Effect of Aging on the Reactivity and Physicochemical Transformation of Sulfidated Nano Zerovalent Iron (S-nZVI)

Aging of nano zerovalent iron (nZVI) in water leads to surface passivation, short reactive lifetime, and ultimately, loss of the zerovalent iron content. Sulfidation of nZVI (S-nZVI) has shown to alleviate some of these shortcomings by altering the surface composition to a sulfur based-coating, providing multiple advantages such as higher reaction rates and greater selectivity. In this study we investigate the effects of aging on S-nZVI and carboxymethyl cellulose (CMC) modified S-nZVI (CMC S-nZVI), synthesized with dithionite (S/Fe = 0.56) and stored under sulfidic environments over short-time intervals (0 - 21 days). SEM, TEM, XRD and XPS were performed to monitor morphological, mineralogical and chemical changes of the two types of particles. Reactivity was tested against organohalide assays (trichloroethylene (TCE) and chloroform (CF)) in a co-contaminated system, typical of field sites. Formation of amorphous iron sulfides immediately after synthesis was confirmed by XPS and XRD. TEM and SEM show nanoparticles retained their spherical morphology throughout the aging period of 21 days, with the acid digestion confirming the presence of Fe⁰. Transformation of iron sulfides to needle-like structures were apparent for S-nZVI, but not for CMC S-nZVI. This seems to suggest that CMC, even at the low concentrations employed in the study (<0.1 % wt/vol), suppresses or slows down the formation of such structures, but the mechanism by which this occurs is not yet clear. CMC also appears to inhibit the reactivity of S-nZVI, possibly by blocking reactive sites, yielding lower k_obs constants than bare S-nZVI. For both cases, however, reactivity against model pollutants increased with aging. This study provides greater insights into the transformations taking place during aging of S-nZVI and the longer reactive lifetime, as reported herein, shows its potential to overcome many of the limitations with traditional nZVI.