An Investigation of the Adsorption Behavior between Silver Nanoparticles (AgNPS) and Corundum (α-Al2O3) at Environmental pH Values using Inductively-Coupled Plasma Optical Emission Spectroscopy (ICP-OES) and Raman Spectroscopy
Steven Higgins (Committee Chair), Audrey McGowin (Committee Member), Ioana Sizemore (Advisor)
Master of Science (MS)
With the increased use of silver nanoparticles (AgNPs) in consumer products, the need to monitor their release into environmental soils has become critical. Although minerals make up a large component of soils, research on their interactions with AgNPs is still limited. Furthermore, the majority of the existing research focused on silica-based minerals rather than metal oxides, which is the second most abundant mineral type in the earth's crust. This study examined for the first time the aqueous interaction between the widely used, spherical Creighton AgNPs and a-corundum (a-Al2O3), a representative non-silica-based mineral, at environmentally relevant pH values (6-11). Samples were prepared by incubating 100 mL of 1 mg L-1 of AgNPs, ~ 1.182 g of a-Al2O3, 100 µL of 5 M of NaNO3 as an ionic strength adjuster (ISA), and 100 µL of pH adjusters (0.1 M of HNO3 or 0.1 M of NaOH) for 30 min. Samples were then centrifuged at 10,000 rpm for 2 min to separate the solid a-Al2O3 with bound AgNPs (pellet) from the aqueous portion containing free AgNPs (supernatant). The supernatant was then analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES) for the quantification of the total amount of AgNPs by difference adsorbed onto a-Al2O3. The pellet was placed on glass microscope slides and large areas were mapped by micro-Raman spectroscopy in order to identify possible molecular interactions between AgNPs and a-Al2O3. The ICP-OES results demonstrated that over 75% of the available AgNPs were adsorbed to a-Al2O3 (sub-monolayer coverage), rapidly reducing their mobility in the environment. This sub-monolayer coverage was also verified using a total surface coverage value (T), with values ranging from 1.269x10-3 to 1.378x10-3. In addition, the adsorption process of the negatively charged Creighton AgNPs (in the 6-11 pH range) was found to be pH independent suggesting that other interaction mechanisms may counteract the electrostatic attractions (< pH 9) and repulsions (> pH 9) with corundum (pHpzc = 9.1). The Raman images collected on the AgNP- a-Al2O3 samples (n = 1,089 spectra at each pH) revealed the appearance of an Ag-O stretching mode at 220-250 cm-1, and confirmed the surface complexation of AgNPs to the terminus oxygen atoms (Ag-O-Al-). The highest chemisorption levels were statistically determined at pH ~ 9, when a-Al2O3 experiences loss of surface charge and is more readily available for direct molecular interactions with AgNPs. Overall, the results of this study demonstrate that both physisorption (electrostatic interactions) and chemisorption (other interactions such as hydrogen bonding, and London-dispersion forces) mechanisms play a role in the significant adsorption of Creighton AgNPs to a-Al2O3 and their reduced mobility into the environment at environmentally relevant pH values.
Department or Program
Department of Chemistry
Year Degree Awarded
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