STANDARD GIBBS ENERGIES OF FORMATION OF ZCC2O4 . 2H2O(S), CDC2O4 . 3H2O(S), HG2C2O4(S), AND PBC2O4(S) AT 298 K AND 1 BAR

Abstract

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 ΔG#f[PbC2O4(s)], ΔG#f[CdC2O4 . 3H2O(s)], ΔG#f[ZnC2O4 . 2H2O(s)], and ΔG#f[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 . H2O(s)#CaCl2(aq,m)#Hg2Cl2(s)#Hg(1)G#f[PbC2O4(s)] from the relatively well-known value of ΔG#f[CaC2O4 . H2O(s)]. This value of ΔG#f[PbC2O4(s)] then allowed us to obtain values of ΔG#f[CdC2O4 . 3H2O(s)], ΔG#f[ZnC2O4 . 2H2O(s)], and ΔG#f[Hg2C2O4(s)] from suitable electrochemical cells.When these values of ΔG#f are incorporated into multicomponent speciation calculations, we find that CdC2O4 . 3H2O(s), Hg2C2O4(s), and ZnC2O4 . 2H2O(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.