Processes Controlling the Time for Orthophosphate to Achieve Effective Corrosion Control
Two sets of lead pipes are being conditioned for use in studying the timing and mechanisms for orthophosphate to effectively control lead concentrations. A set of six new lead pipes has now been conditioned for more than 40 weeks with water designed to simulate water in the Washington, DC distribution system before 2000. The dissolved lead concentrations after stagnation periods have now dropped to values that are more consistent with control of lead solubility by lead(IV) oxides (PbO2) than by the lead carbonate cerussite (PbCO3). Analysis of the pipe scale after 35 weeks confirms the presence of abundant amounts of both forms of PbO2 (plattnerite and scrutinyite) and scales that are much thicker than they had been when a section of pipe was analyzed after 14 weeks. This set of six pipes with welldeveloped scales rich in PbO2 will be valuable to the next stage of the experiments that will test lead release when the disinfectant is switched from free chlorine to chloramine with half of the pipes receiving orthophosphate in an effort to control lead release. The second set of six pipes uses harvested pipes from Providence, RI to examine the effectiveness of orthophosphate at pH values higher than those for which orthophosphate is normally used. From an initial set of ten harvested lead pipes, dump-and-fill conditioning provided data to select six lead pipe assemblies that are now being conditioned with recirculating flow.
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