Quantification of HormoneâHumic Acid Interactions in Nanofiltration

Article pubs.acs.org/est

Quantification of Hormone−Humic Acid Interactions in Nanofiltration Junjie Shen,†,‡ X. Jin Yang,‡ and Andrea I. Schaf̈ er*,† †

School of Engineering, University of Edinburgh, Edinburgh EH9 3JL, United Kingdom Department of Environmental Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China

‡

S Supporting Information *

ABSTRACT: The influence of solute−solute interactions on hormone retention during nanofiltration (NF) was quantified and mechanisms underlying retention identified. A new approach to predict both the mass of hormone sorbed to organic matter and the retention of hormone influenced by solute−solute interactions was applied. Laboratory-scale experiments were carried out in a cross-flow filtration system examining organic matter concentration, solution pH, and hormone type. Solute−solute interactions between HA and estrone improved estrone retention while decreasing estrone adsorption to membranes. HA concentration determined the amount of estrone bound to HA and hence affected estrone retention based on the mechanism of size exclusion. The solution pH influenced both solute−solute as well as solute−membrane interactions. Solute−solute interactions were most important below the pKa of estrone, whereas charge repulsion between estrone and negative functional groups of the membrane dominated estrone retention above the pKa. Of the four hormones studied, progesterone had the greatest affinity for both HA and NF membrane, which was attributed to hydrogen bonding ability. Using partition coefficients KOM from solid-phase microextraction (SPME) resulted in very good agreement between predicted and experimental retention.

1. INTRODUCTION The fate and transport of micropollutants in aquatic environment can be significantly influenced by natural organic matter (NOM).1−3 Through solute−solute interactions, micropollutants form complexes with NOM macromolecules, resulting in an increase in molecular weight and the adaption of negative charges.4,5 Considering that size exclusion and charge repulsion are two main retention mechanisms of nanofiltration (NF),6 solute−solute interactions are expected to have significant impact on the retention of micropollutants during the NF processes. Depending on the diversity in physicochemical properties of the solutes, the interactions can be dominated by either hydrophobic interaction2 or hydrogen bonding.1,3 Previous studies have reported that phenolic groups of steroid hormones and NOM play the predominant role in interactions.3,7,8 Solution chemistry (pH, ionic strength as well as presence matrix compounds) is another key factor influencing their interactions as it can alter the charge and structure of certain solutes.5 NOM can associate with the functional groups present on membrane and thus modify membrane properties such as surface charge9 and molecular weight cutoff (MWCO).10 NOM can hinder the adsorption of micropollutant on membranes due to competition for limited adsorption sites.11 Consequently, retention behavior and the effect of NOM on micropollutant retention might be quite variable in real waters as several of the © 2012 American Chemical Society

above-mentioned mechanisms interplay. Such complexities are reflected by the variability of reported experimental results in the literature4,7,12−14 with NOM both diminishing or enhancing micropollutant retention. In order to better understand the retention mechanism, solute−solute interactions on retention need to be distinguished and quantified. However, few studies are capable of quantifying solute−solute interactions during NF, which may be attributed to the lack in availability of specific partition coefficients.7,15 In consequence, Neale et al.8,16 developed a solid-phase microextraction (SPME) method to measure specific organic matter-water partition coefficients (KOM) and applied these to quantify the interaction between steroidal hormones and organic matter at environmentally relevant concentrations. This method enables the quantification of such solute−solute interactions in NF. This study applied the partition coefficients (KOM) measured by SPME for the same solution chemistry, organic matter, and micropollutant type17 to NF. The role of NOM concentration, solution pH, and hormone type on solute−solute interactions were investigated. The primary objective was to elucidate and Received: Revised: Accepted: Published: 10597

May 21, 2012 July 25, 2012 August 6, 2012 August 6, 2012 dx.doi.org/10.1021/es301843s | Environ. Sci. Technol. 2012, 46, 10597−10604

Environmental Science & Technology

Article

Figure 1. Schematic of cross-flow nanofiltration equipment with flow paths.

neutral and alkaline pH. Contact angle data show the hydrophilic nature of the membrane, which enables NF-270 to be quite resistant to organic fouling.18 2.3. Micropollutant Type and Characteristics. Four types of steroid hormones were used: estrone (E1) and estradiol (E2) as natural estrogens which are produced by all vertebrates, progesterone of the progestogen family, and the androgen testosterone.19 Radiolabeled [2,4,6,7-3H] hormones were purchased from Perkin-Elmer (UK). Nonlabeled hormones were purchased from Sigma Aldrich (UK). Stock solutions of radiolabeled (100 μg·L−1) and nonlabeled (1 mg·L−1) hormones were prepared in pure methanol (Fisher Scientific, UK). The feed concentration of hormone was set to 100 ng·L−1, which contained half labeled and half nonlabeled hormone. Hormone properties such as functional group content, dipole moment, and hydrophobicity have significant implications for solute−solute as well as solute−membrane interactions,3,20,21 and the physicochemical properties of steroid hormones are summarized in Table S2. Estrone and estradiol both contain a phenolic hydroxyl functional group in the C-3 position and hence dissociate at alkaline condition. Progesterone and testosterone do not dissociate within the studied pH range (pH 4 to 12). Previous studies have demonstrated hydrogen bonding as the mechanism for hormone adsorption to membranes22,23 as well as hormone-DOM interactions.3 The hydrogen bonding abilities of different hormones will be discussed in depth later. The octanol−water partition coefficient (KOW) was used to estimate hydrophobicity. Hormones with log KOW above 2.5 were classified as hydrophobic.24 As KOW > 2.5 for all four hormones they are likely to engage in some hydrophobic interactions. 2.4. Organic Matter and Other Chemicals. Aldrich humic acid (HA) was used in this study and obtained from Sigma Aldrich (UK). HA stock solutions of 250 mgC·L−1 were prepared in ultrapure water. The feed HA concentration was typically 12.5 mgC·L−1 except when HA concentration was investigated in the range of 5, 25, 50, and 125 mgC·L−1. Ultrapure water was obtained from a PURELAB Ultra system (ELGA LabWater, UK). The background electrolyte was 1 mM NaHCO3 and 20 mM NaCl, while 1 M NaCl and 1 M HCl were used for pH adjustment. All salts were of analytical grade

quantify the contribution of the hormone−NOM interaction mechanism on hormone retention and membrane sorption.

2. MATERIALS AND METHODS 2.1. Nanofiltration Crossflow System and Experimental Protocol. Filtration experiments were carried out using a laboratory-scale membrane filtration system (Figure 1). This system was composed of a 2.5 L feed tank (TripleT02-FP, MMS, Switzerland), a 46 cm2 membrane cell (TripleC4, MMS, Switzerland), and a high pressure pump (P200, Hydra-Cell, UK) modified in house to remedy the oil leakage problem from the pump into the feed stream. A temperature controller (WK700, Lauda, Germany) was connected to the tank to maintain the internal temperature at 22.0 °C ± 0.5 °C. A thermometer (WTM Pt 100-0-6, Condustrie-Metag, Germany) and two pressure transducers (S model, Swagelok, UK) were used to measure the feed and retentate temperature and pressure, respectively. The flow rate in the feed was measured by a flow meter (M2SSPI, Hydrasun, UK). The permeate volume was measured by an electronic balance (Adventurer Pro, Ohaus, UK) to allow flux calculation. Pressure, flow rate, and temperature data was collected by a datalogger (DAQ 55 Omega, UK). For each test, a new membrane sample was used. Prior to each filtration test, the membrane was gently washed in ultrapure water for at least 12 h. The membrane was then compacted with ultrapure water at 25 bar for two hours. Feed solution of the required composition was then introduced into the system. In order to minimize the possible occurrence of membrane fouling, the experimental pressure was set to 5 bar and the flow rate to 2 L·min−1. Throughout the process, both the retentate and permeate were recycled back to the feed tank. 2.2. Membrane Type and Characteristics. Two commercial NF membranes, namely NF-270 from Dow Film Tec (Minneapolis, USA) and TFC-SR2 supplied by Koch Membrane Systems (San Diego, USA), were used in this study. As indicated by the manufacturers, both membranes are polyamide thin-film composites. It is particularly noteworthy that polyamide is strongly polar and can act as both hydrogen acceptor and donor.6 The membrane properties are summarized in Table S1. Both membranes are negatively charged at 10598

dx.doi.org/10.1021/es301843s | Environ. Sci. Technol. 2012, 46, 10597−10604


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where Vf, Vp, and Vr were the feed, permeate, and retentate volumes (mL), and Cf, Cpi, and Cr were the feed, permeate, and retentate concentrations (ng·L−1), respectively. Based on the definition (data) of KOM8 in eq 5 and the mass balance of total hormone (mTOT) in eq 6, mW and mOM were calculated. The total mass of DOM was mDOM (kg), COM was the concentration of hormones sorbed to organic matter (ng·kg−1), and VW was the volume of aqueous solution (L).

and supplied by Fisher Scientific, UK. The weight-averaged molecular weight of Aldrich HA being 4100 and the percent aromaticity is about 31.25,26 In general, Aldrich HA contains aliphatic, aromatic, carboxylic, and phenolic groups.7,27 Due to the high content of carboxylic groups, the dominant pKa of Aldrich HA is about 4.26.26 The structure of HA is pH dependent; at low pH the HA is curled due to intramolecular hydrogen bonding while at pH above pKa HA is more linear due to intramolecular charge repulsion.28,29 2.5. Analytical Methods. Radiolabeled hormones were analyzed using liquid scintillation counting (LS6500, Beckman Coulter, USA). 0.5 mL of aqueous sample was dispersed in 3.5 mL of scintillation cocktail (Ultima Gold LLT, Perkin-Elmer, UK). Total organic carbon (TOC) was measured using a TOCVCPH analyzer (Shimadzu, UK). The absorbance of HA was determined at 254 nm by a UV−visible spectrophotometer (Cary 100 Scan, Varian. UK). SUVA was calculated as the fraction of UV absorbance/TOC (results are presented in the SI). Electrical conductivity and pH were measured with a pH/ conductivity meter (WTW, Germany). 2.6. Quantification of Solute−Solute Interactions. Organic matter−water partition coefficients (KOM) were used to quantify the interaction of hormone with HA. The coefficients were determined by Neale et al. 17 using solidphase microextration (SPME). The prediction model and methodology for hormone retention was first introduced by de Munari and Schäfer (manuscript in preparation). The retention of hormone in the presence of HA was predicted based on the following assumptions: 1 The retention of freely dissolved hormone in HAcontaining solution is identical to the hormone retention in HA-free solution (this assumption is only valid in ideal systems. In real conditions membrane−HA interactions can modify the membrane surface27 and thus affect hormone retention.). 2 The membrane merely adsorbs freely dissolved hormone, while hormone−HA complexes will remain in solution and be rejected together with HA. 3 No membrane fouling occurs (fouling conditions are minimized by operating conditions of low pressure and high crossflow velocity). Hormone retention R was determined as per eq 1, volumes and concentrations being the directly measured entities R=1−

Cp Cf

=1−

KOM =

mTOT = mW + mOM

(5) (6)

Equations 7 and 8 show mass balances of freely dissolved hormones and sorbed hormones, respectively mW = mpW + mfW + mADS (7) mOM = mpOM + mfOM

(8)

According to the above-mentioned assumptions, mpW and mpOM were calculated by eqs 9 and 10

mpW = mfW ·(1 − R h) mpOM = mfOM ·(1 − R OM )

(9) (10)

where Rh is the retention of hormone in HA-free solution, and ROM is the retention of HA. Therefore the predicted retention for experiments with HA partitioned hormones can be calculated using eq 11. Rp =

mfOM ·R OM + mfW ·R h mfW + mfOM

(11)

The statistical analysis methodology used to determine error bars for both experiments and the modeling can be found in the SI.

3. RESULTS AND DISCUSSION 3.1. Estrone Retention in the Absence of HA. Without HA (Figure 2) estrone retention initially showed constant decline with increasing membrane adsorption, showing that adsorption of estrone to the membrane polymer is the dominant retention mechanism.20 Given the structural properties of both estrone and the membrane, this adsorption is thought to be driven by hydrogen bonding between the estrone

mp mf

COM m V = OM · W CW mDOM mW

(1) −1

Where C was the solute concentration (ng·L ), m was the mass (ng), and the subscripts f and p designate the feed and permeate solutions. Permeate and retentate were recirculated, and thus the volume remains constant. Both feed and permeate solutions contain freely dissolved hormones and the hormones associated with organic matter as is expressed in eqs 2 and 3 mp = mpW + mpOM

(2)

mf = mfW + mfOM

(3)

Hormone adsorption to the membrane was calculated using mass balance in eq 4 n

mADS = Vf Cf − Vp ∑ Cpi − VrCr i=1

Figure 2. Estrone retention and membrane sorption by TFC-SR2 and NF-270 membranes as a function of time (pH 8, 100 ng·L−1 estrone, 20 mM NaCl, and 1 mM NaHCO3).

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Figure 3. Estrone behavior as a function of HA concentration for TFC-SR2 and NF-270 membranes (a) membrane sorption, (b) estrone retention, (c) ratio of permeate flux after 8 h to initial pure water flux, and (d) HA retention (pH 8, 100 ng·L−1 estrone, 20 mM NaCl, and 1 mM NaHCO3).

molecule and the membrane polymer.30,31 While both membranes have similar material composition, higher adsorption is observed with the looser membrane TFC-SR2, which can be attributed to higher adsorption inside the membrane.32 Once the adsorption has reached equilibrium, size exclusion played an overriding role estrone retention at neutral pH.33 It should be noted that the data presented in subsequent figures were collected after the membrane adsorption reached equilibrium (specifically after 8 h). Due to the considerable difference in membrane pore diameter and structure of the active layer, the estrone retention of the two membranes was significantly different (60−80%). 3.2. Influence of HA Concentration on Solute−Solute Interactions in NF. When HA is added to the feed solution (Figure 3), two additional interaction mechanisms are involved in NF, namely estrone−HA and membrane−HA interactions.34 HA has been shown to adsorb to NF membranes and increasing the negative surface charge,27 which will affect charge interactions between the membrane surface and charged micropollutants. In this case, however, estrone is uncharged as experiments were conducted at pH 8. Estrone adsorption decreases for both membranes in the presence of HA (Figure 3a). Solute−solute interactions of estrone with HA result in less estrone being available for membrane sorption. Considering the hydrogen bonding strength and hydrophobicity of both estrone and HA, this interaction is expected to be a combination of hydrogen bonding and hydrophobic interactions.35,36 It should be noted that increasing HA concentration (from 12.5 to 125 mgC·L−1) has insignificant impact on estrone sorption to the membrane. Childress and Elimelech27 have reported no further change in membrane zeta potential from 0.2 to 2 mg·L−1 HA. While estrone may compete with HA for limited adsorption sites on the membrane,11 the vast concentration difference between HA and estrone would suggest that such competition, if important, would not be limited by HA concentration. It is hence hypothesized that it is indeed the estrone−HA interaction that is dominating this adsorption phenomenon in that HA

concentration limits the freely dissolved estrone that can be adsorbed by the membrane polymer. For both membranes, enhanced estrone retention in the presence of HA is observed (Figure 3b). This result indicates the formation of estrone−HA complexes which have considerably larger molecule sizes than free estrone. Because the effective pore diameter of NF-270 (0.84 nm) is very close to the Stokes diameter of estrone (0.8 nm),31 even free estrone cannot easily penetrate the membrane. With the increasing HA concentration, the influence of solute−solute interactions on retention increases from 10% to 33% for the TFC-SR2 membranes and from 6% to 10% for the NF-270 membranes. Ratio of permeate flux J and initial pure water flux J0 was calculated as an indicator for membrane fouling (Figure 3c). Flux ratio is constant for HA concentrations up to 50 mgC·L−1, and fouling is only observed at 125 mgC·L−1. HA sorption has been attributed to enhanced water permeability by forcing the charged membrane pores open,27 alternatively increased hydrophilicity can change water flux. It is important to note that no significant fouling was observed, and hence increased retention cannot be attributed to pore blocking. HA retention in the same background electrolyte was studied using TOC measurements (Figure 3d). Retention is generally >90%, lowest for the TFC-SR2 membrane at the lowest concentration. The high retention is expected to be governed by both size exclusion and charge repulsion.28,37 The incomplete retention of HA is due to the fact that both the NF membranes and the HA have a size distribution. In consequence the smaller fraction of HA can transport across the membrane. Typically, the retention of UV absorbing organic fraction is higher.38 3.3. Influence of pH on Estrone−HA Interactions in NF. The pH determines the ionization of estrone and membrane functional groups. At pH below the pKa (10.34) the estrone molecule in uncharged, while above the pKa estrone assumes a negative charge. The isoelectric point of the TFCSR2 membranes is about pH 4 in background electrolyte, and this isoelectric point is reduced in the presence of HA.27 For 10600



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Figure 4. Estrone behavior as a function of pH for TFC-SR2 membranes (a) membrane sorption, (b) estrone retention, (c) ratio of permeate flux after 8 h to initial pure water flux, and (d) HA retention (12.5 mgC·L−1 Aldrich HA, 100 ng·L−1 estrone, 20 mM NaCl, and 1 mM NaHCO3).

and retention of low molecular weight fractions of HA. At higher pH HA retention is consistently high. 3.4. Influence of Hormone Type on Solute−Solute Interactions in NF. The interaction of estrone, estradiol, progesterone, and testosterone with membrane and HA was studied to establish how different hormone functional groups affect NF behavior. In the absence of HA (at pH 7) the hormones behave distinctly differently (Figure 5). Because all the hormones are undissociated, they are retained by the membrane mainly based on the mechanism of size exclusion after sorption has reached equilibrium. The high retention of

the TFC-SR2 membrane pH was investigated both without HA and with a HA concentration of 12.5 mgC·L−1 (Figure 4). Charge repulsion is expected to affect membrane sorption when estrone becomes dissociated. Indeed, membrane sorption (Figure 4a) is unaffected by pH up to 10, then a sharp decline in adsorption occurs at the pH above the pKa. However, adsorption does not decrease to zero even at pH 12, indicating that estrone−membrane interactions still occur in this condition. The addition of HA leads to slightly lower estrone adsorption at all pH conditions. At low pH, estrone can interact with HA and become a part of a charged complex. In consequence, less freely dissolved estrone is available for membrane sorption. When estrone and HA have similar charge at pH above the pKa, complex formation is reduced. However, the charged HA molecules can still interact with the membrane by hydrophobic interaction at high pH.4 Estrone retention (Figure 4b) at pH 4 is lowest due to the absence of charge repulsion. As mentioned earlier, estrone and HA are both in undissociated forms at pH 4, thus creating an ideal environment for solute−solute interactions. Therefore the highest percentage of estrone is bound to HA at this condition, and the contribution of the solute−solute interactions to estrone retention is about 23%. Then between pH 7 and 10, the contribution of these interactions remains approximately 10%. While estrone retention is consistently higher in the presence of HA, it experiences a dramatic increase above the pKa due to strong electrostatic repulsion. Similar phenomena have been reported on the retention of sulfamethoxazole.39 The presence of HA and the anticipated increase in membrane charge does not cause an increase in retention at high pH. Flux ratio (Figure 4c) increases constantly with higher pH. Previous studies have demonstrated that at high pH the charged functional groups in the membrane increase the intramolecular distance between the polymer chains, thus increasing effective pore size and water permeability.28 HA retention (Figure 4d) is lowest at pH 4 (about 75%) when the HA adopts a coiled structure 28 and experiences no charge repulsion. This could cause some concentration polarization effects that may influence both estrone retention

Figure 5. Hormone adsorption on membrane and hormone retention as a function of hormone type for TFC-SR2 membranes (pH 7, 100 ng·L−1 hormone, 12.5 mgC·L−1 Aldrich HA, 20 mM NaCl, and 1 mM NaHCO3). 10601



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progesterone and testosterone are attributed to their larger molecular weights (314.5 g·mol−1 and 288.4 g·mol−1). Estrone and estradiol have similar molecular weights and chemical characteristics so that their retention values are very similar. The presence of 12.5 mgC·L−1 HA reduced hormone adsorption (Figure 5a). The contribution of HA on membrane sorption varied with hormone type, where progesterone exhibited the strongest influence. This result is associated with certain functional groups of different hormones which will affect their hydrogen bonding ability and hydrophobicity. Previous studies have found that ketone groups are strong hydrogen acceptors, while hydroxyl groups are hydrogen donors.6 Progesterone contains two ketone groups in the C-3 and C-20 positions, while the C-3 ketone moiety has been demonstrated as a triple hydrogen acceptor which can form hydrogen bonds in three positions.17,40 Estrone also contains a ketone group in the C-17 position but a hydroxyl group in the C-3 position. Estradiol and testosterone have similar structures with a hydroxyl moiety in the C-17 position.17 Studies have shown that the hydrogen bonding ability of ketone groups in the C-17 and C-20 positions are stronger than the hydroxyl group in the C-17 position.17,41 Based on the above discussion progesterone is expected to have the greatest affinity for the membrane of the four hormones, thus being most affected by HA in respect of membrane sorption. Hormone retention was only marginally increased for all the four types of hormones, indicating the occurrence of hormone−HA interactions at all conditions. 3.5. Comparison of Experimental and Predicted Hormone Retention. In order to quantify the effect of solute−solute interactions on micropollutant retention, the organic matter-water partition coefficients (KOM) measured by Neale et al.17 were applied. Results are tabulated for HA concentration, pH, and hormone type in Table S3, Table S4, and Table S5, respectively. Experimental conditions are identical in KOM determination and NF. For HA concentration (Figure 6), the result of predicted mass reveals distinctly that larger fraction of estrone complexes with HA at higher HA concentration. The maximum discrepancy between experimental and predicted retention was 10.7% for the TFC-SR2 and 5.8% for the NF-270 membrane. For pH, Log KOM values are affected because the hormonemicropollutant interactions depend on mechanisms such as hydrogen bonding and charge interactions. Log KOM values are generally larger in acidic solutions (Figure 7) when estrone and HA are both uncharged. Considering that estrone remains undissociated in the studied range, the decrease of KOM from acidic to alkaline condition is related to the speciation of HA and consequent changes in conformation.17 Being dissociated and unfolded at neutral and alkaline pH, HA becomes less hydrophobic which in turn reduces partitioning.42 At pH 4 the highest percentage of estrone is partitioned to organic matter. But this does not result in the largest retention as expected. Possible explanations are that HA retention decreases significantly at acidic pH due to the conformational changes in the HA,43 the sorbed estrone may transfer through the membrane together with the nonretained HA, and the absence of charge repulsion. Due to the ionization above the pKa, a higher percentage of estrone becomes freely dissolved in solution, leading to a lower fraction bound to organic matter. However, SPME is unable to quantify the solute−solute interactions at alkaline pH because

Figure 6. Comparison of experimental estrone retention with predicted retention determined using log KOM values (L·kg−1) as a function of HA concentration (TFC-SR2 and NF-270 membranes, pH 8, 100 ng·L−1 estrone, 20 mM NaCl, and 1 mM NaHCO3).

Figure 7. Comparison of experimental estrone retention with predicted retention determined using log KOM values (L·kg−1) as a function of pH (TFC-SR2 membranes, 100 ng·L−1 estrone, 12.5 mgC·L−1 Aldrich HA, 20 mM NaCl, and 1 mM NaHCO3).

the sorption of negatively charged species to the fiber is negligible.17,44,45 Therefore estrone retention above the pKa could not be estimated. For different hormone types (Figure 8), estrone has the highest log KOM value followed by progesterone, while estradiol and testosterone have lower log KOM values. Their predicted sorbed amounts are equally in the same order. In general the prediction and the experiments show very good agreement with a maximum error of 12%. In summary, the retention mechanisms of micropollutants in solution matrix during filtration processes are to date poorly understood as multiple variables (solution chemistry, copresent organic matter, pH) and mechanisms are involved. The research focused on the solute−solute interactions between micropollutants and organic matter. Two aspects of the findings are of particular significance; first, the contribution of solute− solute interactions on the retention of hormones during nanofiltration (NF) was both evidenced and quantified. Second, a novel approach to predict both the mass of 10602



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(4) Agbekodo, K. M.; Legube, B.; Dard, S. Atrazine and simazine removal mechanisms by nanofiltration: influence of natural organic matter concentration. Water Res. 1996, 30 (11), 2535−2542. (5) Dalton, S. K.; Brant, J. A.; Wiesner, M. R. Chemical interactions between dissolved organic matter and low-molecular weight organic compounds: Impacts on membrane separation. J. Membr. Sci. 2005, 266 (1−2), 30−39. (6) Schäfer, A. I.; Akanyeti, I.; Semião, A. J. C. Micropollutant sorption to membrane polymers: a review of mechanisms for estrogens. Adv. Colloid Interface Sci. 2011, 164 (1−2), 100−117. (7) Jin, X.; Hu, J.; Ong, S. L. Influence of dissolved organic matter on estrone removal by NF membranes and the role of their structures. Water Res. 2007, 41 (14), 3077−3088. (8) Neale, P. A.; Escher, B. I.; Schäfer, A. I. Quantification of solute− solute interactions using negligible-depletion solid-phase microextraction: measuring the affinity of estradiol to bulk organic matter. Environ. Sci. Technol. 2008, 42 (8), 2886−2892. (9) Shim, Y.; Lee, H.-J.; Lee, S.; Moon, S.-H.; Cho, J. Effects of natural organic matter and ionic species on membrane surface charge. Environ. Sci. Technol. 2002, 36 (17), 3864−3871. (10) Xu, P.; Drewes, J. E.; Kim, T.-U.; Bellona, C.; Amy, G. Effect of membrane fouling on transport of organic contaminants in NF/RO membrane applications. J. Membr. Sci. 2006, 279 (1−2), 165−175. (11) Comerton, A. M.; Andrews, R. C.; Bagley, D. M.; Yang, P. Membrane adsorption of endocrine disrupting compounds and pharmaceutically active compounds. J. Membr. Sci. 2007, 303 (1−2), 267−277. (12) Devitt, E. C.; Ducellier, F.; Cote, P.; Wiesner, M. R. Effects of natural organic matter and the raw water matrix on the rejection of atrazine by pressure-driven membranes. Water Res. 1998, 32 (9), 2563−2568. (13) Berg, P.; Hagmeyer, G.; Gimbel, R. Removal of pesticides and other micropollutants by nanofiltration. Desalination 1997, 113 (2−3), 205−208. (14) Nghiem, L. D.; Manis, A.; Soldenhoff, K.; Schäfer, A. I. Estrogenic hormone removal from wastewater using NF/RO membranes. J. Membr. Sci. 2004, 242 (1−2), 37−45. (15) Jermann, D.; Pronk, W.; Boller, M.; Schäfer, A. I. The role of NOM fouling for the retention of estradiol and ibuprofen during ultrafiltration. J. Membr. Sci. 2009, 329 (1−2), 75−84. (16) Neale, P. A.; Escher, B. I.; Schäfer, A. I. pH dependence of steroid hormone–organic matter interactions at environmental concentrations. Sci. Total Environ. 2009, 407 (3), 1164−1173. (17) Neale, P. A. Influence of solute-solute interactions on membrane filtration. PhD Thesis, University of Edinburgh, Edinburgh, 2009. (18) Mänttäri, M.; Pekuri, T.; Nyström, M. NF270, a new membrane having promising characteristics and being suitable for treatment of dilute effluents from the paper industry. J. Membr. Sci. 2004, 242 (1− 2), 107−116. (19) Lombardi, P.; Goldin, B.; Boutin, E.; Gorbach, S. L. Metabolism of androgens and estrogens by human fecal microorganisms. J. Steroid Biochem. 1978, 9 (8), 795−801. (20) Nghiem, L. D.; Schäfer, A. I.; Elimelech, M. Removal of natural hormones by nanofiltration membranes: measurement, modeling, and mechanisms. Environ. Sci. Technol. 2004, 38 (6), 1888−1896. (21) Goss, K.-U.; Schwarzenbach, R. P. Rules of thumb for assessing equilibrium partitioning of organic compounds: successes and pitfalls. J. Chem. Educ. 2003, 80, (4), 450-null. (22) Nghiem, L. D.; Schäfer, A. I. Adsorption and transport of trace contaminant estrone in NF/RO membranes. Environ. Eng. Sci. 2002, 19 (6), 441−451. (23) Ng, H. Y.; Elimelech, M. Influence of colloidal fouling on rejection of trace organic contaminants by reverse osmosis. J. Membr. Sci. 2004, 244 (1−2), 215−226. (24) H. Jones, O. A.; Voulvoulis, N.; Lester, J. N. Human pharmaceuticals in wastewater treatment processes. Crit. Rev. Environ. Sci. Technol. 2005, 35 (4), 401−427.

Figure 8. Comparison of experimental hormone retention with predicted retention determined using log KOM values (L·kg−1) as a function of hormone type (TFC-SR2 membranes, pH 7, 100 ng·L−1 hormone, 12.5 mgC·L−1 Aldrich HA, 20 mM NaCl, and 1 mM NaHCO3).

hormones sorbed to organic matter and the retention of hormones influenced by solute−solute interactions was developed. The calculated data of hormone retention show very good agreement with the experimental results. In consequence, a promising prospect for application in other areas of research is revealed, provided the KOM data are available at relevant conditions.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details including material characteristics, sorbed hormone mass calculation and propagation of error methodology. 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 Dow Chemicals and Koch Membrane systems have generously supplied membrane samples, GE Global Research (Munich) donated materials funding, and the Drinking Water Quality Regulator (DWQR) for Scotland project funding. Than Hieu Ngo and Ime Akanyeti are acknowledged for their assistance with crossflow NF and analytical protocols, Andrea Semião and Annalisa De Munari (all University of Edinburgh) for useful discussions on NF transport and solute− solute interactions, respectively.

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REFERENCES

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Quantification of HormoneâHumic Acid Interactions in Nanofiltration

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