Uptake, accumulation and metabolic response of ferricyanide in weeping willows

Abstract

The remediation potential and metabolic responses of plants to ferricyanide were investigated using pre-rooted weeping willows (Salix babylonica L.) grown hydroponically in growth chambers and treated with potassium ferricyanide. Positive responses were observed for the plants exposed to ≤ 274.13 mg CN L⁻¹ as ferricyanide, exhibiting higher chlorophylls and soluble proteins compared with the controls. Visible toxic symptoms were only noted for the treatment exposed to 506.67 mg CN L⁻¹ after 120 h of incubation. Activity of superoxide dismutases (SOD) in leaves showed a slight change to ferricyanide exposure in most treatments. Catalase (CAT) and peroxidase (POD) activities were negatively correlated to the concentrations of ferricyanide. Of all the selected parameters measured, soluble proteins of plants were the most sensitive to ferricyanide, showing a significant linear correlation (R² = 0.952). Between 6.90 and 12.66% of the applied ferricyanide were removed by plants from the hydroponic solution at different treatments over the 192 h of exposure. Small amounts of the applied chemical taken up from the hydroponic solutions were detected in all parts of plant materials: the highest concentration was associated with roots in all treatments, followed by stems; the lowest was observed in leaves. The mass balance analysis showed that the total cyanide recovered in plant biomass was constant in all treatments, indicating that transport is a major limiting step for the uptake of ferricyanide by plants. The majority of the ferricyanide taken up from the growth media was possibly assimilated during transport through plants. The velocity of the removal processes can be described by Michaelis-Menten kinetics, and the half-saturation constant (K_M) and the maximum removal capacity (v_max) were estimated to be 228.1 mg CN L⁻¹ and 36.43 mg CN kg⁻¹d⁻¹, respectively, using non-linear regression methods. These results suggest that weeping willows can take up, transport and assimilate ferricyanide; and phytoremediation is an option for cleaning up the environmental sites contaminated with cyanide complexes.