Investigating the Transition from pH Adjustment to Orthophosphate Corrosion Control Treatment at Circumneutral pH

The U.S. EPA’s Lead and Copper Rule Improvements (LCRI) lowers the lead action level to 10 μg/L, driving utilities to adopt or optimize orthophosphate corrosion control. This particularly affects high-pH systems, many of which may shift to circumneutral orthophosphate treatment (pH 7.2–7.8), a transition that remains understudied. This study used a flow-through pipe rack with lead service lines and pipe scale analysis to evaluate three pH transition strategies: immediate (8.8 → 7.5 in 24 h), gradual (8.8 → 7.5 over 16 weeks), and no change (8.8). Orthophosphate-treated racks reduced lead by 92.5–94.5%, regardless of pH or transition strategy, compared to the control rack (pH 8.8, alkalinity >50 mg/L as CaCO3). Scale composition was assessed via scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS), synchrotron X-ray fluorescence imaging (μ-XRF), and inductively coupled plasma mass spectrometry (ICP-MS), while scale phases were identified with X-ray diffraction (XRD) and X-ray absorption near edge structure imaging (μ-XANES). This was the first use of μ-XRF and μ-XANES imaging for nondestructive 2D analysis of lead pipe scales for spatially resolved highly sensitive elemental detection and phase mapping. Lead was passivated by dissolution–precipitation, replacing soluble carbonates with insoluble phosphates to form a protective barrier that limited its release into drinking water.