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

Environmental behavior, bioavailability and risks posed by TiO, nanomaterials (TiO 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 nanoparticles (TiO NPs) and nanotubes (TiO NTs) were investigated in the presence of Elliott Soil humic acid (HA) and Suwannee River humic acids (HA). The adsorption amount of HA on TiO NMs was inversely proportional to pH of solution. Maximum adsorption amount of HA on the surface of TiO NMs follows the order TiO NPs + HA (236.05 mg/g) > TiO NTs + HA (146.05 mg/g) > TiO NTs + HA (70.66 mg/g) > TiO NPs + HA (37.48 mg/g). Stability of TiO NPs and TiO NTs largely depended on their isoelectric point, morphology and solution pH in the absence of HA. Dispersion of TiO NMs was enhanced with solution pH deviated from the isoelectric point of nanomaterials due to electrostatic repulsion. Moreover, tubular structures of TiO NTs with higher length-diameter ratio seem to aggregate more easily than dose sphere-like TiO 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 NPs and TiO 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 NPs and TiO NTs, and aromatic-rich HA stabilized TiO NMs suspension more aggressively than aliphatic-rich HA.