Experimental Energy Barriers to Anions Transporting through

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Experimental Energy Barriers to Anions Transporting through Nanofiltration Membranes Laura A. Richards,†,‡ Bryce S. Richards,† Ben Corry,§ and Andrea I. Schaf̈ er*,‡ †

School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, United Kingdom School of Engineering, The University of Edinburgh, Edinburgh EH9 3JL, United Kingdom § Research School of Biology, Australian National University, Canberra ACT 0200, Australia ‡

S Supporting Information *

ABSTRACT: Environmentally relevant contaminants fluoride, chloride, nitrate, and nitrite face Arrhenius energy barriers during transport through nanofiltration (NF) membranes. The energy barriers were quantified using crossflow filtration experiments and were in the range of 7−17 kcal·mol−1, according to ion type and membrane type (Filmtec NF90 and NF270). Fluoride faced a comparatively high energy barrier for both membranes. This can be explained by the strong hydration energy of fluoride rather than other ion properties such as bare ion radius, fully hydrated radius, Stokes radius, diffusion coefficient, or ion charge. The energy barrier for fluoride decreased with pressure, indicating an impact of directional force on energy barriers. The influence of temperature-induced pore radius variability and viscosity on energy barriers was considered. The novel link between energy barriers and ion properties emphasizes the importance of ion hydration and/or partial dehydration mechanisms in determining transport in NF.

1. INTRODUCTION A great challenge facing humanity is inadequate access to clean water due to the increasing contamination of freshwater and groundwater systems worldwide with various pollutants.1−4 Certain inorganic pollutants (fluoride, chloride, nitrate, and nitrite) enter water supplies through natural and anthropogenic means and are of particular interest due to their environmental relevance and adverse effects on human health. High fluoride concentration in drinking water is associated with fluorosis and poses a health threat to millions of people globally.5,6 Chloride in water sources is important due to desalination processes.1 Nitrate and nitrite pollution is associated with detrimental health effects,7−9 algal productivity,10 and the formation of nitrogen-based disinfection byproducts.11 Understanding the mechanisms which control the removal of these contaminants in membranes is critically important not only to the development of the next generation of membrane technology but also to protecting public health. The transport of water and solutes through nanofiltration (NF) membranes is substantially hindered because membrane “pores” are similar in size to the ionic and/or hydrated dimensions of partially retained solutes.12 This hindrance creates a net energy barrier, resulting from all mechanisms that affect transport, which must be overcome if solute transport is to occur. Well-accepted transport phenomena include size exclusion,13−15 charge interactions (including electroneutrality maintenance),16−20 sorptive interactions,21−24 diffusion,25−28 and convection.20,29 A more recently proposed phenomenon is the partial or full dehydration of hydrated solutes during transport.14,30−36 In simulations of idealized, neutral pores, © 2013 American Chemical Society

partial dehydration is the dominant contribution to energy barriers when pores are smaller than the fully hydrated ion.30,31,35,36 Experimentally, however, this mechanism has only been discussed by qualitatively correlating retention with trends in hydrated size and/or hydration energy.14,32−34 Although retention is a standard NF performance evaluator, it is limited in describing complex mechanisms as it only provides information about the ratio of feed and permeate concentrations. Energy barriers provide additional information to retention, allowing the linkage of experimental solute transport measurements with fundamental ion properties like hydration free energies and theoretical energy barriers. Determination of energy barriers may therefore provide new insight regarding the dehydration mechanism, where previous studies have been limited. Improving the understanding of solute dehydration during transport is particularly important because hydration is not considered in Extended Nernst− Planck models used to predict transport in membranes.19,20,37−44 Energy barriers can be overcome by any driving force, such as directional pressure, flow and concentration,25−28,40 and nondirectional temperature. An increase in system temperature increases internal energy, solute diffusion, and polymer chain mobility, reduces solvent viscosity, and can lead to changes in pore size/membrane structure.12,23,45−47 Temperature is the Received: Revised: Accepted: Published: 1968

September 27, 2012 December 15, 2012 January 8, 2013 January 8, 2013 dx.doi.org/10.1021/es303925r | Environ. Sci. Technol. 2013, 47, 1968−1976

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M but chloride in seawater averages approximately 0.6 M), in order to reduce experimental variables, to be sufficiently high for large differences in retention and to be consistent with complementary computer simulations,30,36 thus allowing the probe of mechanistic questions. The World Health Organization provides detailed information on typical and extreme environmental concentrations of these contaminants.58 The pH of 6.2 was not adjusted to avoid the addition of any competing chemicals in solution. The salts were NaF (purity 99+%), NaCl (99.9%), NaNO3 (98+%), and NaNO2 (97+%) (all Fisher Scientific, UK). 2.2. Chemical Analysis. Feed and permeate samples were analyzed for ion concentration. Nitrate and nitrite were analyzed with a nutrient analyzer (Lachat QuikChem 8500, USA), fluoride with an ion selective electrode (6.0502.150, Metrohm, UK) with an Ag/AgCl reference electrode and ion meter (781 Ion Meter, Metrohm, UK), and chloride with an ion chromatograph (Metrohm 883 Basic IC Plus, UK). Analytical uncertainty was based on at least six independent replicate sample measurements and was approximately ±2% for fluoride, ±2% for chloride, ±3% for nitrate, and ±4% for nitrite, which inherently includes analytical error and chemical purity. Conductivity and pH were measured with standard probes and meters (WTW, Germany). 2.3. Data Analysis. After experiments had been conducted and samples analyzed, retention was calculated. Real retention

standard driving force used to quantify energy barriers via the Arrhenius method48 and has been used to determine energy barriers in membranes.26,27,45,49−56 The temperature dependence of the flux of a specific solute through a membrane can be described by the Arrhenius relationship if it is linear. The energy barriers represent the net energetic expense of solute transport and thus include contributions from all acting mechanisms. Previously reported experimental energy barriers for solutes transporting across membranes range from −0.5 to 47.5 kcal·mol−1 (Table S1) and have been linked to Stokes radius and molecular weight,12 steric hindrance,56 charge repulsion,56 pressure,26,55 pH,51 concentration,23,51 and enthalpy and entropy.23 However, the link between experimental energy barriers and ion hydration properties has not yet been made. The aim of this work is to link experimental energy barriers with transport mechanisms to gain new insight into the role of hydration in determining selectivity in NF. This will be achieved by experimentally quantifying energy barriers for the transport of anions of identical charge, which allows the isolation of size and hydration based mechanisms. The objectives are to (i) quantify Arrhenius energy barriers using solute flux measurements at different temperatures for the targeted anions (fluoride, chloride, nitrate, and nitrite) and membranes (NF90 and NF270); (ii) determine the effect of applied pressure on energy barriers; (iii) consider the influence of temperature-induced membrane and solution changes on energy barriers; and (iv) relate energy barriers to ion hydration properties (hydration free energy, hydrated size and partial dehydration requirements).

Rr = 1 −

c permeate cmembrane

(1)

depends on the analyzed solute concentration in the permeate (cpermeate, mol·L−1) and at the membrane surface (cmembrane, mol·L−1). cmembrane is the bulk concentration corrected for concentration polarization. Concentration polarization was determined using film theory and methods previously described in detail.59−61 Energy barriers were quantified using the Arrhenius relationship

2. METHODS AND MATERIALS 2.1. Experiments. A bench-scale stainless steel crossflow membrane filtration system was used, consisting of a 46.0 cm2 flat sheet membrane cell, 2.5 L feed tank, and high-pressure diaphragm pump (Hydra-Cell P200, UK).57 Temperature was controlled by a water bath (Lauda WK 700, Germany) piped to a cooling jacket around the feed tanks. Pressure was controlled by a back pressure regulator (Swagelok KPB Series, UK). A datalogger (Omega DAQ55, USA) monitored and recorded inline pressure in the feed and retentate (S model pressure transducer, Swagelok, UK), feed flow rate (Hydrasun M2SSPI, UK), and temperature (Condustrie-Metag WTM Pt 100-0-6, Germany). Pure water permeability was determined from measuring permeate weight with an electronic balance (Ohaus Adventurer Pro, UK) and stopwatch. Prior to each experiment, membranes were compacted for at least one hour, or until pure water flux stabilized, at 15 bar. The feed flow was fixed at 2 L·min−1 and recirculated. First, the temperature was set at 15 °C and the system stabilized for one hour after the temperature was reached. Then, pressure was adjusted in increasing order from 3 to 11 bar in steps of 2 bar. Samples were collected from the feed and permeate, each as a single 20 mL aliquot, after 30 min at a given pressure. The next experiment was then conducted at the next increasing temperature (20, 25, 30, and 35 °C) using the same membrane coupon. Pure water flux was checked in between each experiment. The filtration solution was 0.1 M of a single salt per experiment dissolved in ultrapure water (18.2 MΩ.cm−1, Elga PURELAB Ultra, UK) and was selected for mechanistic simplification. The consistent concentration of 0.1 M was selected for all salts, despite differences in typical environmental concentrations (for example nitrate is typically less than 0.001

ln(ji ) = ln(A) −

⎛ Ea 1 ⎞ ⎜ · ⎟ ⎝R T⎠

(2)

where ji is solute flux (mol·h−1·m−2), A is a constant, R is the gas constant (1.985 × 10−3 kcal·mol−1·K−1), T is temperature (K), and Ea is the energy barrier (kcal·mol−1).48 Energy barriers were determined using four steps: (i) Determination of solute flux using ji = c permeate·Jv

(3)

where Jv represents volumetric flux (L·h−1·m−2). (ii) Plotting solute flux at each temperature against effective transmembrane pressure (applied pressure − osmotic pressure of the boundary layer), which is the effective pressure driving force (Figure S1). A line was fit to this plot and the y-intercept extrapolated to allow for quantification of energy barriers at the zero pressure point where convection and concentration polarization have a minimal impact on solute flux. Nonextrapolated energy barriers were also determined directly at each pressure. (iii) Creation of an Arrhenius plot of the natural log of the solute flux at each pressure against the reciprocal of the temperature. 1969

dx.doi.org/10.1021/es303925r | Environ. Sci. Technol. 2013, 47, 1968−1976

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Table 1. Membrane Characterization of NF90 and NF270 membrane

pure water permeability, 20 °C (Lp, mol·h−1·m−2·bar−1)

effective pore radius, 20 °C (Rpore, 10−10 m)

average active layer thickness to porosity ratio (L/ε, 10−6 m)

zeta potential, pH 6.2 (mV)a

NF90 NF270

11.1 ± 0.2 12.4 ± 0.3

3.4 ± 0.13 3.8 ± 0.13

1.46 ± 0.07 1.01 ± 0.05

−18 ± 2 −10 ± 1

a

Measured in 20 mM NaCl and 1 mM NaHCO3

selectivity of monovalent anions, real retention of ions in single salt solutions was determined for fluoride, chloride, nitrate, and nitrite with NF90 and NF270 (Figure 1). Retention increases

(iv) Determination of the slope of the Arrhenius plot, which is related to the energy barrier divided by the gas constant, as shown in eq 2. Solute flux values were normalized to the solute flux at the lowest temperature (15 °C) to facilitate comparison of membranes with different intrinsic permeabilities. Normalization does not affect the energy barrier calculation. The inherent assumptions are that the process follows Arrhenius behavior, which is validated if the Arrhenius plot is linear, and that the energy barrier is a net effect from all mechanistic, operational, membrane, and environmental contributions. Note that the calculation of energy barriers does not include any calculated compensation for concentration polarization; this effect is only indirectly included due to the impact of concentration polarization on the obtained experimental measurements. Uncertainty (δ) in calculations was assessed using standard propagation techniques.62 Where the propagation calculation could not be directly applied for real retention and the fitting procedure in membrane characterization, uncertainty was estimated by rerunning the calculation with the largest expected errors in input parameters to determine the variation in the outputs. 2.4. Membrane Type and Characterization. Commercially available NF membranes NF90 and NF270 from Dow Filmtec (MN, USA) were selected because their effective pore radius is similar to the solutes’ hydrated radii. NF90 and NF270 consist of a polyester support, a middle layer of microporous polysulphone, and an ultrathin active layer (fully aromatic polyamide for NF90 and a modified, piperazine-based semiaromatic polyamide for NF270).63,64 Pure water permeability (Lp), effective pore radius (Rpore), the ratio of membrane active layer thickness/porosity (L/ε), and zeta potential were determined (Table 1). To reduce the variation in different membrane coupons within the same batch, 46.0 cm2 coupons were selected to have a pure water flux within 10% of the characterized coupon (variation in flux measurement of one coupon ±2%). Rpore and L/ε were obtained by the hydrodynamic model13,43 using methodology as described previously.24,60 Characterization calculations were based on Stokes radius, and standard bulk diffusion coefficients were corrected for hindrance in the membrane. The variation of Rpore with temperature was determined using the inert organic solute xylose, and the diffusion coefficient used in the fitting method was adjusted for temperature assuming the same slope as the linear relationship between temperature and the diffusion coefficient of dextrose.65 Membrane zeta potential was determined using streaming potential measurements at pH 6.2 with an electrokinetic analyzer (EKA, Anton Paar KG, Austria, with Ag/AgCl-electrodes SE 4.2, Sensortechnik Mesinsberg, Germany).

Figure 1. Real retention for each anion as 0.1 M sodium salt at 25 °C and pH 6.2 for (A) NF90 and (B) NF270.

with pressure for all ions and both membranes. This is attributed to real retention approaching the reflection coefficient at high fluxes, and thus the permeate concentration decreases as pressure and concentration polarization increase.44,66,67 As expected, retention is higher for the tighter membrane NF90. High selectivity exists between anions for both membranes, with the retention sequence being fluoride > chloride > nitrate > nitrite. The differences in retention are especially distinguishable in NF90, where fluoride retention is 96%, chloride is 86%, nitrate is 80%, and nitrite is 58%. Similar trends have been observed previously even in an applied study with real groundwater, demonstrating that this same selectivity exists even when there are differences in concentration and competing chemicals in solution.68 Ion characteristics are summarized in Table 2. A similar ordering of anion retention has been observed previously and explained by differences in hydrated radius or hydrated strength.15,23,32,33,66,69 Retention trends (Figure 1) and anion properties (Table 2) highlight the hypothesis that hydration properties control selectivity in NF. The differences in retention of these monovalent anions cannot be adequately explained by the bare ion radii, hydrated radii, Stokes radii, diffusion coefficients, or charge. The bare ion radius does not explain this ordering, as fluoride is the smallest bare ion yet has the highest retention.

3. RESULTS AND DISCUSSION 3.1. Retention of Monovalent Anions (Fluoride, Chloride, Nitrate, and Nitrite) in NF. To establish the 1970

dx.doi.org/10.1021/es303925r | Environ. Sci. Technol. 2013, 47, 1968−1976

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Table 2. Monovalent Anion Propertiesd parameter

fluoride

chloride

nitrate

nitrite

ionic radius (Rion, 10−10 m)a Stokes radius (Rstokes, 10−10 m) hydrated radius (Rhyd, 10−10 m)b hydration free energy (ΔGhyd, kcal·mol−1) bulk diffusion coefficient, 25 °C (Dw, 10−9 m2·s−1)

1.3 71 1.7 72 3.4 30 −119.7

1.8 71 1.2 72 3.8 30 −89.6

3.0 30 1.3 72 5.1 30 −73.1

3.0 30 1.3c 25 5.0 30 −81.0

73

1.46

48

73

2.03

75

74

1.9

75

pore radius is of very similar size to the hydrated radius of fluoride’s first water shell but still applies even when the pore radius is larger than the radius of the first hydration shell of fluoride (for NF270) due to the attraction of more distant water molecules.30 Because the strong hydration energy of fluoride is closely related to both the small ionic radius and the strong electronegativity of the ion, there is an inherent interplay between size, charge, and hydration properties. In order to validate this hypothesis, energy barriers will be quantified and linked to hydration free energies and simulated energy barriers. 3.2. Quantifying Energy Barriers as a Function of Anion and Membrane Type. In order to quantify energy barriers for the transport of monovalent anions in NF using the Arrhenius methodology, the relationship between solute flux and temperature was determined (Figures 2 and 3). Figure 2 shows that the solute flux of fluoride increases both with pressure and temperature. Solute flux increases with pressure due to increased convective flow20,29 and with temperature due to increased diffusion, decreased viscosity, and changes in

74

1.91

76

For nitrate and nitrite, Rion = bN−O (1.22 × 10−10 m) + Rion,oxygen (1.77 × 10−10 m).30 bFor nitrate and nitrite, Rhyd = bN−O (1.22 × 10−10 m) + Radial Distribution Functionmin.30 All hydrated radii signify the radius of the f irst hydration shell only and are defined from one source to avoid ambiguity in definition.30 cCalculated from diffusion coefficient.25 dEach anion was used as a sodium salt. a

The fully hydrated radius yields no explanation either, as nitrate and nitrite are the largest hydrated ions yet their retention is relatively low. Stokes radius, representing the effective size of a theoretical solid sphere diffusing at the same speed as the target ion,70 also provides no explanation. Although fluoride has the highest Stokes radius and the highest retention, chloride, nitrate, and nitrite have very similar Stokes radii, and thus this parameter is unable to distinguish the differing retention of these ions observed for NF90. Bulk diffusion coefficients are equally insufficient to explain the retention order, as chloride has the highest diffusion coefficient, so retention would be expected to be lowest, but this was not observed. It is important to note that Stokes radius and diffusion coefficients are the ionspecific parameters in the Extended Nernst−Planck based models,20,59 and neither of these adequately describes the observed behavior. Charge also cannot explain the ordering, as all have an equal net charge of −1 at this pH and thus would be repelled with equal strength from the negatively charged membrane surface. Differences in charge density or where the charge is located on the ion will not affect the overall charge repulsion based on the same net charge. The electronegativity of fluoride is the largest, which is due to fluoride’s small ionic radius and high hydration energy. These properties are interlinked and not independent. Qualitatively, the high retention of fluoride means that the transport of fluoride has the highest associated energy barrier because transport is the most hindered. However, energy barriers for the transport of these ions in these membranes have not been quantified. Here it is proposed that the ions completely or partially dehydrate from bound water molecules in their hydration shell in order to pass through the membrane, which may offer a suitable explanation for the observed selectivity of the membranes.30,36 Dehydration could occur as a result of limited space availability, when the pore entrance is smaller than the fully hydrated ion, and is due to forces on the ion “squeezing” it through the membrane via pressure, concentration gradient, and/or temperature, resulting in the loss of bound water molecules. If dehydration was occurring during passage, retention trends and the magnitude of energy barriers would correlate with the hydration free energies of the ions and the amount of dehydration required for each. The very strong hydration energy of fluoride means that the energetic expense of stripping water molecules from fluoride’s hydration shell is very high, making transport unfavorable. Hydration energy is the only ion property in Table 2 that can explain the high retention of fluoride observed in Figure 1. This is particularly obvious for the tighter membrane NF90 where the

Figure 2. Solute flux versus applied pressure for fluoride as 0.1 M NaF at pH 6.2 with (A) NF90 and (B) NF270. Note that the scales are different for the membranes.

Figure 3. Solute flux of fluoride, chloride, nitrite, and nitrate as 0.1 M sodium salt at pH 6.2, 25 °C, and 7 bar. 1971

dx.doi.org/10.1021/es303925r | Environ. Sci. Technol. 2013, 47, 1968−1976

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membrane properties.12,23,51,77 Nonzero solute fluxes occurring where there is minimal pressure driving force arise from concentration-driven/diffusive transport. When effective transmembrane pressure is considered, the relationship shown in Figure 2 shifts to the left, but this has no impact on energy barriers. While results are shown for fluoride only, the same trends were observed for each of the salts. The linearity of solute flux and temperature validates the applicability of the Arrhenius relationship. Figure 3 shows the solute flux of each anion and for both membranes for a single pressure and temperature. Solute flux depends on ion type and membrane type. Fluoride has the lowest solute flux and thus faces a larger energy barrier than the other ions, which is consistent with fluoride’s high retention. The ratio of the relative solute flux of the ions is constant with pressure for a particular temperature which is consistent with the principles of the solution-diffusion model for solute transport. Having determined solute flux as a function of temperature, pressure, and ion type, energy barriers can be quantified. Figure 4 shows Arrhenius plots and energy barriers for all solutes and both membranes as determined by relating solute

ion. As noted earlier, the property of hydration energy is inherently linked to other ion properties and thus affects charge and size exclusion mechanisms as well. The high energy barrier of fluoride compared to other monovalent anions cannot be adequately explained by any other transport mechanism. The energy barriers for solutes other than fluoride (chloride, nitrate, and nitrite) are difficult to distinguish outside the error region for NF90, which is consistent with the hydration energies of these solutes which are far closer to each other than to fluoride. More distinction between solutes was observed for NF270, which could be due to differences in the membrane properties of NF270 such as a narrower span of the pore size distribution78 or the modifications in the active layer.64 Energy barriers are higher for the tighter membrane (NF90), which is consistent with the smaller pore size, higher retention, and thus more hindered transport in NF90. Relatively high energy barriers are still observed for fluoride with membrane NF270, even though the membrane pore radius is larger than the inner hydrated radius, due to the displacement or reorientation of water molecules in the second and further hydration shells.30 The experimentally determined energy barriers shown are in the same order of magnitude as those determined with molecular dynamics simulations of the transport of these same anions in an idealized, neutral pore.30,36 Dehydrationbased energy barriers for idealized pores of sizes similar to that of NF90 (Reff,simulation = 3.32 × 10−10) ranged from 6.5 kcal·mol−1 for nitrate to 27.4 kcal·mol−1 for fluoride. For a pore size similar to that of NF270 (Reff,simulation = 3.72 × 10−10), energy barriers ranged from 1.6 kcal·mol−1 for nitrate to 10.6 kcal·mol−1 for fluoride. Energy barriers were shown to originate at the pore entrance,30,79 which parallels the hypothesis that experimental energy barriers arise at the pore entrance and are comparatively small throughout the support layer. Even though these simulations did not consider membrane properties important in NF (for example pore size distributions, charge, and functional groups) it is very noteworthy that the magnitude and ordering of energy barriers are similar to those obtained experimentally. In simulations the energy barriers were shown to be directly attributed to the partial dehydration of the ions required for them to fit in the pore when it was smaller than the fully hydrated radius of the ion. The trends in experimental energy barriers and the good agreement of experimental data with dehydration-based simulations provide strong support that dehydration may be occurring in NF. It is important to note, however, that these energy barriers represent the overall energetic expense of ion transport and thus include all net membrane effects (such as pore size, material and surface charge, which cannot be individually isolated), all operating conditions (including pressure, flow, concentration, solution chemistry), all energetic contributions (entropic, enthalpic), and the properties of the specific ion. Because the membrane characteristics and operating conditions were consistent in all experiments, the differences are due to ion-specific contributions. 3.3. Quantifying Energy Barriers as a Function of Pressure. Energy barriers were directly quantified at each pressure to determine the pressure contributions (Figure 5). Energy barriers reduce with pressure, which the most substantial reduction being for fluoride with NF90. It appears that the directional pressure force applied to the pore makes dehydration more likely for fluoride, the most strongly hydrated solute. This could be explained by pressure driving fluoride to partially separate from its full hydration shell in order to fit

Figure 4. Arrhenius plots for anions as 0.1 M sodium salt at pH 6.2 for (A) NF90 and (B) NF270.

flux at zero pressure to temperature. Energy barriers depend on ion type and membrane type and range from 9.6−17.0 kcal·mol−1 for NF90 and from 7.0−14.3 kcal·mol−1 for NF270. Fluoride, the most strongly hydrated ion, has the highest energy barrier for both membranes, which is seen in the strong temperature dependence of solute flux leading to a steep slope on the Arrhenius plot of fluoride. This energy barrier is attributed to the high hydration strength of fluoride and thus the associated high energetic cost of partial dehydration during transport. Linking energy barriers and the hydration strength of fluoride provides important information that retention cannot. The high energy barriers of fluoride confirm that transport is highly hindered and can be explained by the high energetic expense required to partially dehydrate a very strongly hydrated 1972

dx.doi.org/10.1021/es303925r | Environ. Sci. Technol. 2013, 47, 1968−1976

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Figure 6. (A) Effective pore radius (Reff) and (B) pure water permeability (Lp) for single coupons of NF90 and NF270.

Figure 5. Pressure-dependent energy barriers for (A) NF90 and (B) NF270 as 0.1 M single sodium salt at pH 6.2.

Changes in membrane properties and flux due to temperature are inherently included in the Arrhenius calculations. The increase in pore size and decreased water viscosity leads to the energy barriers previously shown in Figure 4 and 5 being overestimates of what they would be if the pore size remained constant. This is because the increased membrane pore size leads to higher solute flux, higher temperature dependence of solute flux, and thus higher calculated energy barriers than if the pore size did not change. However, this inherent effect cannot be quantitatively isolated. The energy barrier for pure water transport in NF90 and NF270 is 8.5 ± 0.3 kcal·mol−1 and 6.5 ± 0.2 kcal·mol−1, respectively. The pure water energy barriers include the same membrane and viscous effects which cause the measured barriers to be larger than what would be obtained if the pore size and viscosity remained constant. All experimentally determined energy barriers for solute transport are greater than the barriers for pure water transport, which confirms that barriers are ion-specific and more substantial than the membrane and viscous contributions alone. The energy barriers of the transport of single monovalent salts in NF provide novel experimental evidence that hydration effects in NF are significant. Incorporating such effects into models will be beneficial for understanding NF transport mechanisms and for the development of next-generation membranes. This could be done, for example, by developing models which incorporate dehydration energy barriers or an effective “partially hydrated” radius. Further future work would be to conduct a similar experimental study with carbon nanotube composite membranes, which would reduce the variability in pore size and structure inherent in polymeric membranes as well as reducing concentration polarization, thus allowing improved comparison between experiment and simulations of idealized pore structures. Comparing the energy barriers obtained in this study with energy barriers of contaminant transport in real groundwater would allow the

through the pore. The decrease in the energy barrier of fluoride is most notable for the tighter membrane NF90, indicating that membranes with smaller pore sizes are more susceptible to pressure effects and is consistent with the dehydration hypothesis. At pressure above 9 bar, the energy barrier of fluoride has decreased to the extent that the fluoride’s barrier is indistinguishable from that of the other ions. The energy barriers are indistinguishable at high pressure even though retentions are still different due to the different meanings of these parameters. The energy barriers represent the energetics of the process, whereas the retention of the ions approaches the ion-dependent reflection coefficient. For the less hindered membrane NF270, the impact of pressure on energy barriers is less pronounced, and fluoride maintains the highest barrier of the solutes for all pressures. A previous study conducted by Kim et al.55 did not observe an impact of pressure on energy barrier for a membrane with larger pore size than NF270, which supports that the smallest pore sizes are more susceptible to pressure effects. It is important to note that increased pressure leads to increased convection, concentration polarization, and diffusion. Thus, the decrease in the energy barrier of fluoride with pressure cannot be attributed solely to an increased directional force on the solutes. 3.4. Impact of Temperature-Induced Variation in Membrane Pore Size and Water Viscosity on Energy Barriers. The influence of temperature-induced membrane and viscous effects on energy barriers was considered (Figure 6). This impact is expected to be consistent regardless of ion type. The increase in pore size with temperature is due to changes in polymer structure, 12 and the increase in pure water permeability is due to increased pore size, increased diffusion, and decreased viscosity.12,23,45−47 Membrane tortuosity, the thickness/porosity ratio, and effective charge density of membranes can also be influenced by temperature.23,47 1973

dx.doi.org/10.1021/es303925r | Environ. Sci. Technol. 2013, 47, 1968−1976

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determination of the influence of environmentally relevant water matrices and/or membrane fouling on the proposed dehydration mechanism in application-relevant scenarios. These results demonstrate a new way to consider the transport of environmentally relevant contaminants in nanofiltration membranes, thus providing useful information ultimately for desalination and water reuse applications.

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ASSOCIATED CONTENT

S Supporting Information *

A summary of key literature on experimental energy barriers for membranes is provided in Table S1 and a pressure gradient schematic is provided in Figure S1. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +44 131650 7209. Fax: +44 131650 6781. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Viatcheslav Freger (Ben-Gurion University of the Negev), Anthony Szymczyk (Université de Rennes), and Graham Craik (Heriot-Watt University) for valuable discussions and proofreading. Rasha Ruhayel is thanked for suggesting the use of temperature in experiments. Partial project funding was provided by the Leverhulme Royal Society Africa Award (SADWAT-Ghana) and GE Global Research. Membrane samples were kindly provided by Dow Filmtec. Membrane streaming potential measurements were made by Annalisa De Munari (University of Edinburgh) and Kingsley Ho and Alexander Bismarck (Imperial College London). The PhD studentship for Laura Richards was provided by Overseas Research Students Awards Scheme and James Watt Scholarship.

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REFERENCES

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