Partial Oxidation (Aging) and Surface Modification Decrease the Toxicity of Nanosized Zerovalent Iron

Nanoscale zero-valent iron (nZVI) is a redox-active nanomaterial used in the remediation of contaminated groundwater. To assess the effect of aging and surface modification on its potential neurotoxicity, cultured rodent microglia (M) and neurons (N27) were exposed to fresh nZVI, aged (> 11 months) nZVI, magnetite, and polyaspartate surface-modified ISM) nZVI. Increases in various measures of oxidative stress indicated that BV2 microglia responded to these materials in the following rank order: nZVI > aged nZVI > magnetite = SM nZVI. Fresh nZVI produced morphological evidence of mitochondrial swelling and apoptosis. In N27 neurons, ATP levels were reduced in the following rank order: nZVI > SM-nZVI > aged nZVI = magnetite. Ultrastructurally, nZVI produced a perinuclear floccular material and cytoplasmic granularity. Both SM-nZVI produced intracellular deposits of nanosize particles in the N27. The physicochemical properties of each material, measured under exposure conditions, indicated that all had electronegative zeta potentials. The iron content of nZVI (similar to 35%) and SM-nZVI (similar to 25%) indicated high redox activity while that of aged and magnetite was neglibile. Sedimentation and agglomeration occurred in the following rank order: nZVI > aged nZVI > magnetite >> SM-nZVI. Correlating these properties with toxicity indicated that partial or complete oxidation of nZVI reduced its redox activity, agglomeration, sedimentation rate, and toxicity to mammalian cells. Surface modification decreased nZVI toxicity by reducing sedimentation which limited particle exposure to the cells.