Effect of temperature and water chemistry on the dissolution and transformation of lead (II) carbonates.

Caitlin SE Kushnir, University of Western Ontario

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 PbCO₃^o. 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.