Effects of Cu(II) and Zn(II) on PbO Reductive Dissolution under Drinking Water Conditions: Short-term Inhibition and Long-term Enhancement.

Lead oxide (PbO) has the lowest solubility with free chlorine among Pb corrosion products, but depletion of free chlorine or a switch from free chlorine to monochloramine can cause its reductive dissolution. We previously reported that Cu(II) and Zn(II) inhibited PbO reductive dissolution within 12 h. Here, we expanded on this work by performing longer duration experiments and further exploring the underlying mechanisms. Between 12 and 48 h, Cu(II) and Zn(II) had no discernible effect on PbO reductive dissolution. From 48 to 192 h, Cu(II) and Zn(II) enhanced PbO reductive dissolution. Dissolved oxygen (DO) concentrations followed the same trends as PbO reductive dissolution, indicating that the DO was produced by PbO reductive dissolution. On the basis of extended X-ray absorption fine structure spectra, we hypothesize that the inhibitory effect of Cu(II) and Zn(II) on PbO reductive dissolution (<12 h) is caused by decreasing abundance of protonated sites on the PbO surface. The enhanced dissolution (>48 h) may be caused by competitive adsorption of Cu(II) and Zn(II) with Pb(II), which could limit the adsorption of Pb(II) onto PbO that could otherwise inhibit reductive dissolution. This study indicates that stagnation time plays a vital role in determining cations' effects on the stability of Pb corrosion products.