Aggregation and Stability of Reduced Graphene Oxide: Complex Roles of Divalent Cations, pH, and Natural Organic Matter.
The aggregation and stability of graphene oxide (GO) and three successively reduced GO (rGO) nanomaterials were investigated. Reduced GO species were partially reduced GO (rGO-1h), intermediately reduced GO (rGO-2h), and fully reduced GO (rGO-5h). Specifically, influence of pH, ionic strength, ion valence, and presence of natural organic matter (NOM) were studied. Results show that stability of GO in water decreases with successive reduction of functional groups, with pH having the greatest influence on rGO stability. Stability is also dependent on ion valence and the concentration of surface functional groups. While pH did not noticeably affect stability of GO in the presence of 10 mM NaCl, adding 0.1 mM CaCl2 reduced stability of GO with increased pH. This is due to adsorption of Ca(2+) ions on the surface functional groups of GO which reduces the surface charge of GO. As the concentration of rGO functional groups decreased, so did the influence of Ca(2+) ions on rGO stability. Critical coagulation concentrations (CCC) of GO, rGO-1h, and rGO-2h were determined to be ∼ 200 mM, 35 mM, and 30 mM NaCl, respectively. In the presence of CaCl2, CCC values of GO and rGO are quite similar, however. Long-term studies show that a significant amount of rGO-1h and rGO-2h remain stable in Call's Creek surface water, while effluent wastewater readily destabilizes rGO. In the presence NOM and divalent cations (Ca(2+), Mg(2+)), GO aggregates settle from suspension due to GO functional group bridging with NOM and divalent ions. However, rGO-1h and rGO-2h remain suspended due to their lower functional group concentration and resultant reduced NOM-divalent cation bridging. Overall, pH, divalent cations, and NOM can play complex roles in the fate of rGO and GO.
Department of Civil and Environmental Engineering, Washington State University , Pullman, Washington 99164, United States.
Indranil
I
Chowdhury
Departments of Materials Science and Engineering, Chemistry, and Medicine, Northwestern University , Evanston, Illinois 60208, United States.
Nikhita D
ND
Mansukhani
Departments of Materials Science and Engineering, Chemistry, and Medicine, Northwestern University , Evanston, Illinois 60208, United States.
Linda M
LM
Guiney
Departments of Materials Science and Engineering, Chemistry, and Medicine, Northwestern University , Evanston, Illinois 60208, United States.
Mark C
MC
Hersam
National Exposure Research Laboratory, Ecosystems Research Division, United States Environmental Protection Agency , Athens, Georgia 30605, United States.
Dermont
D
Bouchard
D000327:Adsorption; D002413:Cations, Divalent / Q000737:chemistry; D006108:Graphite / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D049329:Nanostructures / Q000737:chemistry*; D009994:Osmolar Concentration; D010084:Oxidation-Reduction; D010087:Oxides / Q000737:chemistry; D012965:Sodium Chloride; D014867:Water; D014874:Water Pollutants, Chemical
D002413:Cations, Divalent; D010087:Oxides; D014874:Water Pollutants, Chemical; D014867:Water; D012965:Sodium Chloride; D006108:Graphite