Article pubs.acs.org/est
Influence of Clay Particles on the Transport and Retention of Titanium Dioxide Nanoparticles in Quartz Sand Li Cai,† Meiping Tong,*,† Xueting Wang,† and Hyunjung Kim‡ †
The Key Laboratory of Water and Sediment Sciences, Ministry of Education, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, P. R. China ‡ Department of Mineral Resources and Energy Engineering, Chonbuk National University, Jeonju, Jeonbuk 561-756, Republic of Korea S Supporting Information *
ABSTRACT: This study investigated the influence of two representative suspended clay particles, bentonite and kaolinite, on the transport of titanium dioxide nanoparticles (nTiO2) in saturated quartz sand in both NaCl (1 and 10 mM ionic strength) and CaCl2 solutions (0.1 and 1 mM ionic strength) at pH 7. The breakthrough curves of nTiO2 with bentonite or kaolinite were higher than those without the presence of clay particles in NaCl solutions, indicating that both types of clay particles increased nTiO2 transport in NaCl solutions. Moreover, the enhancement of nTiO2 transport was more significant when bentonite was present in nTiO2 suspensions relative to kaolinite. Similar to NaCl solutions, in CaCl2 solutions, the breakthrough curves of nTiO2 with bentonite were also higher than those without clay particles, while the breakthrough curves of nTiO2 with kaolinite were lower than those without clay particles. Clearly, in CaCl2 solutions, the presence of bentonite in suspensions increased nTiO2 transport, whereas, kaolinite decreased nTiO2 transport in quartz sand. The attachment of nTiO2 onto clay particles (both bentonite and kaolinite) were observed under all experimental conditions. The increased transport of nTiO2 in most experimental conditions (except for kaolinite in CaCl2 solutions) was attributed mainly to the clay-facilitated nTiO2 transport. The straining of larger nTiO2-kaolinite clusters yet contributed to the decreased transport (enhanced retention) of nTiO2 in divalent CaCl2 solutions when kaolinite particles were copresent in suspensions.
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INTRODUCTION Titanium dioxide nanoparticles (nTiO2), one of the most important metal oxide nanoparticles, have been widely utilized in products including cosmetics, coatings, paints and pigments, textiles, and catalysts.1−4 The increasing applications of nTiO2 will inevitably release nTiO2 into the natural environment (especially in waste disposal sites or accidental leakage occurs during manufacturing and transportation processes). 5−7 Because of their potential risk to the natural ecosystem and human health,8−10 understanding the fate and transport of nTiO2 in natural systems, especially in the subsurface, is therefore imperative for the protection of human health. Great efforts have been devoted to investigating the transport and retention of nTiO2 under environmentally relevant conditions.11−19 Various physicochemical factors such as fluid velocity,11,12 solution chemistry (pH, ionic strength, and ion type),13−15 nanoparticle concentration,12 surfactant,16 and natural organic matter (NOM)17−19 have been shown to affect the transport of nTiO2. By comparing the transport behaviors of nTiO2 with and without suspended bacteria in suspensions, Chowdhury et al.20 recently found that the copresence of suspended bacteria could increase transport (decreased retention) of nTiO2 in porous media due to a combination © 2014 American Chemical Society
of factors including electrosteric and electrostatic effects and the aggregation state of nTiO2 and nTiO2-bacteria. Clay particles, one type of inorganic colloids, are the most abundant colloids in natural aquatic systems. Since they contain surface charge heterogeneity, clay particles are found to have high potential to interact with other (bio)colloids and could serve as carriers of colloids, affecting their distributions in natural environment.21−24 Depending on the physicochemical conditions prevailing in the system considered, clay particles may facilitate or hinder the mobility of colloids in porous media.25−28 For instance, Yang et al.25 found that the presence of bentonite in suspensions significantly decreased the transport of bacteria in packed quartz sand; however, Abdel-Fattah et al.26 reported that the transport of plutonium colloids was facilitated by smectite clay colloids of subsurface soil. By comparing the transport behaviors of nTiO2 in different kinds of soils, Fang et al.27 found that a high amount of clay particles immobilized on soil surfaces decreased the mobility of nTiO2 in Received: Revised: Accepted: Published: 7323
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NaOH. After preparation, nanoparticle suspensions were sonicated at 100 W for 5 min prior to each transport experiment. Zeta potentials of nTiO2 (50 mg L−1) and clay particles (30 mg L−1) under these conditions were measured using a Zetasizer Nano ZS90 (Malvern Instruments, UK). Measurements were performed at room temperature (25 °C) and repeated 9−12 times. The size distribution of nTiO2, clay particles, and nTiO2−clay mixed suspensions was determined by dynamic light scattering (DLS) measurement. Porous Media. Quartz sand (ultrapure with 99.8% SiO2) (Hebeizhensheng Mining Ltd., Shijiazhuang, China) with sizes ranging from 417 to 600 μm, commonly utilized as model porous media to represent coarse sand in natural environments in many previous studies,12,20,36,37 was used as a porous medium for individual nanoparticle transport and nanoparticle cotransport experiments in the present study. The procedure used for cleaning the quartz sand is provided in the previous publication,38 as well as in the Supporting Information (Text S1). The zeta potentials of the crushed quartz sand were also measured under the experimental conditions using the Zetasizer Nano ZS90. The electrophoretic mobility measurements were repeated 9−12 times. Column Experiments. The cylindrical Plexiglas columns (10 cm long and 2 cm inner diameter) were wet-packed with cleaned quartz sand. Prior to packing, the cleaned quartz sand was rehydrated by boiling in Milli-Q water for at least 0.5 h. After the rehydrated quartz sand was cooled, the columns were packed by adding wet quartz sand in small increments (∼1 cm) with mild vibration of the column to minimize any layering or air entrapment. One 80 mesh fabric screen was placed at each end of the column. The porosity of packed column was approximately 0.42.13 After packing, the columns were pre-equilibrated with at least ten pore volumes (10 PV) of NaCl or CaCl2 salt solutions at desired ionic strength. Following pre-equilibration, three pore volumes (3 PV) of nanoparticle suspensions were injected into the column, followed by elution with five pore volumes (5 PV) of salt solution at the same ionic strength. The solutions were injected into the columns in up-flow mode using a syringe pump (Harvard Apparatus Inc., Holliston, MA). To maintain the stability of nanoparticle suspension and avoid the settlement of nanoparticles, the influent nanoparticle suspension was sonicated periodically during the column experiments. Since the typical concentrations of monovalent cations (e.g., Na+, K+) and divalent cations (e.g., Ca2+, Mg2+) in the subsurface are around 1−10 and 0.1−2 mM, respectively,34,35 nTiO2 transport experiments were thus conducted under conditions with ionic strengths from 1−10 mM in NaCl solutions and from 0.1−1 mM in CaCl2 solutions to represent most conditions in natural environments. The pore water velocity of all experiments was set to be 8 m day−1 (0.73 mL min−1) to represent fluid velocities in coarse aquifer sediments, forced-gradient conditions, or engineered filtration systems.39 Samples from the column effluent were collected in centrifuge tubes at the desired time intervals. Following the transport experiment, the sand was excluded from column under gravity and dissected into 10 segments (each 1 cm long). To release the nTiO2 nanoparticles from quartz sand, 5−10 mL 0.01 M NaOH solution was added into each sediment segment, and the mixture was manually and vigorously shaken for a few seconds. It should be noted that Chen et al.31 showed that nTiO2 nanoparticles could be effectively released from quartz sand by employing this method. The effluent samples and the
soil columns. It was worth pointing out that this study focused on the effect of clay particles immobilized on soil surfaces on the transport of nTiO2. Furthermore, the retained profiles, from which more valuable information regarding the mechanisms of colloid and nanoparticle retention can be derived, have not been examined. Since mobile (suspended) clay particles are also ubiquitous in natural environment, the role of suspended clay particles on nTiO2 transport should also be investigated. Hence, this study was designed to fully understand the role of different types of suspended clay particles on the transport and retention behavior of nTiO2 in quartz sand by monitoring both breakthrough curves and retained profiles under a series of ionic strengths at pH 7. Clay particles consist mainly of stacks of two-dimensional aluminosilicate layers: silicon dioxide tetrahedral sheets (T) and aluminum oxyhydroxyl octahedral sheets (O).29 Bentonite and kaolinite are typical examples of T−O−T and T−O clay particles, respectively. Thus, bentonite and kaolinite were employed as model clay particles in this study. Packed column experiments were performed both with and without clay particles in nTiO2 suspensions. Breakthrough curves and retained profiles with clay particles were compared with those without clay particles in solutions. Possible mechanisms by which clay particles (both bentonite and kaolinite) affected the transport behaviors of nTiO2 were proposed and discussed.
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MATERIALS AND METHODS Titanium Dioxide and Clay Suspension Preparation. Anatase titanium dioxide powders (nTiO2, purity greater than 99.7%, in dry form) were purchased from Sigma-Aldrich Corp. (catalog no. 637254). The diameter of nTiO2 provided by the manufacturer is less than 25 nm. On the basis of literature values,12,30−32 a nTiO2 nanoparticle stock suspension (1000 mg L−1) was prepared by suspending nTiO2 nanopowders in MilliQ water and sonicated at 600 W for 10 min with a sonicating probe (Ningboxinzhi Biotechnology Ltd., China). The morphology and size distribution of resulted nTiO2 suspensions after sonication (50 mg L−1 in Milli-Q water) are presented in Figure S1 (Supporting Information). Bentonite (Sigma-Aldrich, St. Louis, MO) and kaolinite (Fluka, Milwaukee, WI), received without further purification, were used as model clay particles in this study. Clay stock suspension (200 mg L−1) was prepared by suspending bentonite and kaolinite powders in Milli-Q water. The suspension was shaken vigorously and subjected to ultrasonic dispersion for 5 min at 300 W (Ningboxinzhi Biotechnology Ltd., China). To obtain stable clay particles with sizes (under 1 μm) that commonly observed in natural environment (e.g., groundwater),33 the suspension was then allowed to settle for 16 h before it was siphoned to prepare the desired experimental suspensions. The clay concentration was determined by measuring the absorbance at a wavelength of 243 nm with an UV spectrophotometer (UV-1800, Shimadzu, Japan). For transport experiments, the influent concentration of nTiO2 and clay suspension was maintained to be 50 and 30 mg L−1, respectively. For transport experiments with copresence of clay particles, a certain amount of clay stock solution was added into nTiO2 suspensions to prepare the mixed influent suspensions. Salt (NaCl or CaCl2) suspensions were added into the nTiO2 nanoparticle suspensions (with or without clay particles). The ionic strength of nanoparticle suspension ranged from 1−10 mM in NaCl and 0.1−1 mM in CaCl2 solutions.34,35 Suspension pH was set to be 7 by adjusting with 0.1 M 7324
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Figure 1. Breakthrough curves (a, c) and retained profiles (b, d) of nTiO2 both with bentonite (solid square), kaolinite (solid triangle) and without clay (open circle) in suspensions in 1 mM (a, b) and 10 mM (c, d) NaCl solutions at pH 7. The data for without clay were adapted from ref 30. C, Co and Cc represent for nTiO2 concentration of effluent, influent and samples recovered from column, respectively. Replicate experiments were performed under all conditions (n ≥ 2).
supernatant samples from recovery of retained nTiO2 particles were analyzed using UV spectrophotometer (UV-1800, Shimadzu, Japan) at a wavelength of 600 nm. The detailed statements of the methods used to determine the concentration of nTiO2 were provided in the Supporting Information (Text S2, Figures S2−S4). It should be noted that the concentrations of nTiO2 determined for experiments with copresence of clay particles in suspensions were the “total” nTiO2 concentrations (including both individual nTiO2 and those attached onto clay particles). The area under the breakthrough-elution curve was integrated to yield the percentage of nanoparticles that exited the column. The percentage of nanoparticles recovered from the sediment was obtained by summing the amounts of nanoparticles recovered from all segments of the sediment and dividing by the total amount of nanoparticles injected. The sum of the percentage of retained particles and particles that exited the column represented the overall recovery (mass balance) of nanoparticles. Detailed information about nanoparticle mass recovery for each experiment is provided in the Supporting Information (Table S1). Scanning Electron Microscopy-Energy Dispersive Xray (SEM-EDX) Analysis. Influent suspensions of nTiO2 nanoparticles with clay particles for cotransport experiments under selected conditions were checked for the morphology of particles (nTiO2 and clay particles) by SEM-EDX (scanning electron microscopy-energy dispersive X-ray) analysis (FEI Nova Nano SEM 430). Prior to the SEM-EDX analysis, 5 μL of influent suspensions were dropped onto a piece of silicon slice and immediately vacuum freeze-dried using a vacuum freeze drier (LGJ-10C, Fourring Science Instrument Plant Co., Ltd., Beijing, China). Detailed information about the vacuum freezedrying process is provided in the Supporting Information (Text
S3). The SEM images were taken at magnifications of ∼20000× and ∼40000×.
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RESULTS AND DISCUSSION Influence of Clay Particles on Breakthrough Curves. The transport behaviors of nTiO2 in quartz sand both with and without clay particles in suspensions were examined at two ionic strengths (1 and 10 mM) in NaCl solutions at pH 7, and the results are presented in Figure 1. At pH 7, zeta potentials of both nTiO2 and quartz sand were negative (Figure S5 and Table S2, Supporting Information), and thus, repulsive electrostatic interaction between nTiO2 and quartz sand were expected. As a result, a portion of nTiO2 passed through the columns at pH 7 in both 1 and 10 mM NaCl solutions (Figure 1, left, open circle). The breakthrough curve of nTiO2 at high ionic strength (Figure 1 c, open circle) was lower than that of low ionic strength (Figure 1 a, open circle), indicating that the transport of nTiO2 was sensitive to ionic strength. The observation is consistent with the less negative zeta potentials of nTiO2 observed at high ionic strength (Table S2, Supporting Information). The decreased nTiO2 transport with the increase of ionic strengths has also been reported previously.12−15,20 The more noteworthy observation is that breakthrough curves for nTiO2 with clay particles present in suspensions were higher than those without clay particles (Figure 1, left, solid symbol vs open symbol) in both 1 and 10 mM NaCl solutions. This was true for both bentonite and kaolinite. For example, in the absence of clay particles, 36% of nTiO2 broke through the column in 1 mM NaCl solutions, whereas under the same solution conditions, 90% and 43% of nTiO2 passed through porous media in the presence of bentonite and kaolinite particles, respectively. The observations clearly showed that the 7325
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Figure 2. Breakthrough curves (a, c) and retained profiles (b, d) of nTiO2 both with bentonite (solid square), kaolinite (solid triangle) and without clay (open circle) in suspensions in 0.1 mM (a, b) and 1 mM (c, d) CaCl2 solutions at pH 7. C, Co and Cc represent for nTiO2 concentration of effluent, influent and samples recovered from column, respectively. Replicate experiments were performed under all conditions (n ≥ 2).
yet the enhancement was more obvious in NaCl solutions. Specifically, at the same ionic strength (1 mM) (Figure 1 a vs Figure 2 c, solid square), bentonite increased 54% of nTiO2 breakthrough the porous media (from ∼36% without bentonite to ∼90% with bentonite) in NaCl solutions. Whereas, bentonite enhanced 38% of nanoparticle passed through the column (from ∼17% without bentonite to ∼55% with bentonite) in CaCl2 solutions. In contrast with the higher breakthrough curves with kaolinite relative to without kaolinite in NaCl solutions, the breakthrough curves of nTiO2 with kaolinite in suspensions (Figure 2, left, solid triangle) were lower than those without kaolinite particles in CaCl2 solutions (Figure 2, left, open circle). This was true under both 0.1 and 1 mM ionic strength conditions. The observation indicated that the presence of kaolinite in nanoparticle suspensions inhibited nTiO2 transport under divalent CaCl2 solutions. Influence of Clay Particles on Retained Profiles. To examine whether the presence of clay particles in nTiO2 suspensions would affect the distributions of nTiO2 retained in quartz sand, which could not be derived from breakthrough curves, the retained profiles of nTiO2 both with and without clay particles in suspensions in both NaCl (Figure 1, right) and CaCl2 solutions (Figure 2, right) were obtained. The magnitudes of the retained profiles for nTiO2 under all examined conditions varied oppositely to the breakthrough plateaus, as expected from mass balance considerations (Table S1, Supporting Information). Without clay particles in suspensions, the retained nTiO2 concentrations in quartz sand decreased hyper-exponentially with transport distance under all examined conditions, with greatest retention being located near column inlet (Figures 1 and 2, right, open circle). The hyper-exponential retained profiles of nTiO2 acquired under all examined conditions (unfavorable conditions) were
presence of clay particles (both bentonite and kaolinite) in suspensions increased the transport of nTiO2 in packed quartz sand under both ionic strength conditions in NaCl solutions. A previous study also found that the presence of clay particles increased transport of plutonium colloids in NaCl solutions.26 However, it is worth pointing out that a few recent studies found that the presence of clay particles in suspensions significantly decreased microbe transport in porous media.23,25 Due to their intrinsic properties, the stabilities of engineered nanoparticles and microbe with clay particles copresent in suspensions might be different. Therefore, the distinct difference in the transport behaviors of nTiO2 and bacteria resulted from the copresence of clay particles was observed. Moreover, for both ionic strength conditions examined, nTiO2 breakthrough curves in the presence of bentonite particles (Figure 1, left, solid square) were higher than those in the presence of kaolinite particles (Figure 1, left, solid triangle). The observation indicated that the enhancement of nTiO2 transport due to the presence of clay particles was more significant when bentonite was present in nTiO2 suspensions. To investigate whether the presence of clay particles would affect the transport of nTiO2 in divalent ion solution conditions, the transport behavior of nTiO2 in quartz sand both with and without clay particles in nanoparticle suspensions was examined at two ionic strengths (0.1 and 1 mM) in CaCl2 solutions at pH 7 (Figure 2). Similar to the observations in NaCl solutions, the breakthrough curves of nTiO2 with bentonite particles in CaCl2 solutions (Figure 2, left, solid square) were also higher than those without clay particles (Figure 2, left, open circle). This held true for both examined ionic strength conditions. Clearly, the presence of bentonite in suspensions also increased the transport of nTiO2 in CaCl2 solutions. Although bentonite increased nTiO2 transport in both NaCl and CaCl2 solutions, 7326
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Figure 3. SEM images and the corresponding EDX spectra of influents of nTiO2 with the presence of bentonite in 1 mM NaCl (a), with the presence of kaolinite in 1 mM NaCl (b), with the presence of bentonite in 1 mM CaCl2 (c), and with the presence of kaolinite in 1 mM CaCl2 (d).
consistent with the observations reported previously.12,13,36 In NaCl solutions, the retained profiles with clay particles in suspensions were lower than those without clay particles (Figure 1, right, solid symbol vs open symbol). This was true for both bentonite and kaolinite particles. Clearly, the copresence of clay particles in nanoparticle suspensions decreased the retention of nTiO2 in porous media in NaCl solutions. Although the retained profiles of nTiO2 in the
presence of clay particles were lower than those without clay particles in suspensions, yet the shapes of retained profiles of nTiO2 with clay particles (both bentonite and kaolinite) were similar as those without clay in suspensions. This observation indicated that in NaCl solutions, the presence of clay particles (both bentonite and kaolinite) in nanoparticle suspensions might not change the mechanisms controlling the retention of nTiO2 in quartz sand. 7327
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Figure 4. Breakthrough curves (a, c) and retained profiles (b, d) of bentonite (a, b) and kaolinite (c, d) particles in NaCl and CaCl2 solutions at pH 7. C, Co and Cc represent for clay particle concentration of effluent, influent and samples recovered from column, respectively. Replicate experiments were performed under all conditions (n ≥ 2).
nanoparticles onto mobile clay particles enhanced the transport of silver nanoparticles in loamy sand soil. Zhou et al.46 found that both Ag and nTiO2 particles could attach onto montmorillonite; thus, their stabilities in the presence of clay particles were found to be different from those without clay particles in aqueous suspensions. nTiO2 particles might also attach onto clay particles in present study. The increased transport behavior of nTiO2 in the presence of clay particles (except for kaolinite in CaCl2 solutions) possibly was due to the clay-facilitated transport. To test our hypothesis, SEM images of nTiO2 with copresence of clay particles in selected suspensions (1 mM ionic strength) were taken and the results were presented in Figure 3. Comparing with the SEM images of individual nTiO2 particles (Figure S1, Supporting Information), SEM images of nTiO2 with bentonite (Figure 3 a and c) and kaolinite (Figure 3 b and d) particles in influents showed that a portion of nTiO2 did attach onto clay particles and formed nTiO2-clay clusters in both NaCl (Figure 3 a and b) and CaCl2 solutions (Figure 3 c and d). To further confirm the attachment of nTiO2 onto clay particles, EDX chemical spectra of nTiO2 with copresence of clay particles were also acquired (Figure 3, right). The presence of Si, Al, Na(Ca), O, and Cl elements are consistent with the basic components of clay particles, background salt solutions (NaCl or CaCl2), and silicon slice. The Ti peak observed in the EDX spectra confirmed that the aggregates were composed of nTiO2 particles. The observations further demonstrated that the attachment of nTiO2 onto clay particles (bentonite and kaolinite) did occur in our study. These observations indicated that similar as the adsorption of colloids onto clay particles reported previously,23−26 the attachment of nTiO2 onto surfaces of both bentonite and kaolinite also occurred in present study. To quantitatively determine the attachment of nTiO2 onto clay particles, batch
Similar as observations in NaCl solutions, the retained profiles of nTiO2 with bentonite in suspensions (Figure 2, right, solid square) were also lower than those without bentonite particles in CaCl2 solutions. Moreover, the shapes of retained profiles with bentonite in suspensions were similar as those without clay particles in suspensions, indicating that the presence of bentonite in nanoparticle suspensions did not change the mechanisms controlling the transport of nTiO2 in CaCl2 solutions. In contrast with the decreased deposition of nTiO2 with bentonite copresent in suspensions, the retained concentrations of nTiO2 in the presence of kaolinite (Figure 2, right, solid triangle) were higher than those without clay particles in suspensions in CaCl2 solutions. Moreover, the excess retention of the nTiO2 observed in the presence of kaolinite in suspensions mainly located at segments near the column inlet, as a result, retained profiles of nTiO2 in CaCl2 solutions were relatively steeper versus those without kaolinite in suspensions. The observation indicated that the presence of kaolinite in suspensions might change the mechanisms dominating the retention of nTiO2 in quartz sand in CaCl2 solutions. Mechanisms Driving the Altered Transport of nTiO2 by Clay Particles. Due to the presence of charge heterogeneity on the surface of clay surfaces,40−44 many previous studies showed that when copresent with clay particles, colloids such as plutonium colloids and biocolloids could attach onto the surfaces of clay particles, and thus their fate and transport would be affected.24−27 For example, Vasiliadou and Chrysikopoulos23 found that the attachment of Pseudomonas putida onto kaolinite particles significantly inhibited the transport of bacteria in water-saturated porous media when clay particles were copresent in suspensions. Very recently, Liang et al.45 reported that the attachment of silver 7328
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with nTiO2 were comparable to those without nTiO2. This observation held true in both NaCl and CaCl2 solutions. The results demonstrated that when copresent with nTiO2 particles, the electrokinetic properties of the bentonite particles did not lead to obvious changes in both NaCl and CaCl2 solutions. Moreover, the sizes of bentonite with nTiO2 (Figure S7, Supporting Information, solid triangle) were also comparable to those without nTiO2 (Figure S7, Supporting Information, solid circle). This observation held true in both NaCl and CaCl2 solutions. The results indicated that even when a portion of nTiO2 attached onto bentonite surfaces, the sizes of bentonite did not have significant change. Since both zeta potentials and sizes of bentonite with copresence of nTiO2 were comparable to those without nTiO2, the transport of bentonite with nTiO2 copresent in suspensions would be similar to that of individual bentonite particles, which were mobile than individual nTiO2 particles. The facilitated transport of nTiO2 particles in the presence of bentonite particles therefore was observed in both NaCl and CaCl2 solutions. Close inspection of the breakthrough curves of nTiO2 with bentonite versus that of individual bentonite particles showed that in both NaCl and CaCl2 solutions the breakthrough curves of nTiO2 in the presence of bentonite were similar to those of individual bentonite particles. For example, the breakthrough plateau of nTiO2 with bentonite in suspensions in 1 mM NaCl was 0.88, which was similar to that of individual bentonite at same conditions (breakthrough plateau of 0.91). Clearly, the transport behaviors of nTiO2 with bentonite in suspensions were consistent with those of bentonite particles, indicating that transport of nTiO2 in quartz sand when bentonite particles were copresent in suspensions were associated with the transport of bentonite (through bonding with the surfaces of bentonite). At the same ionic strength (1 mM), the breakthrough curve of bentonite particles in NaCl solutions (Figure 4a, open diamond) was higher than that in CaCl2 solutions (Figure 4a, solid triangle). Since the transport of nTiO2 with the copresence of bentonite particles was associated with the transport of bentonite, the greater mobility of bentonite particles observed in NaCl solutions relative to that in CaCl2 solutions contributed to the more enhanced nTiO2 transport in NaCl solutions. SEM images of nTiO2 in the presence of bentonite particles (Figure 3, a and c) revealed that in addition to the nTiO2 attached onto bentonite surfaces, separate nTiO2 aggregates (not attached onto bentonite) were also present, which might be attributed to the restabilization induced by the copresent bentonite particles, which was similar as that observed by Zhou et al.46 These separate nTiO2 aggregates dispersed well in suspensions. Previous studies48−51 showed that the sizes of nanoparticles played an important role on the transport of nanoparticles in porous media. The general conclusion from previous studies investigating the size effect on engineered nanoparticles (such as nTiO2, carbon nanotubes, and Fe0) is that decreasing the sizes of nanoparticles greatly increased the transport of nanoparticles in packed coarse sand columns.31,37,52 Zhou et al.46 recently reported that the presence of montmorillonite in suspension could restabilize nTiO2 aggregates and decrease the sizes of nTiO2 particles. The negatively charged bentonite repelled the formation of large nTiO2 aggregates in mixed suspensions and thus enhanced the stability and dispersion of nTiO2 (which did not attach onto the surfaces of bentonite) in present study. The increased stability and dispersion of nTiO2 induced by the presence of bentonite
experiments for nTiO2 adsorption onto both bentonite and kaolinite particles in both NaCl (1 and 10 mM) and CaCl2 (0.1 and 1 mM) solutions were performed and the corresponding attachment efficiencies of nTiO2 particles under all examined conditions were determined (Figure S6, Supporting Information). Detailed information about the batch experiments and determination of nTiO2 attachment efficiencies is provided in the Supporting Information (Text S4). It can be clearly seen from Figure S6 (Supporting Information) that a significant portion of nTiO2 did attach onto clay particles (for both clay types) under all examined conditions. Moreover, for both bentonite and kaolinite, the attachment efficiencies of nTiO2 slightly increased with increasing solution ionic strength in both NaCl and CaCl2 solutions, which was consistent with less negative surface charge of both nTiO2 and clay particles at higher ionic strength (Table S2, Supporting Information). For example, the nTiO2 attachment efficiencies onto bentonite and kaolinite increased from ∼35% to 45% and from ∼44% to 48% with increasing ionic strength from 1 to 10 mM in NaCl solutions, respectively. A comparison of attachment efficiencies onto bentonite with those onto kaolinite showed that the attachment efficiencies for nTiO2 onto kaolinite particles were slightly greater than those onto bentonite particles across all examined conditions. Previous study also showed that the adsorption of bacteria onto kaolinite was greater than that on montmorillonite.47 These observations clearly demonstrated that under all examined solution conditions, a certain amount of nTiO2 did attach onto clay particles (both bentonite and kaolinite). To test whether the increased transport behavior of nTiO2 in the presence of clay particles (except for kaolinite in CaCl2 solutions) was possibly due to the clay-facilitated nTiO2 transport after the attachment of nTiO2 onto clay particles, the transport of clay particles (both bentonite and kaolinite) in packed quartz sand in both NaCl and CaCl2 solutions was investigated, and the results are presented in Figure 4. The transport of clay particles (both bentonite and kaolinite) decreased with the increase of ionic strength in both NaCl and CaCl2 solutions, which was consistent with less negative surface charge and larger particle sizes at higher ionic strength (Table S2, Supporting Information) and thus agreed with DLVO theory. A more important observation was that, under the same solution conditions, the breakthrough curves for both bentonite and kaolinite (Figure 4) were higher than those of individual nTiO2 (Figures 1 and 2). For instance, in 1 mM NaCl solutions, the breakthrough plateau for bentonite and kaolinite was ∼0.9 and ∼0.7, respectively, whereas the breakthrough plateau for nTiO2 only ∼0.3. Clearly, the mobility of clay particles (both bentonite and kaolinite) in packed quartz sand was greater than that of nTiO2. It is therefore expected that if the sizes and zeta potentials of clay particles did not show an obvious change (increase) after being bonded by nanoparticles, the transport behavior of nTiO2-bonded clay particles would be similar to those of sole clay particles. If so, the mobility of nTiO2-bonded clay particles therefore would be greater than that of sole nTiO2 (without clay particles in suspensions), contributing to the facilitated transport of nTiO2 particles in the presence of clay particles observed under most experimental conditions. To confirm our hypothesis, the zeta potentials and size distribution of bentonite both with and without the presence of nTiO2 were measured.27 It can be seen from Table S2 (Supporting Information) that the zeta potentials of bentonite 7329
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caused by the concurrent nanoparticle−nanoparticle aggregation. To verify our hypothesis, the SEM image of nTiO2 with kaolinite retained in segment near column inlet in 1 mM CaCl2 solutions was determined and the result is presented in Figure S9 (Supporting Information). Desorption was performed by adding sand with retained particles in segment near to column inlet into 1 mM CaCl2 solution (with the same ionic strength as nTiO2 and kaolinite injected into the column) and then mildly shaken for a few seconds. To prevent the possible formation of nTiO2−nTiO2 and also nTiO2−kaolinite aggregates after desorbed into salt solution, the sample of the desorbed solution for SEM measurement was immediately dropped into the silicon slice and vacuum freeze-dried. Figure S9 (Supporting Information) showed that nTiO2−kaolinite clusters retained in quartz sand (∼10 μm) was larger than those in influent suspensions (∼ a few μm) (Figure 3d), indicating that concurrent aggregation of nTiO2−kaolinite clusters did occur during the transport process of nTiO2 in the presence of kaolinite in suspensions in CaCl2 solutions. Thus, the enhanced deposition of nTiO2 with kaolinite was observed in CaCl2 solutions. Environmental Implications. The findings from this study give insight into the effects of suspended clay particles on the transport of nTiO2 under the solution chemistry conditions relevant to subsurface environment. The interactions of nanoparticles with abundant natural clay colloids are very plausible and accordingly expected to influence the fate of nanoparticles in porous media. The present study investigated the influence of two representative clay colloids (i.e., both bentonite and kaolinite), which were ubiquitously found in subsurface environment, on the transport of nTiO2 in quartz sand. The core results showed that the presence of bentonite particles increased the transport of nTiO2 in both NaCl and CaCl2 solutions, whereas the presence of kaolinite increased the transport of nTiO2 in NaCl solutions but decreased the transport of nTiO2 in CaCl2 solutions. Although the experiments were performed on small-scale columns and in only one type of porous media, the aforementioned findings clearly showed that the mobility of nTiO2 nanoparticles is very sensitive to the type of clay particles and the surrounding environmental conditions, implying the clay-induced processes in the nTiO2 fate are far more complex and rather coupled, unlike the straightforward observations from many previous studies for bacterial transport with copresence of clay particles.23,25,58 Therefore, to accurately predict transport of nTiO2 in natural aquatic environments where clay particles abundantly present, more attention should be paid to enunciating the role of clay particles on nanoparticles transport. Some examples could be nTiO2 and clay particle concentration, the presence of natural organic matter, and the presence of naturally occurring oxides and hydroxides onto sand surfaces.
might also be a factor contributing to the increased transport of nTiO2 with the presence of bentonite in suspensions. In contrast with the enhanced the stability and dispersion of nTiO2 with bentonite particles copresent in the suspension observed in the present study, Yang et al.25 found that the presence of bentonite induced the formation of bacteria− bentonite clusters especially in CaCl2 solutions; thus, the decreased transport (increased retention) of bacteria with bentonite particles copresent in suspensions was observed. Unlike similar zeta potentials of bentonite observed with and without copresent nTiO2, close comparison of zeta potentials of kaolinite both with and without nTiO2 showed that the zeta potentials of kaolinite with nTiO2 were less negative relative to those without nTiO2 in both NaCl and CaCl2 solutions (Table S2, Supporting Information). The observation indicated that the interaction of nTiO2 with kaolinite decreased the surface charge of kaolinite. Electrostatic force between kaolinite and quartz sand with nTiO2 in solutions, therefore, would be less repulsive compared to those without nTiO2. According to the theory, the mobility of kaolinite would be decreased when copresent in nTiO2 suspensions. Moreover, in both NaCl and CaCl2 solutions, the sizes of kaolinite particles in nTiO2 suspensions (Figure S8, Supporting Information, solid triangle) were larger than those of individual kaolinite particles (Figure S8, Supporting Information, solid circle), which would also decrease the transport of kaolinite particles when copresent with nTiO2 particles. Therefore, although the mobility of individual kaolinite in packed quartz sand was greater than that of nTiO2, the enhanced transport of nTiO2 due to copresence of kaolinite in suspensions was not as significant as that induced by bentonite. It is worth pointing out that the sizes of nTiO2-kaolinite clusters were smaller than those of nTiO2 aggregates without kaolinite in suspensions in NaCl solutions (Figure S8, top, solid triangle vs open square, Supporting Information). Thus, enhanced transport of nTiO2 with kaolinite in suspensions was observed in NaCl solutions (1 and 10 mM). In contrast, nTiO2-kaolinite clusters formed in CaCl2 solutions were larger than nTiO2 aggregates without kaolinite in suspensions (Figure S8, bottom, solid triangle vs open square, Supporting Information). The enhanced aggregation of bacteria due to the presence of kaolinite other than montmorillonite has also been previously reported.47 The larger nTiO2−kaolinite clusters would be more easily retained in quartz sand, especially in segments closer to the column inlet, by straining,53,54 contributing to the significant retention (decreased transport) of nTiO2 with kaolinite in suspensions. Since the sizes of nTiO2−kaolinite clusters were greater in 1 mM CaCl2 solutions (Figure S8d, solid triangle, Supporting Information), straining would be more significant, resulting in the obviously lower breakthrough curve and steeper retained profile of nTiO2 with kaolinite 1 mM CaCl2 solutions (Figure 2, bottom, solid triangle). Previous studies have shown that the concurrent particle− particle aggregation led nanoparticles prone to attach to large aggregates retained in sand by straining, which could further narrow the pore throat (grain to grain contacts) in the porous media and lead to the trap of more nanoparticles.51,55 Moreover, the concurrent aggregation occurred at pore throat would also plug pore throats and retain vast majority of nanoparticles near entry surfaces of porous media.56,57 We speculated the enhanced retention of nTiO2 with the presence of kaolinite particles obtained in CaCl2 solutions might be
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Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
This work was supported by the National Nature Science Foundation of China under Grant No. 21377006 and by the program for New Century Excellent Talents in University under Grant No. NCET-13-0010. We acknowledge the editor and reviewers for their helpful comments.
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