PALLADIUM (Pd) IN URBAN ENVIRONMENTS: EVIDENCE, IMPLICATIONS, AND THE NEED FOR INCLUSION IN SOIL POLLUTION MONITORING.

Palladium (Pd), a platinum group element (PGE), is released into the environment in large quantities due to its widespread application in catalytic converters, electronics, and hydrogenation catalysts. Despite its growing use, Pd remains largely unregulated in current soil pollution frameworks, and its environmental fate is not well characterised. This study indirectly assessed the presence of Pd in urban topsoils from Alcalá de Henares (Spain) by analysing 137 surface samples (0–3 cm depth) for platinum (Pt) and rhodium (Rh) using ICP-MS. The presence of Pd was inferred from well-documented co-emission patterns among PGEs, particularly from automotive catalytic converters, which release Pt, Rh and Pd in relatively stable ratios. Spatial mapping and multivariate statistical analysis demonstrated that Pt and Rh concentrations were elevated in areas affected by traffic and urban gardening activities. These elements clustered together in principal component analysis, suggesting a shared anthropogenic origin that likely includes Pd. Although Pd was not directly quantified, its presence is strongly supported by the overlapping spatial trends of Pt and Rh. The persistence and mobility of PGEs in soil—especially in fine-textured, organic-rich areas—suggest that Pd may similarly accumulate and pose long-term exposure risks. Toxicological reference values for Pd are not yet established, precluding formal risk assessment. Nevertheless, biomedical studies suggest potential immunotoxic, cytotoxic and allergenic effects may occur as a result of chronic Pd exposure, particularly via inhalation of resuspended particles. This study underscores an urgent need for environmental surveillance studies that include Pd and consideration of its toxicological relevance. Given its increasing industrial use and emerging evidence of harm, Pd should no longer be overlooked in urban pollution-monitoring programmes.