Phytotoxicity of seven iron-based materials to mung bean seedlings

Due to the widespread application of various iron-based materials in environmental remediation and agricultural production, it is of significance to assess their environmental risks. Here, seven iron-based materials, including ionic FeCl₃, micro- and nano-sized magnetite (i.e., mFe₃O₄ and nFe₃O₄), micro- and nano-sized zero-valent iron (i.e., mZVI and nZVI), and nZVI modified by starch and activated carbon (i.e., S-nZVI and A-nZVI), were targeted to compare their phytotoxicity in mung bean grown in the soil with different doses (0, 300, 600, and 1000 mg/kg) for 40 days. S-nZVI and A-nZVI severely inhibited plant growth, decreasing shoot and root dry weights by 45.1-79.2 and 26.0-82.3%, respectively, but other materials showed no or minimal effects on plant growth. All the materials decreased chlorophyll content and photosynthesis performance, with more pronounced inhibition from A-nZVI and S-nZVI, especially at 1000 mg/kg. The activities of superoxide dismutase (SOD) and peroxide dismutase (POD) in leaves were stimulated by all the materials, among which S-nZVI enhanced SOD activity by 206.9% at 300 mg/kg and POD activity by 541.1% at 1000 mg/kg. In most cases, Fe accumulation in the roots was increased by all materials, particularly by S-nZVI and A-nZVI. Fe concentrations in roots and shoots exposed to 1000 mg/kg S-nZVI and A-nZVI were 2-3 times higher than those in the control treatments. The disturbance in the homeostasis of minerals (Zn, Mn, Cu, and Mg) was induced by all iron-based materials. To conclude, the phytotoxicity of iron-based materials was dependent on their type and dosage, and stabilization was crucial for the phytotoxicity and bioaccumulation potential of nZVI in plants.