Phytoremediation of cyanide and iron cyanide complexes and the mechanisms involved

This paper reviewed the physical and chemical properties of cyanide species - free cyanide and iron-cyanide complexes, and the potential of cyanide phytoremediation with reference to the phytotoxicity of free cyanide and iron-cyanide complexes in plants. There are three possible pathways, which are β-cyanoalanine synthase, sulfur transferase and formamide hydrolase pathways, for transforming and assimilating endogenous free cyanide in plants. Iron-cyanide complexes are generally resistant to microbial and fungal degradation. It is suggested that there may be undiscovered degradation pathways involved in assimilating iron-cyanide complexes in plants; however the detailed pathways of assimilation of iron-cyanides are still unknown. While uptake of free cyanide is mainly by simple diffusion, as iron-cyanide complexes are membrane-impermeable, it is suggested that the complexes may be transported into the plants through the mode of protein mediated uptake. Upon uptake, biological fates of cyanide species vary with different species of cyanide, depending on their chemical properties and concentrations. Phytotoxicity of free cyanide in plants is much higher than that of iron-cyanide complexes as plants could generally withstand a higher concentration of iron-cyanide complexes comparing with free cyanide. However, it is still unsure if the iron-cyanide complexes are toxic themselves or if they disrupt the metabolism of plants indirectly. It is known that endogenous cyanogenic compounds play a role in providing sources of nitrogen and acting as precursors in some biochemical processes in plants. Studies suggested that exogenous cyanide species, to a certain extent, could benefit the plants through providing nutrition to them. However, there is still no study conclusively indicates that there is a direct acquisition of exogenous cyanide species by plants as their alternative source of nitrogen. Further investigations on the degradation pathways of iron-cyanide complexes and the essential enzymes involved in phyto-assimilation of iron-cyanide complexes are required for better understanding of the degradation and assimilation pathways of cyanogenic compounds in plants.