Evaluating phytotoxicity of bare and starch-stabilized zero-valent iron nanoparticles in mung bean

Zero-valent iron nanoparticles (nZVI) are among the most widely used nanoparticles in nanoremediation of various environmental pollutants. Environmental fate and impact of nZVI has attracted increasing concerns due to their potential risks. However, phytotoxicity of nZVI still remains poorly understood. Here, the phytotoxic effects of bare nZVI (B-nZVI) and starch-stabilized nZVI (S-nZVI) were evaluated on the germination of mung bean seeds exposed to suspensions with different doses of 0-1000 mg/L and the growth of hydroponically cultured seedling at 600 mg/L. In most cases, B-nZVI had no inhibition on seed germination, and even promotion on shoot and root elongation. However, S-nZVI displayed dose-dependent effects, with a decreased germination rate at 600-750 mg/L. B-nZVI at 600 mg/L showed no obvious phytotoxic but even stimulatory effects on seedling growth. Comparatively, S-nZVI at 600 mg/L produced significant phytotoxicity on mung bean plants, leading to decreased seedling growth, altered nutritional balance, and excess Fe accumulation in roots (>400 mg/kg). S-nZVI were observed to form a coating of insoluble Fe(III) compounds on root surface. Simultaneously, some nZVI penetrated and accumulated into root cells, but did not move to shoots. In conclusion, B-nZVI easily aggregate into larger particles in solution, leading to decreased adhesion to root surface and lower uptake by roots, whereas the higher dispersity and hydrophilicity of S-nZVI makes them more readily be adhered to root surface forming a coating, and penetrated into roots, resulting in excess Fe accumulation, consequently interfering with root functions such as the adsorption and transport of water and nutrients.