Polyelectrolyte Functional Bilayers for the Removal of Model Emerging

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Polyelectrolyte Functional Bilayers for the Removal of Model Emerging Contaminants Jain Maria Thomas, Vishnu N. Radhakrishnan, Charuvila T Aravindakumar, and Usha Kulangara Aravind Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02915 • Publication Date (Web): 23 Oct 2017 Downloaded from http://pubs.acs.org on October 26, 2017

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Industrial & Engineering Chemistry Research

Polyelectrolyte Functional Bilayers for the Removal of Model Emerging Contaminants Jain M. Thomas$, Vishnu.N.R¥, C.T. Aravindakumar¥,€and Usha K Aravind*§

School of Chemical Sciences$, Inter University Instrumentation Centre¥, School of Environmental Sciences€, Advanced Centre of Environmental Studies and Sustainable Development§, Mahatma Gandhi University, P.D. Hills P.O., 686560 kottayam, India

ABSTRACT: Microfiltration (MF) membranes with chitosan/polystyrene sulfonate (CHI/PSS) functional layer coated surface can act as a barrier against emerging contaminants (ECs) in water under low pressure. One (one bilayer) or two functional layers are enough for the removal of lidocaine (LD, 100%) and ibuprofen (IBU, >75%). The effect of various ions, surfactant and humic acid is studied along with the influence of feed pH. Phosphate interferes negatively with the removal efficiency of membrane for IBU and sulfamethoxazole (SMZ). The reusability of the membranes are illustrated by the desorption studies of LD. The extended life of membrane and hence the antifouling nature of the selected ECs indicate its potential in small scale filtration units. Effect of ionic strength in feed and in deposition medium on the rejection percentage is presented. This study shows that various mechanisms depending on the properties of ECs are responsible for the rejection of ECs. Key words: emerging contaminants, functional layers, LbL

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1.

INTRODUCTION Low pressure membranes, both ultrafiltration (UF) and microfiltration (MF) act as a

barrier against large, heavy particles such as bacteria, suspended solids and algae. On the other hand small materials, monovalent and polyvalent ions, dissolved organics and micropollutants pass through these membranes. High pressure filtrations, such as reverse osmosis (RO) is useful for the separation of ions and nanofiltration (NF) for emerging contaminants (ECs) having low molecular weight.1 But low pressure filtrations, UF and MF are also extended for the removal of low molecular weight compounds and ions with surface modification.2 In fact, low pressure filtrations are preferred at the municipal and industrial level as the investments are low. However, the low pressure systems at waste water treatment are mostly restricted at the pretreatment level and require further improvement with respect to flux and other filtration properties. Yet another area where they find application is in potable filtration units, where the target is mainly pathogens, turbidity etc. It is worthwhile if such membranes also remove ECs that is found in water matrix lately. Advanced surface coating techniques play a large role in confining such separation surfaces to the conventional membranes. The application of surface modified hollow fiber UF membrane for the removal of certain organic micropollutants was reported in elsewhere.3 Surface coating can be very interesting provided it has molecular level control and ability to upscale at industrial level. Molecular level control is very much attained through Langmuir-Blodgett (LB) and self assembled monolayers (SAMs). But both have certain limitations with respect to substrates.4 Most of the coatings which can be applied at macroscopic scale have drawbacks as they do not have control at molecular level. Hence the new generation two dimensional layering (layer by layer, LbL) and plasma coating are regarded high since they combine both the properties. LbL carry lot of advantages as bottom up controlled assembly. It provides room for control over surface properties.5 LbL is more


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convenient with respect to the simplicity of application and potential to apply to various substrates of different geometries.5 Preparation of composite membranes by coating skin layers on microfiltration membrane by LbL method is an economical and aqueous based cleaner technology.6 This surface modification can be achieved by alternative immersion of membranes on oppositely charged aqueous polyelectrolyte at room temperature.. LbL assembly is primarily driven by electrostatic force7 and could also involve hydrogen bonding,7 hydrophobic,4 covalent bonding,7 charge transfer,4 biological recognition4 etc. Due to these various types of interactions present in the multilayer the resulting LbL films are stable to severe conditions.4 Along with the substrate versatility8 material flexibility4 also increases the popularity of LbL assembly. It has wide range of application especially in water purification,9 gas separation,10 fuel cells,11 controlled drug delivery,11 biosensing,11 implantable materials,11 photocatalysis,12 and metal-organic frame works13. LbL coated membranes also find their way to the removal of biological contaminants. For instance the bacterial antiadhesive properties of layer by layer (LbL) modified MF membrane surface was reported by Tang et al.14 Thickness tunability from few angstroms to micrometers is yet another property suitable for industrial presence.15 Unlike in the case of other surface modification methods where the hydraulic resistance layer decreases the water flux, the LbL method improves the water flux by weakening the hydraulic layer.16 ECs are compounds whose toxicity was unknown or remained rather unclear in the environmental matrices for a long time and was not considered as a potential threat as traditional contaminants. ECs include different types of compounds such as pharmaceuticals, pesticides, herbicides, hormones, endocrine disrupting compounds, personal care products, flame retardants and surfactants.17 They enter in to the environment from various sources.18, 19 Thus they easily enter into drinking water sources and causes serious health effects to aquatic species as well as to humans. The presence of these ECs causes several harmful effects such



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as endocrine disruption, genotoxic effects, chronic damages, reproductive abnormalities on aquatic life.20, 21 A study conducted in Australia reported that some of the pharmaceuticals changes to more toxic form after the water treatment processes.22 The detection and removal of ECs is a great task since they are present in the environment at very low concentrations.23 The various conventional techniques are not effective in completely removing these pollutants.24 Membrane filtration at low transmembrane pressure is getting popular in water purification.2, 25 Emerging contaminants (ECs) poses a real challenge to membrane filters. The interaction between the pollutant and the membrane surface along with the matrix characteristics are the deciding factors in separation. The LbL assembly on microfiltration membrane is expected to remove the ECs without causing fouling. The major focus of the work here is to study the interaction of selected ECs with the few deposited bilayers of polyelectrolytes (CHI and PSS) on polyamide microfiltration membrane. Larger number of bilayers is normally coated for the removal of ions and small molecular weight compounds and hence longer time required for its fabrication.26 In the present work number of functional layers has been restricted to few. The performance of the membranes at different experimental conditions like number of bilayers, pH and ionic strength are studied. Along with this effect of water matrix is taken care of. Antifouling character of LbL coating is also studied on prolonged exposure. 2. EXPERIMENTAL METHODS 2.1. Materials. Ultipor N66 (nylon 6,6; 0.2 µm pore size, uncharged, Pall Life Sciences) microfiltration membrane was used as support for multilayer formation. Chitosan (CHI, medium molecular weight, 75−85% deacetylated, Sigma-Aldrich), Poly (styrene sulfonic acid) sodium salt (PSS, MW 200,000, 30 wt% in water, Sigma Aldrich), NaCl (Merck), Na2HPO4 (SRL), NaNO3 (Sigma-Aldrich), Na2SO4 (Merck), sodium dodecyl sulphate (SDS) 4 ACS Paragon Plus Environment

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(Merck), humic acid (HA) (Sigma Aldrich), HCl (Merck) and NaOH (Merck) were used without further purification. The ECs selected for the study, ibuprofen (IBU), sulfamethoxazole (SMZ), Carbamazepine (CBZ), Estradiol (E2), Lidocaine (LD), Bisphenol A (BPA) and Bisphenol S (BPS) were purchased from Sigma Aldrich. Ultrapure water (MilliQ, 18.2 MΩ cm, Pall Corporation) was used for all experiments. The physicochemical properties of the ECs are shown in the Table 1. 2.2. Preparation of polyelectrolyte multilayers. The supporting membrane was kept in double distilled water overnight for multilayer preparation. The polyelectrolytes used for the preparation of multilayers were chitosan (CHI, 0.01 M) and poly styrene sulfonate (PSS, 0.01 M) solutions (molarities of polyelectrolytes were taken with respect to repeating unit). pH of the polyelectrolyte solutions were adjusted to 1.7 by HCl. Bare membrane was first dipped in positively charged CHI solution for 15 min. After dipping, the membrane was washed with 50 ml distilled water to remove loosely bound polyelectrolytes. Then the membranes were again dipped in negatively charged PSS solution or PSS in different concentrations of NaCl (0.1 M – 0.5M) at pH 1.7 for another 15 min. The membrane was again washed with 50ml distilled water. Thus, one bilayer of oppositely charged polyelectrolyte was obtained by LbL method. By repeating the procedure desired number of CHI/PSS layers is obtained. 2.3. Ultrafiltration. Ultrafiltration experiments of ECs were performed using Amicon 8050dead end ultrafiltration cell (Millipore, 10 psi pressure, 400 rpm) at room temperature. Stock solutions of ECs selected for the study were prepared in methanol. Deionized water was used for the preparation of these pollutants at a concentration of 10-5 M. The amount of pollutants removed by the membrane is calculated using equation 1 as the percentage rejection, R (%), where CP and CF are the solute concentrations in permeate and feed respectively.

R (%) = [(CF - CP)/ CF] x 100


(1)


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2.4. Characterisation methods. 2.4.1. Analysis of emerging contaminants (ECs). The concentration of ECs in the feed and permeate are monitored by high performance liquid chromatography, HPLC (Shimadzu prominence UFLC, LC 20 AD) coupled with diode array detector (SPD-M20A). The mobile phase used for IBU, was 0.1 % formic acid in water/acetonitrile in the ratio 40:60 and for SMZ and CBZ it was in ratio 60:40. For E2, BPA and BPS the eluent composition was water/acetonitrile in the ratios 40:60, 45:55 and 60:40 respectively. For LD, water at pH 3.3 with ortho phosphoric acid/acetonitrile (80:20) was used as the eluent. The flow rate was 1 ml min-1 (250 mm × 4.6 mm ×5 µm Supelcosil C18 column). 2.4.2. Membrane characterization. The polyelectrolyte multilayer formation in the membranes

were

characterized

using

FTIR

analysis

(Shimadzu

IR

Prestige-21

spectrophotometer) in the ATR mode with frequency range between 600 and 4000 cm-1 using ZnSe crystal in air as background at 4 cm-1 resolution and 20 scans. Scanning Electron Microscopy, SEM (JSM-840) and Atomic Force Microscopy, AFM (WITec alpha300 RA) (tapping mode, spring constant of cantilever is 2.8 N/m) were used for the surface characterization of the membranes.

Table 1. Physicochemical properties of ECs.

ECs

Topological

constant

moment

polar surface

(g/mol)

(pKa)

(D)

area h (Å2)

Ibuprofen (IBU)

206.3

4.4-4.9a

3.84

1.80 a

37.30

Sulfamethoxazole (SMZ)

253.3

1.7, 5.6a

0.79

5.40 a

98.22

Carbamazepine (CBZ)

236

2.3a

2.77

3.60 a

46.33

2.84

3.54

b

32.34

0.79

f

40.46

Estradiol (E2)

Dissociation

weight

LogP e

Dipole

Lidocaine (LD)

Molecular

234.34 272

b

8.01

10.5

c

3.75


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a

Bisphenol A (BPA)

228

9.6-10.2c

4.04

0.71f

40.46

Bisphenol S (BPS)

250.3

8.47d

2.32

5.76 g

74.60

Reference27,

b

Reference28,

c

Reference29,

d

Reference30,

f

Reference31,

g

Reference32,

e, h

https://chemicalize.com

3. RESULTS AND DISCUSSION 3.1. Effect of functional layers on surface characteristics. The formation of functional layers on bare membrane is confirmed from the ATR FTIR spectra presented in Figure 1. Symmetrical stretching vibrational frequency of sulfonate group at 1036.77 cm-1 and in-plane vibrations of aromatic ring at 1008.85 cm-1were taken as the reference peaks. The increase in the coating steps increased the area under the functional group peak (Figure 1). Bare membrane has no peak in the above region. The changes in the surface of the bare membrane after bilayer formation can be visible from the SEM images (Figure S1).The topographical images were recorded using AFM (Figure 2A-D). From these images, it is clear that the surface coverage increases with increase in number of bilayers and correspondingly more number of pores are found to be closed. The pore size was calculated from AFM images using the ImageJ software. The pore size value of bare membrane, one bilayer, two bilayers and five bilayers was found to be 0.202±0.0057 µm, 0.182 ±0.0033 µm, 0.161 ±0.0030 and 0.142 ± 0.0022 µm respectively. The RMS roughness value calculated from AFM using Gwyddion software for bare nylon membrane, one, two and five bilayers were 677 nm, 246 nm, 110 nm and 61 nm respectively. CHI/PSS functional layer formation smoothens the surface of bare membrane. This leads to a reduction in fouling. Decrease in fouling due to surface smoothening is reported elsewhere.26 This is because it has less surface area than a rough surface and the number of points for the foulants to contact with the surface is less.33



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One of the major advantages of surface coating by LbL technique is that it makes the membrane more resistant to fouling.

Figure 1. ATR FTIR spectra of bare membrane (black line), CHI/PSS one functional layer (red line), CHI/PSS two functional layers (green line) and CHI/PSS five functional layer (blue line) membranes.

Figure 2. AFM images of bare membrane (A) and CHI/PSS coated membrane (B- D) having one (B) two (C) and five (D) functional layers.


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3.2. Experimental conditions on the performance of membrane. The membrane performance depends on the different experimental conditions of both the membrane as well as the characteristics of ECs under investigation. As given in Table 1 the most hydrophobic EC is BPA (LogP - 4.04) and the hydrophilic one is SMZ (LogP- 0.79).

3.2.1. Effect of number of functional layers. The functional layers constituted a combination of weak/strong (CHI/PSS) polyion combination. Chitosan is a biopolymer obtained from the deacetylation of the natural polysaccharide, chitin.34 It contains free amino groups and exhibits pH dependent ionisation character. PSS, the strong anionic counterpart do not respond to pH and is quite hydrophobic. These combinations are found in LbL coatings for the isolation of proteins, encapsulation of enzymes and other biomolecules.35, 36 The same pair also form effective multilayers for the removal of micro pollutants.37 Identical coating pH is maintained for the polyions so as to avoid the mutual influence on layer formation. Moreover, make up pH of CHI is selected to ensure that it is nearly in fully ionised state. The present make up pH selection is expected to provide a permeable platform. The number of functional layers was increased upto eight. The performance of bare and coated membranes was evaluated for the selected ECs with respect to the permeation characteristics. The percentage rejection values against the number of functional layers are shown in Figure 3. Some of the compounds were fully retained by the bare membrane itself. This included E2, BPA and BPS. In this case the removal percentage remained more or less similar in the single functional bilayered membrane or in the case of bare and their experiments were further carried out using single functional layered membrane. For E2 and BPA the rejection is based on the hydrophobic interaction and hydrogen bonding with the membrane. The factors favouring the removal of these compounds are its own (solute) characteristics such as high hydrophobicity and very low dipole moment. BPS forms hydrogen bonds with the surface of the membrane as it has high polar surface area. For rest of the compounds few bilayers were 9 ACS Paragon Plus Environment


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necessary for either complete or partial removal. It was found that the percentage rejection of bare membrane for LD, IBU, SMZ and CBZ was 3.60 %, 36.00%, 20.99% and 16.66% respectively. Two functional layers were necessary for the nearly complete removal of LD and further increase in number of layers did not influence the rejection behaviour. So, for LD two functional layers were selected for further studies. As the filtration pH (5.8) of LD is below the pKa value (8.01), it carries positive charge. In this case the rejection is based on the interaction between negatively charged PSS with the positively charged form of LD. Single functional layer increased the rejection percentage of IBU to about 79%. Further increase in bilayer did not have much positive influence on membrane performance and hence one functional layer is selected for further studies. SMZ is negatively charged at the filtration pH (6.07). The membranes could partially remove SMZ with the increase in functional layers. For IBU and SMZ electrostatic repulsion between the drugs and the membrane surface is the main rejection mechanism. CBZ, which exist in neutral form mostly remain non interactive with the membrane or functional layer. Size based sieving cannot be expected as the pore size here is high. Similar non interactive behaviour of CBZ was also found in elsewhere where a loose NF membrane was used for the rejection studies under cross flow conditions.27 In the case of SMZ and CBZ, the experiments were carried out using five functional layers since there is no appreciable change in permeability with increasing in number. The possible interactions of the ECs with the membrane are summarized in the Table 2.


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Figure 3. Removal of ECs as a function of number of CHI/PSS functional layers. Table 2. Membrane performance and its interactions with ECs. ECs

Percentage rejection (%)

Ibuprofen (IBU)

79.00


47.15

Carbamazepine (CBZ)

25.33

Lidocaine (LD)

100

Estradiol (E2)

100

Bisphenol A (BPA)

100

Bisphenol S (BPS)

100

Possible interactions with the membrane Electrostatic, Hydrophobic Electrostatic, Hydrogen bonding Non interactive Electrostatic, Hydrophobic Hydrophobic, Hydrogen bonding Hydrophobic, Hydrogen bonding Hydrogen bonding

3.2.2. Effect of commonly present anions, surfactant and humic acid in water. The performance of the membarne is further evaluated in the presence of

various anions,

surfactant and humic acid each at a concentration of 50ppm (Figure 4A). 3.2.2.1. Effect of anions. The performance of the membranes were evaluated in presence of various anions such as chloride, phosphate, sulfate and nitrate.



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IBU showed a positive trend in presence of chloride, sulfate and nitrate. But phosphate had a negative influence in percentage rejection. The positive influence on the membrane performance is due to the accumulation of IBU on the surface of the membrane in presence of these ions. Na2HPO4 is a commonly used pH modifier to increase the solubility of weakly acidic or basic pharmaceuticals.38 In presence of Na2HPO4, IBU, a weakly acidic drug is easily permeated through the membrane because of its increase in solubility due to the change in the micro environmental pH. Also, there is interaction between the Na2HPO4 and negatively charged IBU species. This is evident from the shift in IR spectral peak of C-H in plane deformation of IBU from 1022 cm-1 to 1014 cm-1on interaction of IBU and Na2HPO4 (Figure 7). Topographical analysis reveals changes in the surface characteristics. From the surface roughness value (281 nm) obtained from the AFM image (Figure 5B) it is clear that there is an increase in surface roughness of the membrane after filtration of IBU in presence of Na2HPO4 and it may be due to its change in microenvironmental pH. But in presence of other ions such as chloride, sulphate and nitrate the roughness value of the membrane surfaces after the filtration of IBU was found to be lower ie, 186nm, 209 nm and 204 nm respectively. In this study, the presence of nitrate showed maximum rejection and phosphate gave least rejection of IBU. From the 3d images of membrane surfaces after the filtration of IBU in presence of phosphate (Figure 5G) and nitrate (Figure 5H) also it is clear that the height to depth ratio is greater for that in presence of phosphate than nitrate. It is also clear from their corresponding depth profiles (Figure 5I). In order to get more idea about the structural changes of membrane surfaces after the filtration of IBU in presence of phosphate and nitrate Minkowski functionals were calculated using Gwyddion software and are shown in the Figure 6. In both cases the Minkowski volume (V) and Minkowski surface (S) shows similar trend. The Minkowski connectivity (߯) curve of IBU in presence of phosphate shows a minimum value of −1 x 10-3 at 0.6 µm and a maximum value of 0.75x 10-3 at 1.1µm. But in


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presence of nitrate it shows a minimum value of −0.75 x 10-3 at 0.61µm and maximum value of 0.68x 10-3 at 1.03µm. The minimum values indicate the maximum density of valleys and the maxima indicates the maximum density of peaks. From these values, it is evident that membrane surfaces after the filtration of IBU in presence of phosphate shows maximum density of valleys and peaks than in the case of nitrate. This possibly indicating the formation of individual CHI/PSS complexes which is the reason for the increased roughness.39 This is obvious from the Figure 5G as the pores are largely visible. So, the solution flows freely in the case of phosphate. The percentage rejection of SMZ slightly decreases in presence of these ions. This is because of the screening of the charge on the membrane by the addition of these ions as a result of external compensation.40 Also the charge density of the dissociated species of SMZ is lower than that of IBU.27 Since in SMZ, the O=S=O group shows electron withdrawing properties and thus the negative charge is lowered. This study reveals that a small change in the surface charge of the membrane in presence of salt affects the rejection of SMZ by lowering the electrostatic repulsion. For SMZ also the performance of the membrane was poor in presence of Na2HPO4 as in the case of IBU. In the case of CBZ, the presence of ions does not have much influence as only a slight decrease in percentage rejection was observed. Very significant changes are not observed for LD too. Also, there is not much change in the AFM images and the roughness values of membrane surfaces after the filtration of LD in presence of chloride (149 nm), phosphate (160 nm), sulphate (169 nm) and nitrate (166 nm) (Figure S2A-D). For E2, BPA and BPS the presence of ions did not affect the rejection property. Ion Chromatography studies reveals that chloride, phosphate, sulphate and nitrate ions are permeated during filtration. This is because the hydrated radii of these inorganic species



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are too small compared to the pore radius of this surface modified microfiltration membrane. The membrane preferentially rejects the ECs. The membrane performances and the parameters involved in the rejection of ECs are summarized in the Table 3. Table 3. Effect of anions on the rejection performance of ECs and the possible interactions with the membrane. ECs in presence of anions

Ibuprofen (IBU)


Carbamazepine (CBZ)

Lidocaine (LD)

Estradiol (E2), Bisphenol A (BPA), Bisphenol S (BPS)

Percentage rejection (%)

NaCl Na2HPO4 Na2SO4 NaNO3 NaCl Na2HPO4 Na2SO4 NaNO3 NaCl Na2HPO4 Na2SO4 NaNO3 NaCl Na2HPO4 Na2SO4 NaNO3 NaCl Na2HPO4 Na2SO4 NaNO3

94.41 2.29 94.72 100 34.04 4.55 33.78 34.83 22.64 23.04 24.46 21.29 91.08 97.98 97.46 98.98 100 100 100 100

Interactions Accumulation on the membrane surface Interaction with Na2HPO4 Accumulation on the membrane surface Accumulation on the membrane surface Charge screening Interaction with Na2HPO4 Charge screening Charge screening Non interactive

Competition with Na+ ions

No effect

3.2.2.2. Effect of surfactant. Sodium dodecyl sulphate (SDS) is a widely used anionic surfactant that is discharged into the environment as a result of various processes and it adversely affects the eco system, especially the water bodies. It shows different behaviours with respect to concentration.41 It was reported earlier that the solubility of SDS increases at a concentration above its critical micellar concentration (CMC), 8.1 mM.42 The low concentration of SDS (50ppm) doesn’t have much influence on the solubility on ECs selected for the study. So, there is only a slight change in the percentage rejection of IBU, SMZ and


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CBZ. The corresponding AFM image of IBU is shown in Figure 5E. But for E2, BPA, BPS and LD, there is no effect on the rejection. The corresponding AFM image of IBU is shown in Figure 5E. But for E2, BPA, BPS and LD, there is no effect on the rejection. AFM image of the membrane surface after filtration of LD in presence of SDS is displayed in the Figure S2E. Further the performance of the membrane is evaluated at higher concentrations of SDS in the feed (500 to 2500 ppm) and the results are shown in Figure 4B. In presence of SDS, the rejection of IBU and SMZ decreases at a much higher rate compared to the other ECs selected for the study. For IBU and SMZ, electrostatic repulsion is the main rejection mechanism and the repulsion is greater for IBU than SMZ. On increasing the concentration of SDS, IBU is highly affected than SMZ because IBU is more hydrophobic and hydrophobic sites present in the IBU can interact with the hydrophobic regions present in the SDS (since in aqueous medium, the hydrophobic sites present in the surfactant are away the hydrophilic sites are surrounded by the ionic groups) and it becomes highly soluble compared to SMZ and thus, the rejection sharply decreases. While E2, BPA and BPS, interact with the membrane by their hydrophobic sites or by hydrogen bonding. So, their interaction with SDS is less. In the case of LD, it interacts with the PSS and thus there is lesser interaction with SDS. CBZ remains as non interactive with the membrane and in presence of SDS, it interacts with SDS through its hydrophobic sites and thus its rejection also decreases. 3.2.2.3. Effect of Humic acid (HA). Humic acids are complex organic substances commonly found in water bodies that are mainly formed by the biodegradation of plant residues.43 It shows amphiphilic properties and can act as a natural polyelectrolyte.43 They can complex with certain hydrophobic contaminants through its hydrophobic moieties and increase its water solubility.44 IBU is a hydrophobic drug as evident from the Log P value in the Table 1. So it can form complex with HA due to hydrophobic interaction with the hydrophobic 15 ACS Paragon Plus Environment


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domains of HA. As a result of this interaction the water solubility of the drug increases and it can easily permeates through the membrane which appears as a decrease in percentage rejection. The corresponding AFM image is shown in Figure 5F. Also in the case of LD and CBZ, there is a slight decrease in percentage rejection observed in presence of HA due to this type of interaction and the corresponding AFM image for LD filtered functional layer membrane is given in the Figure S2F. Also, in the case of SMZ, a decrease in rejection was observed. But HA has no effect on the percentage rejection of E2, BPA and BPS.

Figure 4. Removal of ECs in presence of chloride, phosphate, sulphate, nitrate, SDS and HA (A) and SDS at higher concentrations (B) using CHI/PSS functional layer membranes.

Figure 5. AFM 2d images of IBU filtered CHI/PSS one functional layer membrane (A-F) in presence of chloride (A) phosphate (B) sulphate (C) nitrate (D) SDS (E) and HA (F) and the


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corresponding 3d images (G and H) and depth profiles (I) of phosphate(G) and nitrate (H) respectively.

Figure 6. a) The Minkowski volume, ‘V’ b) the Minkowski surface, ‘S’ and c) the Minkowski connectivity, ‘߯’ of IBU filtered CHI/PSS one functional layer membrane in presence of phosphate (black line) and nitrate (red line) respectively.

Figure 7. ATR FTIR spectra of IBU (red line), Na2HPO4 (green line) and IBU in presence of Na2HPO4 (black line).



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3.2.3. Effect of feed pH. Solution pH is an important factor in water treatment processes because the chemical speciation of ECs varies with solution pH. Percentage rejection studies of the ECs at different pH conditions are shown in the Figure 8. The percentage rejection of IBU at pH 4 is 70.11. It enhances to 75.63% till pH 4.5 and there onwards a gradual increase was found upto pH 8. In the present membrane system the outermost functional layer is PSS, excess negative charge is present on the surface and can repel negatively charged species. As an organic acid IBU is highly deprotonated at a pH above the pKa (4.4-4.9) value. Above pKa it has high charge density so that electrostatic repulsion between the negatively charged compound and membrane surface increases.27 In addition the increase in hydrated size of the molecule may have increased the possibility of sieving. Also because of its hydrophobic nature there is hydrophobic interaction between IBU and the hydrophobic sites present in the membrane. The percentage rejection studies of SMZ were carried out at pH 2 to 8. It was observed that the percentage rejection of SMZ at pH 2 is 36.85%. With the increment in feed pH an increase is observed till pH 4 (48.18%) and after that it remains more or less same. The deprotonation of SMZ is caused by amino group and there is not much variation in rejection with pH, though the compound is in the negatively charged state at the pH above the second pKa value (5.6). This compound possess high dipole moment (5.4 D) which results in the attraction between the compound’s pole centres and the membrane surface.27 So it can easily permeate through the membrane and the percentage rejection was found to be lower than that of IBU. SMZ possesses high polar surface area compared to others and hence the chance for hydrogen bonding is greater. But here the predominant factor is dipole moment which decreases the rejection. But in the neutral state (ie, at pH 4) the increase in rejection compared to other pH values may be due to the hydrogen bonding with the membrane surface as a result of higher polar surface area. Electrostatic repulsion is the main rejection 18 ACS Paragon Plus Environment

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mechanism in the negatively charged state of the drug. In another study, similar experiments were conducted using NF membranes made of polyamide composite material and found that NF 270 showed high rejections of IBU and SMZ by electrostatic repulsion in negatively charged state of the drugs under cross flow conditions.45 The pKa value of LD is 8.01. With the increase in feed pH the percentage rejection increases largely and it is completely rejected at pH 6.. From pH 6 onwards the percentage rejection remains constant. Below the pka value (8.01) LD exists in the positively charged form which facilitates its interaction with PSS.46. This is evident from the decrease in the characteristic peaks of PSS as shown in the ATR FTIR spectra at its normal solution pH (5.8) (Figure 10). As a result of this interaction LD is highly rejected by the membrane. But at low pH, the availability of H+ ions in the solution is high and therefore sulfonate group interacts with H+ ions from the solution. As a result, LD easily diffuses through the membrane and percentage rejection was found to be very low. As the pH increases the availability of H+ ions decreases which results in an increased rejection. This is also evident from the desorption studies that about 85% of LD is easily desorbed from the membrane within 30 min using acidic water (pH 2) (Figure 11). But only 10% is desorbed from the membrane even after 6 hours in distilled water. This is because under acidic conditions there is availability of H+ ions in the solution and polystyrene sulfonate easily exchanges LD with H+ ions. This desorbed membrane is in the reusable form as it completely removes LD again. It was reported that uncharged anesthetics have more interaction with hydrophobic sites than the charged ones.47 So it interacts with the hydrophobic sites of the PSS. As per the literature, even in the uncharged form there is partial charge associated with the molecule due to the presence of carbonyl group.48 So, the sulfonate group of PSS may interact with the partial positively charged sites of the anesthetic. That may be the reasons for the high rejection of LD at basic pH. The change in surface morphology of the membrane with change in pH can 19 ACS Paragon Plus Environment


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be seen from the SEM images (Figure S3). At pH 2 the membrane is highly porous in nature. As the pH increases the number of pores decreases. When the pHs of the LD reaches 6 and 8 there are only less number of pores as can be seen from the SEM images. The surface coverage of the membrane increases as a result of increased percentage rejection. This is also evident from the AFM images (Figure 9A-C) as the roughness of the membrane at pH 6 was found to be 156 nm which is less than that at pH 2 (162 nm). This increase in roughness at pH 2 is due to the increase in disorder as a result of the presence of more number of pores. BPS is almost completely rejected by the membrane till a feed pH of 9. Further increase in feed pH causes a sharp decrease in the percentage rejection. Polar surface area is an indication of a compound to form hydrogen bond.49, 50 BPS shows high polar surface area (74.60 Å2) compared to BPA and E2. . From pH 10 onwards the percentage rejection was found to be decreased because at this pH about 98% of BPS exists in the dissociated form and the chances for hydrogen bonding is very less. The percentage rejection of E2 and BPA is not at all affected by the feed pH where as CBZ remained non interactive throughout the study pH. E2 and BPA are neutral compounds and the mechanisms behind the removal are hydrophobic interactions and hydrogen bonding. CBZ exist as a neutral compound under the experimental conditions of pH and does not respond to charge based sieving. Also, its dipole moment (3.6 D) is high and there is greater chance for permeation through the membrane pores. The studies for the removal of CBZ using high pressure membranes such as NF or RO shows that separation is mainly governed by size exclusion. The rejection trend varied according to the pore size.27, 45, 51, 52 But here size based separation is not possible with these fewer number of bilayers.


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Figure 8. Removal of ECs as a function feed pH using CHI/PSS functional layer membranes.

Figure 9. AFM images of LD filtered CHI/PSS two functional layer membrane at feed pHs of 2 (A) 4 (B) and 6 (C).



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Figure 10. ATR FTIR spectra of bare membrane (black line), CHI/PSS two functional layer membrane (green line) and LD filtered CHI/PSS two functional layer membrane (red line).

Figure 11. Releasing studies of LD from its CHI/PSS two functional layer filtered membrane as a function of time. 3.2.4. Influence of makeup salt on membrane permeation. Percentage rejection studies of ECs were carried out using different concentrations of NaCl in PSS solution. For IBU and SMZ percentage rejection were found to be very low with the addition of salt to the deposition solution as illustrated in the Figure 12. This is because of the screening of charge on the membrane due to extrinsic compensation by the addition of salt. Thus, it reduces the electrostatic repulsion between the compound and the membrane. By the addition of salt charge compensation takes place by salt ions.40 In the case of CBZ there is only slight 22 ACS Paragon Plus Environment

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variation in the percentage rejection since it exist as a neutral compound. For E2, BPA, BPS and LD there is no change in the rejection value on the addition of salt in the deposition medium.

Figure 12. Removal of ECs as a function of different concentrations of salt solution in the deposition medium using CHI/PSS functional layer membranes. 3.2.5. Presence of salt in feed solution. The percentage rejection of ECs as a function of different concentration of NaCl in feed solution is illustrated in the Figure 13. It was found that IBU is completely removed in presence of NaCl. There is accumulation of compounds on the membrane surface in presence of NaCl if the compound is having a negatively charged functional group due to electrostatic interaction between sodium ions and negatively charged compounds and this accumulation increases with increase in ionic strength.53 In our study also there is accumulation of IBU on the membrane surface because of the interaction between negatively charged carboxylate ions of IBU and sodium ions from salt solution and as a result the percentage rejection increases. Schafer et al. reported that as ionic strength increases there is transformation from a linear to coiled structure in the case of large molecules and which results in aggregation and increased rejection.54 In the present study, the increase in rejection is due to the transformation of IBU



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from linear to coiled structure and the aggregation increases with ionic strength. This is also evident from the increase in root mean square roughness (RMS) of the membrane surface with increase in aggregation measured using AFM. The surface roughness increases from 136 nm to 198 nm as the concentration of NaCl increases from 0.05 M to 0.2 M (Figure 14A-B). The pore size was also found to be increased to 0.16 µm to 0.19 µm as the concentration increases from 0.05 M to 0.2 M. But in the case of SMZ, the addition of salt screens the interaction between membrane and the dissociated compound which results in lower repulsion and decrease in rejection. Similar rejection pattern for SMZ is observed for NF membrane under cross flow conditions.27 In the case of CBZ only a slight decrease in rejection was observed in presence of salt. But in the case of LD there is a sharp decrease in percentage rejection was observed by the addition of NaCl in the feed solution. This is because in presence of salt, Na+ ion competes with the positively charged form of the drug to interact with the sulfonate group in the PSS and Na+ ion preferentially attacks this site. So, the drug easily permeates through the membrane and as a result percentage rejection decreases. For E2, BPA and BPS there is no effect on percentage rejection by the addition of salt.


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Figure 13. Removal of ECs as a function of different concentration of salt solution in the feed using CHI/PSS functional layer membranes.

Figure 14. AFM images of IBU filtered CHI/PSS one functional layer membrane in presence of NaCl at concentrations (A and B) of 0.05 M (A) and 0.2 M (B). 3.2.6. Performance of membranes on repeated filtration. Generally polyamide membranes have rough surface and there is a greater chance for fouling due to the accumulation of particles on the surface which results in flux decline.55 The presence of fouling layer may even cause the deterioration of membrane material by various interactions.33 For low pressure membranes, fouling occurs due to pore blocking.56 The major factors affecting the membrane fouling are ionic strength and composition, pH and nature of foulants.56 LbL assembly of polyelectrolyte is a useful tool for smoothening the rough surface and thereby increasing the antifouling properties of membranes.57 There are reports for the application of LbL assembly to improve the antifouling nature of NF and RO membranes.58 Polyelectrolyte multilayers also retard the formation of biofilms on the membrane surface.14 Thus the surface modification by alternate deposition of polyelectrolyte increases the endurance of membranes.59 The life of bilayered membranes during the repeated filtrations of ECs is evaluated in terms of the variation in percentage rejection and is presented in Figure 15. The performance of the membrane declines immediately on second filtration for IBU. As a result of accumulation of IBU on the surface modified membrane during prolonged exposure, the roughness values calculated from the AFM images of the membrane surfaces decreases from 25 ACS Paragon Plus Environment


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157 nm to 133 nm after repeated filtrations (Figure S4). Decrease in surface roughness after filtration was also observed in another study.60 But for LD, BPA and E2 the percentage rejection remains above 80% till 5th filtration. At the filtration pH, BPA and E2 are neutral, and are highly hydrophobic. Hence the hydrophobic interaction keeps the membrane interactive and can be used for their removal. The removal of LD remains more satisfactory since it has additional electrostatic interaction. As a result, the LD filtered membrane surface was found to be more resistant to fouling. This is also evident from the almost same roughness values of the filtered membranes (157 nm) even after different times of filtrations (157 nm) (Figure S4). The removal of BPS also declines on each filtration. It is almost non interactive for CBZ.

Figure 15. Variations in the rejection of ECs as a function of repeated filtrations using CHI/PSS functional layer membranes. 4. CONCLUSIONS This study investigated the removal of various ECs from water by surface functionalization of microfiltration membranes using polyelectrolytes. It is evident that only few functional layers are enough for the complete or near complete removal of almost all the ECs under study. These results indicated that the performance of the membrane is strongly 26 ACS Paragon Plus Environment

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affected by various experimental conditions of pH, ionic strength, presence of various substances etc. As a result of change in speciation with pH, the rejection performance alters with respect to pKa values of the selected compounds. Complete removal of IBU is achieved in presence of salt as a result of the combined effect of various processes such as interaction between the drug and the salt and change in structural properties of the drug. But the addition of salt to the polyelectrolyte solution shows a reverse effect on rejection of IBU because of the screening of the charge on the membrane. In the case of LD the membrane performance was poor on the addition of salt to the feed solution but it does not affect the rejection property on adding salt to the deposition medium. In conclusion, LbL is a very effective and eco friendly surface modification technique for the removal of various ECs from water. This study explores the properties of a MF membrane with surface modification. Thus, this method finds its application in MF based potable units. ASSOCIATED CONTENT Supporting Information 1. SEM images of bare and CHI/PSS coated membranes 2. AFM images of LD filtered CHI/PSS coated membranes 3. SEM images of LD filtered membranes at different feed pHs 4. AFM images of CHI/PSS coated membranes after filtration and repeated filtrations with LD and IBU. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes



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The authors declare no competing financial interest. ACKNOWLEDGEMENTS JMT and UKA are thankful to UGC and DST, respectively, for the financial support. REFERENCES (1) Guo, H.; Deng, Y.; Tao, Z.; Yao, Z.; Wang, J.; Lin, C.; Zhang, T.; Zhu, B.; Tang, C. Y. Does Hydrophilic Polydopamine Coating Enhance Membrane Rejection of Hydrophobic Endocrine-Disrupting Compounds? Environ. Sci. Technol. Lett. 2016, 3, (9), 332-338. (2) Huang, H.; Schwab, K.; Jacangelo, J. G., Pretreatment for Low Pressure Membranes in Water Treatment: A Review. Environ. Sci. Technol. 2009, 43, (9), 3011-3019. (3) Ojajuni, O.; Holder, S.; Cavalli, G.; Lee, J.; Saroj, D. P. Rejection of Caffeine and Carbamazepine by Surface-Coated PVDF Hollow-Fiber Membrane System. Ind. Eng. Chem. Res. 2016, 55, (8), 2417-2425. (4) Tang, Z.; Wang, Y.; Podsiadlo, P.; Kotov, N. A. Biomedical Applications of Layer-byLayer Assembly: From Biomimetics to Tissue Engineering. Adv. Mater. 2006, 18, (24), 3203-3224. (5) Xiao, F.-X.; Pagliaro, M.; Xu, Y.-J.; Liu, B. Layer-by-layer assembly of versatile nanoarchitectures with diverse dimensionality: a new perspective for rational construction of multilayer assemblies. Chem. Soc. Rev. 2016, 45, (11), 3088-3121. (6) Gopalakrishnan, A.; Mathew, M. L.; Chandran, J.; Winglee, J.; Badireddy, A. R.; Wiesner, M.; Aravindakumar, C. T.; Aravind, U. K. Sustainable polyelectrolyte multilayer surfaces: possible matrix for salt/dye separation. ACS Appl. Mater. Interfaces 2015, 7, (6), 3699-707. (7) Choi, J.; Konno, T.; Takai, M.; Ishihara, K. Controlled drug release from multilayered phospholipid polymer hydrogel on titanium alloy surface. Biomaterials 2009, 30, (28), 52018. 28 ACS Paragon Plus Environment

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Graphical Abstract Rejection pattern of ECs



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177x90mm (300 x 300 DPI)

ACS Paragon Plus Environment

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Polyelectrolyte Functional Bilayers for the Removal of Model Emerging

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