Lead oxide (PbO 2 ) 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 2 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 2 reductive dissolution. From 48 to 192 h, Cu(II) and Zn(II) enhanced PbO 2 reductive dissolution. Dissolved oxygen (DO) concentrations followed the same trends as PbO 2 reductive dissolution, indicating that the DO was produced by PbO 2 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 2 reductive dissolution (<12 h) is caused by decreasing abundance of protonated sites on the PbO 2 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 2 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.

The dynamics of Pb­(II) at mineral surfaces affect its mobility in the environment. Pb­(II) forms inner- and outer-sphere complexes on mineral surfaces, and this adsorbed pool often represents a large portion of the bioaccessible Pb in contaminated soils. To assess the lability of this potentially reactive adsorbed Pb­(II) pool at metal oxide surfaces, we performed Pb­(II) isotope exchange measurements between dissolved Pb­(II) enriched in 207Pb and natural isotopic abundance Pb­(II) adsorbed to rutile at pH 5, 6, and 7. We find that ∼95% of the adsorbed lead is exchangeable. An initially fast exchange (<1 h) is followed by a slower exchange that occurs on a time scale of hours to days. Pb LIII-edge extended X-ray absorption fine structure spectra indicate that similar binding mechanisms are present at all pH values and Pb­(II) loadings, implying that differences in exchange rates across the pH range examined are not attributable to changes in the coordination environment. The slower exchange at pH 5 may be associated with interparticle and intraparticle diffusion resulting from particle aggregation. These findings demonstrate that the dissolved Pb­(II) pool can be rapidly replenished by adsorbed Pb­(II) if this pool is drawn down incrementally by biological uptake or a shift in chemical conditions.

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