Abstract
Titanium dioxide nanoparticle (TiO2 NP) discharged into water bodies can affect ecosystems and human health adversely. We studied the properties of residual TiO2 NPs with and without gentle mixing (to simulate a natural environment more closely) and after settling for 12-h periods. Surface complexation, dynamic particle size changes, and TiO2 NP destabilization in synthetic and lake waters were investigated. The accumulation of inert ions (Na+ and Cl−) in the diffuse layer which was not discussed in other studies was supposed to be the main reason that aggregation occurred slowly and continuously. PO43− stabilized and destabilized TiO2 NPs at 10 mM and 100 mM, respectively. Destabilization occurred because high ionic strength overwhelmed increased negative charges of TiO2 NPs by complexation with PO43−. TiO2 NP destabilization was achieved in approximately 12 h in synthetic and lake waters, and is attributed to the slow diffusion of ions into aggregates. Despite the presence of moderately high concentrations of natural organic matter, which tends to stabilize TiO2 NPs, the addition of 20 mM PO43− destabilized the TiO2 NPs in lake water. Smaller aggregate sizes formed compared with those before destabilization, which indicates that stable residual TiO2 NPs could exist in aquatic environments after extended periods.
Highlights
The increased production and widespread use of nanoparticles has raised concerns regarding their environmental fate and ecological toxicity[1, 2]
The complexation of PO43− in Milli-Q water caused a lowering in zeta potential of TiO2 NPs in Milli-Q water as shown in Fig. 4(d), there was no effect of PO43− addition on the change in zeta potential of TiO2 NPs in the lake water. This result is attributed to the fact that the complexation of PO43− with the limited numbers of sites on the TiO2 NPs was interrupted by competition among various anions and organic matter, and because the negative surface charge of the complexed PO43− was shielded by the complexation of other ions and a compaction of the electrical double layer, which is caused partly by an increased ionic strength from PO43− addition
An understanding of TiO2 NP behavior in natural water sources is important for human health
Summary
The increased production and widespread use of nanoparticles has raised concerns regarding their environmental fate and ecological toxicity[1, 2]. Factors that affect NP stability include their physico-chemical characteristics, such as surface chemistry[14], particle size[15], structure, and composition[16] Their stability is affected by water properties, such as the solution pH, ionic strength (IS) and ionic species, and dissolved organic matter (DOM)[11, 13, 17,18,19,20]. Some studies have reported that NP aggregation and settling is slow, and can take up to 24 h10, 14, 20, 25; inorganic ion adsorption destabilizes hydrous oxides with a range of equilibrium times from minutes to weeks[28] It is still unclear how properties such as particle size and the zeta potential of TiO2 NPs change dynamically in the water body
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