Impact of orthophosphate on the solubility and properties of lead orthophosphate nanoparticles†

Orthophosphate (PO₄) is a commonly used corrosion control treatment to reduce lead (Pb) concentrations in drinking water. PO₄ reduces Pb concentrations by forming relatively insoluble lead phosphate (Pb–PO₄) minerals. In some cases, however, Pb–PO₄ minerals have been observed to form nanoparticles, and if suspended in water, these nanoparticles can be mobile and reach consumer taps. Although recent research on Pb–PO₄ particles has been performed, there remains a need to improve our understanding of the nature of Pb–PO₄ nanoparticles. For that reason, Pb precipitation experiments were conducted to generate Pb–PO₄ nanoparticles in bench scale studies for analysis. The study objective was to observe how pH, dissolved inorganic carbon (DIC), and PO₄ impacted the properties of Pb–PO₄ particles. Specifically, particle size, surface charge, mineralogy, and solubility were analysed. Hydrocerussite was precipitated when no PO₄ was present, hydroxypyromorphite (Pb₅(PO₄)₃OH) nanoparticles (<100 nm diameter) were precipitated when excess PO₄ relative to Pb necessary to completely precipitate the mineral was present, and a mixture of the two minerals was precipitated when an insufficient amount of PO₄ was present. Hydroxypyromorphite particles were less soluble than hydrocerussite by up to two orders of magnitude. The estimated K_sp,OH of 10^−66.87 in this work closely aligned with previous K_sp,OH estimates that ranged from 10^−66.77 to 10^−62.79. Hydroxypyromorphite particles would not settle in water which was likely due to their small size and high negative charge. The mobility and size of these particles indicates that there are potential implications for such particulate Pb to remain suspended in water and thus be present in the tap water.