Density Functional Theory-Based Thermodynamic Model for Stable Scale Formation in Lead-Water Systems

Current safety standards for water treatment to reduce indigestible lead in drinking water utilize decades-old data and models to define risk mitigation. Limitation is achieved in part by utilizing additives to control particulate and soluble Pb(II) release through scale formation in lead pipes that rely on thermodynamic and kinetic factors. Advances in state-of-the-art computational materials science techniques allow fresh comparisons to older thermochemical data and thereby enable the formulation of new ab initio-based thermodynamic models. Here, we perform first-principles calculations to simulate electrochemical Pb Pourbaix and stability diagrams that include a limited number of non-ideal solution effects and account for variable concentration, additives, and temperature. Our data show that lead carbonates are thermodynamically driven to form a cerussite scale across a large range of conditions. Orthophosphate is a known lead corrosion inhibitor; however, the formation of lead phosphate scales, such as Pb5(PO4)3(OH)2(s), may rely on transient high Pb(II) concentrations with trace alkaline-earth elements. Moreover, their previously reported stability may be due to overestimated free energies of formation. Acceptable thermodynamically stabilized soluble lead levels in water can only be maintained under specific thermal and aqueous boundary conditions. Our work defines critical environmental limits on lead scale formation and provides a renewed look of E-pH lead stability and solid stabilities of lead (II) carbonate and phosphate species as a function of Pb(II) and pH using here-to-fore unavailable computational approaches for comparison to other data.