Introduction: Open‑pit mining operations generate substantial environmental disturbances, most notably the formation of massive waste rock piles and deep excavation pits. These waste deposits, composed largely of fragmented rock material, typically exhibit low moisture‑holding capacity, high bulk density, poor nutrient content, lower pH, and elevated concentrations of potentially toxic elements, including heavy metals. Such characteristics pose significant risks to both environmental and human health. Consequently, developing effective remediation strategies is essential. Among available approaches, phytoremediation leverages the capacity of plant species to extract, stabilize, or accumulate heavy metals from contaminated substrates. The present study aimed to evaluate the uptake of heavy metals by the leaves of Robinia pseudoacacia (black locust) and Fraxinus excelsior (ash) trees planted around the Sungun copper mine in Varzeqan, Iran.
Methods: Leaf samples were collected from mature black locust and ash trees (approximately 15–20 years old) established within the green belt surrounding the Sungun copper mine. Sampling trees were selected randomly from healthy, pest‑free individuals with intact crowns, using five trees per species and three replicates. From each tree, 20–30 leaves were harvested from the upper third of the canopy in a single sampling event. Samples were transported to the laboratory in paper bags and digested prior to analysis. Concentrations of copper (Cu), molybdenum (Mo), lead (Pb), and cadmium (Cd) were quantified using ICP‑MS. In addition, bioconcentration factor (BCF), comprehensive bioconcentration index (CBCI), and metal accumulation index (MAI) were calculated. Data normality was assessed using the Shapiro–Wilk test, followed by t‑tests to determine significant differences between species.
Results: Statistical analyses revealed significant differences in the bioconcentration of Mo, Cu, Pb, and Cd between black locust and ash. According to t‑test results, black locust exhibited significantly higher leaf concentrations of all four metals compared with ash. Overall, the highest extraction levels across both species occurred in the order Cu > Mo > Pb > Cd. Concentrations of Cu, Pb, and Mo in all leaf samples exceeded standard permissible limits. Based on bioconcentration indices, black locust was identified as a specialized hyperaccumulator of Mo, whereas ash functioned as a Mo indicator species. For other metals, differences between species were less pronounced. MAI values further confirmed that black locust had the highest overall heavy‑metal accumulation capacity, and CBCI results similarly ranked black locust above ash. Species classification based on heavy‑metal uptake indicated that black locust acts as a Mo hyperaccumulator, while ash falls within the indicator category for Mo. Additionally, black locust showed strong affinity for Cu, Pb, and Cd, whereas ash demonstrated greater resistance to these metals.
Conclusion: The combined evidence from bioconcentration indices demonstrates that black locust possesses superior capacity for heavy‑metal accumulation compared with ash and can be classified as a hyperaccumulator species in this context. Nevertheless, despite its lower index values, ash remains a viable candidate for phytoremediation, particularly in environments requiring species with moderate uptake but higher ecological resilience. This study provides an initial insight into the phytoremediation potential of these two species in mining‑impacted landscapes and highlights future research pathways for improving contaminated‑site management through biomonitoring and bioaccumulation assessment.