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Ground Truthing the 'Conventional Wisdom' of Lead Corrosion Control Using Mineralogical Analysis
Citation:
Schock, M. AND Mike DeSantis. Ground Truthing the 'Conventional Wisdom' of Lead Corrosion Control Using Mineralogical Analysis. Presented at AWWA Water Quality Technology Conference, New Orleans, LA, November 16 - 19, 2014.
Impact/Purpose:
The purpose of this presentation is to organize the information gathered since 1989 on the analyses of over 310 lead pipe samples from 53 water systems in the US and Canada to determine how well accepted drinking water treatment theory fits the observed pipe scales, to assess the state of corrosion control optimization under the Lead and Copper Rule, and to uncover research and implementation gaps.
Description:
For drinking water distribution systems (DWDS) with lead-bearing plumbing materials some form of corrosion control is typically necessary, with the goal of mitigating lead release by forming adherent, stable corrosion scales composed of low-solubility mineral phases. Conventional wisdom, based on geochemical modeling, experimental studies, and practical evidence of water lead levels, predicts what phase ‘should’ control lead solubility, given certain water chemistry parameters. However, due to uncertainties in model parameters and DWDS-specific conditions and treatment history, the actual corrosion scales may or may not correspond to the ideal. Over the last twenty years, 301 lead pipe samples have been received and analyzed by the USEPA. This library represents a broad, though not random, cross-section of DWDSs including 51 systems from 21 US states and Canadian provinces. These are mainly surface water sourced, but include a few groundwater systems. Finished water pHs range from ~7 to >10, and alkalinities from low to high (~10 to ~360 mg/L). Corrosion control methods include: pH/alkalinity adjustment, lime softening, and phosphate addition -- including orthophosphate, zinc orthophosphate, zinc polyphosphate, blended phosphates, and hexametaphosphate. Mineralogical analysis using powder X-ray diffraction, supplemented by elemental analysis via SEM/EDS or XRF, allows comparison of observed scale minerals to those predicted to be present. Observed pipe scales can be mineralogically classified into several categories: 1) Pb(II) carbonates -- Though examples of pure phase scales exist, many show mixed Pb3(CO3)3(OH)2 (hydrocerussite) and PbCO3 (cerussite), despite pH/alkalinity adjustment to favor the former, indicating incomplete conversion of preexisting scales. Plumbonacrite [(Pb10(CO3)6(OH)6O] in one high-pH system is consistent with modeling predictions. However, plumbonacrite was also observed, mixed with other Pb carbonates, in several systems at pHs much lower than that predicted for its stability based on thermodynamic modeling. 2) Pb(II) phosphates -- Observed phases include Pb5(PO4)3OH (hydroxypyromorphite), Pb3(PO4)2 (tertiary lead phosphate), Pb9(PO4)6, and Ca-substituted hydroxypyromorphite. Hydroxypyromorphite and tertiary lead phosphate occurrences are predicted by modeling. In many cases, lead phosphate phases coexist with lead carbonates, indicating incomplete conversion of the latter. 3) Pb(IV) -- A high ORP is necessary to actively form and maintain PbO2 phases, though apparently not the only determining factor. Destabilized PbO2 scales may persist for years after ORP reduction, potentially contributing to particulate lead release. 4) Significant non-Pb external layer deposits dominated by poorly crystalline Mn, Fe, Al, or Si-rich phases were observed in several systems. These can either improve lead release by acting as a barrier or aggravate it due to unstable particle release or by consuming phosphate inhibitors.
URLs/Downloads:
http://www.awwa.org/Portals/0/files/education/conferences/wqtc/wqtc14/pdfs/WQTC14FinalProgramWeb.pdf