A PITZER MODEL FOR HYDROLYSIS OF LEAD TO HIGH IONIC STRENGTH: APPLICATIONS TO LOW TEMPERATURE GEOCHEMICAL PROCESSES

In this study, the hydrolysis model for lead applicable to high ionic strength is developed based on lead oxide solubilities as a function of ionic strength. Solubility measurements on lead oxide, a-PbO (tetragonal, red, mineral name litharge), as a function of ionic strength are conducted in NaClO₄ solutions up to I = 0.45 mol•kg^–1, in NaCl solutions up to I = 5.0 mol•kg^–1, and in Na₂SO₄ solutions up to I = 5.4 mol•kg^–1, at room temperature (22.5 ± 0.5^oC). The lead hydroxyl species considered in this work include the following,

PbO(cr) + 2H⁺ ⇄ Pb²⁺ + H₂O(l) (1)

Pb²⁺ + H₂O(l) ⇄ PbOH⁺ + H⁺ (2)

Pb²⁺ + 2H₂O(l) ⇄ Pb(OH)₂(aq) + 2H⁺ (3)

Pb²⁺ + 3H₂O(l) ⇄ Pb(OH)₃^– + 3H⁺ (4)

The equilibrium constants for Reactions (1) and (2) were taken from literature. The equilibrium constants for Reactions (3) and (4) are determined in this study as –17.05 and –27.99, respectively, with a set of Pitzer parameters describing the interactions with Na⁺, Cl^–, and SO₄^2–.

In combination with the parameters from literature including those that have already been published by our group, the solution chemistry of lead in a number of media including NaCl, MgCl₂, NaHCO₃, Na₂CO₃, Na₂SO₄, NaClO₄, their mixtures, and natural multiple component underground waters, can be accurately described in a wide range of ionic strengths.

The model is expected to find applications for describing the geochemical behavior of lead in a wide range of low temperature geological processes, such as lead chemistry in geological repositories for nuclear waste, modeling the formation of oxidized, non-sulfide lead-zinc deposits in supergene environments, lead chemistry in acid mine drainage, lead chemistry in soils contaminated by heavy metals, and modeling the interactions of human-made lead artifacts with soils in various archaeological sites.

Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. This research is funded by WIPP programs administered by the Office of Environmental Management (EM) of the U.S Department of Energy. SAND2017-6560A