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Lead in the Environment – Air, Water, and Soil Lead is a naturally occurring element that is classified as a heavy metal. Most heavy metals are considered to be toxic in one form or another (usually as compounds). Lead itself has been problematic in all parts of the biosphere. However, with an increasing number of regulations being imposed, lead concentrations have been decreasing. This does not mean that lead in the environment can be considered negligible. There still is plenty of lead being emitted from industrial processes and automobiles among other sources. The amount of Pb found in an area depends on multiple factors including its proximity to heavy industries and the geology of a region. Today, natural sources only contribute a small amount of lead. Anthropogenic sources are the primary contributor of lead emissions, amounting to more than 95% of lead deposits (Marcantonio et al ., 2002). There are distinctive chemical characteristics of lead that influence its behavior. According to Goldschmidt’s classification of elements, Pb is a chalcophile (Faure, 1998) which means lead has a high affinity for sulfur or sulfur containing compounds. It is also strongly attracted to phosphorus, especially for phosphate ions (PO4 -3) (Yoon, 2005). Pb is also known to readily substitute for certain metals. Often, Pb+2 ions replace K+ and Ca+2. This explains why lead is known to accumulate in the bones of the body, if high concentrations of lead get into the bloodstream. With minerals, this process occurs as well. Lead is able to mimic the behavior of other metals and undergo an exchange with another cation. Additionally, lead tends to attach itself to other compounds or particles. Lead adsorbs strongly to small particles (clays, fine particles, etc) (EPA, 1986). In particular, Pb usually attaches itself to organic compounds (forming alkyl leads), which can be problematic.
Pb is not a commonly found element within the earth, but it does form naturally. Lead is found within the earth’s core and as the end-product of the radiometric decay of three naturally- occurring radioactive elements: uranium, thorium, and actinium (Faure, 1998). Lead is most often found as PbS, or galena, which makes up about 0.002 % of the earth's crust (Calvert, 2004). Significant amounts of lead are also found as byproducts or bound together with other metallic ore deposits such as zinc, silver, and copper. Cerussite (PbCO3) and angelsite (PbSO4) are products of the chemical alteration of galena (Reuer and Weiss, 2002). Most alterations of galena are a result of chemical weathering: PbS + H2CO3 → PbCO3 + H2S. Carbonic acid that is formed by the interaction of carbon dioxide with water is able to undergo a substitution reaction with PbS, forming lead carbonate. Furthermore, lead carbonate can dissociate into PbO and CO2. Galena can also be altered as it is extracted as an ore via smelting. Lead compounds can be created by human activities. Adding hydrogen sulfide (or sulfide salts) to a mixture of lead ions (Pb+2) gives a fairly insoluble black product consisting of PbS (galena): Pb2+ + H2S → PbS + 2 H+ (Calvert, 2004). This is one way in which galena can form. Since PbS is the main ore of lead, much attention has been given to developing a means of processing PbS. The smelting of PbS and then reducing the resulting oxide is commonly done in industrial processes. The chemical smelting process is a reduction process overall. 2 PbS(s) + 3 O2(g) → 2 PbO(s) + 2 SO2(g) - oxidation PbO + C → Pb + CO – reduction in presence of charcoal 2 PbCO3(s) + C(s) → 2 Pb(s) + 2 CO2(g) – formation of purified lead (Brady et al ., 1996) Oxidation of lead may also occur upon lead coming into contact with oxygen: 2Pb(s) + O2(g) → 2PbO(s). An important feature that limits the spread of lead through the environment is the low solubility of many lead based compounds. Native lead (PbS) does not dissolve in water under normal conditions. Lead is also insoluble in many other forms (ex: PbSO4). In these forms lead is present as an immobile (insoluble) compound in the environment. This means that the lead
PBr2 (Nriagu, 1990). Lead produced can oxidize further when coming into contact with the atmosphere: 2 Pb + O2 → 2 PbO. This was a major form of lead, along with PbClBr that contributed to atmospheric lead. It should be noted that the lead produced is not in the form of a gas, but is a solid. The solid form of lead is produced as very fine microscopic particles. Today, mining and smelting lead produces lots of PbSO4 and PbS (Spear et al ., 1998). PbSO4 and PbCO3 are the most common forms of lead that form in the air, resulting from chemical alterations. In cars, lead from batteries causes lead salts (chlorines, bromines, etc) to form. These lead salts attached to fine particles enter into the atmosphere via a cars exhaust. This lead-cycle caused by human production is much more extended than the natural lead-cycle. It has caused lead pollution to be a worldwide issue. Lead can end up directly in bodies of water by a variety of processes. Surface runoff may contain Pb in solution (usually from agricultural lead containing compounds or even lead paint chips). Under normal conditions lead does not react with water. When lead comes into contact with moist air, reactivity with water gradually increases. A small lead oxide (PbO) layer develops on the surface of the metal (Yoon, 2005). When both oxygen and water are present, metallic lead can be changed into lead hydroxide by the subsequent reaction: (Pb(OH)2): 2Pb(s)+ O2(g) + 2H2O(l) - > 2 Pb(OH)2(s) (Yoon, 2005). Another source of lead is from wet deposition (Reuer and Weiss, 2002). Lead containing aerosols released from industrial processes can be ‘washed out’ of the atmosphere. Contamination of water, from lead batteries, is another way in which lead can be leached into water. The downward movement of Pb ions, or Pb compounds through soil (although not very common) can potentially lead to contamination of groundwater. Solubility, which is related to pH, is the ultimate factor that determines how lead reacts in water. The amount of soluble lead depends on pH. When pH>5.4 carbonates limit solubility, while with a pH<5.4 sulfates control lead precipitation (EPA, 1979). Lead compounds dissociated best under very acidic conditions.
Sources of lead that contribute to Pb in soil include: smelting, mining, vehicle exhaust, lead based paint ((PbCO3)2·Pb(OH)2), and pesticides. PbHAsO4 (lead arsenate) is a potent pesticide that contains not only lead but also arsenic. This is a highly toxic pesticide that ends up in the soil and can eventually infiltrate bodies of water. Lead based paint is not as big of a problem today as it was in the past. Older homes with exteriors of leaded paint could be a source of some lead contamination. Lead paint chips can fall off onto ground, break down into fine particles, and infiltrate into the soil. Lead from smelting and vehicle exhaust typically goes into the atmosphere first and then may end up in the soil. Soils have varying effects on lead and lead based compounds. Within soil, lead ions or compounds could be adsorbed onto minerals, precipitate out of solution, form stable organic- metal complexes, or undergo chelation. All processes depend on: soil pH, soil type, particle size of Pb compound, organic matter content of soil and cation exchange capacity (Reddy et al ., 1995). Ph plays a large factor as the mobility of lead increases in low pH environments while high pH limits mobility. Lead tends to be immobilized by ion exchange with hydrous oxides, clays, and chelating agents (Olson and Skogerboe, 1975). Soil acts as a sink for lead and lead containing compounds as most lead stays in soils. Clays, silts, iron, and manganese oxides bind to Pb electrostatically (cation exchange) and chemically (adsorption). Lead does takes a long time to permeate down through soil layers and into groundwater. Much lead gets ‘trapped’ in upper layers (Wang et al ., 1995). This occurs as results of much clays and other materials capable of ‘capturing’ lead existing in the upper layers of soil. Layers of soil appear to help filter out heavy metals. There does appear to be a general pattern in terms of the chemical pathways by which lead species move. Galena can undergo chemical weathering and be altered to lead carbonate. Lead carbonate can then undergo dissociation and Pb+2 ions can dissociate. The lead ions may then come into contact with oxygen (along with moisture) to form PbO. Very fine particles may remain airborne for a few hours or maybe days, and then end up in the hydrosphere or on land. In
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