Aging of zero-valent iron-based nanoparticles in aqueous environment and the consequent effects on their reactivity and toxicity.

A fundamental understanding of the long-term fate of nanoscale zero-valent iron (nZVI)-based particles in aqueous environment and the corresponding impacts on their reactivity and toxicity is essential for the responsible use and management of the nanoparticles in environmental applications. This paper comprehensively reviews the physicochemical transformations of nZVI-based particles and the consequent effects on the particle's reactivity and toxicity. The corrosions of nZVI in water under both anaerobic and aerobic conditions are summarized. The transformation of contaminant-bearing nZVI is also discussed. Besides, the factors influencing the transformation of nZVI (i.e., pH, typical anions and cations, natural organic matter, surface stabilizers, bimetal decoration, and sulfidation treatment) are summarized and discussed. In addition, the effects of particle aging on its reactivity and toxicity are discussed. Generally, the aging of nZVI-based particles would have negative impact on the removal of contaminants, especially for the degradation of organic pollutants. However, the aging process of nZVI-based particles would cause a significant reduction in their toxicity. It is suggested that the nZVI-based particles would finally transform to less toxic or benign materials (i.e., iron (oxyhydr)oxides) over time. Finally, future perspectives are proposed to better quantify and predict the transformation of nZVI-based particles in aqueous environment. PRACTITIONER POINTS: The corrosion rates and products of nZVI in water varied much under anaerobic and aerobic conditions. Typical anions and cations, natural organic matter, and iron types are critical factors influencing the physicochemical transformation of nZVI. The aging of nZVI would have negative impact its reactivity, especially for the degradation of organic pollutants. Although the fresh nZVI exhibits obvious toxicity, the aging process would cause a significant reduction in its toxicity.