|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.
In what way and in what form does lead react with water?
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)
Solubility of lead and lead compounds
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 frequently binds to sulphur in sulphide form (S2-), or to phosphor in phosphate form (PO43-). In these forms lead is extremely insoluble, and is present as immobile compounds in the environment. Lead compounds are generally soluble in soft, slightly acidic water.
Why is lead present in water?
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.
Despite its toxicity, the above-mentioned lead(II)acetate was applied far into the 19th century, to sweeten wine and other beverages and food stuffs.
Water pollution containing lead compounds derived from lead ores in the mining industry was first mentioned by architect Vitruvius, in 20 B.C., when he gave out a warning of its health effects. In Rome lead was often released as a by-product of silver mining.
Lead white, an alkalic lead carbonate (2PbCO3.Pb(OH)2), is a white pigment. It is no longer applied because of its extreme toxicity. Selling tubes of lead white is now prohibited in the European Union.
Organic lead is applied in petroleum production, and inorganic lead compounds are applied for battery and paint production.
Most industrially processed lead is applied for fabricating computer and TV screens. The lead compound tetra-ethyl lead is applied as an additive in fuels. This organic lead compounds is quickly converted to inorganic lead, and ends up in water, sometimes even in drinking water. Fortunately, this form of release of lead is less and less abundant.
In architecture lead is applied in roofs and in stained glass windows. Generally, lead dissolved or suspended in wastewater mostly stems from streets, pipes and soils.
In The Netherlands, lead in food causes about half of the human lead exposure.
What are the environmental effects of lead in water?
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.
Lead limits plant chlorophyll synthesis. Nevertheless, plants can take up high levels of lead up to 500 ppm from soils. Higher concentrations negatively influence plant growth. Through plant uptake, lead enters food chains. Consequently, lead pesticide application is prohibited in most countries. Lead accumulates in organisms, sediments and sludge. Lead in wastewater mostly stems from streets and roofs.
Lead exists as four stable isotopes, and no less than 26 instable isotopes.
What are the health effects of lead in water?
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.
Children may absorb a larger amount of lead per unit body weight than adults (up to 40%). Consequently, children are generally more susceptible for lead poisoning than adults. Symptoms include lower IQs, behavioural changes and concentration disorder.
Lead accumulates in leg tissue. The most severe type of lead poisoning causes encephalopathy.
Lead toxicity is induced by lead ions reacting with free sulfydryl groups of proteins, such as enzymes. These are deactivated. Furthermore, lead may interact with other metal ions.
Which water purification technologies can be applied to remove lead from water?
Lead removal from water may be established applying coagulation, sand filtration and ion exchange. Additionally, active carbon, KDF media filtration and reverse osmosis may be applied.
Literature and the other elements and their interaction with water