THE INFLUENCE OF ACETATE AND OXALATE AS SIMPLE ORGANIC LIGANDS ON THE BEHAVIOR OF PALLADIUM IN SURFACE ENVIRONMENTS

Abstract

Acetate and oxalate are simple organic ligands that occur naturally in surface waters, groundwaters, and soils; they may play a role in the transport of palladium in such environments. Mass transfer of Pd in surface environments has implications for geochemical exploration as well as the environmental impact of the platinum-group elements. The complexation of divalent palladium and acetate was studied via measurement of the solubility of amorphous Pd(OH)2 in 1 molal NaClO4 as a function of concentration of acetate (0.0001 to 0.1 molal) at 25°C. From these data, the predominant Pd 2+ acetate species was determined to be Pd(ac)2 0 (where ac denotes the acetate anion), with a conditional stability-constant of log � 2 * = 9.3 ± 0.3. Palladium oxalate complexes were investigated using UV-visible spectrophotometry. Experiments were conducted at 25°C as a function of concen- tration of NaCl (0.1 to 2.0 molal) and oxalate (0.1 to 100 millimolal). The results demonstrate that oxalate-bearing Pd species become important at relatively low concentrations of oxalate. At 1 molal NaCl, the existence of an isosbestic point indicates an equilibrium between one oxalate-bearing Pd species and PdCl4 2- . At lower concentrations of NaCl, more than two Pd species may be present under the experimental conditions. Calculation of speciation as a function of pH in a model soil solution shows that Pd(ac)2 0 never exceeds 40% total Pd, even in the absence of oxalate and at acetate concentrations as high as 5 millimolal. At lower concentrations of acetate or in the presence of oxalate, the proportion of Pd(ac)2 0 is negligible. However, Pd(ox) 0 appears to be an important Pd 2+ species, predominating at low pH, with its field of predominance expanding with increasing concentration