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Aggregation, Deposition and Release of Graphene Oxide Nanomaterials in the Aquatic Environment
Citation:
Chowdhury, I., M. Duch, N. Mansukhani, M. Hersam, AND D. Bouchard. Aggregation, Deposition and Release of Graphene Oxide Nanomaterials in the Aquatic Environment. Presented at Gordon Conference: Environmental Nanotechnology - Novel Approaches to Meet Global Challenges, Stowe, VT, June 02 - 07, 2013.
Impact/Purpose:
Presentation for Gordon Conference: Environmental Nanotechnology -Novel Approaches to Meet Global Challenges, June 2-7, 2013 Stowe, VT
Description:
Graphene is an atomically thin two dimensional carbon-based nanomaterial that is composed of a single layer of sp2 – hybridized carbon atoms as found in graphite.1, 2 Usage of graphene-based nanomaterials is increasing rapidly and these materials are predicted to be the most abundant carbon-based nanomaterials in the future.3, 4,5 In different applications, graphenes are usually oxidized at different levels and also different synthesis processes can result in varying graphene oxidation states.6 Three common forms of graphenes are pristine graphene (G), graphene oxide (GO), and reduce graphene oxide (rGO).6 GO is an insulating material; hence, GO is often reduced to increase the electrical conductivity for applications in electronic devices. Relative transport behavior of differently oxidized graphene nanomaterials needs to be understood to accurately predict the fate of graphene-based nanomaterials in the environment as well as to inform the synthesis of environmentally friendly graphene-based nanomaterials. Therefore, in this study the influence of oxidation on the fate of graphene-based nanomaterials was investigated. Three types of graphene-based nanomaterials were used in this study. A GO suspension was synthesized using the modified Hummers method and reduced by hydrazine to partially oxidized GO (PrGO) and highly reduced GO (HrGO). This reduction process resulted in similar size dimensions among GO, PrGO, and HrGO, with observed variations in oxidation level and electrical conductivity of the nanomaterials. Physical characterization of synthesized graphenes included atomic force microscopy for physical dimension, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy for oxidation level. Electrokinetic and hydrodynamic properties of graphenes were determined as a function pH, ionic strength, salt types and in the presence of natural organic matter (NOM). Suwannee River Humic Acid Standard II (SRHA) was used as model NOM. Aggregation kinetics was measured using time-resolved dynamic light scattering. Interactions of GO, PrGO, and HrGO with natural surfaces were investigated using a Quartz Crystal Microbalance with Dissipation monitoring (QCM-D). Two types of natural surfaces were investigated in this study, silica and NOM-coated surfaces, as these are commonly found in the aquatic environment. Both deposition and release of GO, PrGO, and HrGO were monitored in three different salt types (NaCl, CaCl2, and MgCl2) as a function of IS. Preliminary results showed that aggregation and stability of GO in the aquatic environment follows colloidal theory, even though the shape of GO is not spherical. pH did not have notable influence on the stability of GO; however, salt type (NaCl, CaCl2, MgCl2) and concentration did affect GO stability. Critical coagulation concentration (CCC) values of GO were lower than reported fullerenes CCC values and higher than reported carbon nanotube CCC values. CaCl2 destabilized GO more aggressively than MgCl2 and NaCl due to the binding capacity of Ca2+ ions with the GO hydroxyl and carbonyl functional groups. Natural organic matter (NOM) can significantly improve the stability of GO in water. Deposition studies with QCM-D show that under favorable conditions (PLL-coated positive surface), deposition of GO showed two distinct slopes due to the two dimensional shape of GO. However, this dual mode deposition of GO diminished with increased IS due to formation of large GO aggregates. A negligible amount of GO deposited on the silica surface in CaCl2, though we observed a notable amount of GO deposition in MgCl2. We also observed significant GO release after addition of deionized water at the end of the experiment. Release of GO showed significant dependence on salt type, ionic strength, favorable and unfavorable conditions. Ongoing studies are underway for PrGO and HrGO. Detailed findings and mechanisms will be presented.
