The Ability of Phosphate To Prevent Lead Release from Pipe Scale When Switching from Free Chlorine to Monochloramine

For lead pipes that contain PbO_2(s) as a major component of their scales, a change in the residual disinfectant from free chlorine to monochloramine can destabilize the PbO_2(s) and result in dramatic increases in aqueous lead concentrations. Such a scenario occurred in Washington, D.C., in late 2000. That problem was ultimately addressed by the addition of phosphate as a corrosion inhibitor, but it took several months for lead levels to drop below regulatory values. This study sought to determine whether adding phosphate prior to switching the disinfectant could mitigate lead release. Using synthetic tap water and new lead pipes, we developed a set of lead pipes with scales rich in PbO_2(s) and then studied their response to a change from free chlorine to monochloramine. Total lead concentrations remained below 10 μg/L for pipes that received phosphate prior to and during the switch. In contrast, total lead concentrations increased from less than 5 μg/L to more than 150 μg/L as a result of the disinfectant switch when phosphate was not present. Characterization of the pipe scales demonstrated that plattnerite (β-PbO_2(s)) was the dominant component of the scale prior to the switch, and that the scale gradually transformed into one containing a lead phosphate solid chemically similar to phosphohedyphane (Ca₂Pb₃(PO₄)₃(Cl,F,OH)_(s)) when phosphate was present.