Lead is one of the RCRA metals. Low concentration of lead is found in sedimentary and rocks soils. This page documents the very basic geochemistry and important concepts that you should be aware of when dealing with lead contamination. I like to think about this page as LEAD 101!
What is the Maximum Contaminant Limit (MCL) for lead?
- EPA has set the maximum contaminant level for lead at 0.015 mg/L
Lead concentration in common geologic strata:
|Average Lead Cponcentration||Concentration||Source|
|Shale||20 mg/Kg||Turekian and Wedepohl, 1961|
|Sandstone||7 mg/Kg||Turekian and Wedepohl, 1961|
|Carbonates||9 mg/Kg||Turekian and Wedepohl, 1961|
|Soil||17-26 mg/Kg||Pendias and Pendias (1984)|
Lead is similar to K in in ionic size. Ionic substitution is common between lead and potassium in silicates (K-feldspars and biotite in particular). Lead content generally increases from ultramafic to granitic rocks. Average lead concentration in granites is about 23 mg/Kg
Did you know these FACTS about LEAD?
- Lead was ranked second on the CERCLA Priority List of Hazardous Substances in 1999 and 2001 (First being Arsenic).
- Lead is the most commonly RECYCLED metal – 50% of lead production is secondary in nature.
- 70% of lead produced is used in lead-acid storage batteries.
- Lead was widely used in the construction of water pipes in 20th century.
- Lead picments were common in paints prior to 1978!
- Leaded gasoline (tetraethyl lead) was available between 1923 to 1980s .
- Most of the lead produced in the USA comes from Missouri.
Sources of lead contamination:
- fall out of atmospheric dust
- Industrial/municipal discharge
- lead based paints
- mining waste
Lead geochemistry is controlled by
- adsorption at the solid/water interface.
- complexation with organic components
RULE of LEAD: Lead has strong affinity towards soils (adsorbed to soils) and rarely released to surface or ground water.
- Exception to the rule – low pH system or high dissolved organic carbon (DOC).
- Lead Hydroxide – Pb(OH)2
- Cerussite -PbCO3
- Hydrocerussite – Pb3(CO3)2(OH)2)
- Angelsite – PbSO4
- Galena – PbS
- Lead Oxide – PbO
Remedial technologies for lead contamination:
- Containment (caps and vertical barriers)
- Solidification/ stabilization
- Separation/ concentration
Solubility of Lead:
Lead is highly soluble in water over a wide range of pH conditions. In pure water lead forms stable cation Pb+2 below about pH 7. Lead can also form aqueous complexes with OH-, Cl-, SO4– etc. As the pH increases of the solution, lead forms more stable species such as PbOH+, Pb(OH)2, and Pb(OH)3-.
Lead geochemistry and solubility is highly controlled by solution pH and Eh of the environment.
- Lead is MOBILE in LOW pH and HIGH Eh conditions.
- Lead is usually NOT a metal of concern in at mining related sites. Acid mine drainage produces sulfate apart from low pH condition which allows lead to precipitate as angelsite (PbSO4).
- Lead carbonate (Cerrussite) is highly soluble below pH 6; but highly insoluble above pH 8.
- Thermodymanic data predicts lead hydroxide and lead oxide to be stable. However, kinetically they are difficult to precipitate in room temperature.
- Lead phosphate minerals are highly insoluble and remedial technologies uses this strategy.
Eh-pH diagram for Lead:
Eh-pH diagrams are not specific to every field conditions that you would encounter. The Eh-pH diagram for lead generated below uses the following activities -
- Pb activity = 1e-5
- HCO3- activity= .001
- SO4– activity = .001
It is evident from the Eh-pH diagram that lead is mobile at low pH condition (pH<2). With increasing pH, the lead sulfate Anglesite becomes first to precipitate if enough sulfate is available below pH 6. Above pH 6, the carbonates cerussite and hydrocerussite stable. In a reducing condition, galena could also be stable over wide range of pH.
Figure 3 is generated with lead activity = 1.E-6. Notice, how lower concentration of lead leads to higher field of soluble phase.
Any Eh-pH diagram could be easily converted in to a pe-pH diagram. The figure below converts the Eh-pH diagram from figure 3 to a pe-pH diagram:
Adsorption of LEAD:
- Hydrous Ferric Oxide (HFO) plays major role in lead mobility as lead adsorbs to HFO stronger than any other divalent metal ions.
- Adsorption increases as pH in increased between 3 (0% adsorbed) to 6 (100% adsorbed).
- Lead could also adsorbs to iron sulfide in reducing environment.
Lead Valence states
- 0, + 2 and +4
- Lead transport in colloidal form could be significant in both surface and ground water. Colloidal particles could be either organic or inorganic.
- If you are high dissolved lead in your sample, try using 0.1 micron filters for “dissolved” samples.