Lead (Pb) and water
Lead and water: reaction mechanisms, environmental impact and health effects
|Seawater contains trace amounts of lead (2-30 ppt). On average rivers contain between 3 and 30 ppb. Phytoplankton contains approximately 5-10 ppm lead (dry mass), freshwater fish approximately 0.5-1000 ppb, and oyster approximately 500 ppb. |
The World Health Organization (WHO) stated a legal limit of 50 ppb for lead in 1995, which is decreased to 10 ppb in 2010.
Under normal conditions lead does not react with water. However, when lead comes in contact with moist air reactivity with water increases. A small lead oxide (PbO) layer forms at the surface of the metal. When both oxygen and water are present, metallic lead is converted to lead hydroxide (Pb(OH)2):
2Pb(s)+ O2(g) + 2H2O(l) -> 2 Pb(OH)2(s)
Elementary lead does not dissolve in water under normal conditions (20oC, and pressure = 1 bar). It may however occur dissolved in water as PbCO3 or Pb(CO3)22-. A well-known example of a water soluble lead compound is lead sugar (lead(II)acetate), which derived its name from its sweet nature.
Lead waterworks were often applied in former days, and these may still be present in old buildings. Lead from pipes may partially dissolve in the water flowing through. Lead may bind to carbonate, therefore lower amounts of lead dissolve in hard water. Inside the pipes, a layer of hardly soluble alkalic lead carbonate is formed. This layer functions as a protective coating for the underlying lead of the pipes. The Romans often filled the pipes with wine on holidays, causing the layer to dissolve and form lead sugar.
Lead and lead compounds are generally toxic pollutants. Lead(II)salts and organic lead compounds are most harmful ecotoxicologically. Lead salts are attributed to water hazard class 2, and consequently are harmful. The same applies to lead compounds such as lead acetate, lead oxide, lead nitrate, and lead carbonate.
The human body contains approximately 120 mg of lead. About 10-20% of lead is absorbed by the intestines. Symptoms over overexposure to lead include colics, skin pigmentation and paralysis. Generally, effects of lead poisoning are neurological or teratogenic. Organic lead causes necrosis of neurons. Inorganic lead causes axonal degeneration and demyelination. Both species of lead may cause cerebral oedema and congestion. Organic lead compounds are absorbed quicker, and therefore pose a greater risk. Organic lead derivates may be carcinogenic. Women are generally more susceptible to lead poisoning than men. Lead causes menstrual disorder, infertility and spontaneous abortion, and it increases the risk of stillbirth. Foetuses are more susceptible to lead poisoning than mothers, and generally foetuses even protect mothers from lead poisoning. A long time ago lead was applied as a measure of birth control, for example as a spermicidal, and to induce abortion.