Effects of solution chemistry on the attachment of graphene oxide onto clay minerals.

With the increase in production and wide application of graphene oxide (GO), colloidal GO particles are expectantly released into soil and groundwater, where a large number of mineral particles exist. In addition, the porewater chemistry (e.g. organic acid, valence of cation) is a neglected but important aspect to comprehensively investigate the fate of GO. The interactions of GO with three ubiquitous clay minerals (i.e., montmorillonite, kaolinite and diatomite) have been systematically investigated through batch experiments across different solution chemistry conditions. In general, the affinity towards GO is in the order of montmorillonite > kaolinite > diatomite under the same experimental conditions. This observation can be explained by the characteristics of different clay minerals, such as surface charge and surface area. The results indicated that increasing the ionic strength or decreasing the pH enhanced the attachment of GO nanoparticles onto clay minerals as a result of electrostatic interactions. With the increase in concentration of Ca2+, more GO particles were attached onto clay mineral particles. This is caused by complexation between the surface oxygen functional groups of both GO nanoparticles and clay minerals. The presence of 0.1 mM tartaric acid significantly inhibited the attachment of GO onto clay minerals. This is possibly linked to the increased negative charges of the organic acids and the competition between organic acids and GO. The interaction energies were also calculated by applying the classical DLVO theory. The results of this study have helped to understand the behavior and fate of GO in subsurface formations.