EXPERIMENTAL DETERMINATION OF SOLUBILITY CONSTANT OF EARLANDITE (Ca3[C3H5O(COO)3]2•4H2O) AND PITZER INTERACTION PARAMETERS OF CaC3H5O(COO)3– IN NaCl–H2O AND MgCl2–H2O SYSTEMS

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 panels in which each panel has 7 disposal rooms.

In the WIPP, there are significant inventories of organic ligands. One of these, present in the waste inventory, is citrate. Citrate is considered a significant chelating agent. Other organic ligands in the inventory include acetate, oxalate and EDTA. An understanding of interactions of major ions in brines (of high ionic strengths) with organic ligands is important to the representation of the near-field geochemical conditions in the WIPP. A series of experimental programs have been initiated at Sandia National Laboratories Carlsbad Facility to determine the thermodynamic properties of interactions of major ions in the brines with the specified organic ligands.

Under the WIPP conditions, the concentration of oxalate is controlled by whewellite (CaC₂O₄•H₂O). The concentration of citrate could be limited by the precipitation of earlandite (Ca₃[C₃H₅O(COO)₃]₂•4H₂O). In this study, the solubility constant of earlandite is determined as –17.54 in logarithmic unit based on a series of solubility experiments in NaCl and MgCl₂ solutions. The formation constant of Ca[C₃H₅O(COO)₃]^– is determined as 5.69. The Pitzer interaction parameters of Na⁺–Ca[C₃H₅O(COO)₃]^– are derived as b⁽⁰⁾ = –0.2574, b⁽¹⁾ = ‑0.7868 and C^f = –0.07600.

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.