es9b06019_si_001.pdf (4.31 MB)
The Ability of Phosphate To Prevent Lead Release from Pipe Scale When Switching from Free Chlorine to Monochloramine
journal contribution
posted on 2019-12-13, 19:08 authored by Yeunook Bae, Jill D. Pasteris, Daniel E. GiammarFor lead pipes that contain PbO2(s) as a major
component
of their scales, a change in the residual disinfectant from free chlorine
to monochloramine can destabilize the PbO2(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 PbO2(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-PbO2(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.