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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Civil and Environmental Engineering

Supervisor

Robinson, Clare E.

Abstract

Lead in drinking water is a serious concern. While lead corrosion in water distribution systems has been well studied, major research gaps remain. Prior research has shown thermodynamic models may be used to predict lead concentrations in drinking water, however, use of these models is limited due to uncertainties including which lead phases to consider in the models and associated equilibrium constants. Further, although field data often show dissolved lead concentrations are higher when temperature is higher, there is limited fundamental understanding how temperature affects lead aqueous-solid phase interactions for varying water chemistry conditions. There is also limited understanding of the influence of temperature on the effectiveness of phosphate-based corrosion inhibitors. To address these knowledge gaps, the thesis objectives were to i) evaluate uncertainty in thermodynamic models of the lead (II) carbonate system, ii) evaluate the effects of temperature, pH and dissolved inorganic carbon (DIC) on the dissolution of lead (II) carbonates, and iii) evaluate the effect of temperature on the dissolution and transformation of lead (II) carbonate to lead (II) phosphate phases. These objectives were addressed by combining laboratory batch dissolution experiments with thermodynamic equilibrium modeling.

For the first objective, it was shown that uncertainty in thermodynamic models of the lead (II) carbonate system is mainly associated with the equilibrium constants of five species: hydrocerussite, cerussite, PbCO3-2, PbOH+, and PbCO3o. For the second objective, the effect of temperature in increasing dissolved lead was found to be greater for high pHs and DIC conditions. Temperature was also found to influence the pH at which the dominant lead (II) carbonate phase switches from hydrocerussite to cerussite. The observed temperature effects were simulated well using a thermodynamic model despite limitations on available thermodynamic data. For the third objective, orthophosphate was found to be more effective at decreasing dissolved lead concentrations at low temperatures. At high temperatures, dissolved lead was related to the formation of the lead (II) phosphate phase hydroxylpyromorphite, but overall data indicate multiple mechanisms influence the formation of this lead (II) phosphate phase under varying temperature, pH, and DIC conditions.

Summary for Lay Audience

High lead concentrations in drinking water is a major concern due to the harmful effects on human health. Lead in drinking water mainly comes from lead pipes installed to provide water to homes built prior to the 1980s. Over time, the interior of the lead pipes corrodes and forms a corrosion scale. This corrosion scale is typically made up of a combination of lead solids; with lead (II) carbonate solid phases typically a major solid phase within the scale.

While replacing lead pipes to prevent lead contamination is a priority, pipe replacement programs progress slowly and are costly. Alternative approaches used by water utilities to reduce lead contamination include adjusting water chemistry parameters or adding specific chemicals to promote the formation of corrosion scales that limit the release of lead into the drinking water. Prior studies have developed and applied computer models to understand and predict the release of lead in water distribution systems, but these models have considerable limitations and uncertainties. This thesis aims to i) understand the uncertainty associated with computer models used to predict lead dissolution in drinking water, ii) examine the effect of temperature on the dissolution of lead solids exposed to different water chemistry conditions, and iii) examine how temperature may affect the effectiveness of using the chemical orthophosphate to reduce lead concentrations in drinking water.

The findings from this thesis provide insight into the causes of uncertainty in lead aqueous-solid phase computer models and provide recommendations for improving these models for predicting the dissolution of lead solids. Experiments show that the dissolution of lead carbonate solids is generally greater at higher temperatures, especially when the water has high pH and dissolved inorganic carbon concentrations. Temperature also influences the specific lead carbonate solids that form, switching from the lead carbonate hydrocerussite to the lead carbonate cerussite. This is an important finding as hydrocerussite is generally more stable than cerussite at moderate to high pHs that are common for drinking water. Finally, experiments conducted showed that temperature influences the dissolution and transformation of lead (II) carbonate solid phases in the presence of orthophosphate. Orthophosphate was found to be more effective at lowering dissolved lead concentrations at low temperatures. At higher temperatures, dissolved lead concentration were controlled by the formation of the lead (II) phosphate solid phase hydroxylpyromorphite.

While this thesis focuses on a simplified lead aqueous-solid phase system compared to the chemistry within real water distribution systems, the findings provide important new understanding on how temperature and water chemistry can affect lead concentrations and the transformation of lead solids that are commonly present in lead pipes. Understanding these interactions is crucial in safeguarding drinking water from lead contamination.

Creative Commons License

Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

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Available for download on Monday, September 01, 2025

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