High molecular weight components of natural organic matter preferentially adsorb onto nanoscale zero valent iron and magnetite.

Nanoscale zero valent iron particles (nano-Fe0 ) are attractive for in-situ groundwater remediation due to their high reactivity and ability to degrade many different classes of environmental contaminants. It is expected that adsorbed natural organic matter (NOM), which is heterogeneous and typically has a wide molecular weight (MW) distribution, will affect the reactivity and performance of nano-Fe0 as a remediation agent. However, the interaction of NOM with nano-Fe0 has not been well-studied. In this study, we used high performance size exclusion chromatography (HPSEC) to determine if there was preferential sorption of the high MW fraction of NOM onto nano-Fe0 that have a Fe0 core and a Fe-oxide shell (predominantly magnetite). Adsorption of two types of NOM, Suwannee River Humic Acid (SRHA) and Fulvic Acid (SRFA), to nano-Fe0 was compared to magnetite of similar size (nano-Fe3 O4 ) to also assess the effect of the Fe0 core on adsorption of NOM. The results showed that the surface area normalized adsorbed mass (mg/m2 ) of both SRHA and SRFA onto nano-Fe0 is almost three times than that of nano-Fe3 O4 . This is attributed to a greater number of reactive sites on nano-Fe0 compared to nano-Fe3 O4 , and indicates that the surface properties of nano-Fe0 are different that nano-Fe3 O4 despite the shell of magnetite on nano-Fe0 . The sorption capacity of both SRHA and SRFA onto nano-Fe0 were similar. However, the intermediate sized MW fractions (2-6 kDa) of SRHA were preferentially adsorbed onto the nano-Fe0 surface, whereas the large MW fractions (>3.5 kDa) of SRFA were preferentially adsorbed. These results suggest that NOM interaction with nano-Fe0 are a function of the MW distribution of the NOM in the system studied and indicate that the MW distributions of NOM should be taken into consideration when predicting the fate and performance of nano-Fe0 in environmental remediation.