Elucidating the Rejection Mechanisms of PPCPs by Nanofiltration and

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Elucidating the Rejection Mechanisms of PPCPs by Nanofiltration and Reverse Osmosis Membranes Yi-Li Lin* and Chung-Hsiang Lee Department of Safety, Health and Environmental Engineering, National Kaohsiung First University of Science and Technology, Kaohsiung 824, Taiwan, R.O.C. S Supporting Information *

ABSTRACT: In this study, the rejection mechanisms of six commonly detected pharmaceutical and personal care products (PPCPs) were systematically studied with three commercial thin film composite polyamide reverse osmosis (RO, XLE) and nanofiltration (NF, NF90, and NF270) membranes at pH 3−10. The amount of PPCP adsorption on membrane surfaces was also extracted and calculated so as to determine the contribution of adsorption mechanism on PPCP rejection using adsorption kinetics and adsorption isotherms. At low pHs, PPCP rejection was the highest for XLE followed by NF90 and NF270. As pH increased to 10, PPCP rejection increased significantly for NF90 and NF270, attributed to the enhanced electrostatic repulsion between the negatively charged membrane surface and ionized PPCPs, while being slightly increased for XLE due to the dominant mechanism of steric hindrance. The simplified charge concentration polarization model predicted well for most cases, which demonstrated the contribution of steric hindrance and electrostatic repulsion mechanisms in PPCP rejection by NF and RO membranes. However, two groups of bias were observed during model prediction. One is the underestimated group of small ionized PPCPs at high pH values (8 and 10) by NF270 because of the electrostatic repulsion between the small ionized PPCPs and NF270 overwhelmed the increase in permeate flux by membrane swelling. The other group is the overestimated rejection of triclosan (TRI), a highly hydrophobic compound adsorbing onto membrane surfaces leading to its diffusion and penetration through membranes, which can be confirmed by the extraction of TRI mainly from the top polyamide plus polysulfone membrane layers and also the bottom polyester layer after filtration experiments even at high pH values. The competitive adsorption of TRI and ibuprofen was observed in static adsorption kinetic experiments, and the adsorption can be explained well by the first-order reaction model. The adsorption isotherm data fitted best with the Freundlich model in all cases with the n value indicating chemical adsorption, which would be a hydrophobic interaction between PPCPs and the membrane surface in this study. zil, and mefenamic acid. However, Comerton, et al.6 reported high rejection of acetaminophen (85%) and gemfibrozil (98%) in lake water by polyamide NF membranes. Discrepancy in PPCP rejections by NF/RO happen in literature, which is attributed to the complex interaction between membranes, solutes, and water matrix. The rejection of organic compounds by membrane filtration can be affected by the physicochemical properties of the membrane (e.g., molecular weight cutoff (MWCO), surface charge, and hydrophobicity), solute (e.g., molecular size, geometry, charge, and hydrophobicity), characteristics of feedwater (e.g., pH, ionic strength, and presence of other organics/inorganics), as well as hydrodynamic conditions (e.g., transmembrane pressure, cross-flow velocity, and channel configuration).3,6,7,8 Three major rejection mechanisms by NF/RO are reported, including size exclusion (steric hindrance), electrostatic repulsion, and hydrophobic interactions between membrane and solute (e.g., adsorption and cake layer formation).7 Many research explained PPCP rejections using one or two of the major rejection mechanisms,9−13 but

1. INTRODUCTION The occurrence of pharmaceutically active compounds (PhACs) and pharmaceuticals and personal care products (PPCPs) in the aquatic environment has caused much attention in the past few years. Although the concentration of PPCPs are usually detected at trace levels (ng/L to μg/L) after entering the environment via the major source of effluent from hospitals and wastewater treatment plants,1,2 bioconcentration and interaction of these compounds could cause adverse physiological consequences on organisms after long-term exposure.3,4 In order to prevent potential risk to humans and wildlife as well as to increase the possibility of water reuse, it is essential to implement advanced treatment technologies, such as membrane processes, as an extra barrier to remove PPCPs from treated effluent. With the development of membrane materials in this decade, new nanofiltration (NF) and reverse osmosis (RO) membranes are invented with low cost, low power consumption, high flux, and high permeate quality so that NF/RO membrane processes become an attractive technology for the removal of small organic micropollutants. For instance, Radjenović et al.5 studied the removal of 31 pharmaceuticals in groundwater by polyamide NF and RO in a full-scale drinking water treatment plant and reported high rejection for almost all the investigated pharmaceuticals (>85%) except for acetaminophen, gemfibro© 2014 American Chemical Society

Received: Revised: Accepted: Published: 6798

November 18, 2013 March 10, 2014 March 11, 2014 March 11, 2014 dx.doi.org/10.1021/ie500114r | Ind. Eng. Chem. Res. 2014, 53, 6798−6806

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Figure 1. Filtration setup for PPCP rejection experiments.

thoxazole (SMX), and sulfamethazine (SMZ) are frequently detected in Taiwan’s surface waters14 and were selected as the model PPCPs. According to the acidic dissociation constant (pKa) and hydrophobicity (log Kow), these PPCPs can be classified as ionic (I)/nonionic (N) and hydrophobic (HPO)/ hydrophilic (HPI), respectively, and the details of the physicochemical properties of model PPCPs are shown in Table S2 in the Supporting Information. PPCPs concentrations were analyzed with a high-performance liquid chromatograph (LC-20A Prominence, Shimadzu, Japan) equipped with a RP-C18 column (Mightysil, Kanto Taiwan Corp., particle size 5 μm, 4.6 mm × 250 mm), a C18 Gemini guard column (Phenomenex, U.S.A), and a photodiode array detector (SPD-M20A, Shimadzu, Japan). The detection wavelength was 270 nm for DIA, SMZ, SMX, and CBZ and 213 nm for IBU and TRI. The mobile phase consisted of 30% acetonitrile (A) and 70% 0.025 M KH2PO4 buffer solution (B) for the analysis of DIA, SMZ, SMX, and CBZ and 70% A/30% B for the analysis of IBU and TRI with a flow rate of 1 mL/min. The detection limit of each PPCP was around 4 μg/L with an injection volume of 100 μL 2.3. Filtration System and Protocol. Three flat-sheet cross-flow membrane cells with a channel height of 2 mm and an active membrane surface area of 137.75 cm2 were configured in parallel and operated in recycled mode, and the scheme of setup is shown in Figure 1. The mass transfer coefficient was calculated as 2.0 × 10−5 m/s in the filtration cell. Membrane coupons were soaked in distilled (DI) water over 24 h before use and then were compacted using DI water for 12 h to reach steady-state permeate flux. The experiments were conducted at cross-flow velocity and transmembrane pressure of 0.1 m/s and

comprehensive elucidation and quantification for each of them are rare. The objectives of this study were to elucidate comprehensively the rejection mechanisms of PPCPs by two NF and one RO membranes. Six commonly detected PPCPs in Taiwan with different physicochemical properties, including carbamazepine (CBZ), ibuprofen (IBU), sulfadiazine (DIA), sulfamethoxazole (SMX), sulfamethazone (SMZ), and triclosan (TRI), were investigated systematically at pH 3−10. The amount of PPCP adsorption on membrane surfaces was also extracted and calculated so as to determine the contribution of the adsorption mechanism on PPCP rejections using adsorption kinetics and adsorption isotherm.

2. MATERIALS AND METHODS 2.1. Membranes. One RO (XLE) and two NF membranes (NF90 and NF270) manufactured by Dow FilmTec (U.S.A.) were used in this study. These membranes are made with a thin-film composite (TFC) polyamide layer with a different modification of functional groups on top of an asymmetric polysulfone supporting layer and backed by a nonwoven polyester layer. Details of the physicochemical properties of the membranes are showed in Table S1 in the Supporting Information. NF270 shows the lowest roughness and largest pore size and can be classified as a loose NF membrane, while the pure water permeability and NaCl rejection of NF90 is close to those of the RO membrane and can be classified as a tight NF membrane. All membranes are negatively charged at pH 7 with an isoelectric point at around pH 4. 2.2. Chemicals and Analyses. Carbamazapine (CBZ), triclosan (TRI), ibuprofen (IBU), sulfadiazine (DIA), sulfame6799

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6.9 bar, respectively, and the temperature was kept at 25 ± 0.5 °C using a recirculating chiller (D-606, Deng Yng, Taiwan). A 30 L feed solution spiked with 800 μg/L of each PPCP at once as well as 20 mM NaCl and 1 mM NaHCO3 as background electrolytes was supplied by a Hydracell diaphragm pump (Wanner Engineering, Minneapolis, MN, U.S.A.). One M of NaOH and HCl was used to adjust solution pH to 3, 5, 8, or 10 to test the impact of electrostatic repulsion, an important rejection mechanism in NF membranes. The filtration experiments were conducted for 24 h, and samples were taken regularly from the tank and the permeate for the analysis of PPCPs to ensure steady-state rejection. Observed PPCP rejection (Ro) is defined as ⎛ Cp ⎞ R o (%) = 100 × ⎜1 − ⎟ Cf ⎠ ⎝

Therefore, the rejection of charged PPCPs can be calculated as follows ⎛ Cp ⎞ R charged (%) = 100 × ⎜1 − ⎟ Cf ⎠ ⎝ ⎛ Cp ⎞ ≅ 100 × ⎜1 − βcharge × ⎟ Cm ⎠ ⎝ ⎡ ⎛ R uncharged) ⎞⎤ = 100 × ⎢1 − βcharge⎜1 − ⎟⎥ 100 ⎠⎦ ⎝ ⎣

Knowing Runcharged at low pHs and calculating βcharged using eq 3, the theoretical rejection of PPCPs can be predicted. 2.5. Adsorption Protocol. Static adsorption experiments were performed without transmembrane pressure for the quantification of PPCP adsorption on membrane surfaces only by membrane−solute contact in 1 L glass bottles with the aqueous solution contacting the top layer of the membrane. The initial PPCP cocktail concentrations ranged from 800 to 10,000 μg/L, and solution pHs ranged from 3 to 10. The bottles were shaken for 72 h at 25 ± 0.5 °C, and small amounts of samples were taken for the measurement of PPCP concentrations as time went by. Each experiment was performed in triplicate. Control experiments for TRI alone were carried out for the confirmation of competitive adsorption with IBU in the PPCP cocktail solution. Blank experiments were also carried out for each PPCP concentration without contact with the membrane, and no PPCPs loss due to adsorption onto the tested glass bottles were observed. The amount of PPCP adsorption, Q (μg/cm2) was calculated as follows

(1)

where Cp and Cf are the permeate and the feed concentrations of PPCPs, respectively. After each filtration experiment, each membrane coupon was cut in 4 in. diameter and manually separated as two layers, polyamide plus polysulfone (PA + PS) and polyester (PET), for the extraction using methanol to quantify the adsorbed PPCPs according to the method proposed by Tang et al.15,16 2.4. Simplified Charge Concentration Polarization Model. This model was based on the concept of “charge concentration polarization”, which can be calculated theoretically but also determined experimentally by comparing the PPCP rejections with or without the presence of charge interactions.17 At low pHs where PPCPs are nonionized and membranes are uncharged (or slightly charged), the rejection of PPCPs is regarded without the influence of electrostatic interactions (Runcharged). With the assumption of low hydrodynamic concentration polarization (β not much higher than 1, with 1.4−1.7 for NF90 and XLE and 1.8−5.2 for NF270 calculated using the concentration polarization model), the second and third term of eq 2 are equal.

Q=

(2)

where Cm is the PPCP concentration at the membrane surface. At higher pHs, membranes and some PPCPs will be negatively charged because of the ionization of functional groups so that the rejection will increase by the electrostatic repulsion mechanism. The Gouy−Chapman and DLVO theories for colloids are used to calculate the concentration profile of charged PPCPs close to a charged surface,18 which leads to the Boltzmann distribution shown in eq 3. ⎛ −Z VF ⎞ Cm = exp⎜ m ⎟ ⎝ RT ⎠ Cf

(Ci − Ce) × Vs A

(5)

where Ci and Ce are the initial and equilibrium PPCP concentrations, respectively (μg/L), Vs is the solution volume (L), and A is the effective membrane surface area (cm2). Under the assumptions of limited active sites on the membrane surface and that the adsorption can reach a state of equilibrium as time increases, the adsorption can be expressed using a first-order kinetic model shown in the following

⎛ ⎛ Cp ⎞ Cp ⎞ R unchanged (%) = 100 × ⎜1 − ⎟ ⎟ ≅ 100 × ⎜1 − Cf ⎠ Cm ⎠ ⎝ ⎝

βcharged =

(4)

Q = Q e(1 − e−kt )

(6)

where Qe is the maximum mass adsorbed on the membrane surface at equilibrium (μg/cm2), and k is the rate constant (t−1). 2.6. Freundlich Isotherm Modeling. The PPCP adsorption isotherms were described using the Freundlich model, which assumes that the surface of adsorbent is heterogeneous with an exponential decrease in adsorption energy on the completion of the adsorption sites.21 The Freundlich isotherm is not restricted to monolayer adsorption and is widely applied in heterogeneous systems with interactions between adsorbed molecules. This isotherm is expressed as follows

(3)

where βcharged is the charge concentration polarization, Zm is the PPCP’s ion valence, V is the membrane potential (using zetapotential, mV), F is Faraday’s constant, R is the universal gas constant, and T is the temperature (K). The ion valence for each PPCP can be determined from the pKa in Table S2 of the Supporting Information. The zeta potentials of NF90, NF270, and XLE were cited from the studies of Nghiem et al.,3 Lin,19 and McCloskey,20 and the values for each membrane are summarized in Table S2 of the Supporting Information.

Q = KFCe n

(7)

where KF (L/μg) and n (−) are empirical constants for the measurement of adsorption capacity and reversibility of adsorption interactions, respectively. 6800

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The value of n ranges between 0 and infinity; n = 1 means linear adsorption, n < 1 means chemical adsorption, n > 1 means physical adsorption. The closer the value of n is to 0, the more heterogeneous the surface is.21 KF and n were calculated using SYSTAT 10.2 with the nonlinear regression function (SYSTAT Software Inc., U.S.A.).

3. RESULTS AND DISCUSSION 3.1. Effect of pH on PPCP Rejections and Permeate Flux. Figure 2 shows the rejection of PPCPs by NF90, NF270,

Figure 3. Permeate flux of NF90, NF270, and XLE as a function of solution pH. Error bars represent one standard deviation of triplicate samples.

Figure 4. Rejection of PPCPs predicted by the simplified charge concentration of the polarization model as well as measured from the filtration experiments. Error bars represent one standard deviation of triplicate measurements.

Supporting Information). Along with the fact that pH did not have much impact on PPCP rejections by XLE (2−4% increase in PPCP rejections at pH 10 compared to those at pH 3, Figure 2c), steric hindrance (size exclusion) was the dominant rejection mechanism for tight XLE. However, as pH increased from 3 to 10, the rejection of HPI-I (SMX and DIA) and HPOI (IBU) increased significantly by NF90 (17−29%, Figure 2a) and NF270 (5−64%, Figure 2b), which could be attributed to the enhanced electrostatic repulsion between the negatively charged membrane surface and ionized PPCPs.5,19 This effect can also be observed for HPI-N (SMZ, pKa = 7.6) and HPO-N (TRI, pKa = 8.0) at pHs higher than their pKa values. As pH increased from 8 to 10, SMZ rejection increased from 76% to 97%, 70% to 93%, and 89% to 99% by NF90, NF270, and XLE, respectively, and TRI rejection increased from 57% to 92%, 38% to 45%, and 87% to 98% by NF90, NF270, and XLE, respectively. NF270 showed poor and moderate rejections for PPCPs smaller than (i.e., IBU, DIA, SMX and TRI) or similar to (i.e., CBZ and SMZ) its pore radius at pH 3 (Tables S1 and S2, Supporting Information, respectively), at which membrane

Figure 2. Rejection of PPCPs by NF90 (a), NF270 (b), and XLE (c) at pH 3−10. Error bars represent one standard deviation of triplicate samples.

and XLE as a function of solution pH and solute properties. It is obvious that the rejection of PPCPs was the highest by XLE (86−99% of all PPCPs, Figure 2c) at tested pHs, followed by NF90 (57−97%, Figure 2a) and NF270 (23−92%, Figure 2b), which fits the order of pore radius decrease (Table S1, 6801

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mentioned above. However, CBZ remained uncharged (pKa = 13.9; Table S2, Supporting Information), and its rejection at pH 10 by NF270 further decreased to 16%. This particular phenomenon was related to the change of permeate flux as a function of pH, which is shown in Figure 3. In Figure 3, as pH increased from 3, the permeate flux of tight NF90 and XLE was relatively stable with a 17% decrease and 4% increase at pH 10, respectively, which might be due to the mild disproportionate polymerization between different membrane coupons used. However, the permeate flux of loose NF270 increased 41% at pH 8 and 71% at pH 10 compared to that at pH 3, which could be explained by the swelling of the membrane due to the dissociation of carboxyl and amide functional groups on it, increasing the electrostatic repulsion between each other and leading to the expansion of membrane pores.22,23 Therefore, this inevitable membrane swelling effect at high pHs compromised the rejections of HPI-N and HPO-N by NF270. 3.2. Simulation of PPCP Rejections Using the Simplified Charge Concentration Polarization Model. Figure 4 shows the rejection of PPCPs predicted by the simplified charge concentration polarization model as well as measured from the filtration experiments. The predicted rejections correlate well with the measured rejections for most PPCPs by NF90, NF270, and XLE, with the inaccuracy less than 10%. The good correlation between the measured and predicted PPCP rejections in Figure 4 demonstrated the effect of electrostatic repulsion between PPCPs and these three membranes. However, two groups of bias can be observed in Figure 4. The first group is the underestimated rejections of HPI-I (DIA and SMX) and HPO-I (IBU) by NF270 at pH 8 and 10, which are the three smaller PPCPs with Stokes radius less than 0.412 nm in Table S2 of the Supporting Information. The reason could be due to the experimental error during the measurement of streaming potential for the determination of zeta potential.17 Another reason could be that the zeta potential does not include membrane surface conductance so that it does not correctly represent the membrane potential in eq 3.17 Although the current theoretical model does not account for the membrane swelling mechanism (see Figure 3, mentioned previously in Section 3.1) due to electrostatic repulsion within pore structures for NF270, the effect of electrostatic repulsion between the small ionized PPCPs and NF270 at high pHs overwhelmed the increase in permeate flux so that the actual rejections were higher than the predicted ones. The other group is the overestimated rejections of HPO-N (TRI) at pH 8 and 10 by all three membranes, which could be due to the adsorption of TRI onto membrane surfaces leading to its diffusion and penetration through membranes so that the effect of electrostatic repulsion was overcame. The adsorption of TRI will be further discussed in the following sections. 3.3. Adsorption of PPCPs on NF and RO from Extraction Experiments. Figure 5 shows the adsorption of PPCPs extracted from different membrane layers after filtration experiments at pH 3 to 10. For the three tested membranes, only IBU and TRI could be extracted from the top PA + PS layers (Figure 5(a)) and the bottom PET layer (Figure 5(b)), which are the two most hydrophobic compounds with log Kow values of 3.14 and 4.86, respectively, in Table S2 of the Supporting Information. Most adsorption happened on the PA + PS layers, which was 1−6 and 8−10 folds higher than that on the PET layer for IBU and TRI, respectively, on each

Figure 5. Adsorption of IBU and TRI extracted from PA + PS layers (a) and PET layer (b) of NF90, NF270, and XLE at pH 3−10. Error bars represent one standard deviation of triplicate samples.

Figure 6. Normalized adsorption of IBU and TRI extracted from PA + PS layers (a) and PET layer (b) of NF90, NF270, and XLE at pH 3− 10.

surface and solutes were neutral or slightly protonated, implying that steric hindrance was not a supreme mechanism for NF270. One interesting phenomenon was observed for NF270 that the rejections of neutral SMZ and HPO-N (TRI and CBZ) at pH 8 decreased significantly compared to those at pH 3 and 5 (Figure 2b). At pH 10, SMZ and TRI were ionized and their rejections raised because of the electrostatic repulsion 6802

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Figure 7. Adsorption kinetics of IBU and TRI in a PPCP cocktail solution at pH 3−10 and 25 °C on NF90 (a, b), NF270 (c, d), and XLE (e, f). Adsorption values are averaged over three samples with error bars denoting one standard deviation, which are small and covered by the symbols. Initial concentration = 800 μg/L. Temperature = 25 °C.

membrane. Steinle-Daring et al.24 reported that adsorption happened predominantly within the PA layer, while McCallum et al.10 reported that adsorption occurred predominantly at the PS layer. In this study, the two statements might work coherently by adsorbing IBU and TRI to the PA layer as they approached the membrane surface and then diffusing to the PS layer, which was more hydrophobic than the PA layer.10 The diffusive transport can be demonstrated by the extraction of IBU and TRI from the PET layer for each membrane (Figure 5(b)). The adsorption of TRI on different membrane layers was much higher than that of IBU at all pHs (Figure 5). Moreover,

the adsorption of IBU and TRI on different membrane layers was much lower at pHs higher than their pKa values for each membrane. At pH above 5, IBU was ionized as well as the membrane surface so that the effect of electrostatic repulsion prevents PPCPs approaching the membrane surface,10 which also happened for TRI at pH 10. The results are consistent with the observation of feed concentration decline (7−8% and 52− 59% for IBU and TRI, respectively, at pH 3 and less than 1% for both PPCPs at pH 10 for the three membranes during 24 h filtration, data not shown). It is noteworthy that at pH 10, there is still a considerable amount of TRI adsorbed on each membrane (Figure 5), which is unusual because all TRI existed 6803

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be due to the competitive adsorption of active sites on the membrane surface.10,12 However, the adsorbed TRI on membranes follows the order of NF270 > NF90 ≥ XLE, which fits the order of the membrane pore radius but not the order of surface roughness in Table S1 of the Supporting Information, implying that TRI adsorption happened on the membrane surface as well as into the membrane pores. Because the permeate flux of NF270 was much higher that those of NF90 and XLE (Figure 3), the adsorption of IBU and TRI in Figure 5 was normalized to the total permeate volumn of each membrane at pH 3−10, and the results are shown in Figure 6. In Figure 6, it is notable that the IBU and TRI adsorption on each membrane was very similar, which follws the order of NF90 ≈ NF270 ≥ XLE. Therefore, the much higher TRI adsorption on NF270 in Figure 5 could be explained by the more chances TRI has to be brought to the membrane surface because of the much higher permeate flux of NF270 compared to that of NF90 and XLE (Figure 3). Higher adsorption will lead to a breakthrough in permeate concentration as indicated by McCallum et al.10 so as to decrease rejections by membranes (Figure 2). 3.4. Adsorption of PPCPs on NF and RO from Static Adsorption Experiments. 3.4.1. Adsorption Kinetics. During the batch static adsorption experiments with PPCP concentrations of 800 μg/L for 72 h, only the concentrations of the three HPO compounds (TRI, IBU, and CBZ) were observed to decrease exponentially in the batch reactor. The decreasing trend in the time evolution was stabilized after 24 h, indicating that equilibrium between membrane and PPCP solution was reached. Because the amount of CBZ adsorption onto the membranes was relatively low (less than 0.4 μg/cm2 at low pHs for each membrane), only the adsorption kinetics of TRI and IBU within 24 h were calculated and are shown in Figure 7. In Figure 7(a), (c), and (e), IBU adsorption decreased significantly as pH increased from 3 to 5, and no adsorption was observed at higher pHs. TRI adsorption decreased gradually in Figure 7(b), (d), and (f), and a relatively large amount of TRI adsorption (5 μg/cm2) was observed even at pH 10 for each membrane. It is interesting to note that TRI adsorption at pH 3 was slightly lower than that at pH 5 in the static adsorption experiments, which could be due to the competitive adsorption of IBU at pH 3 for the number of active membrane sites.26 The static adsorption experiments with TRI alone showed higher TRI adsorption than that in the presence of the other PPCPs, especially at pH 3 (Figure 8), which confirms the competitive adsorption between TRI and IBU. The amounts of static IBU and TRI adsorption on the membranes were much lower than those in the pressurized filtration experiments (Figure 5), which was also observed in the study of McCallum et al.10 On the other hand, the amounts of static IBU and TRI adsorption followed the same order of NF90 ≈ NF270 ≥ XLE as that in the normalized adsorption (Figure 6). These two observations demonstrated that the hydrophobic affinity between PPCPs and the membrane surface was the driving force for PPCP adsorption at static conditions, while the pressurized filtration could bring more IBU and TRI to the membrane surface so as to enhance their adsorption. IBU and TRI adsorption at pHs higher than their pKa values was much lower that that at low pHs for all membranes, which also demonstrates the effect of electrostatic repulsion as mentioned in the previous sections.

Figure 8. Adsorption kinetics of TRI alone at pH 3−10 and 25 °C on NF90 (a), NF270 (b), and XLE (c). Adsorption values are averaged over three samples with error bars denoting one standard deviation, which are small and covered by the symbols. Initial concentration = 800 μg/L. Temperature = 25 °C.

as a deprotonated and negatively charged species at this pH. This partition of deprotonated TRI could be due to its large hydrophobic moiety and dissolving into the polymer phases. Therefore, the negatively charged TRI could form ion pairs with the sodium ion from the background electrolytes, which hinders its electrostatic interaction with the membrane surface so that it can approach, adsorb to, and even penetrate through the membrane surface.25 The amount of IBU adsorbed on each membrane is relatively similar and much less than that of TRI (Figure 5), which could 6804

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Table 1. Kinetic Parameters of Pseudo-First-Order Reaction Model NF90 IBU

TRI

NF270

XLE

pH

Qe (μg/cm2)

k (h−1)

R2

Qe (μg/cm2)

k (h−1)

R2

Qe (μg/cm2)

k (h−1)

R2

3 5 8 10 3 5 8 10

1.21 0.37 0.02 0.00 4.24 4.43 4.25 2.09

0.30 1.25 1.01 9.57 0.37 0.41 0.15 0.14

0.995 0.995 0.953 0.216 0.992 0.994 0.998 0.997

1.53 0.30 0.01 0.01 4.67 4.83 4.94 2.42

1.04 1.96 5.87 5.87 0.67 0.69 0.30 0.10

0.993 0.979 0.996 0.345 0.998 0.998 0.998 0.999

1.21 0.36 0.04 0.03 4.29 4.82 4.07 1.10

0.20 0.92 3.08 7.47 0.19 0.26 0.13 0.10

0.993 0.998 0.974 0.931 0.995 0.997 0.991 0.995

Table 2. Freundlich Isotherm Parameters of PPCPs Derived from Static Adsorption Experiments for Tested Membranes NF90 CBZ

IBU

TRI

a

NF270

XLE

pH

n

KF

R2

n

KF

R2

n

KF

R2

3 5 8 10 3 5 8 10 3 5 8 10

0.66 0.78 0.52 0.77 0.46 0.66 nd 1.73 0.64 0.41 0.44 0.75

0.36 0.43 0.43 0.55 1.82 0.38 nd 0.02 16.48 19.71 14.01 2.79

0.958 0.993 0.637 0.902 0.950 0.925 nd 0.937 0.973 0.793 0.908 0.848

nd 0.63 0.95 0.82 1.10 0.36 nd nd 0.54 0.49 0.49 0.48

nd 0.58 0.38 0.37 0.73 0.52 nd nd 27.76 32.11 20.58 5.69

nd 0.939 0.870 0.972 0.695 0.664 nd nd 0.998 0.992 0.979 0.563

0.73 1.01 0.71 0.79 0.53 nd nd 0.57 0.56 0.49 0.57 0.72

0.36 0.23 0.42 0.41 2.07 nd nd 0.04 19.45 23.60 11.64 2.61

0.948 0.863 0.949 0.937 0.942 nd nd 0.882 0.980 0.930 0.955 0.843

nd: not determined because of poor model fitness (R2 < 0.5).

One is the underestimated group of small ionized PPCPs at high pH values (8 and 10) by NF270 because of the strong electrostatic repulsion between the small ionized PPCPs and NF270. The other group is the overestimated rejection of highly hydrophobic TRI at high pH values because of its adsorption and penetration through the membrane surface, which can be confirmed by the extraction of TRI mainly from the top PA + PS layers and also the PET layer after filtration experiments. Static adsorption kinetic experiments showed competitive adsorption of TRI and IBU at pH 3 for each membrane. IBU adsorption decreased significantly as pH increased, which demonstrates the effect of electrostatic repulsion, while a relatively large amount of TRI adsorption was observed even at pH 10. The amounts of static IBU and TRI adsorption followed the same order of NF90 ≈ NF270 ≥ XLE as those in the pressurized filtration experiments normalized to the total permeate volume, while the amounts of adsorption were much lower. The adsorption of TRI and IBU can be explained well by the first-order reaction model for pH 3−10. The adsorption isotherm data fitted best with the Freundlich model in all cases, suggesting that chemical adsorption (hydrophobic interaction in this study) onto the membrane surface was the major mechanism. By clarifying the rejection mechanisms of PPCPs by NF/RO, we can select a particular membrane and operate the filtration at suitable pH to ensure satisfactory permeate quality and flux. Moreover, this information can be used for design of new membranes with more ionizable functional groups to provide stronger electrostatic repulsion, which can compensate for the insufficiency of steric hindrance as well as decrease compound adsorption and penetration through membranes.

The kinetic parameters of the pseudo-first-order reaction model were calculated and are shown in Table 1 for each membrane. The correlation coefficients were high for TRI adsorption at all pHs and IBU adsorption at low pHs (R2 > 0.99), indicating that adsorption can be explained well by the first-order reaction model. 3.4.2. Adsorption Isotherm. In order to get deep insights into the PPCP adsorption mechanisms, the adsorption isotherms were calculated from the results of triplicated static batch experiments with initial PPCP concentrations from 800 to 10,000 mg/L using different models including Langmuir, Freundlich, Redlich-Peterson and Temkin models (data not shown). The Freundlich model showed the best fit with parameters and regression coefficients for most of the three HPO PPCPs, and the results are summarized in Table 2. In Table 2, the most adsorbed TRI showed the smallest fitted n and the largest KF at each tested pH compared to those of IBU and CBZ. It is obvious that most of the fitted n values were less than 1, which means chemical adsorption and supports the statement of hydrophobic affinity between PPCPs and membranes made in the previous sections.

4. CONCLUSIONS PPCP rejection was the highest for XLE without much influence of pH, suggesting that steric hindrance was the dominant mechanism, while the effect of electrostatic repulsion between the negatively charged membrane surface and ionized PPCPs increased with an increase in pH for NF90 and NF270. The good prediction of PPCP rejection using the simplified charge concentration polarization model demonstrated the contribution of steric hindrance and electrostatic repulsion mechanisms in most cases with only two groups of exceptions. 6805

dx.doi.org/10.1021/ie500114r | Ind. Eng. Chem. Res. 2014, 53, 6798−6806

Industrial & Engineering Chemistry Research

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ASSOCIATED CONTENT

S Supporting Information *

Physicochemical properties of membranes and PPCPs used in this study. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +886-7-6011000, ext. 2328. Fax: +886-7-6011061. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Science Council of Taiwan (NSC 98-2221-E-327-002 and NSC 99-2221-E-327-019) for financial support as well as Dow Co. for providing the tested membranes.

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