Standard Gibbs energies of formation of ZnC2O4 center dot 2H(2)O(s), CdC2O4 center dot 3H(2)O(s), Hg2C2O4(s), and PbC2O4(s) at 298 K and 1 bar

We test the hypothesis that cycling of some heavy metals in soils and aquifers is affected by equilibrium with oxalate solids. The hypothesis was tested using electrochemical cells that allow us to determine Delta G(f) degrees[PbC2O4(s)], Delta G(f) degrees[CdC2O4 . 3H(2)O(s)], Delta G(f) degrees[ZHC(2)O(4) . 2H(2)O(s)] and Delta G(f) degrees[Hg2C2O4(s)] and hence, the solubilities in soil solutions. The approach was stepwise. First, reversible equilibria was achieved using an electrochemical cell: Pb(Hg),2-phase\PbC2O4(s), CaC2O4 . H2Os)\CaCl2(aq,m)\Hg-2(s)\Hg(1), in order to calculate Delta G(f) degrees[PbC2O4(s)] from the relatively well-known value of G(f) degrees[CaC2O4 . H2O(s)]. This value of Delta G(f) degrees[PbC2O4(s)] then allowed us to obtain values of Delta G(f) degrees[CdC2O4 . 3H(2)O(s)], Delta G(f) degrees[ZnC2O4 . 2H(2)O(s)], and Delta G(f) degrees[Hg2C2O4(s)] from suitable electrochemical cells. When these values of Delta G(f) degrees are incorporated into multicomponent speciation calculations, we find that CdC2O4 . 3H(2)O(s), Hg2C2O4(s), and ZnC2O4 . 2H(2)O(4)(s) are unlikely to form except in highly contaminated soils, or in intercellular environments where the concentration of dissolved oxalate is very high. Of these heavy-metal-oxalate minerals, it is conceivable that the PbC2O4(s) may reach equilibrium in soil solutions. Although the metal-oxalate solids may precipitate locally in the rhizosphere, these solids would not be in equilibrium with the adjacent soil solution.