LEAD PRESENTATIONRaymond Fellerman

Biology 464 Aquatic Toxicology

Professor Zed Mason

03 May, 2011

Picture from scienceblogs.com

WHAT IS LEAD?Not the stuff on your pencil mind you!

Lead is defined as a bluish-white lustrous metal that is very soft, highly malleable, ductile, and a poor conductor of electricity.

It is fairly resistant to corrosion.

It tarnishes upon exposure to air, but an oxide will form to prevents deeper corrosion to the air.

Basic Information Name of the element: lead Symbol: Pb Atomic Number: 82 Atomic mass: 207.2 amu Melting point: 327.5 degrees Celsius,

621.5 degrees Fahrenheit Boiling point: 1,740 degrees Celsius,

3,164 degrees Fahrenheit Number of protons and electrons: 82 Number of Neutrons: 125 Natural isotopes : 204, 206, 207, and 208. (204Pb, 206Pb, 207Pb, and 208Pb) Radiogenic isotope: 202Pb (half life of

52,500 years) Lead isotopes are the end products of

each of naturally occurring radioactive elements.

www.chemicalelements.com

History and Uses

Lead has been used for thousands of years. It is fairly common, easy to extract via smelting and is highly

malleable and ductile. Earliest use for lead was in jewelry as beads, but it was later used

as pigments in pottery. Romans used lead in piping, from which the term plumbing

comes from. Lead used to be in paint as it was a primer. Lead used to be used in gasoline as an additive. Lead was banned from consumer paint in the U.S. in 1977, but is

still used in bridge paint. Lead is still used in commercial products such as automotive

batteries, computers, jewelry, pewter, some ceramic glazes. Oddly it is also used in dental fillings.

Contamination.

Lead can contaminate water, food, and beverages, but it’s presence can’t be seen, tasted, or smelled.

Imported home remedies and cosmetics can contain lead.

Lead concentrations in soil, air, and water can be high near the sites of current or historic mining.

Environmental Modes of Entry

Lead can enter organisms by being ingested, inhaled, or absorbed through the skin.

Anthropogenic emissions are the leading cause of lead pollution and lead to an alteration of the natural lead cycle.

Deposition from the atmosphere is the major contributor to lead in aquatic systems.

Lead can also be carried in runoff from mines or contaminated soils in dissolved or particulate forms.

Only a few lead compounds can dissolve in water as most lead is precipitated and incorporates into the sediments.

Bioavailability Knowing the total lead content of a soil is not helpful in

assessing the potential risk. The degree of exposure depends on the form of the lead. Common compounds: Lead Azide (PbN6)

Lead Bromate (Pb(Bro3)2*H2O)

Lead Chloride (PbCl2)

Lead Dioxide (PbO2)

Lead Oxide (Pb3O4)

Lead Nitrate (Pb(NO2) 2)

Tertraethyl Lead (Pb(C2H5) 4)

Tetramethyl Lead (Pb(CH3) 4 the latter being the most toxic form. Lead is most soluble under acidic conditions such as those

present in the stomach.

Lead in Gas

Tetraethyl lead (TEL) gets a special mention as it was used in leaded fuels to prevent engine knocking.

Unfortunately it could react with compounds in the gasoline to produce the more toxic organic forms and be inhaled.

The burning of TEL may cause the formation of various free radicals.

Entry into Organisms

Lead enters most organisms via ingestion, typically only 20% is absorbed. Though if it is inhaled nearly all of it is absorbed.

Some organisms (such as molluscs) can absorb solid lead compounds from sediments, but there hasn’t been many studies on it.

Inorganic lead is not metabolized in the liver while all organic lead is metabolized in the liver.

The half-life of lead in adults is 28-36 days. Lead can be deposited in the blood, bone, and soft

tissues.

Effects of Lead

Lead has the ability to inhibit or mimic the actions of calcium at lower concentrations

Lead can act like other metals in enzymatic reactions and binds with the sulfur groups called sulfhydryls on proteins or enzymes and inactivates them.

Lead can also bind to DNA and is a known Carcinogen.

10 μg/dL was adopted by CDC in 1991 as an action level for children, and is also an advisory level for the environment.

The nervous system is the most sensitive target of lead exposure, while renal effects, and increased susceptibility to disease may also occur.

Lead effects Delta-aminolvulic acid dehydratase (ALAD) which is important in the biosynthesis of heme.

Lead also inhibits the enzyme ferrochelatase which is also involved in heme production.

Heme is a cofactor in hemoglobin and by lowering its production causes symptoms of anemia.

ALAD

Detoxification

Detoxification systems for lead include metallothionein, cytochrome P-450, and bile.

One of the primary ways the body gets rid of lead is from the liver into the bile where it is taken to the small intestine and excreted in the feces.

As mentioned earlier lead can also be incorporated into the bones, inactivating it for years.

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