Reports: G2
45879-G2 Stabilization of Metal-Sulfide Nanoparticles by Natural Organic Matter
This research seeks to identify interactions occurring between dissolved humic materials and nanoparticulate metal-sulfides in the natural environment. Natural petroleum deposits contain impurities such as sulfur and trace metals that can interfere with the production and utilization of petroleum-based products. When metals such as Zn, Hg, Cd, and Cu are present during diagenesis of carbonaceous material, they are likely to be in the form of metal-sulfides. Nanoscale metal-sulfide particles (<100 nm diameter) are formed as intermediates of precipitation reactions in sediment and soil porewater. These species can persist at the nanoscale if further crystal growth and aggregation are kinetically hindered. The objective of this ACS PRF project was to investigate how nanoparticles of metal sulfide can form as they coprecipitate with humic substances and common natural organic acids that are substrates for early diagenesis processes.
In the most recent funding period (September 08 to August 09), we accomplished the following:
- We conducted studies on coprecipitation of zinc (Zn) and mercury (Hg) sulfides with natural organic matter (NOM). We utilized dynamic light scattering to monitor the size of particles growing over time. Our finding demonstrate that thiol-containing organics (such as amino acids, humic substances) are capable of adsorbing to the surface of nucleated particles, slow their growth, and promote their stability in suspension as nanoparticles. Two journal articles were published from this work
- We performed follow-up studies investigating the aggregation of ZnS nanoparticles in suspension with model organic compounds. We investigated aggregation, which is one component of the precipitation process that enables nucleated nanoparticles to remain in the nano size range. Our findings show that surface sorption of the amino acid cysteine induced negative surface charge on the nanoparticles, leading to reduced attachment efficiencies and slow aggregation. This interaction was quantified as a function of pH, ionic strength, and adsorbed cysteine density. We are currently preparing a manuscript describing this work and expect to submit this fall (Gondikas et al.).
- We utilized synchrotron X-ray absorption spectroscopy to investigate molecular-scale interactions occurring between NOM and ZnS particles. The objective of experiments was to identify the coordination environment of surface Zn atoms on these nanoparticles. Our XAS data indicate that cysteine reduced growth rate of ZnS nanoparticles by directly coordinating through thiol and carboxylate functional groups to surface Zn atoms on the nanoparticles. We are planning complementary experiments with synchrotron small angle X-ray scattering (SAXS) to provide information regarding the size of the nuclei and aggregate structure (i.e., fractal dimension). We currently have beam time scheduled at the European Synchrotron Radiation Facility (ESRF) in December 2009.
