title: Geochemical behaviour of palladium in soils and Pd/PdO model substances in the presence of the organic complexing agents L-methionine and citric acid.
abstract: Risk assessments of platinum group metal (PGE) emissions, notably those of platinum (Pt), palladium (Pd) and rhodium (Rh), have been mostly based on data regarding the metallic forms used in vehicular exhaust converters, known to be virtually biologically inert and immobile. To adequately assess the potential impacts of PGE, however, data on the chemical behaviour of these metals under ambient conditions post-emission is needed. Complexing agents with a high affinity for metals in the environment are hypothesized to contribute to an increased bioaccessibility of PGE. The purpose of this study is to examine the modulating effects of the organic complexing agents, L-methionine and citric acid, on the geochemical behavior of Pd in soils and model substances (Pd black and PdO). Batch experimental tests were conducted with soils and model substances to examine the impacts of the concentration of complexing agents, pH and length of extraction period on Pd solubility and its chemical transformation. Particle surface chemistry was examined using X-ray photoelectron spectroscopy (XPS) on samples treated with solutions under various conditions, including low and high O2 levels. Pd was observed to be more soluble in the presence of organic complexing agents, compared to Pt and Rh. Pd in soils was more readily solubilized with organic complexing agents compared to the model substances. After 7 days of extraction, L-methionine (0.1 M) treated soil and Pd black samples, for instance, had mean soluble Pd fractions of 12.4 ± 5.9% and 0.554 ± 0.024%, respectively. Surface chemistry analyses (XPS) confirmed the oxidation of metallic Pd surfaces when treated with organic complexing agents. The type of organic complexing agent used for experimental purposes was observed to be the most important factor influencing solubility, followed by solution pH and time of extraction. The results demonstrate that metallic Pd can be transformed into more bioaccessible species in the presence of organic complexing agents which are ubiquitous in the environment.
authors.affiliation: Institute for Atmospheric and Environmental Sciences, Department of Environmental Analytical Chemistry, Goethe University Frankfurt am Main, Frankfurt am Main, Germany.
authors.forename: Fathi
authors.initials: F
authors.lastname: Zereini
authors.affiliation: School of the Environment, Earth Sciences Centre, University of Toronto, Rm. 1016V, 33 Willcocks St., Toronto, Ontario, Canada M5S 3E8. clare.wiseman@utoronto.ca and Dalla Lana School of Public Health, University of Toronto, Toronto, Canada.
authors.forename: Clare L S
authors.initials: CL
authors.lastname: Wiseman
authors.affiliation: Institute for Atmospheric and Environmental Sciences, Department of Environmental Analytical Chemistry, Goethe University Frankfurt am Main, Frankfurt am Main, Germany.
authors.forename: My
authors.initials: M
authors.lastname: Vang
authors.affiliation: AQura GmbH, Hanau-Wolfgang, Germany.
authors.forename: Peter
authors.initials: P
authors.lastname: Albers
authors.affiliation: Umicore AG & Co. KG, Hanau-Wolfgang, Germany.
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Schneider
authors.affiliation: Institute for Analytical and Bioanalytical Chemistry, University of Ulm, Ulm, Germany.
authors.forename: Roland
authors.initials: R
authors.lastname: Schindl
authors.affiliation: Institute for Analytical and Bioanalytical Chemistry, University of Ulm, Ulm, Germany.
authors.forename: Kerstin
authors.initials: K
authors.lastname: Leopold
mesh_terms: D019343:Citric Acid / Q000737:chemistry*; D004785:Environmental Pollutants / Q000737:chemistry*; D008715:Methionine / Q000737:chemistry*; D008956:Models, Chemical; D010165:Palladium / Q000737:chemistry*; D012987:Soil / Q000737:chemistry*
chemical_list: D004785:Environmental Pollutants; D012987:Soil; D019343:Citric Acid; D010165:Palladium; D008715:Methionine