RESUMO
Environmental levels of platinum group elements (PGEs) are rising due to emissions of vehicle catalytic converter (VCC) materials containing palladium, platinum and rhodium. When these PGE-containing VCC materials are exposed to soil and water, coordination complex formation with ligands present in the environment may mobilize PGEs into solution, particularly Pd. Road de-icing salt contains two ligands with high affinities for Pd2+: chloride (Cl-) from NaCl and cyanide (CN-) from ferrocyanide (Fe(CN)64-) anti-caking agents. Batch leaching studies of VCC materials were conducted with solutions representative of de-icer-contaminated road runoff at pH 8 and room temperature for 48 h. Ferrocyanide (FC) concentrations of 0 µM, 1 µM, 2 µM and 10 µM were tested with background electrolyte concentrations of 0.028 M NaCl (1000 mg/L Cl-) or 0.028 M NaClO4. Palladium release increased with FC concentration, ranging from 0.014 ± 0.002 µM Pd without FC to 5.013 ± 0.002 µM Pd at 10 µM FC. At 0 µM, 1 µM and 2 µM FC, chloride induced further Pd release, but had no effect at 10 µM FC. PHREEQC modeling predicted that the predominant species present in equilibrium with Pd(OH)2(s) were Pd(OH)20 and Pd(CN)42-, and that PdClx2-x complexes had only a minor effect on the total concentration of dissolved palladium. The effect of FC on Pd release was predicted but not the effect of Cl-, indicating possible kinetic control. Platinum was measured above limits of detection (LODs) only at 10 µM FC, and rhodium levels were below LODs, consistent with their slower complexation kinetics.