Labille J., Harns C., Bottero J.Y. Brant J., Heteroaggregation of titanium dioxide nanoparticles with natural clay colloids Environmental Science and Technology (2015) in press.
To better understand and predict the fate of engineered nanoparticles in the water column, we assessed the heteroaggregation of TiO2 nanoparticles with a smectite clay as analogs for natural colloids. Heteroaggregation was evaluated as a function of water salinity (10-3 and 10-1 M NaCl), pH (5 and 8), and selected nanoparticle concentration (0-4 mg/L). Time-resolved laser diffraction was used, coupled to an aggregation model, to identify the key mechanisms and variables that drive the heteroaggregation of the nanoparticles with colloids. Our data show that at relevant concentration, nanoparticle behavior is mainly driven by heteroaggregation with colloids, while homoaggregation remains negligible. TiO2 nanoparticle affinity for clay is driven by electrostatic interactions. Opposite surface charges and/or high ionic strength favored the formation of primary heteroaggregates via the attachment of nanoparticles to the clay. The initial shape and dispersion state of the clay as well as the nanoparticle/clay concentration ratio also affected the nature of heteroaggregation mechanism. With dispersed clay platelets (10-3 M NaCl), secondary heteroaggregation driven by bridging nanoparticles occured at a nanoparticle/clay number ratio > 0.5. In 10-1 M NaCl, the clay was pre-aggregated into larger and more spherical units. This favored secondary heteroaggregation at lower nanoparticle concentration that correlated to the nanoparticle/clay surface area ratio. In this latter case, a nanoparticle to clay sticking efficiency could be determined.