Cotransport of Titanium Dioxide and Fullerene Nanoparticles in

May 13, 2013 - This study investigated the cotransport of titanium dioxide nanoparticles (nTiO2) and fullerene nanoparticles (nC60), two of the most w...
0 downloads 0 Views 822KB Size
Download PDF
Article pubs.acs.org/est

Cotransport of Titanium Dioxide and Fullerene Nanoparticles in Saturated Porous Media Li Cai,† Meiping Tong,*,† Hanyu Ma, 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 cotransport of titanium dioxide nanoparticles (nTiO2) and fullerene nanoparticles (nC60), two of the most widely utilized nanoparticles, in saturated quartz sand under a series of ionic strengths in NaCl solutions (0.1−10 mM) at both pH 5 and 7. Under all examined ionic strengths at pH 5, both breakthrough curves and retained profiles of nTiO2 in the copresence of nC60 were similar to those without nC60, indicating that nC60 nanoparticles copresent in suspensions did not significantly affect the transport and retention of nTiO2 in quartz sand at pH 5. In contrast, under all examined ionic strengths at pH 7, the breakthrough curves of nTiO2 in the copresence of nC60 in suspensions were higher and the retained profiles were lower than those without nC60, which demonstrated that the presence of nC60 in suspensions increased the rate of transport (decreased retention) of nTiO2 in quartz sand at pH 7. Competition of deposition sites on quartz sand surfaces by the copresence of nC60 was found to contribute to the increased nTiO2 transport at pH 7. Under all examined ionic strength conditions at both pH 5 and 7, the breakthrough curves of nC60 were reduced in the copresence of nTiO2, and the corresponding retained profiles were higher than those without nTiO2, indicating that the presence of nTiO2 decreased the transport of nC60 in quartz sand. Co-deposition of nC60 with nTiO2 in the form of nTiO2-nC60 clusters as well as the deposition of nC60 onto previously deposited nTiO2 were responsible for the increased nC60 deposition in the presence of nTiO2 at pH 5, whereas deposition of nC60 onto surfaces of predeposited nTiO2 was found to be responsible for the increased nC60 deposition at pH 7.

■

INTRODUCTION With the rapid growth of the nanotechnology industry, different types of nanomaterials such as metal/metal oxide based nanoparticles and carbon-based nanoparticles have been fabricated and used increasingly in various fields including biotechnology,1 optics and electronic engineering,2,3 environmental protection,4,5 and the cosmetic industry.6−8 Among the metal/metal oxide based nanoparticles, such as Ag nanoparticles, zerovalent iron, CuO, ZnO, and Al2O3 nanoparticles, titanium dioxide nanoparticles (nTiO2) have been regarded as one of the most important metal oxide nanoparticles due to their great range of applications in consumer products including sunscreens, cosmetics, paints, and other products.6,8−10 Fullerene nanoparticles (nC60) have become one of the most important carbon-based nanomaterials for a wide range of existing applications in commercial products and are used in products ranging from tennis racquets to epidermal growth factors and facial antioxidant creams.11−14 The waste product containing nanoparticles could be disposed of informally, treated in waste disposal sites, or processed in a waste treatment/recycling system. Especially for sites of some © XXXX American Chemical Society

electronic waste (e-waste), nanoparticles in e-waste are readily released into the environment during the primitive disposal process.15,16 Due to their potential risk to the natural ecosystem and to human health,12,17−24 understanding the fate and transport of nTiO2 and nC60 particles in natural systems, especially in the subsurface, is therefore of great interest, as well as being necessary for the protection of human health. Many previous studies have investigated the transport and deposition of nTiO2 and nC60 under environmentally relevant conditions (e.g., refs 25−41). Factors such as fluid velocity,25−27 solution chemistry (pH, ionic strength, and ion component),28−30 nanoparticle concentration,27,31 porous media heterogeneity,31,32 surfactant,33,34 natural organic matter (NOM),35−38 bacteria,39 and biofilm40,41 have been found to affect the transport and deposition kinetics of nTiO2 or nC60. For example, Chowdhury et al.39 recently showed that the Received: January 17, 2013 Revised: May 5, 2013 Accepted: May 13, 2013

A

dx.doi.org/10.1021/es400256d | Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Environmental Science & Technology

Article

presence of natural organic matter and E. coli cells in suspensions significantly increased the transport of nTiO2 in porous media, with NOM having a greater stabilizing influence than bacteria. Very recently, Wang et al.33 demonstrated that the presence of NOM (humic acid or fulvic acid) or surfactant dramatically increased the transport of nC60 in quartz sand. Although the transport and deposition kinetics of nTiO2 or nC60 nanoparticles in porous media have been extensively explored, these previous studies mainly focused on understanding the transport of one type of nanoparticle (either nTiO2 or nC60) in porous media. Due to their large scale of applications in a wide variety of fields, it will be inevitable that different types of nanoparticles will enter natural environment simultaneously. However, to date, the cotransport behaviors of different types of nanoparticles especially the two most utilized nanoparticles, nTiO2 and nC60 nanoparticles, in porous media have never been explored. Several studies on the cotransport of (bio)colloids (e.g., clay, virus, bacteria),42,43 which did not particularly deal with engineered nanomaterials, have been recently reported, and they found that the cotransport behavior of (bio)colloids was more complex than that of individual particle, indicating that the study on the cotransport of nanoparticles is worth investigating. Hence, this study was designed to fully understand the cotransport behaviors of the two most widely utilized nanoparticles, nTiO2 and nC60, in porous media under a series of environmentally relevant conditions (pH 5 and 7, 0.1−10 mM NaCl). Both breakthrough curves and retention profiles of nTiO2 and nC60 in cotransport experiments were compared with those obtained from individual nanoparticle transport experiments. Our results showed that nTiO2 transport at pH 5 was not significantly affected by nC60 copresent in solution, whereas, the transport of nTiO2 was increased in the presence of nC60 at pH 7. At both pH 5 and 7, the transport of nC60 was found to be decreased by nTiO2 copresent in suspensions.

Transmission electron microscopy (TEM, JEM-200CX, JEOL) and FEI Nova Nano scanning electron microscopic 430 (SEM) were employed to characterize the size and morphology of individual nanoparticles in either single or mixed suspensions. The corresponding results and discussion regarding the zeta potential and the size (distribution) of nanoparticles are provided in the Supporting Information. Porous Media. Quartz sand (ultrapure with 99.8% SiO2) (Hebeizhensheng Mining Ltd., Shijiazhuang, China) with sizes ranging from 417 to 600 μm was used as the porous media for all nanoparticle transport experiments. The procedure used for cleaning the quartz sand is provided in a previous publication,45 as well as in the Supporting Information. The zeta potentials of the crushed quartz sand were also measured in NaCl solutions 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. The detailed information on the column protocols could be found in Tong et al.46 Briefly, prior to packing, the cleaned quartz sand was rehydrated by boiling in Milli-Q water for at least 1 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, so as to minimize any layering or air entrapment. A single 140 mesh stainless steel screen was placed at each end of the column. The gravimetrically measured bed porosity was approximately 0.42.28 After packing, the columns were pre-equilibrated with at least ten pore volumes of NaCl solutions at desired ionic strength and pH. Following pre-equilibration, three pore volumes of nanoparticle suspensions were injected into the column, followed by elution with five pore volumes of NaCl solution at the same ionic strength and pH. For selected experiments, prior to the injection of nanoparticle suspension (nTiO2 or nC60), the columns were pre-equilibrated with three pore volumes of nanoparticle suspension (nC60 or nTiO2) at desired ionic strength and pH, which was followed by the introduction of two pore volumes of salt solution at the same ionic strength and pH. The solutions were injected into the columns in upflow mode using a syringe pump (Harvard Apparatus Inc., Holliston, MA). During column experiments, the input nanoparticle suspension was periodically sonicated to avoid any settlement of nanoparticles and maintain the stability of suspension at desired solution chemistry conditions. The transport experiments were conducted at three ionic strengths (0.1, 1, and 10 mM) in NaCl solutions, and at two pH conditions (pH 5 and 7). The pore water velocity of all experiments was set to 8 m day−1 (0.73 mL min−1) to represent fluid velocities in coarse aquifer sediments, forced-gradient conditions, or engineered filtration systems.47,48 Samples from the column effluent were collected (∼9 mL) in centrifuge tubes at predetermined 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 nanoparticles from quartz sand, approximately 5−10 mL of 0.1 M NaOH solution was added into each sediment segment, and the mixture was continuously shaken at 300 rpm overnight and then manually and vigorously shaken for a few seconds. The effluent samples and supernatant samples from recovery of retained nTiO2 particles were analyzed using a UV spectrophotometer (UV-1800, Shimadzu,

■

MATERIALS AND METHODS Nanoparticle Suspension Preparation. Titanium dioxide nanoparticle powder (nTiO2, anatase, less than 25 nm in diameter, purity greater than 99.9%) was purchased from Sigma-Aldrich Corp, whereas fullerene nanoparticle powder (nC60, 99.9%, purified by sublimation) was purchased from the Materials Electronics Research Corp. (Tuscon, AZ). A nTiO2 nanoparticle stock suspension (1000 mg L−1) was prepared by suspending nTiO2 nanopowder in Milli-Q water (Q-Gard 1, Millipore Inc., MA) and sonicating for 10 min with a probe (Ningboxinzhi Biotechnology Ltd., China). The aqueous suspensions of nC60 were prepared from fullerene powders following the method provided by Deguchi et al.44 The detailed information of nC60 preparation is provided in the Supporting Information. The concentration of nC60 stock suspension, as determined using a TOC-meter (TOC-VCPN, Shimadzu, Japan), was ∼30 mg L−1 as TOC. For both individual nanoparticle transport and nanoparticle cotransport experiments, the influent concentration of nTiO2 and nC60 suspensions was maintained to be 50 and 10 mg L−1, respectively. The ionic strength of nanoparticle suspensions ranged from 0.1 to 10 mM in NaCl solutions. The suspension pH was set to be 5 and 7 by adjusting with 0.1 M HCl or 0.1 M NaOH. The zeta potentials and sizes of nanoparticles under these conditions were measured using Zetasizer Nano ZS90 (Malvern Instruments, UK). Measurements were performed at room temperature (25 °C) and repeated 9−12 times. B

dx.doi.org/10.1021/es400256d | Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Environmental Science & Technology

Article

Japan), whereas the corresponding samples for nC60 were determined by a TOC-meter. The detailed statements of the methods used to determine the concentration of nanoparticles are provided in the Supporting Information. 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.

(Figure 1a, open symbol). Although zeta potentials and particle sizes of nTiO2 varied at different solution conditions (Table S2), the BTCs of nTiO2 in quartz sand were similar (9−11% breakthrough) under all examined ionic strength conditions at pH 5 (favorable conditions), indicating that the influence of solution ionic strength on the transport of nTiO2 was minimal at pH 5. Moreover, despite the size distributions of nTiO2 in the presence of nC60 were different from those in its absence at pH 5 (Figure S4), nTiO2 BTCs with nC60 copresent in suspensions (8−10%) (Figure 1a, solid symbol) were almost the same as those without nC60 (9−11%) (Figure 1a, open symbol). This observation was true under all ionic strength conditions (0.1−10 mM). The results indicated that the presence of nC60 did not significantly alter the transport of nTiO2 in quartz sand at pH 5. Zeta potentials of both nTiO2 and quartz sand were negative at pH 7 (Table S2), and thus the electrostatic interaction between nTiO2 and quartz sand was repulsive. In contrast with the low BTCs obtained at pH 5, higher BTCs of nTiO2 without nC60 copresent in suspensions were observed at pH 7 (especially at low ionic strength) (e.g., ∼11% and 78% breakthrough at 0.1 mM for pH 5 and 7, respectively), and the BTCs were sensitive to solution chemistries, as indicated by the decrease of the breakthrough plateau with increasing ionic strength (Figure 1b, open symbol). The lower BTCs of nTiO2 with increasing ionic strength as observed at pH 7 were consistent with less negative zeta potentials observed at high ionic strength (Table S2) and thus agreed with the classic Derjaguin−Landau−Verwey−Overbeek (DLVO) theory.50,51 Similar observations have also been reported previously.27,28 In contrast with the equivalent transport behaviors of nTiO2 observed regardless of the presence of nC60 in solutions at pH 5, the BTCs of nTiO2 acquired at pH 7 in the presence of nC60 in suspensions (Figure 1b, solid symbol) were higher than those without nC60 (Figure 1b, open symbol). This observation was true under all examined ionic strength conditions (0.1−10 mM). For example, at 0.1 mM ionic strength and pH 7, the breakthrough plateau for nTiO2 in the absence of nC60 was 0.75, whereas the breakthrough plateau in the presence of nC60 was 0.90. The results indicated that, unlike the negligible change of nTiO2 transport induced by the presence of nC60 at pH 5, the copresence of nC60 in suspensions increased the transport of nTiO2 in quartz sand at pH 7. Similar to the competition of the deposition sites between NOM and microbes,52,53 we proposed that nC60 in suspension might compete with nTiO2 for deposition sites on quartz sand, contributing to the enhanced transport of nTiO2 in quartz sand at pH 7. To test our hypothesis, additional experiments were performed by pre-equilibrating the columns with three pore volumes of 10 mg L−1 nC60 suspensions prior to the injection of nTiO2 suspensions. If competition for deposition sites by nC60 occurred when the nC60 was present in the nTiO2 suspension, pre-equilibration of the columns with nC60 suspensions would allow nC60 to preferably deposit onto the quartz sand surfaces. Thereby, increased transport (reduced deposition) of nTiO2 relative to that without pre-equilibration would be observed. The results of the pre-equilibration experiments performed in 1 mM NaCl solutions at pH 7 are presented in Figure 2. A direct comparison of nTiO2 BTCs for columns that were preequilibrated with nC60 suspensions (Figure 2, solid square) versus those without pre-equilibration (Figure 2, open triangle) revealed that the former (∼45% breakthrough) were higher

■

RESULTS AND DISCUSSION Transport of nTiO2 Both in the Absence and the Presence of nC60. The transport of nTiO2 in quartz sand both with and without nC60 in suspension was investigated under a series of ionic strengths (0.1−10 mM) in NaCl solutions at two pH conditions (5 and 7), and the corresponding breakthrough curves (BTCs) for nTiO2 are presented in Figure 1. Similar to

Figure 1. Breakthrough curves of nTiO2 in quartz sand both with (solid symbol) and without (open symbol) nC60 copresent in suspensions at 0.1, 1, and 10 mM ionic strength in NaCl solutions at pH 5 (a) and pH 7 (b). Replicate experiments were performed under all conditions (n ≥ 2).

the observations of previous studies,27,49 the transport of nTiO2 without nC60 in suspensions in quartz sand was strongly dependent on pH. As shown in Table S2, the zeta potentials of nTiO2 were positive, yet the zeta potentials of quartz sand were negative under the examined ionic strengths at pH 5. Attractive electrostatic interactions were therefore present between nTiO2 and quartz sand under all examined ionic strengths at pH 5. Thereby, almost all of the injected nTiO2 was retained in quartz sand, and only a very small portion of nTiO2 could pass through the column under all ionic strength conditions at pH 5 C

dx.doi.org/10.1021/es400256d | Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Environmental Science & Technology

Article

Figure 2. Breakthrough curves of nTiO2 in quartz sand in 1 mM NaCl solutions at pH 7 with nC60 copresent yet without pretreatment of the columns with nC60 suspensions (solid triangle) and without nC60 copresent yet with (solid square) and without (open triangle) pretreatment of the columns with nC60 suspensions. Data without pretreatment are reproduced from Figure 1. Replicate experiments were performed under all conditions (n ≥ 2).

than the latter (∼36% breakthrough). This observation showed that, when quartz sand was pretreated with nC60, the deposition sites on quartz sand would be occupied by nC60, leaving less sites for the subsequent nTiO2 deposition, and as a result, increased transport of nTiO2 was observed. Hence, when nC60 was copresent with nTiO2 in suspension, nC60 would compete with nTiO2 for the deposition sites, and thus the available sites on quartz sand surfaces for nTiO2 deposition decreased, increasing the transport of nTiO2 at pH 7. Moreover, BTCs for nTiO2 without nC60 copresent in suspension yet with preequilibration with nC60 suspensions (Figure 2, solid square) were similar to those with nC60 copresent in nTiO2 suspension, yet without pre-equilibration with nC60 (Figure 2, solid triangle). This observation showed that the pretreatment of quartz sand with nC60 had a similar influence on the transport of nTiO2 as that of the copresence of nC60 in nTiO2 suspension. The results indicated that, when nC60 was present in solutions, the competition of deposition sites by nC60 was a major (and possibly a sole) contributor to the enhanced nTiO2 transport in porous media at pH 7. Transport of nC60 Both in the Absence and the Presence of nTiO2. To examine the influence of nTiO2 copresent in suspensions on the transport behavior of nC60, transport experiments of nC60 in packed quartz sand were performed both in the absence and presence of nTiO2 at three ionic strength conditions (0.1−10 mM NaCl) and two pH conditions (5 and 7). As shown in Table S2, at both pH conditions, the zeta potentials of both nC60 and quartz sand were negative under all examined ionic strength conditions. Thereby, the electrostatic interaction between nC60 and quartz sand were repulsive at both pH 5 and 7. Obvious breakthrough of nC60 in quartz sand was observed under all examined ionic strength conditions at both pH 5 and 7 (Figure 3). The BTC plateau of nC60 decreased with increasing solution ionic strength at both pH 5 and 7, which was consistent with less negative zeta potentials observed at higher ionic strengths (Table S2) and, thus, was in agreement with DLVO theory. This observation was also consistent with those of previous studies.25,26,54 Unlike the negligible change of BTCs of nTiO2 induced by the copresence of nC60 at pH 5 (Figure 1a), BTCs of nC60 in the copresence of nTiO2 in suspensions (Figure 3a, solid symbol) were lower than those without nTiO2 (Figure 3a,

Figure 3. Breakthrough curves of nC60 in quartz sand both with (solid symbol) and without (open symbol) nTiO2 copresent in suspensions at 0.1, 1, and 10 mM ionic strength in NaCl solutions at pH 5 (a) and pH 7 (b). Replicate experiments were performed under all conditions (n ≥ 2).

open symbol) under all examined ionic strength conditions at pH 5 (e.g., ∼70% and 97% breakthrough at 0.1 mM for with and without nTiO2, respectively). Meanwhile, BTCs of nC60 in the copresence of nTiO2 in suspensions (Figure 3b, solid symbol) were also lower than those without nTiO2 under all examined ionic strength conditions at pH 7 (Figure 3b, open symbol) (e.g., ∼86% and 100% breakthrough at 0.1 mM for with and without nTiO2, respectively). This observation demonstrated that, at both pH 5 and 7, the copresence of nTiO2 in nC60 suspensions decreased the transport of nC60 in quartz sand under all examined ionic strength conditions. At pH 5, positively charged nTiO2 is expected to attract a portion of nC60 to form nTiO2−nC60 clusters (Table S2 and Figures S4 and S5) with a heterogeneous surface charge. The presence of surface charge heterogeneity of nTiO2−nC60 clusters can lead to the attractive electrostatic interaction between nTiO2−nC60 clusters and quartz sand.55 The nTiO2− nC60 clusters therefore would be easily retained in quartz sand. The retention of nTiO2−nC60 clusters increased the overall nC60 deposition in quartz sand at pH 5. Meanwhile, comparing with deposition onto negatively charged quartz sand, nC60 would more likely deposit on the surfaces of positively charged nTiO2 which had been previously deposited on quartz sand, leading to a greater deposition of nC60. A similar trend was observed from a previous study in which the enhanced deposition of negatively charged bacteria onto goethite coated quartz sand that was positively charged.56 Hence, decreased transport of nC60 in the presence of nTiO2 in suspensions was observed in all examined conditions (0.1−10 mM) at pH 5. Although the zeta potentials of nTiO2 were negative at pH 7, they were less negative than those of quartz sand (Table S2). The electrostatic interaction between nC60 and nTiO2 therefore would be less repulsive than that between nC60 and quartz sand. The retention of nC60 by nTiO2 which was predeposited onto the surfaces of quartz sand might occur. Herein, we propose D

dx.doi.org/10.1021/es400256d | Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Environmental Science & Technology

Article

that nTiO2 deposited onto quartz sand surfaces might serve as additional sites for nC60 deposition and thus decrease the transport of nC60 in quartz sand at pH 7. To test whether predeposited nTiO2 would decrease the transport of nC60 in quartz sand at pH 7, experiments were performed by pre-equilibrating the columns with three pore volumes of 50 mg L−1 nTiO2 suspensions prior to the injection of nC60. Pre-equilibration of the columns with nTiO2 would allow nTiO2 to preferably deposit onto the quartz sand surfaces. If the predeposited nTiO2 provided additional sites for nC60 deposition, lower nC60 BTCs for columns precovered with nTiO2 relative to those without pre-equilibration with nTiO2 would be observed. The results of the pre-equilibration experiments performed in 1 mM NaCl solutions at pH 7 are presented in Figure 4. A comparison of nC60 BTCs obtained

retained profiles were the inverse of the plateaus of BTCs, as expected from mass balance considerations (Table S3). In the absence of nC60 in suspensions, the retained concentration of nTiO2 in quartz sand decreased log−linearly with distance under all examined ionic strength conditions at pH 5 (Figure 5a, open symbol), which agreed with classical filtration theory (CFT).57 The log−linear retained profiles of nTiO2 acquired at pH 5 were also observed previously.27 Conversely, retained concentrations of nTiO2 in quartz sand decreased nonexponentially with transport distance at 1 and 10 mM ionic strength conditions at pH 7, with the greatest retention being located near the column inlet (Figure 5b, open symbol). The hyperexponential retained profiles of nTiO2 acquired under unfavorable conditions were consistent with previously reported observations.27,28,49 The mechanism(s) including DLVO interaction, straining, and concurrent aggregation, which control nTiO2 transport and retention in porous media under unfavorable condition, have been revealed.27,28,49 These factors also played important roles on nTiO2 transport and retention without nC60 at pH 7 in the present study. Specifically, the transport and retention of nTiO2 in quartz sand at pH 7 were sensitive to solution ionic strength (zeta potential), indicating that DLVO interaction plays an important role in the transport behavior of nTiO2. As shown in the BTCs, ripening was present at higher ionic strengths (i.e., 10 mM), indicating the occurrence of concurrent aggregation on the solid phase.49,58 As nTiO2 was continuously being injected into the column, the attached aggregates would grow more dramatically, which further compressed the pore throats in the column and thereby induced particle-straining.49,58 A comparison of the retained profiles of nTiO2 in the presence of nC60 versus those without nC60 in suspensions showed that, at pH 5, the retained profiles of nTiO2 with nC60 copresent in suspensions (Figure 5a, solid symbol) were similar to those acquired in the absence of nC60 (Figure 5a, open symbol). The observation demonstrated that the copresence of nC60 in suspensions did not affect the retention of nTiO2 in porous media at pH 5. In contrast, the retained concentrations of nTiO2 in the presence of nC60 in suspensions acquired at pH 7 (Figure 5b, solid symbol) were lower relative to those in the absence of nC60 (Figure 5b, open symbol), demonstrating that, at pH 7 (unfavorable conditions), the copresence of nC60 in suspensions decreased the retention of nTiO2 in porous media. A close comparison of the shapes of retained profiles of nTiO2 in the presence of nC60 in suspensions versus those in the absence of nC60 showed that under all three ionic strength conditions, the shapes of retained profiles of nTiO2 in the presence of nC60 were similar to those without nC60 in suspension. The observation implied that although the copresence of nC60 in suspension decreased the retention of nTiO2 in porous media at pH 7, nC60 copresent in suspensions might not change the deposition mechanisms of nTiO2 in quartz sand. Unlike nTiO2, the hyper-exponential retained profiles of nC60 were observed at all three ionic strength conditions examined at pH 5 and two relatively high ionic strengths (1 and 10 mM) at pH 7 (Figure 5 c and d, open symbol). The retained concentration of nC60 did not change much with travel distance at 0.1 mM and pH 7. The relatively flat retained profile of nC60 observed has also been reported previously under similar conditions.30,31 At relatively high ionic strength conditions (1 and 10 mM), since the zeta potentials of nC60 were less negative than those of quartz sand, the interaction

Figure 4. Breakthrough curves of nC60 in quartz sand in 1 mM NaCl solutions at pH 7 with nTiO2 copresent yet without pretreatment of the columns with nTiO2 suspensions (solid triangle) and without nTiO2 copresent yet with (solid square) and without (open triangle) pretreatment of the columns with nTiO2 suspensions. Data without pretreatment are reproduced from Figure 3. Replicate experiments were performed under all conditions (n ≥ 2).

with precovering quartz sand with nTiO2 versus those without pre-equilibration revealed that nC60 BTCs (∼80% breakthrough) for columns pre-equilibrated with nTiO2 and without nTiO2 in nC60 suspensions (Figure 4, solid square) were lower than those (∼95% breakthrough) without pre-equilibration with nTiO2 and without nTiO2 in nC60 suspensions (Figure 4, open triangle). This observation clearly showed that nTiO2 predeposited on quartz sand favored the retention of nC60 and thus decreased transport of nC60 was observed. Moreover, nC60 BTC for columns with pretreated with nTiO2 yet without nTiO2 in nC60 suspensions (Figure 4, solid square) was almost similar with that without pre-equilibration yet with nTiO2 in nC60 suspensions (Figure 4, solid triangle) (e.g., ∼80% and 86% breakthrough for with pre-equilibration yet without nTiO2 and without pre-equilibration yet with nTiO2, respectively). The results clearly demonstrated that, when nTiO2 was copresent in solution, nTiO2 deposited onto quartz sand could serve as additional sites for nC60 deposition, enhancing the deposition of nC60 in quartz sand (decreased transport) under all examined ionic strength conditions at pH 7. Retention of Nanoparticles in Porous Media. To examine whether the cotransport of nTiO2 and nC60 would affect the distribution of nanoparticles retained in quartz sand, the retained profiles of nC60 and nTiO2 under all ionic strength conditions both for the individual and cotransport experiments were obtained, and the results are provided in Figure 5. The E

dx.doi.org/10.1021/es400256d | Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Environmental Science & Technology

Article

Figure 5. Retention profiles of nTiO2 (a for pH 5 and b for pH 7) and nC60 (c for pH 5 and d for pH 7) in quartz sand for the individual nanoparticle transport (open symbol) and cotransport experiments (solid symbol) at 0.1, 1, and 10 mM ionic strengths in NaCl solutions. Replicate experiments were performed under all conditions (n ≥ 2).

nC60 deposition, enhancing the deposition of nC60 in the presence of nTiO2. At both pH 5 and 7, the interactions of nC60 with previously deposited nTiO2 was more pronounced near the column inlet as observed from the greatest retention of nTiO2 at these segments (Figure 5 a and b). Environmental Implications. With rapid applications of various types of engineered nanoparticles, their simultaneous release into surface and subsurface environments is inevitable.59 Thereby, studies which are focused on the cotransport of nanoparticles are urgently required in order to understand the fate and transport mechanisms of nanoparticles in complex and realistic environments. This study investigated, for the first time, the cotransport and retention behaviors of nTiO2 and nC60 nanoparticles, as the two most commonly utilized engineered nanoparticles, in saturated porous media under various ionic strength conditions relevant to subsurface/ groundwater environments. The findings from this study suggest that the cotransport of nTiO2 and nC60 nanoparticles in water saturated porous media is far more complex than the transport of single component nanoparticles, and it also gives us an attention on the importance of understanding cotransport of multicomponent nanoparticles as well as transport of individual nanoparticles in real aquatic environments, which are more complex than those simulated in our study. Therefore, to better understand the cotransport behavior of nTiO2 and nC60 nanoparticles, further research is required to incorporate the effect of other factors (e.g., nanoparticle concentration, NOM, and ion valence) which could influence the stability and interaction of nanoparticles.27,39

between nC60 particles would be less repulsive than that of nC60-sand. As a result, nC60 would more likely be deposited onto the surfaces of previously deposited nC60 particles. Previous studies25,31 also found that interactions of nC60 with previously deposited aggregates near the column inlet resulted in the observation of hyper-exponential retained profiles under unfavorable conditions. Under both examined pH conditions (pH 5 and 7), the concentrations of nC60 retained in quartz sand with nTiO2 copresent in suspensions (Figure 5 c and d, solid symbol) were greater relative to those in the absence of nTiO2 (Figure 5 c and d, open symbol). The observation showed that the copresence of nTiO2 in suspension increased the deposition of nC60 in porous media under all examined conditions. A close comparison of the retained profiles of nC60 in the presence of nTiO2 versus those in the absence of nTiO2 showed that, under all examined conditions, the increased retention of nC60 induced by the copresence of nTiO2 in suspension occurred across the entire column. Moreover, the shapes of nC60 retained profiles in the presence of nTiO2 were similar to those in the absence of nTiO2 in suspensions. This observation implied that the major mechanisms controlling the deposition of nC60 with nTiO2 copresent in suspension were similar to those without the nTiO2 in suspension. In addition to the codeposition of nC60 with nTiO2 (i.e., nTiO2-nC60 clusters) onto quartz sand, positively charged nTiO2 previously deposited onto the quartz sand could provide additional deposition sites for nC60. As a result, greater deposition of nC60 was observed when nTiO2 was copresent in suspension at pH 5. Likewise, at pH 7, nTiO2 predeposited onto quartz sand could provide additional sites for F

dx.doi.org/10.1021/es400256d | Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Environmental Science & Technology

■

Article

Nelson, J. Morphology evolution via self-organization and lateral and vertical diffusion in polymer:fullerene solar cell blends. Nat. Mater. 2008, 7, 158−164. (15) Li, R.; Yang, Q.; Qiu, X.; Li, K.; Li, G.; Zhu, P.; Zhu, T. Reactive oxygen species alteration of immune cells in local residents at an electronic waste recycling site in northern China. Environ. Sci. Technol. 2013, 47, 3344−3352. (16) Bystrzejewska-Piotrowska, G.; Golimowski, J.; Urban, P. L. Nanoparticles: Their potential toxicity, waste and environmental management. Waste Manage. 2009, 29, 2587−2595. (17) Wiesner, M. R.; Lowry, G. V.; Alvarez, P.; Dionysiou, D.; Biswas, P. Assessing the risks of manufactured nanomaterials. Environ. Sci. Technol. 2006, 40, 4336−4345. (18) Baun, A.; Hartmann, N. B.; Grieger, K. D.; Hansen, S. F. Setting the limits for engineered nanoparticles in European surface waters - are current approaches appropriate? J. Environ. Monitor. 2009, 11, 1774− 1781. (19) Scown, T. M.; van Aerle, R.; Tyler, C. R. Review: Do engineered nanoparticles pose a significant threat to the aquatic environment? Crit. Rev. Toxicol. 2010, 40, 653−670. (20) Adams, L. K.; Lyon, D. Y.; Alvarez, P. J. J. Comparative ecotoxicity of nanoscale TiO2, SiO2, and ZnO water suspensions. Water Res. 2006, 40, 3527−3532. (21) Lyon, D. Y.; Fortner, J. D.; Sayes, C. M.; Colvin, V. L.; Hughes, J. B. Bacterial cell association and antimicrobial activity of a C-60 water suspension. Environ. Toxicol. Chem. 2005, 24, 2757−2762. (22) Fang, J.; Shan, X.-q.; Wen, B.; Lin, J.-m.; Owens, G.; Zhou, S.-r. Transport of copper as affected by titania nanoparticles in soil columns. Environ. Pollut. 2011, 159, 1248−1256. (23) Lunliang, Z.; Lilin, W.; Ping, Z.; Amy, T. K. Facilitated transport of 2,2′,5,5′-polychlorinated biphenyl and phenanthrene by fullerene nanoparticles through sandy soil columns. Environ. Sci. Technol. 2011, 45, 1341. (24) Song, M.; Yuan, S.; Yin, J.; Wang, X. Size-dependent toxicity of nano-C60 aggregates: More sensitive indication by apoptosis-related bax translocation in cultured human cells. Environ. Sci. Technol. 2012, 46, 3457−3464. (25) Li, Y.; Wang, Y.; Pennell, K. D.; Abriola, L. M. Investigation of the transport and deposition of fullerene (C60) nanoparticles in quartz sands under varying flow conditions. Environ. Sci. Technol. 2008, 42, 7174−7180. (26) Lecoanet, H. F.; Wiesner, M. R. Velocity effects on fullerene and oxide nanoparticle deposition in porous media. Environ. Sci. Technol. 2004, 38, 4377−4382. (27) Chowdhury, I.; Hong, Y.; Honda, R. J.; Walker, S. L. Mechanisms of TiO2 nanoparticle transport in porous media: Role of solution chemistry, nanoparticle concentration, and flowrate. J. Colloid Interface Sci. 2011, 360, 548−555. (28) Chen, G. X.; Liu, X. Y.; Su, C. M. Transport and retention of TiO2 rutile nanoparticles in saturated porous media under low-ionicstrength conditions: Measurements and mechanisms. Langmuir 2011, 27, 5393−5402. (29) Ben-Moshe, T.; Dror, I.; Berkowitz, B. Transport of metal oxide nanoparticles in saturated porous media. Chemosphere 2010, 81, 387− 393. (30) Wang, Y.; Li, Y.; Pennell, K. D. Influence of electrolyte species and concentration on the aggregation and transport of fullerene nanoparticles in quartz sands. Environ. Toxicol. Chem. 2008, 27, 1860− 1867. (31) Wang, Y. G.; Li, Y. S.; Fortner, J. D.; Hughes, J. B.; Abriola, L. M.; Pennell, K. D. Transport and retention of nanoscale C-60 aggregates in water-saturated porous media. Environ. Sci. Technol. 2008, 42, 3588−3594. (32) Zhang, L.; Hou, L.; Wang, L.; Kan, A. T.; Chen, W.; Tomson, M. B. Transport of fullerene nanoparticles (nC60) in saturated sand and sandy soil: Controlling factors and modeling. Environ. Sci. Technol. 2012, 46, 7230−7238.

ASSOCIATED CONTENT

S Supporting Information *

Information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 1062756491 (M.T.); +82632702370 (H.K.). Fax: +86 1062756526 (M.T.); +82632702366 (H.K.). E-mail address: [email protected] (M.T.); kshjkim@jbnu. ac.kr (H.K.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Nature Science Foundation of China under grant no. 40971181 as well as the National Research Foundation of Korea Grant funded by the Korean Government (MEST) (NRF-2010-0023782). The authors wish to acknowledge the editor and reviewers for their helpful comments.

■

REFERENCES

(1) Bosi, S.; Da Ros, T.; Spalluto, G.; Prato, M. Fullerene derivatives: an attractive tool for biological applications. Eur. J. Med. Chem. 2003, 38, 913−923. (2) Eckert, J. F.; Nicoud, J. F.; Nierengarten, J. F.; Liu, S. G.; Echegoyen, L.; Barigelletti, F.; Armaroli, N.; Ouali, L.; Krasnikov, V.; Hadziioannou, G. Fullerene-oligophenylenevinylene hybrids: Synthesis, electronic properties, and incorporation in photovoltaic devices. J. Am. Chem. Soc. 2000, 122, 7467−7479. (3) Venturini, J.; Koudoumas, E.; Couris, S.; Janot, J. M.; Seta, P.; Mathis, C.; Leach, S. Optical limiting and nonlinear optical absorption properties of C-60-polystyrene star polymer films: C-60 concentration dependence. J. Mater. Chem. 2002, 12, 2071−2076. (4) Mueller, N. C.; Nowack, B. Nanoparticles for remediation: Solving big problems with little particles. Elements 2010, 6, 395−400. (5) Savage, N.; Diallo, M. Nanomaterials and water purification: Opportunities and challenges. J. Nanopart. Res. 2005, 7, 331−342. (6) Chen, X.; Mao, S. S. Titanium dioxide nanomaterials: Synthesis, properties, modifications, and applications. Chem. Rev. 2007, 107, 2891−2959. (7) Erickson, B. Nanomaterials in food, cosmetics. Chem. Eng. News 2012, 90, 8. (8) Dunphy Guzman, K. A.; Finnegan, M. P.; Banfield, J. F. Influence of surface potential on aggregation and transport of titania nanoparticles. Environ. Sci. Technol. 2006, 40, 7688−7693. (9) Reddy, K. M.; Manorama, S. V.; Reddy, A. R. Bandgap studies on anatase titanium dioxide nanoparticles. Mater. Chem. Phys. 2003, 78, 239−245. (10) Lorenz, C.; Tiede, K.; Tear, S.; Boxall, A.; Von Goetz, N.; Hungerbühler, K. Imaging and characterization of engineered nanoparticles in sunscreens by electron microscopy, under wet and dry conditions. Int. J. Occup. Environ. Heal. 2010, 16, 406−428. (11) Hotze, E. M.; Labille, J.; Alvarez, P.; Wiesner, M. R. Mechanisms of photochemistry and reactive oxygen production by fullerene suspensions in water. Environ. Sci. Technol. 2008, 42, 4175−4180. (12) Sayes, C. M.; Fortner, J. D.; Guo, W.; Lyon, D.; Boyd, A. M.; Ausman, K. D.; Tao, Y. J.; Sitharaman, B.; Wilson, L. J.; Hughes, J. B.; West, J. L.; Colvin, V. L. The differential cytotoxicity of water-soluble fullerenes. Nano Lett. 2004, 4, 1881−1887. (13) Mauter, M. S.; Elimelech, M. Environmental applications of carbon-based nanomaterials. Environ. Sci. Technol. 2008, 42, 5843− 5859. (14) Campoy-Quiles, M.; Ferenczi, T.; Agostinelli, T.; Etchegoin, P. G.; Kim, Y.; Anthopoulos, T. D.; Stavrinou, P. N.; Bradley, D. D. C.; G

dx.doi.org/10.1021/es400256d | Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Environmental Science & Technology

Article

(33) Wang, Y.; Li, Y.; Costanza, J.; Abriola, L. M.; Pennell, K. D. Enhanced mobility of fullerene (C60) nanoparticles in the presence of stabilizing agents. Environ. Sci. Technol. 2012, 46, 11761−11769. (34) Godinez, I. G.; Darnault, C. J. G. Aggregation and transport of nano-TiO2 in saturated porous media: Effects of pH, surfactants and flow velocity. Water Res. 2011, 45, 839−851. (35) Chen, Z.; Westerhoff, P.; Herckes, P. Quantification of C(60) fullerene concentrations in water. Environ. Toxicol. Chem. 2008, 27, 1852−1859. (36) Chen, K. L.; Elimelech, M. Relating colloidal stability of fullerene (C-60) nanoparticles to nanoparticle charge and electrokinetic properties. Environ. Sci. Technol. 2009, 43, 7270−7276. (37) Thio, B. J. R.; Zhou, D. X.; Keller, A. A. Influence of natural organic matter on the aggregation and deposition of titanium dioxide nanoparticles. J. Hazard. Mater. 2011, 189, 556−563. (38) Aiken, G. R.; Hsu-Kim, H.; Ryan, J. N. Influence of dissolved organic matter on the environmental fate of metals, nanoparticles, and colloids. Environ. Sci. Technol. 2011, 45, 3196−3201. (39) Chowdhury, I.; Cwiertny, D. M.; Walker, S. L. Combined factors influencing the aggregation and deposition of nano-TiO2 in the presence of humic acid and bacteria. Environ. Sci. Technol. 2012, 46, 6968−6976. (40) Tong, M. P.; Ding, J. L.; Shen, Y.; Zhu, P. T. Influence of biofilm on the transport of fullerene (C-60) nanoparticles in porous media. Water Res. 2010, 44, 1094−1103. (41) Tripathi, S.; Champagne, D.; Tufenkji, N. Transport behavior of selected nanoparticles with different surface coatings in granular porous media coated with Pseudomonas aeruginosa biofilm. Environ. Sci. Technol. 2012, 46, 6942−6949. (42) Vasiliadou, I. A.; Chrysikopoulos, C. V. Cotransport of Pseudomonas putida and kaolinite particles through water-saturated columns packed with glass beads. Water Resour. Res. 2011, 47. (43) Syngouna, V. I.; Chrysikopoulos, C. V. Cotransport of clay colloids and viruses in water saturated porous media. Colloids Surf., A 2013, 416, 56−65. (44) Deguchi, S.; Alargova, R. G.; Tsujii, K. Stable dispersions of fullerenes, C60 and C70, in water. Preparation and characterization. Langmuir 2001, 17, 6013−6017. (45) Li, X.; Johnson, W. P. Nonmonotonic variations in deposition rate coefficients of microspheres in porous media under unfavorable deposition conditions. Environ. Sci. Technol. 2005, 39, 1658−1665. (46) Tong, M. P.; Camesano, T. A.; Johnson, W. P. Spatial variation in deposition rate coefficients of an adhesion-deficient bacterial strain in quartz sand. Environ. Sci. Technol. 2005, 39, 3679−3687. (47) Tong, M. P.; Long, G. Y.; Jiang, X. J.; Kim, H. N. Contribution of extracellular polymeric substances on representative gram negative and gram positive bacterial deposition in porous media. Environ. Sci. Technol. 2010, 44, 2393−2399. (48) Harter, T.; Wagner, S.; Atwill, E. R. Colloid transport and filtration of Cryptosporidium parvum in sandy soils and aquifer sediments. Environ. Sci. Technol. 1999, 34, 62−70. (49) Solovitch, N.; Labille, J.; Rose, J.; Chaurand, P.; Borschneck, D.; Wiesner, M. R.; Bottero, J. Y. Concurrent aggregation and deposition of TiO2 nanoparticles in a sandy porous media. Environ. Sci. Technol. 2010, 44, 4897−4902. (50) Verwey, E. J. W.; Overbeek, J. T. G. Theory of the Stability of Lyophobic Colloids: The Interaction of Sol Particles Having an Electric Double Layer; Elsevier: Amsterdam, 1948. (51) Derjaguin, B.; Landau, L. Theory of the stability of strongly charged lyophobic sols and of the adhesion of strongly charged particles in solution of electrolytes. Acta Physicochim. URSS 1941, 14, 633−662. (52) Redman, J. A.; Grant, S. B.; Olson, T. M.; Hardy, M. E.; Estes, M. K. Filtration of recombinant norwalk virus particles and bacteriophage MS2 in quartz sand: Importance of electrostatic interactions. Environ. Sci. Technol. 1997, 31, 3378−3383. (53) Yang, H.; Kim, H.; Tong, M. Influence of humic acid on the transport behavior of bacteria in quartz sand. Colloids Surf., B 2012, 91, 122−129.

(54) Brant, J.; Lecoanet, H.; Wiesner, M. R. Aggregation and deposition characteristics of fullerene nanoparticles in aqueous systems. J. Nanopart. Res. 2005, 7, 545−553. (55) Tong, M.; Johnson, W. P. Colloid population heterogeneity drives hyperexponential deviation from classic filtration theory. Environ. Sci. Technol. 2007, 41, 493−499. (56) Kim, S.-B.; Park, S.-J.; Lee, C.-G.; Choi, N.-C.; Kim, D.-J. Bacteria transport through goethite-coated sand: Effects of solution pH and coated sand content. Colloids Surf., B 2008, 63, 236−242. (57) Yao, K.-M.; Habibian, M. T.; O’Melia, C. R. Water and waste water filtration. Concepts and applications. Environ. Sci. Technol. 1971, 5, 1105−1112. (58) Jiang, X.; Tong, M.; Lu, R.; Kim, H. Transport and deposition of ZnO nanoparticles in saturated porous media. Colloids Surf., A 2012, 401, 29−37. (59) Delay, M.; Frimmel, F. Nanoparticles in aquatic systems. Anal. Bioanal. Chem. 2012, 402, 583−592.

H

dx.doi.org/10.1021/es400256d | Environ. Sci. Technol. XXXX, XXX, XXX−XXX