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Effects of temperature on aggregation kinetics of graphene oxide in aqueous solutions
Accepted Manuscript Title: Effects of temperature on aggregation kinetics of graphene oxide in aqueous solutions Authors: Mei Wang, Bin Gao, Deshan Tang, Huimin Sun, Xianqiang Yin, Congrong Yu PII: DOI: Reference:
S0927-7757(17)30960-3 https://doi.org/10.1016/j.colsurfa.2017.10.061 COLSUA 22019
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
28-8-2017 11-10-2017 24-10-2017
Please cite this article as: Mei Wang, Bin Gao, Deshan Tang, Huimin Sun, Xianqiang Yin, Congrong Yu, Effects of temperature on aggregation kinetics of graphene oxide in aqueous solutions, Colloids and Surfaces A: Physicochemical and Engineering Aspects https://doi.org/10.1016/j.colsurfa.2017.10.061 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 aggregation kinetics of graphene oxide in aqueous solutions
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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Abstract Laboratory experiments were conducted to determine the effects of temperature (6, 25, and 40 °C) on GO aggregation kinetics under different combinations of ionic strength, cation type, humic acid (HA) concentration by monitoring GO hydrodynamic radii and attachment efficiencies. The results showed that, without HA, temperature increase promoted GO aggregation in both monovalent (Na+ and K+) and divalent (Ca2+) solutions. This phenomenon might be caused by multiple processes including enhanced collision frequency, enhanced cation dehydration, and reduced electrostatic repulsion. The presence of HA introduced steric repulsion forces that enhanced GO stability and temperature showed different effects GO aggregation kinetics in monovalent and divalent electrolytes. In monovalent electrolytes, cold temperature diminished the steric repulsion of HA-coated GO. As a result, the fastest increasing rate of GO hydrodynamic radius and the smallest critical coagulation concentration value appeared at the 2
lowest temperature (6 °C). Conversely, in divalent electrolyte solutions with HA, high temperature favored GO aggregation, probably because the interactions between Ca2+ and HA increased with temperature resulting in lower HA coating on GO. Findings of this work emphasized the importance of temperature as well as solution chemistry on the stability and fate of GO nanoparticles in aquatic environment.
Keywords: Nanoparticles; Coagulation; Stability; Critical coagulation concentration
1. Introduction Graphene and its derivatives have been extensively applied to various advanced applications in the sectors of energy storage, material science and engineering, environmental remediation, biosensor, and biomedicine due to its unique mechanical, electrical, thermal, and optical properties and chemical stability [1-5]. Graphene oxide (GO), one derivative of graphene, contains abundant oxygen-containing groups, such as carbonyl and carboxyl groups located at the edges and hydroxyl and epoxy groups on the basal planes [6, 7]. These oxygen functionalities make GO an exceptional precursor for large-scale production of graphene, reduced GO, and chemically modified graphene for industrial usages. Given their wide applications and popularity in industry, graphene-based nanomaterials including GO may enter the environment and finally end up in human bodies through food chains, breathing or skin exposure [8-10]. Some research has showed that GO sheets exhibit significant toxicity to human cells and aquatic organisms even 3
at a low concentration [8, 11, 12]. The fate and transport of GO in the environment, especially in the aquatic systems, thus have attracted much research interest recently. Because of the abundant surface functional groups, GO nanoparticles often have good dispersibility in water. Nevertheless, GO nanoparticles may still experience complicated environment behaviors (dispersion, aggregation, transformation, adsorption or sedimentation) after being released into water bodies [13]. The stability and particle size of GO nanoparticles are important factors that control their fate and transport in aqueous systems and thus strongly affect their environmental impact [14]. It has also been reported that the lateral size of GO nanosheets decisively determines their toxicity by affecting the interactions between GO and biological systems [11, 15]. Therefore, the aggregation kinetics and stability of GO particles should be carefully investigated to better predict their subsequent environmental behaviors and assess the exposure risks. Only few investigations have been carried out to measure the aggregation kinetics of GO in aqueous solutions [16-22]. The results suggest that the stability of GO in water is strongly influenced by pH, electrolyte types, ionic strength (IS) and natural organic matter (NOM) concentration. The aggregation of GO in water surges remarkably with increasing IS and divalent cations (e.g., Ca2+, Mg2+, and Ba2+) destabilize GO much more aggressively than monovalent cations (e.g., Na+ and K+). The presence of NOMs also strongly affects GO stability and NOMs can reduce GO aggregation even under relatively high IS conditions [16, 18, 19]. Solution pH may also affect GO stability and aggregation behavior in aqueous solutions. 4
Increasing solution pH promotes the deprotonation of carboxylic functional groups on GO surface to increase the electrostatic repulsion force among GO sheets and thus to enhance GO stability [17]. In addition, some studies have suggested the presence of natural colloids including aluminum oxide, goethite, and clay colloids may promote GO aggregation through various interactions [23-26]. Natural water temperature alters seasonally or diurnally according to ambient conditions, such as the shifting angle of solar radiation, atmospheric or soil heat transfer, and anthropogenic activities [27]. In a specific geographic area, seasonal groundwater temperature fluctuation tends to palliate with the increase of soil depth; while surface water temperature can generally change over a wide range of factors. Temperature can alter the physicochemical properties of water, such as pH, conductivity and salinity, water density, NOM concentration, etc., which may affect the environmental behaviors of nanoparticles in natural waters. It has been reported that temperature can affect the retention and transport of GO in porous media under relatively high IS conditions [27]. Few studies have investigated the effect of temperature on aggregation kinetics of several types of engineered nanoparticles including carbon nanotubes, ZnO, and CeO2 in aqueous solutions [28-30]. The results indicate that high temperature generally favors nanoparticle aggregation in aqueous solutions, except the study on CeO2 which suggest the temperature effect is nonmonotonic and closely related to IS conditions. To the best of the authors’ knowledge, however, none of previous studies has systematically examined the influences of temperature on the aggregation kinetics of GO nanoparticles in aqueous solutions. 5
NOMs are ubiquitous in soils and natural waters and play a key role in governing nanoparticle fate and transport in aquatic environments. Humic acid (HA), the main existing form of NOMs, can interact extensively with various engineered nanoparticles to improve their stability in aqueous solutions. Hydrogen bonding, π-π interaction and Lewis acid−base interaction are suggested as the principal mechanisms contributing to GO interaction with HA. Changes in water temperature may affect not only physicochemical properties of NOMs in water, but also the interactions between NOMs and nanoparticles. Previous studies have shown that elevated temperature can promote the growth of HA aggregates through dehydration and diminished electrostatic repulsion [31, 32] and thus decrease HA’s ability to stabilize nanoparticles in water. Given that both GO nanoparticles and HA molecules possess abundant surface oxygen-containing functional groups that are impressionable to temperature, the presence of HA in water could potentially alter the effect of temperature on GO aggregation kinetics. The combined effects of temperature and HA on GO aggregation and stability in aqueous solutions, however, are still unclear. The overarching objective of this work was to provide a clear picture of the role of temperature on GO aggregation kinetics in common monovalent and divalent electrolyte solutions with or without HA. Laboratory experiments were conducted to monitor the hydrodynamic radius of GO under different combinations of temperature, IS, cation type, and HA concentration conditions to determine GO aggregation kinetics. The specific objectives were as follows: (i) determine temperature-induced changes of GO aggregation under the influence of 6
different monovalent (Na+ and K+) or divalent (Ca2+) cations without HA, (ii) determine temperature-induced changes of GO aggregation under the influence of different monovalent (Na+ and K+) or divalent (Ca2+) cations in presence of HA, and finally (iii) understand the governing mechanisms.
2. Materials and methods 2.1. Preparation of GO Suspension Single-layer GO, synthesized through the modified Hummer’s method, was purchased from ACS Material (Medford, MA). According to the manufacturer, the diameter range and the thickness range of the GO particles are 1-5 µm and 0.8-1.2 nm, respectively. A previous study has confirmed that the nanosheets have an average sheet area of 338724±12365 nm2 and an average thickness of 0.92±0.13 nm through AFM image analysis [17]. HA, purchased from Aldrich Co., was used as standard NOM in this study. The HA stock solution was prepared as follows: adding a small volume (10-15 ml) of 0.1 M NaOH into 100 mg dry HA powder; transferring the supernatant into 1000 mL volumetric flask; repeating these two steps until all the HA was dissolved thoroughly; diluting the dissolved HA to 1 L; and finally using 0.22 µm filter membranes to remove oversize HA molecules. The concentration of the filtrate (i.e., the stock HA solution) was determined using a UV–vis Spectrophotometer (Thermo Scientific, Waltham, MA). To prepare the GO stock solution, 100 mg of the GO were thoroughly dispersed in 1 L 7
deionized (DI) water by ultrasonic treatment for 2 h with a Misonix S3000 ultrasonicator (QSonica, Newtown, CT). Prior to each test, the stock solution was diluted to a desired GO concentration (20 mg/L) with different combinations of IS (NaCl, KCl, or CaCl2) and NOM concentrations (0, 1, and 10 mg/L). The pH values of the GO suspensions were adjusted to 6.0 ± 0.2 with 0.1 M sodium hydroxide (NaOH) and 0.1 M hydrochloric acid (HCl).
2.2. GO hydrodynamic radius GO hydrodynamic radius (Dh) was measured with a Zetasizer Nano ZS (Malvern Instruments, Worcestershire, U.K.) which employed Dynamic Light Scattering (DLS) technology equipped with a 633nm laser and a 173° detection optics for particle size characterization [17]. The DLS technology measures the diffusion coefficients of particles undergoing Brownian motion and then converts the diffusion coefficient data to a hydrodynamic size by using the Stokes-Einstein’s equation [33]. The Multiple Narrow Modes algorithm based on non-negative least squares (NNLS) fit was used to obtain particle size distributions [33, 34]. For each aggregation experiment, about 1 mL of diluted GO suspension was added into a quartz cuvette that had been thoroughly rinsed at least twice by the sample. All the experiments were performed at three different temperatures, representing cold (6 °C), temperate (25 °C), and tropical (40 °C) water bodies. The quartz cuvette was equipped with a thermal cap to help keep temperature stability when heating and cooling the sample during size measurements. The equilibration time was set to 120 seconds to ensure the sample equal to the desired temperature 8
after being inserted into the cell area. Each datum point of GO Dh as a function of time was determined by one measurement that included five runs that took 6 seconds per run. For most GO samples of specific solution chemistries, the data collection of Dh by DLS measurements lasted over a period ranging from 30 to 150 min, in which Dh values exceeded at least 1.3 times the initial hydrodynamic radius (Dh0). Every GO sample was diluted from stock GO suspension with different combinations of electrolyte solutions, HA solutions, and DI water just before DLS measurement. The stability of GO nanoparticles over a range of solution chemistries was visually compared over a period of 24-48 h under three experimental temperatures: 6, 25 and 40 ℃. At time zero, about 5 mL of GO samples were introduced into a glass vial. Every few hours, the vials were arranged in a specific order and then photographed to visually compare the samples.
2.3. Aggregation kinetics The initial rate of growth in the hydrodynamic radius Dh(t) with time (t) was used to determine the early stage aggregation kinetics of GO nanoparticles [19, 35, 36]. The initial aggregation rate constant (ka) is proportional to the initial growth of Dh as a function of t, but inversely proportional to the initial primary particle concentration (N0) [17]:
ka
1 dDh t N 0 dt t 0
(1)
Dh(t) is obtained through linear fitting with the Dh data over a time range when the 9
hydrodynamic radius increases from Dh0 to 1.3Dh0 and determined by the slope of the fitted line. Under some low IS conditions where GO aggregation is nearly negligible and an approximately 30% increase hardly occurs, the (dD h(t)/dt)t→0 is obtained by determining the slope up to the point where maximum Dh is achieved. The attachment efficiency (α), also known as the inverse stability ratio (1/W), can be determined by normalizing the initial aggregation rate constant of interest to the initial aggregation rate (ka,fav) under the favorable aggregation (or diffusion-controlled) condition where the IS is generally believed to be greater than or equal to the critical coagulation concentration (CCC value, i.e., the minimum electrolyte concentration to reach the diffusion-controlled regime) [37]. For all aggregation experiments, the GO concentration is identical, so the attachment efficiency (α) is equal to the ratio of the ka at a given solution chemistry condition to that at the favorable aggregation condition [14]:
=
1 dDh t N 0 dt t 0
k 1 a W ka,fav 1 dDh t （N 0）fav dt t 0,fav
dDh t dt t 0 = dDh t dt t 0,fav
(2)
where the subscript “fav” denotes favorable aggregation conditions. The CCC values were analyzed on a log-log plot with the IS of a certain kind of electrolyte as the horizontal axis and the attachment efficiency as the vertical axis. They were then determined by the intersection of a fitted line of the data points that showed an obvious rise of the attachment efficiencies with the 10
IS (i.e., reaction-limited aggregation conditions), with an extrapolation of the horizontal line of the favorable aggregation conditions where the attachment efficiencies changed little with the IS increase. HA aggregation in aqueous solutions is also strongly dependent on surrounding environmental conditions, especially with respect to IS and temperature [28, 31]. In view of the possibility of disruption from HA aggregation on GO Dh measurement, the aggregation kinetics of HA (10mg/L) was monitored in varied electrolytes at different temperatures.
3. Results and discussion 3.1. Effect of temperature on GO aggregation in absence of HA The aggregation of the GO nanoparticles in absence of HA was studied over NaCl concentrations ranging from 50 to 250 mM, KCl concentrations ranging from 20 to 100 mM, or CaCl2 concentrations ranging from 0.3 to 0.8 mM. Fig. 1A-C show the examples of the changes of hydrodynamic radius with time in NaCl, KCl, and CaCl2, respectively. Detailed aggregation profiles of all the experiments can be found in Fig. S1 in the Support Information. The initial aggregation kinetic rates at different IS were determined from the aggregation profiles (Fig. 1D-F). The results showed that the kinetic rates increased with temperature, suggesting GO aggregation was temperature dependent and high temperature promoted aggregation. The aggregation attachment efficiencies as functions of NaCl, KCl and CaCl2 concentrations at different temperatures are presented in Fig. 1G-I, respectively. The CCC values 11
of GO declined from 156 to 132 mM in NaCl, from 72 to 60 mM in KCl, and from 0.62 to 0.52 mM in CaCl2 solutions when the temperature increased from 6 to 40°C. These further confirmed that increasing temperature decreased GO stability in electrolyte solutions. Examples of visual observation of GO stability in Fig. 1J-L show that that GO aggregates first appeared at 40 ℃ in the absence of HA, followed by 25 and 6 ℃. The photos of all the visual experiments are listed in Fig. S2 in the Supporting information, which show GO aggregations in the electrolytes without HA (i.e., HA = 0 mg/L) often occurred at high temperature and high IS. Previous studies have observed the similar temperature effects on the aggregation kinetics of carbon nanotubes [28, 38]. Several potential mechanisms including reduction of electrostatic repulsion among the nanoparticles caused by temperature increase and increase of random Brownian motion and collision frequency have been proposed to explain this phenomenon [28, 38]. In this work, increase in temperature would also lead to less negative zeta potential and weaker electrostatic repulsion. Measurements of zeta-potential of GO in a previous study indicate that GO nanoparticles at higher temperature are generally less negative than that at lower temperature under the same solution chemistry conditions [27]. The temperature dependence of the zeta potential was also observed for other nanoparticles or colloids in previous studies [39, 40]. The observed temperature dependence of GO aggregation rate in the electrolytes without HA thus might be attributed to the reduction of electrostatic repulsion induced by increasing temperature. Enhanced dehydration effect with increasing temperature might also play an important role. The existence of the hydration shells around cations can imped them to attach 12
and destabilize GO. The hydration numbers of both monovalent and divalent metal cations decrease when temperature increases [41, 42]. This is consistent with the observed results that increasing temperature enhanced GO aggregation in this work. Regardless of temperature and electrolyte type, GO attachment efficiencies showed a consistent trend of ascending with the IS at the beginning (i.e., at the reaction-limited regime) and then keeping stable (i.e., at the diffusion-controlled regime). The enhanced GO aggregation with the increase of IS at the reaction-limited regime can be attributed to the relatively weak repulsive electrostatic interaction between GO particles. At the reaction-limited regime, the degree of surface charge screening reached the maximum, which means the surface charge of GO particles was completely screened, thus, GO aggregation rate would stop increasing with the IS. Both Dh values and the order of magnitude of CCC values suggest that the divalent ion (Ca2+) was far more aggressive than the monovalent ions (Na+ and K+) in destabilizing GO in aqueous solutions. The CCC values of NaCl and KCl for GO are at least two-order magnitude larger than that of CaCl2, which is correlated well with findings from the literature [21, 22]. The divalent Ca2+ can build cation bridges between GO particles via forming inner-sphere complexes with oxygen-containing functional groups on GO surfaces [22]. Fig. 2A shows three types of the Ca2+ bridging mechanisms: (1) intraparticle bridging between carboxyl and phenolic groups to form tube-shaped GO structure [21]; (2) face-to-face bridging between hydroxyl and carbonyl functional groups to form multi-piled structure [21]; and (3) edge-to-edge bridging through 13
chelating carboxyl groups to form chain structure [17]. These cation bridges can effectively neutralize the negative surface charge of GO and thus lead to significant aggregation. High temperature may enhance the deprotonation of the carboxyl groups and thus aggravate the interactions of GO particles with Ca2+, especially for the edge-to-edge bridging. The more profound bridge-linking effect of Ca2+ at higher temperatures can cause greater GO aggregation. In contrast, Na+ and K+ can only form outer-sphere complexes that are incapable of serving as bridging agents. Although Na+ and K+ possess the same valence, they showed different effects on GO aggregation in aqueous solutions. This result deviates from the Schulze-Hardy rule that cations of the same valent should have the same destabilization capability, but follows the rank of the Hofmeister series that K+ is more effective than Na+ in salting out colloids [43]. The CCC value of NaCl is generally more than double that of KCl at the same temperature, indicating that K+ had a stronger effect to destabilize GO than Na+. The variation of Dh in NaCl and KCl at the same IS (Fig. 1A and 1B) further confirmed that GO aggregation kinetics was also influenced by electrolyte types. Xia et al. [22] suggest that cations with larger ionic radii (e.g., K+) have thinner hydration shell thickness and hold their hydration shell relatively less strongly when they approach nanoparticle surface, thus resulting in stronger charge screen effects than those with smaller ionic radii (e.g., Na+).
3.2. Effect of temperature on HA aggregation 14
Previous studies have demonstrated that humic substances can form molecular aggregates under various experimental conditions, such as high temperature, high IS, and multivalent cation [31, 44, 45]. To understand the temperature effect on HA aggregation kinetics, changes in Dh of HA with time were measured at 6, 25, and 40 ℃ with 200-2000 mM NaCl, 100-2000 mM KCl, or 1-20 mM CaCl2 (Fig. S3, Supporting Information). For all the three temperatures, HA was very stable with negligible changes of Dh under most of the tested conditions (e.g., Fig. 3A-B). Compared with the monovalent cations, Ca2+ was more effective in destabilizing HA (e.g., Fig. 3C). When CaCl2 concentration raised from 3 to 20 mM, the hydrodynamic size of HA increased significantly with IS. The visual appearance of dark colored precipitates at the bottom of glass vials confirmed the negative relationship of both temperature and IS to the HA stability in CaCl2 (Fig. S4C, Supporting Information). The enhanced HA aggregation by increasing IS was also observed by previous studies [46, 47]. Benincasa et al. [45] have attributed HA aggregation to enhanced hydrophobic interactions due to the collapse of the humic coil induced by the increase of IS. For NaCl and KCl, no obvious HA aggregation was observed even when the IS reached 2000 mM (Fig. 3A-B). Visual observation of HA stability over 24 h also confirmed this (Fig. S4A-B, Supporting information). The observed difference in the destabilizing ability of monovalent and divalent cations suggest that electrostatic shielding alone was not sufficient for HA aggregation. HA molecules bridge-linked with divalent cations might be the principle mechanism [31]. Intramolecule or intermolecule bridging caused by Ca2+ as shown in Fig. 2B can rapidly promote the formation of calcium humates (i.e., HACax, where x stands for the mean 15
number of Ca2+ ions bound to each HA molecule), which are insoluble in water [46]. It is more likely for HACax molecules to approach each other due to their less negative charges. Subsequently, the small aggregates formed by HACax molecules can further aggregate with Ca2+ acting as intermolecule bridges [46]. Temperature effect on HA aggregation kinetics was much more remarkable in the presence of Ca2+ than that of Na+ or K+. The decrease of the CCC values of Ca2+ from 4.94 mM to 3.99 mM in the 6–40 ℃ range (Fig. 3E), as well as the increased initial aggregation rates (Fig. 3D), suggests that the HA aggregation became more prevalent in the presence of Ca2+ as the temperature was raised. For Na+ and K+, the corresponding CCC values must be over 2000 mM. The CCC ratios of Na+ and K+ to Ca2+ are more than 400, inconsistent with the predictions of the Schulze-Hardy rule (i.e., 26=64), further confirming HA aggregation is a more complex process than just electrostatic shielding. A very slight increase in HA hydrodynamic size with temperature in the presence of Na+ and K+ was found. This observation was attributed to the reduction of hydration resulted from the decrease of the relative permittivity and viscosity of water with temperature [30, 31]. A previous study has proposed that the adsorption of divalent cations on HA molecules is an endothermic process [48]. Thus, in case of divalent cation, Ca2+, the dehydration effect played a more active role in stimulating the crosslinking at high temperature. Temperature and IS appeared to exert a combined effect on the ability of divalent cation to enhance HA aggregation, with higher temperature and IS being more effective. Diminished electrostatic repulsion and 16
enhanced crosslinking were responsible for HA aggregation in CaCl2 solution.
3.3. Effect of temperature on GO aggregation in presence of monovalent cation and HA Based on the results from the HA aggregation experiments, HA in all the monovalent cation solutions used in the experiments was stable without any aggregations. Aggregation profiles of GO in varying concentrations of NaCl and KCl with HA are presented in Fig. S5, Supporting Information. The GO stability was positively correlated to HA concentration. At a fixed IS (compare Fig. 4A-B, and Fig. 5A-B), hydrodynamic sizes of GO particles were much smaller with 10 mg/L HA than that with 1 mg/L HA, indicating HA had strong ability to stabilize GO in aqueous solutions. The lowest CCC values of NaCl with 1 mg/L HA and 10 mg/L HA were 233 mM and 299 mM, respectively, approximately double that without HA (132 mM NaCl). A similar trend of CCC values changing with HA concentrations was also observed for KCl. Overall, addition of HA improved GO stability under all investigated conditions (Fig. 4 and 5). The stabilizing ability of HA to GO and other nanoparticles in monovalent electrolytes has also been reported in other studies [19, 49]. HA is adsorbed on GO surface mainly through π-π interactions [50, 51]. Previous studies have demonstrated that the adsorbed HA can introduce negative charges to GO surface, thus promoting GO stability by the increased electrostatic repulsion [50, 52]. It has also been demonstrated that the coating of NOMs such as HA on nanoparticle surface can introduce strong non-DLVO forces (mainly steric repulsions) to reduce their aggregations [16, 19, 49]. 17
Temperature showed strong effect on the aggregation kinetics of GO in presence of monovalent cation and HA (Fig. 4 and 5). At the reaction-limited regime, the growth rate of hydrodynamic sizes (Fig. 4C-D and 5C-D) and attachment efficiency (Fig. 4E-F and 5E-F) at 6 ℃ were remarkably higher than those at 25 and 40 ℃. Additionally, the smallest CCC values for both NaCl and KCl all occurred at the lowest temperature (i.e., 6 ℃). These results indicate that GO aggregation was relatively aggressive in cold temperature conditions in the presence of HA, which is contradictory to the experimental results without HA. The presence of HA complicated the temperature effect on GO aggregation in monovalent cation solutions, probably due to the introduction of steric interactions by the HA coating of GO particles. Previous studies have proven that steric effect of polymer-coated particles in aqueous solutions closely depended on temperature [53]. Increasing temperature would promote the steric repulsion to enhance particle stability in the presence of HA because adsorption process of HA on carbon materials was endothermic [54]. On the other hand, temperature increase would also reduce particle stability through several mechanisms including reducing electrostatic repulsion, enhancing collision frequency, and promote dehydration. The effect of temperature on GO aggregation kinetics in presence of monovalent cation and HA, therefore, would rely on the balance of all these interactions. In this work, elevated steric repulsion by temperature increase from 25 to 40 ℃ probably neutralized the destruction of GO stability caused by the other mechanisms (i.e., reduction of electrostatic repulsion, enhanced collision frequency, and enhanced dehydration). Consequently, the CCC values at 25 and 40 ℃ are close to each other. 18
The CCC values at 6 ℃ are notably smaller than those at 25 and 40 ℃, suggesting cold temperature greatly weakened steric repulsion. For all the experiments, the CCC values ascended with the increase of HA concentration. This suggests changes in steric repulsion with temperature in the presence of monovalent cation and HA in all the experiments still surpassed that of other interactions. Visual observations of GO in 200 mM NaCl and 100 mM KCl with HA concentration of 0, 1 and 10 mg/L are presented in Fig. S2A-B (Supporting Information), respectively. The electrolyte concentrations were above the CCC values of the cations without HA (0 mg/L), but below the CCC values of the cations with HA (1 and 10 mg/L). Therefore, the stability of GO with HA was observed in the reaction-limited regime, indicating that GO precipitation in the presence of HA might be slower than that without HA, which was in the diffusion-controlled regime. The GO aggregates were first observed 1 hour after sample preparation in NaCl in the presence of 0 and 1 mg/L HA, but did not show up after 24 hour at 6 ℃ with10 mg/L HA. Similarly, it took 6 hours to only observe the GO precipitation in KCl at 6 ℃ with 10 mg/L HA; while GO had completely precipitated at the same time in the case of no HA or 1mg/L HA. These visual observations confirmed not only the strong stabilizing effect of HA and but also that the effect would be weakened by cold temperature in monovalent electrolyte solution. However, GO precipitation did not first show up at 6 ℃ in both NaCl and KCl in the presence of 1 mg/L HA, which contradicts the fact that 6 ℃ had the lowest CCC values in the corresponding experimental conditions. Furthermore, GO particles with 1 mg/L HA at 40 ℃ formed large flocs 19
instead of aggregate, indicating the involvement of flocculation process. The formation of the large flocs during GO aggregation might affect the measurements of the hydrodynamic radius and thus might cause the contradictories. Nevertheless, most of the visual observations are consistent with the trends of the GO aggregation kinetics and attachment efficiencies determined through the hydrodynamic radius measurements.
3.4. Effect of temperature on GO aggregation in presence of divalent cation and HA The changes in Dh of GO with time measured at 6, 25, and 40 ℃ in CaCl2 with 1 mg/L and 10 mg/L HA are presented in Fig. S6, Supporting Information. Even though HA can stabilize the nanoparticles, CaCl2 was effective in promoting GO aggregation for most of the tests (Fig. 6). Different from the monovalent cation system, increase of temperature clearly enhanced GO aggregation in presence of CaCl2 and HA (Fig. 6A-D). The CCC values of CaCl2 for GO aggregation measured in CaCl2 with HA decreased with the temperature (Fig. 6E-F). The GO aggregation was the fastest at 40 ℃, which was confirmed by visual observation experiments showing rapid GO precipitation (Fig. S2C). Although the concentration of divalent cation in this study (within 2 mM) would not result in HA aggregating, Ca2+ might interact with HA to reduce its adsorption on GO surface. It is speculated that high temperature would favor the interaction between Ca2+ and HA [48], and thus would reduce GO stability. Overall, HA continued to behave as effective stabilizing agents but was less effective at higher temperature. 20
4. Conclusions The aggregation kinetics of GO in aqueous solutions were temperature dependent, and the influences of the temperature varied among different combinations of IS, electrolyte type and valence, and HA concentration. Without HA, increasing temperature enhanced GO aggregation in both monovalent (Na+ and K+) and divalent (Ca2+) cation solutions. The temperature effect on GO aggregation was more complicate in the presence of HA. For monovalent electrolytes with HA, the lowest temperature (6 ℃) destabilized GO most efficiently while the aggregation kinetics at 25 and 40 ℃ were close to each other and were notably weaker than that of 6 ℃. This was attributed to that cold temperature greatly reduced the steric repulsion between the HA-coated GO particles. For divalent electrolyte with HA, however, GO aggregation was accelerated by the rise of temperature, probably because of the enhanced interaction between Ca2+ and HA that reduced HA coating on GO. Because the natural water bodies have wide range of temperatures, the effects of temperature on GO aggregation kinetics should be considered as an important factor in the prediction and assessment of its environmental fate and impacts.
Acknowledgment This work was partially supported by the NSF (1213333) and the China Scholarship Council (CSC).
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29
Fig. 1 Effects of temperature on GO aggregation without HA: (A-C) Changes of GO hydrodynamic radii in 100 mM NaCl, 100 mM KCl, and 0.8 mM CaCl2, respectively; (D-F) initial aggregation rates of GO in NaCl, KCl, and CaCl2, respectively;(G-I) attachment efficiencies of GO in NaCl, KCl, and CaCl2, respectively; (J-L) photograph illustrations of GO suspension at different temperatures after a given time. The numbers in the colored rounded rectangles in (G-I) represent the CCC values of GO in the corresponding electrolyte solutions, and red, yellow and blue colors stand for 40, 25 and 6 ºC, respectively. 30
Fig. 2 Aggregation mechanisms of (A) GO and (B) HA in the presence of divalent cations
31
Fig. 3 Effects of temperature on HA aggregation: (A-C) Changes of HA hydrodynamic radii in 2000 mM NaCl, 2000 mM KCl, and 5 mM CaCl2, respectively; (D) Initial aggregation rates of HA in CaCl2; (E) attachment efficiencies of HA in CaCl2. The numbers in the colored rounded rectangles in (E) represent the CCC values of HA in the corresponding electrolyte solutions, and red, yellow and blue colors stand for 40, 25 and 6 ºC, respectively.
32
Fig. 4 Effects of temperature on GO aggregation in the presence of HA and Na+: (A-B) Changes of GO hydrodynamic radii in 200 mM NaCl with 1 and 10 mg/L HA, respectively; (C-D) initial aggregation rates of GO with 1 and 10 mg/L HA, respectively; (E-F) attachment efficiencies of GO with 1 and 10 mg/L HA, respectively;(G-H) photograph illustrations of GO in 200 mM NaCl solution with 1 and 10 mg/L HA at different temperatures after a given time. The numbers in the colored rounded rectangles in (E-F) represent the CCC values of GO in the corresponding electrolyte solutions, and red, yellow and blue colors stand for 40, 25 and 6 ºC, respectively. 33
Fig. 5 Effects of temperature on GO aggregation in the presence of HA and K+: (A-B) Changes of GO hydrodynamic radii in 100 mM KCl with 1 and 10 mg/L HA, respectively; (C-D) initial aggregation rates of GO with 1 and 10 mg/L HA, respectively; (E-F) attachment efficiencies of GO with 1 and 10 mg/L HA, respectively;(G-H) photograph illustrations of GO in 100 mM KCl solution with 1 and 10 mg/L HA at different temperatures after a given time. The numbers in the colored rounded rectangles in (E-F) represent the CCC values of GO in the corresponding electrolyte solutions, and red, yellow and blue colors stand for 40, 25 and 6 ºC, respectively. 34
Fig. 6 Effects of temperature on GO aggregation in the presence of HA and Ca2+: (A-B) Changes of GO hydrodynamic radii in 0.8 mM CaCl2 with 1 and 10 mg/L HA, respectively; (C-D) initial aggregation rates of GO with 1 and 10 mg/L HA, respectively; (E-F) attachment efficiencies of GO with 1 and 10 mg/L HA, respectively;(G-H) photograph illustrations of GO in 0.8 mM CaCl2 solution with 1 and 10 mg/L HA at different temperatures after a given time. The numbers in the colored rounded rectangles in (E-F) represent the CCC values of GO in the corresponding electrolyte solutions, and red, yellow and blue colors stand for 40, 25 and 6 ºC, respectively. 35
S0927-7757(17)30960-3 https://doi.org/10.1016/j.colsurfa.2017.10.061 COLSUA 22019
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
28-8-2017 11-10-2017 24-10-2017
Please cite this article as: Mei Wang, Bin Gao, Deshan Tang, Huimin Sun, Xianqiang Yin, Congrong Yu, Effects of temperature on aggregation kinetics of graphene oxide in aqueous solutions, Colloids and Surfaces A: Physicochemical and Engineering Aspects https://doi.org/10.1016/j.colsurfa.2017.10.061 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 aggregation kinetics of graphene oxide in aqueous solutions
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
1
40℃ 25℃ 6℃
0.1
0.52mM
0.01
0.56mM 0.62mM
0.001 0.2
Electrolyte concentration (mM)
2
Hydrodynamic Radius (nm)
Attachment efficiency (α)
Graphical abstract 1000
40℃,0.45mM 25℃,0.45mM 6℃,0.45mM
800 600 400 200
0 0
500
1000 1500 Time (s)
2000
Abstract Laboratory experiments were conducted to determine the effects of temperature (6, 25, and 40 °C) on GO aggregation kinetics under different combinations of ionic strength, cation type, humic acid (HA) concentration by monitoring GO hydrodynamic radii and attachment efficiencies. The results showed that, without HA, temperature increase promoted GO aggregation in both monovalent (Na+ and K+) and divalent (Ca2+) solutions. This phenomenon might be caused by multiple processes including enhanced collision frequency, enhanced cation dehydration, and reduced electrostatic repulsion. The presence of HA introduced steric repulsion forces that enhanced GO stability and temperature showed different effects GO aggregation kinetics in monovalent and divalent electrolytes. In monovalent electrolytes, cold temperature diminished the steric repulsion of HA-coated GO. As a result, the fastest increasing rate of GO hydrodynamic radius and the smallest critical coagulation concentration value appeared at the 2
lowest temperature (6 °C). Conversely, in divalent electrolyte solutions with HA, high temperature favored GO aggregation, probably because the interactions between Ca2+ and HA increased with temperature resulting in lower HA coating on GO. Findings of this work emphasized the importance of temperature as well as solution chemistry on the stability and fate of GO nanoparticles in aquatic environment.
Keywords: Nanoparticles; Coagulation; Stability; Critical coagulation concentration
1. Introduction Graphene and its derivatives have been extensively applied to various advanced applications in the sectors of energy storage, material science and engineering, environmental remediation, biosensor, and biomedicine due to its unique mechanical, electrical, thermal, and optical properties and chemical stability [1-5]. Graphene oxide (GO), one derivative of graphene, contains abundant oxygen-containing groups, such as carbonyl and carboxyl groups located at the edges and hydroxyl and epoxy groups on the basal planes [6, 7]. These oxygen functionalities make GO an exceptional precursor for large-scale production of graphene, reduced GO, and chemically modified graphene for industrial usages. Given their wide applications and popularity in industry, graphene-based nanomaterials including GO may enter the environment and finally end up in human bodies through food chains, breathing or skin exposure [8-10]. Some research has showed that GO sheets exhibit significant toxicity to human cells and aquatic organisms even 3
at a low concentration [8, 11, 12]. The fate and transport of GO in the environment, especially in the aquatic systems, thus have attracted much research interest recently. Because of the abundant surface functional groups, GO nanoparticles often have good dispersibility in water. Nevertheless, GO nanoparticles may still experience complicated environment behaviors (dispersion, aggregation, transformation, adsorption or sedimentation) after being released into water bodies [13]. The stability and particle size of GO nanoparticles are important factors that control their fate and transport in aqueous systems and thus strongly affect their environmental impact [14]. It has also been reported that the lateral size of GO nanosheets decisively determines their toxicity by affecting the interactions between GO and biological systems [11, 15]. Therefore, the aggregation kinetics and stability of GO particles should be carefully investigated to better predict their subsequent environmental behaviors and assess the exposure risks. Only few investigations have been carried out to measure the aggregation kinetics of GO in aqueous solutions [16-22]. The results suggest that the stability of GO in water is strongly influenced by pH, electrolyte types, ionic strength (IS) and natural organic matter (NOM) concentration. The aggregation of GO in water surges remarkably with increasing IS and divalent cations (e.g., Ca2+, Mg2+, and Ba2+) destabilize GO much more aggressively than monovalent cations (e.g., Na+ and K+). The presence of NOMs also strongly affects GO stability and NOMs can reduce GO aggregation even under relatively high IS conditions [16, 18, 19]. Solution pH may also affect GO stability and aggregation behavior in aqueous solutions. 4
Increasing solution pH promotes the deprotonation of carboxylic functional groups on GO surface to increase the electrostatic repulsion force among GO sheets and thus to enhance GO stability [17]. In addition, some studies have suggested the presence of natural colloids including aluminum oxide, goethite, and clay colloids may promote GO aggregation through various interactions [23-26]. Natural water temperature alters seasonally or diurnally according to ambient conditions, such as the shifting angle of solar radiation, atmospheric or soil heat transfer, and anthropogenic activities [27]. In a specific geographic area, seasonal groundwater temperature fluctuation tends to palliate with the increase of soil depth; while surface water temperature can generally change over a wide range of factors. Temperature can alter the physicochemical properties of water, such as pH, conductivity and salinity, water density, NOM concentration, etc., which may affect the environmental behaviors of nanoparticles in natural waters. It has been reported that temperature can affect the retention and transport of GO in porous media under relatively high IS conditions [27]. Few studies have investigated the effect of temperature on aggregation kinetics of several types of engineered nanoparticles including carbon nanotubes, ZnO, and CeO2 in aqueous solutions [28-30]. The results indicate that high temperature generally favors nanoparticle aggregation in aqueous solutions, except the study on CeO2 which suggest the temperature effect is nonmonotonic and closely related to IS conditions. To the best of the authors’ knowledge, however, none of previous studies has systematically examined the influences of temperature on the aggregation kinetics of GO nanoparticles in aqueous solutions. 5
NOMs are ubiquitous in soils and natural waters and play a key role in governing nanoparticle fate and transport in aquatic environments. Humic acid (HA), the main existing form of NOMs, can interact extensively with various engineered nanoparticles to improve their stability in aqueous solutions. Hydrogen bonding, π-π interaction and Lewis acid−base interaction are suggested as the principal mechanisms contributing to GO interaction with HA. Changes in water temperature may affect not only physicochemical properties of NOMs in water, but also the interactions between NOMs and nanoparticles. Previous studies have shown that elevated temperature can promote the growth of HA aggregates through dehydration and diminished electrostatic repulsion [31, 32] and thus decrease HA’s ability to stabilize nanoparticles in water. Given that both GO nanoparticles and HA molecules possess abundant surface oxygen-containing functional groups that are impressionable to temperature, the presence of HA in water could potentially alter the effect of temperature on GO aggregation kinetics. The combined effects of temperature and HA on GO aggregation and stability in aqueous solutions, however, are still unclear. The overarching objective of this work was to provide a clear picture of the role of temperature on GO aggregation kinetics in common monovalent and divalent electrolyte solutions with or without HA. Laboratory experiments were conducted to monitor the hydrodynamic radius of GO under different combinations of temperature, IS, cation type, and HA concentration conditions to determine GO aggregation kinetics. The specific objectives were as follows: (i) determine temperature-induced changes of GO aggregation under the influence of 6
different monovalent (Na+ and K+) or divalent (Ca2+) cations without HA, (ii) determine temperature-induced changes of GO aggregation under the influence of different monovalent (Na+ and K+) or divalent (Ca2+) cations in presence of HA, and finally (iii) understand the governing mechanisms.
2. Materials and methods 2.1. Preparation of GO Suspension Single-layer GO, synthesized through the modified Hummer’s method, was purchased from ACS Material (Medford, MA). According to the manufacturer, the diameter range and the thickness range of the GO particles are 1-5 µm and 0.8-1.2 nm, respectively. A previous study has confirmed that the nanosheets have an average sheet area of 338724±12365 nm2 and an average thickness of 0.92±0.13 nm through AFM image analysis [17]. HA, purchased from Aldrich Co., was used as standard NOM in this study. The HA stock solution was prepared as follows: adding a small volume (10-15 ml) of 0.1 M NaOH into 100 mg dry HA powder; transferring the supernatant into 1000 mL volumetric flask; repeating these two steps until all the HA was dissolved thoroughly; diluting the dissolved HA to 1 L; and finally using 0.22 µm filter membranes to remove oversize HA molecules. The concentration of the filtrate (i.e., the stock HA solution) was determined using a UV–vis Spectrophotometer (Thermo Scientific, Waltham, MA). To prepare the GO stock solution, 100 mg of the GO were thoroughly dispersed in 1 L 7
deionized (DI) water by ultrasonic treatment for 2 h with a Misonix S3000 ultrasonicator (QSonica, Newtown, CT). Prior to each test, the stock solution was diluted to a desired GO concentration (20 mg/L) with different combinations of IS (NaCl, KCl, or CaCl2) and NOM concentrations (0, 1, and 10 mg/L). The pH values of the GO suspensions were adjusted to 6.0 ± 0.2 with 0.1 M sodium hydroxide (NaOH) and 0.1 M hydrochloric acid (HCl).
2.2. GO hydrodynamic radius GO hydrodynamic radius (Dh) was measured with a Zetasizer Nano ZS (Malvern Instruments, Worcestershire, U.K.) which employed Dynamic Light Scattering (DLS) technology equipped with a 633nm laser and a 173° detection optics for particle size characterization [17]. The DLS technology measures the diffusion coefficients of particles undergoing Brownian motion and then converts the diffusion coefficient data to a hydrodynamic size by using the Stokes-Einstein’s equation [33]. The Multiple Narrow Modes algorithm based on non-negative least squares (NNLS) fit was used to obtain particle size distributions [33, 34]. For each aggregation experiment, about 1 mL of diluted GO suspension was added into a quartz cuvette that had been thoroughly rinsed at least twice by the sample. All the experiments were performed at three different temperatures, representing cold (6 °C), temperate (25 °C), and tropical (40 °C) water bodies. The quartz cuvette was equipped with a thermal cap to help keep temperature stability when heating and cooling the sample during size measurements. The equilibration time was set to 120 seconds to ensure the sample equal to the desired temperature 8
after being inserted into the cell area. Each datum point of GO Dh as a function of time was determined by one measurement that included five runs that took 6 seconds per run. For most GO samples of specific solution chemistries, the data collection of Dh by DLS measurements lasted over a period ranging from 30 to 150 min, in which Dh values exceeded at least 1.3 times the initial hydrodynamic radius (Dh0). Every GO sample was diluted from stock GO suspension with different combinations of electrolyte solutions, HA solutions, and DI water just before DLS measurement. The stability of GO nanoparticles over a range of solution chemistries was visually compared over a period of 24-48 h under three experimental temperatures: 6, 25 and 40 ℃. At time zero, about 5 mL of GO samples were introduced into a glass vial. Every few hours, the vials were arranged in a specific order and then photographed to visually compare the samples.
2.3. Aggregation kinetics The initial rate of growth in the hydrodynamic radius Dh(t) with time (t) was used to determine the early stage aggregation kinetics of GO nanoparticles [19, 35, 36]. The initial aggregation rate constant (ka) is proportional to the initial growth of Dh as a function of t, but inversely proportional to the initial primary particle concentration (N0) [17]:
ka
1 dDh t N 0 dt t 0
(1)
Dh(t) is obtained through linear fitting with the Dh data over a time range when the 9
hydrodynamic radius increases from Dh0 to 1.3Dh0 and determined by the slope of the fitted line. Under some low IS conditions where GO aggregation is nearly negligible and an approximately 30% increase hardly occurs, the (dD h(t)/dt)t→0 is obtained by determining the slope up to the point where maximum Dh is achieved. The attachment efficiency (α), also known as the inverse stability ratio (1/W), can be determined by normalizing the initial aggregation rate constant of interest to the initial aggregation rate (ka,fav) under the favorable aggregation (or diffusion-controlled) condition where the IS is generally believed to be greater than or equal to the critical coagulation concentration (CCC value, i.e., the minimum electrolyte concentration to reach the diffusion-controlled regime) [37]. For all aggregation experiments, the GO concentration is identical, so the attachment efficiency (α) is equal to the ratio of the ka at a given solution chemistry condition to that at the favorable aggregation condition [14]:
=
1 dDh t N 0 dt t 0
k 1 a W ka,fav 1 dDh t （N 0）fav dt t 0,fav
dDh t dt t 0 = dDh t dt t 0,fav
(2)
where the subscript “fav” denotes favorable aggregation conditions. The CCC values were analyzed on a log-log plot with the IS of a certain kind of electrolyte as the horizontal axis and the attachment efficiency as the vertical axis. They were then determined by the intersection of a fitted line of the data points that showed an obvious rise of the attachment efficiencies with the 10
IS (i.e., reaction-limited aggregation conditions), with an extrapolation of the horizontal line of the favorable aggregation conditions where the attachment efficiencies changed little with the IS increase. HA aggregation in aqueous solutions is also strongly dependent on surrounding environmental conditions, especially with respect to IS and temperature [28, 31]. In view of the possibility of disruption from HA aggregation on GO Dh measurement, the aggregation kinetics of HA (10mg/L) was monitored in varied electrolytes at different temperatures.
3. Results and discussion 3.1. Effect of temperature on GO aggregation in absence of HA The aggregation of the GO nanoparticles in absence of HA was studied over NaCl concentrations ranging from 50 to 250 mM, KCl concentrations ranging from 20 to 100 mM, or CaCl2 concentrations ranging from 0.3 to 0.8 mM. Fig. 1A-C show the examples of the changes of hydrodynamic radius with time in NaCl, KCl, and CaCl2, respectively. Detailed aggregation profiles of all the experiments can be found in Fig. S1 in the Support Information. The initial aggregation kinetic rates at different IS were determined from the aggregation profiles (Fig. 1D-F). The results showed that the kinetic rates increased with temperature, suggesting GO aggregation was temperature dependent and high temperature promoted aggregation. The aggregation attachment efficiencies as functions of NaCl, KCl and CaCl2 concentrations at different temperatures are presented in Fig. 1G-I, respectively. The CCC values 11
of GO declined from 156 to 132 mM in NaCl, from 72 to 60 mM in KCl, and from 0.62 to 0.52 mM in CaCl2 solutions when the temperature increased from 6 to 40°C. These further confirmed that increasing temperature decreased GO stability in electrolyte solutions. Examples of visual observation of GO stability in Fig. 1J-L show that that GO aggregates first appeared at 40 ℃ in the absence of HA, followed by 25 and 6 ℃. The photos of all the visual experiments are listed in Fig. S2 in the Supporting information, which show GO aggregations in the electrolytes without HA (i.e., HA = 0 mg/L) often occurred at high temperature and high IS. Previous studies have observed the similar temperature effects on the aggregation kinetics of carbon nanotubes [28, 38]. Several potential mechanisms including reduction of electrostatic repulsion among the nanoparticles caused by temperature increase and increase of random Brownian motion and collision frequency have been proposed to explain this phenomenon [28, 38]. In this work, increase in temperature would also lead to less negative zeta potential and weaker electrostatic repulsion. Measurements of zeta-potential of GO in a previous study indicate that GO nanoparticles at higher temperature are generally less negative than that at lower temperature under the same solution chemistry conditions [27]. The temperature dependence of the zeta potential was also observed for other nanoparticles or colloids in previous studies [39, 40]. The observed temperature dependence of GO aggregation rate in the electrolytes without HA thus might be attributed to the reduction of electrostatic repulsion induced by increasing temperature. Enhanced dehydration effect with increasing temperature might also play an important role. The existence of the hydration shells around cations can imped them to attach 12
and destabilize GO. The hydration numbers of both monovalent and divalent metal cations decrease when temperature increases [41, 42]. This is consistent with the observed results that increasing temperature enhanced GO aggregation in this work. Regardless of temperature and electrolyte type, GO attachment efficiencies showed a consistent trend of ascending with the IS at the beginning (i.e., at the reaction-limited regime) and then keeping stable (i.e., at the diffusion-controlled regime). The enhanced GO aggregation with the increase of IS at the reaction-limited regime can be attributed to the relatively weak repulsive electrostatic interaction between GO particles. At the reaction-limited regime, the degree of surface charge screening reached the maximum, which means the surface charge of GO particles was completely screened, thus, GO aggregation rate would stop increasing with the IS. Both Dh values and the order of magnitude of CCC values suggest that the divalent ion (Ca2+) was far more aggressive than the monovalent ions (Na+ and K+) in destabilizing GO in aqueous solutions. The CCC values of NaCl and KCl for GO are at least two-order magnitude larger than that of CaCl2, which is correlated well with findings from the literature [21, 22]. The divalent Ca2+ can build cation bridges between GO particles via forming inner-sphere complexes with oxygen-containing functional groups on GO surfaces [22]. Fig. 2A shows three types of the Ca2+ bridging mechanisms: (1) intraparticle bridging between carboxyl and phenolic groups to form tube-shaped GO structure [21]; (2) face-to-face bridging between hydroxyl and carbonyl functional groups to form multi-piled structure [21]; and (3) edge-to-edge bridging through 13
chelating carboxyl groups to form chain structure [17]. These cation bridges can effectively neutralize the negative surface charge of GO and thus lead to significant aggregation. High temperature may enhance the deprotonation of the carboxyl groups and thus aggravate the interactions of GO particles with Ca2+, especially for the edge-to-edge bridging. The more profound bridge-linking effect of Ca2+ at higher temperatures can cause greater GO aggregation. In contrast, Na+ and K+ can only form outer-sphere complexes that are incapable of serving as bridging agents. Although Na+ and K+ possess the same valence, they showed different effects on GO aggregation in aqueous solutions. This result deviates from the Schulze-Hardy rule that cations of the same valent should have the same destabilization capability, but follows the rank of the Hofmeister series that K+ is more effective than Na+ in salting out colloids [43]. The CCC value of NaCl is generally more than double that of KCl at the same temperature, indicating that K+ had a stronger effect to destabilize GO than Na+. The variation of Dh in NaCl and KCl at the same IS (Fig. 1A and 1B) further confirmed that GO aggregation kinetics was also influenced by electrolyte types. Xia et al. [22] suggest that cations with larger ionic radii (e.g., K+) have thinner hydration shell thickness and hold their hydration shell relatively less strongly when they approach nanoparticle surface, thus resulting in stronger charge screen effects than those with smaller ionic radii (e.g., Na+).
3.2. Effect of temperature on HA aggregation 14
Previous studies have demonstrated that humic substances can form molecular aggregates under various experimental conditions, such as high temperature, high IS, and multivalent cation [31, 44, 45]. To understand the temperature effect on HA aggregation kinetics, changes in Dh of HA with time were measured at 6, 25, and 40 ℃ with 200-2000 mM NaCl, 100-2000 mM KCl, or 1-20 mM CaCl2 (Fig. S3, Supporting Information). For all the three temperatures, HA was very stable with negligible changes of Dh under most of the tested conditions (e.g., Fig. 3A-B). Compared with the monovalent cations, Ca2+ was more effective in destabilizing HA (e.g., Fig. 3C). When CaCl2 concentration raised from 3 to 20 mM, the hydrodynamic size of HA increased significantly with IS. The visual appearance of dark colored precipitates at the bottom of glass vials confirmed the negative relationship of both temperature and IS to the HA stability in CaCl2 (Fig. S4C, Supporting Information). The enhanced HA aggregation by increasing IS was also observed by previous studies [46, 47]. Benincasa et al. [45] have attributed HA aggregation to enhanced hydrophobic interactions due to the collapse of the humic coil induced by the increase of IS. For NaCl and KCl, no obvious HA aggregation was observed even when the IS reached 2000 mM (Fig. 3A-B). Visual observation of HA stability over 24 h also confirmed this (Fig. S4A-B, Supporting information). The observed difference in the destabilizing ability of monovalent and divalent cations suggest that electrostatic shielding alone was not sufficient for HA aggregation. HA molecules bridge-linked with divalent cations might be the principle mechanism [31]. Intramolecule or intermolecule bridging caused by Ca2+ as shown in Fig. 2B can rapidly promote the formation of calcium humates (i.e., HACax, where x stands for the mean 15
number of Ca2+ ions bound to each HA molecule), which are insoluble in water [46]. It is more likely for HACax molecules to approach each other due to their less negative charges. Subsequently, the small aggregates formed by HACax molecules can further aggregate with Ca2+ acting as intermolecule bridges [46]. Temperature effect on HA aggregation kinetics was much more remarkable in the presence of Ca2+ than that of Na+ or K+. The decrease of the CCC values of Ca2+ from 4.94 mM to 3.99 mM in the 6–40 ℃ range (Fig. 3E), as well as the increased initial aggregation rates (Fig. 3D), suggests that the HA aggregation became more prevalent in the presence of Ca2+ as the temperature was raised. For Na+ and K+, the corresponding CCC values must be over 2000 mM. The CCC ratios of Na+ and K+ to Ca2+ are more than 400, inconsistent with the predictions of the Schulze-Hardy rule (i.e., 26=64), further confirming HA aggregation is a more complex process than just electrostatic shielding. A very slight increase in HA hydrodynamic size with temperature in the presence of Na+ and K+ was found. This observation was attributed to the reduction of hydration resulted from the decrease of the relative permittivity and viscosity of water with temperature [30, 31]. A previous study has proposed that the adsorption of divalent cations on HA molecules is an endothermic process [48]. Thus, in case of divalent cation, Ca2+, the dehydration effect played a more active role in stimulating the crosslinking at high temperature. Temperature and IS appeared to exert a combined effect on the ability of divalent cation to enhance HA aggregation, with higher temperature and IS being more effective. Diminished electrostatic repulsion and 16
enhanced crosslinking were responsible for HA aggregation in CaCl2 solution.
3.3. Effect of temperature on GO aggregation in presence of monovalent cation and HA Based on the results from the HA aggregation experiments, HA in all the monovalent cation solutions used in the experiments was stable without any aggregations. Aggregation profiles of GO in varying concentrations of NaCl and KCl with HA are presented in Fig. S5, Supporting Information. The GO stability was positively correlated to HA concentration. At a fixed IS (compare Fig. 4A-B, and Fig. 5A-B), hydrodynamic sizes of GO particles were much smaller with 10 mg/L HA than that with 1 mg/L HA, indicating HA had strong ability to stabilize GO in aqueous solutions. The lowest CCC values of NaCl with 1 mg/L HA and 10 mg/L HA were 233 mM and 299 mM, respectively, approximately double that without HA (132 mM NaCl). A similar trend of CCC values changing with HA concentrations was also observed for KCl. Overall, addition of HA improved GO stability under all investigated conditions (Fig. 4 and 5). The stabilizing ability of HA to GO and other nanoparticles in monovalent electrolytes has also been reported in other studies [19, 49]. HA is adsorbed on GO surface mainly through π-π interactions [50, 51]. Previous studies have demonstrated that the adsorbed HA can introduce negative charges to GO surface, thus promoting GO stability by the increased electrostatic repulsion [50, 52]. It has also been demonstrated that the coating of NOMs such as HA on nanoparticle surface can introduce strong non-DLVO forces (mainly steric repulsions) to reduce their aggregations [16, 19, 49]. 17
Temperature showed strong effect on the aggregation kinetics of GO in presence of monovalent cation and HA (Fig. 4 and 5). At the reaction-limited regime, the growth rate of hydrodynamic sizes (Fig. 4C-D and 5C-D) and attachment efficiency (Fig. 4E-F and 5E-F) at 6 ℃ were remarkably higher than those at 25 and 40 ℃. Additionally, the smallest CCC values for both NaCl and KCl all occurred at the lowest temperature (i.e., 6 ℃). These results indicate that GO aggregation was relatively aggressive in cold temperature conditions in the presence of HA, which is contradictory to the experimental results without HA. The presence of HA complicated the temperature effect on GO aggregation in monovalent cation solutions, probably due to the introduction of steric interactions by the HA coating of GO particles. Previous studies have proven that steric effect of polymer-coated particles in aqueous solutions closely depended on temperature [53]. Increasing temperature would promote the steric repulsion to enhance particle stability in the presence of HA because adsorption process of HA on carbon materials was endothermic [54]. On the other hand, temperature increase would also reduce particle stability through several mechanisms including reducing electrostatic repulsion, enhancing collision frequency, and promote dehydration. The effect of temperature on GO aggregation kinetics in presence of monovalent cation and HA, therefore, would rely on the balance of all these interactions. In this work, elevated steric repulsion by temperature increase from 25 to 40 ℃ probably neutralized the destruction of GO stability caused by the other mechanisms (i.e., reduction of electrostatic repulsion, enhanced collision frequency, and enhanced dehydration). Consequently, the CCC values at 25 and 40 ℃ are close to each other. 18
The CCC values at 6 ℃ are notably smaller than those at 25 and 40 ℃, suggesting cold temperature greatly weakened steric repulsion. For all the experiments, the CCC values ascended with the increase of HA concentration. This suggests changes in steric repulsion with temperature in the presence of monovalent cation and HA in all the experiments still surpassed that of other interactions. Visual observations of GO in 200 mM NaCl and 100 mM KCl with HA concentration of 0, 1 and 10 mg/L are presented in Fig. S2A-B (Supporting Information), respectively. The electrolyte concentrations were above the CCC values of the cations without HA (0 mg/L), but below the CCC values of the cations with HA (1 and 10 mg/L). Therefore, the stability of GO with HA was observed in the reaction-limited regime, indicating that GO precipitation in the presence of HA might be slower than that without HA, which was in the diffusion-controlled regime. The GO aggregates were first observed 1 hour after sample preparation in NaCl in the presence of 0 and 1 mg/L HA, but did not show up after 24 hour at 6 ℃ with10 mg/L HA. Similarly, it took 6 hours to only observe the GO precipitation in KCl at 6 ℃ with 10 mg/L HA; while GO had completely precipitated at the same time in the case of no HA or 1mg/L HA. These visual observations confirmed not only the strong stabilizing effect of HA and but also that the effect would be weakened by cold temperature in monovalent electrolyte solution. However, GO precipitation did not first show up at 6 ℃ in both NaCl and KCl in the presence of 1 mg/L HA, which contradicts the fact that 6 ℃ had the lowest CCC values in the corresponding experimental conditions. Furthermore, GO particles with 1 mg/L HA at 40 ℃ formed large flocs 19
instead of aggregate, indicating the involvement of flocculation process. The formation of the large flocs during GO aggregation might affect the measurements of the hydrodynamic radius and thus might cause the contradictories. Nevertheless, most of the visual observations are consistent with the trends of the GO aggregation kinetics and attachment efficiencies determined through the hydrodynamic radius measurements.
3.4. Effect of temperature on GO aggregation in presence of divalent cation and HA The changes in Dh of GO with time measured at 6, 25, and 40 ℃ in CaCl2 with 1 mg/L and 10 mg/L HA are presented in Fig. S6, Supporting Information. Even though HA can stabilize the nanoparticles, CaCl2 was effective in promoting GO aggregation for most of the tests (Fig. 6). Different from the monovalent cation system, increase of temperature clearly enhanced GO aggregation in presence of CaCl2 and HA (Fig. 6A-D). The CCC values of CaCl2 for GO aggregation measured in CaCl2 with HA decreased with the temperature (Fig. 6E-F). The GO aggregation was the fastest at 40 ℃, which was confirmed by visual observation experiments showing rapid GO precipitation (Fig. S2C). Although the concentration of divalent cation in this study (within 2 mM) would not result in HA aggregating, Ca2+ might interact with HA to reduce its adsorption on GO surface. It is speculated that high temperature would favor the interaction between Ca2+ and HA [48], and thus would reduce GO stability. Overall, HA continued to behave as effective stabilizing agents but was less effective at higher temperature. 20
4. Conclusions The aggregation kinetics of GO in aqueous solutions were temperature dependent, and the influences of the temperature varied among different combinations of IS, electrolyte type and valence, and HA concentration. Without HA, increasing temperature enhanced GO aggregation in both monovalent (Na+ and K+) and divalent (Ca2+) cation solutions. The temperature effect on GO aggregation was more complicate in the presence of HA. For monovalent electrolytes with HA, the lowest temperature (6 ℃) destabilized GO most efficiently while the aggregation kinetics at 25 and 40 ℃ were close to each other and were notably weaker than that of 6 ℃. This was attributed to that cold temperature greatly reduced the steric repulsion between the HA-coated GO particles. For divalent electrolyte with HA, however, GO aggregation was accelerated by the rise of temperature, probably because of the enhanced interaction between Ca2+ and HA that reduced HA coating on GO. Because the natural water bodies have wide range of temperatures, the effects of temperature on GO aggregation kinetics should be considered as an important factor in the prediction and assessment of its environmental fate and impacts.
Acknowledgment This work was partially supported by the NSF (1213333) and the China Scholarship Council (CSC).
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Fig. 1 Effects of temperature on GO aggregation without HA: (A-C) Changes of GO hydrodynamic radii in 100 mM NaCl, 100 mM KCl, and 0.8 mM CaCl2, respectively; (D-F) initial aggregation rates of GO in NaCl, KCl, and CaCl2, respectively;(G-I) attachment efficiencies of GO in NaCl, KCl, and CaCl2, respectively; (J-L) photograph illustrations of GO suspension at different temperatures after a given time. The numbers in the colored rounded rectangles in (G-I) represent the CCC values of GO in the corresponding electrolyte solutions, and red, yellow and blue colors stand for 40, 25 and 6 ºC, respectively. 30
Fig. 2 Aggregation mechanisms of (A) GO and (B) HA in the presence of divalent cations
31
Fig. 3 Effects of temperature on HA aggregation: (A-C) Changes of HA hydrodynamic radii in 2000 mM NaCl, 2000 mM KCl, and 5 mM CaCl2, respectively; (D) Initial aggregation rates of HA in CaCl2; (E) attachment efficiencies of HA in CaCl2. The numbers in the colored rounded rectangles in (E) represent the CCC values of HA in the corresponding electrolyte solutions, and red, yellow and blue colors stand for 40, 25 and 6 ºC, respectively.
32
Fig. 4 Effects of temperature on GO aggregation in the presence of HA and Na+: (A-B) Changes of GO hydrodynamic radii in 200 mM NaCl with 1 and 10 mg/L HA, respectively; (C-D) initial aggregation rates of GO with 1 and 10 mg/L HA, respectively; (E-F) attachment efficiencies of GO with 1 and 10 mg/L HA, respectively;(G-H) photograph illustrations of GO in 200 mM NaCl solution with 1 and 10 mg/L HA at different temperatures after a given time. The numbers in the colored rounded rectangles in (E-F) represent the CCC values of GO in the corresponding electrolyte solutions, and red, yellow and blue colors stand for 40, 25 and 6 ºC, respectively. 33
Fig. 5 Effects of temperature on GO aggregation in the presence of HA and K+: (A-B) Changes of GO hydrodynamic radii in 100 mM KCl with 1 and 10 mg/L HA, respectively; (C-D) initial aggregation rates of GO with 1 and 10 mg/L HA, respectively; (E-F) attachment efficiencies of GO with 1 and 10 mg/L HA, respectively;(G-H) photograph illustrations of GO in 100 mM KCl solution with 1 and 10 mg/L HA at different temperatures after a given time. The numbers in the colored rounded rectangles in (E-F) represent the CCC values of GO in the corresponding electrolyte solutions, and red, yellow and blue colors stand for 40, 25 and 6 ºC, respectively. 34
Fig. 6 Effects of temperature on GO aggregation in the presence of HA and Ca2+: (A-B) Changes of GO hydrodynamic radii in 0.8 mM CaCl2 with 1 and 10 mg/L HA, respectively; (C-D) initial aggregation rates of GO with 1 and 10 mg/L HA, respectively; (E-F) attachment efficiencies of GO with 1 and 10 mg/L HA, respectively;(G-H) photograph illustrations of GO in 0.8 mM CaCl2 solution with 1 and 10 mg/L HA at different temperatures after a given time. The numbers in the colored rounded rectangles in (E-F) represent the CCC values of GO in the corresponding electrolyte solutions, and red, yellow and blue colors stand for 40, 25 and 6 ºC, respectively. 35