Palladium release from catalytic converter materials induced by road de-icer components chloride and ferrocyanide

Environmental levels of platinum group elements (PGE5) 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 PGE5 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)g ) 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 mu M, 1 mu M, 2 mu M and 10 mu M were tested with background electrolyte concentrations of 0.028 M NaCl (1000 mg/L CI-) or 0.028 M NaC104. Palladium release increased with FC concentration, ranging from 0.014 +/- 0.002 mu M Pd without FC to 5.013 +/- 0.002 mu M Pd at 10 mu M FC. At 0 mu M, 1 mu M and 2 mu M FC, chloride induced further Pd release, but had no effect at 10 mu M FC. PHREEQC modeling predicted that the predominant species present in equilibrium with Pd(OH)(2)(s) were Pd(OH)(2)(0) and Pd(CN)(4)(2-), 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 mu M FC, and rhodium levels were below LODs, consistent with their slower complexation kinetics. (C) 2019 Elsevier Ltd. All rights reserved.