Assessing the potential of inorganic anions (Cl⁻, NO₃⁻, SO₄²⁻ and PO₄³⁻) to increase the bioaccessibility of emitted palladium in the environment: Experimental studies with soils and a Pd model substance

Palladium (Pd) emitted from vehicles equipped with exhaust catalytic converters has been accumulating at a greater rate relative to other platinum group elements (PGE) in the last 10–20 years. Little is known, however, regarding the various environmental factors and conditions which are likely to modulate the chemical behavior and bioaccessibility of this element post-emission. To meet data needs, soils and a Pd model substance were treated with solutions containing common anions (Cl⁻, NO₃⁻, SO₄²⁻ und PO₄³⁻) to shed light on the geochemical behavior of emitted Pd under ambient conditions. As part of this, the particle surface chemistry of treated residues (insoluble phase) and solutions (soluble phase) was examined using XPS to assess the chemical transformation of Pd in the presence of inorganic anions. The results show that Pd is the most soluble in the presence of anionic species, followed by rhodium (Rh) and platinum (Pt). Pd in field-collected samples was found to be considerably more soluble than the metallic Pd in the model substance, Pd black, when treated with anionic species. The results also demonstrate that the solubility of Pd black is strongly dependent on solution pH, concentration and the duration of reaction. The outer 3–4 atomic layers of metallic Pd was determined via XPS to be partially oxidized when treated with anion solutions, with the degree being dependent on anion type. The concentration of dissolved O₂ in solution was found to have little impact on the transformation of metallic Pd. Given the ubiquitous nature of the anions examined, we can expect that Pd will become more bioaccessible post-emission. - Highlights: • Pd species in soils and a model substance treated with anions was examined. • Pd in field soils most soluble compared to Pt and Rh. • The outer atomic surfaces of metallic Pd black oxidized with anion treatment. • Oxygen in anionic solutions had little effect on species status of metallic Pd. • Pd likely to become more bioaccessible when in contact with anions in environment. - Inorganic anions commonly present in the environment are likely to interact with emitted metallic Pd to yield species that are more bioaccessible in soils and aquatic systems.