Environmental fate of sulfidated nZVI particles: the interplay of nanoparticle corrosion and toxicity during aging†
Nanomaterials have attracted research attention due to their unique properties, which have also made them increasingly useful for various applications, including remediation technologies. In this work, we tried to explore the aging phenomenon of several newly developed nanoscale zero-valent iron (nZVI) particles containing 0.25, 1, and 5% sulfur and to monitor the possible toxicity changes related to the nZVI particles before and during the aging period. We used several relevant techniques for monitoring nZVI aging, including transmission electron microscopy, X-ray powder diffraction, and particle size analysis. The toxicity of the nZVI particles was monitored using a new oxidative stress assay based on lipid peroxidation, a simple bacterial cultivation test. A test using activated sludge respiration and phospholipid fatty acid analysis of microbial populations was also employed. The results of the material analyses showed that the sulfidated nZVI particles were more stable than bare nZVI particles in terms of Fe0 content and particle diameter. The stability was more pronounced with the increasing content of sulfur in the nanoparticles; the content of α-Fe was only 35.3% in the nZVI particles compared with 78.6% in the 5% sulfur-doped nanoparticles after 60 days of aging. The results of the oxidative stress assay revealed that the toxicity was also lower with increasing sulfur content; nevertheless, the oxidative stress marker increased substantially after 7 days of aging, reaching a similar level to that caused by bare nZVI in some cases. Other results showed that even incubation in media used for the respective tests caused extensive aging; therefore, this information must be considered before the selection of toxicity assays for nanomaterial evaluation.