Adsorptive removal of humic acids and microbes by polyacrylonitrile- chitosan composite membrane

Swapna Rekha panda, Munmun Mukherjee, Sirshendu De*

Department of Chemical Engineering, Indian Institute of Technology Kharagpur, Kharagpur â?? 721302, India

Abstract

The present study describes fabrication of PAN UF membrane using chitosan (CS) as an antibacterial coating material. The efficacy of cast membrane to remove natural organic matter in terms of humic acid (HA) content and microbes was evaluated using pond water as feed. The presence of amino groups in CS provides an appropriate adsorption site to the anionic HA as well as microbes. Chitosan coating lowers the MWCO and makes the cast membrane dense. Chitosan coating also increases the surface hydrophilicty of the composite UF membranes. SEM images confirm the formation of CS layer on the top membrane surface. The cast membrane exhibited high antimicrobial property and good rejection of humic acid. The flux decline was marginal due to the nominal hydraulic resistance offered by the humic acid deposits on the membrane surface. Reversible membrane fouling was identified. Whatever flux declination occurred due to irreversible fouling was, because of the adsorption of HA on the pores of PAN UF membranes. The adsorption of humic acid and microbes on composite membrane was confirmed by SEM and FTIR respectively.

Introduction:

The use of membrane filtration is increasing day by day at an alarming rate to meet the stringent drinking water quality regulations [1]. Microfiltration (MF) membranes are capable to remove the suspended particles, turbidity, micro-organisms like bacteria, algae, and protozoan; while, ultrafiltration (UF) membranes can remove water born viruses and dissolved organic matters. [2]. Major issue in membrane separation is membrane fouling [3]. Recent literature highlights that, humic acids (HA) as a major natural occurring foulant in surface as well as stilled
water [4, 5]. In addition to that, the presence of HA beyond an acceptable limit gives bad odor, color problems to water [6]. Additionally, HA also form complexes with heavy metals, synthetic organic chemicals [7], and chlorine present in water bodies during water treatment process forming carcinogenic disinfectant by-products [8, 9]. These major issues dragged into a motivated extensive research focusing on the use of a novel separation technique i.e. â??membrane filtrationâ?, an easy, economic and lower waste generating method. Still flux declination and membrane fouling are the major inherent problems associated with membrane filtration. Coating, blending and grafting (by hydrophilic polymers) are the different means of membrane modification methods that could be adopted to enhance the antifouling properties of the membranes [10]. Chitosan is a hydrophilic, biocompatible, non toxic, antibacterial and biodegradable biopolymer of our interest [11, 12], used in our present study to coat the PAN hydrophobic membrane. Chitosan a natural adsorbent, having amino and hydroxyl groups in their active sites, used mostly to remove heavy metals, fluorides and dyes from polluted water [13].

Membrane preparation:

In our present study, polyacrylonitrile (PAN) UF membrane at a polymer concentration of 15 wt% in N,N-dimethylformamide (DMF) solvent was cast using phase inversion method. Membranes were cast at room temperature (25±20 C). The laboratory cast membrane was then subjected to coating with a hydrophilic biopolymer chitosan (CS), to enhance the hydrophilicity, filtering efficiency, permeability and antimicrobial activity of the membranes. A layer by layer method was adopted for the preparation of the final PAN-CS coated composite UF membrane using glutalaldehyde as a cross linking agent. The coating layer of chitosan was prepared by dissolving 0.6 gm of CS in 200 ml of 0.4 % (v/v) acetic acid solution with continuous stirring.
Once the coating process was completed, the membrane was immediately dipped into a gelation bath containing 1N NaOH solution for 30 min to ensure the completion of neutralization process. Then the membranes were cleaned thoroughly to remove the excess NaOH adhered to the membrane surface. Finally, the membranes are cut into appropriate size having an effective area of 100 cm2 for undertaking the cross flow experimental runs, and an effective area of 34 cm2 in batch mode operation.

Characterization of membranes

The membranes were characterized in terms of pure water flux (PWF), surface hydrophilicity (contact angle) etc. The contact angle was measured by a Goniometer using sessile drop method. at six different locations of the membrane and the average value was reported. The pure water flux and measurement of membrane permeability was carried out in a batch cell. The pure water flux Jw, was calculated, using the parameters like, Q the volumetric flow rate of permeating water, A is effective membrane area, ?T is the sampling time. A plot of Jw with transmembrane pressure drop (TMP) resulted into a straight line through the origin. From the slope of this curve, membrane permeability was estimated. The molecular weight cutoff (MWCO) of membranes was estimated using solute rejection measurements carried out in a stirred batch cell. Molecular weight corresponding to 90% rejection was estimated as MWCO of the membrane. The membrane morphology was analyzed using a scanning electron microscope (SEM). The pore radius value of membrane was determined by Brunauerâ??Emmettâ?? Teller (BET) image analysis. BET analysis provides average pore diameter along with the pore size distribution plot using nitrogen adsorption desorption technique by degassing the sample at
650 C prior to the measurement.
The results indicated the use of CS affected the pore size distribution and the membrane morphology significantly. The corresponding membrane permeability and contact angle were
2.84 Ã? 10-11 m/Pa.s and 650 respectively. The extent of flux decline with pond water as a feed
was marginal for the cast membrane due to the nominal hydraulic resistance offered by the humic acid deposits on the membrane surface. Reversible membrane fouling was identified. Whatever scant flux declination occurred due to irreversible fouling was, because of the adsorption of HA on the pores of PAN UF membranes. The SEM images confirm the formation of CS layer on the top membrane surface (Fig. 1a). BET analysis of composite membrane provides an average pore diameter of 23 nm.

Methodology

The adsorption study of HA on the membrane was evaluated spectrophotometrically at
254 nm. The amount of HA (solutes qe) adsorbed was calculated from mass balance and plotted against equilibrium concentration to get the desired Langmuir isotherm. The maximum
adsorption capacity of the composite membrane was found to be 53 mg/g (Fig. 1b). FTIR study confirms the adsorption of humic acid on composite membrane.
The antibacterial activity of cast membrane was explored by means of colony forming unit (CFU) count and most probable number (MPN) method using pond water as a feed. In feed the concentration of total microorganisms (CFU/ml) and total coli forms (MPN/100ml) is
9.2Ã?107 CFU/ml and 300/100ml respectively. However, the composite membrane completely
removed total microorganisms (CFU/ml) and total coliforms (MPN/100ml). Adsorption of Escherichia coli on PAN-CS composite membrane was also quantified. The bacterial adsorption using PAN-CS coated composite membranes followed Langmuir isotherm (Fig. 1c). The

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maximum adsorption capacity ( vm ) and adsorption constant (k) were found to be 1.2Ã?10
CFU/g
and 1.1Ã?10-6 ml/CFU, respectively. SEM image after bacterial adsorption of the composite membrane, was reported at 5000X magnification. SEM image confirms adsorption of bacteria (E. coli) on the membrane surface (Fig. 1d).

Reference:

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Ce(ppm)

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1200000 1500000 1800000 2100000 2400000

Equilibrium of E. coli concentration Ce, CFU/ml

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Fig 1: (a) Cross section of PAN-CS membrane (b) Adsorption of humic acid onto PAN-CS membrane (c) Adsorption of E. coli onto PAN-CS membrane (d) SEM showing adsorption of E. coli on PAN-CS membrane.