THE MINERALOGY OF PB SCALES IN DRINKING WATER DISTRIBUTION SYSTEMS AS REVEALED BY COMBINED XRD AND MICRO-RAMAN SPECTROSCOPY

Dissolving Pb from lead service lines and Pb-containing brasses and solders has become a major health issue for many water distribution systems. Knowledge of the mineralogy of scales in these pipes is key to modeling this dissolution. The traditional method of determining their mineralogy has been bulk X-ray diffraction (XRD). However, the minerals in these scales are often heterogeneously distributed in layers, and identification of each layer is difficult because they are thin and difficult to separate. There is also horizontal heterogeneity and some minerals are only present in low concentrations or occur as amorphous phases, which limit the use of XRD. Micro-Raman spectroscopy is a powerful complement to XRD because it allows mineral identification of micron-sized individual grains and can identify amorphous phases. This study evaluates the applicability of micro-Raman spectroscopy for identifying and mapping the mineralogy of the layers and surface topology of lead pipe scales.

One lead pipe and its scales, provided by Utility X, was used for this study. This pipe was examined with a stereomicroscope to identify the spatial arrangements of the mineral phases. The scale minerals displayed a great deal of variation both vertically and horizontally. Scale deposits from portions of the pipe were harvested as a single sample for pressed-powder XRD analyses. The mineralogy determined by XRD included: hydrocerussite, (Pb3(CO₃)₂(OH)₂), cerussite (PbCO₃), litharge (PbO), and massicot (PbO). Minor amounts of plattnerite (PbO₂) were also detected. Plumbonacrite (Pb₁₀O(CO₃)₆(OH)₆) and/or quartz may also be present. Micro- Raman spectra confirmed the presence of these same minerals. In addition it showed that these minerals occur in radial patterns, with hydrocerussite at the center, followed by massicot, then litharge all of which is surrounded by plattnerite. The large number of Pb phases shows that these pipes do not constitute an equilibrium system. The Raman mapping suggests the evolution in Pb chemistry with time that may reflect changes in water treatment practices.