Geochemical associations of metals in corrosion scales from lead and copper drinking water pipes: Assessing the dissolution and bioaccessibility potentials

Abstract

This study investigated the speciation of metals accumulated in corrosion scales within drinking water distribution pipes by comparing two methods: X-ray diffraction (XRD) and an operationally defined chemical sequential extraction technique. Additionally, two single-step chemical extraction methods were employed to target metal fractions that are considered potentially bio-accessible if scale particulates are ingested through tap water consumption. The first is a 1.0 M CH₃COONa solution adjusted to pH 5 with CH₃COOH to target metal fractions operationally defined as exchangeable and bound to carbonate minerals. The second is a solution mimicking gastric juice (USEPA Method 1340). The corrosion scales analyzed were obtained from lead (Pb) and copper (Cu) drinking water pipes excavated from U.S. municipalities, where the metal concentrations released into the water are expected to meet the thresholds established by the Lead and Copper Rule (LCR). Although the amorphous phases were not detectable by XRD, our findings highlighted their potential contribution to the measured concentrations of dissolved metals, as inferred from the high concentrations of chemically extracted metals that were not associated with the minerals identified in the diffractograms. Therefore, identifying crystalline minerals using XRD alone is insufficient for predicting the potential of corrosion scales to contaminate potable water flowing through municipal distribution pipes with metals that pose human health risks. The assessment of potentially bio-accessible metal fractions showed that for Pb, Cu, and to some extent zinc (Zn), extraction with CH₃COONa resulted in dissolved metal concentration trends similar to those obtained with the US-EPA Method 1340, although the correlations were not strong. Overall, the US-EPA Method 1340 dissolved significantly higher amounts of Pb, Cu, Mn, Fe, Cr, and Sn compared to the CH₃COONa method, suggesting that these methods, if assumed acceptable for predicting the pool of bio-accessible metals, cannot be used interchangeably for corrosion scales.