Dynamics of lead release from pipes containing PbO2 in response to free chlorine depletion during water stagnation

Lead(IV) oxide (PbO₂) is an important lead corrosion product found on many lead service lines (LSLs). Because of its low solubility, PbO₂ formation on the surface of lead pipes can potentially minimize the release of lead to water. However, PbO₂ is only stable in the presence of free chlorine, and it undergoes reductive dissolution in the absence of free chlorine. A laboratory-scale pipe loop test with harvested LSLs that had pipe scales rich in PbO₂ investigated how the PbO₂-rich layer responded to free chlorine depletion. Depletion of free chlorine resulted in substantial increases in both dissolved and total lead concentrations. However, the rise in dissolved lead was not immediate and was delayed by 1-2 days, eventually exceeding 15 μg/L after chlorine depletion. Total lead release posed a greater concern, characterized by sporadic and high concentrations (>100 μg/L) when free chlorine was depleted. A free chlorine concentration of 0.4 mg/L as Cl₂ was able to maintain PbO₂ stability for the full duration of a 10-day experiment. Although the reductive dissolution of PbO₂ after free chlorine decay resulted in substantial increases in lead concentrations in the water, the vast majority (greater than 99.93 %) of the PbO₂ remained in the scale. This research provides new information on the effects of key factors on the reductive dissolution of PbO₂ present in the scales of LSLs, and it offers insights into how PbO₂ formation and stability can govern the dynamics of lead concentrations in water delivered through lead pipes.