Takahiro
Fujioka
*a,
Stuart J.
Khan
b,
James A.
McDonald
b and
Long D.
Nghiem
c
aGraduate School of Engineering, Nagasaki University, Nagasaki 852-8521, Japan. E-mail: tfujioka@nagasaki-u.ac.jp
bUNSW Water Research Centre, School of Civil and Environmental Engineering, The University of New South Wales, NSW 2052, Australia
cStrategic Water Infrastructure Laboratory, School of Civil Mining and Environmental Engineering, The University of Wollongong, NSW 2522, Australia
First published on 30th July 2015
This study aims to provide further insights to the rejection mechanisms of trace organic chemicals (TrOCs) by nanofiltration (NF). The separation mechanisms of TrOCs by an NF membrane were elucidated by assessing the role of molecular properties and the impact of caustic cleaning on their rejection. All charged TrOCs were rejected by the NF270 membrane by more than 80%. However, the rejection of positively charged TrOCs was lower than that of negatively charged TrOCs with similar molecular sizes and was similar to the rejection of neutral TrOCs. The results suggest that size exclusion, rather than electrostatic repulsion, was a major factor attributing to the rejection of these positively charged TrOCs. The results also showed that the minimum projection area was a better surrogate parameter for molecular dimensions than molecular weight. Our study highlights the need to monitor the rejection of neutral and positively charged TrOCs (particularly those that are normally moderately rejected by the membrane) following caustic cleaning.
Water impactThe impact of this work to water research is demonstrated by the additional insights to the rejection mechanisms of trace organic chemicals (TrOCs) by nanofiltration (NF) membranes. Results reported here revealed several subtle factors that could influence the rejection of TrOCs by NF membranes, in addition to the three well-known rejection mechanisms. It was shown that the minimum projection area was a better surrogate parameter for molecular dimensions than molecular weight. In addition, the possible impact of caustic cleaning on the rejection of neutral and positively charged TrOCs (particularly those that are normally moderately rejected by the membrane) was highlighted. |
There are two major factors contributing to the public awareness of TrOCs in the environment. Firstly, the increasing number and concentration of TrOCs have been released into the aquatic environment, in particular since World War II, due to the large quantities of produced and consumed pharmaceuticals in modern societies.1,3 Secondly, there has been tremendous technological progress in the field of analytical chemistry, which has allowed the quantification of TrOCs at trace levels.7 TrOCs can be detected in a water sample at concentrations as low as 1 nanogram per litre (ng L−1) or less. The majority of TrOCs are released into the environment by effluent discharged from private households, hospitals, and industrial and farming activities.8,9 These TrOCs are often poorly removed from wastewaters by conventional wastewater treatment facilities.8,9 Significant progress in process engineering and materials science have facilitated effective removal of TrOCs by membrane filtration processes such as nanofiltration (NF) and reverse osmosis (RO). Indeed, NF/RO membranes have become an integral part of many water reuse facilities. Water reuse is commonly considered to be more cost effective and environmentally friendly than seawater desalination or long-distance water transfers for regions experiencing regular droughts and water scarcity.1
The increasing use of NF/RO for drinking water purification and potable water reuse has spurred many dedicated studies to assess the rejection mechanism of TrOCs by these membrane processes. As an example, Mery-sur-Oise is the world's largest NF plant (capacity of 140000 m3 per day) specifically designed and built for the removal of pesticides from the Paris river for drinking water production.10 NF process also shows an excellent performance on softening and removing natural organic matter for drinking water applications.10 On the other hand, RO has been extensively used for potable water reuse applications.
Although the distinction between NF and RO membranes is not clear, it is widely accepted that the removal mechanisms of TrOCs by these membranes are similar. In addition, because TrOC rejection by NF membranes is lower compared to RO membranes, variations in TrOC rejection due to changes in the operating condition can be better observed with NF membranes. Bellona et al.,11 provided an early, and arguably one of the most comprehensive, reviews on the rejection of TrOCs by NF/RO membranes. However, the review by Bellona et al.,11 and most subsequent studies only cover a small number of TrOCs and often heavily rely on investigations with concentrations well above typical for these compounds due to difficulties associated with their analysis. To date, key mechanisms governing the separation of TrOCs by NF membranes, namely size exclusion, electrostatic interaction, and adsorption (e.g., due to hydrophobic interaction or hydrogen bonding), have been discussed.12–14
The lack of comprehensive data obtained from consistent conditions has hindered the identification of more subtle factors that can also influence the rejection of TrOCs by NF membranes. As a notable example, the effects of membrane fouling and chemical cleaning on TrOC rejection have only been recently investigated. It has been observed that membrane fouling can compromising the rejection of TrOCs by altering the surface hydrophobicity, charge, pore size and by hindering back diffusion of the solute.15–17 Membrane fouling has to be managed by periodic caustic and acidic chemical cleaning, which in turn can compromise the membrane properties temporary or permanently. It has been reported that caustic cleaning may exert considerable impact on the rejection of some TrOCs by NF membranes.18 Simon et al.,18 suggested that caustic cleaning causes the swelling of the membrane polymer matrix due to the increased electrostatic repulsion among the deprotonated carboxylic functional groups in the polymer matrix, which was identified by zeta potential analysis. The swelling effect caused by caustic cleaning ultimately results in an enlargement of membrane pore structure and an increase in solute and solution permeation. However, in comparison to membrane fouling, studies focusing on the impact of chemical cleaning on TrOC rejection remain very limited.19 Since fouling and subsequent cleaning (particularly caustic cleaning) to restore the water flux are inevitable in most if not all membrane filtration processes, it is essential to understand the impact of chemical cleaning on TrOC rejection. Thus, the aim of this study was to provide further insights to the rejection mechanisms of TrOCs by an NF membrane, allowing for an estimation of TrOC removal by chemically cleaned NF membranes. By examining the role of molecular properties and the impact of caustic cleaning on their rejection, the separation mechanisms of TrOCs by an NF membrane were assessed.
Compound | Molecular weight [Da] | LogD at pH 8a | pKa (pKb)a | Ionisation at pH 8a [%] | MPAa [Å2] | MDLb [ng L−1] | ||
---|---|---|---|---|---|---|---|---|
a Chemaxon (http://www.chemicalize.org/). b MDL: method detection limit. | ||||||||
Neutral | Hydrophilic (HL) | Paracetamol | 151.2 | 0.91 | 9.5 | 3 | 21.8 | 5 |
Caffeine | 194.2 | −0.55 | (0.9) | 0 | 30.0 | 10 | ||
Simazine | 201.7 | 1.78 | (3.2) | 0 | 35.8 | 5 | ||
Atrazine | 215.7 | 1.32 | (3.2) | 0 | 39.0 | 5 | ||
Primidone | 218.3 | 1.12 | 11.5 | 0 | 42.7 | 5 | ||
Meprobamate | 218.3 | 0.93 | 15.2 | 0 | 45.8 | 5 | ||
Triamterene | 253.3 | 1.11 | (1.9) | 0 | 35.2 | 5 | ||
Trimethoprim | 290.3 | 1.28 | (7.2) | 12 | 51.1 | 5 | ||
Hydrophobic (HP) | N,N-Diethyl-meta-toluamide (DEET) | 191.3 | 2.50 | (0.1) | 0 | 40.1 | 5 | |
Bisphenol A | 228.3 | 4.04 | 9.8; 10.4 | 2 | 44.0 | 20 | ||
Diuron | 233.1 | 2.53 | 13.2 | 0 | 28.6 | 10 | ||
Carbamazepine | 236.3 | 2.77 | 16.0 | 0 | 38.8 | 5 | ||
Linuron | 249.1 | 2.68 | 12.0 | 0 | 30.8 | 5 | ||
Dilantin | 252.3 | 2.18 | 9.5 | 3 | 47.3 | 5 | ||
Diazepam | 284.7 | 3.08 | (2.9) | 0 | 47.8 | 5 | ||
Tris(2-chloroethyl)phosphate (TCEP) | 285.5 | 1.96 | n.a. | 0 | 49.9 | 10 | ||
Diazinon | 304.4 | 4.25 | (4.2) | 0 | 50.7 | 5 | ||
Triclocarban | 315.6 | 4.93 | 11.4 | 0 | 50.1 | 10 | ||
Clozapine | 326.3 | 3.40 | (3.9; 7.8) | 36 | 55.5 | 5 | ||
Omeprazole | 345.4 | 2.43 | (4.8); 9.3 | 2 | 43.5 | 5 | ||
Hydroxyzine | 374.9 | 3.24 | (2.1; 7.8) | 40 | 64.7 | 5 | ||
Charged | (−) | Ibuprofen | 206.3 | 0.97 | 4.9 | 100 | 35.4 | 5 |
Naproxen | 230.3 | −0.16 | 4.2 | 100 | 34.8 | 5 | ||
Gemfibrozil | 250.3 | 1.33 | 4.4 | 100 | 43.4 | 5 | ||
Sulfamethoxazole | 253.3 | 0.39 | 6.2 | 99 | 45.2 | 5 | ||
Ketoprofen | 254.3 | 0.48 | 3.9 | 100 | 41.7 | 5 | ||
Triclosan | 289.5 | 4.57 | 7.7 | 68 | 38.5 | 5 | ||
Diclofenac | 296.1 | 1.16 | 4.0 | 100 | 43.3 | 5 | ||
Enalapril | 376.5 | −0.91 | 3.7; (5.2) | 100 | 60.0 | 5 | ||
Simvastatin hydroxy acid | 436.6 | 0.63 | 4.2 | 100 | 65.1 | 5 | ||
(+) | Atenolol | 266.3 | −1.18 | (9.7) | 98 | 36.9 | 5 | |
Amitriptyline | 277.4 | 3.02 | (9.8) | 98 | 58.2 | 5 | ||
Fluoxetine | 309.3 | 2.46 | (9.8) | 98 | 44.3 | 5 | ||
Verapamil | 454.6 | 3.44 | (9.7) | 98 | 81.2 | 5 |
Fig. 2 Conceptual figure of minimum projection area of simazine. The line perpendicular to the circular disk represents the centre axis of the minimum projection area. |
A stock solution containing 10 mg L−1 of each of the selected TrOCs was prepared in methanol. Deuterated analogues of each TrOC were obtained from CDN isotopes (Pointe-Claire, Quebec, Canada) and used as surrogate standards to account for matrix effects and incomplete recoveries during sample preparation and analysis of TrOCs. A surrogate stock solution containing contained 50 μg L−1 of each deuterated TrOC was also prepared in methanol. Both stock solutions were kept in the dark at −18 °C. Analytical grade NaCl, CaCl2 and NaHCO3 were purchased from Ajax Finechem (Australia) and were used to prepare the synthetic feed solutions.
Compound rejection (R) was calculated using , where Cp and Cf are measured concentrations in the permeate and feed solutions, respectively. When TrOC concentrations in the permeate were detected at below their detection limits, the analytical detection limit was used for the (minimum) rejection calculation.
The pH, electrical conductivity and temperature of permeate and feed solutions were measured by an Orion 4-Star Plus pH/conductivity meter (Thermo Fisher Scientific, Waltham, MA, USA).
It is noteworthy that triclocarban (logD = 4.93) and triclosan (logD = 4.57) were excluded from Fig. 3. They are the most hydrophobic compound, respectively, among the neutral and negatively charged TrOCs investigated in this study. The concentration of triclocarban in the feed after 20 hours filtration decreased to below the detection limit (10 ng L−1) in all experiments. Similarly, the concentration of triclosan in the feed decreased to less than 40 ng L−1 after 20 hours filtration. The decrease in feed concentration of these two TrOCs can be attributed to their adsorption onto the membrane due to hydrophobic interaction. The adsorption of hydrophobic TrOCs onto polyamide NF/RO membranes have also been reported in several previous studies.14,23
Although the rejection of neutral TrOCs did increase as their molecular weight increased, the data are quite scattered. In an early study, Meireles et al.,24 investigated the rejection of several organic solutes (i.e., dextrans, proteins, and polyethylene glycol) by ultrafiltration and microfiltration membranes and suggested that the hydrodynamic volume of these organic solutes rather than molecular weight should be used to characterise their rejection. In their study,24 the hydrodynamic volume parameter is the product between molecular weight and intrinsic viscosity of the solute. It is noteworthy that the intrinsic viscosity of TrOCs may not be readily available. More importantly, Meireles et al.,24 did not account for the 3 dimensional nature of the solute and thus their findings are only valid for microporous membranes (i.e. ultrafiltration and microfiltration). As can be seen in Fig. 4, results reported here show that the minimum projection area is a better surrogate parameter to assess the rejection of neutral TrOCs by the NF270 membrane in comparison to molecular weight. The correlation between minimum projection area and the rejection of neutral TrOCs by the NF270 membrane was generally consistent with that by another NF membrane (NF90, Dow/Filmtec) that was reported in a previous study.25 However, data presented in Fig. 4 also show three exceptions (or outliners) including bisphenol A, caffeine, and TCEP, and their rejection values do not follow the other neutral compounds investigated here.
Fig. 4 Rejection of neutral and hydrophobic (HP) and hydrophilic (HL) TrOCs by the NF270 membrane as a function of the compound minimum projection area. Experimental conditions are described in Fig. 3. The rejection trendline of neutral TrOCs does not include caffeine, TCEP, and bisphenol A. The logD of these three TrOCs is shown in the parentheses. |
Bisphenol A (logD = 4.0; MPA = 44 Å2) that showed a lower rejection than the other compounds with equivalent minimum projection areas (Fig. 4). The rejection of bisphenol A (62%) by the NF270 membrane was much lower than that of omeprazole (94%; logD = 2.4; MPA = 44 Å2). Although bisphenol A is the third most hydrophobic compound among the selected neutral TrOCs, the degree of hydrophobic property is not the only factor explaining its low rejection. In fact, the other hydrophobic and neutral TrOCs including diazinon (logD = 4.3; MPA = 51 Å2) generally fitted well with the correlation between minimum projection area and rejection (Fig. 4). There can possibly be mechanisms other than electrostatic, steric and hydrophobic interactions that govern the separation of TrOCs by NF membrane. It is interesting to note that one of the three exceptions involved a hydrophilic and neutral TrOC (i.e., caffeine). Caffeine (logD = −0.6; MPA = 30 Å2) – the most hydrophilic compound among the selected TrOCs – exhibited a higher rejection than the other neutral TrOCs with equivalent minimum projection area values: diuron (logD = 2.5; MPA = 29 Å2) and linuron (logD = 2.7; MPA = 31 Å2).
The rejection of positively charged TrOCs generally followed the rejection trendline of neutral TrOCs with an exception of amitriptyline that has a hydrophobic property (logD = 3.0) (Fig. 5). The results suggest that the main mechanism of the rejection of positively charged TrOCs is the size exclusion like neutral TrOCs. Positively charged TrOCs may exhibit lower rejections than uncharged compounds due to their electrostatic attraction with the negatively charged membrane surface that causes the ‘charge concentration polarisation’ phenomenon.12 Nevertheless, in this study, the effect of ‘charge concentration polarisation’ could not be confirmed (Fig. 5). In fact, the electrostatic attraction effect on rejection can vary considerably and can be negligible as previously reported by van der Bruggen et al.26 Although the cause of the difference in the electrostatic attraction among NF membranes still remains unclear, it is noted that the positively charged TrOCs may show lower rejections than neutral TrOCs when used with the other NF membranes. In contrast to the positively charged TrOCs investigated here, the rejection of negatively charged TrOCs was high and was independent of their minimum projection area. The observed high rejection of all negatively charged TrOCs can be attributed to the electrostatic repulsion occurred between these negatively charged TrOCs and the negatively charged NF270 membrane surface (zeta potential = −14 mV at pH 8).18
Fig. 5 Rejection of charged TrOCs by the NF270 membrane as a function of the compound minimum projection area. Experimental conditions are described in Fig. 3. The line “Neutral” is the rejection trendline of neutral TrOCs described in Fig. 4. The logD of amitriptyline is shown in the parentheses. |
Name | Permeabilitya [L m−2 h−1 bar−1] | Conductivity rejectionb [%] |
---|---|---|
a Determined with Milli-Q water at 1000 kPa and 20 °C feed temperature. Values reported here are the average of duplicate experiments. b Determined with feed solution containing 20 mM NaCl, 1 mM NaHCO3, 1 mM CaCl2, at permeate flux 42 L m−2 h−1, feed pH 8.0 ± 0.1 and feed temperature 20.0 ± 0.1 °C. | ||
NF270 virgin | 15.3 | 38 |
NF270 cleaned with pH 11 | 18.2 | 22 |
NF270 cleaned with pH 12 | 23.6 | 18 |
Fig. 6 (a) Rejection of neutral and hydrophobic (HP) and hydrophilic (HL) TrOCs by the virgin NF270 membrane, and (b) differences in rejection after being exposed to pH 11 and pH 12 solutions for 25 h at 30 °C. Experimental conditions are described in Fig. 3. The minimum projection area (Å2) is shown in the parentheses. |
Minimum projection area also allows for a better assessment of the impact of operating condition variation on TrOC rejections by the NF270 membrane. The strong correlation between minimum projection area of neutral TrOCs and their rejections could still be observed after caustic cleaning (Fig. 7b & c). Once again, there were three outline TrOCs (i.e., caffeine, bisphenol A, and TCEP) as previously discussed in section 3.1. However, a similar conclusion can be made for these compounds. For example, as can be seen in Fig. 7, caffeine rejection by the NF270 membrane decreased from 86% (virgin condition) to 54% (immediately after caustic chemical cleaning at pH 12).
Fig. 7 Rejection of neutral and hydrophobic (HP) and hydrophilic (HL) TrOCs by (a) the virgin NF270 membrane, and the NF 270 membranes after being exposed to (b) pH 11 and (c) pH 12 caustic solutions as a function of their minimum projection area. Experimental conditions are described in Fig. 3. The rejection trendline of neutral TrOCs does not include caffeine, TCEP, and bisphenol A. The logD of these three TrOCs is shown in the parentheses. |
Fig. 8 (a) Rejection of positively and negatively charged TrOCs by the virgin NF270 membrane, and (b) differences in rejection after being exposed to pH 11 and pH 12 solutions for 25 h at 30 °C. Experimental conditions are described in Fig. 3. The minimum projection area (Å2) is shown in the parentheses. |
Fig. 9 Rejection of positively and negatively charged TrOCs by (a) the virgin NF270 membrane, and the NF 270 membranes after being exposed to (b) pH 11 and (c) pH 12 caustic solutions as a function of their minimum projection area. Experimental conditions are described in Fig. 3. The rejection trendline of neutral TrOCs does not include caffeine, TCEP, and bisphenol A. |
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