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

Abstract

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.