DISTRIBUTION AND ENVIRONMENTAL RISK OF RHODIUM (Rh) IN URBAN AND GARDEN SOILS: EVIDENCE FROM ALCALÁ DE HENARES, SPAIN.

Rhodium (Rh) is a platinum group element (PGE) increasingly found in urban environments due to traffic-related emissions. In this study, Rh was quantified via ICP-MS in 137 topsoil samples from urban, industrial and garden areas in Alcalá de Henares (Spain). Despite being below detection limits in 15.5% of urban samples, Rh concentrations were significantly higher in garden soils (median: 0.168 mg/kg) than in urban parks (0.084 mg/kg; p-value<0.05). Principal component and cluster analyses grouped Rh with molybdenum (Mo) and antimony (Sb), suggesting shared anthropogenic origins, likely related to vehicle emissions and industrial activities. Rh levels showed substantial spatial variability, with high coefficients of variation and skewed distributions that point to localised hotspots. Notably, Rh concentrations were strongly associated with sand and silt fractions, indicating its potential mobility and influence of soil texture. Toxicological characterisation could not be performed due to the absence of US EPA reference values (RfDo/RfCi). Nonetheless, given Rh’s increasing environmental prevalence, potential for human exposure via resuspended particles, and growing use in industrial processes, its monitoring is essential. This study highlights a regulatory and research gap surrounding Rh, emphasising the need for its inclusion in environmental assessments. Further research is warranted to elucidate its toxicokinetics, environmental persistence, and possible health effects, especially in vulnerable populations exposed to urban garden soils.