SOLUBILITY OF FERROUS OXALATE DIHYDRATE IN MAGNESIUM CHLORIDE AND SODIUM CHLORIDE SOLUTION

Oxalate (C₂O₄^-2) combines with various divalent metal ions to form sparingly soluble minerals such as CaC₂O₄·H₂O (Whewellite), CaC₂O₄·2H₂O (Weddellite), MgC₂O₄·2H₂O (Glushinskite), MnC₂O₄·2H₂O (Lindbergite), and FeC₂O₄·2H₂O (Humboldtine). These minerals are typically found at the lichen-rock interfaces, and sometimes serve as indicators of active chemical weathering driven by oxalate-producing microorganisms. At the Waste Isolation Pilot Plant (WIPP) near Carlsbad, New Mexico, U.S.A., oxalate is present as a waste component, and a significant amount of iron is present in the form of steel waste canisters. Oxalate can solubilize actinides by forming aqueous complexes, increasing their mobility. A favorable removal mechanism for oxalate is its precipitation with benign cations. Under the reducing conditions present at WIPP after closure, it is possible that ferrous oxalate dihydrate (FeC₂O₄·2H₂O, humboldtine) could form if oxalate is available after the formation of whewellite. A limited number of literature articles have dealt with the solubility product for FeC₂O₄·2H₂O. To determine the solubility product for FeC₂O₄·2H₂O, we examined the dissolution of FeC₂O₄·2H₂O by measuring the concentration of dissolved ferrous iron ([Fe(II)]_diss) with the Ferrozine method in MgCl₂ and NaCl solutions of incremental concentrations. Some reactors took more than 400 days to achieve apparent equilibrium in terms of [Fe(II)]_diss. Higher [Fe(II)]_diss was observed in MgCl₂ and NaCl solutions of higher concentration. At comparable ionic strengths, the [Fe(II)]_diss in MgCl₂ solution was higher than in NaCl solution. In MgCl₂ solution, phenomena such as (i) a color change in reactors of lower solid-to-liquid ratio and (ii) a higher [Fe(II)]_diss in reactors of higher solid-to-liquid ratio were observed, which could indicate the presence of reaction(s) other than simple dissolution of FeC₂O₄·2H₂O. Further chemical and theoretical analyses are in progress and will be presented in the meeting together with conclusions.