Scale Formation Under Blended Phosphate Treatment for a Utility With Lead Pipes

Abstract

US corrosion control practice often assumes that the orthophosphate component of blended phosphate corrosion inhibitors causes the formation of low‐solubility lead–orthophosphate solids that control lead release into drinking water. This study identified the solids that formed on the interior surface of a lead service line and a galvanized steel pipe excavated from a system using a proprietary blended phosphate chemical. The scale was analyzed by X‐ray diffraction, X‐ray fluorescence, and scanning electron microscopy/energy dispersive spectroscopy. Instead of crystalline lead–orthophosphate solids, a porous amorphous layer rich in aluminum, calcium, phosphorus, and lead was observed at the lead pipe scale–water interface. Thus, the mechanism inhibiting lead release into the water was not a thermodynamically predictable passivating lead–orthophosphate scale, but rather an amorphous barrier deposit that was possibly vulnerable to disturbances. Galvanized pipe scales showed relatively crystalline iron and zinc compounds, with additional surface deposition of aluminum, phosphorus, calcium, and lead.

Water quality–pipe deposit relationships in Midwestern lead pipes

“…Theoretically, higher concentrations of orthophosphate, with no interfering polyphosphate complexation, could potentially improve lead release by the formation of a less‐soluble Pb(II) orthophosphate scale. This notion of amorphous phases functioning as protection mechanisms has been previously discussed by Wasserstrom et al ().…”

“…Crystalline phases in the outer layers of a pipe scale act more as passivating films formed by the direct pipe–water interaction, whereas amorphous phases were more analogous to accumulating deposits on the pipe scale surface. Wasserstrom et al () observed the dominance of mixed‐metal amorphous phases within lead pipe scales from a blended‐phosphate system in the Great Lakes Region (United States). In this study, a little over one‐third of the systems (four GW and five SW systems) had an amorphous phase detectable by XRD, coexisting with crystalline phases (Figure ).…”

“…The corrosion scales in this study were analyzed over the course of the past 25 years, using multiple analytical techniques that have previously been described in detail (DeSantis & Schock, ; DeSantis, Schock, & Bennett‐Stamper, ; DeSantis, Welch, & Schock, ; Schock et al, ; Schock & Lytle, ; Schock, Scheckel, DeSantis, & Gerke, ; Triantafyllidou, Schock, DeSantis, & White, ; Wasserstrom et al, ). Although there has been some variation in preanalysis sample handling and in analytical procedures available over the time study samples were collected, the general procedure was to ship the LSLs to the USEPA's AMSARC in Cincinnati for evaluation.…”

“…Among all the systems predicted to form Pb(II) orthophosphate phases, only one had an orthophosphate concentration >1.2 mg/L as PO 4 . Potentially exacerbating the low concentrations of orthophosphate used in these systems is the presence of other metals that may outcompete lead to form complexes with the available phosphate, such as calcium and aluminum (DeSantis et al, 2012;Snoeyink et al, 2003;Wasserstrom et al, 2017). Amorphous aluminosilicate phases have also been found to occur on LSLs in systems without phosphate treatment (Hayes et al, 2016;Kim & Herrera, 2010;Kim, Herrera, Huggins, Braam, & Koshowski, 2011).…”

“…For example, studies have not conclusively determined the factors causing the specific formation of observed Pb(II) orthophosphate solids such as Pb 5 (PO 4 ) 3 OH (hydroxypyromorphite), Pb 5 (PO 4 ) 3 Cl (pyromorphite), or tertiary lead phosphates (Pb 3 (PO 4 ) 2 , Pb 9 (PO 4 ) 6 ) (Hopwood et al, ). Furthermore, studies (DeSantis & Schock, ; Hayes, Croft, Phillips, Craik, & Schock, ; Kim & Herrera, ; Schock, Cantor, Triantafyllidou, Desantis, & Scheckel, ; Snoeyink et al, ; Wasserstrom, Miller, Triantafyllidou, Desantis, & Schock, ) have noted the extensive presence of amorphous solid phases in a number of drinking water distribution systems (DWDSs). The conditions that govern when amorphous solid phases, rather than defined crystalline phases, form have not been systematically determined.…”