Foliar litters : Sources of contaminants in phytoremediation sites by returning potentially toxic metals (PTMs) back to soils

Phytoremediation is regarded as one of the most cost-effective and environmentally friendly strategies for potentially toxic metals (PTMs) contaminated soil remediation. However, uncertainties still existed about the contribution of foliar litter on metal accumulation and mobility in phytoremediation sites. Thus, in this study, fallen leaves, decomposed leaves, and soils at different depths (i.e., 0–5 and 5–10 cm) were collected from a phytoremediation site near a Zn smelter factory. Metals content and mobility were evaluated. Results indicated that upper-layer soils (0–5 cm) were higher in the electrical conductivity (EC) and soil organic matter (SOM) content than the deeper-layer soils (5–10 cm). However, the pH was relatively lower in the upper-layer soils. Fallen leaves were sources of metals in the phytoremediation site, and significantly high amounts of Cd (16.08 ± 0.21 mg kg⁻¹) and Zn (1130.30 ± 60.10 mg kg⁻¹) were found in the decomposed leaves. Metals in the upper-layer soils demonstrated higher accumulation and mobility than the deeper-layer soils. Moreover, the accumulated metals in leaves would gradually return to the soil as the contents of extractable metals increased with the rising decomposition degree of leaves (i.e., cold-water extraction < 80 °C hot water extraction < 1 M HCl extraction). Results from this research are helpful for the guidance of phytoremediation site management.