Abstract
The impact of pH and ionic strength on the mobility (individual and co-transport) and deposition kinetics of TiO2 and ZnO NPs in porous media was systematically investigated in this study. Packed column experiments were performed over a series of environmentally relevant ionic strengths with both NaCl (0.1−10 mM) and CaCl2 (0.01–0.1mM) solutions and at pH 5, 7, and 9. The transport of TiO2 NPs at pH 5 was not significantly affected by ZnO NPs in solution. At pH 7, a decrease in TiO2 NP transport was noted with co-existence of ZnO NPs, while at pH 9 an increase in the transport was observed. At pH 5 and 7, the transport of ZnO NPs was decreased when TiO2 NPs was present in the solution, and at pH 9, an increase was noted. The breakthrough curves (BTC) were noted to be sensitive to the solution chemistries; the decrease in the breakthrough plateau with increasing ionic strength was observed under all examined pH (5, 7, and 9). The retention profiles were the inverse of the plateaus of BTCs, as expected from mass balance considerations. Overall, the results from this study suggest that solution chemistries (ionic strength and pH) are likely the key factors that govern the individual and co-transport behavior of TiO2 and ZnO NPs in sand.
Highlights
Various studies have shown that TiO2 NPs is toxic to organisms like rats, algae, microbes, invertebrates, and fish
The transport of TiO2 NPs in absence and presence of ZnO NPs in sand was examined under a series of ionic strengths in both NaCl (0.1, 1, and 10 mM) and CaCl2 (0.01, 0.05, and 0.1 mM) solutions at three different pH conditions, 5, 7, and 9
The breakthrough curves of TiO2 NPs without ZnO NPs are presented in Figs 1, 2 and 3
Summary
Various studies have shown that TiO2 NPs is toxic to organisms like rats, algae, microbes, invertebrates, and fish. The yearly production of TiO2 NPs is expected to expand to *2.5 million metric tons by 2025 in the United States alone [1]. Due to their extensive production and use in household and industrial commodities, some of the TiO2 NPs utilized have been discharged into the natural aquatic environment directly or indirectly. Reports suggest that 10–100 mg/L of Ti is present in municipal wastewater treatment plant effluents [2]. Mechanisms controlling the transport and deposition of these nanomaterials in porous media are critical to understand in order to thoroughly evaluate the mobility and persistence of PLOS ONE | DOI:10.1371/journal.pone.0134796 August 7, 2015
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