Impacts of orthophosphate-polyphosphate blends on the dissolution and transformation of lead (II) carbonate

Abstract

Phosphate addition is a popular strategy to minimize lead release. Despite continuous research on orthophosphate for lead control, studies exploring the complexity of the interaction between lead and orthophosphate-polyphosphate blends in drinking water are scarce. Three model polyphosphates—tripoly-, trimeta- and hexametaphosphate— were used to examine the structural impacts of polyphosphate on lead release. We used a continuously-stirred tank reactor with a lead (II) carbonate solid to evaluate the impact of orthophosphate-polyphosphate blends compared to orthophosphate on lead solubility, speciation, and mineralogy under conditions relevant to drinking water. Tripolyphosphate was a stronger complexing agent for lead than trimetaphosphate (1 ± 0.01 vs 0.07 ± 0.01 molPb/molPolyphosphate), and hexametaphosphate was associated with greater lead solubility (1.6-2.1 ± 0.1 molPb/molPolyphosphate). At equivalent orthophosphate and polyphosphate concentrations (as P), orthophosphate-trimetaphosphate had minimal impact on lead release, while orthophosphate-tripolyphosphate increased dissolved by 554 ± 29 and 213 ± 22 μg Pb0.2μm m-2 at 30-min and 24-hr reaction times, respectively. Meanwhile, orthophosphate-hexametaphosphate increased dissolved lead only at the 24-hr reaction time (by 256 ± 13 μg Pb0.2μm m-2). Both orthophosphate-tripolyphosphate and orthophosphate-hexametaphosphate increased small colloidal lead concentrations over a 24-hr stagnation. Except with orthophosphate-trimetaphosphate, having more polyphosphate than orthophosphate increased dissolved lead release. All three polyphosphates inhibited the formation of hydroxypyromorphite and reduced the phosphorous content of the resulting lead solids. We attribute the impacts of orthophosphate-polyphosphates to a combination of complexation, adsorption, colloidal dispersion, polyphosphate hydrolysis, and lead mineral precipitation.