Phytotoxicity of iron-based materials in mung bean: Seed germination tests

Environmental applications and potential risks of iron-based materials have attracted increasing attention. However, most previous studies focused on a single material. Comparative research using different iron-based materials under the same experimental conditions is still lacking. Here, six iron-based materials, including micro-sized and nanoscale Fe₃O₄ (i.e., mFe₃O₄ and nFe₃O₄), bulk and bare nanoscale zero-valent iron (i.e., mZVI and B-nZVI), starch-supported nZVI (S-nZVI), and activated carbon-supported nZVI (A-nZVI), were studied to compare their phytotoxicity in mung bean grown in suspensions with doses of 0, 300, 600 and 1000 mg/L. Taking the four toxicology parameters (seed germination rate, germination index, seedling elongation and biomass) together, the iron-based materials except mFe₃O₄ generally produced no significant phytotoxicity to mung bean even at 1000 mg/L. nFe₃O₄ and B-nZVI showed no higher phytotoxicity than their micro-sized counterparts (mFe₃O₄ and mZVI). All the materials resulted in increased Fe concentrations in seedlings particularly in roots, and mZVI and B-nZVI produced more significant effects. However, the Fe in the roots was difficultly translocated to the shoots. Compared to B-nZVI, nFe₃O₄ had lower bioavailability and bioaccumulation potential. XRD results confirmed that most Fe₃O₄ and B-nZVI remained unchanged during seedling growth, while support materials accelerated the corrosion and transformation of S-nZVI and A-nZVI. In conclusion, the tested nanoscale iron-based materials generally possess no obvious phytotoxicity within the dose range, but cause excess Fe accumulation in seedlings. Introduction of support materials may reduce such risk, allowing safer applications of these iron-based materials.