Impact of montmorillonite clay on the homo- and heteroaggregation of titanium dioxide nanoparticles (nTiO_2) in synthetic and natural waters

摘要

The homoaggregation of titanium dioxide nanoparticles (nTiO_2) and their heteroaggregation with ubiquitous natural clay colloids are crucial processes affecting the environmental transport and fate of nTiO_2, whereas the latter has received less attention. In this study, the effects of pH, electrolytes, natural organic matter (NOM), and montmorillonite on the homo- and heteroaggregation of nTiO_2 were systematically investigated. The isoelec-tric point of bare nTiO_2 was 6.98, whereas TiO_2-montmorillonite mixtures remained negative charged due to the reduced particle surface potential by heteroaggregation. Homoaggregation of nTiO_2 was mainly affected by anions, whereas heteroaggregation in TiO_2-montmorillonite mixtures was mainly affected by cations. Heteroaggregation between nTiO_2 and montmorillonite involved the adsorption of C-C/C-H. Intensive aggregation of nTiO_2 was observed with 4 mg/L montmorillonite, whereas with 20 mg/L montmorillonite. the aggregation was significantly inhibited by the over-coverage of montmorillonite. NOM was attached to the surface of nTiO_2 with the adsorption of functional groups involving C-C/C-H and 0-C=O. The addition of NOM effectively reduced the homo- and heteroaggregation of nTiO_2, and the stabilizing effect was enhanced with the increased molecular weight and aromatic/aliphatic fraction in NOM. Besides electrostatic repulsion, steric repulsion could also be one of the main stabilization mechanisms of NOM. With Ca~(2+) in the solutions, the stabilizing effect of NOM was significantly weakened through cation bridging. The addition of montmorillonite could facilitate the aggregation of nTiO_2 in the presence of NOM. The homo- and heteroaggregation of nTiO_2 were also observed in 7 different types of natural waters. Homoaggregation predominated in waters with high ionic strength and low NOM contents (seawater and groundwater), whereas heteroaggregation predominated in surface freshwater and wastewater systems. The results reflect the instability of nTiO_2 in natural aquatic environments and the potential risk they pose to benthic organisms.