ABSTRACT
The assessment of the quality of the geophagious ‗Nzu Clay‘ obtained from two hills in Ozanogogo, Ika South LGA, Delta State and Uzella river, Owan West LGA in Edo State Nigeria, with the highest cation exchange capacity, bulk density and dispersibility index, 10.800±0.424 meq/100g, 0.915±0.120 gcm-3, 80.00±1.414 respectively was investigated while the pH range from 4.4 to 4.6. The concentrations of heavy metals in ‗Nzu clay‘ obtained from the hill side were significantly higher at (P < 0.05) compared to the clay from Uzella river. The concentration of heavy metals in the clay from the river side was in the order: Zn ˃ Pb ˃ Cu ˃ Ni ˃ Cr ˃ Cd, while the one from hill side showed that Cr ˃ Cu ˃ Zn ˃ Pb ˃ Ni ˃ Cd. The heavy metals in all the ‗Nzu clay‘ exceeded their standard limit in soil according to WHO, 2010. The hair of addicted consumers of the clay above 5 years had mean values greater than that in addicted consumers of the clay below 5 years with concentrations of the heavy metals greater than the non addicted consumers as follows, As: 25.60 ± 1.1mgkg-1, Cd: 4.35 ± 0.82, Cr: 112.47 ± 22.9, Cu: 4.04 ± 0.72, Ni: 7.62 ± 1.46, Pb: 2.99 ± 0.68 and Zn: 0.17 ± 0.22 mg/kg For consumers of the clay above 5 years, similarly the concentration of 22.44 ± 0.39, 2.59 ± 0.09, 66.06 ± 3.18, 0.52 ± 0.38, 3.95 ± 0.08, 1.14 ± 0.04, 0.12 ± 0.03 mgkg-1 were recorded for As, Cd, Cr, Cu, Ni, Pb and Zn respectively for consumers below 5 years. These values exceeded the recommended dietary allowance in solid food by World Health Organization (WHO) National Agency for Food and Drug Administration and Control (NAFDAC). The levels of radioactive elements in ‗Nzu Clay‘ obtained using Hyper Pure Germanium detector indicated that the average specific activities of 40K, 238U and 232Th ranged from 54.45 ± 32.45 to 127.60 ± 14.7, 21.35 ± 6.28 to 38.75 ± 4.67 and 26.83 ±
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13.94 to 44.51 ± 1.16 Bq/kg respectively. The radionuclides detected in ‗Nzu Clay‘ are all lower than the worldwide averages by United Nations Scientific Committee on the Effects of Atomic Radiation, (UNSCEAR) 2000. The mean absorbed dose rate by the ‗Nzu Clay‘ was 48.86 nGy/h, and is within the world average of 60nGy/h. The mean Annual Gonadal Equivalent Dose (AGED) in the ‗Nzu Clay‘ was 207.1Sv/y, which is relatively high. The radium equivalent activity index, representative Gamma Index (Iyr), external hazard index (Hex) and internal hazard index (Hin) indicated a negligible health hazard in consuming the ‗Nzu Clay‘. The XRD analysis revealed that the ‗Nzu clay‘ was dominantly kaolinite and quartz. Based on the results obtained in this study, it is recommended that the ‗Nzu Clay‘ be continually banned with appropriate defined measures.
TABLE OF CONTENTS
Title page i
Cover page iii Declaration iv Certification v
Dedication vi
Acknowledgement vii Abstract viii
CHAPTER ONE
1.0 INTRODUCTION 1
1.1 Clay 1
1.2 ‗Nzu Clay‘ 1
1.3 Some of the Proffered Reasons for Geophagy in Humans 3
1.4 History of Clay 5
1.5 Physical and Chemical Properties of Clay 7
1.6 Uses of cay 9
1.7 Radionuclides of Specific Interest in Clays and the Environment 11
1.7.1 Pathways of radionuclides in the environment 12
1.7.2 Hazardous radionuclides and their impacts on human health 13
1.8 Gamma-ray Spectroscopy 15
1.8.1 Assessments of radioactivity levels 17
1.8.2 Biomonitor of Accumulated Heavy Metals in Human 17
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1.9 Justification for the Study 18
1.10 Aim and Objectives of the study 19
CHAPTER TWO
2.0 LITERATURE REVIEW 21
2.1 Kaolin 21
2.2 Organoleptic and Granulometric Properties of Edible Clays 24
2.2.1 Cation exchange capacity 24
2.2.2 Bulk density 24
2.2.3 Water absorption index of the clay 25
2.2.4 Dispersibility of the clay 25
2.3 Heavy Metals in Clay 26
2.3.1 Arsenic 27
2.3.2 Chromium 27 2.3.3 Lead 29
2.3.4 Nickel 29
2.3.5 Zinc in clay samples 29
2.4 Health Impact of Heavy Metal 31
2.4.1 Arsenic 31
2.4.2 Cadmium 32
2.4.3 Lead 32
2.5 Radioactivity in Soil, Clay, Sand Samples 33 2.6 Selected Instrumentation Used in the Study 39
2.6.1 Theory and Instrumentation of X-Ray Diffraction Analysis 39
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2.6.2 Procedure for XRD analysis 40 2.7 Theory and Instrumentation of Hyper Pure Gammanium Detector For
Radioactivity Measurement 41
2.7.1 Hyper-pure germanium detector configuration 42
2.7.2 Planar configuration 42
2.7.3 Electric field and capacitance 44 2.7.4 Operational characteristics of HPGe 45
2.7.5 Energy resolution 46
2.8 Theory and Instrumentation of Atomic Absorption Spectroscopy for Metal
Analysis 47
2.9 Structure of Hair 49
2.9.1 Hair as a superior bio-indicator of heavy metals compared to blood and
urine 50 2.9.2 Hair analysis as a tool in assessing human exposure to heavy metals through
geophagy and the limitations 52
CHAPTER THREE
3.1 MATERIALS 55 3.1.1 List of apparatus/equipments 55 3.1.2 List of reagent 55
3.1.3 Sampling of the clay types 56
3.2 Method 58
3.2.1 Quality assurance/Preparation of standard solution 58
3.2.2 Sampling of hair from addicted and non-addicted consumers of ‗Nzu clay‘ 60
3.2.2a Processing of raw ‗Nzu Clay‘ Sample 60
3.2.2b Sample preparation 62
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3.2.3 Determination of some physico-chemical properties of the ‗Nzu Clay‘ samples 62
3.2.4 Determination of the cation exchange capacity of the clay by the Bacl2
compulsive exchange method 65
3.2.5 Digestion of ‗Nzu clay‘ Samples for determination of total Cd, Cr, Cu, Ni, Pb
and Zn contents 66
3.2.6 Measurement of the radioactivity in the clay samples 68 3.2.6 Radiation hazard indices 69
3.2.7 X-ray diffraction analysis of the ‗Nzu Clay‘ samples using X-ray diffractometer 73
3.2.8 Statistical analysis 74
CHAPTER FOUR
4.0 RESULTS 75
CHAPTER FIVE
5.0 DISCUSSION 91 5.1 Physicochemical Parameters of the Raw and Processed/Finished ‗Nzu clay‘
Samples 92 5.1.1 Correlation between similar physiochemical properties of the various
sampling sites 96 5.2 Heavy metals concentration in ‗Nzu clay‘ and in the hair of addicts and non
addicts 97
5.2.1 Concentration of Arsenic in hair of addicts and in ‗Nzu clay‘ samples 97
5.2.2 Concentration of Cadmium in the ‗Nzu clay‘ samples and in hair of addicts 98
5.2.3 Concentration of Chromium in the ‗Nzu clay‘ samples and in hair of addicts 99
5.2.4 Concentration of Copper in the ‗Nzu clay‘ samples and in hair of addicts 100
5.2.5 Concentration of Nickel in the ‗Nzu clay‘ samples and in hair of addicts 101
5.2.6 Concentration of Lead in the ‗Nzu clay‘ samples and in hair of addicts 103
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5.2.7 Concentration of Zinc in the ‗Nzu clay‘ samples and in hair of addicts 104
5.3 Concentration (Bq/kg) of Radioactive Elements in ‗Nzu clay‘ Samples 105
5.3.1 Level of the health risk hazard of radioactive elements in ‗Nzu Clay‘ samples 107
5.4 Mineral composition of ‗Nzu clay‘ by XRD method 110
CHAPTER SIX
6.0 SUMMARY, CONCLUSION AND RECOMMENDATION 111
6.1 Summary 111
6.2 Conclusion 113 6.3 Recommendation 115 REFERENCE 116
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CHAPTER ONE
1. INTRODUCTION
1.1 CLAY Animals are instinctively drawn to clay, especially when it is in the form of mud. Animals lick the clay or if injured, roll around in it to obtain relief from their injuries. It has now been observed that many varieties of species have evolved to ingest clay to counteract environmental and man-made toxins. Many herbivorous animals will eat clay after ingesting herbs loaded with tannins, a toxic substance (Domini et al., 2004). The practice of eating clay for gastrointestinal ailments and applying clay topically as bandaids for skin infections is as old as mankind and one that continues today among traditional ethnic groups as well as numerous animal species (Carretero, 2002). 1.2 Nzu Clay ‗Nzu clay‘ – also known (according to language/locality) as Argile, Calabar stone, Calabash clay, Ebumba, La Craie, Mabele, Ndom, Poto and Ulo – is a generic term used for naming these Nigerian geophagical materials. This material is available in a variety of forms including powder, moulded shapes and blocks. Though native to Africa, it is available in the UK in ethnic stores and markets. ‗Nzu clay‘ is used for facial masks or soap. Though the consumption of this chalk cuts across sex and age, it is greater among women, especially during pregnancy (Callahan, 2003). Geophagia in Nigeria is noted to be especially associated with pregnant women who consume earth materials to alleviate the symptoms of morning sickness (Abraham, 2002).
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This chalk which occurs naturally and made up of fossilized sea shells may be prepared artificially. It is prepared as a combination of clay and mud, with other ingredients like sand, wood ash and sometimes salt being mixed to them, it is then moulded and heated to produce the final product (Callahan, 2003). Calabash chalk has the structure of aluminium silicate hydroxide, which is a member of the kaolin clay group with a possible formula Al2Si2O5(OH)4 (Dean et al., 2004). Multi-elemental analysis by energy dispersive x-ray fluorescence spectroscopy (EDXRF) quantified 22 elements in ‗Nzu clay‘ including lead and aluminium, as well as persistent organic pollutants (Dean et al., 2004). Another report had established the presence of arsenic (Campbell and Belfast, 2002). Lead and other toxic elements present in the clay have been reported to be associated with numerous gastrointestinal disorders including nausea, ulcer, gastritis etc. (Caraterro, 2002). This geophagic material is consumed by oral route, and several pollutants have been reported to be present in ‗Nzu clay‘. The most severe risk of eating clay is a total blockage of the lower intestine, which can only be remedied by surgery (Padilla and Torre, 2006).
Pica is the ingestion of any unusual substance. Many definitions have been proposed for this word, among them “the craving for oral ingestion of a given substance that is unusual in kind or quantity‖ and it is defined as manifestations of false or craving appetite leading to the deliberate ingestion of a bizarre selection of foods, non-nutritive substances and non-food items (Parry and Parry, 1992). Commonly cited items of pica are hair (trichophagia), burnt matches (cautopyreiophagia), feaces (coprophagia), lead paint chips (plumbophagia), leaves, grass and plant sterns (foliophagia), starch (arnylophagia), stones (lithophagia) and soil (geophagia) (Callahan, 2003).
3
People have often speculated that geophagy occurs as a result of a desire or need for salt. Examination of the literature, however, indicates this is unlikely in most cases for example, in some areas salt is actually added to clay before ingestion. (Vermeer, 1998) noted that among certain tribes in Nigeria salt is added to soil before it is eaten.
1.3 Some of the Proffered Reasons for Geophagy in Humans
Soil has been eaten for a variety of reasons throughout history, these can be grouped accordingly, as; medicinal use, for cosmetics, use during pregnancy, as a famine food, as a food additive or condiment, and for religious or ceremonial purposes (Dean et al., 2004). Worldwide, the medicinal properties ascribed to clays are numerous. Soil has been used to treat a multitude of diseases and ailments. Soil has been used both internally and externally, for example, it has been used externally to treat bubonic plague, eczema and herpes and internally for treatment of such ailments as diarrhea and nausea which formed basis for other digestive and antibiotics production usage (Williams et al., 2008). This belief can be attributed to the fact that clay acts as coating material on the gastrointestinal tract and may absorb dangerous toxins. In pharmaceuticals/cosmetics, bentonite is used as a binder in tablet manufacturing. Clays are used as thickeners in a wide variety of cosmetics including facial creams, lipsticks, shampoos and calamine lotion (Doel et al., 2012). Bentonite was reported to be used in 78 different cosmetics in the USA, usually at concentrations between 1% and 10%, but reaching 80% in some paste masks (CIREP, 2003).
It is generally assumed that children ingestion rates are higher than adult rates, due primarily to children‘s mouthing behaviour. Consumption of clay is more predominant
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among women than men. It was observed that social pattern of soil eating was less frequent among young boys than in girls where no age trend was apparent (Calabrese et al., 1997). Geophagy is most commonly cited as occurring during pregnancy (Saathoff et al., 2002). From the 16th to the 20th century, it was widely believed to be dangerous to stop pregnant women from eating whatever they wanted. Another widely held belief is that women practising geophagy will have children which are more handsome and that clay keeps the body from being marked at birth (O’Rourke et al., 1967). Prevalence of geophagy among pregnant women of Malawi, Zambia, Zimbabwe, Swaziland and South Africa is estimated to be 90% (Hunter, 1993). Soil ingestion has often been related to famine (Callahan, 2003) gives numerous examples of soil being used in this manner in China, New Zealand, New Caledonia, India, Mongolia Germany, Finland, Africa, North and South America. (Vermeer, 1998) noted that among the Tiv of Nigeria it is common to qualify a bad famine by saying that “one only had dirt to feed his children‖. Not only that, soil has also been used by various people as a food additive or condiment. Soil is sometimes added to breads and wild potatoes to decrease bitterness. In 1998, bentonite was approved for use as a ―Generally regarded as safe‖ (GRAS) food additive in the USA, (US. FDA, 2004).
Bentonite is used to bind tiny particles of iron ore, which are then formed into pellets for use as feed material for blast furnaces. Finely ground clays are used in paint industry to disperse pigment evenly throughout the paint. Without clays, it would be extremely difficult to evenly mix the paint base and colour pigment (Callahan, 2003).
5
In addition, soils are eaten in religious ceremonies in Mexico, Barbados, China, Burma, and Malaysia. Diatomaceous earth in China was often hailed as having a supernatural origin, and the finding and ingestion of this earth was viewed as a happy omen. Geophagy also occurs in various cultures in conjunction with the swearing of oaths (Laurie, 2008).
1.4 History of Clay
Clay has been used in bricks and pottery for millennia. Sun-dried brick were used from possibly over 10,000 years ago and kiln-fired bricks were used in the construction of a temple in the Euphrates region, considered to be more than 5000 years old. Sumerian and Babylonian builders constructed ziggurats, palaces and city walls of sun-dried bricks and covered them with more durable kiln-baked bricks, often brilliantly glazed and arranged in decorative pictorial friezes. The earliest form of pottery was earthenware (porous and coarse), which has been made for at least 9000 years. The earliest pottery yet discovered in the Middle East comes from Çatal Hüyük, in Anatolia (near modern Çumra, Turkey), and dates from 8500 years ago. The oldest translucent porcelain originated from China during the T‘ang dynasty (618-907 AD). The porcelain best known in the West (where it is called chinaware) was not produced until the Yuan dynasty (1278-1368 AD). This ―hard-paste‖ porcelain was made from petuntse, or china stone (a crushed kaolinised granite consisting of a mixture of kaolinite, sericite, feldspar and quartz), ground to powder, mixed with kaolin and fired at a temperature of about 1450 ºC. Porcelain import from China was considered a great luxury in Europe and attempts to imitate it led to discovery in Florence during 1575 of ―soft-paste‖ porcelain (or frit porcelain), a mixture
6
of clay and ground glass fired at about 1200 ºC. The secret of hard-paste porcelain was discovered in about 1707 at the Meissen factory in Saxony (Germany) by Johann Böttger and Ehrenfried von Tschirnaus. English bone china was first produced around 1800, when Josiah Spode added calcined bones to the hard-paste porcelain formula (Konta, 1995). The use of clays (probably smectite) as soaps and absorbents was reported in Natural History by the Roman author Pliny the Elder (AD 77). The use of kaolin-bearing surface on paper began in China about 400 AD when powdered kaolin was added to the pigment of paper coating (Konta, 1995). Clays are present in soils, they may be formed in soils during soil development through the weathering of various minerals or can be found essentially without change in the parent material from which the soils were formed. Parent material, climate, topography and vegetation determine the kinds of clays that are found. Hydrothermal alteration may also lead to clay formation. As erosion acts on the landscape, clay may be suspended in water and carried until they are deposited by sedimentation (Redmond, 2009).
Clays are generally mined by highly selective open pit methods using hydraulic excavators, front-end loaders, or draglines. The clay is then processed using either a dry (air flotation) or a wet process (water washing). The wet process produces a higher cost and higher quality product than the dry process. The dry process involves crushing, drying, pulverizing and air flotation, to remove the grit particles (mostly quartz and feldspar). In the wet process, the first step is to remove the non-clay minerals, usually by extracting the grit from clay slurry in drag boxes, classifiers, and/or hydro cyclones. The
7
clay slurry is centrifuged and then thickened to about 30% solids in settling tanks. Further processing may involve ultra flotation and screening/filtering. In some cases flotation or high-intensity magnetic separation is used to remove iron and titanium impurities (Pattanyak et al., 2014).
1.5 Physical and Chemical Properties of Clay
Clays are colloidal – sub-microscopic and held in suspension in solution. When wet, they are viscous gelatinous and sticky. When dry, clay is hard, packed and cohesive. It is composed of micelles (flat, sheet-like plates laminated into stacks). They are chemically very complex, negatively charged with very high cation exchange capacity (CEC): 10-100 meq/100g and these charge allows flocculation or de-flocculation.
Clay minerals are fine-particle-size hydrous alumino-silicates which develop plasticity when mixed with water. They vary over quite wide limit in chemical, mineralogical and physical characteristics. They are all composed of electrical neutral alumino-silicate layers which move readily over each other, giving rise to such physical properties as softness, soapy feel and easy cleavage (Manukaji, 2004). All clay minerals are of secondary geologic origin that is, they are formed as alteration products of alumino-silicate rocks in an environment in which water is present (Ijagbemi, 2002). Clay minerals are produced mainly from the weathering of feldspars and micas. They form part of a group of complex alumino-silicates of potassium, magnesium and iron, known as layer-lattice minerals (Abifarin, 1999). They are very small in size and very flaky in shape, and so have considerable surface area. These surfaces carry a negative electrical charge, a phenomenon that has great significance in the understanding of the engineering
8
properties of clay soils (Agha, 1998). Their layer-lattice structure comprises of
tetrahedral unit, which is made up of a central silicon ion with four surrounding oxygen
ions and the octahedral unit comprising of a central ion of either aluminum or
magnesium, surrounded by six hydroxyl ions. In both, the metal is on the inside and the
non-metallic ions form the outside (Akinbode, 1996). The layer structure is formed when
the oxygen ions covalently link between units. Thus, a silica layer is a form of linked
tetrahedral unit, having a general formula of nSi4O.10(OH). The spacing between the
outer ions in the tetrahedral and octahedral layers is sufficiently similar for them to link
together via mutual oxygen or hydroxyl ions. Two stacking arrangements are possible,
giving either a two-layer or a three-layer structure (Mahmoud et al., 2003). In a two-layer
lattice, tetrahedral and octahedral layers alternate, while a three-layer arrangement
consists of an octahedral layer sandwiched between two tetrahedral layers as seen in fig
1.0. Mineral particles are built up when the layers are linked together to form stacks (Li
and Zhou, 2001; Mahmoud et al., 2003).
Figure 1.0: Silica tetrahedral and aluminum octahedron structure (Li and Zhou, 2001)
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1.6 Uses of Clay
Clay as raw materials are used in industry and other areas of human activity. The most extensive applications are naturally in industrial branches. Many applications however, include the area called the ‗formation and protection of the environment‘. It is enough to be aware of the fact that industrial products utilizing clay raw materials such as porcelain, various ceramic goods, plastics, rubber goods, innumerable sorts of paper and other products influence the environment of mankind. In the formation of modern human environment, all ceramic products have a substantial significance. The building ceramic parts, manufactured bricks and roofing tiles, sanitary ceramics, easily washable tiles for exteriors and interiors in subways, airports and railway stations, shopping centres, private flats, all of them are mainly manufactured from clay raw materials (Manukaji, 2004). Furthermore, clay minerals are of use in various paints and varnishes where they act as filler, provide protection against weathering and improves the flatting effect and adhesion of paint. Paints and varnishes filled with clay minerals also protect against corrosion and erosion (Pattanayak et al., 2014). A vast area of utilization of clay minerals in the protection of the environment includes their role as sorbents and retention-insulation materials. Certain clay minerals are noted for their specific adsorption properties. Kaolinite is suitable for the sorption of fluoride ions from water (Pattanayak et al., 2014)
Radioactive alkaline metals are most effectively sorbed by mica clay minerals, while chlorite is suitable for divalent radionuclides. Clay minerals in rivers, both in suspension and settled in muds, are important adsorbents of toxic substances in solution. The stirring properties of clay in water (including adsorption) have been known since the days of
10
ancient Greece and Rome. The role of clay minerals is also fundamental in agriculture, fruit growing and forestry. Clay minerals in soils are important sources of nutrients. As negatively charged colloidal bridges, they encourage a long-term proton exchange from plant roots for necessary cation released from weathering primary minerals (Speight, 2006) In the building industries, clays and bricks are used as construction raw materials. Bricks are made up of 100% earth materials which include shales, clays, and fine-grained lateritic soils (Obaje et al., 2013). In archeology, clay minerals can serve as archaeological thermometers in the investigation of ancient ceramics. The investigation of clay minerals and argillaceous rocks in geological sciences today is an extensive region of theoretical and applied research. Clay minerals and argillaceous sediments play an important role in the oil geology and oil industry. They are of use partly as adsorbents of unwanted compounds and partly as catalysts for cracking (Speight, 2006; Ismadji, 2015). In the southern part of Nigeria, most regions are richly blessed with natural resources like clay. These clay-based materials occur both in the plain and riverine areas (Amaefule, 1990). The use of these material resources was made possible by the application of heat in transforming the soft clay deposit into something malleable, hard, and durable. The use of these material resources was made possible by the application of heat in transforming the soft clay deposit into something malleable, hard and durable (Dinadle, 1986; Amaefule, 1990; Nweke and Ugwu, 2007).
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It is believed that the history of ceramics really began with the recognition of clay as a useful raw material. The characteristic features of interest include plasticity, resistance to high temperature, malleability and complex composite formulations. The various characteristic properties of most clay products are believed to be the consequence of impurity additions and sintering firing procedure (Nweke and Ugwu, 2007). It is clear and well known that clay–based materials are especially abundant in high temperature areas like Nigeria, since it possesses the ability to resist compressional forces to a reasonable extent. Based on the stacking arrangements and chemical composition of clay; six main groups of clay minerals may be identified: kaolinite, montmorillonite, illite, chlorite, attapulgite and vermiculite (Karpiński and Szkodo, 2015). The presence of minor amounts of mineral or soluble salt impurities in clays can restrict their uses. The more common mineral impurities are quartz, mica, carbonates, iron oxides, sulphides and feldspar (Mohammed et al., 2013).
1.7 Radionuclides of Specific Interest in Clays and the Environment
With regard to internal exposure from ingestion of clay being part of the immediate pathways leading to contamination of food and the environment, the most important radionuclides to be assessed in the environment are:
i. Alpha emitters: 238Pu, 239+240Pu, 241Am, 242Cm
ii. Beta emitters: 89Sr, 90Sr and tritium
iii. Gamma emitters: 134Cs, 137Cs (137mBa), I31I
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In general, the radionuclides of major importance in the contamination of clay/soil or food and environmental samples (materials which are part of the pathways leading to food) are: Air: 131I, 134Cs, 137Cs Water: 3H, 89Sr, 90Sr, 131I, 134Cs, and 137Cs Milk: 89Sr, 90Sr, 131I, 134Cs, 137Cs Meat: 134Cs, 137Cs Other foods: 89Sr, 90Sr, 134Cs, 137Cs Vegetation: 89Sr, 90Sr, 95Zr, 95Nb, 103Ru, 106Ru, 131I, 134Cs, 137Cs, 144Ce Soil/Clay: 90Sr, 134Cs, 137Cs, 238Pu, 239+240Pu, 241Am, 242Cm, 234Th, 235U, 40K The levels of radionuclides in the environment/clay and food have been extensively compiled by UNSCEAR (2008). This group of radionuclides is most likely to be of concern in terrestrially produced foods. Biological concentration processes in fresh water and marine systems can result in very rapid transfer and enrichment of specific radionuclides (EPA, 2008).
1.7.1 Pathways of radionuclides in the environment
Some levels of radiation are naturally present in surface and ground water, but other degrees of radiation exposure come from contact with rocks, clay and soil that have been contaminated with artificially produced radionuclides such as 239Pu, 137Cs90Sr, 241Am and
13
131I. Release of radionuclides into environmental materials is part of the immediate pathways to the commonly encountered hazardous radionuclides through accidents, poor waste disposal or other means. Contamination of clay/soil and water sources can occur from dust transported by wind from uranium mine sites and waste deposits (Neves et al., 2008).
1.7.2 Hazardous radionuclides and their impacts on human health
Natural sources of radioactivity are all around and man-made radioactive materials are vital part of medicine and industry. Exposure to some radiation, natural or man-made is inevitable. We live with radiation every day, therefore we must understand its risks. Radiation is known to cause cancer in humans and other adverse health effects, including genetic defects in children of exposed parents or mental retardation in the children of mothers exposed during pregnancy (EPA US, 2007).
Eighty percent of that exposure comes from natural sources of radon gas, the human body, outer space, rocks and clay/soil. The remaining 20 percent comes from man-made radiation sources, primarily medical x-rays. Radioactive materials that decay spontaneously produce ionizing radiation, which have sufficient energy to strip away electrons from atoms (creating two charged ions) or to break some chemical bonds. Any living tissue in the human body can be damaged by ionizing radiation in a unique manner. When ionizing radiation, it strikes an organism‘s cells and may injure the cells. If radiation affects a significant number of cells, it can eventually lead to cancer. At extremely high doses, this type of exposure can cause death. In general, there is no safe level of radiation exposure (EPA, 2008).
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In the Chernobyl nuclear plant disaster which occurred in Ukraine 1986, some 60 radionuclides were emitted from the reactor (Balonov, 2008), but only a few were considered to present serious health hazards to humans and animals. Immediately after the disaster, the main health concern involved is radioactive iodine-131, with a short half-life (eight days). It can be relatively transferred to humans rapidly from the air and through consumption of contaminated milk and leafy vegetables. Iodine becomes localized in the thyroid gland (EHP, 2010). Today, there is concern about contamination of the soil with strontium-90 (half-life approximately 30 years) which occurs in the clay/soil, food and water. It can be deposited in bones and remain in the body for long periods. In addition, 90Sr could cause problems in areas close to the reactor, but at greater distances its deposition levels are low (EHP, 2010). Uranium mining and the use of nuclear reactors are common sources of radionuclides. They are primarily contained in radioactive wastes, which present serious threats to human health. It also causes lung cancer and death in uranium miners. Long-term exposure to radon leads to an elevated risk of leukemia (Yablokov, 2009).
Cesium-137 and 134 isotopes of cesium have relatively longer half-lives (134Cs has a half-life of 2 years while that of 137Cs is 30 years). These radionuclides cause longer-term exposures through the ingestion pathway and through external exposure from their deposition on the ground. The radionuclide can be ingested or inhaled and resides in muscle, tissue, bones and fat. The geochemical characteristics of this radionuclide are fairly similar to those of nonradioactive 55Cs (El-Reefy, 2004). Therefore, 137Cs released into the atmosphere becomes strongly adsorbed by clay minerals and also by organic matter in soils. It has received particular attention in the marine environment due to its
15
long environmental half-life, high radio-toxicity and easy assimilation by animal and plant tissues. It is usually present as simple cation with high solubility and mobility in marine environments, depending particularly on the sorption of 137Cs to sediment surfaces (El-Reefy, 2004).
1.8 Gamma-ray Spectroscopy
There are a lot of methods and techniques that are applied in the determination of naturally occurring radionuclide in geological, biological and environmental media such as rocks, soil, air and natural wastewater. However, quantitative gamma-ray spectroscopy is a powerful technique available for the non-destructive analysis of samples from such media (Yousef et al., 2007). The basic principles involved in the use of gamma-ray spectroscopy include:
i. Energy Calibration: Energy calibration is simply to assign the correct energy value to the corresponding channel number. The pulse height is assumed to be proportional to the energy of the incident particle. In gamma-ray spectroscopy, the pulse height scale must be calibrated in terms of absolute gamma-ray energy if various peaks in the spectrum are to be properly identified. In many applications, the gamma-rays expected to appear in the spectrum are well known in advance and the corresponding peaks can be identified by inspection. In other applications, unknown gamma-ray spectra may occur and hence a spectra calibration gamma-ray source is used to supply peaks of known energy in the spectrum. Accurate calibration should involve a standard source with gamma-ray energies that are not widely different from those to be measured in the unknown spectrum. It is also useful to have multiple calibration peaks at various points
16
along the measured energy range of interest. The selection of standards to be used for gamma-ray spectrometer calibration depends on the energy range of interest (Bernard et al., 1998; Glenn et al., 2000).
ii. Detection Efficiency: In principle, all detectors give rise to an output pulse or signal for a quantum of radiation, which interacts within its active volume. Radiation such as gamma-ray must first undergo a considerable interaction in the detector crystal before detection is possible because gamma photons can travel large distance between interactions, detectors are often less than 100% efficient. It then becomes necessary to have a precise figure for the detector efficiency in order to relate the number of pulses counted to the number of photons incident on the detector (Gilmore and Hemingway, 1995; Knoll, 2002).
iii. Intrinsic efficiency:
1.1 The intrinsic efficiency of a detector is a detector property and is independent of the geometry. The intrinsic efficiency of a detector depends on the detector material, the radiation energy and the physical thickness of the detector in the direction of the incident radiation.
iv. Absolute Efficiency (photopeak)
Ɛabs = 1.2
The absolute efficiency is dependant not only on detector properties but also on the details of the counting geometry such as the distance from the source to the detector. By
17
using many calibration sources with known activities, the absolute efficiency of the detector for each gamma-ray line can be calculated from the formula:
1.3 (Gilmore and Hemingway, 1995; Knoll, 2002).
1.8.1 Assessments of radioactivity levels
Identification and assessment of low radioactivity levels in different samples emitting gamma rays by High purity Germanium detectors (HPGe) requires two types of calibration such as energy calibration and photopeak detection efficiency calibration. Energy calibration is necessary to identify different isotopes from the respective gamma ray energy lines, while photopeak detection efficiency determination is necessary for quantitative assessment of the radioactivity levels for each radioisotope (Gilmore and Hemingway, 1995).
1.8.2 Biomonitor of accumulated heavy metals in human
Trace element analysis on hair samples has been widely used to assess wildlife and human exposure to different contaminants present in the environment or at the workplace. Several advantages were mentioned for the use of this biological material in monitoring studies (Wenning, 2000; ATSDR, 2001), namely:
(i) The less invasive character of hair collection procedures that avoid veninpucture,
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(ii) The stability of hair, as a biological material, which facilitates the storage and transport processes,
(iii) The higher concentrations of residues usually found in hair samples, when compared to those on blood and urine, and
(iv) The capacity of hair to accumulate metals during extended periods, at least 1 year of exposure.
The World Health Organization (WHO), Environmental Protection Agency (EPA), and International Atomic Energy Agency (IAEA) have recommended the use of hair as an important biological material for worldwide environmental monitoring (Samanta et al., 2004).
1.9 Justification for the Study
Clay eating is a global practice that exists among humans as well as animal species. Several researches carried out on ‗Nzu clay‘, a geophagious material has revealed abnormally high amount of heavy metals. These heavy metals have the capacity to bioaccumulate in human system. Hair testing can complement conventional blood and urine analysis as it enlarges the window of detection and by segmentation, permits differentiation between long-term therapeutic use and single exposure. So, there is a sound motivation for investigating the elements contained in hair of group of people that ingest ‗Nzu clay‘ habitually. The levels of the selected heavy metals in the hair of addicts (long and short term addicts) of ‗Nzu clay‘ will signal the long and short term implication of consuming the clay.
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Exposure to some radiation, natural or man-made is inevitable. We live with radiation everyday; therefore, we must understand its risks. Radiation is known to cause cancer in humans and other adverse health effects, including genetic defects in the children of exposed parents or mental retardation in the children of mothers exposed during pregnancy (EPA, US, 2007). The presence of natural radioactivity in soil results in external and internal exposure to humans. People who ingest ‗Nzu clay‘ are exposed to radioactive element that is natural in soil. There is the need for the clay/soil to be analyzed to determine the radioactive contaminants and the level of contamination to its low levels since radioactive contamination pose a health risk. This would provide information on the safety of consuming the products which is sourced from different environment in the South Eastern part of Nigeria.
1.10 Aim and Objectives of the study
The aim of this work was to evaluate the activity concentration of radionuclides, physico-chemical properties of ‗Nzu clay‘ at the raw and processed stages and to assess the level of heavy metal exposure in ‗Nzu clay‘ addicts using biological samples (hair). This aim was achieved through the following set objectives:
i. Ascertaining the physico-chemical parameters of ‗Nzu clay‘ (swelling power, dispersibility, bulk density, water absorption index, cation exchange capacity and pH);
ii. determining the crystallographic particle size distribution of the raw and finished product of the clay using X-ray Diffraction (XRD);
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iii. determining the activity concentration of radioactive elements in ‗Nzu clay‘ using Hyper-pure Germanium Detector and
iv. assessing the level of As, Cd, Cr, Cu, Pb, Ni, and Zn in ‗Nzu clay‘ samples and in the hair of addicted consumers (less than and above five (5) years) of the clay to compare with those of non-consumers of the clay.
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