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

Highlights: • Adsorption capacity of nano-Fe⁰ is 3 times more than nano-Fe₃O₄ for SRHA and SRFA. • Similar amounts of SRHA and SRFA were adsorbed onto nano-Fe⁰. • Nano-Fe⁰ preferentially adsorbed intermediate molecular weight fractions of SRHA. • Nano-Fe⁰ preferentially adsorbed larger molecular weight fractions of SRFA. 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-Fe₃O₄) 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-Fe₃O₄. This is attributed to a greater number of reactive sites on nano-Fe⁰ compared to nano-Fe₃O₄, and indicates that the surface properties of nano-Fe⁰ are different that nano-Fe₃O₄ 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.