EXPERIMENTAL DETERMINATION OF THERMODYNAMIC PROPERTIES OF PbC2O4(AQ)

The Waste Isolation Pilot Plant (WIPP) is a U.S. Department of Energy geological repository for the permanent disposal of defense-related transuranic (TRU) waste. The repository is geographically located 42 km east of Carlsbad in southeastern New Mexico, and is geologically located at a subsurface depth of 655 m in the Salado Formation, a Permian bedded-salt formation composed predominantly of halite, and of lesser amounts of anhydrite, gypsum, polyhalite, magnesite, clays and quartz. The repository is planned to eventually comprise 56 disposal rooms, each 91.4 m long by 9.14 m wide by 3.96 m high, arranged in 8 7-room panels.

In the WIPP, there are significant inventories of lead in waste packages for radiation shielding and iron in waste containers. Oxalate is one of the organic ligands present in the waste inventory. Other organic ligands in the inventory include acetate, citrate and EDTA. An understanding of interactions of lead and iron with organic ligands in brines with high ionic strengths is important to the representation of the near-field geochemical conditions in the WIPP. A series of experimental programs have been initiated at SNL Carlsbad Facility to determine the thermodynamic properties of interactions of lead and iron with the specified organic ligands.

Under the WIPP conditions, the concentration of oxalate is controlled by whewellite (CaC₂O₄•H₂O). Lead and iron could potentially form a relatively strong complex with oxalate, and therefore compete with actinides for soluble oxalate. In this presentation, the formation constant of PbC₂O₄(aq), the Pitzer interaction parameters of PbC₂O₄(aq)–Na⁺, PbC₂O₄(aq)–Mg²⁺, and PbC₂O₄(aq)–Cl^– are presented. The formation constant determined in this study is comparable to the value in the literature. The lambda parameters of PbC₂O₄(aq)–Mg²⁺ and PbC₂O₄(aq)–Cl^– are –0.675 and –0.560, respectively. The zeta parameter of PbC₂O₄(aq)–Mg²⁺–Cl^– is –0.0856.

Acknowledgement: Sandia is a multiprogram laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. This research is funded by WIPP programs administered by the Office of Environmental Management (EM) of the U.S. Department of Energy.