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Effects of temperature on graphene oxide deposition and transport in saturated porous media
Accepted Manuscript Title: Effects of temperature on graphene oxide deposition and transport in saturated porous media Authors: Mei Wang, Bin Gao, Deshan Tang, Huimin Sun, Xianqiang Yin, Congrong Yu PII: DOI: Reference:
S0304-3894(17)30095-X http://dx.doi.org/doi:10.1016/j.jhazmat.2017.02.014 HAZMAT 18369
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
24-12-2016 9-2-2017 10-2-2017
Please cite this article as: Mei Wang, Bin Gao, Deshan Tang, Huimin Sun, Xianqiang Yin, Congrong Yu, Effects of temperature on graphene oxide deposition and transport in saturated porous media, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2017.02.014 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effects of temperature on graphene oxide deposition and transport in saturated porous media
Mei Wang1, 2, Bin Gao2*, Deshan Tang1, Huimin Sun3, 2, Xianqiang Yin3, 2, Congrong Yu4
1. College of Water Conservancy and Hydropower Engineering, Hohai University, Nanjing 210098, China 2. Department of Agricultural and Biological Engineering, University of Florida, Gainesville, FL 32611, USA 3. College of Resources and Environment, Northwest A&F University, Yangling, Shaanxi 712100, China 4. College of Hydrology and Water Conservancy and Water Resources, Hohai University, Nanjing 210098, China
_______________________ * Corresponding author, phone: (352) 392-1864 ext. 285, Fax: (352) 392-4092, email: [email protected]
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Graphical abstract
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Research highlights Temperature affected GO sorption onto sand for all of the tested conditions Deposition of GO in saturated porous media is endothermic Temperature had little effect on GO retention and transport in porous media at low IS Temperature showed notable effects on GO retention and transport at high IS For all combinations of sand type and grain size, high temperature promoted GO mobility
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Abstract Laboratory batch sorption and sand column experiments were conducted to examine the effects of temperature (6 and 24 oC) on the retention and transport of GO in water-saturated porous media with different combination of solution ionic strength (IS, 1 and 10 mM), sand type (natural and acid-cleaned), and grain size (coarse and fine). Although results from batch sorption experiment showed that temperature affected the sorption of GO onto the sand grains at the low IS, the interactions between GO and the sand were relatively weak, which did make the temperature effect prominent. When the IS was 1 mM, experimental temperature showed little effect on GO retention and transport regardless of the medium properties. GO was highly mobile in the sand columns with mass recovery rates ranged from 77.3% to 92.4%. When the IS increased to 10 mM, temperature showed notable effects on GO retention and transport in saturated porous media. For all the combinations of sand type and grain size, the higher the temperature was, the less mobile GO particles were. The effects of temperature on GO retention and transport in saturated porous media were further verified though simulations from an advection–dispersion-reaction model. Keywords: graphene oxide; transport modeling; temperature; retention; ionic strength
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1. Introduction Nanomaterials have brought a new technological revolution in the last decade and will still be in the spotlight of engineered materials in the foreseeable future. Among the existing engineered nanomaterials, carbon-based family consisting of different members such as fullerene, graphene, carbon nanotube, etc., is a crucial branch [1]. Graphene, a new nanomaterial that was first manufactured in 2004 [2], is projected to revolutionize the 21st century because of its supernormal properties embodied in many aspects, such as electronics, thermotics, mechanics, magnetics and optics. However, strong hydrophobicity and easy aggregation limit the applications of graphene in a certain degree. Compared with graphene, graphene oxide (GO), an important derivative of graphene [3], shows good dispersibility in water due to the presence of a number of oxygen-containing functional groups on its surface [4]. Additionally, these functional groups also expand the inter-laminar spacing between carbon layers, providing favorable conditions for GO to be used as an ideal precursor of preparing graphene-based nanocomposites. GO thus is being applied widely in many promising applications (e.g., biosensors, electronic devices, drug delivery, energy storage, composite materials, etc. [5, 6]). Similar to other engineered nanomaterials, GO has also been released into the environment during the manufacturing, using and disposing processes and eventually into the food chain, posing a potential threat to the health of humans and other living beings. Some research [7-9] has reported that GO nanoparticles can serve as superior carriers for organic contaminants and heavy metals in saturated soil and thus further threaten the safety of groundwater resources. In addition, 5
numerous toxicity studies have documented that GO nanoparticles may be toxic to the environment and human health. Previous investigations have also demonstrated that carbon nanoparticles entering the food chains may cause severe and persistent lung injuries that can induce the risk of airways disease, fibrosis, cardiovascular disease, or cancer [10, 11]. With the knowledge of the potential risks of GO in the subsurface systems, it is in an urgent need to advance current understanding of the fate and transport of GO in porous media. To date, many factors including ionic strength [12], pH [13], surfactants [14], organic matters [15], flow velocity and direction [8, 16], input concentration [17], grain size [17], moisture content [18], etc., have been found to play important roles in governing GO fate and transport in porous media. However, several other possible important factors, such as temperature, sunlight, redox potential, and air pressure have not yet been addressed [19]. Soil temperature varies periodically with diurnal or seasonal variations of solar radiation and is accordingly disturbed in different degrees as a function of depth. The temperature of shallow groundwater in a small forested catchment (New Brunswick, Canada) ranges from 4°C in February to approximately 14°C in August at a depth of 1.5 m, and stays at the range of 6-8 °C all year around at a depth of 8.75 m [20]. Previous studies have already demonstrated that temperature affects soil organic matter decomposition and stability [21, 22], potassium ion (K+) release [23], cation valence [24], and soil porosity [25]. Temperature is projected to be also an influential factor that cannot be neglected for the nanoparticle transport in porous media. A previous study conducted a theoretical analysis of the effects of solution temperature on particle 6
transport in aquifers and soils and predicted that higher solution temperature, generally accompanied by more severe Brownian motion and lower solution viscosity, leads to weaker mobility of colloids [26]. Despite its importance, none of the previous studies has systematically investigated the effect of temperature on the retention and transport of engineered nanoparticles including GO in porous media. In this work, a series of laboratory batch sorption and column experiments were conducted to determine effects of temperature on the retention and transport of GO in water-saturated porous media under different ionic strength (IS), sand type, and grain size conditions. Two types of sand (natural sand and acid-cleaned sand) with two size ranges were selected as experimental porous medium. Two laboratory temperatures and two IS conditions were also considered in these studies. The specific objectives were as follows: (1) examine the stability of GO at different temperatures in porous media; (2) compare the sorption of GO onto the sand medium under different temperature conditions; (3) determine the effects of temperature on the retention and transport of GO in saturated porous media under various conditions; and (4) model the retention and transport of GO in saturated porous media under the tested conditions.
2. Materials and methods 2.1. GO The single layer GO nanosheets (ACS Material, Medford, MA) used in this study were produced using modified Hummers method [27]. Based on the information provided by the 7
manufacture, the GO particles are in the diameter range of 1-5 um and the thickness range of 0.8-1.2 nm. The stock suspension was prepared by ultra-sonicating 100 mg of original GO (as received from the manufacture) in 1000 mL deionized (DI) water with Misonix S3000 ultrasonicator (QSonica, Newtown, CT) for 2 hours. Two concentrations of electrolyte solutions (i.e., 10 and 100 mM KCl) were independently used to dilute the stock GO suspension to a constant GO concentration of 20 mg/L at a desired IS (1 mM or 10 mM) in this study. Subsequently, in each experiment, a small part of the diluted GO suspensions was set aside and served in cuvettes for stability test, and the left were used as working suspensions of GO (i.e., for sorption and column experiments) under varying physicochemical conditions. The hydrodynamic diameter and zeta potential of GO particles were determined using a ZetaSizer (Malvern Instruments, Worcestershire, U.K.) and a ZetaPlus (Brookhaven Instrument Co., Holtsville, NY), respectively. Stability of GO suspensions in varying electrolyte solutions and under two temperature conditions was monitored with an Evolution 60 UV-Vis Spectrophotometer (Thermo Scientific, Waltham, MA) at a wavelength of 230 nm over time for 7 h. The cuvette with about 3.5 ml GO sample standing in the cell holder was measured at set intervals to obtain the GO concentrations. The stability experiments were performed in duplicate.
2.2. Porous media Quartz sand (Standard Sand & Silica Co.) was used as the experimental porous medium and 8
was sieved into two size ranges of 0.3-0.4 mm (fine) and 0.9-1.0 mm (coarse). The sand experienced a sequential wash by tap water and deionized (DI) water and then was oven dried at 105 ºC. After these steps, half of the sand was stored for future use and labeled as natural sand; and the other half was subsequently soaked with 10% nitric acid (v/v) for 24 h, rinsed with DI water, dried again in an oven at 105 ºC and finally marked as acid-cleaned sand. According to SEM-EDS and ICP-AES analyses in a previous study [28], metal impurities, such as Fe, Al and Ni, did exist on the surface of natural sand while the acid-cleaned sand surface did not have any impurities. The zeta potential of sand was measured by the ZetaPlus.
2.3. Sorption experiment Batch experiments were performed to determine whether temperature can affect GO sorption onto the sand, thereby verifying the possibility of temperature dependence of GO transport in porous media. The experimental conditions of the various sorption experiments performed in this study are summarized in Table 1, keeping a one-to-one relationship with the corresponding transport experiments. All samples were prepared in triplicate, so nine 8-ml plastic tubes were used for each attachment experiment, of which every three tubes contained 6 ml GO suspension at a desired IS with 2 g sand, 6 ml GO suspension at the corresponding IS without sand, and 6 ml KCL solution at the corresponding IS with 2 g sand, respectively. The second group of samples acted as a baseline to determine the attachment capacity of GO in sand by comparing the concentrations before and after attachment. The third group of samples was set to 9
help eliminate the disturbance caused by sand in the shaking process on the first group of samples. Two sheets of parafilm were employed to cover each tube for avoiding liquid leakage. When experimental samples were ready, all samples were gently shaken at 50 rpm in a shaker (Heidolph Promax 1020, Buch & Holm) for 2 hours in a desired temperature condition. After that, UV-vis Spectrophotometer was used to measure the concentrations of samples at 230 nm. The sorption experiments were performed in triplicate.
2.4. Column experiment The quartz sand of different sizes (fine and coarse) and surface properties (natural and acid-cleaned) was wet-packed into an acrylic column of 2.5 cm inner diameter and 12.5 cm length. Stainless membranes with 50 um mesh (Spectra/Mesh, Spectrum Laboratories, Inc) were used on both sides of the column to stop the sand from escaping the column and to distribute the flow. Prior to a transport experiment, hydrodynamic stabilization was established by flushing the column with 5 pore volumes (PVs) of DI water and then 5 PVs of background electrolyte solution at a chosen IS which matched the GO working suspension. A peristaltic pump (Masterflex L/S, Cole Parmer Instrument, Vernon Hills, IL) was used for solution injection at the constant flow rate of 1 mL/min (Darcy velocity of 0.2 cm/min). Then, 2 PVs of working suspension of GO were introduced to the sand column, followed by flushing with about 3 PVs of the corresponding background electrolyte solution. A fraction collector (IS-95 Interval Sampler, 10
Spectrum Chromatography, Houston, TX) was used to collect the effluent at 4-min intervals. The concentrations of GO in the effluent were measured immediately after sample collection with UV-vis Spectrophotometer. In addition, a number of tracer tests with potassium nitrate (KNO3) as the tracer were conducted separately to obtain the breakthrough curves (BTCs) with different temperatures and collector grain sizes. The injected KNO3 concentration was 14 mg/L and the breakthrough concentrations were quantified using UV-vis Spectrophotometer. Because the nitrate ion (NO3-) has the advantage of highest absorbance at the wavelength of 201 nm and no absorption observed near the wavelength of 275 nm, the actual absorbance of the effluent was determined by the difference between the absorptions measured at 201 nm and 275 nm. In this way could be erased the possible interferences of impurities. All the column experiments were performed in duplicate.
2.5. Mathematical models The extended Derjaguin–Landau–Verwey–Overbeek (XDLVO) theory was used to better interpret the stability and deposition of GO nanoparticles in GO-water-GO and GO-water-sand systems. This theory considers van der Waals attraction, electric double layer repulsion, and Lewis acid-base interactions [17]. Details of the calculation of the three interaction forces can be found in previous works [13, 17]. Retention and transport of GO in saturated sand columns were simulated with Hydrus-1D 11
package. The governing equation of GO retention and transport processes can be written as:
C 2C C S D 2 q b t z t z
b
S kC t
(1
S S max
(1) (2)
)
(3)
where θ is the volumetric water content (dimensionless), C is the nanoparticle concentration in the aqueous phase (ML-3), D is the hydrodynamic dispersion coefficient (L2T-1), q is the Darcy velocity (LT-1), ρb is the porous medium bulk density (ML-3), S is the solid phrase nanoparticle concentration (MM-1), Smax is the maximum solid phrase nanoparticle concentration (MM-1), ψ is a dimensionless nanoparticle retention function, k is the first-order deposition (attachment) coefficient (T-1), t is the time (T), and z is the travel distance in the direction of flow (L). This model was first applied to the NO3- breakthrough data (obtained from tracer tests) to optimize D. Then, measured concentration data of GO (breakthrough curves) were simulated inversely fitted using the Levenberg-Marquardt algorithm to botain the best-fit values of Smax and k.
3. Results and discussion 3.1 Effects of temperature on GO stability under different IS conditions The surface properties of GO particles and sand under different conditions are listed in Table 2. Fig. 1 shows the stability of GO as a function of time in the 1 and 10 mM KCl solution at 6 and 24 ºC. Across all the experimental conditions, the GO nanoparticles showed excellent 12
stability, which is consistent with the predictions from XDLVO theory (Fig. 2) that the experimental conditions were unfavorable for GO aggregation. The XDLVO energy profiles showed strong energy barriers over 10.6 mJ/m2 between GO particles for both temperatures, almost impossible for the GO particles in the size range of 1-5 um to overcome for aggregation in the primary minima. As a result, temperature showed little effect on the stability of GO in this work. After a 7-hour-long observation period, the largest absolute temporal change of relative GO concentration was only 0.025C0 at 10 mM KCl under 24 ºC. Even though the effects of temperature and IS on the stability of GO were both negligible in this study, the stability curves suggest that GO was slightly more stable at lower temperature and lower IS. The IS effects on GO stability can be related to difference in thickness of the electric double layer that affects the electrostatic repulsion forces and thus DLVO interactions [29]. In this study, the energy barriers for the IS of 1mM (16.6 mJ/m2 and 16.5 mJ/m2 at 6 ºC and 24 ºC, respectively) were higher than those for the IS of 10 mM (11.1 mJ/m2 and 10.6 mJ/m2 ). Further, higher IS (i.e., 10 mM) also introduced secondary minima to the XDLVO profiles at both tested temperatures, which might reduce GO stability. It should be noted that none of the previous studies have examined the temperature dependence of GO stability. According to the XDLVO calculations, changing temperature had little effect on the energy profiles (Fig. 2). Lowering temperature at 10 mM from 24 ºC to 6 ºC, however, slightly reduced the depths of the secondary minimum wells, suggesting that 13
temperature might show some effects on GO suspensions that are less stable. In an unstable system, low temperature can improve GO stability by lowering the internal energy of the particles because the particles have low internal energy.
3.2. Effects of temperature on GO sorption In a batch sorption study, Sotirelis and Chrysikopoulos [2] have shown slight temperature dependence of GO sorption onto quartz sand. They found the thermodynamic attachment equilibrium constant of GO slightly increases with increasing temperature and thus concluded that the GO sorption on sand is an endothermic process. In this work, temperature also showed strong influences on the sorption of GO onto the sand media under various conditions (Fig. 3). For all the tested conditions, the amounts of GO sorbed on the sand at 24 ºC were more than double of that at 6 ºC. In addition to temperature, other experimental conditions such as IS and sand properties also affected the sorption of GO. As shown in Fig. 3, fine natural sand had the highest GO sorption rate at 24 ºC when the IS was 10 mM. The zeta-potential measuring results indicated that the surfaces of both sand and GO were negatively charged for all tested experimental conditions (Table 2), suggesting that there were electrostatic repulsion forces between GO and sand surfaces. The zeta potential values of sand and GO at higher temperature (24ºC) were generally less negative than those at lower temperature (6ºC) under the same IS background. This is consistent with sorption experimental results that GO sorption rates were higher at higher temperature (Fig. 3). Previous studies have 14
also shown the temperature dependence of the zeta-potential of other materials but the trend may vary over the chemistry of background solutions [30, 31], probably because both temperature and water chemistry affect the ion adsorption/desorption and protonation/deprotonation processes on charged surface.
The XDLVO calculation also indicated that all the experimental
conditions were unfavorable for GO sorption onto the sand media (Fig. 4). In particular, the energy barriers were too high for the GO particles to overcome to attach on the sand surface at primary minima. However, the batch sorption experiments showed GO attachment onto the sand media under all of the conditions. This contradictory was probably due to either the involvement of other forces or the deposition of GO onto the sand surfaces via secondary minima.
3.3. Effects of temperature on GO retention and transport in porous media Observed and simulated breakthrough curves (BTCs) of GO in the sand columns are shown in Fig. 5. In these BTCs, normalized effluent concentration (C/C0) is plotted as a function of pore volumes (PVs) passed through the sand columns. A summary of the fitted model parameters and recovery rates are displayed in Table 1.
3.3.1. Combined effects of temperature and IS Fig. 5 and mass balance calculations (Table 1) indicate that the combination of temperature and IS had significant effects on the GO retention and transport in sand columns. Mass balance calculations showed that 77.33%-92.42% and 5.17%-40.64% of GO particles were recovered in 15
the effluents at IS of 1mM and 10mM, respectively. This confirms the important of IS to GO retention and transport, as indicated in the literature [12, 18, 32]. Temperature showed different effects on GO retention and transport in the sand columns under low and high IS conditions. When the IS was low (i.e., 1 mM), GO had relatively high mobility (C/C0 >0.85) regardless of the temperature (Fig. 5 and Table 1), indicating temperature only had little effect on deposition of GO in the porous media. This result is consistent with the predictions of the XDLVO theory, which showed no signs of secondary minimum (Fig. 4). However, the result is contradictory to findings of the batch sorption experiments (Fig. 3). The different effects of temperature on GO sorptive behaviors in the batch and column systems were probably due to different interaction kinetics and behaviors of GO and sand in the two systems. When the IS was 10 mM, the temperature showed notably influences on the retention and transport of GO in the porous media (Fig. 5). An obvious increase of recovery rates at 10 mM when the temperature lowed from 24ºC to 6 ºC was also observed (Table 1). The BTCs at 6 ºC were generally located over those at 24 ºC in the same type of sand columns at 10mM. This trend agrees well with the findings from the sorption experiments (Fig. 3). The endothermic dynamic attachment process of GO might be the main reason why GO mobility in sand columns at 6 ºC was higher than that at 24 ºC. This trend was in good agreement with the XDLVO prediction that second minimum wells in Fig. 4 were all deepened when the temperature increased from 6 ºC to 24 ºC at the IS of 10 mM. The best-fit k values at 1 mM for 6 and 24 ºC were virtually equal; while most k values at 10 16
mM for 24 ºC were one third bigger than those for 6ºC. A similar situation also prevailed in the best-fit values of Smax. Furthermore, Smax at higher IS were almost one order of magnitude larger than those at lower IS. The model results confirmed that GO transport was strongly sensitive to solution IS and that temperature dependence of GO transport only happened at high IS.
3.3.2. Combined effects of temperature and grain size When combined with grain size, the temperature also showed different effects on GO transport (Fig. 5). The BTCs in the coarse sand columns at 6ºC and 24ºC were closer to each other than that in the fine sand columns. When temperature decreased from 24 to 6 ºC at 10 mM, GO recovery rates were doubled or tripled in fine porous media, only increased less than half in coarse media (Table 1). These results suggest that lowing temperature was more remarkable in porous media with smaller grain sizes in enhancing GO mobility. In most conditions, except in acid-cleaned sand columns at 1mM, fine sand had more GO retention than coarse sand. Mass balance calculations showed 5.17%-17.44% of GO were recovered in the effluents from fine sand at 10 mM. Under the same conditions, the recovery rates in coarse sand columns were much higher and ranged from 28.32% to 40.64 %. These results are consistent with the predictions of the classic filtration theory and findings of previous studies [17, 33]. The best-fit Smax values ranged from 13.29-17.50 ug/g and 9.49-11.95 ug/g for fine grains and coarse grains at the IS of 10 mM, respectively, suggesting that the attachment capacity of the medium increased as the grain size decreased. This trend of Smax was also found 17
in natural sand with IS of 1 mM. In acid-clean sand with IS of 1 mM, slightly more GO retention in coarse media was observed than that in fine media, which contradicts with the predictions of the classic filtration theory. In a previous study, Phenrat, Kim, Fagerlund, Illangasekare and Lowry [34] have observed that zero-valent iron nanoparticles showed higher mobility in saturated porous media with smaller grain sizes. They proposed that hydrodynamic forces, such as local fluid shear, may be responsible for this unusual phenomenon. Fluid shear helps restrain nanoparticle aggregation and should be higher for finer grains at a given flow velocity [34]. In this work, hydrodynamic forces might play a more pronounced role than physical straining in the acid-clean sand at IS of 1 mM to increase GO retention in coarse sand.
3.3.3. Combined effects of temperature and sand type When IS was 10 mM, GO mobility increased as temperature decreased in both natural sand and acid-cleaned sand columns (Fig. 5). Temperature dependence of GO retention in acid-cleaned sand columns was slightly stronger than that in columns packed with natural sand. When the temperature reduced from 24 to 6 ºC, GO recovery rates increased from 7.80 -29.95% to 17.44-40.64% in acid-cleaned sand columns; while only from 5.17-28.32% to 16.13-32.87% in natural sand columns. Under the same conditions, GO recovery rates in acid-cleaned sand were generally higher than that in that natural sand (Table 1). Previous studies have shown that natural sand may provide additional attachment sites for 18
nanoparticles than acid-cleaned sand [28], which may explain the higher retention of GO in the natural sand columns in this study. Metal impurities, particularly oxyhydroxides of Fe and Al, on the surface of natural sand may introduce positive charge and thus locally favorable conditions for GO attachment [13]. The differences in zeta potential values (Table 2) between natural and acid-cleaned sand under the same IS and temperature conditions (i.e., the zeta potential of natural sand was less negative than that of acid-cleaned sand) verified the presence of local positive charge on natural sand surface. The best-fit values of Smax in natural sand were also generally larger than those in acid-cleaned sand, which well tallies with the observation of GO mobility in the porous media. However, the XDLVO calculation did not show perceived difference between natural sand and acid-cleaned sand at the same temperature. This is because XDLVO theory does not take surface-charge heterogeneity into account when calculating the interaction forces.
4. Conclusions The study provides new information about the effects of a critical but under-discussed factor, temperature, on GO retention and transport in saturated porous media. Batch sorption experimental results showed that GO attachment on sand was an endothermic process. As a result, temperature had notable effects on GO retention in porous media, particularly at relatively high IS (10 mM), and more GO was recovered from the effluents as temperature decreased. In addition to the temperature effects, other factors including IS, grain size, sand surface properties 19
also strongly influenced the retention and transport of GO in porous media. When temperature was the same, IS was a dominant factor for GO retention and transport, and the higher the IS was, the less mobile the GO particles were; grain size was also important and finer grains size led to more retention for most of the cases; and GO was generally less mobile in natural sand than in acid-cleaned sand. Findings from this work indicate the importance of temperature on GO retention in porous media. It thus should be considered, particularly in combinations with other important factors, to accurately predict and monitor the environmental fate and transport of GO and other engineered nanoparticles.
Acknowledgment This work was partially supported by the NSF (1213333), the China Scholarship Council (CSC), and the Fundamental Research Funds for the Central Universities (2015B33714)
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in Saturated Quartz Sand: Cation-Dependent Retention Mechanisms, Environ Sci Technol, 49 (2015) 11468-11475. [25] H.B. Gao, M.G. Shao, Effects of temperature changes on soil hydraulic properties, Soil Till Res, 153 (2015) 145-154. [26] M. Elimelech, J. Gregory, X. Jia, R.A. Williams, Particle deposition and aggregation: measurement, modelling and simulation, Butterworth-Heinemann, 1995. [27] W.S. Hummers, R.E. Offeman, Preparation of Graphitic Oxide, J Am Chem Soc, 80 (1958) 1339-1339. [28] Y. Tian, B. Gao, Y. Wang, V.L. Morales, R.M. Carpena, Q. Huang, L. Yang, Deposition and transport of functionalized carbon nanotubes in water-saturated sand columns, J Hazard Mater, 213-214 (2012) 265-272. [29] L. Wu, L. Liu, B. Gao, R. Munoz-Carpena, M. Zhang, H. Chen, Z.H. Zhou, H. Wang, Aggregation kinetics of graphene oxides in aqueous solutions: Experiments, mechanisms, and modeling, Langmuir, 29 (2013) 15174-15181. [30] D. Al Mahrouqi, J. Vinogradov, M.D. Jackson, Temperature dependence of the zeta potential in intact natural carbonates, Geophys Res Lett, 43 (2016) 11578-11587. [31] J. Vinogradov, M.D. Jackson, Zeta potential in intact natural sandstones at elevated temperatures, Geophys Res Lett, 42 (2015) 6287-6294. [32] J.D. Lanphere, C.J. Luth, S.L. Walker, Effects of solution chemistry on the transport of graphene oxide in saturated porous media, Environ Sci Technol, 47 (2013) 4255-4261. 24
[33] X.Y. Lv, B. Gao, Y.Y. Sun, J.C. Wu, B.L. Jiang, X.Q. Shi, Effects of grain size and structural heterogeneity on the transport and retention of nano-TiO2 in saturated porous media, Sci Total Environ, 563 (2016) 987-995. [34] T. Phenrat, H.J. Kim, F. Fagerlund, T. Illangasekare, G.V. Lowry, Empirical correlations to estimate agglomerate size and deposition during injection of a polyelectrolyte-modified Fe0 nanoparticle at high particle concentration in saturated sand, J Contam Hydrol, 118 (2010) 152-164.
25
Table 1. Summary of best-fit model results and recovery rates of GO transport in saturated sand columns under various experimental conditions. Experimental conditions Sand type
Sand size
Acid-cleaned Coarse
IS (mM) 1 10
Fine
1 10
Natural
Coarse
1 10
Fine
1 10
T ( ºC) 6 24 6 24 6 24 6 24 6 24 6 24 6 24 6 24
Smax (ug/g)
k (min-1)
R2
Recovery rate (%)
1.11 1.12 9.49 9.53 0.90 0.89 13.29 15.14
0.0194 0.0215 0.0609 0.0997 0.0117 0.0122 0.1252 0.1981
0.9977 0.9976 0.9903 0.9971 0.9953 0.9961 0.9872 0.9894
88.97 88.47 40.64 29.25 92.42 92.17 17.44 7.80
1.62 1.60 10.25 11.95 2.34 2.53 13.99 17.50
0.0304 0.0304 0.0798 0.0873 0.0236 0.0242 0.1226 0.2151
0.9956 0.9966 0.9950 0.9857 0.9886 0.9903 0.9702 0.9911
85.65 84.49 32.87 28.32 78.38 77.33 16.13 5.17
26
Table 2. Surface characteristics of GO and sand in background solutions. Hydro-dynamic diameter (nm)
Electrophoretic mobility (×10−8 m2/(Vs))
Zeta-potential (mV)
6ºC
24ºC
6ºC
6ºC
GO (1 mM)
160.30
205.20 -2.24±0.06 -2.14±0.13 -43.31±1.18 -27.95±1.72
GO (10 mM)
187.72
242.40 -1.99±0.04 -1.03±0.10 -38.69±0.72 -13.42±1.28
24ºC
24ºC
Grain size (mm) Natural sand (1 mM)
0.3-0.4
-1.82±0.09 -1.93±0.12 -35.31±1.82 -25.12±1.62
Natural sand (10 mM)
0.3-0.4
-1.19±0.12 -1.52±0.09 -22.98±2.3
Natural sand (1 mM)
0.9-1.0
-2.35±0.03 -2.74±0.07 -45.50±0.60 -35.73±0.85
Natural sand (10 mM)
0.9-1.0
-1.95±0.06 -1.82±0.03 -37.88±1.08 -23.76±0.38
Acid-cleaned sand (1 mM)
0.3-0.4
-2.52±0.03 -2.59±0.07 -48.88±0.66 -33.74±0.88
Acid-cleaned sand (10 mM) 0.3-0.4
-1.90±0.05 -1.81±0.07 -36.84±0.95 -23.63±0.89
Acid-cleaned sand (1 mM)
0.9-1.0
-2.47±0.07 -2.65±0.09 -47.89±0.13 -34.56±1.16
Acid-cleaned sand (10 mM) 0.9-1.0
-2.00±0.05 -1.95±0.05 -38.89±0.98 -25.48±0.71
-19.836±1.21
27
Fig. 1 Stability of GO in aqueous solutions under different temperature and IS conditions.
28
Interaction enery (mJ/m 2 )
18
6℃,1mM
24℃,1mM
6℃,10mM
24℃,10mM
0.001
14
0.0005
10
0 0 20 40 60 80 100120 -0.0005
6
-0.001 2
-2
0
5
10
15
20
Separation distance (nm)
Fig. 2 XDLVO energy between GO particles under different temperature and IS conditions.
29
Fig. 3 Effects of temperature on GO sorption onto sand
30
Natural Sand
Acid-cleaned Sand
(a)
(b)
(c)
(d)
Fine
Coarse
Fig. 4 XDLVO energy between GO particles and (a) fine-size natural sand, (b) fine-size acid-cleaned sand, (c) coarse-size natural sand and (d) coarse-size acid-cleaned sand at different ISs (1 and 10 mM) and temperatures (4 and 24 ºC).
31
Natural Sand
Acid-cleaned Sand
1
1 (b)
(a)
0.8
0.8
0.6
C/C0
C/C0
Fine
0.6
0.4 0.2
0.2
0
0 0
1
2
PV 3
4
5
0
1 (c)
2
PV 3
4
5
1
2
PV 3
4
5
(d)
0.8 0.6
C/C0
0.6
C/C0
1
1
0.8
Coarse
0.4
0.4 0.2
0.4 0.2
0
0 0
1
2
PV 3
4
5
0
Fig. 5 Transport of GO at different ISs (1 and 10 mM) and temperatures (6 and 24 ºC) in saturated columns packed with (a) fine-size natural sand, (b) fine-size acid-cleaned sand, (c) coarse-size natural sand, and (d) coarse-size acid-cleaned sand.
32
S0304-3894(17)30095-X http://dx.doi.org/doi:10.1016/j.jhazmat.2017.02.014 HAZMAT 18369
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
24-12-2016 9-2-2017 10-2-2017
Please cite this article as: Mei Wang, Bin Gao, Deshan Tang, Huimin Sun, Xianqiang Yin, Congrong Yu, Effects of temperature on graphene oxide deposition and transport in saturated porous media, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2017.02.014 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effects of temperature on graphene oxide deposition and transport in saturated porous media
Mei Wang1, 2, Bin Gao2*, Deshan Tang1, Huimin Sun3, 2, Xianqiang Yin3, 2, Congrong Yu4
1. College of Water Conservancy and Hydropower Engineering, Hohai University, Nanjing 210098, China 2. Department of Agricultural and Biological Engineering, University of Florida, Gainesville, FL 32611, USA 3. College of Resources and Environment, Northwest A&F University, Yangling, Shaanxi 712100, China 4. College of Hydrology and Water Conservancy and Water Resources, Hohai University, Nanjing 210098, China
_______________________ * Corresponding author, phone: (352) 392-1864 ext. 285, Fax: (352) 392-4092, email: [email protected]
1
Graphical abstract
2
Research highlights Temperature affected GO sorption onto sand for all of the tested conditions Deposition of GO in saturated porous media is endothermic Temperature had little effect on GO retention and transport in porous media at low IS Temperature showed notable effects on GO retention and transport at high IS For all combinations of sand type and grain size, high temperature promoted GO mobility
3
Abstract Laboratory batch sorption and sand column experiments were conducted to examine the effects of temperature (6 and 24 oC) on the retention and transport of GO in water-saturated porous media with different combination of solution ionic strength (IS, 1 and 10 mM), sand type (natural and acid-cleaned), and grain size (coarse and fine). Although results from batch sorption experiment showed that temperature affected the sorption of GO onto the sand grains at the low IS, the interactions between GO and the sand were relatively weak, which did make the temperature effect prominent. When the IS was 1 mM, experimental temperature showed little effect on GO retention and transport regardless of the medium properties. GO was highly mobile in the sand columns with mass recovery rates ranged from 77.3% to 92.4%. When the IS increased to 10 mM, temperature showed notable effects on GO retention and transport in saturated porous media. For all the combinations of sand type and grain size, the higher the temperature was, the less mobile GO particles were. The effects of temperature on GO retention and transport in saturated porous media were further verified though simulations from an advection–dispersion-reaction model. Keywords: graphene oxide; transport modeling; temperature; retention; ionic strength
4
1. Introduction Nanomaterials have brought a new technological revolution in the last decade and will still be in the spotlight of engineered materials in the foreseeable future. Among the existing engineered nanomaterials, carbon-based family consisting of different members such as fullerene, graphene, carbon nanotube, etc., is a crucial branch [1]. Graphene, a new nanomaterial that was first manufactured in 2004 [2], is projected to revolutionize the 21st century because of its supernormal properties embodied in many aspects, such as electronics, thermotics, mechanics, magnetics and optics. However, strong hydrophobicity and easy aggregation limit the applications of graphene in a certain degree. Compared with graphene, graphene oxide (GO), an important derivative of graphene [3], shows good dispersibility in water due to the presence of a number of oxygen-containing functional groups on its surface [4]. Additionally, these functional groups also expand the inter-laminar spacing between carbon layers, providing favorable conditions for GO to be used as an ideal precursor of preparing graphene-based nanocomposites. GO thus is being applied widely in many promising applications (e.g., biosensors, electronic devices, drug delivery, energy storage, composite materials, etc. [5, 6]). Similar to other engineered nanomaterials, GO has also been released into the environment during the manufacturing, using and disposing processes and eventually into the food chain, posing a potential threat to the health of humans and other living beings. Some research [7-9] has reported that GO nanoparticles can serve as superior carriers for organic contaminants and heavy metals in saturated soil and thus further threaten the safety of groundwater resources. In addition, 5
numerous toxicity studies have documented that GO nanoparticles may be toxic to the environment and human health. Previous investigations have also demonstrated that carbon nanoparticles entering the food chains may cause severe and persistent lung injuries that can induce the risk of airways disease, fibrosis, cardiovascular disease, or cancer [10, 11]. With the knowledge of the potential risks of GO in the subsurface systems, it is in an urgent need to advance current understanding of the fate and transport of GO in porous media. To date, many factors including ionic strength [12], pH [13], surfactants [14], organic matters [15], flow velocity and direction [8, 16], input concentration [17], grain size [17], moisture content [18], etc., have been found to play important roles in governing GO fate and transport in porous media. However, several other possible important factors, such as temperature, sunlight, redox potential, and air pressure have not yet been addressed [19]. Soil temperature varies periodically with diurnal or seasonal variations of solar radiation and is accordingly disturbed in different degrees as a function of depth. The temperature of shallow groundwater in a small forested catchment (New Brunswick, Canada) ranges from 4°C in February to approximately 14°C in August at a depth of 1.5 m, and stays at the range of 6-8 °C all year around at a depth of 8.75 m [20]. Previous studies have already demonstrated that temperature affects soil organic matter decomposition and stability [21, 22], potassium ion (K+) release [23], cation valence [24], and soil porosity [25]. Temperature is projected to be also an influential factor that cannot be neglected for the nanoparticle transport in porous media. A previous study conducted a theoretical analysis of the effects of solution temperature on particle 6
transport in aquifers and soils and predicted that higher solution temperature, generally accompanied by more severe Brownian motion and lower solution viscosity, leads to weaker mobility of colloids [26]. Despite its importance, none of the previous studies has systematically investigated the effect of temperature on the retention and transport of engineered nanoparticles including GO in porous media. In this work, a series of laboratory batch sorption and column experiments were conducted to determine effects of temperature on the retention and transport of GO in water-saturated porous media under different ionic strength (IS), sand type, and grain size conditions. Two types of sand (natural sand and acid-cleaned sand) with two size ranges were selected as experimental porous medium. Two laboratory temperatures and two IS conditions were also considered in these studies. The specific objectives were as follows: (1) examine the stability of GO at different temperatures in porous media; (2) compare the sorption of GO onto the sand medium under different temperature conditions; (3) determine the effects of temperature on the retention and transport of GO in saturated porous media under various conditions; and (4) model the retention and transport of GO in saturated porous media under the tested conditions.
2. Materials and methods 2.1. GO The single layer GO nanosheets (ACS Material, Medford, MA) used in this study were produced using modified Hummers method [27]. Based on the information provided by the 7
manufacture, the GO particles are in the diameter range of 1-5 um and the thickness range of 0.8-1.2 nm. The stock suspension was prepared by ultra-sonicating 100 mg of original GO (as received from the manufacture) in 1000 mL deionized (DI) water with Misonix S3000 ultrasonicator (QSonica, Newtown, CT) for 2 hours. Two concentrations of electrolyte solutions (i.e., 10 and 100 mM KCl) were independently used to dilute the stock GO suspension to a constant GO concentration of 20 mg/L at a desired IS (1 mM or 10 mM) in this study. Subsequently, in each experiment, a small part of the diluted GO suspensions was set aside and served in cuvettes for stability test, and the left were used as working suspensions of GO (i.e., for sorption and column experiments) under varying physicochemical conditions. The hydrodynamic diameter and zeta potential of GO particles were determined using a ZetaSizer (Malvern Instruments, Worcestershire, U.K.) and a ZetaPlus (Brookhaven Instrument Co., Holtsville, NY), respectively. Stability of GO suspensions in varying electrolyte solutions and under two temperature conditions was monitored with an Evolution 60 UV-Vis Spectrophotometer (Thermo Scientific, Waltham, MA) at a wavelength of 230 nm over time for 7 h. The cuvette with about 3.5 ml GO sample standing in the cell holder was measured at set intervals to obtain the GO concentrations. The stability experiments were performed in duplicate.
2.2. Porous media Quartz sand (Standard Sand & Silica Co.) was used as the experimental porous medium and 8
was sieved into two size ranges of 0.3-0.4 mm (fine) and 0.9-1.0 mm (coarse). The sand experienced a sequential wash by tap water and deionized (DI) water and then was oven dried at 105 ºC. After these steps, half of the sand was stored for future use and labeled as natural sand; and the other half was subsequently soaked with 10% nitric acid (v/v) for 24 h, rinsed with DI water, dried again in an oven at 105 ºC and finally marked as acid-cleaned sand. According to SEM-EDS and ICP-AES analyses in a previous study [28], metal impurities, such as Fe, Al and Ni, did exist on the surface of natural sand while the acid-cleaned sand surface did not have any impurities. The zeta potential of sand was measured by the ZetaPlus.
2.3. Sorption experiment Batch experiments were performed to determine whether temperature can affect GO sorption onto the sand, thereby verifying the possibility of temperature dependence of GO transport in porous media. The experimental conditions of the various sorption experiments performed in this study are summarized in Table 1, keeping a one-to-one relationship with the corresponding transport experiments. All samples were prepared in triplicate, so nine 8-ml plastic tubes were used for each attachment experiment, of which every three tubes contained 6 ml GO suspension at a desired IS with 2 g sand, 6 ml GO suspension at the corresponding IS without sand, and 6 ml KCL solution at the corresponding IS with 2 g sand, respectively. The second group of samples acted as a baseline to determine the attachment capacity of GO in sand by comparing the concentrations before and after attachment. The third group of samples was set to 9
help eliminate the disturbance caused by sand in the shaking process on the first group of samples. Two sheets of parafilm were employed to cover each tube for avoiding liquid leakage. When experimental samples were ready, all samples were gently shaken at 50 rpm in a shaker (Heidolph Promax 1020, Buch & Holm) for 2 hours in a desired temperature condition. After that, UV-vis Spectrophotometer was used to measure the concentrations of samples at 230 nm. The sorption experiments were performed in triplicate.
2.4. Column experiment The quartz sand of different sizes (fine and coarse) and surface properties (natural and acid-cleaned) was wet-packed into an acrylic column of 2.5 cm inner diameter and 12.5 cm length. Stainless membranes with 50 um mesh (Spectra/Mesh, Spectrum Laboratories, Inc) were used on both sides of the column to stop the sand from escaping the column and to distribute the flow. Prior to a transport experiment, hydrodynamic stabilization was established by flushing the column with 5 pore volumes (PVs) of DI water and then 5 PVs of background electrolyte solution at a chosen IS which matched the GO working suspension. A peristaltic pump (Masterflex L/S, Cole Parmer Instrument, Vernon Hills, IL) was used for solution injection at the constant flow rate of 1 mL/min (Darcy velocity of 0.2 cm/min). Then, 2 PVs of working suspension of GO were introduced to the sand column, followed by flushing with about 3 PVs of the corresponding background electrolyte solution. A fraction collector (IS-95 Interval Sampler, 10
Spectrum Chromatography, Houston, TX) was used to collect the effluent at 4-min intervals. The concentrations of GO in the effluent were measured immediately after sample collection with UV-vis Spectrophotometer. In addition, a number of tracer tests with potassium nitrate (KNO3) as the tracer were conducted separately to obtain the breakthrough curves (BTCs) with different temperatures and collector grain sizes. The injected KNO3 concentration was 14 mg/L and the breakthrough concentrations were quantified using UV-vis Spectrophotometer. Because the nitrate ion (NO3-) has the advantage of highest absorbance at the wavelength of 201 nm and no absorption observed near the wavelength of 275 nm, the actual absorbance of the effluent was determined by the difference between the absorptions measured at 201 nm and 275 nm. In this way could be erased the possible interferences of impurities. All the column experiments were performed in duplicate.
2.5. Mathematical models The extended Derjaguin–Landau–Verwey–Overbeek (XDLVO) theory was used to better interpret the stability and deposition of GO nanoparticles in GO-water-GO and GO-water-sand systems. This theory considers van der Waals attraction, electric double layer repulsion, and Lewis acid-base interactions [17]. Details of the calculation of the three interaction forces can be found in previous works [13, 17]. Retention and transport of GO in saturated sand columns were simulated with Hydrus-1D 11
package. The governing equation of GO retention and transport processes can be written as:
C 2C C S D 2 q b t z t z
b
S kC t
(1
S S max
(1) (2)
)
(3)
where θ is the volumetric water content (dimensionless), C is the nanoparticle concentration in the aqueous phase (ML-3), D is the hydrodynamic dispersion coefficient (L2T-1), q is the Darcy velocity (LT-1), ρb is the porous medium bulk density (ML-3), S is the solid phrase nanoparticle concentration (MM-1), Smax is the maximum solid phrase nanoparticle concentration (MM-1), ψ is a dimensionless nanoparticle retention function, k is the first-order deposition (attachment) coefficient (T-1), t is the time (T), and z is the travel distance in the direction of flow (L). This model was first applied to the NO3- breakthrough data (obtained from tracer tests) to optimize D. Then, measured concentration data of GO (breakthrough curves) were simulated inversely fitted using the Levenberg-Marquardt algorithm to botain the best-fit values of Smax and k.
3. Results and discussion 3.1 Effects of temperature on GO stability under different IS conditions The surface properties of GO particles and sand under different conditions are listed in Table 2. Fig. 1 shows the stability of GO as a function of time in the 1 and 10 mM KCl solution at 6 and 24 ºC. Across all the experimental conditions, the GO nanoparticles showed excellent 12
stability, which is consistent with the predictions from XDLVO theory (Fig. 2) that the experimental conditions were unfavorable for GO aggregation. The XDLVO energy profiles showed strong energy barriers over 10.6 mJ/m2 between GO particles for both temperatures, almost impossible for the GO particles in the size range of 1-5 um to overcome for aggregation in the primary minima. As a result, temperature showed little effect on the stability of GO in this work. After a 7-hour-long observation period, the largest absolute temporal change of relative GO concentration was only 0.025C0 at 10 mM KCl under 24 ºC. Even though the effects of temperature and IS on the stability of GO were both negligible in this study, the stability curves suggest that GO was slightly more stable at lower temperature and lower IS. The IS effects on GO stability can be related to difference in thickness of the electric double layer that affects the electrostatic repulsion forces and thus DLVO interactions [29]. In this study, the energy barriers for the IS of 1mM (16.6 mJ/m2 and 16.5 mJ/m2 at 6 ºC and 24 ºC, respectively) were higher than those for the IS of 10 mM (11.1 mJ/m2 and 10.6 mJ/m2 ). Further, higher IS (i.e., 10 mM) also introduced secondary minima to the XDLVO profiles at both tested temperatures, which might reduce GO stability. It should be noted that none of the previous studies have examined the temperature dependence of GO stability. According to the XDLVO calculations, changing temperature had little effect on the energy profiles (Fig. 2). Lowering temperature at 10 mM from 24 ºC to 6 ºC, however, slightly reduced the depths of the secondary minimum wells, suggesting that 13
temperature might show some effects on GO suspensions that are less stable. In an unstable system, low temperature can improve GO stability by lowering the internal energy of the particles because the particles have low internal energy.
3.2. Effects of temperature on GO sorption In a batch sorption study, Sotirelis and Chrysikopoulos [2] have shown slight temperature dependence of GO sorption onto quartz sand. They found the thermodynamic attachment equilibrium constant of GO slightly increases with increasing temperature and thus concluded that the GO sorption on sand is an endothermic process. In this work, temperature also showed strong influences on the sorption of GO onto the sand media under various conditions (Fig. 3). For all the tested conditions, the amounts of GO sorbed on the sand at 24 ºC were more than double of that at 6 ºC. In addition to temperature, other experimental conditions such as IS and sand properties also affected the sorption of GO. As shown in Fig. 3, fine natural sand had the highest GO sorption rate at 24 ºC when the IS was 10 mM. The zeta-potential measuring results indicated that the surfaces of both sand and GO were negatively charged for all tested experimental conditions (Table 2), suggesting that there were electrostatic repulsion forces between GO and sand surfaces. The zeta potential values of sand and GO at higher temperature (24ºC) were generally less negative than those at lower temperature (6ºC) under the same IS background. This is consistent with sorption experimental results that GO sorption rates were higher at higher temperature (Fig. 3). Previous studies have 14
also shown the temperature dependence of the zeta-potential of other materials but the trend may vary over the chemistry of background solutions [30, 31], probably because both temperature and water chemistry affect the ion adsorption/desorption and protonation/deprotonation processes on charged surface.
The XDLVO calculation also indicated that all the experimental
conditions were unfavorable for GO sorption onto the sand media (Fig. 4). In particular, the energy barriers were too high for the GO particles to overcome to attach on the sand surface at primary minima. However, the batch sorption experiments showed GO attachment onto the sand media under all of the conditions. This contradictory was probably due to either the involvement of other forces or the deposition of GO onto the sand surfaces via secondary minima.
3.3. Effects of temperature on GO retention and transport in porous media Observed and simulated breakthrough curves (BTCs) of GO in the sand columns are shown in Fig. 5. In these BTCs, normalized effluent concentration (C/C0) is plotted as a function of pore volumes (PVs) passed through the sand columns. A summary of the fitted model parameters and recovery rates are displayed in Table 1.
3.3.1. Combined effects of temperature and IS Fig. 5 and mass balance calculations (Table 1) indicate that the combination of temperature and IS had significant effects on the GO retention and transport in sand columns. Mass balance calculations showed that 77.33%-92.42% and 5.17%-40.64% of GO particles were recovered in 15
the effluents at IS of 1mM and 10mM, respectively. This confirms the important of IS to GO retention and transport, as indicated in the literature [12, 18, 32]. Temperature showed different effects on GO retention and transport in the sand columns under low and high IS conditions. When the IS was low (i.e., 1 mM), GO had relatively high mobility (C/C0 >0.85) regardless of the temperature (Fig. 5 and Table 1), indicating temperature only had little effect on deposition of GO in the porous media. This result is consistent with the predictions of the XDLVO theory, which showed no signs of secondary minimum (Fig. 4). However, the result is contradictory to findings of the batch sorption experiments (Fig. 3). The different effects of temperature on GO sorptive behaviors in the batch and column systems were probably due to different interaction kinetics and behaviors of GO and sand in the two systems. When the IS was 10 mM, the temperature showed notably influences on the retention and transport of GO in the porous media (Fig. 5). An obvious increase of recovery rates at 10 mM when the temperature lowed from 24ºC to 6 ºC was also observed (Table 1). The BTCs at 6 ºC were generally located over those at 24 ºC in the same type of sand columns at 10mM. This trend agrees well with the findings from the sorption experiments (Fig. 3). The endothermic dynamic attachment process of GO might be the main reason why GO mobility in sand columns at 6 ºC was higher than that at 24 ºC. This trend was in good agreement with the XDLVO prediction that second minimum wells in Fig. 4 were all deepened when the temperature increased from 6 ºC to 24 ºC at the IS of 10 mM. The best-fit k values at 1 mM for 6 and 24 ºC were virtually equal; while most k values at 10 16
mM for 24 ºC were one third bigger than those for 6ºC. A similar situation also prevailed in the best-fit values of Smax. Furthermore, Smax at higher IS were almost one order of magnitude larger than those at lower IS. The model results confirmed that GO transport was strongly sensitive to solution IS and that temperature dependence of GO transport only happened at high IS.
3.3.2. Combined effects of temperature and grain size When combined with grain size, the temperature also showed different effects on GO transport (Fig. 5). The BTCs in the coarse sand columns at 6ºC and 24ºC were closer to each other than that in the fine sand columns. When temperature decreased from 24 to 6 ºC at 10 mM, GO recovery rates were doubled or tripled in fine porous media, only increased less than half in coarse media (Table 1). These results suggest that lowing temperature was more remarkable in porous media with smaller grain sizes in enhancing GO mobility. In most conditions, except in acid-cleaned sand columns at 1mM, fine sand had more GO retention than coarse sand. Mass balance calculations showed 5.17%-17.44% of GO were recovered in the effluents from fine sand at 10 mM. Under the same conditions, the recovery rates in coarse sand columns were much higher and ranged from 28.32% to 40.64 %. These results are consistent with the predictions of the classic filtration theory and findings of previous studies [17, 33]. The best-fit Smax values ranged from 13.29-17.50 ug/g and 9.49-11.95 ug/g for fine grains and coarse grains at the IS of 10 mM, respectively, suggesting that the attachment capacity of the medium increased as the grain size decreased. This trend of Smax was also found 17
in natural sand with IS of 1 mM. In acid-clean sand with IS of 1 mM, slightly more GO retention in coarse media was observed than that in fine media, which contradicts with the predictions of the classic filtration theory. In a previous study, Phenrat, Kim, Fagerlund, Illangasekare and Lowry [34] have observed that zero-valent iron nanoparticles showed higher mobility in saturated porous media with smaller grain sizes. They proposed that hydrodynamic forces, such as local fluid shear, may be responsible for this unusual phenomenon. Fluid shear helps restrain nanoparticle aggregation and should be higher for finer grains at a given flow velocity [34]. In this work, hydrodynamic forces might play a more pronounced role than physical straining in the acid-clean sand at IS of 1 mM to increase GO retention in coarse sand.
3.3.3. Combined effects of temperature and sand type When IS was 10 mM, GO mobility increased as temperature decreased in both natural sand and acid-cleaned sand columns (Fig. 5). Temperature dependence of GO retention in acid-cleaned sand columns was slightly stronger than that in columns packed with natural sand. When the temperature reduced from 24 to 6 ºC, GO recovery rates increased from 7.80 -29.95% to 17.44-40.64% in acid-cleaned sand columns; while only from 5.17-28.32% to 16.13-32.87% in natural sand columns. Under the same conditions, GO recovery rates in acid-cleaned sand were generally higher than that in that natural sand (Table 1). Previous studies have shown that natural sand may provide additional attachment sites for 18
nanoparticles than acid-cleaned sand [28], which may explain the higher retention of GO in the natural sand columns in this study. Metal impurities, particularly oxyhydroxides of Fe and Al, on the surface of natural sand may introduce positive charge and thus locally favorable conditions for GO attachment [13]. The differences in zeta potential values (Table 2) between natural and acid-cleaned sand under the same IS and temperature conditions (i.e., the zeta potential of natural sand was less negative than that of acid-cleaned sand) verified the presence of local positive charge on natural sand surface. The best-fit values of Smax in natural sand were also generally larger than those in acid-cleaned sand, which well tallies with the observation of GO mobility in the porous media. However, the XDLVO calculation did not show perceived difference between natural sand and acid-cleaned sand at the same temperature. This is because XDLVO theory does not take surface-charge heterogeneity into account when calculating the interaction forces.
4. Conclusions The study provides new information about the effects of a critical but under-discussed factor, temperature, on GO retention and transport in saturated porous media. Batch sorption experimental results showed that GO attachment on sand was an endothermic process. As a result, temperature had notable effects on GO retention in porous media, particularly at relatively high IS (10 mM), and more GO was recovered from the effluents as temperature decreased. In addition to the temperature effects, other factors including IS, grain size, sand surface properties 19
also strongly influenced the retention and transport of GO in porous media. When temperature was the same, IS was a dominant factor for GO retention and transport, and the higher the IS was, the less mobile the GO particles were; grain size was also important and finer grains size led to more retention for most of the cases; and GO was generally less mobile in natural sand than in acid-cleaned sand. Findings from this work indicate the importance of temperature on GO retention in porous media. It thus should be considered, particularly in combinations with other important factors, to accurately predict and monitor the environmental fate and transport of GO and other engineered nanoparticles.
Acknowledgment This work was partially supported by the NSF (1213333), the China Scholarship Council (CSC), and the Fundamental Research Funds for the Central Universities (2015B33714)
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Table 1. Summary of best-fit model results and recovery rates of GO transport in saturated sand columns under various experimental conditions. Experimental conditions Sand type
Sand size
Acid-cleaned Coarse
IS (mM) 1 10
Fine
1 10
Natural
Coarse
1 10
Fine
1 10
T ( ºC) 6 24 6 24 6 24 6 24 6 24 6 24 6 24 6 24
Smax (ug/g)
k (min-1)
R2
Recovery rate (%)
1.11 1.12 9.49 9.53 0.90 0.89 13.29 15.14
0.0194 0.0215 0.0609 0.0997 0.0117 0.0122 0.1252 0.1981
0.9977 0.9976 0.9903 0.9971 0.9953 0.9961 0.9872 0.9894
88.97 88.47 40.64 29.25 92.42 92.17 17.44 7.80
1.62 1.60 10.25 11.95 2.34 2.53 13.99 17.50
0.0304 0.0304 0.0798 0.0873 0.0236 0.0242 0.1226 0.2151
0.9956 0.9966 0.9950 0.9857 0.9886 0.9903 0.9702 0.9911
85.65 84.49 32.87 28.32 78.38 77.33 16.13 5.17
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Table 2. Surface characteristics of GO and sand in background solutions. Hydro-dynamic diameter (nm)
Electrophoretic mobility (×10−8 m2/(Vs))
Zeta-potential (mV)
6ºC
24ºC
6ºC
6ºC
GO (1 mM)
160.30
205.20 -2.24±0.06 -2.14±0.13 -43.31±1.18 -27.95±1.72
GO (10 mM)
187.72
242.40 -1.99±0.04 -1.03±0.10 -38.69±0.72 -13.42±1.28
24ºC
24ºC
Grain size (mm) Natural sand (1 mM)
0.3-0.4
-1.82±0.09 -1.93±0.12 -35.31±1.82 -25.12±1.62
Natural sand (10 mM)
0.3-0.4
-1.19±0.12 -1.52±0.09 -22.98±2.3
Natural sand (1 mM)
0.9-1.0
-2.35±0.03 -2.74±0.07 -45.50±0.60 -35.73±0.85
Natural sand (10 mM)
0.9-1.0
-1.95±0.06 -1.82±0.03 -37.88±1.08 -23.76±0.38
Acid-cleaned sand (1 mM)
0.3-0.4
-2.52±0.03 -2.59±0.07 -48.88±0.66 -33.74±0.88
Acid-cleaned sand (10 mM) 0.3-0.4
-1.90±0.05 -1.81±0.07 -36.84±0.95 -23.63±0.89
Acid-cleaned sand (1 mM)
0.9-1.0
-2.47±0.07 -2.65±0.09 -47.89±0.13 -34.56±1.16
Acid-cleaned sand (10 mM) 0.9-1.0
-2.00±0.05 -1.95±0.05 -38.89±0.98 -25.48±0.71
-19.836±1.21
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Fig. 1 Stability of GO in aqueous solutions under different temperature and IS conditions.
28
Interaction enery (mJ/m 2 )
18
6℃,1mM
24℃,1mM
6℃,10mM
24℃,10mM
0.001
14
0.0005
10
0 0 20 40 60 80 100120 -0.0005
6
-0.001 2
-2
0
5
10
15
20
Separation distance (nm)
Fig. 2 XDLVO energy between GO particles under different temperature and IS conditions.
29
Fig. 3 Effects of temperature on GO sorption onto sand
30
Natural Sand
Acid-cleaned Sand
(a)
(b)
(c)
(d)
Fine
Coarse
Fig. 4 XDLVO energy between GO particles and (a) fine-size natural sand, (b) fine-size acid-cleaned sand, (c) coarse-size natural sand and (d) coarse-size acid-cleaned sand at different ISs (1 and 10 mM) and temperatures (4 and 24 ºC).
31
Natural Sand
Acid-cleaned Sand
1
1 (b)
(a)
0.8
0.8
0.6
C/C0
C/C0
Fine
0.6
0.4 0.2
0.2
0
0 0
1
2
PV 3
4
5
0
1 (c)
2
PV 3
4
5
1
2
PV 3
4
5
(d)
0.8 0.6
C/C0
0.6
C/C0
1
1
0.8
Coarse
0.4
0.4 0.2
0.4 0.2
0
0 0
1
2
PV 3
4
5
0
Fig. 5 Transport of GO at different ISs (1 and 10 mM) and temperatures (6 and 24 ºC) in saturated columns packed with (a) fine-size natural sand, (b) fine-size acid-cleaned sand, (c) coarse-size natural sand, and (d) coarse-size acid-cleaned sand.
32