(578d) Nanostructured Au/Organoclay Materials for Methylmercury Adsorption

Authors: 
Nigra, M. M., The University of Utah
Fink, K., The University of Utah
Yang, S., The University of Utah
Chica, A., The University of Utah
Johnson, W. P., The University of Utah
In recent decades, the increase in aqueous methylmercury (MeHg) levels due to anthropogenic activity has raised significant public health concerns, and has emphasized the need for an effective, low-cost means of MeHg removal from contaminated wastewater. Highly expandable clays have been investigated as MeHg sorbents; additionally, the affinity of gold nanoparticles (Au NPs) for MeHg has been documented. In several instances, organically modified clays (organoclays) have been successfully doped with Au NPs, but a comprehensive molecular understanding of these materials, which exist at the interface of natural materials and nanoscience, is still lacking.

In the current work, natural montmorillonite (STx-1b) was modified with hexadecyltrimethylammonimum (HDTMA) and doped with Au NPs in the range of 0.5 – 5 wt%, and batch sorption experiments were conducted to test the efficacy of MeHg uptake on these novel Au-organoclays. All synthesized Au-organoclay species were characterized to determine organic composition (total C and N analysis), interlayer spacing (XRD), interlayer crystallinity (FT-IR), Au loading (ICP-MS), and Au NP morphology and size distribution (UV-Vis; HAADF-STEM). In addition, the effect of MeHg sorption on both interlayer crystallinity and Au NP morphology was measured at several MeHg concentrations.

Our results suggest that the presence of Au NPs drastically increases the sorption of MeHg relative to both pure and organomodified clays. MeHg uptake, on a mass-percent basis, was found to consistently exceed 90% for initial MeHg concentrations of between 40 ng/L and 10 μg/L (vs 20-30% uptake for pure clay and 30-40% uptake for organomodified clay). Increased gold loading was found to be roughly correlated with an increase in sorption capacity across all MeHg concentrations. Further, our results suggest that the interlayer environment of the clay is significantly altered by the sorption process, even at contaminant levels as low as 20 ng/L. Aggregation of the gold nanoparticles was observed for all Au-organoclay species following exposure to MeHg; the degree of aggregation appeared to be largely independent of initial MeHg concentration. Additionally, the crystallinity (relative ordering) of HDTMA in the organoclay interlayer was found to decrease following MeHg sorption; but again, crystallinity did not appear to vary significantly with initial MeHg concentration. These findings imply a general rearrangement of the interlayer upon sorption of MeHg, and may suggest that a new stable arrangement is achieved beyond which additional MeHg sorption does not further alter the interlayer environment.

The extremely high sorption capacity and low cost of these novel nanostructured Au-organoclays makes them a highly promising alternative to existing remediation techniques. We aim to further develop insights into material properties and sorption mechanisms in order to maximize MeHg uptake with a minimum of material expense.