1.1 Nature of Lead
According to WHO (2010) Lead is a heavy metal with a bluish-grey colour. It has low melting point, is easily moulded and shaped, and can be combined with other metals to form alloys.WHO (2010) classified lead in three as given below:
Elemental lead: The chemical symbol for lead is Pb (from the Latin name for lead, plumbum). Lead has an atomic number of 82 and an atomic weight of 207.2. It is a bluish-grey metal that tarnishes easily in air to a dark grey. The density of lead is 11.34 g/cm3. It has a low melting point of 327.46 °C or 621.43 °F.
Naturally occurring ores: Lead ores comprise 0.002% (15g/t) of the earth’s crust. They include galena (lead sulfide), anglesite (lead sulfate), cerussite (lead carbonate), mimetite (lead chloroarsenate) and pyromorphite (lead chlorophosphate).
Inorganic lead: This is the form of lead found in old paint, soil, dust and various consumer products. The colour varies, depending on the chemical form, and the most common forms are white lead (a lead carbonate compound), yellow lead (lead chromate, lead monoxide) or red lead (lead tetraoxide). Lead acetate has a sweetish taste.
Organic lead: Tetra-ethyl lead is the form of lead used in leaded gasoline. Organic forms of lead are extremely dangerous, as they are absorbed through the skin and are highly toxic to the brain and central nervous system, much more than inorganic lead. The combustion of organic lead – when it is added to petrol as a fuel additive – results in the release of lead into the atmosphere.
According to Flora et al. (2012), lead is ubiquitous and one of the earliest metals discovered by the human race. Lead is a soft, malleable metal included in the group of heavy metals. It has a lustrous silver-blue appearance when freshly cut, but darkens to a dull grayish color when exposed to moist air. This occurs due to the immediate formation of an oxide film that protects the metal from further oxidation or corrosion. A dense, ductile metal with low tensile strength, lead has a face-centered cubic crystalline structure, and poor electrical conductivity. It is highly resistant to corrosion and can be toughened by adding a small quantity of antimony or other metals to it. Natural occurrence of lead is very rare; it is found in ores with zinc, copper, and silver, and is later extracted from these elements. The most important mineral that lead is extracted from is Galena (PbS), which contains 86.6% lead. Cerussite (PbCO3), or lead carbonate, is another important ore of lead, as is anglesite (PbSO4), a lead sulfate mineral that occurs after oxidation of the primary lead sulfide ore, Galena.
1.2 Lead as a Heavy Metal
|According to Lenntech (2017), heavy metals refer to any metallic chemical element that has a relatively high density and is toxic or poisonous at low concentrations. Fergusson (1990) also defined heavy metals as metallic elements that have a relatively high density compared to water. Lead as a heavy metal is considered as trace element because it occurs in trace concentrations (ppb range to less than 10ppm) in various environmental matrices (Kabata-Pendia, 2001).|
|1.3 Isotopes of Lead|
Lead has high number of isotopes and this is attributed to the even nature of its atomic number. Four naturally occurring and stable isotopes (204Pb, 206Pb, 207Pb and 208Pb) are identified (Haack et al., 2003). The decay chains of uranium-238, uranium-235, and thorium-232 produces three of the isotopes lead-206, lead-207, and lead-208 respectively as their final products. Thedecay chains are, therefore, called the uranium series, the actinium series and the thorium series (Beeman et al., 2013). The concentrations of these isotopes in natural rock sample depends greatly on the presence of these three parent uranium and thorium isotopes. For example, the relative abundance of lead-208 can range from 52% in normal samples to 90% in thorium ores (Smirnov et al., 2012). Relative to 204Pb, the half-lives of the decay schemes of 238U, 235U and 232Th to yield 206Pb, 207Pb and 208Pb are 4.47 x 109, 7.04 x 108 and 1.4 x 1010 years respectively (Rosman and Taylor, 1999). Some traces of radioactive isotopes of lead exist apart from the stable ones. The most stable of the radioisotopes of lead is lead-205 with a half-life of about 1.5 x 107 years, followed by lead-202 which has half-life of 5.3 x 104 years. Lead-210 is produced by long decay series that starts with uranium-238 and it has a half-life of 22.3 years (International Atomic Energy Agency, 2017; Fiorini, 2010). Others include lead-211, -212, and -214 which are also present in the decay chain of uranium-235, thorium-232, and uranium-238 (Levin, 2009).
Although the various sources of lead have specific isotopic signatures, stable lead isotopes have been used generally to distinguish between lead originating from natural or anthropogenic sources (Erel et al., 1997).
Lead isotope analysis has proved to be an effective technique for identifying the origin of lead in different terrestrial, marine and aquatic ecosystems (Bacon et al., 2004). Lead-210 is particularly useful for helping to identify the ages of samples by measuring its ratio to lead-206 (both isotopes are present in a single decay chain) (Fiorini, 2010).In the past two decades, research has examined lead isotopes signatures to trace emission sources and assessed spatial and temporal changes of recent lead pollution originating from lead smelters and manufacturing plants and from the use of alkyllead in petroleum products, particularly before 1990 (Bollhofer and Rosman, 2001; Pacyna et al., 1995).Stable lead isotope ratios can be used as a complementary tool in dating contamination events and for the evaluation of sedimentation rates, including the use of lead-210 (Saint-Laurent et al., 2008). In the recent years, a growing number of environmental science studies on soil and sediment contamination were characterized by the use of lead stable isotopes to determine the source and origin of this element (geogenic or anthropic) and to evaluate its persistence in the environment (Komarek et al., 2008).The decay of uranium and thorium to lead permits geological age determinations to be made of minerals containing the heavy radioactive elements. Extensive use of lead over the history of mankind has led to widespread pollution, and the isotope-abundance variations reflected in the atomic weights enable historical and modern sources to be identified (De Laeter et al., 2003).
1.4 Physical Properties of Lead
Lead has a face-centered cubic structure like the similarly sized divalent metals, calcium and strontium. This is because the p-electrons are delocalized and shared between the Pb2+ ions (Christensen, 2002). The face-centered cubic structure is closely packed resulting in a high density of 11.34g/cm3 (Thornton et al., 2001). Hence, it has greater density than that of common metals such as iron which has density of 7.87g/cm3, copper (8.939/cm3) and zinc (7.14g/cm3) (Lide, 2005). Other properties of lead include malleability and high resistance to corrosion (due to passivity). It is also a very soft metal and can be scratched with nails (Vogel and Achilless, 2013). Gale and Totemeier (2003) reports that lead has a bulk modulus of 137.8GPa which is lower than that of some common metals such as aluminum (75.2GPa), copper (137.8GPa) and mild steel (160-169GPa). Bulk modulus is a measure of the compressibility of a substance. In the same vein, the tensile strength of lead which is 12-17MPa is 6 times less than that of aluminum, 10 times less than that of copper and 15 times less than that of mild lead. The strength of lead can, however, be improved by addition of small amounts copper or antimony (Thornton et al., 2001). Lead has low melting and boiling points compared to other metals. The melting point of lead is 327.5oC (621.5oF) while the boiling point is 1749oC (3180oF). It has electrical resistivity of 192nΩm at 20oC and is higher than other industrial metals like aluminum (24.15nΩm), copper (15.43nΩm) and gold (20.51nΩm) (Lide, 2005). In terms of superconductivity, lead is the third highest elemental superconductor and it exhibits superconductivity at temperatures lower than
-265.81oC (7.19K) (Webb et al., 2015; Blakemore, 1985).
1.5 Chemical Properties of Lead
Bulk lead is chemically inert when exposed to moist air due to the formation of a protective layer of varying compositions. The common constituents of the layer is carbonate, sulfate and chloride of lead (Thürmer et al., 2002; Greenwood and Earnshaw, 1998). However, lead in its finely powdered form is pyrophoric and burns with a bluish-white flame (Brethrick, 2016).
1.5.1 Oxidation States of Lead
Lead mainly exists in two oxidation states of +2 and +4. However, it is predominantly tetravalent with elements of similar electronegativity such as carbon (as in organolead compounds). This arises because the similarity in the sizes of the 6S and 6P orbitals energetically favours SP3 hybridization in lead (Kaupp, 2014). This is, however different in its divalent state which are rare for carbon and silicon. In this divalent state there is significant partial positive charge on lead arising from the wide margin in the differences in electronegativity between lead and for example its oxides, halides or nitrides anions. Hence, the 6S orbital in lead is more contracted than that of its 6P orbital. The electronegativity of lead (II) at 1.87 highly differs from that of lead (IV) at 2.33 (Dieter and Watson, 2009).
1.5.2 Compounds of Lead.
Lead compounds can be in the forms of lead (II), lead (IV) and organolead compounds. Mostly the inorganic compounds of lead are at the lead (II) oxidation state. Lead combines with fluorine, chlorine and sulfur in the +2 oxidation state to form the respective fluoride, chloride and sulfide (Hunt, 2014; Bunker and Casey, 2016). Lead also forms two polymorphs of lead (II) oxide namely: red α-PbO and yellow ß-PbO, with ß-PbO being stable only at temperature above 488oC (Greenwood and Earnshaw, 1998). Lead (II) ions usually forms colourless solutions which partially hydrolyze to form Pb(OH)+ and finally Pb4(OH)4+ with the hydroxyl ions acting as bridging ligands, but not reducing agents. However, the lead (II) hydroxide formed by the solution of lead (II) oxide remains in solution and forms plumbite anion (Hunt, 2014; King, 1995; Greenwood and Earnshaw, 1998).
The heavier chalcogens such as sulfur, selenium and tellurium to react with lead form characteristic photoconductive lead chalcogenides, that is, sulfides, selenides and tellurides of lead respectively (Greenwood and Earnshaw, 1998). Lead also forms dihalides with the halogens including the diastatide and mixed halides such as PbFCl. The mixed halides, PbFCl is relatively insoluble and this forms the basis for gravimetric determination of fluorine (Funke, 2013).
According to the ATSDR (2007a), the inorganic lead (IV) compounds known are few. An example of the compounds is a mixed oxide, lead (II, IV) oxide which is formed from further oxidation of lead (II) oxide. Other examples of lead (IV) compounds are lead disulfide and diselenide which are only stable at high pressures (Macintyre, 1992). Lead tetrahalides exist as lead (IV). Examples are lead tetrafluoride which is less stable than its difluoride; lead tetrachloride which decomposes at room temperature; lead tetrabromide which is even less stable while lead tetraiodide does not exist (Greenwood and Earnshaw, 1998).
Other than +4 and +2, lead exists in other oxidation states. For instance, lead (III) is an unstable ion of lead obtained as an intermediate between lead (II) and lead (IV) and is found in larger organolead complexes (Becker et al., 2008; Konu and Chivers, 2011). Others are some mixed oxides of lead when lead (II) oxide is heated in air at various temperatures. Examples are Pb12O19 at 293oC, Pb12O17 at 351oC. At high pressure a sesquioxide Pb2O3 is obtained (Greenwood and Earnshaw, 1998).
Lead exists in negative oxidation states in some of its compounds either as free lead anions, e.g. Ba2Pb in which case lead has oxidation number (-IV). Also in its polyhedral cluster ions such as in trigonal bipyramidal Pb52- ion in which case two of the lead atoms are lead (-1) and three are lead (0) (Röhr, 2017; Alsfasser, 2007). Each atom of lead in such anion is at a polyhedral vertex where it contributes 2 electrons to each covalent bond along an edge from their SP3hybrid orbitals, the other two being an external lone pair (King, 1995).
Another class of lead compounds referred to as organolead compounds are formed due to the ability of lead to form multiple bonded chains such as with carbon which is a lighter homolog of lead. Lead can build metal-metal bonds of an order up to three itself (Stabenow et al., 2003). However, the organometallic chemistry of lead is far less wide-ranging than that of tin. This is because the Pb-C bond is a weaker bond. As such the organolead compounds with carbon is less stable than typical organic compounds (Polyanskiy, 1986; Greenwood and Earnshaw, 1998).
Plumbane (PbH4) also called lead tetrahydride or lead (IV) hydride or tetrahydridolead is a lead analog of the simplest organic compound, methane and may be obtained in reaction between lead and atomic hydrogen. It is an unstable colourless gas and the heaviest group IV halide (Hein et al., 1993). Some derivatives of plumbane which are relatively stable are tetramethyllead [Pb(CH3)4] and tetraethyllead [(CH3CH2)4Pb], lead tetrafluoride (PbF4) (Wilberg et al., 2001; Toxicological Profile for Lead, 2007). Other examples of stable organolead compounds include tetraphenyllead (C24H20Pb) which is a stable compound below 270oC, lead tetraacetate [Pb(C2H3O2)4]which is an important laboratory reagent used in organic chemistry as oxidizing agent (Schürmann and Huber, 1994).
1.6 Uses of Lead
Lead has served several functions to mankind especially several years ago. Flora et al. (2012) stated that some unique properties of lead, including its softness, high malleability, ductility, low melting point and resistance to corrosion, have resulted in its widespread usage in different industries like automobiles, paint, ceramics, plastics, etc. Lead was used extensively in the past in building construction (Kanika, 2016).
The most important use of lead is seen in the automobile industry in the form of batteries. As they are inexpensive compared to newer technologies, lead-acid batteries are widely used even when surge current is important and other designs could provide higher energy densities. Most of the world’s lead-acid batteries are automotive starting, lighting and ignition (SLI) batteries (Linden and Reddy, 2002). They are also used in emergency lighting and to power sump pumps in case of power failure (Crompton, 2000) and in backup power supplies in many situations, including storing the energy captured from solar panels as large and industrial lead-acid batteries (Planet Ark Research Report, 2011)
Because of its resistance to corrosion and discoloration, lead was widely used early in pipes for the collection, transport, and distribution of water and containers for the storage of food and beverages. Although the use of lead in drinking water pipes has been largely discontinued in developed countries, lead-based systems are still found in old buildings in older cities (Wright and Welbourn, 2002).
Another use of lead is in paint pigments and in ceramics as colouring elements. Lead-based paints are widely used because they cling well to wood and the presence of lead imparts brightness to the colour (Angier, 2007). This use continues in developing countries. In 2007, US based toy companies and the US Consumer Product Safety Commission recalled toy trains, toy cars, and inexpensive children’s jewelry manufactured in China because they contained lead-based paint (Lipton and Story, 2007).
The organolead derivative of lead compounds called tetraethyl lead (TEL) is used as an additive to gasoline used in vehicle engines. This serves as a patented octane rating booster or anti-knock agent that allowed engine compression to be raised substantially. In turn, the use of higher compression ratios which enhances the performance of the vehicle is achieved (Loeb, 1995; Seyferth, 2003). The use of TEL as octane rating booster became more promoted than ethanol, though ethanol was comparatively cheaper and less toxic, because it was uniquely profitable to the patent holders and also the oil industry was generally hostile to ethanol, (Kitman, 2000). Though the use of lead has been banned in so many countries, however, leaded gasoline are still used in some other countries (Schnaas et al., 2004).
Due to its low melting temperature and wide availability, lead, along with tin and other alloys, act as the most commonly used solder material for electronics. The alloy commonly used for electrical soldering are 60/40 Sn-Pb, which melts at 188 oC (370 oF) and 63/37 Sn-Pb used principally in electrical/electronic work. Lead-tin solders readily dissolve gold plating and form brittle intermetallics (Howard, 2001). Lead-free solders have been increasing in use due to regulatory requirements plus the health and environmental benefits of avoiding lead-based electronic components. They are almost exclusively used today in consumer electronics (Ogunseitan, 2007).
Lead is also used in aprons to shield patients during X-rays. This safeguards against scatter radiation. Lead is the more preferred material for radiation shielding because it is dense and can be used against various high-energy applications (Canada Metal North America,2016).Shielding is mainly achieved by wearing protective lead aprons of 0.25 or 0.5 mm thickness which have been cited to attenuate over 90% and 99% of the radiation dose respectively (Bushberg et al., 1994). However, reduced radiation use (e.g. by using robotic guidance) is concluded as a more effective strategy for minimizing exposure to radiation than reliance on protection by lead aprons (Seung-Jae et al., 2016).
However, notwithstanding the various and widespread use of lead in the industries, it serves no useful function in the human body (Rastogi, 2008). Also, it is not easily degradable in the human body.
1.7 Sources and Routes of Lead Exposure to Humans
Basically, the major sources of lead to human exposure arise from its extensive use by mankind in the household and in industrial processes and productions. According to USEPA (2006), Lead is used in thousands of applications, and each of these uses has constituted a potential exposure source.Global consumption of lead is increasing today, because of increasing demand for energy-efficient vehicles. The use of lead in storage batteries currently has far exceeded its use in petrol (International Lead and Zinc Study Group, 2009).The extensive use of lead in several industrial productions due to its unique properties results in a manifold rise in the occurrence of free lead in biological systems and the inert environment. Also, due to the non-biodegradable nature of lead it persists in the environment, thereby increasing chances of its exposure to humans (Flora et al., 2012). Human exposure to lead occurs through various sources like leaded gasoline, industrial processes such as lead smelting and coal combustion, lead-based paints, lead containing pipes or lead-based solder in water supply systems, battery recycling, grids and bearings, etc. Children and adults are still routinely exposed to very high levels of lead in developing countries, particularly in regions with a long mining history (Plumlee and Morman, 2011; Fontúrbel et al., 2011).
However, the largest contributor to global environmental lead contamination has been the use of lead in petrol (OECD, 1999; Landrigan et al., 2002). After lead in petrol, lead in paint is the next largest sources of exposure to lead. Leaded paint can remain a source of exposure to lead and lead poisoning for many years after the paint has been applied to surfaces. As lead-based residential paint deteriorates with age or as homes undergo renovation, lead-containing dust is generated. As a result, lead can be found in lead-painted homes in high concentrations in three media to which children may be directly or indirectly exposed: (a) the paint itself; (b) interior dust; and (c) exterior soil or dust (WHO, 2010).
Humans are exposed to lead via inhalation of lead-contaminated dust particles or aerosols, drinking contaminated water, and eating contaminated food; acidic foods and beverages will solubilize lead from containers. Children are also exposed to lead when they eat leaded paint chips from toys, jewelry, and the walls of old buildings (Bradl, 2005; ATSDR, 1999a).Adults absorb 35 to 50% of lead through drinking water and the absorption rate for children may be greater than 50%. Lead absorption is influenced by factors such as age and physiological status. In the human body, the greatest percentage of lead is taken into the kidney, followed by the liver and the other soft tissues such as heart and brain, however, the lead in the skeleton represents the major body fraction (Flora et al., 2006).The relative importance of these various potential sources of exposure to lead varies both within and between countries and regions. In the United States, for example, lead-based paint is an important source of exposure, while in Mexico, lead-glazed ceramics used for food storage and preparation are much more important (Rojas-López et al., 1994). In the low-income world the informal recovery of lead from car batteries and the open burning of waste are very important sources of environmental lead contamination. WHO (2010) states that the major sources of children’s exposure to lead are:
- lead added to petrol
- lead from an active industry, such as mining (especially in soils)
- lead-based paints and pigments
- lead solder in food cans
- ceramic glazes
- drinking-water systems with lead solder and lead pipes
- lead in products, such as herbal and traditional medicines, folk remedies, cosmetics and toys
- lead released by incineration of lead-containing waste
- lead in electronic waste (e-waste)
- lead in the food chain, via contaminated soil
- lead contamination as a legacy of historical contamination from former industrial sites.
1.7.1 Lead in Water
According to Baran et al. (2014), human exposure to metals is predominantly associated with contaminated groundwater and soils. Lead in drinking water is of intermediate significance as a source of lead intake, but is highly significant for both children and the fetuses of pregnant women. Water service lines made from lead, lead solder, or plumbing materials that contain lead are means through which drinking water is contaminated (Brown and Margolis, 2012). Lead from the atmosphere contaminates bodies of water, thereby making water an important source of lead poisoning (Murata et al., 2009).
Brown and Margolis (2012), posits that water treatment and disinfection practices using chloramines instead of free chlorine makes the transformed highly insoluble lead scale minerals no longer stable and makes it to dissolve. This makes a substantial amount of lead to be released from the lead service lines into drinking water at the tap. Therefore, there is a relationship between blood lead levels (BLLs) in children, the presence of a lead service line, and water disinfection practices. Partial lead service line replacement has been associated with short-term increases in lead levels in drinking water and has not been found to decrease risk for BLLs ≥5 µg/dL in children (Brown, 2012).In children, lead in drinking water has been associated both with BLLs ≥10 µg/dL (Cosgrove et al., 1989) as well as levels that are higher than the U.S. GM level for children (1.4 µg/dL) but are <10 µg/dL (Miranda et al., 2007; Lanphear et al., 1998; CDC, 2004). The nature of plumbing also may be important in this regard. Although use of lead pipes (largely replaced by copper or polyvinyl pipes) has declined considerably since the 1950s, old public water systems continue to have networks that include lead piping. Because the use of lead-based soldering of copper pipes was permitted until 1986, homes with copper plumbing may have substantial lead in the water. In May 2015, at least 28 children under the age of five have been killed by drinking stream water contaminated with lead in Niger state, Nigeria (Berg, 2009).