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Interrelationships between Blended Phosphate Treatment and Scale Formation for a Utility with Lead Pipes
Citation:
Miller, S., L. Wasserstrom, AND M. Schock. Interrelationships between Blended Phosphate Treatment and Scale Formation for a Utility with Lead Pipes. Presented at 42nd Annual WMAO Conference, Columbus, OH, November 13 - 14, 2013.
Impact/Purpose:
Reveals that the corrosion inhibition mechanism in Chicago, where a blended ortho/polyphosphate chemical is used, does not follow simple Pb(II) crystalline scale mineral solubililty control, but is governed by an amorphous phosphate-rich layer with aluminum, calcium and iron as main metal components. This deviation from conventioal corrosion inhibiton theory has important implications for water systems and EPA's corrosion control guidance.
Description:
Lead (Pb) in tap water (released from Pb-based plumbing materials) poses a serious public health concern. Water utilities experiencing Pb problems often use orthophosphate treatment, with the theory of forming insoluble Pb(II)-orthophosphate compounds on the pipe wall to inhibit Pb release into the water. Characterization of corrosion scales on plumbing materials can show the relationship between scale formation and treatment history, and how this may influence Pb release over time. This study characterized the composition of scales formed on the surface of a Pb service line and a galvanized steel pipe in a distribution system that utilizes a chemical that is a blend of ortho- and polyphosphate as a corrosion control strategy. Scales were harvested by layers and analyzed using X-ray diffraction and X-ray fluorescence. Cross sectional areas of pipe samples were prepared for in-situ analysis using scanning electron microscopy and energy dispersive spectroscopy. Results were compared to the literature, and to theoretical predictions for a system treated with orthophosphate. Analysis of the Pb pipe corrosion scales revealed no crystalline Pb-phosphate solids. Instead, an amorphous layer rich in Al, Ca, P, and Pb was observed at the scale-water interface. Thus, the mechanism inhibiting Pb release into the water is not a passivating low-solubility Pb(II)-orthophosphate scale, but rather an amorphous diffusion barrier that was also porous and not well-adhered to the pipe wall. Therefore, scales could easily slough off with a small hydraulic disturbance, sporadically releasing particulate Pb and exposing underlying layers high in Pb. Galvanized pipe scales showed relatively well-crystallized Fe and Zn compounds, with additional surface deposition of Al, P, and Ca. Furthermore, corrosion scales showed accumulation of Pb (likely from the upstream Pb pipe), creating a latent source of particulate Pb release and a potential for long-term Pb exposure, even if the Pb pipe were to be removed.
