Adsorption, aggregation and sedimentation of titanium dioxide nanoparticles and nanotubes in the presence of different sources of humic acids

Abstract Environmental behavior, bioavailability and risks posed by TiO 2 , nanomaterials (TiO 2 NMs) in surface waters are affected by morphologies of the particles and geochemistry, including pH, inorganic and organic matter. Here, the adsorption, aggregation and sedimentation of TiO 2 nanoparticles (TiO 2 NPs) and nanotubes (TiO 2 NTs) were investigated in the presence of Elliott Soil humic acid (HA E ) and Suwannee River humic acids (HA S ). The adsorption amount of HA on TiO 2 NMs was inversely proportional to pH of solution. Maximum adsorption amount of HA on the surface of TiO 2 NMs follows the order TiO 2 NPs + HA E (236.05 mg/g) > TiO 2 NTs + HA E (146.05 mg/g) > TiO 2 NTs + HA S (70.66 mg/g) > TiO 2 NPs + HA S (37.48 mg/g). Stability of TiO 2 NPs and TiO 2 NTs largely depended on their isoelectric point, morphology and solution pH in the absence of HA. Dispersion of TiO 2 NMs was enhanced with solution pH deviated from the isoelectric point of nanomaterials due to electrostatic repulsion. Moreover, tubular structures of TiO 2 NTs with higher length-diameter ratio seem to aggregate more easily than dose sphere-like TiO 2 NPs. This might be due to their spherical structure enhancing steric repulsion. Notably, the adsorption of HA led to disagglomeration and significant stability of TiO 2 NPs and TiO 2 NTs due to steric hindrance under varying solution pH. In addition, adsorption time, concentration and sources of HA also influenced suspension/sedimentation behavior of TiO 2 NPs and TiO 2 NTs, and aromatic-rich HA E stabilized TiO 2 NMs suspension more aggressively than aliphatic-rich HA S .