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Transport and Retention of TiO2 Rutile Nanoparticles in Saturated Porous Media: Influence of Solution pH, Ionic Strength, and the Presence of Humic Acid
Chen, G., X. Liu, E. H. Jones, AND C. SU. Transport and Retention of TiO2 Rutile Nanoparticles in Saturated Porous Media: Influence of Solution pH, Ionic Strength, and the Presence of Humic Acid. Presented at Gordon Research Conference, Waterville Valley, NH, May 29 - June 03, 2011.
Poster presentation for the Gordon Research Conference in Waterville Valley, NH (May 29 - June 3, 2011)
The influence of solution pH, ionic strength, and varying concentrations of the Suwannee River Humic Acid (SRHA) on the transport of titanium dioxide (TiO2, rutile) nanoparticle aggregates (nTiO2) in saturated porous media was investigated through systematically examining the transport and retention kinetics of the nTiO2 over a broad range of environmentally relevant aquatic conditions (i.e. pH: 6 and 9; ionic strength: 3 – 100 mM NaCl; concentration of SRHA: 0 – 10 mgL-1). Transport experimental results showed considerably different effects of SRHA on the stability and mobility of nTiO2 at pH 6 versus pH 9. Specifically, the stability and transport of nTiO2 in porous media was substantially increased in the presence of SRHA at pH 6, even at a concentration of 1 mgL-1 SRHA. Greater stability and mobility of nTiO2 were observed with increasing the concentration of SRHA. In contrast, this enhancement of nTiO2 stability and mobility due to the presence of SRHA was minor at pH 9, regardless of the concentration of SRHA tested in this study. This difference may be explained by the different amount of the adsorption of SRHA to nTiO2 and sand grains at the two pH conditions. It is well documented in the literature that the adsorption of SRHA to solid surfaces is highly sensitive to solution pH and ionic strength. Qualitatively, low pH and high ionic strength facilitate the adsorption of SRHA to nTiO2 and sand grains in this study. Adsorbed SRHA to nTiO2 and sand grains provided additional electrostatic and electrosteric repulsion to stabilize and mobilize nTiO2 in porous media. Interestingly, comparable transport and retention patterns were observed for nTiO2 in the absence and the presence of SRHA with the concentration of 1 mgL-1 at pH 6, but at much higher ionic strength in the presence of SRHA. Similar temporal and spatial variations of nTiO2 transport and deposition in the column were obtained. In contrast, different transport and retention characteristics were observed for nTiO2 at 3 and 10 mgL-1 SRHA. Presumably, this is due to partial coverage of SRHA on TiO2 and sand grains not sufficiently shielding the effect of the bare nTiO2 and sand grains at a low SRHA concentration of 1 mgL-1. In addition, the deposition rates of the nTiO2 in both pH 6 and 9 solutions increased with increasing ionic strength. Overall, the experimental results presented here suggest that nTiO2 suspension is often stable and mobile under typical environmentally-relevant conditions of solution pH, ionic strength, and natural organic matter. This study implies that solution pH and natural organic matter are one of the key factors governing the stability and mobility of nTiO2 in the aquatic environment.