Metal-organic complexes in geochemical processes: Temperature dependence of the standard thermodynamic properties of aqueous complexes between metal cations and dicarboxylate ligands

By combining results from regression and correlation methods, standard stat e thermodynamic properties for aqueous complexes between metal cations and divalent organic acid ligands (oxalate, malonate, succinate, glutarate, and adipate) are evaluated and applied to geochemical processes. Regression of experimental standard-state equilibrium constants with the revised Helgeso n-Kirkham-Flowers (HKF) equation of state yields standard partial molal ent ropies ((S) over bar degrees) of aqueous metal-organic complexes, which all ow determination of thermodynamic properties of the complexes at elevated t emperatures. In cases where (S) over bar degrees is not available from eith er regression or calorimetric measurement, the values of (S) over bar degre es can be estimated from a linear correlation between standard partial mola l entropies of association (Delta (S) over bar(r)degrees) and standard part ial molal entropies of aqueous cations ((S) over bar(M)degrees). The correl ation is independent of cation charge, which makes it possible to predict ( S) over bar degrees for complexes between divalent organic acids and numero us metal cations. Similarly, correlations between standard Gibbs free energ ies of association of metal-organic complexes (Delta (G) over bar(r)degrees ) and Gibbs free energies of formation (Delta (G) over bar(f)degrees) for d ivalent metal cations allow estimates of standard-state equilibrium constan ts where experimental data are not available. These correlations are found to be a function of ligand structure and cation charge. Predicted equilibri um constants for dicarboxylate complexes of numerous cations were included with those for inorganic and other organic complexes to study the effects o f dicarboxylate complexes on the speciation of metals and organic acids in oil-field brines. Relatively low concentrations of oxalic and malonic acids affect the speciation of cations more than similar concentrations of succi nic, glutaric, and adipic acids. However, the extent to which metal-dicarbo xylate complexes contribute to the speciation of dissolved metals depends o n the type of dicarboxylic acid ligand; relative concentration of inorganic , mono-, and dicarboxylate ligands; and the type of metal cation. As an exa mple, in the same solution, dicarboxylic acids have a greater influence on the speciation of Fe+2 and Mg+2 than on the speciation of Zn+2 and Mn+2. Co pyright (C) 1999 Elsevier Science Ltd.