It is important to determine the assembly configuration of engineered nanomaterials (ENMs) because assembly configuration influences their fate and transport behavior in the aquatic environment. Aggregated particles are more subject to segregation upon changes of environmental conditions (and vice versa) than agglomerated particles. As a strategic tool for investigating the time-resolved reversible segregating and assembling behavior of ENMs and thus estimating their assembly configuration, a controlled sonication process was proposed. It was hypothesized that the unique colloidal response of ENMs to sonication, with respect to changes in size, might be their intrinsic property associated with assembly configuration. As a model ENM, three different TiO2 particles with unique properties (commercial P-25 and UV 100 and home-made (HM) TiO2) were examined with programmed sonication processes under various environmental settings. When they were dispersed in water, all TiO2 particles tested obviously assembled to form much larger clusters. Size of P-25 decreased immediately upon sonication and did not change under the subsequent quiescence step while sizes of UV 100 and HM gradually decreased and then slowly recovered back to their initial sizes. The trend was generally observed in all conditions tested. The unique colloidal response of TiO2 could be explained by its properties associated with assembly configuration.
Engineered nanomaterials (ENMs) have been utilized in many new applications. Particularly, metal and metal oxide nanoparticles (NPs) are widely studied in the remediation of contaminated water and soil (Mueller & Nowack 2010). With the increasing usage of NPs in commercial products and environmental remediation areas, concern about their potential environmental risk has been also raised (Hessler et al. 2012). Since not all NPs released to the environment are available for exposure of human and living organisms to them, it is important to understand their colloidal behavior under various conditions, which controls their fate and transport in the environment (Sygouni & Chrysikopoulos 2015).
As one of the most important and widely used ENMs, titanium dioxide (TiO2) has been studied as a photocatalyst and pigment. TiO2 particles are insoluble in water and they assemble together naturally to form larger clusters like other ENMs. As a result, overall size of TiO2 particles is much larger than their crystal size and thus they tend to precipitate quickly and filter out easily more than what is extrapolated based on their primary particle size (Keller et al. 2010). The reactivity, mobility, availability of TiO2 particles can change significantly due to their assembling in aqueous media (Domingos et al. 2009). This implies the assembly configuration of TiO2 particles should be always considered when their fate and transport behavior is assessed.
Many studies have been conducted to understand the stability and mobility of TiO2 particles under various treatment conditions and in natural water resources (Zhang et al. 2008; Gottschalk et al. 2009; Keller et al. 2010; Brunelli et al. 2013). However, there have been few systematic studies to understand the assembly configuration of TiO2 particles (i.e. how TiO2 particles form assemblages and segregates over time), not to mention other ENMs. Particularly, it is important to determine whether particles are in aggregate configuration or agglomerate configuration because assembly configuration often has more influence on the fate and transport behavior of particles in the environment than their primary size.
An experimental approach is needed to investigate the segregating and assembling behavior of TiO2 particles and thus to estimate their assembly configuration. In this study, we propose to use the sonication process to achieve the task. Sonication breaks particles in suspension via either erosion or fracturing. Erosion is effective to break weak bonds between particles mostly in aggregate configuration, while fracturing attacks weak structures mostly in agglomerated particles such as imperfections and cracks (Mandzy et al. 2005; Taurozzi et al. 2011). Sonication energy, once optimized, is believed to be strong enough to agitate particles mostly by mechanically segregating aggregated particles (i.e. from Figure 1(b) to 1(a)) but not enough to break down agglomerated particles (i.e. from Figure 1(c) to 1(a)).
We hypothesize that the unique colloidal response of TiO2 particles to controlled sonication, with respect to changes in particle size, might be their intrinsic property associated with their assembly configuration. Based on the configurations shown in Figure 1, nanoscale particles in aggregate configuration might be more sensitive to sonication and more mobile in the environment and thus they can be practically called NPs in comparison to nanoscale particles in agglomerate configuration which can be called nanostructured particles (NSPs) (Virkutyte et al. 2014).
To prove the hypothesis, this study focuses on investigating the time-resolved reversible segregating and assembling behavior of TiO2 particles under programmed sonication. Three different TiO2 particles with unique properties were tested with programmed sonication processes (sonication followed by quiescence, sonication intensity) under different environmental settings (TiO2 concentration and pH). Understanding the unique reversible segregating and assembling behavior of TiO2 particles and thus their assembly configuration would be beneficial to interpreting their mobility, availability, and treatability in natural environmental systems as well as water and wastewater treatment facilities.
MATERIALS AND METHODS
Three types of TiO2 ENMs were tested: P-25 (Degussa), UV 100 (Hombikat), and home-made (HM) TiO2. P-25 purchased from Evonik Degussa GmbH, Germany shows primary crystal size of 28.2 nm, specific surface area of 53.8 m2/g, and mixture of 70% anatase and 30% rutile. P-25 is synthesized by flame hydrolysis. UV 100 purchased from Sachtleben Chemie GmbH, Germany shows primary crystal size of 5.50 nm, specific surface area of 292 m2/g, and 100% anatase crystal phase. UV 100 is synthesized by the sulfate process. HM synthesized by the sol-gel process in our laboratory has surface area of 231 m2/g and amorphous phase (Choi et al. 2006). Other chemicals include hydrogen chloride (HCl) and sodium hydroxide (NaOH) purchased from Sigma-Aldrich, USA.
Each of the TiO2 ENMs (P-25, UV 100, and HM) was dispersed in water at a fixed concentration of 50 mg/l to briefly investigate their response to sonication and thus to estimate their assembly configuration. The sonication process was applied to the TiO2 suspension to investigate how particles uniquely segregate and assemble as a function of sonication time followed by quiescence time. A sonicator (Misonix Sonicator S-4000) with the capability to accurately manipulate the total energy delivered to particle suspension was utilized. Based on preliminary studies, sonication was programmed at a probe energy intensity of 60 W and sonication for 15 min followed by quiescence for 15 min (standard conditions), where TiO2 particles showed well-developed segregating and assembling behavior (Taurozzi et al. 2011).
The effect of concentration of TiO2 particles in a range of 20–200 mg/L on their response to sonication was investigated. The test was conducted without pH control and thus pH was naturally maintained at 5.4 for P-25, 5.2 for UV 100, and 5.8 for HM. The experiment above under standard conditions was also resumed under controlled pH conditions in a range of 3–11 adjusted by adding HCl or NaOH in order to examine the effect of pH on the response of TiO2 particles to sonication. Ionic species and strength was not controlled in this particular study. For the examination, variation of the zeta potential and hydrodynamic size of TiO2 particles was monitored and their point of zero charge (pHPZC) was also determined. Lastly, in order to investigate the effect of sonication energy, different probe sonication intensities were tested in a range of 30–100 W under standard conditions except for sonication time. Instead of 15 min, 10 min for sonication and 10 min for quiescence were applied because high intensities caused severe erosion of the sonication probe tip.
Size measurement and zeta potential analysis
The hydrodynamic size and zeta potential (i.e. electric potential at the shear layer) of TiO2 particles in suspension were monitored upon their segregation and assembling in response to sonication under different pH conditions. A particle size and zeta potential analyzer (SZ-100, Horiba, Japan) utilizing dynamic light scattering and laser Doppler electrophoresis, respectively, was applied. Based on light scattering of particles, their hydrodynamic diameter was calculated by using the Stoke-Einstein equation. Particle size was measured at the 173° detection angle. Zeta potential was calculated from the mobility by using the Smoluchowski model.
RESULTS AND DISCUSSION
Response of TiO2 particles to sonication
Since TiO2 particles are synthesized by different processes, they might exhibit inherently different assembly configurations and thus unique segregation behavior in response to sonication (Taurozzi et al. 2011). Size of P-25 decreased from 653 nm to around 210 nm immediately within 5 minutes of sonication and the size did not further decrease significantly. The similar result for P-25 was also observed by Jiang et al. (2009). They reported that size of P-25 decreased to 180 nm after 5 minutes of sonication and then became fixed. Taurozzi et al. (2011) pointed out that size of particles is stabilized after they break down to a specific size. Sonication increases collision frequency of particles and enhances particle-particle interactions, resulting in particle segregation (so-called peaking behavior) (Taurozzi et al. 2011). The response of UV 100 and HM to sonication was different from that of P-25. Their sizes kept decreasing over 15 minutes of sonication. Size of UV 100 almost continuously decreased from 3,854 nm to 1,675 nm and size of HM also continuously decreased from 6,284 nm to 1,797 nm. The result investigated under the specific sonic intensity implies that bindings between particles for UV 100 and HM might be stronger than particles for P-25. P-25 presumably in aggregate configuration segregated easily and immediately upon application of sonication energy while UV 100 and HM presumably in agglomerate configuration continued to disassemble over sonication time.
Size of P-25 at 133 nm after 15 minutes of sonication is relatively close to its primary crystal size of 28.2 nm. This implies that P-25 particles initially at 653 nm were effectively segregated upon sonication and several primary particles formed a small assemblage. The observation that P-25 particles were not completely broken to its primary particles might be explained by many factors associated with their synthesis method, storage state, sonication intensity, and erosion mechanism (Ding & Pacek 2008; Zhang et al. 2008). Meanwhile, size of UV 100 at 1,675 nm after sonication is still much larger than its primary crystal size of 5.50 nm. This implies UV 100 is considered to be in agglomerate configuration although UV 100 successfully disassembled by both segregation and fracturing mechanisms during sonication. This also applies to HM.
Response of TiO2 particles to quiescence conditions
After sonication, TiO2 suspension stayed under quiescence conditions to investigate how TiO2 particles re-assemble over time back to their initial size, as shown in Figure 2. Size of P-25 did not change significantly during the quiescence step. Size at around 133 − 170 nm was maintained, indicating no significant re-assembling. Taurozzi et al. (2011) and Horst et al. (2012) also observed the similar behavior. They explained that size of particles decreases only to size of their primary aggregates which are larger than their primary particles and those primary aggregates are hard to re-assemble. The result for UV 100 and HM is interesting. Their sizes under quiescence conditions increased almost back to their initial sizes before sonication. There was a continuous re-assembling process for UV 100 and HM. Those particles seemed unstable after sonication compared to stable P-25. The sonication intensity applied to this specific test might have physically and/or chemically altered the surface characteristics of TiO2 particles differently (Taurozzi et al. 2011; Horst et al. 2012).
Effect of sonication intensity
The intensities investigated in this specific study, ranging 30 − 100 W, were not able to make any significant difference in the unique colloidal behavior of P-25. However, the intensities significantly impacted UV 100. High intensity at 100 W greatly changed size of UV 100 from 3,854 nm to 1,721 nm while low intensity at 30 W did not make any significant changes. P-25 was more sensitive to sonication even at low intensities and thus P-25 seemed to be in aggregate configuration while UV 100 in agglomerate configuration was less sensitive, requiring high sonication intensities for particle segregation. The behavior of HM was similar to that of UV 100.
Effect of TiO2 concentrations
Effect of pH conditions
The time-resolved reversible segregating and assembling behavior of TiO2 particles under a controlled sonication program was investigated to estimate their assembly configuration. When they were dispersed in water, all the TiO2 tested obviously assembled to form much larger particles. Size of P-25 decreased immediately upon sonication and did not change significantly under subsequent quiescence conditions, while sizes of UV 100 and HM decreased gradually and then recovered slowly back to their initial sizes. The trends were generally observed in all cases under different experimental conditions. The unique colloidal response of TiO2 particles to sonication could be explained by their properties associated with assembly configuration. P-25 was in aggregate configuration while UV 100 and HM were in agglomerate configuration. This implies that P-25 particles can be practically called NPs while UV 100 and HM should be called NSPs. Those TiO2 particles are expected to behave differently with respect to mobility, availability, and treatability in natural environmental systems as well as water and wastewater treatment facilities that work based on size-dependent exclusion and settling mechanisms, e.g. P-25, in comparison to UV 100 and HM, is obviously more mobile and available in the environment and is harder to remove by treatment processes. Examination on actual transport behavior of a series of TiO2 particles under different colloidal states manipulated by the controlled sonication should be followed in future. This study should also be extended to include more TiO2 particles with different crystal phases and primary sizes and to test them under various ionic species and strengths so that we are able to set up a database for the aggregation and segregation behavior of a wide variety of TiO2 particles in actual water matrix. Since TiO2 was selected just as a model ENM for the concept demonstration, the hypothesis and the experimental approach might be widely applicable to many other ENMs assembled in various media to better understand and predict their colloidal behavior.
This research was supported in part by the Texas Higher Education Coordinating Board through the Norman Hackerman Advanced Research Program (THECB13311).