Aggregation and transport of rutile titanium dioxide nanoparticles with montmorillonite and diatomite in the presence of phosphate in porous sand.

Crop soil is inevitably contaminated by the excess of phosphate (P) fertilizers. A large amount of nanoparticle titanium dioxide (nTiO2) entered soils as well due to the wide use of engineered nanomaterials. It is of great urgency and a high priority to investigate the mechanisms of nTiO2 deposition with the presence of P in crop soils. This study investigated the transport behavior of (1.0 g L-1) rutile nTiO2 with two representative clay particles (montmorillonite or diatomite) in the presence of P through the saturated quartz sand. In 10 mM NaCl electrolyte solution at pH 6.0, the recovery percentage of nTiO2 was 36.3% from sand column. Nevertheless, it was reduced to 18.6% and 11.1% while montmorillonite and diatomite present in suspensions, respectively. Obviously, the improvement of nTiO2 retention in sand was more pronounced by diatomite than montmorillonite. The likely mechanism for this result was that large aggregates were formed due to the attachment of nTiO2 to montmorillonite and diatomite. Moreover, the surface of diatomite with the larger hydrodynamic radius was less negatively charged by comparison with montmorillonite. However, this phenomenon disappeared with the addition of P. P adsorption increases the repulsive force between particles and sand and the fast release of attached nTiO2-montmorillonite and diatomite from sand. The two-site kinetic retention model and the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory suggested that the combination of k1/k1d, k2 and secondary minimum energy can be used to accurately describe the attachment of nTiO2-montmorillonite and diatomite to sand in the presence of P.