Aggregation of Graphene Oxide in Natural Waters: Role of Solution Chemistry and Specific Interactions

Abstract

In this paper, the impact of pH, ionic strength, and dissolved organic matter [humic acid (HA) and tannic acid (TA)] on graphene oxide (GO) stability was investigated. The results showed that the GO is negatively charged over a pH range from 2 to 11. pH did significantly affect GO stability at a level of 4 or higher, but the particles became unstable below pH 3 due to protonation of −COOH at the edge. Ionic strength (IS) and salt type had observable effects on stability as a result of electrical double layer compression and specific interactions. CaCl2 affects GO more noticeably than NaCl because of the binding ability of Ca2+ ions with carboxyl and hydroxyl functional groups. pH had negligible effects in the presence of 10 mM NaCl, but 1 mM CaCl2 decreased GO stability as pH increased owing to the adsorption of Ca2+ ions on the surface functional groups of GO and the consequent decrease of surface charge. The applicability of DLVO theory as a predictive tool was investigated by modeling GO sheets in two different geometries; three-dimensional spherelike particles and two-dimensional particles at different values of pH and IS. Overall, the specific interactions and chemical structure of adsorbed organics had a dominant role in GO stability.