollution
There has been increasing concern chemicals in the environment. Such concern has arisen in response to the widespread distribution of chemicals stemming from human activities and the potentially harmful effects of those chemicals on humans or on the ecological systems. As nations develop their industrial activities, the production and use of chemicals rise in parallel to the standard of living and the consequent increase in the life expectancy (Adepetu & Eziashi, 1998). Technology has made it possible for people to live longer in comfort and with greater leisure. Environmental deterioration threatens our well being when air, water or food become contaminated (Eisenbud, 1979).
2
United Nations Conference on Environment and Development (1992) has identified the following adverse global environmental impacts, which have caused
deterioration of the planet earth due to developmental processes: eutrophication, acid precipitation, ozone layer depletion, deforestation, soil erosion, global warming, climate change, air, water and land pollution, from toxic and hazardous industrial wastes, depletion of natural resources, land degradation, ill health and death (human), loss of biodiversity and loss in beauty and aesthetic value of the physical environment. At its ninth session in 1981, the Governing Council of the United Nations Environment Program (UNEP) recognized that the list of selected environmental dangerous chemical substances harmful at the global level should be prepared (Andersen & Sarma, 2002). Harmful chemicals are defined as those, which enter the environment as product, or by-product of human activities, threat man’s health, the environment and which can be eliminated with difficulty from the environment (Redwood & Dixon, 1992).
The causes of most environmental problems have their origins in the development process or in its failures and inadequacies. Technological advancement and increasing industrial which is satisfying human needs and comforts, and to improve on civilization and human life styles, have created unexpected adverse effects. Man and nature have been at odds since the industrial revolution, and especially in this century (Odiette, 1993).
Environmental and toxicological concern over heavy metals, such as cadmium, lead, copper, and zinc etc, has been documented in recent years in the large volume of analytical literature presenting methods for their determination (Taylor, Branch, Halls, Owen & White, 1998). Environmental degradation and pollution threaten not only the development process, but the safety of man and other organisms. Heavy metal
3
contamination of soil, water, and food are causes of concern because of the toxicological effects of such metals on humans and other living systems (Matin, 1995).
As urban areas increase, there is increase of wastes such as water and solid wastes, and other demands. Urban and industrial areas are polluted by the increasing number of motor vehicles, the expansion of existing industrial areas and the aging in old factories (Pham, Dang & Nguyen, 1995). Increasing industrialization and globalization have spawned the unprecedented generation of environmental toxic and hazardous wastes (Pandey & Carney, 1989). Hazardous waste management is now a major and urgent global concern because hazardous substances significantly contribute to destruction of life. Far more controversial is the issue about the quantities of wastes from the developed nations shipped to developing countries in Asia, Africa and other parts of the world. The existence of known hazardous waste such as toxic spills and river pollution put pressure on each national government to take an extensive role in hazardous waste control (Kellog, 1977).
Bartley and Gardiner (1977) described the level of heavy metals in a water body as a pointer to the cause. These are natural sources based on the geochemical nature of the soil and the rock beds within the water basin including the presence of mineral deposits, and the discharge of untreated industrial wastes. Both tend to cause the enrichment in metals depending on the nature and scope of the industry or geology of the area.
Water pollution is a problem in the wake of rising population, rapid urbanization, and industrialization. The urban growth has increased domestic wastes, while new industries have augmented industrial wastes. The discharge of these chemicals into water courses have resulted in hazard of water pollution (Itanna, 1998). Undesirable results from the discharge of inorganic materials include changes in the pH and toxicity by heavy metals or other toxic materials.
4
Industrial emissions are potential sources of environmental contamination. Major industries including fertilizer, pulp and paper mills, dyeing, printing, textile, tanneries, cement and pesticides, plastic, food processing, distilleries etc. generate wastes with varying pollution burdens. Monitoring these emissions helps to improve industrial processes, and consequently, to protect the environment (Maier, 1996).
1.1.2 Sources of Pollution
The advanced technology as well as the rapid urbanization in recent years has affected the quality of the ecosystem. Due to human activities the chemical compositions of soil, water and air have been altered. Pollution can also be as the consequence of a natural disaster (Duggal, 1988).
1.1.2.1 Land Pollution
Environmental problems associated with the land degradation are caused by mining activity. Exploration affects landscape specifically aesthetic deterioration of the landscape, path-construction and trampling in wilderness areas. The land degradation and ecosystem destabilization caused by mineral extraction has specific impacts, such as land surface devastation (including erosion), disruption of drainage system, deforestation, and contamination of the water table. Processing, transportation, storage and consumption of mining activities are also of concern, because oil spills and land pollution (acidification of soil) are pronounced in mining regions. Environmental problems connected with petroleum exploitation is oil spillage, both on-shore and offshore. The rate of the spill has been with the increasing tempo of petroleum production (Igbozurike, 1978).
Mining and smelting wastes contain heavy metals such as cadmium, copper, lead and zinc. Shaheen (1995) observed that mining wastes can pollute streams and ground water, harm wildlife, fishing and agriculture in areas influenced by mining activity.
5
Erosion is the detachment and transportation of soil particles by running water, wind and waves. Wind erosion affects northwest Sokoto, northern Kano and Borno. Coastal erosion affects Lagos (especially Bar Beach at Victoria Island), Ogun, Ondo, Rivers, Akwa Ibom, and Cross River States. Bigger sizes of gully erosion are mainly to be found in Imo and Anambra states, where a combination of weak, sandy soils, widespread deforestation, and high rainfall has promoted accelerated erosion (Adefolalu, Ade-Odutola, Afolayan, Agunbiade & Aina, 1991).
1.1.2.2 Water Pollution
Water pollution could be from plant nutrient used to fertilize farmlands, ashes, and detergents. When these plant nutrients are washed into water bodies, they encourage growth of algae, and phytoplankton. The unregulated use of fertilizers, pesticides, and herbicides to increase crop production has led to environmental pollution. Medical wastes generated in the diagnosis, treatment, and immunization of human or animals pose a great danger to the environment (Okoronkwo, 1998). Also, industries discharge of their effluents without prior treatment, into rivers, estuaries or lagoons. Solid wastes such as metal (scraps), plastics and other objects pollute water bodies. The ever increasing solid wastes being generated are disposed indiscriminately, and managed poorly by the relevant government authority (Umeh & Uchegbu, 1997).
1.1.2.3 Air Pollution
Air pollution involving the release of chemicals and particulates into the atmosphere, in sufficient concentrations endanger human health. The increasing skin cancer, cataracts, weakening human immune systems, and damaging crops and natural ecosystem) or produce other physical effects on living matter, and other materials (Britton & Greeson, 1994). The sources contributing to the pollution of the atmospheric air are:
6
a) Smoke due to incomplete combustion of coal in industrial plants, furnaces and hearths.
b) Finely divided dust particles, salt particles from oceans, pollens, spores etc, remain suspended in air.
c) Gaseous impurities from chemical manufacturing industries and including sulphur dioxide, benzene, carbon monoxide, acid vapours, fumes etc.
d) Automobile exhaust gases, in particular, exhaust from trucks and buses. These contain products of incomplete combustion, carbon monoxide, hydrogen, methane, unburnt carbon, and partially burnt hydrocarbons; oxides of sulphur, and nitrogen contribute significantly to air pollution (Raina & Aggrawal, 2004). Motor vehicles have been regarded as the primary source of air pollution in the urban areas, and accounts for 60- 70% of the pollution found in the urban environment (Singh et al., 1995).
The atmosphere, a relatively thin layer of gas enveloping our globe, is basically of constant composition, temperature, and pressure (Duggal, 1988). The minor changes that do occur, however, have an enormous influence on man’s life.
1.1.2.4 Noise Pollution
Another source of environmental pollution is noise. Noise pollution affects the physical and mental health of man and, reduces working efficiency (Maruthi, Rao & Ravindra, 2004). The rapid urbanization and the increased transportation in the recent years have helped the people to develop but on the other hand left impacts that have affected the human environment (Arutchelvan, Venkatash, Damodharan & Elangovan, 2004). The growing needs of human life in this fast track society and the increased automobiles have contributed to increase in noise pollution.
7
1.1.3 Heavy Metals in Environmental Pollution
Heavy metals are not biodegradable, but persist in the environment for a long time. They are known to be causative for many diseases including cancer, immune diseases, allergies, and asthma. Toxic effects of heavy metals on biological systems are very variable, and are related to their chemical form. Certain heavy metals forms, such as bio-available forms, are potentially toxic, and hazardous to the biota of receiving water. However, recent studies have shown that the free ions, and weakly complexed forms as being the bio-available, and more toxic forms of metal (Mendoza, Cories & Munoz, 1996). The metals, with the highest impact on organisms are copper, chromium, cadmium, tin, lead, vanadium, molybdenum, cobalt, and nickel. From an environmental point of view, all these elements are important because they cannot be biodegraded in water, soils and sediments (Carson & Munford, 1994).
The heavy metals are given consideration in monitoring activity to prevent acute toxicity and destruction to ecosystems. Though there are some natural sources of heavy metal contamination, the greatest to the quality of the environment is posed by human activity. Some chemicals even in trace quantities are carcinogenic, mutagenic, teratogenic and toxic to man and other biological substances of economic importance (Moriarity, 1990).
Itanna (1998) described contamination by heavy metals as contributors of toxic substances, is not restricted to the soil alone. The effect goes beyond that, affecting every component in the food chain, namely; plants, grazing animals and ultimately man. Air and aquatic environments including the sea foods of both plant and animal origin like fish are subject to this hazard.
Many heavy metals affect the vegetation in terresterial habitat (Pahlsson, 1989). The uptake of heavy metals by plants from contaminated soils is of interest because an
8
excess dietry intake of some of these metals could be deleterious to the health of a consumer (Davies, 1992).
The problem of heavy metal contamination in water bodies is widespread threatening an ever-increasing portion of the global population. The presence of toxic heavy metals in water resources poses unacceptable chronic and acute health risks (Roy, Greenlaw & Shane, 1993).Heavy metals are known to be essential for living organisms. They are used in respiratory pigments – (iron, copper, and vanadium), enzyme – zinc, vitamins – cobalt and other metabolic processes. It is only when normal concentrations are exceeded, that they become potentially toxic (Kielly, 1988).
Excess of dietary intake of these heavy metals might be deleterious to the man (Dudka, Piotrowska & Chlopecka, 1994). It is therefore considered that removals as well as recovery of toxic metals are the desirable approach to control pollution and safeguard the environment and health (Wasay et al., 1994). The maximum tolerable levels of trace elements set by the World Health Organisation are as shown in Table 1.
According to Koul, Zutsch and Dubey (1988), studies pertaining to the toxicity of trace metals follow the general trend that an undersupply leads to deficiency, sufficient supply results in optimum conditions but an oversupply leads to toxic effects, and ultimately death.
1.1.4. Effects of Heavy Metals on Man
Heavy metals are known to be toxic to man, as their concentrations in the environment has been progressively increasing, and might cause severe health hazard to man. Lead and cadmium are trace metals with no known beneficial physiological effect, but they are toxic to plants and animals if their concentrations exceed certain values (Adriano, 1986).
9
Table 1. The Maximum Tolerable Levels for Trace Elements Proposed
by World Health Organization (1984)
* Source: Recommendations, World Health Organization.
Element
Concentration (mg/L)
Copper
1.0
Lead
0.05
Zinc
5
Cadmium
0.005
Manganese
0.1
Nickel
0.5
Cobalt
0.05
10
A study by Abdel-Saheb, Schwab, Banks and Hetrick (1994) indicated that people exposed to lead and cadmium from contaminated drinking water and surface soils suffer from increased chronic kidney disease and anaemia.
There are many ways in which human are exposed to lead through air, drinking water, food, contaminated soil, deteriorating paint, and dust. Lead is an important pollutant in emissions from industries, and automobiles. Much of the stable lead pollution is of atmospheric origin. Drinking water can become contaminated with significant amounts of lead in its distribution system as a result of corrosion, and leaching from lead pipes, and lead/tin soldered joints associated with the copper service lines used in household plumbing (Subramanian & Connor, 1991). Lead is responsible
for serious damage to the health of human, and birds. Lead affects all systems within the body. It is bio-accumulative and 30 – 50% of inhaled lead lodges in the respiratory system with the remainder absorbed into the body.
Elevated lead levels in blood could lead to hematological problems, particularly at blood levels in excess of 0.2μg/ml (O’Halloran, Myers & Duggan, 1988). Lead at high levels (80 μg/dl of blood) can cause convulsions, coma, and even death. Lower levels of lead can cause adverse health effects on the central nervous system, kidney, and blood cells. A study carried out by Needleman, Schell, Bellinger, Levinton and Allred (1990) has shown that exposure of children to lead is extensive, and ingrained public health problem. The effects of lead exposure on fetuses and young children can be severe. Fetuses, infants and children are more vulnerable to lead exposure than adults. Since lead is easily absorbed into growing bodies, tissues of small children are more sensitive to its damaging effects. At low levels, it poses a public health hazard (Subramanian, 1988). Wolverton and Mcdonald (1978) reported that excess levels of lead can cause anaemia,
11
kidney, liver disease, paralysis, brain damage, convulsions, and death. Low levels of this metal may contribute to hyperactivity, learning disabilities, night blindness, and suppression of the body’s immune response.
Cadmium is probably the heavy metal of most environmental concern due to its high toxicity, relative high mobility in the terrestrial environment, and its occurrence in
the human diet at the highest percentage of the provisional tolerable intake (De Haan, Van Der Zee & Giraldez, 1989). A recent study by Cvetković, Arpadjan, karadjova and Stafilov (2006) described that the sources of cadmium pollution being the non- ferrous metal production, waste incineration, phosphate fertilizer manufacture, wood, coal, oil and gasoline combustion, iron and steel production and industrial cadmium application. These authors indicated that in industrialized areas cadmium in air varies from 1 to 50 ng/m3, while in rural air is 0.1 to 6 ng/m3. Cadmium affects the agricultural soil via air deposition (41%), phosphate fertilizers (54%) and sludge application (5%), also in unusual conditions via liquid effluents and solid wastes from cadmium processing plants. The authors reported that cadmium enters the human body via plants and animal food products and is readily absorbed by plants than lead. Cadmium is widely distributed in soil and water and is a by-product of many industrial processes (Pahlsson, 1989).
Food and cigarette smoke are contributors to non-occupational cadmium exposure. It is considered non-essential, and highly toxic element, with a serious cumulative effect (Berman, 1980). Cadmium toxicity is comparable to that of arsenic and mercury, but its lethal potential is higher than that of any other metallic element. Interests in its potential link with carcinogenicity have drawn attention to its concentration in body fluids, tissues, and foods. Although reports of its carcinogenic activity are inconclusive, measurement of cadmium in body fluids is still used for exposure monitoring, because of its very toxic effects. Besides this, the analyses of
12
different kinds of samples are important in order to know other sources of its contamination (Yaman, 1999). Although Friberg, Piscator and Norberg (1992) earlier reported that cadmium toxicity is associated with hypertension, emphysema, renal tubular damage, liver dysfunction, and cancer. After the World War II in Japan, Itai-Itai disease, a severe bone disease related in part to cadmium exposure, was diagnosed in postmenopausal women who lived in Toyama prefecture downstream from a lead/zinc mine. People in this area were exposed to cadmium from the river water, which was used for drinking and rice paddy irrigation (Bhattacharyya, 1991).
There are many known sources of contamination by cadmium, owing to the large number of its inorganic salts used in catalytic and synthetic reactions, in Ni-Cd battery manufacturing, stabilizers for plastics, and pigments and still many unknown sources (Tsalev & Zaprianov, 1985). Spehar, Anderson and Finnat, (1978) reported that cadmium can cause permanent damage to the kidney, and give rise to nephril protenium. The accumulation of cadmium in the body gives cause for concern because of its long biological half life and the damage that high levels can do to the kidney. Although high levels of cadmium can occur from natural causes but may be elevated by localized pollution (Dellar, 1989).
Copper is biologically essential, a balance between absorption and excretion has to be maintained, otherwise retention in the body may cause diseases of liver and central nervous system. Hepatic diseases are known to arise from exposure to copper (Duffus, 1980). Theophanides and Anastassopoulou (2002) reported that copper is a trace element essential for the activity of such mammalian metalloenzymes as ceniloplasmin cytochrome C oxidase, dopamine and tyrosinate. They reported that copper helps to form haemoglobin in the blood, facilitating the absorption and use of iron so that red blood cells can transport oxygen to tissues. Copper assists in regulating blood pressure and
13
heart rate. Although copper is an essential element for human and animals, a high concentration of copper could induce growth proliferation, and cancer by damaging deoxyribonucleic acid with toxic-free hydroxyl radicals. Massive accumulation of copper occurs in the liver, and brain in patients with Wilson’s disease due to the genetically determined metabolic defect in copper metabolism. A normal individual absorbs sufficient amount of copper to meet the body’s needs, and excess copper is readily eliminated (Parker, Weil &Richman, 1987). Gupta, Sinha and Chandra, (1994) reported that excess copper causes alteration in plant metabolism, and poses health hazards.
Nickel is an essential element and anaemia, prenatal mortality, growth retardation, acute stroke in rats, and primates have been attributed to its deficiency. Nickel is a co-factor of jackbean urease, glycoprotein and albumin. Accumulation results in damage of alveoli of the lungs (Birch & Saddler, 1979). High levels of nickel elevated red blood cells, haemoglobin content and packed cell volume can cause leucopoenia and lymphopenia. Ghazaly (1992) reported that the presence of nickel results in its accumulation in the blood, kidney, liver and muscle. Nickel markedly elevates the blood zinc content of the other tissues. Sen and Bhattacharyya (1994) reported that nickel causes dermatitis, dizziness, headache, nausea, and carcinogenesis.
Halls and Fell (1980) in occupational health have focused on the relationship between manganese exposure and early signs and symptoms of health effect in active working populations. Accumulation of manganese resulting from prolonged industrial exposure has been implicated in the incidence of hepatitis, liver and necrotic cirrhosis. In general, occupational and animal studies have identified three major types of effect: respiratory, reproductive and neurotoxic disorders. Exposure to manganese has been associated particularly with extra pyramidal signs and symptoms and with the development of Parkinson’s disease (Loranger, Zayed & Forget, 1994).
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Cobalt is unique in that, only one combined form cyanocobalamin or vitamin B12, is physiologically active in man. This vitamin is essential to deoxyribonucleic acid synthesis and propionate metabolism to the avoidance, and control of pernicious anaemia. In cases of iron deficiency anaemia, inorganic cobalt may be added to the diet to increase the rate of haemoglobin synthesis. However, cobalt in doses of about 5mg per day can be toxic to man and may in the long term, cause heart disease. Cobalt can cause allergic and non-allergic asthma (Barfoot and Pritchard, 1980). Cherian and Gupta (1991) reported that a deficiency of zinc uptake, in food, leads to dwarfism, hypogonadism, and sickle cell anemia. High levels of zinc salts are corrosive and irritating to the gastrointestinal tract and ingestion may lead to vomiting and fever.
Chromium (III) is an essential micronutrient for both humans and experimental animals. This element was once considered to potentiate the action of insulin by forming a ternary complex with it at the membrane receptor. In essence, this element is a co-factor for insulin. The side effects of chromium deficiency include renal opacity in primated and impaired glucose tolerance in human beings leading to diabetes (Ottaway, 1981). Birch and Saddler (1979) described the toxic nature of chromium (VI) arising from industrial exposure to dichromates or chromic acid as evident in skin diseases, and increased incidents of various forms of cancer experienced by chrome industrial workers.
Mise and Shanta (1993) attributed the over exposure to chromium dust and mist as causes of irritation, lung carcinoma, corrosion of skin and respiratory tract. High concentrations of chromium (VI) present in water bodies were lethal to various fishes. In a biological system chromium undergoes the process of bioaccumulation resulting in chromium laden biological sludges, which cause most concern.
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1.1.5. Industrial Effluent Discharges
Industries are the major sources of environmental contamination arising primarily from the discharge of untreated industrial effluents into the environment. These sources from the industries are the use of herbicides and insecticides, agricultural chemicals, industrial effluents, metal processing factories, dust emissions from metallurgical industries, cement, coal grinding, asbestos, and related industries are contributors of metal pollution (Albasel, 1985).
Industries contribute to environmental pollution by discharging toxic chemicals as effluents in to the environment. Hutzinger, Van Lelywed and Zoeteman, (1978) reported the polluting ability of these compounds depends on their intrinsic properties determined by their structure and non- chemical factors like production and pattern of use. Contamination may have just occurred or may already have been caused centuries ago. Maier (1996) described all types of industrial activities as sources of contamination whether by organic or inorganic chemicals, natural or artificial compounds. They may be produced by small or medium sized enterprises or large production plants.
Jyoti, Pandey and Singh, (1994) reported that the industrial effluents discharged directly into the streams and rivers are frequently used for drinking, cooking or dry season farming of vegetables. As such there is the tendency that the heavy metals will pass through this drinking or eating of the food prepared using such water. With the increase in industrial activities, release of heavy metals in the environment cannot be prevented. The industrial wastewater is specific and particular to the industries, each having its own undesirable waste constituents and their negative effect (Eckenfelder, 1989).
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Increased industrial activities have led to urbanization and pollution stress on the environment both from industrial and domestic sources. Ajayi and Osibanjo (1981) indicated that the major streams in the industrial estates of cities like Lagos, Kano and Kaduna were already polluted by wastes from industrial sources and all the streams flowing through the densely populated city of Ibadan were polluted by wastes from domestic wastes.
1.1.6 Regulations and Monitoring Of Pollutants
Water quality monitoring has focused upon surface water heavy metal concentrations to safeguard drinking water supplies and to characterize the state of the aquatic environment (Bubb and Lester, 1994).
The World Health Organization Expert Committee on Environmental Pollution Control (WHO, 1983) in relation to development, reported amongst many sections, the health hazards related to development with special reference to chemical pollution in developing countries. Many of the recommendations involved policy issues, which were to be decided by each government (Andersen, 1985). The global approach to monitor the world environment in order to protect human health and preserve essential natural resources has become important. Thus, the United Nation Environmental Programme in 1979 established the Global Environment Monitoring system in view of monitoring the air and water quality as well as food contamination (Walters, 1987).
Human interactions and over-exploitation of resources are increasingly degrading the potential land, water, and air resources, due to the use of metal contaminated sewage sludge, as toxic metals may accumulate in some soils, and foods. Like many developing nations, Nigeria has had to face new problems of chemical pollution, beside traditional ones such as poor sanitation, poor quality drinking water, and deforestation. Adequate legislation, guidance, and monitoring for generation of toxicity data are important
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(Dung-Gwom, 1999). The control option requires emissions suppression from the point of generation until the affected process wastewater streams are properly treated or reduced.
Nigeria has laudable legislative and policy initiatives in environmental management, which led to the creation of Federal Ministry of Environment (Formerly Federal Environmental Protection Agency in 1988), charged with the responsibility for the protection, and development of the Nigerian environment including policy initiation in relation to environmental research and technology (Aina, 1991). The agency has come up with the interim national water quality guidelines and standards (Table 2).
Nigeria has taken active part in all major international initiative aimed at alleviating global environmental problems, and is a signatory to a number of international treaties, protocols, and conventions. A number of programs and projects have been funded through utilization of official development assistance, financial, and technical aids, which supplement internal resources gaps following the downturn in the economy, and the rising external debts. One of the on-going bilateral assistance activities include, the Environmental Management Project being sponsored by the World Bank in Nigeria through an IDA loan, of which a major component is the State Environmental Action Plans (SEAP) (Onwuka, 1995).
There are many individual sources of water pollution. The major sources in Nigeria are soil erosion, urban wastes, industrial wastes, and oil spillages. Waste loads from urban areas tend to increase owing to growing populations and greater per capita water usage. Wastewaters from industries are also expanding due to increasing water-using factories and industries. Accidental spillage of oil and hazardous substances into
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Table 2. Interim Guidelines and Standards for Water Quality
* Source: Federal Environment Protection Agency, 1991.
Parameter
Drinking water
(mg/L)
Cd
0.01
Co
0.05
Cu
0.1
Pb
0.05
Mn
0.05
Ni
0.05
Zn
5.0
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watercourses is a fairly recent interest. These pollutants can cause devastating and extensive damage to the aqueous environment. In 1988, accidental discharge of water containing high ammonia level into Okrika river from the National Fertilizer Company of Nigeria (NAFCON), a fertilizer company near Port Harcourt, Rivers State caused massive fish kill, and socio-economic problems for the artisan fishery industry in the surrounding village (Umeh & Uchegbu, 1997).
Odiette (1993) observed that the deplorable state of solid waste disposal in cities in Nigeria demands the urgent actions by the various states of the Waste Disposal Board. It has become a concern because of the accumulation of refuse on all the cities threaten health. Epidemic diseases like malaria fever have been associated with the indiscriminate dumping of refuse, and with the added problem of high temperatures, which encourage the spread of many infections. Also the several forms of enteric infections like cholera, typhoid fever, and schistosomiasis are linked to the exposure of the sufferers to garbage.
According to Odubela, Adeleke-Adedoyin, Ene-Ita and Baiyeroh (1990), much of the drinking water in Nigeria is supplied without effective treatment; the primary concern in relation to pollution is with the dangers arising from the entry of pathogenic
microorganisms to the environment, and the consequent hazards of water related diseases. Thus a rational pollution control measures are vital factors in improving the health of the Nation. The degree of water quality could only be ascertained through periodical physical, bacteriological, and chemical monitoring.
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1.1.7 Atomic Absorption Spectrometry
The flame atomic absorption spectrometry is the technique for the determination of trace elements in a variety of samples (Brown, Roberts & Kahokola, 1987). This technique is used for the analysis of a variety of materials containing trace elements (parts per million concentrations) to (> 1%) inorganic constituents, such as agricultural, biological, geological, petroleum products, glass and its raw materials, cement, ferrous metals and alloys, water effluents, and air (Bauer, Christian and O’Reilly, 1985). This technique became a reality when Walsh (1955) succeeded in making radiation sources with narrow bandwidths, whose wavelengths matched the absorption wavelengths of different analytes (Hargis, 1988).Walsh (1955) recognized the importance of a high-intensity, narrow line-width source for sensitivity, and suggested the use of hollow cathode discharge lamps as meeting these requirements. A flame was used to produce neutral atoms, through which the energy emitted by the hollow cathode could be passed to produce the absorption signal. The theoretical factors governing the relationship between atomic absorption and atomic concentration, also the potential and practical realization of atomic absorption spectrometry as a method of analysis appeared in 1955.
Alkemade and Milatz (1983) independently proposed an atomic absorption system utilizing a flame as radiation source and a second flame as the sample cell. The analyte concentration is related to the difference in radiation between two intensities. They indicated the potential of atomic absorption spectral techniques to the field of analytical chemistry. The
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atomic absorption technique extended the range of flame methods to elements with best lines in the deep ultraviolet region. This enhanced the detection of additional elements, and made the method less susceptible to interferences than emission methods. The absorption methods gave precision equal to the best flame emission methods (1-2%), and the instrumentation could be inexpensive (Koirtyohann, 1980).
In general, the flame atomic absorption spectrometry is used to determine elements with sensitivity down to parts per million, and measurement precision of one percent (Wassall, 1986). The advantages of the technique are specificity, low detection limit, many elements can be determined in one solution, there is rarely any lapsed time requirement (as in colour development, drying of precipitates, etc) in sample preparation, and data output in a directly readable form. These advantages include versatility with regard to both types of samples, and concentration ranges. Among the attractive features of flame atomization are its comparatively low capital and operation costs besides its ease of operation (Alvarado & Jaffe, 1988).
The technique provides fast, accurate, and reliable method for determining the concentrations of metals. Flame atomic absorption spectrometry (FAAS) is used for the determination of metals, showing good selectivity but low detectability, which may be improved by a preconcentration step. Several methods for the separation and preconcentration of metals have been developed, enhancing the detection, and the versatility of the techniques (Bortoleto, Macarovscha & Cadore, 2004). For many elements, detection limits for their atomic absorption with flame atomization lie in the range of 1 to 20 ng/ml or 0.001 to 0.020 ppm. Under the usual conditions, the relative error associated with a flame absorption analysis is of the order of 1% to 2% (Skoog, 1998).
The proportion of ground state to excited state atoms, suggests that absorption are direct measurement of the number of ground state atoms. Analytical methods based
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on atomic absorption are specific because atomic absorption lines are narrow (0.002 to 0.005 nm) and electronic transition energies are unique for each element.
The basic instrumental components needed to make absorption measurements are the source of radiation, flame (atomizer or sample holder), monochromator (a wavelength selector), detector, and readout system (Fig. 1).
Double-beam instruments are required for background correction using deuterium continuum lamps. For broadband absorption a background correction can be made in the ultraviolet region of the spectrum (where most elements absorb and background absorption is serious) with a hydrogen or deuterium continuum source. In the visible region, a tungsten continuum source may be used. Sharp line absorption of the continuum source by the test element is assumed negligible, compared to that by the broad background over the bandwidth of the monochromator, so the absorbance of the continuum source can be subtracted from the absorbance of the resonance line from the hollow cathode lamp (Christian, 2004).
The source beam is sent through the flame and around the flame by the chopper. The detector measures these alternately and the logarithm of the ratio of the incident radiation versus the transmitted radiation (I0/I) is displayed. The detector amplifier is tuned to receive only radiation modulated at the frequency of the chopper, and so direct current radiation emitted by the flame is discriminated against (Robinson, 1996).
This is the basis of a Philips Pye –Unicam SP9 Atomic Absorption Spectrometer used in the atomic absorption investigations in this Thesis, as well as for those commercially available automatic background correctors. A mirror alternately passes the hollow cathode radiation and the continuum radiation, the absorption of each is measured, and the continuum source absorbance is automatically subtracted from the absorbance to obtain the net sharp line absorbance by the test element. So only a single measurement is required (Christian & Feldman, 1970).
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Fig. 1. Block Diagram of Atomic Absorption Spectrometer
Flame
Sample
Monochromator
Detector
Source
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The common source for atomic absorption measurements is the hollow cathode lamp, which consists of a cylindrical metallic cathode, and tungsten anode sealed in a glass tube containing neon or argon at a pressure of about 1 to 5 torr. When a high voltage is applied between the anode and cathode, the filler gas becomes ionized, and positive ions are accelerated toward the cathode. They strike the cathode with enough energy to “sputter” metal atoms from the cathode into the gas phase. Many of the sputtered atoms are in excited states; they emit photons, and then return to the ground state. This atomic radiation is of the same frequency as that absorbed by the atoms of the analyte. The line width is sufficiently narrow, with respect to that of the high temperature analyte, to be nearly `monochromatic’ (Harris, 1987).
Light, from the lamp generating a sharp line spectrum characteristic of the desired element, passes through the flame, into which the sample solution is sprayed as a fine mist (Hargis, 1988). In a flame atomizer, a sample solution is aspirated by a flow of gaseous oxidant mixed with a gaseous fuel; the heat of the flame first causes the solvent to evaporate producing a finely divided solid molecular aerosol. The micro crystals remaining are partially or wholly dissociated into elements in the gaseous form or atomic gas (Christian & O’Reilly, 1986).
The dissociated atoms can react with other atoms or molecules in the flame to produce molecular species and/or radicals. These in turn may give rise to molecular spectra. Also for elements of low ionization energy, ions and electrons may be produced. The ions may be excited and emission lines of the excited ions are produced. Such lines are not useful analytically, but in certain cases can cause interference with the desired process (Schrenk, 1975).
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The flame converts the sample, introduced into it as an aerosol. This aerosol generated using a pneumatic nebulizer connected to the flame burner by a spray chamber (Fritz & Schenk, 1979). The ground state atoms must be obtained from the sample in a reproducible manner if quantitative data are to be obtained. The atomic vapour forms a cloud of atoms; only atoms of one particular element will absorb the radiant energy of a characteristic wavelength, and become excited to a higher electronic state leading to a decrease in the radiant power transmitted.
The neutral atom distribution is variable in the flames. Therefore the maximum absorption signal from a flame cell will only be obtained if the optical path traverses the flame through the region of maximum neutral atom population. The optimum path through the flame is different for different elements, varies with the fuel-oxidant combination used and the fuel to oxidant ratio. Flame temperatures and fuel – to – oxidant ratios are two important parameters to consider, when using a flame as an atomic absorption sample cell (Svehla, 1975).
The region of the spectrum in the immediate neighborhood of the resonance line to be measured is selected by the monochromator, the functions of which is to isolate the resonance line from non-absorbing lines close to the source spectrum. Such line may originate from the cathode metal or the lamp fill gas. A further function of the monochromator is to isolate the measured resonance line from molecular emission, and other background which originate in the flame (Ebdon, 1998).
The isolated resonance line falls onto the detector, a photomultiplier, the output of which consists of a pulse of electrons, for each photon that reaches the detector surface. A single photon that strikes the cathode of a photomultiplier ultimately leads to a cascade of 106 to 107 electrons, which produce a pulse of current that can be amplified, and drives a readout device (e.g. meter, strip- chart recorder, to a digital display unit,
26
printer). Most modern spectrometers now provide effective microprocessor – based measurements of the signals (Price, 1983).
The intensity of the resonance line is measured with and without the sample passing into the flame. The log ratio of these readings is a measure of the absorbance, and therefore proportional to the amount of the element being determined. It is more suitable for routine analysis that has higher sample quantity reaching the flame, of simpler operation, and the instrumentation has a lower total cost than electrothermal atomization-atomic absorption spectrometry.
In spite of the present state of electrothermal atomization, the most convenient, stable source of atomic vapor remains the combustion flame. The important parameters in the production of free atoms from a given element are the flame temperature, the chemical effects of radicals, and other substances present in the flame. The dissociation equilibrium of a molecule MA containing the analyte element, M
MA M + A
is characterized by a temperature – dependent equilibrium constant KD, where
KD = [M][A]
[MA]
The atoms of the analyte element, M, may be bound as molecules with one of the bulk constituents of the flame gases (e.g. O or OH) or with one of the atoms introduced into the flame in the sample nebulized (e.g. Cl or F).
The widely used of fuel/oxidant mixtures is air-acetylene, as it enables (about 30) of the common metals to be determined. These are elements that do not form refractory oxides. Calcium, iron, cobalt, nickel, magnesium, molybdenum, strontium and the noble metals are among the elements normally determined with this flame.
27
Both the gas flow rates and thermal expansion of the flame gases after combustion contribute to the dilution of the absorbing species and to the length of time of the absorbing atom in the radiation beam. The best sensitivity at any given temperature is by flame having the smallest combustion gas / sample volume ratio (Price, 1983).
The flame is a convenient and reproducible source of heat, but is less ideal as a sampling device for atomic absorption in that the two endothermic processes, (solvent evaporation followed by atomization), must take place within short time interval for a particle to shoot through the flame. In addition, the flame introduces significant random fluctuations in the effective optical path length, because of turbulence, and this causes excessive noise in the signal (Ewing, 1975).
The precision and accuracy of atomic absorption spectrometric method is critically dependent upon the atomization step and the method of introduction of the sample to the atomization region (Skoog, 1998). Innovation in atomic absorption spectrometry sample introduction ways have been employed in order to solve challenging analytical problems in science and technology (Bings, Bogaerts & Broekaert, 2004).
Magda, Khayyal and Farrag (1985) reported new approaches for the application of atomic absorption spectrometry in drug analysis. The technique was based on the measurements of the metal content of the precipitated complex after treatment with the complexing agent. Holen, Bye and Lund (1981) determined selenium in technical sulphuric acid by electrochemical preconcentration on a platinum filament followed by atomization in an argon-hydrogen flame with simultaneous electrothermal heating of the filament.
28
Multiple pass devices have been constructed, so that effective optical path length through the flame could be increased, which increased the sensitivity of the method. Various attempts have been made to use simple cell other than flame to produce the atomic vapor in the optical path of the spectrometer.
The graphite furnace devised (L’vov, 1961) is electrically heated tube furnace as an atomizer for atomic absorption, the carbon rod analyzers; this device can be used to convert a powdered sample into atomic vapour. An electric current is applied to a very thin, heated carbon rod that contains the solid sample in order to vaporise it (West & Williams, 1969). The tantalum boat devices enabled the introduction of the entire sample into the flame without nebulization (Hwang, Ullucci & Smith, 1971). High intensity sources other than hollow cathode tubes have been investigated like the electrodless discharge lamps (Barnett, Voller & De Nuzzo, 1976). Also new and better atomic absorption systems have been developed to readout the data.
The limitations of flame atomic absorption spectrometry are the low efficiency of the nebulizer system. That is, not all the samples reach the atomizer and hence, sample volume requirements are greater with the technique (Milner & Whiteside, 1984). In a typical nebulizer, liquid droplets will be produced with particle sizes ranging from a few micrometers. The droplets have a diameter of 5 to 10 m, but the sample volume is contained in droplets of 20m in diameter. In a premix burner the larger droplets never reach the flame at all. Particles larger than 10 to 20μm are used inefficiently in the flame or are deposited on the premix chamber walls to flow out the drain tube. From 85-90% of the sample literally goes down the drain (Bauer, Christian & O’Reilly, 1985).
The sample is drawn into the nebulizer by the low pressure created around the end of the capillary by the flow of the carrier or oxidant gas. The resulting droplets are ejected with the carrier gas into the spray chamber. The design of the chamber is such
29
that droplets with a diameter greater than 5 μm fallout onto the sides of the chamber and flow to waste.
Another way of improving a flame atomic absorption spectrometry is to increase the efficiency of the nebulizer system (Nygren, Nilsson & Gustavsson, 1988). These authors developed an improved system for the determination of lead in blood by flame atomic absorption spectrometry. The system was based on flow injection, a nebulizer interface, and a computer signal evaluation system. The system improved the detection limit 12-fold compared with that obtained by unmodified instrumentation.
To improve the nebulization efficiency various ways and means of altering the normal droplet size distribution have been employed (Rawson, 1966);
i. The application of heat – either to the sample and gases before they enter the spray chamber or in the spray chamber itself.
ii. Impact beads are placed close to the orifice of the nebulizer. The droplets, whose speed at this point is near sonic, are fragmented by the impact and the mass of material vaporized in the flame is increased by 50 – 100%.
iii. Counter flow nebulizers – the oxidant/sample aerosol and fuel nozzles are placed opposite each other within the spray chamber. It results in a high-speed turbulence, which produces larger proportion of the small size droplets. Increases in sensitivity of two or three times have been obtained with this system.
The maximum useful population of droplet sizes constitutes about 10% of the total mass of sample nebulized and an absorbing atom remains in the radiation beam usually no more that 10-4 sec. These limitations of atomic absorption spectrometry with insufficient sensitivity could be overcome if the atomic vapour are constrained to remain longer in the resonance beam, and if the sample could be introduced without inefficient nebulization process, or indeed without any pretreatment involving dilution at all.
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Ideal liquid sample introduction implies reproducible transfer of the total sample solution into the atomization cell. The elements with sensitivity improvements are the volatile elements, which decompose thermally in the primary reaction zone of an air–acetylene flame. The sensitivity improvement is due to the increased residence time of analyte atoms in the light path of the atomic absorption spectrophotometer (Brown & Milner, 1985).
1.1.7.1 Problems Encountered in the Conventional Flame Atomic Absorption Spectrometry
The conventional flame atomic absorption method is a versatile analytical technique like other techniques. It is not an absolute method, as it is plagued with the low efficiency of the nebulizer system. The nebulization process is of importance, as it is meant to convert the sample solution into aerosol with 100% efficiency. The sensitivity of the analysis depends upon its correct function and efficiency. The sample introduction step limits the accuracy, the precision, and the detection limits of atomic absorption spectrometric measurements (Browner & Boorn, 1990).
The droplet diameters should be less than 10μm, and preferably approaching 1 to 2μm. However, most nebulizers are only about 3 to 15% efficient which means that 85 to 90% of the sample goes down the drain. This remains an unsolved problem for flame atomic absorption spectrometry. The absorbing atom remains in the radiation beam for not more that 10-4 seconds, even though gas flow rates and thermal expansion of the flame gases after combustion both contribute to the total dilution of the absorbing species in the flame and to the length of time (Price, 1983).
1.1.7.2 Methods for Improving the Sensitivity of Flame Atomic Absorption Spectrometry
An analytical chemist is often required to quantify an element in a sample, and is always seeking new and improved analytical methods. The sensitivity of the atomization
31
of some elements is insufficient for many sample materials, so that some degree of enrichment is needed (Daniel, Mats & Stig, 1981). Suvardahan, Suresh Kumar, Reddy and Chiranjeevi (2003) observed that the drawback can be overcome by a combination of preconcentration techniques with subsequent atomic absorption spectrometry determination.
When lower concentrations are sought, the sensitivity of flame Atomic Absorption Spectrometry could be increased if more of the sample is introduced into the flame, or if the analyte could be constrained longer in the light path. The sensitivity of flame atomic absorption spectrometry is limited by several factors. The analyte atoms generated in the flame pass rapidly and comtinously through the measurement zone during sample aspiration. A further limitation is the poor efficiency of the nebulizer/burner system.
There has been increasing focus on the development of a number of devices to increase the sensitivity of flame Atomic Absorption Spectrometry (Rawson, 1966, Kahn, 1968, Delves, 1970, Brown, 1984, Alvarado, 1998). Advances in Atomic Absorption Spectrometry have continued to improve the nebulization, process and are exemplified by the introduction of a circular flame atomizer, the use of radiofrequency, the effect of long chains surfactants, and discrete sample nebulization (Jackson & Qiao, 1992). The atom – trapping technique used to increase the sensitivity of Flame Atomic Absorption Spectrometry could be described as the slotted quartz tube and the water – cooled silica tube (Brown, Roberts & Kahokola, 1987).
The slotted quartz tube is used to increase the sensitivity of the volatile elements that decompose thermally in the primary reaction zone of an air-acetylene flame (Plate i). The sensitivity improvement is due to an increased residence time of analyte atoms in the light path of the atomic absorption spectrometer and the reduction of potential
32
interferences. It is inexpensive and the required quartz tube is easily manufactured (Brown & Taylor, 1984). In the water – cooled silica tube technique atoms are trapped on a water – cooled silica tube. After a fixed collection time, the tube rapidly heats up releasing the trapped metals species, resulting in an increase in sensitivity (Brown & Milner, 1985).
1.2. CONCEPT
Atomic absorption spectrometry is considered a versatile laboratory technique. The method renders itself for improved instrumentation, more reliable source of resonance radiation, hotter flames and non flame atomizers. A key disadvantage of the method is the inefficient use of sample by the nebulizer spray chamber burner system. Thus considerable effort has been directed toward sample introduction step by many researchers. The atom trap technique is used to increase the sensitivity of the conventional flame method by constraining the analyte atoms to remain longer in the measurement zone. As a consequence, low concentrations in parts per billion are determined.
1.3. AIMS/OBJECTIVES OF THE STUDY
This thesis would be devoted to the design and the development of atom – trapping collecting tubes as a method for analysis in the determination of trace elements
in environmental samples, and where appropriate comparisons would be made with the conventional flame atomic absorption spectrometry. Various sizes of the developed atom trap would be checked. The method developed would be compared with the existing conventional flame atomic absorption spectrometry.
Detailed interference study, which describes the effect of NaCl on lead and cadmium atomic absorption signals with the developed atom – trapping technique would be carried out. Standard reference materials with known concentrations would be
33
analysed to ascertain the validity of the developed procedure. Precision, detection limit and sensitivity check would be performed to assess the developed method.
The method would be used to analyse heavy metals from industrial wastes namely from Naraguta village (local leather processing outfit), effluents discharged from NASCO group of companies and Jos International Breweries all located in Jos, Plateau State. Also effluents discharged from leather tanneries located in Kano state, Majema area of Kofar Wambai, and Sharada Industrial Estate Phase III would be analysed.
1.4. SCOPE AND LIMITATIONS OF THE STUDY
The research covers the designing of nickel tube atom trap for the determination of low concentrations of heavy metals in industrial effluents. The developed method provides increased sensitivity particularly at parts per billion (ppb) level, which otherwise could have been undetected by the conventional flame atomic absorption spectrometry. The sensitivity increase achieved is due to the analyte atoms being constrained in the light path for longer period. The developed method is used to assess the environmental exposure of heavy metals in effluents obtained from various sources. As in every method development analytical performance data such as precision, accuracy, detection limit and sensitivity would be checked.
The developed method is limited to the study of elements that are decomposed in air acetylene flame. One of the drawbacks of the developed method is manually reproducing the exact position of the atom trap tube with respect to the source radiation. This lowers the reproducibility of the method.
1.5. SIGNIFICANCE OF THE STUDY
The developed nickel tube would provide a direct measurement of trace heavy metals at very low concentrations without preconcentration. When these samples are analysed the time consuming preconcentration step is avoided. Sample volume
34
requirement is small when compared to the conventional flame atomic absorption spectrometry method. The nickel tube can be fabricated locally and easily fitted to the equipment. The nickel tube atom trap method is characterized by simplicity, efficiency and low cost. It is an added advantage to many laboratories such as industries, research institutes and universities with limited resources.
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Air/Acetylene Burner
Slotted Quartz Tube
Plate i. The Slotted Quartz Tube Atom Trap
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