Heteroaggregation and sedimentation of graphene oxide with hematite colloids: Influence of water constituents and impact on tetracycline adsorption.

Because the transport of graphene oxide nanosheets (GO) from water to sediments is influenced by their heteroaggregation and sedimentation with natural colloids, knowledge on the interdependence of heteroaggregation and sedimentation for GO is needed to gain a better insight on the environmental fate of these nanosheets. However, this phenomenon is still not well understood. In this study, the heteroaggregation and sedimentation behaviors of GO with hematite nanoparticles (HemNPs) were investigated at various conditions. It has been found that negatively charged GO rapidly underwent heteroaggregation with positively charged HemNPs, leading to the sedimentation of GO. Significant sedimentation occurred when the net charge of the GOHemNP mixture was close to zero. The presence of various natural organic matters suppressed the sedimentation of the heteroaggregates through various mechanisms. Specifically, adsorption of humic acid and alginate reversed HemNP surface charge from positive to negative, leading to a slow sedimentation of the GOHemNP mixtures due to the increase in nanoparticle electrostatic repulsion. Adsorption of bovine serum albumin raised steric hindrance effect between GO and HemNP, which in turn inhibited their heteroaggregation and sedimentation. At high ionic strength conditions, the sedimentation of GO and HemNP was enhanced, possibly through the combination of homo- and hetero-aggregation. At elevated pH, the heteroaggregates were partially disaggregated, probably due to the weakening of GOHemNP bonds as the surface charges of these nanomaterials became more negative. Moreover, heteroaggregation of GO with HemNP likely to occupy the adsorption sites on GO surfaces, thus greatly reduced the adsorption of tetracycline on GO. These findings highlighted the important roles of natural colloids on the fate and transport of GO, together with the importance of heteroaggregation on the adsorption of co-existing pollutants to GO in natural aquatic environments.