IMPACT OF DISSOLVED ORGANIC CARBON ON THE SIZE-DEPENDENT SURFACE REACTIVITY OF MAGNETITE NANOPARTICLES: LESSONS LEARNED IN THE FIELD AND LAB

Nano-scale minerals have garnered increasing interest from scientists due to their size-dependent physical and chemical properties, their abundance in nature, and the many potential applications for these materials. Mineral nanoparticles are typically studied in the laboratory, as the very thing that makes nanoparticles special, their extremely small size, makes them difficult to study in the field. As a result, how the chemical behavior of nanoparticles in a field setting compares to laboratory results is not well understood.

Using custom-built TEM grid-holders, synthetic magnetite nanoparticles with average diameters of 6 nm, 44 nm, and 90 nm were emplaced in the anoxic groundwater zone of the leachate plume in the subsurface of the USGS Norman Landfill Site (OK). To investigate the role of dissolved organic carbon (DOC), laboratory analog experiments were also conducted using magnetite nanoparticles and synthetic groundwater modeled on the chemistry of the groundwater from the landfill site, but omitted DOC. In addition, a series of laboratory magnetite-chromate adsorption experiments were conducted to investigate changes in surface reactivity with varying DOC amounts.

Results from the field investigation revealed that a thin coating of organic matter (OM) developed on the particles surfaces, largely inhibiting dissolution of the magnetite. The laboratory analog results found that magnetite dissolved in the in the absence of DOC, with the 44 nm and 6 nm particles experiencing the greatest amount of dissolution. Results supportive of this interpretation were obtained from the magnetite-chromate sorption experiments, which indicated that the competitive adsorption of OM onto the magnetite surfaces inhibited the removal of chromate from solution.

The results of this investigation revealed that under the field conditions, magnetite dissolution decreased as particle size decreased, whereas this trend was inverse for the laboratory experiments. Sorption experiments indicate that this reversal is likely due to dissolved OM. Together, these experiments show that OM can play a significant role in the surface reactivity of nanoscale minerals and remains integrally important to understanding the environmental fate of nanomaterials.