Improved Control of Lead Release from Boosting Orthophosphate and Removing Polyphosphate from a Blended Phosphate Corrosion Inhibitor

Abstract

Orthophosphate addition can be an effective lead corrosion control method. Commercial phosphate-based inhibitors used in water treatment are often a blend of orthophosphate and polyphosphate. Orthophosphate (PO 4 ) can limit lead release from pipes by forming low-solubility lead phosphate compounds on the inner surfaces. In contrast, polyphosphate can increase lead release by forming soluble lead phosphate complexes. In a controlled laboratory study using recirculating pipe loops, the effects of boosting the orthophosphate concentration and removing polyphosphate on lead release were investigated. The study began with conditioning of six harvested lead service lines from locations in the service area of Buffalo Water with artificial Buffalo water that received a 70%/30% poly/ortho-blended phosphate chemical (orthophosphate concentration: 0.2 mg/L of PO 4 ). Three of the pipes were tested in two treatment stages, while the other three remained as controls. During treatment stage 1, the orthophosphate concentration in the test pipes was boosted to 1.5 mg/L for PO 4 while maintaining the original blended phosphate concentration. Both the total and dissolved lead concentrations decreased and became more stable during this treatment stage. During treatment stage 2, blended phosphate was no longer added to the test pipes, and the test pipes received only orthophosphate (1.5 mg/L of PO 4 ). During treatment stage 2, the total and dissolved lead concentrations continued to decrease from their values at the end of treatment stage 1. Scale analysis revealed that the phosphorus content of the scales on the test pipes increased during treatment. A crystalline lead/calcium phosphate was observed that was phosphohedyphane (Ca 2 Pb 3 (PO 4 ) 3 Cl) or a similar solid. The lead phosphate increased in abundance in moving from treatment stage 1 to treatment stage 2.