Lead Scale Formation and Solubility

The Carbonates

Many distribution systems have calcium carbonate scales, usually as the mineral calcite, which does not present a health problem, but can cause serious problems for equipment maintenance. Carbonate minerals are also widely distributed in nature and accordingly have been thoroughly studied. There are several Pb carbonates but two are dominant, cerussite, which has an orthorhombic crystal structure, and hydrocerussite which is trigonal. The orthorhombic form is more symmetrical and cerussite tends to form elongate, blocky crystals, whereas hydrocerussite tends to form in intersecting plates. Both minerals are frequently found as weathering products of Pb ore minerals, so again there is a wide literature on their mineralogy and chemistry.

A useful way to compare different Pb minerals is to use concentration vs pH diagrams (Figure 1). These assume equilibrium between the mineral and the solution and also require mathematically solving a set of simultaneous equations involving ion-pair formation in the solution. These equations are embodied in computer codes such as PHREEQC, which is the code used for this set of diagrams. The y-axis is total Pb, which comprises Pb²⁺, Pb⁴⁺, PbCO₃⁰, PbCl⁺, etc.  Moving up on the diagram, there is first a region of solution only, with no minerals stable. At about 10^-6 moles/liter, one or the other of the carbonates should precipitate. At this point, Pb levels in the solution should not go any higher, because any addition of Pb would be compensated by growth of additional Pb carbonate. If for some reason the carbonate does not form (for example in very low alkalinity waters), the solution continues to increase in Pb content until litharge forms at Pb concentrations in the neighborhood of 10^-3 moles/liter. This Pb level is high and most utilities strive to convert any PbO in the distribution system into one of the carbonates. Even equilibrium values with respect to the lead carbonates are still above the LCR action limit, however. In practice, a well-developed lead carbonate scale does provide protection, because only a portion of the water in the system is in contact with the lead service lines or Pb-containing plumbing materials. Normally, utilities with well-developed lead carbonate scales will be able to keep lead levels at the tap below the action level. Pb in equilibrium with PbCO₃ is virtually independent of pH at dissolved inorganic carbonate (DIC) levels of 60 mgC/L and higher. However, at 20 mg C/L, there is a strong solubility minimum at pH about 9.8 (Schock et al 1996, their fig. 4-14). Also note that if there is an underlayer of litharge, mechanical removal of the cerussite layer could lead to a sudden increase in Pb levels.

Figure 1. pH-concentration diagram for the Pb carbonates in alkaline distribution water. (PbO is shown for comparison. Speciation based on the PHREEQC code using the MINTEQ database and the solution parameters given in Table 2.)