Competition between fulvic acid and phosphate-mediated surface properties and transport of titanium dioxide nanoparticles in sand porous media

Abstract

Fulvic acid (FA) is a series of organic macromolecular compounds with many oxygen-containing functional groups and is easy to complex with nanoparticles (NPs) in the environment, thus affecting the stability and mobility of NPs in the water environment. Therefore, it is necessary to investigate the transport behavior of nano titanium dioxide (nTiO2) particles with FA in the presence of phosphate (P). A series of transport experiments were conducted by using nTiO2 suspended in phosphate background electrolyte with and without the addition of FA passing through water-saturated sand columns. The zeta potentials, hydrodynamic radii, and aggregation kinetics were measured, and the extended Derjaguin-Landau-Verwey-Overbeek (DLVO) theory and two-site kinetic attachment model (TSKAM) were employed to explain the underlying mechanism. The experimental results showed that the presence of FA compressed the transport of nTiO2 in phosphate electrolyte due to formation of larger complex via the adsorption of organic FA molecules onto nTiO2, which can strongly compete with phosphate adsorption, leading to reduced electrostatic repulsive forces between nTiO2 and sand. In the absence of FA at low pH (6.0), transport of individual nTiO2 was increased by low phosphate adsorption in response to increased phosphate levels, and then, transport was decreased with the increased number of cations (Na+) from the phosphate (P ≥ 1.0 mM) electrolyte due to the compressed electric double layer. From the goodness-of-fit of the modeling results, the TSKAM provided a good prediction for the retention and transport of nTiO2 in the copresence of FA and phosphate. The compressed transport of nTiO2 in the copresence of FA and phosphate was distinct from the synergistic transport in the copresence of humic acid.