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Influence of gravity on transport and retention of representative engineered nanoparticles in quartz sand
CONHYD-03094; No of Pages 8 Journal of Contaminant Hydrology xxx (2015) xxx–xxx
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Influence of gravity on transport and retention of representative engineered nanoparticles in quartz sand Li Cai a, Jinghan Zhu b, Yanglong Hou b, Meiping Tong a,⁎, Hyunjung Kim c,⁎⁎ a b c
The Key Laboratory of Water and Sediment Sciences, Ministry of Education, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, PR China Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, PR China Department of Mineral Resources and Energy Engineering, Chonbuk National University, Jeonju, Jeonbuk 561-756, Republic of Korea
a r t i c l e
i n f o
Article history: Received 21 November 2014 Received in revised form 30 January 2015 Accepted 8 February 2015 Available online xxxx Keywords: Engineered nanoparticle Gravity Transport Retention Quartz sand
a b s t r a c t Four types of NPs: carbon nanotubes and graphene oxide (carbon-based NPs), titanium dioxide and zinc oxide metal-oxide NPs, were utilized to systematically determine the influence of gravity on the transport of NPs in porous media. Packed column experiments for two types of carbonbased NPs were performed under unfavorable conditions in both up-flow (gravity-negative) and down-flow (gravity-positive) orientations, while for two types of metal-oxide NPs, experiments were performed under both unfavorable and favorable conditions in both up-flow and down-flow orientations. Both breakthrough curves and retained profiles of two types of carbon-based NPs in up-flow orientation were equivalent to those in down-flow orientation, indicating that gravity had negligible effect on the transport and retention of carbon-based NPs under unfavorable conditions. In contrast, under both unfavorable and favorable conditions, the breakthrough curves for two types of metal-oxide NPs in down-flow orientation were lower relative to those in up-flow orientation, indicating that gravity could decrease the transport of metal-oxide NPs in porous media. The distinct effect of gravity on the transport and retention of carbon-based and metaloxide NPs was mainly attributed to the contribution of gravity to the force balance on the NPs in quartz sand. The contribution of gravity was determined by the interplay of the density and sizes of NP aggregates under examined solution conditions. © 2015 Elsevier B.V. All rights reserved.
1. Introduction With the rapid growth of the nanotechnology industry, different types of nanomaterials such as carbon-based nanoparticles (NPs) and metal/metal-oxide NPs have been fabricated and used increasingly in various fields. Carbon nanotubes (CNTs), graphene oxide (GO), titanium dioxide (nTiO2), and zinc oxide (nZnO) NPs, have been regarded as the most important nanomaterials due to their various applications (Hu et al., 2011; Mauter and Elimelech, 2008; Soares et al., 2008). The massive applications would eventually result in the
⁎ Corresponding author. Tel.: +86 1062756491; fax: +86 1062756526. ⁎⁎ Corresponding author. Tel.: +82 632702370; fax: +82 632702366. E-mail addresses: [email protected] (M. Tong), [email protected] (H. Kim).
entrance of these NPs into the natural environment. To date, physicochemical factors such as fluid velocity (Lecoanet and Wiesner, 2004), solution chemistry (pH, ionic strength, and ion type) (French et al., 2009; Jiang et al., 2012; Lanphere et al., 2013; Liu et al., 2009), NP concentration (Chowdhury et al., 2011; Jaisi and Elimelech, 2009), natural organic matter (Chowdhury et al., 2012; Jones and Su, 2014), surfactant (Fang et al., 2013; Lu et al., 2013, 2014), as well as bacteria (Chowdhury et al., 2012; Jones and Su, 2014) have been shown to significantly affect the transport behavior of NPs in porous media. A few previous studies found that gravity could also affect the transport and retention of (bio)colloids (Chen et al., 2009; Chrysikopoulos and Syngouna, 2014; Ma et al., 2011; Wan et al., 1995). For instance, by using a parallel plate flow chamber system, Chen et al. (2009) investigated the effects of gravity on
http://dx.doi.org/10.1016/j.jconhyd.2015.02.005 0169-7722/© 2015 Elsevier B.V. All rights reserved.
Please cite this article as: Cai, L., et al., Influence of gravity on transport and retention of representative engineered nanoparticles in quartz sand, J. Contam. Hydrol. (2015), http://dx.doi.org/10.1016/j.jconhyd.2015.02.005
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the deposition of both bacteria and microspheres on glass surfaces under both unfavorable and favorable conditions. They found under both unfavorable and favorable conditions, gravity could increase the deposition of both bacteria and microsphere on glass collectors. By employing unit cell models, Ma et al. (2011) examined the effects of gravity on colloid transport and retention in porous media and demonstrated that gravity could also enhance the retention of colloid (in down-flow orientation). Moreover, the effect of gravity was more significant for large (N2 μm) and/or dense (N 1.1 g cm−3) colloids. Unlike the observation that gravity could increase the deposition of micron-sized colloids (Chen et al., 2009), Chrysikopoulos and Syngouna (2014) recently found that gravity could decrease the retention (increased transport) of clay particles in porous media. Obviously, contradicted observations on the effect of gravity on the transport and retention of (bio)colloids have been reported. Moreover, to date, the effect of gravity on the fate and transport of engineered NPs in porous media has not been studied systematically and thus requires further investigation. Hence, this study was designed to fully investigate the role of gravity on the transport and retention of engineered NPs in quartz sand by monitoring both breakthrough curves (BTCs) and retained profiles (RPs) of NPs under various solution conditions. CNTs and GO were used as typical carbon-based nanomaterials; while, nTiO2 and nZnO were employed as typical metal-oxide nanomaterials. Packed column experiments were performed in both up-flow and down-flow orientations. BTCs and RPs of NPs in up-flow orientations were compared with those in down-flow orientations. Possible mechanisms by which gravity affected the transport behavior of NPs were proposed and discussed. 2. Material and methods 2.1. Preparation and characterization of NPs Carboxyl (−COOH) functionalized multi-walled carbon nanotubes (CNTs, purity greater than 95%, in dry form) synthesized by CVD method with 0.7 wt.% metal residual (0.5% Fe and 0.2% Ni) were purchased from Chengdu Organic Chemicals Co. Ltd., China. Graphene oxide (GO) with a planar structure was prepared by a modified Hummers method (Cote et al., 2009) and the detailed synthesis method has been reported previously (Zhang et al., 2013). The oxygen contents of CNTs and GO were 7 at.% and 32 at.%, respectively. Based on previous studies (Jaisi and Elimelech, 2009; Lanphere et al., 2013; Qi et al., 2014; Tian et al., 2012), the CNTs and GO NP stock suspension were prepared by suspending 20 mg CNTs or GO powders in 200 mL Milli-Q water (Q-Gard 1, Millipore Inc., MA) and sonicated for 20 min with a sonicating probe (Ningboxinzhi Biotechnology Ltd., China) at 500 W. The resulting materials were immediately centrifuged (15 min, 5000 g) to remove aggregates/bundles. The supernatant was then transferred carefully into a clean bottle and then sonicated for another 20 min. The total organic carbon (TOC) of the CNTs and GO stock solution was determined to be ~50 and 60 mg L−1, respectively, using a TOC-meter (TOC-VCPN, Shimadzu, Japan). Anatase titanium dioxide powders (nTiO2, purity greater than 99.7%, in dry form) and zinc oxide powders (nZnO, purity
greater than 97%, in dry form) were purchased from SigmaAldrich Corp. (catalog no. 637254 for nTiO2 and 677450 for nZnO). Similar to many previous studies (Cai et al., 2013; Chen et al., 2012; Chowdhury et al., 2011), nTiO2 and nZnO NP stock suspension (1000 mg L−1) was prepared by suspending nTiO2 or nZnO nanopowders in Milli-Q water and sonicating with a sonicating probe (Ningboxinzhi Biotechnology Ltd., China). The morphologies of the prepared CNTs, GO, nTiO2, and nZnO suspensions after sonication (in Milli-Q water) were determined by SEM analysis (FEI Nova Nano SEM 430) and the results were presented in Fig. S1. For transport experiments, the influent concentrations of CNTs and GO were maintained at 10 mg L−1 TOC, while the influent concentration of nTiO2 and nZnO was set at 50 mg L−1. The isoelectric point (IEP) for both CNTs and GO was not found in the wide pH range tested (from pH 3 to 11; Fig. S2). Thus, transport experiments for CNTs and GO were performed at the unadjusted pH (pH 6 and 5 for CNTs and GO, respectively), which could make the experimental conditions to be unfavorable for CNTs and GO deposition. Unlike the carbon-based nanomaterials, nTiO2 and nZnO had IEPs right in the midst of the environmentally relevant pH range, which could create electrostatically favorable or unfavorable conditions for their deposition. Specially, nTiO2 particles used in our previous study had a near-neutral IEP value (~pH 6) (Cai et al., 2013) (Fig. S2), while, the IEP value of nZnO was about pH 9.5 (Fig. S2), which was consistent with previous studies (Kim et al., 2012; Li et al., 2011, 2014). To achieve both favorable and unfavorable conditions for the deposition of NPs, the nTiO2 suspension pH was set to be 5 (favorable deposition) and 7 (unfavorable deposition) by adjusting with 0.1 M HCl or NaOH. For nZnO, an unadjusted solution pH (~7.5) was selected to make the experimental condition to be favorable for nZnO deposition, while, the pH was set to be 10 to make unfavorable condition. It should be noted that the dissolutions of nZnO NPs were quite low at these two pHs (~0.6 wt.%). The ionic strengths of the NP suspensions were 0.1 and 10 mM in NaCl solutions. After preparation, NP suspensions were sonicated at 100 W for 5 min prior to each transport experiment. The zeta potentials of the CNTs (10 mg L−1), GO (10 mg L−1), nTiO2 (50 mg L−1), and nZnO (50 mg L−1), under these conditions, were measured using a Zetasizer Nano ZS90 (Malvern Instruments, UK) (Table S2). Measurements were performed at room temperature (25 °C) and repeated 9–12 times. The particle sizes of CNTs, GO, nTiO2, and nZnO were determined by dynamic light scattering (DLS) measurement (Table S2). The resulted hydrodynamic diameters of CNTs, GO, nTiO2, and nZnO in Milli-Q water after sonication were 189.3 ± 15.9, 396.2 ± 11.4, 436.0 ± 9.9, and 502.4 ± 16.2 nm, respectively. 2.2. Porous media Quartz sand (ultrapure with 99.8% SiO2; Hebeizhensheng Mining Ltd., Shijiazhuang, China) with sizes ranging from 417 to 600 μm, commonly used as model porous media in many previous studies (Chrysikopoulos et al., 2012; Solovitch et al., 2010; Syngouna and Chrysikopoulos, 2012), was used for NP transport experiments in the present study. The procedure used for cleaning the quartz sand was provided in the Supplementary Information (Text S1). The zeta potentials of the crushed quartz sand were also measured under the experimental conditions
Please cite this article as: Cai, L., et al., Influence of gravity on transport and retention of representative engineered nanoparticles in quartz sand, J. Contam. Hydrol. (2015), http://dx.doi.org/10.1016/j.jconhyd.2015.02.005
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using the Zetasizer Nano ZS90 (Table S2). The electrophoretic mobility measurements were repeated 9–12 times. 2.3. Column experiments 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 the packed column was ~0.42. After packing, the columns were pre-equilibrated with at least ten pore volumes (10 PV) of NaCl salt solutions of the desired ionic strength. Following pre-equilibration, 3 PV of NP suspensions were injected into the column, followed by elution with 5 PV of salt solution at the same ionic strength. The same input solutions were injected into the columns in both up-flow and down-flow modes using a syringe pump (Harvard Apparatus Inc., Holliston, MA) to determine the effect of gravity. To maintain the stability of NP suspension and avoid the settlement of NPs, the influent NP suspension was sonicated periodically during the column experiments. 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. Samples from the column effluent were collected in centrifuge tubes at 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 CNTs, GO, nTiO2, and nZnO NPs from the quartz sand, 5–20 mL 0.01 M NaOH solution was added into each sediment segment, and the mixture was manually and vigorously shaken and vortexed for a few seconds. The effluent samples and the supernatant samples from recovery of the retained CNTs, GO, nTiO2, and nZnO NPs were analyzed using UV spectrophotometer (UV-1800, Shimadzu, Japan) at a wavelength of 260, 228, 600, and 380 nm, respectively. The calibration curves of CNTs, GO, nTiO2, and nZnO NPs are presented in Fig. S3. The area under the breakthrough–elution curve was integrated to yield the percentage of NPs that exited the column. The percentage of NPs recovered from the sediment was obtained by summing the amounts of NPs recovered from all segments of the sediment and dividing by the total amount of NPs injected. The sum of the percentage of retained particles and particles that exited the column represented the overall recovery (mass balance) of NPs. Detailed information about NP mass recovery for each experiment is provided in the Supplementary Information (Table S1). 3. Results and discussion 3.1. Influence of gravity on the transport and retention of carbon-based NPs To test whether gravity could affect the transport behavior of carbon-based NPs in quartz sand, the transport behavior of CNTs and GO, two types of representative carbon-based NPs,
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were examined in both up-flow and down-flow orientations. Transport experiments were performed under unfavorable deposition conditions in both 0.1 and 10 mM NaCl solutions (at ~ pH 6 for CNTs and pH 5 for GO). The corresponding BTCs of CNTs and GO are presented in Fig. 1 (a and c). Under the examined solution conditions, zeta potentials of CNTs, GO, and quartz sand were negative (Table S2). Thus, interactions between NPs (both CNTs and GO) and quartz sand were electrostatically repulsive (Fig. S4), which was unfavorable for NP deposition. As a result, relatively high breakthrough of CNTs and GO (with C/Co N 0.5) in quartz sand was observed under all examined conditions. This was true for both up-flow and down-flow orientations (Fig. 1, a and c). For example, in both flow orientations, around 98% of carbon-based NPs (both CNTs and GO) broke through the quartz sand columns in 0.1 mM NaCl solutions. Although the BTCs of both CNTs and GO in 10 mM NaCl solutions were lower than those in 0.1 mM NaCl solutions (consistent with the less negative zeta potentials at high ionic strength), still more than 50% of NPs (~ 55% for CNTs and ~ 80% for GO) passed through the sand columns for both flow orientations. The observation indicated that under unfavorable conditions, both CNTs and GO exhibited high mobility through packed quartz sand in both flow orientations. The obvious breakthrough of the two types of examined carbon-based NPs (CNTs and GO) through porous media columns under unfavorable conditions has been reported previously (Lanphere et al., 2013; Lu et al., 2014). The more noteworthy observation is that for both CNTs and GO, the BTCs in up-flow orientation (Fig. 1, a and c, open symbols) were equivalent to those in down-flow orientation (Fig. 1, a and c, solid symbols). This was true in both 0.1 and 10 mM NaCl solutions. For example, in 10 mM NaCl solutions, 54.43% and 55.28% of CNTs broke through the columns in up-flow and down-flow orientation, respectively. The equivalent BTCs obtained in both flow orientations clearly demonstrated that gravity did not obviously affect the transport of CNTs and GO in either ionic strength under unfavorable conditions. To examine whether gravity would affect the distributions of carbon-based NPs retained in quartz sand, the RPs of both CNTs and GO in two flow orientations (up-flow and downflow) were determined (Fig. 1, b and d). The magnitudes of the retained profiles for both CNTs and GO under all examined conditions varied oppositely to the breakthrough plateaus, as expected from mass balance considerations (Table S1). Specifically, due to ~98% of CNTs and GO passed through columns in both flow orientations, the retention of CNTs and GO in columns in 0.1 mM NaCl solutions was therefore minimal (Fig. 1, b and d, triangle). At high ionic strength (10 mM NaCl), obvious retention was observed for the two types of carbon-based NPs in both up and down flow orientations. Moreover, the retained concentration of CNTs decreased hyper-exponentially with increasing transport distance (Fig. 1b, square). Such hyper-exponential RPs of CNTs observed under unfavorable conditions have also been reported in many previous studies (Jaisi et al., 2008; Wang et al., 2012a). Unlike the hyper-exponential RPs observed for CNTs, the retained concentration of GO did not change with travel distance in 10 mM NaCl solution (Fig. 1d, square). This relatively flat RPs corresponding to high BTCs have also been reported for the transport and deposition of fullerene (nC60) in previous studies
Please cite this article as: Cai, L., et al., Influence of gravity on transport and retention of representative engineered nanoparticles in quartz sand, J. Contam. Hydrol. (2015), http://dx.doi.org/10.1016/j.jconhyd.2015.02.005
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Fig. 1. Breakthrough curves (a, c) and retained profiles (b, d) of CNTs (a, b) and GO (c, d) in both up-flow (open symbol) and down-flow (solid symbol) orientations in 0.1 (triangle) and 10 mM (square) NaCl solutions under unfavorable conditions. C, Co and Cc represent the concentration of effluent, influent and samples recovered from columns, respectively. Replicate experiments were performed under all conditions (n ≥ 2).
(Wang et al., 2008, 2012b). The relatively flat RPs of GO might be due to the high transportability of GO in 10 mM NaCl (as high as ~80% breakthrough in both flow orientations). Similar to the comparable BTCs observed in both flow orientations (Fig. 1, a and c), the retained profiles for both CNTs and GO in up-flow orientation were also equivalent to those in down-flow orientation (Fig. 1, b and d). This held true in both 0.1 and 10 mM NaCl solutions. These results suggested that gravity had negligible effect on the retention of either CNTs or GO in packed quartz sand under unfavorable conditions. Ma et al. (2011) previously showed that for dense colloids (e.g., N1.1 g cm− 3), gravity was a major driver of colloid deposition in unit cell and thus gravity might increase the retention of colloids in porous media. In contrast, very recently, Chrysikopoulos and Syngouna (2014) found that gravity could decrease the retention (increase the transport) of clay particles (with density greater than 1.1 g cm−3) in porous media. It is worth pointing out that, although the densities of both CNTs and GO were greater than 1.1 g cm−3 in the present study (Table S3), the above observations showed that gravity had negligible effect on the transport and retention of both CNTs and GO in porous media under the examined conditions (unfavorable conditions). 3.2. Influence of gravity on the transport and retention of metal-oxide NPs To investigate whether gravity would affect the transport behavior of metal-oxide NPs in quartz sand, the transport behavior of nTiO2 and nZnO, two types of representative metal-
oxide NPs, in quartz sand were examined in both up-flow and down-flow orientations. Column experiments were conducted under unfavorable conditions in both 0.1 and 10 mM NaCl solutions (at pH 7 for nTiO2 and pH 10 for nZnO). The corresponding BTCs of nTiO2 and nZnO are presented in Fig. 2 (a and c). Since zeta potentials of nTiO2, nZnO, and quartz sand were all negative under the examined conditions (Table S2), repulsive electrostatic interaction existed between NPs (both nTiO2 and nZnO) and quartz sand, resulting in unfavorable condition for NP deposition (Fig. S4, c and e). As a result, a portion of nTiO2 and nZnO passed through the columns in both up-flow and down-flow orientations under both ionic strength conditions (especially at low ionic strength). For example, in 0.1 mM NaCl in up-flow orientation, ~78% of nTiO2 and ~65% of nZnO passed through the quartz sand columns. Similar to the observation for carbon-based NPs, the transport of metal-oxide NPs in both flow orientations also decreased with increasing ionic strength from 0.1 to 10 mM NaCl, as indicated by the lower breakthrough plateaus at higher ionic strength (Fig. 2, a and c, square vs. triangle). The lower BTCs of nTiO2 and nZnO with increasing ionic strength observed under unfavorable conditions were consistent with less negative zeta potentials observed at higher ionic strength (Table S2), which were consistent with the previously reported observations (Cai et al., 2014; Jiang et al., 2010). Unlike the equivalent BTCs of carbon-based NPs in both flow orientations (Fig. 1, a and c), the BTCs of nTiO2 and nZnO in down-flow orientation were slightly lower than those in the up-flow orientation under unfavorable conditions (Fig. 2, a and c, solid vs. open symbol). For example, in 0.1 mM NaCl
Please cite this article as: Cai, L., et al., Influence of gravity on transport and retention of representative engineered nanoparticles in quartz sand, J. Contam. Hydrol. (2015), http://dx.doi.org/10.1016/j.jconhyd.2015.02.005
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Fig. 2. Breakthrough curves (a, c) and retained profiles (b, d) of nTiO2 (a, b) and nZnO (c, d) in both up-flow (open symbol) and down-flow (solid symbol) orientations in 0.1 mM (triangle) and 10 mM (square) NaCl solutions under unfavorable conditions. C, Co and Cc represent the concentration of effluent, influent and samples recovered from columns, respectively. Replicate experiments were performed under all conditions (n ≥ 2).
solutions, the percentages of nTiO2 and nZnO that passed through the quartz sand columns in up-flow orientation were 78.40% and 64.65%, respectively, while, the percentages of nTiO2 and nZnO that broke through the columns in down-flow orientation decreased to 70.45% and 58.36%, respectively. The slightly decreased transport of both nTiO2 and nZnO under down-flow orientation versus those in up-flow orientation held true in both 0.1 and 10 mM NaCl solutions. These observations clearly demonstrated that unlike the negligible effect of gravity on the two types of carbon-based NPs, gravity decreased the transport of both nTiO2 and nZnO, two types of representative metal-oxide NPs, in quartz sand under unfavorable conditions in both 0.1 and 10 mM in NaCl solutions. The RPs of both nTiO2 and nZnO under unfavorable conditions were also obtained (Fig. 2, b and d). In both up-flow and down-flow orientations, the retained concentration of both nTiO2 and nZnO decreased hyper-exponentially with increasing transport distance in both 0.1 and 10 mM NaCl solutions (Fig. 2, b and d). The hyper-exponential RPs of both types of metal-oxide NPs were in agreement with previously reported observations (Cai et al., 2013; Chen et al., 2011; Chowdhury et al., 2011; Solovitch et al., 2010). Previous studies found that the mechanisms controlling the transport and retention of nTiO2 and nZnO in porous media under unfavorable attachment conditions included DLVO interaction, straining, and concurrent aggregation (Chen et al., 2011; Chowdhury et al., 2011; Jiang et al., 2012). These factors also played important roles on nTiO2 and nZnO transport and retention under unfavorable conditions in both flow orientations in the present study. As expected from mass balance considerations (Table S1), the amounts of NPs (both
nTiO2 and nZnO) retained in quartz sand were greater in downflow orientation than those in up-flow orientation. Moreover, the excess retention of NPs in down-flow orientation mainly occurred at segments near the column inlet, as indicated by the relatively higher concentration of retained NPs at segments near the column inlet (Fig. 2, b and d, solid vs. open symbol), resulting in relatively steeper RPs of nTiO2 and nZnO in down-flow orientation. These observations demonstrated that unlike the negligible effect of gravity on both transport and retention of the two types of carbon-based NPs, gravity not only had slight influence on the transport of metal-oxide NPs (nTiO2 and nZnO), but also affected the distribution of retained NPs in quartz sand under unfavorable conditions. To examine whether gravity would also affect the transport behavior of nTiO2 and nZnO under favorable conditions, column experiments of nTiO2 and nZnO in both up-flow and down-flow orientations were conducted in 0.1 and 10 mM NaCl solutions at pH 5 for nTiO2 and pH 7.5 for nZnO. Under these conditions, the zeta potentials of nTiO2 and nZnO were positive, while the zeta potentials of quartz sand were negative (Table S2); attractive electrostatic interactions were therefore present between NPs (both nTiO2 and nZnO) and quartz sand, which was favorable for the deposition of NPs (Fig. S4, d and f). Fig. 3 showed that the amount of nTiO2 and nZnO that broke through columns were minimal under favorable conditions in both up-flow and down-flow orientations. This held true in both 0.1 and 10 mM NaCl solutions. The minimal breakthrough of nTiO2 and nZnO in porous media under favorable conditions has also been reported previously (Cai et al., 2013; Jones and Su, 2014; Petosa et al., 2012; Solovitch et al., 2010). Similar to
Please cite this article as: Cai, L., et al., Influence of gravity on transport and retention of representative engineered nanoparticles in quartz sand, J. Contam. Hydrol. (2015), http://dx.doi.org/10.1016/j.jconhyd.2015.02.005
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Fig. 3. Breakthrough curves (a, c) and retained profiles (b, d) of nTiO2 (a, b) and nZnO (c, d) in both up-flow (open symbol) and down-flow (solid symbol) orientations in 0.1 mM (triangle) and 10 mM (square) NaCl solutions under favorable conditions. C, Co and Cc represent the concentration of effluent, influent and samples recovered from columns, respectively. Replicate experiments were performed under all conditions (n ≥ 2).
the observations obtained under unfavorable conditions, the BTCs of nTiO2 and nZnO in down-flow orientation were also slightly lower than those in the up-flow orientation under favorable conditions (Fig. 3, a and c, solid vs. open symbol). For example, in 0.1 mM NaCl solutions, 12.73% of nTiO2 and 11.57% of nZnO passed through the quartz sand columns in up-flow orientation, while, 6.02% of nTiO2 and 7.41% of nZnO broke through the columns in down-flow orientation. The slightly decreased transport of both nTiO2 and nZnO in down-flow orientation versus that in up-flow orientation held true in both 0.1 and 10 mM NaCl solutions. These observations showed that similar to the unfavorable conditions, gravity also decreased the transport of nTiO2 and nZnO under favorable conditions in both 0.1 and 10 mM NaCl solutions. RPs of nTiO2 and nZnO were also obtained in both up-flow and down-flow orientations under favorable conditions, and the results are provided in Fig. 3 (b and d). In both flow orientations, the retained concentration of both nTiO2 and nZnO in quartz sand decreased exponentially with distance under favorable conditions (Fig. 3, b and d), which agreed well with the classic filtration theory prediction. The observation held true in both 0.1 and 10 mM NaCl solutions. The exponential RPs of nTiO2 acquired under favorable conditions have also been reported previously (Cai et al., 2013; Chowdhury et al., 2011). As expected from mass balance (Table S1), the overall retained concentrations of both nTiO2 and nZnO under favorable conditions were higher in down-flow orientation relative to that in up-flow orientation (Fig. 3, b and d, solid vs. open symbol). Moreover, similar to the observations under unfavorable conditions, the excess retention of metal-oxide NPs under favorable conditions
in down-flow orientation also located at segments near the column inlet, which induced relatively steeper retained profiles of NPs in down-flow orientation. The observations indicated that under favorable conditions, gravity affected the transport and retention of nTiO2 and nZnO in quartz sand. The above results demonstrated that unlike the negligible effect of gravity on the transport and retention of carbon-based NPs, gravity decreased the transport and increased the retention of nTiO2 and nZnO NPs in both 0.1 and 10 mM NaCl solutions under both unfavorable and favorable conditions. Our results were consistent with the findings of Chen et al. (2009). By investigating the deposition of microspheres (with different sizes) and bacteria in a parallel plate flow chamber system, Chen et al. (2009) also found that gravity had a considerable effect on the deposition of micrometer-sized colloids on glass surfaces under both unfavorable and favorable conditions. 3.3. Mechanisms affecting the transport and retention of NPs by gravity The effect of gravity on the transport of NPs in different flow orientations would be different. Specifically, in up-flow orientation, gravity is in the downward direction, whereas fluid flow is in upward direction. Thus, gravity would be against with fluid drag and inhibit NPs approaching the quartz sand surfaces. However, in down-flow orientation, gravity and fluid flow are in the same direction. Jointly with fluid drag, gravity thus would help the NPs approaching toward the surfaces of quartz sand for attachment (Ma et al., 2011). Thus, the contribution of gravity on NPs striking the quartz sand surfaces would be
Please cite this article as: Cai, L., et al., Influence of gravity on transport and retention of representative engineered nanoparticles in quartz sand, J. Contam. Hydrol. (2015), http://dx.doi.org/10.1016/j.jconhyd.2015.02.005
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η0¼ ηD þ ηI þ ηG
ηD ¼ 2:4As
1=3
NR
ηI ¼ 0:55As NR
ηG ¼ 0:22NR
ð1Þ −0:081
1:55
−0:24
NPe
NG
NPe
−0:715
−0:125
1:11
Nvdw
N vdw
Nvdw
0:125
0:053
0:052
ð2Þ
ð3Þ
ð4Þ
The detailed calculation equations for ηD, ηI, ηG, and ηo are provided in text S2. The parameter values used in calculations are provided in Table S3. The results of ηD, ηI, and ηG in the size range of 0.01 to 10 μm for CNTs, GO, nTiO2, and nZnO are presented in Tables S4–S7, respectively. The fraction of ηG to ηo represents the degree of gravity term contributing to the transport and retention of colloidal particles in porous media. The fractions of ηG to ηo as function of particle sizes for CNTs, GO, nTiO2, and nZnO were calculated and are shown in Fig. 4. For instance, under unfavorable conditions, the sizes of CNTs, GO, nTiO2, and nZnO in 10 mM NaCl solutions were ~150, ~450, ~1100, and ~800 nm, respectively (Table S2). The corresponding fractions of ηG to ηo for CNTs, GO, nTiO2, and nZnO were found to be ~10%, ~15%, ~80%, and ~85%, respectively (Fig. 4). Clearly, for carbon-based NPs (both CNTs and GO), the fractions of ηG to ηo were only ~10–15%. This observation indicated that the contribution of gravity to carbon-based NPs transport was small, which theoretically explained the similar transport behavior observed in both up-flow and down-flow orientations for both CNTs and GO under unfavorable conditions. In contrast, for metal-oxide NPs, the fractions of ηG to ηo were higher (~80%), indicating that gravity could play an important role in affecting the transport behavior of metal-oxide NPs. As a result, for both nTiO2 and nZnO, increased retention of metal-oxide NPs was obtained in down-flow orientation under unfavorable conditions. It should be noted that due to the comparable NP size (Table S2) and the same density as those under unfavorable conditions, the magnitudes of ηG (derived from the interplay of NP size and density) to ηo for metal-oxide NPs under favorable conditions were similar to those under unfavorable conditions.
CNTs
GO
1.0
nTiO 2
nZnO
0.8
ηG to ηo
different for up-flow and down-flow orientations. It could be hypothesized that whether the gravity would affect the transport behavior of NPs would mainly depend on the gravity contribution to the force balance on NPs. In order to quantitatively determine the contribution of gravity to the transport behavior of engineered NPs in porous media, theoretical calculations were performed for different types of NPs. Colloid transport in porous media is commonly quantified by the single collector contact efficiency (ηo), the ratio of particles striking the collector to particles flowing toward the collector. Correlation equations such as the R–T equation (Rajagopalan and Tien, 1976) and the T–E equation (Tufenkji and Elimelech, 2004) have been previously utilized to calculate ηo by integrating individual contributions of different transport mechanisms (Brownian diffusion, ηD; interception, ηI; and gravitational sedimentation, ηG). The η0 for NPs in the present study was calculated using the following equations (Eqs. (1)–(4)) according to the T–E correlation equation (Tufenkji and Elimelech, 2004).
7
0.6 0.4 0.2 0.0 0.01
0.1
1
10
dp (μμ m) Fig. 4. The fraction of ηG to ηo for different types of engineered NPs in the size range of 0.01 to 10 μm (calculated with the T–E equation (Tufenkji and Elimelech, 2004)).
Thereby, increased retention of metal-oxide NPs was also obtained in down-flow orientation under favorable conditions. Moreover, as shown in Eq. (4), ηG is a function of aspect ratio (NR), gravity number (NG), and the vander Waals number (Nvdw). The variables in determining the ηG are NP size (dp) and density (ρp). Thus, the role of gravity on the transport of NPs involves the interplay of NP size and density. By employing unit cell models, Ma et al. (2011) previously reported that with NG N 0.01, colloid deposition was different for up-flow and down-flow orientations. Under unfavorable conditions, the calculated NG values for CNTs, GO, nTiO2, and nZnO NPs in 10 mM NaCl solutions were 0.0004, 0.0025, 0.0550, and 0.0460, respectively. Obviously, for both CNTs and GO, NG was less than 0.01. The transport of CNTs and GO in up-flow orientation thus was similar to that in down-flow orientation. For both types of metal-oxide NPs, NG was larger than 0.01. Accordingly, transport behavior of metal-oxide NPs in up-flow orientation differed from those in down-flow orientations, which is in well agreement with that previously predicted based on unit cell models (Ma et al., 2011). 4. Conclusion Using packed quartz sand columns, we directly compared the BTCs and RPs of carbon-based NPs (CNTs and GO) and metal-oxide NPs (nTiO2 and nZnO) in both up-flow (gravitynegative) and down-flow (gravity-positive) orientations. Results showed that gravity had negligible effect on the transport and retention of carbon-based NPs (e.g., CNTs and GO) under unfavorable conditions (typically encountered in natural environment), whereas it increased the retention (decreased transport) of metal-oxide based NPs (e.g., nTiO2 and nZnO) under both unfavorable and favorable conditions. Theoretical calculations were performed to help interpret the role of gravity on the transport and retention behaviors of NPs in quartz sand. This study showed that whether gravity would affect the transport and retention of NPs was controlled by the contribution of gravity to the force balance on NPs, which can be determined by the interplay of the density and sizes of aggregates under the examined solution conditions. Therefore, for metal-oxide NPs especially those with high density and
Please cite this article as: Cai, L., et al., Influence of gravity on transport and retention of representative engineered nanoparticles in quartz sand, J. Contam. Hydrol. (2015), http://dx.doi.org/10.1016/j.jconhyd.2015.02.005
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large sizes, the effects of gravity on metal-oxide NPs transport could not be negligible. Acknowledgment This work was supported by the National Natural Science Foundation of China under grant no. 41422106 and 21377006, and the program for New Century Excellent Talents in University under grant no. NCET-13-0010. We also acknowledge the editor and four anonymous reviewers for their valuable help in the review and revision process. Appendix A. Supplementary data Mass balance of all column experiments (Tables S1); zeta potentials and particle sizes of CNTs, GO, nTiO2, and nZnO as well as zeta potentials of bare quartz sand in various background solutions (Tables S2); Parameters used for the Tufenkji-Elimelech correlation equation (Table S3); Calculated magnitude of diffusion, interception and gravity terms of CNTs, GO, nTiO2, and nZnO (Tables S4-S7); SEM images of CNTs, GO, nTiO2, and nZnO in suspensions (Fig. S1); Zeta potentials of CNTs, GO, nTiO2, and nZnO as a function of pH (Fig. S2); Calibration curves of CNTs, GO, nTiO2, and nZnO (Fig. S3); DLVO energy profiles (Fig. S4); and additional details on methods are provided in the Supplementary Information. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jconhyd.2015.02.005. References Cai, L., Tong, M., Ma, H., Kim, H., 2013. Cotransport of titanium dioxide and fullerene nanoparticles in saturated porous media. Environ. Sci. Technol. 47 (11), 5703–5710. Cai, L., Tong, M., Wang, X., Kim, H., 2014. Influence of clay particles on the transport and retention of titanium dioxide nanoparticles in quartz sand. Environ. Sci. Technol. 48 (13), 7323–7332. Chen, G., Hong, Y., Walker, S.L., 2009. Colloidal and bacterial deposition: role of gravity. Langmuir 26 (1), 314–319. Chen, G.X., Liu, X.Y., Su, C.M., 2011. Transport and retention of TiO2 rutile nanoparticles in saturated porous media under low-ionic-strength conditions: measurements and mechanisms. Langmuir 27 (9), 5393–5402. Chen, G., Liu, X., Su, C., 2012. Distinct effects of humic acid on transport and retention of TiO2 rutile nanoparticles in saturated sand columns. Environ. Sci. Technol. 46 (13), 7142–7150. Chowdhury, I., Hong, Y., Honda, R.J., Walker, S.L., 2011. Mechanisms of TiO2 nanoparticle transport in porous media: role of solution chemistry, nanoparticle concentration, and flowrate. J. Colloid Interface Sci. 360 (2), 548–555. Chowdhury, I., Cwiertny, D.M., Walker, S.L., 2012. Combined factors influencing the aggregation and deposition of nano-TiO2 in the presence of humic acid and bacteria. Environ. Sci. Technol. 46 (13), 6968–6976. Chrysikopoulos, C.V., Syngouna, V.I., 2014. Effect of gravity on colloid transport through water-saturated columns packed with glass beads: modeling and experiments. Environ. Sci. Technol. 48 (12), 6805–6813. Chrysikopoulos, C., Syngouna, V., Vasiliadou, I., Katzourakis, V., 2012. Transport of Pseudomonas putida in a 3-D bench scale experimental aquifer. Transp. Porous Media 94 (3), 617–642. Cote, L.J., Kim, F., Huang, J., 2009. Langmuir–Blodgett assembly of graphite oxide single layers. J. Am. Chem. Soc. 131 (3), 1043–1049. Fang, J., Shan, X., Wen, B., Huang, R., 2013. Mobility of TX100 suspended multiwalled carbon nanotubes (MWCNTs) and the facilitated transport of phenanthrene in real soil columns. Geoderma 207, 1–7. French, R.A., et al., 2009. Influence of ionic strength, pH, and cation valence on aggregation kinetics of titanium dioxide nanoparticles. Environ. Sci. Technol. 43 (5), 1354–1359. Hu, W., et al., 2011. Protein corona-mediated mitigation of cytotoxicity of graphene oxide. ACS Nano 5 (5), 3693–3700. Jaisi, D.P., Elimelech, M., 2009. Single-walled carbon nanotubes exhibit limited transport in soil columns. Environ. Sci. Technol. 43 (24), 9161–9166.
Jaisi, D.P., Saleh, N.B., Blake, R.E., Elimelech, M., 2008. Transport of single-walled carbon nanotubes in porous media: filtration mechanisms and reversibility. Environ. Sci. Technol. 42 (22), 8317–8323. Jiang, X., Tong, M., Li, H., Yang, K., 2010. Deposition kinetics of zinc oxide nanoparticles on natural organic matter coated silica surfaces. J. Colloid Interface Sci. 350 (2), 427–434. Jiang, X., Tong, M., Lu, R., Kim, H., 2012. Transport and deposition of ZnO nanoparticles in saturated porous media. Colloids Surf. A Physicochem. Eng. Asp. 401, 29–37. Jones, E.H., Su, C., 2014. Transport and retention of zinc oxide nanoparticles in porous media: effects of natural organic matter versus natural organic ligands at circumneutral pH. J. Hazard. Mater. 275, 79–88. Kim, K.-M., et al., 2012. Colloidal behaviors of ZnO nanoparticles in various aqueous media. Toxicol. Environ. Heal. Sci. 4 (2), 121–131. Lanphere, J.D., Luth, C.J., Walker, S.L., 2013. Effects of solution chemistry on the transport of graphene oxide in saturated porous media. Environ. Sci. Technol. 47 (9), 4255–4261. Lecoanet, H.F., Wiesner, M.R., 2004. Velocity effects on fullerene and oxide nanoparticle deposition in porous media. Environ. Sci. Technol. 38 (16), 4377–4382. Li, M., Zhu, L., Lin, D., 2011. Toxicity of ZnO nanoparticles to Escherichia coli: mechanism and the influence of medium components. Environ. Sci. Technol. 45 (5), 1977–1983. Li, Y., Niu, J., Zhang, W., Zhang, L., Shang, E., 2014. Influence of aqueous media on the ROS-mediated toxicity of ZnO nanoparticles toward green gluorescent protein-expressing Escherichia coli under UV-365 irradiation. Langmuir 30 (10), 2852–2862. Liu, X., O'Carroll, D.M., Petersen, E.J., Huang, Q., Anderson, C.L., 2009. Mobility of multiwalled carbon nanotubes in porous media. Environ. Sci. Technol. 43 (21), 8153–8158. Lu, Y., Xu, X., Yang, K., Lin, D., 2013. The effects of surfactants and solution chemistry on the transport of multiwalled carbon nanotubes in quartz sand-packed columns. Environ. Pollut. 182, 269–277. Lu, Y., Yang, K., Lin, D., 2014. Transport of surfactant-facilitated multiwalled carbon nanotube suspensions in columns packed with sized soil particles. Environ. Pollut. 192, 36–43. Ma, H., Pazmino, E.F., Johnson, W.P., 2011. Gravitational settling effects on unit cell predictions of colloidal retention in porous media in the absence of energy barriers. Environ. Sci. Technol. 45 (19), 8306–8312. Mauter, M.S., Elimelech, M., 2008. Environmental applications of carbon-based nanomaterials. Environ. Sci. Technol. 42 (16), 5843–5859. Petosa, A.R., Brennan, S.J., Rajput, F., Tufenkji, N., 2012. Transport of two metal oxide nanoparticles in saturated granular porous media: role of water chemistry and particle coating. Water Res. 46 (4), 1273–1285. Qi, Z., Hou, L., Zhu, D., Ji, R., Chen, W., 2014. Enhanced transport of phenanthrene and 1-naphthol by colloidal graphene oxide nanoparticles in saturated soil. Environ. Sci. Technol. 48 (17), 10136–10144. Rajagopalan, R., Tien, C., 1976. Trajectory analysis of deep-bed filtration with the sphere-in-cell porous media model. AlChE J. 22 (3), 523–533. Soares, J.W., Whitten, J.E., Oblas, D.W., Steeves, D.M., 2008. Novel photoluminescence properties of surface-modified nanocrystalline zinc oxide: toward a reactive scaffold. Langmuir 24 (2), 371–374. Solovitch, N., et al., 2010. Concurrent aggregation and deposition of TiO2 nanoparticles in a sandy porous media. Environ. Sci. Technol. 44 (13), 4897–4902. Syngouna, V.I., Chrysikopoulos, C.V., 2012. Transport of biocolloids in water saturated columns packed with sand: effect of grain size and pore water velocity. J. Contam. Hydrol. 129, 11–24. Tian, Y., et al., 2012. Deposition and transport of functionalized carbon nanotubes in water-saturated sand columns. J. Hazard. Mater. 213–214 (0), 265–272. Tufenkji, N., Elimelech, M., 2004. Correlation equation for predicting singlecollector efficiency in physicochemical filtration in saturated porous media. Environ. Sci. Technol. 38 (2), 529–536. Wan, J., Tokunaga, T.K., Tsang, C.F., 1995. Bacterial sedimentation through a porous medium. Water Resour. Res. 31 (7), 1627–1636. Wang, Y., et al., 2008. Transport and retention of nanoscale C60 aggregates in water-saturated porous media. Environ. Sci. Technol. 42 (10), 3588–3594. Wang, Y., Kim, J.-H., Baek, J.-B., Miller, G.W., Pennell, K.D., 2012a. Transport behavior of functionalized multi-wall carbon nanotubes in water-saturated quartz sand as a function of tube length. Water Res. 46 (14), 4521–4531. Wang, Y., Li, Y., Costanza, J., Abriola, L.M., Pennell, K.D., 2012b. Enhanced mobility of fullerene (C60) nanoparticles in the presence of stabilizing agents. Environ. Sci. Technol. 46 (21), 11761–11769. Zhang, C., Hao, R., Liao, H., Hou, Y., 2013. Synthesis of amino-functionalized graphene as metal-free catalyst and exploration of the roles of various nitrogen states in oxygen reduction reaction. Nano Energy 2 (1), 88–97.
Please cite this article as: Cai, L., et al., Influence of gravity on transport and retention of representative engineered nanoparticles in quartz sand, J. Contam. Hydrol. (2015), http://dx.doi.org/10.1016/j.jconhyd.2015.02.005
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Journal of Contaminant Hydrology journal homepage: www.elsevier.com/locate/jconhyd
Influence of gravity on transport and retention of representative engineered nanoparticles in quartz sand Li Cai a, Jinghan Zhu b, Yanglong Hou b, Meiping Tong a,⁎, Hyunjung Kim c,⁎⁎ a b c
The Key Laboratory of Water and Sediment Sciences, Ministry of Education, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, PR China Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, PR China Department of Mineral Resources and Energy Engineering, Chonbuk National University, Jeonju, Jeonbuk 561-756, Republic of Korea
a r t i c l e
i n f o
Article history: Received 21 November 2014 Received in revised form 30 January 2015 Accepted 8 February 2015 Available online xxxx Keywords: Engineered nanoparticle Gravity Transport Retention Quartz sand
a b s t r a c t Four types of NPs: carbon nanotubes and graphene oxide (carbon-based NPs), titanium dioxide and zinc oxide metal-oxide NPs, were utilized to systematically determine the influence of gravity on the transport of NPs in porous media. Packed column experiments for two types of carbonbased NPs were performed under unfavorable conditions in both up-flow (gravity-negative) and down-flow (gravity-positive) orientations, while for two types of metal-oxide NPs, experiments were performed under both unfavorable and favorable conditions in both up-flow and down-flow orientations. Both breakthrough curves and retained profiles of two types of carbon-based NPs in up-flow orientation were equivalent to those in down-flow orientation, indicating that gravity had negligible effect on the transport and retention of carbon-based NPs under unfavorable conditions. In contrast, under both unfavorable and favorable conditions, the breakthrough curves for two types of metal-oxide NPs in down-flow orientation were lower relative to those in up-flow orientation, indicating that gravity could decrease the transport of metal-oxide NPs in porous media. The distinct effect of gravity on the transport and retention of carbon-based and metaloxide NPs was mainly attributed to the contribution of gravity to the force balance on the NPs in quartz sand. The contribution of gravity was determined by the interplay of the density and sizes of NP aggregates under examined solution conditions. © 2015 Elsevier B.V. All rights reserved.
1. Introduction With the rapid growth of the nanotechnology industry, different types of nanomaterials such as carbon-based nanoparticles (NPs) and metal/metal-oxide NPs have been fabricated and used increasingly in various fields. Carbon nanotubes (CNTs), graphene oxide (GO), titanium dioxide (nTiO2), and zinc oxide (nZnO) NPs, have been regarded as the most important nanomaterials due to their various applications (Hu et al., 2011; Mauter and Elimelech, 2008; Soares et al., 2008). The massive applications would eventually result in the
⁎ Corresponding author. Tel.: +86 1062756491; fax: +86 1062756526. ⁎⁎ Corresponding author. Tel.: +82 632702370; fax: +82 632702366. E-mail addresses: [email protected] (M. Tong), [email protected] (H. Kim).
entrance of these NPs into the natural environment. To date, physicochemical factors such as fluid velocity (Lecoanet and Wiesner, 2004), solution chemistry (pH, ionic strength, and ion type) (French et al., 2009; Jiang et al., 2012; Lanphere et al., 2013; Liu et al., 2009), NP concentration (Chowdhury et al., 2011; Jaisi and Elimelech, 2009), natural organic matter (Chowdhury et al., 2012; Jones and Su, 2014), surfactant (Fang et al., 2013; Lu et al., 2013, 2014), as well as bacteria (Chowdhury et al., 2012; Jones and Su, 2014) have been shown to significantly affect the transport behavior of NPs in porous media. A few previous studies found that gravity could also affect the transport and retention of (bio)colloids (Chen et al., 2009; Chrysikopoulos and Syngouna, 2014; Ma et al., 2011; Wan et al., 1995). For instance, by using a parallel plate flow chamber system, Chen et al. (2009) investigated the effects of gravity on
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Please cite this article as: Cai, L., et al., Influence of gravity on transport and retention of representative engineered nanoparticles in quartz sand, J. Contam. Hydrol. (2015), http://dx.doi.org/10.1016/j.jconhyd.2015.02.005
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the deposition of both bacteria and microspheres on glass surfaces under both unfavorable and favorable conditions. They found under both unfavorable and favorable conditions, gravity could increase the deposition of both bacteria and microsphere on glass collectors. By employing unit cell models, Ma et al. (2011) examined the effects of gravity on colloid transport and retention in porous media and demonstrated that gravity could also enhance the retention of colloid (in down-flow orientation). Moreover, the effect of gravity was more significant for large (N2 μm) and/or dense (N 1.1 g cm−3) colloids. Unlike the observation that gravity could increase the deposition of micron-sized colloids (Chen et al., 2009), Chrysikopoulos and Syngouna (2014) recently found that gravity could decrease the retention (increased transport) of clay particles in porous media. Obviously, contradicted observations on the effect of gravity on the transport and retention of (bio)colloids have been reported. Moreover, to date, the effect of gravity on the fate and transport of engineered NPs in porous media has not been studied systematically and thus requires further investigation. Hence, this study was designed to fully investigate the role of gravity on the transport and retention of engineered NPs in quartz sand by monitoring both breakthrough curves (BTCs) and retained profiles (RPs) of NPs under various solution conditions. CNTs and GO were used as typical carbon-based nanomaterials; while, nTiO2 and nZnO were employed as typical metal-oxide nanomaterials. Packed column experiments were performed in both up-flow and down-flow orientations. BTCs and RPs of NPs in up-flow orientations were compared with those in down-flow orientations. Possible mechanisms by which gravity affected the transport behavior of NPs were proposed and discussed. 2. Material and methods 2.1. Preparation and characterization of NPs Carboxyl (−COOH) functionalized multi-walled carbon nanotubes (CNTs, purity greater than 95%, in dry form) synthesized by CVD method with 0.7 wt.% metal residual (0.5% Fe and 0.2% Ni) were purchased from Chengdu Organic Chemicals Co. Ltd., China. Graphene oxide (GO) with a planar structure was prepared by a modified Hummers method (Cote et al., 2009) and the detailed synthesis method has been reported previously (Zhang et al., 2013). The oxygen contents of CNTs and GO were 7 at.% and 32 at.%, respectively. Based on previous studies (Jaisi and Elimelech, 2009; Lanphere et al., 2013; Qi et al., 2014; Tian et al., 2012), the CNTs and GO NP stock suspension were prepared by suspending 20 mg CNTs or GO powders in 200 mL Milli-Q water (Q-Gard 1, Millipore Inc., MA) and sonicated for 20 min with a sonicating probe (Ningboxinzhi Biotechnology Ltd., China) at 500 W. The resulting materials were immediately centrifuged (15 min, 5000 g) to remove aggregates/bundles. The supernatant was then transferred carefully into a clean bottle and then sonicated for another 20 min. The total organic carbon (TOC) of the CNTs and GO stock solution was determined to be ~50 and 60 mg L−1, respectively, using a TOC-meter (TOC-VCPN, Shimadzu, Japan). Anatase titanium dioxide powders (nTiO2, purity greater than 99.7%, in dry form) and zinc oxide powders (nZnO, purity
greater than 97%, in dry form) were purchased from SigmaAldrich Corp. (catalog no. 637254 for nTiO2 and 677450 for nZnO). Similar to many previous studies (Cai et al., 2013; Chen et al., 2012; Chowdhury et al., 2011), nTiO2 and nZnO NP stock suspension (1000 mg L−1) was prepared by suspending nTiO2 or nZnO nanopowders in Milli-Q water and sonicating with a sonicating probe (Ningboxinzhi Biotechnology Ltd., China). The morphologies of the prepared CNTs, GO, nTiO2, and nZnO suspensions after sonication (in Milli-Q water) were determined by SEM analysis (FEI Nova Nano SEM 430) and the results were presented in Fig. S1. For transport experiments, the influent concentrations of CNTs and GO were maintained at 10 mg L−1 TOC, while the influent concentration of nTiO2 and nZnO was set at 50 mg L−1. The isoelectric point (IEP) for both CNTs and GO was not found in the wide pH range tested (from pH 3 to 11; Fig. S2). Thus, transport experiments for CNTs and GO were performed at the unadjusted pH (pH 6 and 5 for CNTs and GO, respectively), which could make the experimental conditions to be unfavorable for CNTs and GO deposition. Unlike the carbon-based nanomaterials, nTiO2 and nZnO had IEPs right in the midst of the environmentally relevant pH range, which could create electrostatically favorable or unfavorable conditions for their deposition. Specially, nTiO2 particles used in our previous study had a near-neutral IEP value (~pH 6) (Cai et al., 2013) (Fig. S2), while, the IEP value of nZnO was about pH 9.5 (Fig. S2), which was consistent with previous studies (Kim et al., 2012; Li et al., 2011, 2014). To achieve both favorable and unfavorable conditions for the deposition of NPs, the nTiO2 suspension pH was set to be 5 (favorable deposition) and 7 (unfavorable deposition) by adjusting with 0.1 M HCl or NaOH. For nZnO, an unadjusted solution pH (~7.5) was selected to make the experimental condition to be favorable for nZnO deposition, while, the pH was set to be 10 to make unfavorable condition. It should be noted that the dissolutions of nZnO NPs were quite low at these two pHs (~0.6 wt.%). The ionic strengths of the NP suspensions were 0.1 and 10 mM in NaCl solutions. After preparation, NP suspensions were sonicated at 100 W for 5 min prior to each transport experiment. The zeta potentials of the CNTs (10 mg L−1), GO (10 mg L−1), nTiO2 (50 mg L−1), and nZnO (50 mg L−1), under these conditions, were measured using a Zetasizer Nano ZS90 (Malvern Instruments, UK) (Table S2). Measurements were performed at room temperature (25 °C) and repeated 9–12 times. The particle sizes of CNTs, GO, nTiO2, and nZnO were determined by dynamic light scattering (DLS) measurement (Table S2). The resulted hydrodynamic diameters of CNTs, GO, nTiO2, and nZnO in Milli-Q water after sonication were 189.3 ± 15.9, 396.2 ± 11.4, 436.0 ± 9.9, and 502.4 ± 16.2 nm, respectively. 2.2. Porous media Quartz sand (ultrapure with 99.8% SiO2; Hebeizhensheng Mining Ltd., Shijiazhuang, China) with sizes ranging from 417 to 600 μm, commonly used as model porous media in many previous studies (Chrysikopoulos et al., 2012; Solovitch et al., 2010; Syngouna and Chrysikopoulos, 2012), was used for NP transport experiments in the present study. The procedure used for cleaning the quartz sand was provided in the Supplementary Information (Text S1). The zeta potentials of the crushed quartz sand were also measured under the experimental conditions
Please cite this article as: Cai, L., et al., Influence of gravity on transport and retention of representative engineered nanoparticles in quartz sand, J. Contam. Hydrol. (2015), http://dx.doi.org/10.1016/j.jconhyd.2015.02.005
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using the Zetasizer Nano ZS90 (Table S2). The electrophoretic mobility measurements were repeated 9–12 times. 2.3. Column experiments 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 the packed column was ~0.42. After packing, the columns were pre-equilibrated with at least ten pore volumes (10 PV) of NaCl salt solutions of the desired ionic strength. Following pre-equilibration, 3 PV of NP suspensions were injected into the column, followed by elution with 5 PV of salt solution at the same ionic strength. The same input solutions were injected into the columns in both up-flow and down-flow modes using a syringe pump (Harvard Apparatus Inc., Holliston, MA) to determine the effect of gravity. To maintain the stability of NP suspension and avoid the settlement of NPs, the influent NP suspension was sonicated periodically during the column experiments. 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. Samples from the column effluent were collected in centrifuge tubes at 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 CNTs, GO, nTiO2, and nZnO NPs from the quartz sand, 5–20 mL 0.01 M NaOH solution was added into each sediment segment, and the mixture was manually and vigorously shaken and vortexed for a few seconds. The effluent samples and the supernatant samples from recovery of the retained CNTs, GO, nTiO2, and nZnO NPs were analyzed using UV spectrophotometer (UV-1800, Shimadzu, Japan) at a wavelength of 260, 228, 600, and 380 nm, respectively. The calibration curves of CNTs, GO, nTiO2, and nZnO NPs are presented in Fig. S3. The area under the breakthrough–elution curve was integrated to yield the percentage of NPs that exited the column. The percentage of NPs recovered from the sediment was obtained by summing the amounts of NPs recovered from all segments of the sediment and dividing by the total amount of NPs injected. The sum of the percentage of retained particles and particles that exited the column represented the overall recovery (mass balance) of NPs. Detailed information about NP mass recovery for each experiment is provided in the Supplementary Information (Table S1). 3. Results and discussion 3.1. Influence of gravity on the transport and retention of carbon-based NPs To test whether gravity could affect the transport behavior of carbon-based NPs in quartz sand, the transport behavior of CNTs and GO, two types of representative carbon-based NPs,
3
were examined in both up-flow and down-flow orientations. Transport experiments were performed under unfavorable deposition conditions in both 0.1 and 10 mM NaCl solutions (at ~ pH 6 for CNTs and pH 5 for GO). The corresponding BTCs of CNTs and GO are presented in Fig. 1 (a and c). Under the examined solution conditions, zeta potentials of CNTs, GO, and quartz sand were negative (Table S2). Thus, interactions between NPs (both CNTs and GO) and quartz sand were electrostatically repulsive (Fig. S4), which was unfavorable for NP deposition. As a result, relatively high breakthrough of CNTs and GO (with C/Co N 0.5) in quartz sand was observed under all examined conditions. This was true for both up-flow and down-flow orientations (Fig. 1, a and c). For example, in both flow orientations, around 98% of carbon-based NPs (both CNTs and GO) broke through the quartz sand columns in 0.1 mM NaCl solutions. Although the BTCs of both CNTs and GO in 10 mM NaCl solutions were lower than those in 0.1 mM NaCl solutions (consistent with the less negative zeta potentials at high ionic strength), still more than 50% of NPs (~ 55% for CNTs and ~ 80% for GO) passed through the sand columns for both flow orientations. The observation indicated that under unfavorable conditions, both CNTs and GO exhibited high mobility through packed quartz sand in both flow orientations. The obvious breakthrough of the two types of examined carbon-based NPs (CNTs and GO) through porous media columns under unfavorable conditions has been reported previously (Lanphere et al., 2013; Lu et al., 2014). The more noteworthy observation is that for both CNTs and GO, the BTCs in up-flow orientation (Fig. 1, a and c, open symbols) were equivalent to those in down-flow orientation (Fig. 1, a and c, solid symbols). This was true in both 0.1 and 10 mM NaCl solutions. For example, in 10 mM NaCl solutions, 54.43% and 55.28% of CNTs broke through the columns in up-flow and down-flow orientation, respectively. The equivalent BTCs obtained in both flow orientations clearly demonstrated that gravity did not obviously affect the transport of CNTs and GO in either ionic strength under unfavorable conditions. To examine whether gravity would affect the distributions of carbon-based NPs retained in quartz sand, the RPs of both CNTs and GO in two flow orientations (up-flow and downflow) were determined (Fig. 1, b and d). The magnitudes of the retained profiles for both CNTs and GO under all examined conditions varied oppositely to the breakthrough plateaus, as expected from mass balance considerations (Table S1). Specifically, due to ~98% of CNTs and GO passed through columns in both flow orientations, the retention of CNTs and GO in columns in 0.1 mM NaCl solutions was therefore minimal (Fig. 1, b and d, triangle). At high ionic strength (10 mM NaCl), obvious retention was observed for the two types of carbon-based NPs in both up and down flow orientations. Moreover, the retained concentration of CNTs decreased hyper-exponentially with increasing transport distance (Fig. 1b, square). Such hyper-exponential RPs of CNTs observed under unfavorable conditions have also been reported in many previous studies (Jaisi et al., 2008; Wang et al., 2012a). Unlike the hyper-exponential RPs observed for CNTs, the retained concentration of GO did not change with travel distance in 10 mM NaCl solution (Fig. 1d, square). This relatively flat RPs corresponding to high BTCs have also been reported for the transport and deposition of fullerene (nC60) in previous studies
Please cite this article as: Cai, L., et al., Influence of gravity on transport and retention of representative engineered nanoparticles in quartz sand, J. Contam. Hydrol. (2015), http://dx.doi.org/10.1016/j.jconhyd.2015.02.005
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upflow, 0.1mM upflow, 10 mM 1.2
1
a) unfav
CNT (Cc/Co) /g Sand
1.0 0.8
C/Co
downflow, 0.1mM downflow, 10mM
0.6 0.4
b) unfav
CNT
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GO
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1E-3 0.00 1
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GO
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0.01
1E-3 0.00
0.02
0.04
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0.08
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Distance (m)
Fig. 1. Breakthrough curves (a, c) and retained profiles (b, d) of CNTs (a, b) and GO (c, d) in both up-flow (open symbol) and down-flow (solid symbol) orientations in 0.1 (triangle) and 10 mM (square) NaCl solutions under unfavorable conditions. C, Co and Cc represent the concentration of effluent, influent and samples recovered from columns, respectively. Replicate experiments were performed under all conditions (n ≥ 2).
(Wang et al., 2008, 2012b). The relatively flat RPs of GO might be due to the high transportability of GO in 10 mM NaCl (as high as ~80% breakthrough in both flow orientations). Similar to the comparable BTCs observed in both flow orientations (Fig. 1, a and c), the retained profiles for both CNTs and GO in up-flow orientation were also equivalent to those in down-flow orientation (Fig. 1, b and d). This held true in both 0.1 and 10 mM NaCl solutions. These results suggested that gravity had negligible effect on the retention of either CNTs or GO in packed quartz sand under unfavorable conditions. Ma et al. (2011) previously showed that for dense colloids (e.g., N1.1 g cm− 3), gravity was a major driver of colloid deposition in unit cell and thus gravity might increase the retention of colloids in porous media. In contrast, very recently, Chrysikopoulos and Syngouna (2014) found that gravity could decrease the retention (increase the transport) of clay particles (with density greater than 1.1 g cm−3) in porous media. It is worth pointing out that, although the densities of both CNTs and GO were greater than 1.1 g cm−3 in the present study (Table S3), the above observations showed that gravity had negligible effect on the transport and retention of both CNTs and GO in porous media under the examined conditions (unfavorable conditions). 3.2. Influence of gravity on the transport and retention of metal-oxide NPs To investigate whether gravity would affect the transport behavior of metal-oxide NPs in quartz sand, the transport behavior of nTiO2 and nZnO, two types of representative metal-
oxide NPs, in quartz sand were examined in both up-flow and down-flow orientations. Column experiments were conducted under unfavorable conditions in both 0.1 and 10 mM NaCl solutions (at pH 7 for nTiO2 and pH 10 for nZnO). The corresponding BTCs of nTiO2 and nZnO are presented in Fig. 2 (a and c). Since zeta potentials of nTiO2, nZnO, and quartz sand were all negative under the examined conditions (Table S2), repulsive electrostatic interaction existed between NPs (both nTiO2 and nZnO) and quartz sand, resulting in unfavorable condition for NP deposition (Fig. S4, c and e). As a result, a portion of nTiO2 and nZnO passed through the columns in both up-flow and down-flow orientations under both ionic strength conditions (especially at low ionic strength). For example, in 0.1 mM NaCl in up-flow orientation, ~78% of nTiO2 and ~65% of nZnO passed through the quartz sand columns. Similar to the observation for carbon-based NPs, the transport of metal-oxide NPs in both flow orientations also decreased with increasing ionic strength from 0.1 to 10 mM NaCl, as indicated by the lower breakthrough plateaus at higher ionic strength (Fig. 2, a and c, square vs. triangle). The lower BTCs of nTiO2 and nZnO with increasing ionic strength observed under unfavorable conditions were consistent with less negative zeta potentials observed at higher ionic strength (Table S2), which were consistent with the previously reported observations (Cai et al., 2014; Jiang et al., 2010). Unlike the equivalent BTCs of carbon-based NPs in both flow orientations (Fig. 1, a and c), the BTCs of nTiO2 and nZnO in down-flow orientation were slightly lower than those in the up-flow orientation under unfavorable conditions (Fig. 2, a and c, solid vs. open symbol). For example, in 0.1 mM NaCl
Please cite this article as: Cai, L., et al., Influence of gravity on transport and retention of representative engineered nanoparticles in quartz sand, J. Contam. Hydrol. (2015), http://dx.doi.org/10.1016/j.jconhyd.2015.02.005
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upflow, 0.1 mM upflow, 10 mM 1.0
TiO2 (Cc/Co) /g Sand
C/Co
0.8 0.6 0.4 0.2 0
1.0
2
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0.01 0.00
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TiO2
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a) unfav
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0.08
0.10
ZnO
0.1
0.01 0.00
0.02
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Distance (m)
Fig. 2. Breakthrough curves (a, c) and retained profiles (b, d) of nTiO2 (a, b) and nZnO (c, d) in both up-flow (open symbol) and down-flow (solid symbol) orientations in 0.1 mM (triangle) and 10 mM (square) NaCl solutions under unfavorable conditions. C, Co and Cc represent the concentration of effluent, influent and samples recovered from columns, respectively. Replicate experiments were performed under all conditions (n ≥ 2).
solutions, the percentages of nTiO2 and nZnO that passed through the quartz sand columns in up-flow orientation were 78.40% and 64.65%, respectively, while, the percentages of nTiO2 and nZnO that broke through the columns in down-flow orientation decreased to 70.45% and 58.36%, respectively. The slightly decreased transport of both nTiO2 and nZnO under down-flow orientation versus those in up-flow orientation held true in both 0.1 and 10 mM NaCl solutions. These observations clearly demonstrated that unlike the negligible effect of gravity on the two types of carbon-based NPs, gravity decreased the transport of both nTiO2 and nZnO, two types of representative metal-oxide NPs, in quartz sand under unfavorable conditions in both 0.1 and 10 mM in NaCl solutions. The RPs of both nTiO2 and nZnO under unfavorable conditions were also obtained (Fig. 2, b and d). In both up-flow and down-flow orientations, the retained concentration of both nTiO2 and nZnO decreased hyper-exponentially with increasing transport distance in both 0.1 and 10 mM NaCl solutions (Fig. 2, b and d). The hyper-exponential RPs of both types of metal-oxide NPs were in agreement with previously reported observations (Cai et al., 2013; Chen et al., 2011; Chowdhury et al., 2011; Solovitch et al., 2010). Previous studies found that the mechanisms controlling the transport and retention of nTiO2 and nZnO in porous media under unfavorable attachment conditions included DLVO interaction, straining, and concurrent aggregation (Chen et al., 2011; Chowdhury et al., 2011; Jiang et al., 2012). These factors also played important roles on nTiO2 and nZnO transport and retention under unfavorable conditions in both flow orientations in the present study. As expected from mass balance considerations (Table S1), the amounts of NPs (both
nTiO2 and nZnO) retained in quartz sand were greater in downflow orientation than those in up-flow orientation. Moreover, the excess retention of NPs in down-flow orientation mainly occurred at segments near the column inlet, as indicated by the relatively higher concentration of retained NPs at segments near the column inlet (Fig. 2, b and d, solid vs. open symbol), resulting in relatively steeper RPs of nTiO2 and nZnO in down-flow orientation. These observations demonstrated that unlike the negligible effect of gravity on both transport and retention of the two types of carbon-based NPs, gravity not only had slight influence on the transport of metal-oxide NPs (nTiO2 and nZnO), but also affected the distribution of retained NPs in quartz sand under unfavorable conditions. To examine whether gravity would also affect the transport behavior of nTiO2 and nZnO under favorable conditions, column experiments of nTiO2 and nZnO in both up-flow and down-flow orientations were conducted in 0.1 and 10 mM NaCl solutions at pH 5 for nTiO2 and pH 7.5 for nZnO. Under these conditions, the zeta potentials of nTiO2 and nZnO were positive, while the zeta potentials of quartz sand were negative (Table S2); attractive electrostatic interactions were therefore present between NPs (both nTiO2 and nZnO) and quartz sand, which was favorable for the deposition of NPs (Fig. S4, d and f). Fig. 3 showed that the amount of nTiO2 and nZnO that broke through columns were minimal under favorable conditions in both up-flow and down-flow orientations. This held true in both 0.1 and 10 mM NaCl solutions. The minimal breakthrough of nTiO2 and nZnO in porous media under favorable conditions has also been reported previously (Cai et al., 2013; Jones and Su, 2014; Petosa et al., 2012; Solovitch et al., 2010). Similar to
Please cite this article as: Cai, L., et al., Influence of gravity on transport and retention of representative engineered nanoparticles in quartz sand, J. Contam. Hydrol. (2015), http://dx.doi.org/10.1016/j.jconhyd.2015.02.005
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upflow, 0.1 mM upflow, 10 mM
(Cc/Co) /g Sand
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TiO2
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Fig. 3. Breakthrough curves (a, c) and retained profiles (b, d) of nTiO2 (a, b) and nZnO (c, d) in both up-flow (open symbol) and down-flow (solid symbol) orientations in 0.1 mM (triangle) and 10 mM (square) NaCl solutions under favorable conditions. C, Co and Cc represent the concentration of effluent, influent and samples recovered from columns, respectively. Replicate experiments were performed under all conditions (n ≥ 2).
the observations obtained under unfavorable conditions, the BTCs of nTiO2 and nZnO in down-flow orientation were also slightly lower than those in the up-flow orientation under favorable conditions (Fig. 3, a and c, solid vs. open symbol). For example, in 0.1 mM NaCl solutions, 12.73% of nTiO2 and 11.57% of nZnO passed through the quartz sand columns in up-flow orientation, while, 6.02% of nTiO2 and 7.41% of nZnO broke through the columns in down-flow orientation. The slightly decreased transport of both nTiO2 and nZnO in down-flow orientation versus that in up-flow orientation held true in both 0.1 and 10 mM NaCl solutions. These observations showed that similar to the unfavorable conditions, gravity also decreased the transport of nTiO2 and nZnO under favorable conditions in both 0.1 and 10 mM NaCl solutions. RPs of nTiO2 and nZnO were also obtained in both up-flow and down-flow orientations under favorable conditions, and the results are provided in Fig. 3 (b and d). In both flow orientations, the retained concentration of both nTiO2 and nZnO in quartz sand decreased exponentially with distance under favorable conditions (Fig. 3, b and d), which agreed well with the classic filtration theory prediction. The observation held true in both 0.1 and 10 mM NaCl solutions. The exponential RPs of nTiO2 acquired under favorable conditions have also been reported previously (Cai et al., 2013; Chowdhury et al., 2011). As expected from mass balance (Table S1), the overall retained concentrations of both nTiO2 and nZnO under favorable conditions were higher in down-flow orientation relative to that in up-flow orientation (Fig. 3, b and d, solid vs. open symbol). Moreover, similar to the observations under unfavorable conditions, the excess retention of metal-oxide NPs under favorable conditions
in down-flow orientation also located at segments near the column inlet, which induced relatively steeper retained profiles of NPs in down-flow orientation. The observations indicated that under favorable conditions, gravity affected the transport and retention of nTiO2 and nZnO in quartz sand. The above results demonstrated that unlike the negligible effect of gravity on the transport and retention of carbon-based NPs, gravity decreased the transport and increased the retention of nTiO2 and nZnO NPs in both 0.1 and 10 mM NaCl solutions under both unfavorable and favorable conditions. Our results were consistent with the findings of Chen et al. (2009). By investigating the deposition of microspheres (with different sizes) and bacteria in a parallel plate flow chamber system, Chen et al. (2009) also found that gravity had a considerable effect on the deposition of micrometer-sized colloids on glass surfaces under both unfavorable and favorable conditions. 3.3. Mechanisms affecting the transport and retention of NPs by gravity The effect of gravity on the transport of NPs in different flow orientations would be different. Specifically, in up-flow orientation, gravity is in the downward direction, whereas fluid flow is in upward direction. Thus, gravity would be against with fluid drag and inhibit NPs approaching the quartz sand surfaces. However, in down-flow orientation, gravity and fluid flow are in the same direction. Jointly with fluid drag, gravity thus would help the NPs approaching toward the surfaces of quartz sand for attachment (Ma et al., 2011). Thus, the contribution of gravity on NPs striking the quartz sand surfaces would be
Please cite this article as: Cai, L., et al., Influence of gravity on transport and retention of representative engineered nanoparticles in quartz sand, J. Contam. Hydrol. (2015), http://dx.doi.org/10.1016/j.jconhyd.2015.02.005
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η0¼ ηD þ ηI þ ηG
ηD ¼ 2:4As
1=3
NR
ηI ¼ 0:55As NR
ηG ¼ 0:22NR
ð1Þ −0:081
1:55
−0:24
NPe
NG
NPe
−0:715
−0:125
1:11
Nvdw
N vdw
Nvdw
0:125
0:053
0:052
ð2Þ
ð3Þ
ð4Þ
The detailed calculation equations for ηD, ηI, ηG, and ηo are provided in text S2. The parameter values used in calculations are provided in Table S3. The results of ηD, ηI, and ηG in the size range of 0.01 to 10 μm for CNTs, GO, nTiO2, and nZnO are presented in Tables S4–S7, respectively. The fraction of ηG to ηo represents the degree of gravity term contributing to the transport and retention of colloidal particles in porous media. The fractions of ηG to ηo as function of particle sizes for CNTs, GO, nTiO2, and nZnO were calculated and are shown in Fig. 4. For instance, under unfavorable conditions, the sizes of CNTs, GO, nTiO2, and nZnO in 10 mM NaCl solutions were ~150, ~450, ~1100, and ~800 nm, respectively (Table S2). The corresponding fractions of ηG to ηo for CNTs, GO, nTiO2, and nZnO were found to be ~10%, ~15%, ~80%, and ~85%, respectively (Fig. 4). Clearly, for carbon-based NPs (both CNTs and GO), the fractions of ηG to ηo were only ~10–15%. This observation indicated that the contribution of gravity to carbon-based NPs transport was small, which theoretically explained the similar transport behavior observed in both up-flow and down-flow orientations for both CNTs and GO under unfavorable conditions. In contrast, for metal-oxide NPs, the fractions of ηG to ηo were higher (~80%), indicating that gravity could play an important role in affecting the transport behavior of metal-oxide NPs. As a result, for both nTiO2 and nZnO, increased retention of metal-oxide NPs was obtained in down-flow orientation under unfavorable conditions. It should be noted that due to the comparable NP size (Table S2) and the same density as those under unfavorable conditions, the magnitudes of ηG (derived from the interplay of NP size and density) to ηo for metal-oxide NPs under favorable conditions were similar to those under unfavorable conditions.
CNTs
GO
1.0
nTiO 2
nZnO
0.8
ηG to ηo
different for up-flow and down-flow orientations. It could be hypothesized that whether the gravity would affect the transport behavior of NPs would mainly depend on the gravity contribution to the force balance on NPs. In order to quantitatively determine the contribution of gravity to the transport behavior of engineered NPs in porous media, theoretical calculations were performed for different types of NPs. Colloid transport in porous media is commonly quantified by the single collector contact efficiency (ηo), the ratio of particles striking the collector to particles flowing toward the collector. Correlation equations such as the R–T equation (Rajagopalan and Tien, 1976) and the T–E equation (Tufenkji and Elimelech, 2004) have been previously utilized to calculate ηo by integrating individual contributions of different transport mechanisms (Brownian diffusion, ηD; interception, ηI; and gravitational sedimentation, ηG). The η0 for NPs in the present study was calculated using the following equations (Eqs. (1)–(4)) according to the T–E correlation equation (Tufenkji and Elimelech, 2004).
7
0.6 0.4 0.2 0.0 0.01
0.1
1
10
dp (μμ m) Fig. 4. The fraction of ηG to ηo for different types of engineered NPs in the size range of 0.01 to 10 μm (calculated with the T–E equation (Tufenkji and Elimelech, 2004)).
Thereby, increased retention of metal-oxide NPs was also obtained in down-flow orientation under favorable conditions. Moreover, as shown in Eq. (4), ηG is a function of aspect ratio (NR), gravity number (NG), and the vander Waals number (Nvdw). The variables in determining the ηG are NP size (dp) and density (ρp). Thus, the role of gravity on the transport of NPs involves the interplay of NP size and density. By employing unit cell models, Ma et al. (2011) previously reported that with NG N 0.01, colloid deposition was different for up-flow and down-flow orientations. Under unfavorable conditions, the calculated NG values for CNTs, GO, nTiO2, and nZnO NPs in 10 mM NaCl solutions were 0.0004, 0.0025, 0.0550, and 0.0460, respectively. Obviously, for both CNTs and GO, NG was less than 0.01. The transport of CNTs and GO in up-flow orientation thus was similar to that in down-flow orientation. For both types of metal-oxide NPs, NG was larger than 0.01. Accordingly, transport behavior of metal-oxide NPs in up-flow orientation differed from those in down-flow orientations, which is in well agreement with that previously predicted based on unit cell models (Ma et al., 2011). 4. Conclusion Using packed quartz sand columns, we directly compared the BTCs and RPs of carbon-based NPs (CNTs and GO) and metal-oxide NPs (nTiO2 and nZnO) in both up-flow (gravitynegative) and down-flow (gravity-positive) orientations. Results showed that gravity had negligible effect on the transport and retention of carbon-based NPs (e.g., CNTs and GO) under unfavorable conditions (typically encountered in natural environment), whereas it increased the retention (decreased transport) of metal-oxide based NPs (e.g., nTiO2 and nZnO) under both unfavorable and favorable conditions. Theoretical calculations were performed to help interpret the role of gravity on the transport and retention behaviors of NPs in quartz sand. This study showed that whether gravity would affect the transport and retention of NPs was controlled by the contribution of gravity to the force balance on NPs, which can be determined by the interplay of the density and sizes of aggregates under the examined solution conditions. Therefore, for metal-oxide NPs especially those with high density and
Please cite this article as: Cai, L., et al., Influence of gravity on transport and retention of representative engineered nanoparticles in quartz sand, J. Contam. Hydrol. (2015), http://dx.doi.org/10.1016/j.jconhyd.2015.02.005
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large sizes, the effects of gravity on metal-oxide NPs transport could not be negligible. Acknowledgment This work was supported by the National Natural Science Foundation of China under grant no. 41422106 and 21377006, and the program for New Century Excellent Talents in University under grant no. NCET-13-0010. We also acknowledge the editor and four anonymous reviewers for their valuable help in the review and revision process. Appendix A. Supplementary data Mass balance of all column experiments (Tables S1); zeta potentials and particle sizes of CNTs, GO, nTiO2, and nZnO as well as zeta potentials of bare quartz sand in various background solutions (Tables S2); Parameters used for the Tufenkji-Elimelech correlation equation (Table S3); Calculated magnitude of diffusion, interception and gravity terms of CNTs, GO, nTiO2, and nZnO (Tables S4-S7); SEM images of CNTs, GO, nTiO2, and nZnO in suspensions (Fig. S1); Zeta potentials of CNTs, GO, nTiO2, and nZnO as a function of pH (Fig. S2); Calibration curves of CNTs, GO, nTiO2, and nZnO (Fig. S3); DLVO energy profiles (Fig. S4); and additional details on methods are provided in the Supplementary Information. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jconhyd.2015.02.005. References Cai, L., Tong, M., Ma, H., Kim, H., 2013. Cotransport of titanium dioxide and fullerene nanoparticles in saturated porous media. Environ. Sci. Technol. 47 (11), 5703–5710. Cai, L., Tong, M., Wang, X., Kim, H., 2014. Influence of clay particles on the transport and retention of titanium dioxide nanoparticles in quartz sand. Environ. Sci. Technol. 48 (13), 7323–7332. Chen, G., Hong, Y., Walker, S.L., 2009. Colloidal and bacterial deposition: role of gravity. Langmuir 26 (1), 314–319. Chen, G.X., Liu, X.Y., Su, C.M., 2011. Transport and retention of TiO2 rutile nanoparticles in saturated porous media under low-ionic-strength conditions: measurements and mechanisms. Langmuir 27 (9), 5393–5402. Chen, G., Liu, X., Su, C., 2012. Distinct effects of humic acid on transport and retention of TiO2 rutile nanoparticles in saturated sand columns. Environ. Sci. Technol. 46 (13), 7142–7150. Chowdhury, I., Hong, Y., Honda, R.J., Walker, S.L., 2011. Mechanisms of TiO2 nanoparticle transport in porous media: role of solution chemistry, nanoparticle concentration, and flowrate. J. Colloid Interface Sci. 360 (2), 548–555. Chowdhury, I., Cwiertny, D.M., Walker, S.L., 2012. Combined factors influencing the aggregation and deposition of nano-TiO2 in the presence of humic acid and bacteria. Environ. Sci. Technol. 46 (13), 6968–6976. Chrysikopoulos, C.V., Syngouna, V.I., 2014. Effect of gravity on colloid transport through water-saturated columns packed with glass beads: modeling and experiments. Environ. Sci. Technol. 48 (12), 6805–6813. Chrysikopoulos, C., Syngouna, V., Vasiliadou, I., Katzourakis, V., 2012. Transport of Pseudomonas putida in a 3-D bench scale experimental aquifer. Transp. Porous Media 94 (3), 617–642. Cote, L.J., Kim, F., Huang, J., 2009. Langmuir–Blodgett assembly of graphite oxide single layers. J. Am. Chem. Soc. 131 (3), 1043–1049. Fang, J., Shan, X., Wen, B., Huang, R., 2013. Mobility of TX100 suspended multiwalled carbon nanotubes (MWCNTs) and the facilitated transport of phenanthrene in real soil columns. Geoderma 207, 1–7. French, R.A., et al., 2009. Influence of ionic strength, pH, and cation valence on aggregation kinetics of titanium dioxide nanoparticles. Environ. Sci. Technol. 43 (5), 1354–1359. Hu, W., et al., 2011. Protein corona-mediated mitigation of cytotoxicity of graphene oxide. ACS Nano 5 (5), 3693–3700. Jaisi, D.P., Elimelech, M., 2009. Single-walled carbon nanotubes exhibit limited transport in soil columns. Environ. Sci. Technol. 43 (24), 9161–9166.
Jaisi, D.P., Saleh, N.B., Blake, R.E., Elimelech, M., 2008. Transport of single-walled carbon nanotubes in porous media: filtration mechanisms and reversibility. Environ. Sci. Technol. 42 (22), 8317–8323. Jiang, X., Tong, M., Li, H., Yang, K., 2010. Deposition kinetics of zinc oxide nanoparticles on natural organic matter coated silica surfaces. J. Colloid Interface Sci. 350 (2), 427–434. Jiang, X., Tong, M., Lu, R., Kim, H., 2012. Transport and deposition of ZnO nanoparticles in saturated porous media. Colloids Surf. A Physicochem. Eng. Asp. 401, 29–37. Jones, E.H., Su, C., 2014. Transport and retention of zinc oxide nanoparticles in porous media: effects of natural organic matter versus natural organic ligands at circumneutral pH. J. Hazard. Mater. 275, 79–88. Kim, K.-M., et al., 2012. Colloidal behaviors of ZnO nanoparticles in various aqueous media. Toxicol. Environ. Heal. Sci. 4 (2), 121–131. Lanphere, J.D., Luth, C.J., Walker, S.L., 2013. Effects of solution chemistry on the transport of graphene oxide in saturated porous media. Environ. Sci. Technol. 47 (9), 4255–4261. Lecoanet, H.F., Wiesner, M.R., 2004. Velocity effects on fullerene and oxide nanoparticle deposition in porous media. Environ. Sci. Technol. 38 (16), 4377–4382. Li, M., Zhu, L., Lin, D., 2011. Toxicity of ZnO nanoparticles to Escherichia coli: mechanism and the influence of medium components. Environ. Sci. Technol. 45 (5), 1977–1983. Li, Y., Niu, J., Zhang, W., Zhang, L., Shang, E., 2014. Influence of aqueous media on the ROS-mediated toxicity of ZnO nanoparticles toward green gluorescent protein-expressing Escherichia coli under UV-365 irradiation. Langmuir 30 (10), 2852–2862. Liu, X., O'Carroll, D.M., Petersen, E.J., Huang, Q., Anderson, C.L., 2009. Mobility of multiwalled carbon nanotubes in porous media. Environ. Sci. Technol. 43 (21), 8153–8158. Lu, Y., Xu, X., Yang, K., Lin, D., 2013. The effects of surfactants and solution chemistry on the transport of multiwalled carbon nanotubes in quartz sand-packed columns. Environ. Pollut. 182, 269–277. Lu, Y., Yang, K., Lin, D., 2014. Transport of surfactant-facilitated multiwalled carbon nanotube suspensions in columns packed with sized soil particles. Environ. Pollut. 192, 36–43. Ma, H., Pazmino, E.F., Johnson, W.P., 2011. Gravitational settling effects on unit cell predictions of colloidal retention in porous media in the absence of energy barriers. Environ. Sci. Technol. 45 (19), 8306–8312. Mauter, M.S., Elimelech, M., 2008. Environmental applications of carbon-based nanomaterials. Environ. Sci. Technol. 42 (16), 5843–5859. Petosa, A.R., Brennan, S.J., Rajput, F., Tufenkji, N., 2012. Transport of two metal oxide nanoparticles in saturated granular porous media: role of water chemistry and particle coating. Water Res. 46 (4), 1273–1285. Qi, Z., Hou, L., Zhu, D., Ji, R., Chen, W., 2014. Enhanced transport of phenanthrene and 1-naphthol by colloidal graphene oxide nanoparticles in saturated soil. Environ. Sci. Technol. 48 (17), 10136–10144. Rajagopalan, R., Tien, C., 1976. Trajectory analysis of deep-bed filtration with the sphere-in-cell porous media model. AlChE J. 22 (3), 523–533. Soares, J.W., Whitten, J.E., Oblas, D.W., Steeves, D.M., 2008. Novel photoluminescence properties of surface-modified nanocrystalline zinc oxide: toward a reactive scaffold. Langmuir 24 (2), 371–374. Solovitch, N., et al., 2010. Concurrent aggregation and deposition of TiO2 nanoparticles in a sandy porous media. Environ. Sci. Technol. 44 (13), 4897–4902. Syngouna, V.I., Chrysikopoulos, C.V., 2012. Transport of biocolloids in water saturated columns packed with sand: effect of grain size and pore water velocity. J. Contam. Hydrol. 129, 11–24. Tian, Y., et al., 2012. Deposition and transport of functionalized carbon nanotubes in water-saturated sand columns. J. Hazard. Mater. 213–214 (0), 265–272. Tufenkji, N., Elimelech, M., 2004. Correlation equation for predicting singlecollector efficiency in physicochemical filtration in saturated porous media. Environ. Sci. Technol. 38 (2), 529–536. Wan, J., Tokunaga, T.K., Tsang, C.F., 1995. Bacterial sedimentation through a porous medium. Water Resour. Res. 31 (7), 1627–1636. Wang, Y., et al., 2008. Transport and retention of nanoscale C60 aggregates in water-saturated porous media. Environ. Sci. Technol. 42 (10), 3588–3594. Wang, Y., Kim, J.-H., Baek, J.-B., Miller, G.W., Pennell, K.D., 2012a. Transport behavior of functionalized multi-wall carbon nanotubes in water-saturated quartz sand as a function of tube length. Water Res. 46 (14), 4521–4531. Wang, Y., Li, Y., Costanza, J., Abriola, L.M., Pennell, K.D., 2012b. Enhanced mobility of fullerene (C60) nanoparticles in the presence of stabilizing agents. Environ. Sci. Technol. 46 (21), 11761–11769. Zhang, C., Hao, R., Liao, H., Hou, Y., 2013. Synthesis of amino-functionalized graphene as metal-free catalyst and exploration of the roles of various nitrogen states in oxygen reduction reaction. Nano Energy 2 (1), 88–97.
Please cite this article as: Cai, L., et al., Influence of gravity on transport and retention of representative engineered nanoparticles in quartz sand, J. Contam. Hydrol. (2015), http://dx.doi.org/10.1016/j.jconhyd.2015.02.005