Heteroaggregation between graphene oxide and titanium dioxide particles of different shapes in aqueous phase

Graphene oxide (GO) inevitably interacts with engineered and/or natural nanoparticles (NPs) after releasing into aquatic environments. It is observed that GO could form heteroaggregates with differently shaped TiO2 NPS at pHs 4.0 and 7.0 rather than pH 10.0. Optimal pHs for the maximal heteroaggregation were around 6.63 and 4.92 for TiO2 spheres and fibers, respectively, which was dominated by electrostatic attraction. The optimal concentration ratio for TiO2 spheres was 10 times higher than TiO2 fibers, due to cross-linking configuration and stronger van der Waals force of TiO2 fibers with GO. Pre-homoaggregation of TiO2 NPs could promote subsequent heteroaggregation while the promotion was negligible at high ionic strength (e.g., > critical coagulant concentrations), and a longer pre-homoaggregation time showed a stronger promotion (750 s > 150 s > 0 s). Natural organic matter (NOM) obviously inhibited GO-TiO2 heteroaggregation at pH 4.0 compared with pH 7.0 due to stronger steric hindrance. Moreover, strong disturbance (e.g., sonication) could break down the heteroaggregates, and the disaggregation and re-aggregation processes were shape-independent. This work provides new insights into the role of NPs shapes and homoaggregation on the interaction of GO with both engineered and natural particles/minerals, and the fate of GO in aquatic environments.