Overview: Equilibrium Chemistry of Pb in Drinking Water

Lead Scale Formation and Solubility

 The response of iron, copper, and lead source materials in premise and distribution systems to changes in water treatment is strongly dependent on the mineralogy of the corrosion scales attached to various parts of the system. These responses are particularly critical for lead compounds, because so many utilities are close to the lead action level in tap samples. Knowledge of the mineralogy of scale compounds is a key contribution to models of lead behavior and to an understanding of lead control via passivation. Accordingly it is important to develop an understanding of the solid-solution interactions at work involving Pb components in distribution systems. (Further discussion)

Equilibrium Modeling

The first step to evaluating the relationship between scale mineralogy and water quality is equilibrium modeling. Pipe scale -- water interactions generally do not reach equilibrium in the time the water spends in the pipe, but for some minerals the approach is fairly close during stagnation episodes and the models tell us the direction of the reactions. A useful approach is to employ existing computer codes to handle aqueous speciation and to estimate mineral saturation states. Two common codes are PHREEQC from the US Geological Survey and MINTEQ from the US Environmental Protection Agency. The PHREEQC code can be used with the MINTEQ database, which is what we have done to generate the diagrams shown here.

Solubility Diagrams

In the diagrams below, the PHREEQC code has been used along with average compositions from a typical water utility to generate models of scale solubility. Click on the tumbnail for a full-size image.

Pb carbonates(concentration-pH): There are two common Pb carbonates, cerussite and hydrocerussite (note that PHREEQC spells these minerals "cerrusite") plus the less common plumbonacrite. In water of moderate to high alkalinity like the one shown here, there is only a gentle variation of carbonate solubility with pH. Hydrocerussite is somewhat less soluble than cerussite and therefore more desirable as a scale mineral.
Pb oxides (pe-pH): There are also several lead oxides found in distribution systems. They can have the formula PbO (Pb2+) or PbO2 (Pb4+). PbO minerals are litharge and massicot; PbO2 minerals are plattnerite and scrutinyite.  
Pb oxide-carbonates (pe-pH): In most distribution systems, there is an initial layer of PbO overlain by one of the carbonates or by PbO2. If so, the PbO is separated from the water, barring a scale disruption, and equilibrium involves the carbonates and plattnerite. Very high Eh values are required for plattnerite stability, hence chlorinated systems might have plattnerite whereas those using chloramine will generally not.
Pb oxide-carbonate (concentration-pH): Two lessons from this diagram: (1) higher pHs favor plattnerite over Pb carbonate, but raising too high will clog the system with CaCO3; (2) plattnerite is less soluble than cerussite at any pH above about 7.2. To get below the action level requires a pH of 8.4 or greater. The upshot is that relying on pH elevation alone to solve Pb problems will likely not work in systems with moderate or high alkalinity because CaCO3 precipitation will become severe before that water goes below the action level for Pb
Pb phosphates: The remedy most distribution systems turn to is some form of dosing with phosphate. If polyphosphates are used,however, there may be unintended consequences caused by increased release of iron into the system. There are many Pb-containing phosphate minerals, here we show calculations for pyromorphite.