The effects of palladium coordination complex speciation and concentration upon the ubiquitous bacterial species Pseudomonas aeruginosa.

The toxicity of three different palladium (Pd) species to Pseudomonas aeruginosa, an environmentally ubiquitous bacterial species, is reported. Palladium was added to chemically-defined minimal media as three complex ion salts, namely sodium tetrachloropalladate (Na2[PdCl4]), tetraamminepalladium(II) chloride ([Pd(NH3)4]Cl2), and potassium hexachloropalladate(IV) (K2[PdCl6]), inoculated with log-phase cultures and incubated for 24 h at 25 °C. Toxicity was tested for Pd concentrations ranging from 6.55 µg/L (0.06 µM Pd) to 250 µg/L (2.33 µM Pd). Minimum inhibitory concentrations (MICs) were determined and growth tracked via optical absorption at 600 nm. Viability and minimum bactericidal concentrations (MBCs) were measured in parallel with dilution, plating and colony forming unit (CFU) counting. MICs for all forms of Pd were 62.5 µg Pd/L, approximately 1000 times lower than previously reported values. The MBCs for PdCl42- and Pd(NH3)42+ were 62.5 µg Pd/L and 125 µg Pd/L for PdCl62-. Pd(NH3)42+ and PdCl62- culture viability at 7.8-31.3 µg Pd/L was not different from controls. However, PdCl42- culture viability was different from the other additives, with decreasing viability at sub-MBC concentrations down to 6.55 µg Pd/L. To understand the possible effect of speciation upon toxicity, the equilibrium speciation of Pd was modeled for all solutions using PHREEQC and found to be dominated by Pd(NH3)3Cl+ (65.6 % of total Pd) and Pd(NH3)42+ (34.2 % total Pd). The juxtaposition of the equilibrium calculations and the toxicity results indicates that the kinetics of ligand exchange between the palladium complexes and the growth medium could influence bacterial response.

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

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.

Enhanced release of palladium and platinum from catalytic converter materials exposed to ammonia and chloride bearing solutions.

The environmental levels of platinum group elements (PGEs) are steadily rising, primarily due to exhaust emissions of vehicle catalytic converter (VCC) materials containing solid PGEs. Once these VCC materials reach soil and water, the PGEs may be transported in the form of nanoparticles (dimensions 1-100 nm) or they may be mobilized by forming coordination complexes with ligands in the environment. Chloride (Cl-) and ammonia (NH3) are two ligands of particular concern due to their ubiquity as well as their potential to form the chemotherapy drug cisplatin (Pt(NH3)2Cl2) or other potentially bioactive complexes. This initial study examines the release of Pd and Pt into solutions exposed to VCC materials at pH 8 and 25 °C, using elemental analysis of metal content in post-exposure extracts. The solutions had total ammonia nitrogen concentrations (TAN, [NH4+] + [NH3]) of 0 µM, 5.56 µM, 55.6 µM and 1.13 × 105 µM (0 ppm, 0.1 ppm, 1 ppm, and 2147 ppm). The former three represent background environmental levels had a minimal effect on release. However, when combined with 1.13 × 105 µM Cl- (4000 ppm Cl-), 55.6 µM TAN induced a marked increase in metal release (∼41× for Pd). High TAN solutions induced more Pd and Pt release than equimolar NaCl solutions. Materials characterization revealed that ∼4 nm palladium-containing nanoparticles were present, spatially associated with nanoparticles of γ-Al2O3; ceria-zirconia nanoparticles were also present but did not have any metal associated with them. Platinum-containing nanoparticles were not observed.