Preparation and characterization of poly (ethersulfone) nanofiltration membranes for amoxicillin removal from contaminated water
© Omidvar et al.; licensee BioMed Central Ltd. 2014
Received: 4 April 2013
Accepted: 2 November 2013
Published: 8 January 2014
Nowadays, antibiotics such as amoxicillin have been entered in water bodies. Nanofiltration has been proposed as an attractive technology for removal of antibiotics from aquatic environment instead of conventional wastewater treatment. In this paper, novel asymmetric flat sheet nanofiltration membranes were prepared via immersion precipitation technique and by using the poly(ethersulfone)/Brij®S100/Poly(vinylpirrolidone)/1-methyl-2-pyrolidone casting solutions. The effect of addition of Brij®S100 as a non-ionic surfactant additive as well as concentration of poly (ethersulfone) on morphology, wettability, pure water flux and rejection of amoxicillin were studied using the scanning electron microscopy, water contact angle apparatus and experimental set-up. The results indicated that the addition of Brij®S100 to the casting solutions resulted in the formation of membranes with higher hydrophilicity and relatively noticeable rejection of amoxicillin up to 99% in comparison with unmodified poly(ethersulfone) membrane. Contrary to amoxicillin rejection, pure water flux was decreased when higher poly(ethersulfone) concentration was employed.
Among all the pharmaceutical drugs that cause contamination of the environment, antibiotics occupy an important place due to their high consumption rates in both veterinary and human medicine . Antibiotics as an important group of pharmaceutically active compounds (PhACs) were first produced in early 1940s and widely used in fighting against infectious bacteria and fungi . Recently, antibiotics were quantified in hospital sewage water and wastewater, in rivers and in wastewater treatment plants (WTPs) .
The presence of antibiotics in the aquatic environment has created two issues. The immediate concern is the potential toxicity to aquatic organisms, and also to humans through drinking water. In addition, there is growing alarm that release of antibiotics to the environment contributes to the emergence of strains of disease-causing bacteria, resistant to high doses of these drugs . Consequently, removal of antibiotics before they enter the aquatic environment, as well as for water reuse is very pertinent . The molecular mass (MW) of antibiotics are in the range of 200 to 1,200 Daltons, coincident with the range of molecular mass cutoffs of NF membranes . Membrane filtration using nanofiltration (NF) and reverse osmosis (RO) membranes is shown to be one of the most promising techniques for the removal of antibiotics .
Fouling in pressure-driven membrane processes like NF is a key design and operational concern; thus several control strategies have evolved to minimize its occurrence and impact. Fouling reduction involves one or a combination of three approaches viz. feed pre-treatment, controlling the system hydrodynamics and modifying the membrane characteristics . Many investigations have demonstrated that increasing membrane surface hydrophilicity could effectively inhibit membrane fouling . Therefore membrane with hydrophilic characteristics has drawn considerable attention in practical use in recent years because of its better fouling resistance .
Poly(ethersulfone) (PES), a transparent and amorphous polymer, is well-known due to its excellent heat deflection temperature, excellent toughness, dimensional stability, and resistance to steam, boiling water and mineral acids. Its other desirable properties include thermal stability, creep resistance, inherent flame resistance, and status as an approved material for use in food, water and medical applications. This polymer demonstrates moderate chemical resistance against many alkalis, and exhibits excellent biology and blood compatibility. All these properties make PES as an attractive material for membrane preparation. Its amorphous phase provides membrane flexibility while the crystalline phase provides the desired thermal stability . The main disadvantage of PES membrane is the low hydrophilicity of the prepared membrane. Membrane surface properties often cause intense fouling when solutions containing substances like proteins are filtered. Therefore the modification of PES membrane is necessary for reducing the membrane fouling .
A promising in situ membrane surface modification approach can be obtained by addition of hydrophilic additives to the membrane casting solution. To improve the performance of PES membrane, researchers investigated the effect of some surfactants such as tetronic 1307 , sodium dodecyl sulphate (SDS), cetyle three methyl ammonium bromide (CTAB), triton x-100  and tween 80  on the properties and performance of PES membranes. They found out that addition of surfactant to the casting solution increased porosity of the membrane support layer and enhanced pure water permeability through the membranes. Surfactants constitute the most important group of detergents which are generally surface active agents. They are comprised of a hydrophobic portion attached to a hydrophilic functional group. Surfactants can be categorized according to the charge present in the hydrophilic portion of the molecule (after dissociation in the aqueous solution): anionic, cationic, non-ionic and zwitterionic surfactants [18, 19].
There has been no prior study on the effect of Brij®S100 surfactant as a hydrophilic additive in order to improve the hydrophilic property of the PES nanofiltration membranes. As such, this research work investigates the preparation and characterization of these improved PES membranes. Membrane performance was evaluated in terms of concentrations of Brij®S100, PES and amoxicillin.
Materials and methods
Preparation of the membrane
Composition of PES casting solution
PES (wt. %)
PVP (wt. %)
Brij®S100 (wt. %)
Membranes test by an experimental setup
Where R is the rejection (%), and Cf and Cp are the solute concentration in feed and permeate samples, respectively. Amoxicillin concentration in the samples was determined by reacting amoxicillin with N, N-dimethyl-p-phenylenediamine in the presence of potassium hexacynoferrate (III) in an alkaline medium. The absorbance of the blue water-soluble reaction product was measured at 660 nm, using a UV–vis Spectrophotometer (T60, China) .
Scanning electron microscopy (SEM)
Membrane structure was examined by a scanning electron microscope (KYKY-EM 3200, China). To obtain a generally consistent and clean cut, membrane samples were held under liquid nitrogen and then snapped by flexing in one direction until it broke. After sputtering with gold, they were viewed with the microscope at 25 KV.
To determine the hydrophilicity of a membrane, the contact angle between a drop of distilled water and the membrane surface was measured at room temperature of 25 ± 2°C, using a contact angle measuring instrument (G10, KRUSS, Germany).
Results and discussion
Morphological studies of prepared membranes
Effect of Brij®S100 concentration
Effect of PES concentration
The effect of variation of PES concentration on the membrane morphology is detected by comparison between Figures 3 and 4 as observed increase in PES concentration from 17 to 21wt. % results in the formation of smaller macrovoids and increase of thickness of the membrane top layer. Increase in the PES concentration from17 wt. % to 21 wt. % results in noticeable increase in viscosity values and consequently reduces mutual diffusivities between the nonsolvent (water) and solvent (NMP) in the system during solidification of the casting solution. Thus, using higher values of PES concentration, the precipitation process is stopped after a longer time and this leads to preparation of denser membranes [26, 27].
Susanto and Ulbricht  determined that a PES membrane without an additive had a lower contact angle than that typically measured for a non-porous PES film. They state that this is due to the porous structure in the outer membrane surface. Therefore, care should be taken to interpret the contact angle results because wettability is influenced not only by membrane material but also by the surface porosity. Thus the greater contact angle of the 21 wt % PES membranes can be attributed to the decreased surface porosity.
In fact higher porosity of membrane surface can reduce the contact angle of water drops on membrane surface. In addition, Figure 6 shows that the addition of Brij®S100 decreases the contact angle and hence increases hydrophilicity of the membranes. This can be attributed to the hydrophilic head of Brij®S100.
Rejection of amoxicillin
Effect of Brij®S100 and PES concentration
As observed in Figures 7 and 8, increase in PES concentration from 17 wt. % to 21 wt. % along with increase in Brij®S100 concentration from 0 wt. % to 6 wt. %, results in higher rejection of amoxicillin. So that M5 membrane, prepared from 17 wt. % PES and without addition of Brij®S100 in the casting solution showed the lowest rejection of amoxicillin, whereas maximum rejection of amoxicillin was obtained for M4, the membrane which contains maximum concentration of PES and Brij®S100 in its casting solution, i.e. 21 wt. % and 6 wt. %, respectively. These observations are in agreement with the literature [19, 21, 32].
Generally rejection of organic compounds by NF membranes is performed based on the size exclusion (steric hindrance), electrostatic charge repulsion and adsorption on the membrane surface which are related to the membrane and solute properties and solution conditions [19, 32]. Because some pharmaceutical compounds such as amoxicillin are hydrophilic  they are not mostly adsorbed on the membrane surface. Consequently, the removal can occur through steady-state rejection due to either steric effects for uncharged solutes or combined steric and electrostatic effects for charged solutes.
By comparison between the rejection values obtained from Figures 7 and 8 with SEM images, it is found out that the main mechanism governing separation of amoxicillin is steric hindrance, because generally, the membranes with denser structures i.e. ones prepared with higher concentration of PES and Brij®S100 show higher rejection of amoxicillin. Nghiem et al. reported that the retention of pharmaceuticals by a tight NF membrane is dominated by steric exclusion, whereas both electrostatic repulsion and steric exclusion govern the retention of ionizable pharmaceuticals by a loose NF membrane. This is fully in line with our results and interpretation.
Effect of amoxicillin concentration
As shown in Figures 7 and 8, the increase in amoxicillin concentration results in the reduction of amoxicillin rejection. This may be due to concentration polarization. Shahtalebi et al.. investigated the effect of amoxicillin concentration on the performance of commercial NF membranes. They discovered that the increase of amoxicillin concentration results in lower flux. They found out that the concentration polarization occurs in the membrane separation process and has an important influence on the membrane separation performance. When the phenomenon of concentration polarization takes place, a layer is formed at the membrane-liquid interface. The concentration of solute in this layer is higher than that of the solution bulk on the high pressure side. The concentration polarization layer holds up the transport through the membrane, because the increase in osmotic pressure reduces the driving force of mass transfer. Consequently, flux decreases . Also the rejection of amoxicillin reduces.
The modification of PES nanofiltration membrane was carried out by the addition of different concentrations of Brij®S100 hydrophilic surfactant into the casting solution. The membranes performance was studied in terms of PWF and rejection of amoxicillin. The addition of 2 wt. % Brij®S100 to the casting solution increased the formation of macrovoid in the sub-layer of these membranes and consequently resulted in increasing PWF. With further increase in Brij®S100 concentrations from 2 wt. % to 6 wt. % and because of the importance of viscosity effects, the membranes structure, particularly top-layer zone, become denser and consequently PWF decreased. The morphological and experimental studies revealed that the addition of Brij®S100 to the casting solutions resulted in the formation of membranes with higher hydrophilicity and rejection of amoxicillin in comparison with net PES membrane. Lower PES concentrations resulted in the simultaneous increase in PWF and the transmission of amoxicillin through the membranes. Also amoxicillin rejection was decreased by increasing the concentration of amoxicillin in feed.
The authors wish to acknowledge technical assistance provided by Ahmad Moarefian at the Young Researcher Club. Furthermore we thank Ehsan Saljoughi for his support in the development of the analytical methods and Dana Pharmacy Company for Antibiotic supply.
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