Synergistic effects of phosphorus and humic acid on the transport of anatase titanium dioxide nanoparticles in water-saturated porous media

Abstract

The (un)intentional release of titanium dioxide nanoparticles (TiO2 NPs) poses potential risks to the environment and human health. Phosphorus (P) and humic acid (HA) usually coexist in the natural environments. This study aims at investigating the transport and retention behaviors of TiO2 NPs in the single and binary systems of P and HA in water-saturated porous media. The experimental results showed that HA alone favored the transport of TiO2 NPs in sand columns to a greater extent than that of P alone at pH 6.0. Interestingly, the co-presence of P and HA acting in a synergistic fashion enhanced the transport of TiO2 NPs in sand-packed columns more significantly compared to that in the single-presence of P or HA. Particularly, P plays a dominant role in the synergistic effect. This is largely due to the competitive effect between P and HA for the same adsorption sites on the sand surfaces favorable for TiO2 NPs retention. A two-site kinetic attachment model that considers Langmuirian blocking of particles at one site provided a good approximation of TiO2 NPs transport. Modeled first-order attachment coefficient (k2) and the maximum solid-phase retention capacity on site 2 (Smax2) for P or HA alone were larger than those in the co-presence of P and HA, suggesting a less retention degree of TiO2 NPs in the binary system of P and HA. Our findings indicate that the mobility of TiO2 NPs is expected to be appreciable in soil and water environments, where P and HA are rich and always co-present at low pH conditions.