Co-transport of graphene oxide and titanium dioxide nanoparticles in saturated quartz sand: Influences of solution pH and metal ions

Abstract

Increasing production and application of nanomaterials lead to their environmental release possible. The nanomaterials with different properties may transport together in porous media, and consequently affect their environmental fates. In this study, column experiments were conducted to investigate the co-transport of two typical nanomaterials, graphene oxide (GO) and nano-titanium dioxide (nTiO2), in saturated quartz sand in NaCl and CaCl2 electrolyte solutions under both favorable and unfavorable conditions. The breakthrough curves as well as the retained profiles of single and binary nanoparticles were examined. The results indicated that nTiO2 significantly enhanced the GO retention under all examined conditions, especially at lower pH, higher ionic strength and the presence of divalent cation Ca2+. This might be attributed to the formation of less negatively charged and larger-sized GO-nTiO2 agglomerates as well as the increased retention sites on sand surface by preferentially deposited nTiO2. However, GO merely slightly enhanced the transport of nTiO2 in NaCl solutions, whereas had negligible effect on nTiO2 transport and retention in CaCl2 solutions. The highly hydrophilic and mobile GO served as a carrier and facilitated the transport of nTiO2 in NaCl solutions. In CaCl2 solutions, the strong attachment affinity between positively charged nTiO2 and negatively charged quartz sand (at pH 4.5), and dramatical accumulation of large nTiO2 agglomerates near the column inlets (at pH 6.5) led to significant deposition of nTiO2 on quartz sand. The co-presence of GO failed to counteract the retention of nTiO2 particles on sand.