Lead partitioned readily to suspended iron oxide colloids when added to the sodium silicate and hexametaphosphate suspensions (Figure ), with variation in 57 Fe explaining r 2 = 99 and 93% of variation in 208 Pb, respectively. The strong apparent relationship between iron and lead is consistent with research documenting lead adsorption to iron (oxyhydr)oxides and, more specifically, elevated lead in iron-rich drinking water. Given that iron colloids with diameters smaller than 450 nm were not detectable in the control suspension, it was excluded from the partitioning experiment. The larger aggregates it contained, though, would also be expected to adsorb lead. There was little evidence here of lead complexation by free hexametaphosphate: the primary 31 P signal at a retention time of approximately 14 minutes did not co-occur with Pb. The preferential interaction of lead with iron over hexametaphosphate suggests that the colloids that result from iron corrosion in the presence of a sequestrant may be a particularly effective transport vector for lead. Preferential partitioning of lead to iron (oxyhydr)oxide colloids was reported in a recent paper, where a much larger fraction of total lead was associated with colloidal iron than colloidal dissolved organic matter (although both appeared to bind lead). In another study, a greater fraction of dissolved lead adsorbed to iron oxide particles than precipitated with orthophosphate. 50 The interaction among corroding iron, sequestrant additives, and lead has important implications for drinking water quality. When polyphosphate or sodium silicate is used to minimize the aesthetic effects of iron corrosion, the iron colloids that result may increase lead mobility. That is, complexation of lead by polyphosphate is not the only risk that sequestrant use poses: depending on the relative affinity of lead for the dispersed colloids versus the free sequestrant-and the relative concentrations of the two adsorbents-the dispersion of iron may have a greater effect on lead levels.