Lead Phosphate Particles in Tap Water: Challenges for Point-of-Use Filters

To lower Pb concentrations in tap water, water systems implement corrosion control treatment that often involves the addition of orthophosphate. Because NSF/ANSI 53 (2018) certified point-of-use (POU) filters can remove Pb from tap water to less than 10 μg Pb/L, they are often distributed in emergencies or as follow-up steps to lead service line removals or disturbances. While these filters are certified for the removal of both soluble and particulate lead, this study reported that Pb phosphate particles can penetrate POU filters under certain conditions. The small sizes and negative surface charges of suspended Pb phosphate particles make them more mobile through the solid block activated carbon media of POU filters. When tested using challenging water containing 150 μg Pb/L, 2 mM ionic strength (IS), and no hardness at pH 7.0, 41% of the Pb passed through the POU filters. However, when IS was increased to 50 mM or Ca was increased to 0.5 mM, the amount of Pb penetrating the POU filters decreased to 32% and 17%, respectively. Adsorption of Ca onto Pb phosphate particles and high IS promotes aggregation and enhances POU filter performance.