1. Introduction
Membrane technology has gained significant attention for application in wastewater treatment due to high quality water requirements and environmental concerns. Membrane technology has proven to be a promising alternative to conventional processes due to its high separation efficiency and inexpensiveness [
1]. However, during operation, membrane fouling restricts full performance of the membrane due to accumulation and deposition of contaminants on the membrane surface and within the pores. Numerous studies reviewed by [
2,
3] deduced that membrane hydrophilicity is directly related to membrane fouling. Therefore, hydrophilic modification of membranes during wastewater treatment is essential in membrane science and technology [
4]. Natural or synthetic polymeric materials have attracted vast attention to be employed as membrane materials. Polymeric materials are attractive due to their non-toxic, and biodegradable properties and they cost effective [
5]. Polyethersulphone (PES) has emerged as one of the most important polymeric material due to its high thermal and chemical resistance and mechanical stability [
6]. The main disadvantage characterizing PES and PES based membranes is related to its hydrophobic character generated by the sulfonyl group linking the two phenylene rings [
7]. Several alterations such as chemical, physical and surface modification and blending have been research to improve the hydrophilic property of PES membranes [
8]. The ultimate aim of membrane modification with hydrophilic agents is to localize the hydrophilic material on the membrane surface and within the pores to positively influence membrane permselectivity and reduce fouling [
4].
This study opted to blend hydrophilic chitosan within the PES membrane matrix to not only modify the membrane surface but also inside the pores. The synthesized membrane was performance was tested during the treated of acid mine drainage. Chitosan is obtained from chitin, which is a higher molecular weight linear polymer and is the second most abundant natural fiber after cellulose. Chitin is found in many different living organisms such as shellfish, crabs, insects, crustacean shells and arthropods [
9]. Chitosan is obtained by partial N-deacetylation of chitin and it contains two free hydroxyl functional groups and one primary amino [
10]. The large number of amino (–NH
2) and hydroxyl (–OH) groups, which can act as contaminate binding sites, makes chitosan attractive, as it will improve both the membrane’s hydrophilic property and permselectivity. Under acidic medium, the amino group gets protonated and attract anions and repel cations through electrostatic repulsion [
11]. Microporous chitosan/polyethylene glycol mixed matrix membrane was tested by Reiad et al. [
12] for removal of iron and manganese from aqueous solution. The study showed improved iron and manganese rejection by the blend membrane and the authors concluded that its reusable after successful desorption of the rejected metal ions. Boricha and Murthy [
13] synthesized and compared PES membrane coated chitosan and acrylonitrile butadiene styrene. It was found that increased chitosan content had a positive effect on the amorphous nature of the membrane. Permeate flux is directly related to amorphous nature of membranes.
Chitosan is obtained from chitin through a deacetylation process, that is treating chitin with a strong alkaline solution. The most important parameter characterizing chitosan sample is the degree of deacetylation [
14]. Chitosan which is a principal derivative of chitin refers to partially or fully deacetylated chitin, which means the degree of acetylation is around or lower than 50%. This also means the degree of deacetylation is around or higher than 50%. Degree of deacetylation influences the physical, biological and chemical properties of the synthesized chitosan. Degree of deacetylation determines the free amino groups exposed due to the removal of the acetyl groups from the molecular chain of chitin, hence the name deacetylation. It is a parameter used to differentiate between chitin and chitosan [
15]. Deacetylation process involves the removal of acetyl group from the molecular chain of chitin, leaving behind a complete amino group (–NH
2) and chitosan versatility depends mainly on this high degree of chemical reactive amino groups. Chitosan with different chemical structures can be synthesized by manipulating reaction time, synthesis temperature and strength of the alkaline solution utilized during the deacetylation process. When chitosan is used to modify polymeric membranes for metal ion removal from solution, it is expected that a high number of available amino groups on the chitosan structure should translate into more effective sorption capacity. However, the influence of chitosan’s degree of deacetylation on the effectiveness of metal ion binding during AMD treatment is non-existent. The study first synthesizes chitosan with different degree of deacetylation by manipulating temperature and the strength of the alkaline solution. Then, chitosan with different degree of deacetylation was blended with PES membrane to evaluate the effect during acid mine drainage (AMD) treatment.
2. Experimental Setup
2.1. Materials and Chemicals
Chitosan used in this study was synthesized from chitin which was obtained by processing seashells collected from Durban South Beach, Rutherford in KwaZulu Natal, South Africa. Chemicals such as Solvent Dimethyl Sulfoxide (DMSO), polyethersulphone (PES) granules (3mm), piperazine (PIP), trimesoyl chloride (TMC), triethylamine (TEA), acetone (C3H6O), sodium hydroxide (NaOH), hexane (C6H14), ethanol (C2H6O), sulphuric (H2SO4) and hydrochloric (HCl) acids and metal sulphates salts were obtained from Sigma-Aldrich (Pty), Johannesburg, South Africa. The chemicals were analytical grade; therefore, they were used without purification. Deionized water was prepared in-house by-passing tap water through Ion exchange polymer resins. The water had pH of 6.89 and conductivity of 0.19 mS/cm.
2.2. Production of Chitosan from Chitin
The seashells were washed and dried in an oven at 120 °C for 1 hour before crushing and milling into fine powder (chitin). The following steps were carried out chronologically:
- (i)
Deproteinization: The crushed and milled seashells (chitin) were treated with an alkaline NaOH (6 w/w%) solution in a 500 mL Erlenmeyer flask at 60 °C. The concoction was stirred on a heating plate fitted with a magnetic stirrer for 2 h. After 2 h of stirring, the chitin was separated from the solution by decanting the alkaline solution. The collected chitin was rinsed with deionized water until the pH was measured neutral.
- (ii)
Demineralization: After deproteinization, the resulting chitin was mixed for 2 h with 6% HCl solution in a 500 mL Erlenmeyer flask at 60 °C. After 2 h of mixing on heating plate equipped with a magnetic stirrer, the demineralized chitin was separated from the acidic solution by decanting the supernatant solution. The demineralized chitin was then washed with deionized water until neutral pH.
- (iii)
Deacetylation: The deproteinized and demineralized chitin was treated with various NaOH concentration (20, 40 and 60 wt%) and temperature (80, 100 and 120 °C) to manipulate the degree of deacetylation (DD) of chitosan. Nine chitosan samples were synthesized and stored inside airtight containers. The solid to liquid ratio for all processes was set at 1:20. Nine chitosan samples were obtained and
Table 1 shows synthesis process conditions and corresponding chitosan sample No.
2.3. Fabrication of PES and Modified PES Membranes
PES granules were dissolved in Dimethyl Sulfoxide at room temperature measured at 26.8 °C on a magnetic stirrer. Once the PES granules dissolved, chitosan was added to the mix and was left for 24 h to obtain a homogenous gel. Before casting, the casting solution was left at ambient conditions to remove any air bubbles. The gel was cast at 250 µm thickness using a casting knife on a glass plate. The membrane was kept inside deionized water to allow for complete desorption of the solvent from the membrane sheet. The membranes were placed in oven at 60 °C to allow evaporation of any trapped water and/or solvent from the membrane for 15 min.
2.4. Characterization of Chitosan and Membranes
Fourier Transform Infrared (FTIR) spectroscopy (Bruker Tensor 27 Spectrometer from LightMachinery Inc, Ottawa, Canada) was used to confirm success in chitosan synthesis from chitin and identifying chemical groups on the membrane surface. FTIR used a DTGS KBr dector and KBr beam splitter, set in a wavelength between 500 to 4500 cm
−1 at an optical velocity of 0.6329. Thermogravimetric analysis was employed to determine the thermal stability of the PES membranes infused with chitosan samples having various degree of deacetylation. Thermal stability of the membranes was performed by heating the samples to 700 °C in air and the temperature. The TGA was performed using a TA SDT Q600 (TA Instruments, DE, USA) The wettability of the membranes was measured water drop using Dataphysics Optical contact angle analyzer (OCA 15 EC GOP from DataPhysics Instruments, Filderstadt, Germany) to quantify the hydrophilic property of the membranes. Ten random measurement were taken at different places on the membrane surface and the average value was utilized. The membranes bulk porosity was estimated gravimetrically. 2-propanol was used as a solvent to investigate the wettability of the membranes. Pieces of membranes were cut and placed inside the solvent for 24 h at room temperature. After 24 h, the pieces of the membranes were removed and placed between two filter papers to remove excess solvent on the surface. The pieces were weighed to obtain wet weight (W
w). Subsequently, the wet membranes were heated in an oven for 2 h at 50 °C. The dried pieces were weighed to obtained dry weight (W
d). The membranes bulk porosity was obtained using Equation (1):
where,
is the average thickness of the membranes measured using a digital Micrometer, A is the membrane effective areas,
is 2-propanol density (0.786 g/cm
3).
Chitosan was characterized using FTIR. Although there are various techniques for chitosan characterization, infrared spectroscopy is the most discussed due to its simplicity [
16]. As such, FTIR was employed to characterize chitosan and to determine its degree of deacetylation of the chitosan samples. Absorption band rations such as A
1655/A
3450, A
1560/A
897, A
1320/A
3450, A
1655/A
2875 and A
1655/A
3450 have been previously used to determine the DD of chitosan samples [
17]. However, the absorption band ration of A
1320/A
1420 have proven to shows superior agreement between the absolute and estimated DD values (Habiba et al., 2017). The DD of the chitosan samples was determined using the following Equations (2) and (3) [
18,
19]:
DD% is the degree of deacetylation. Duplicate chitosan samples were prepared, and average values were taken. DA% is percentage degree of acetylation, and Percentage yield was determined to understand the efficiency of the synthesis process and conditions in terms of chitosan quantity which was obtained. Equation 4 was used to determine the percentage yield:
2.5. Performance Evaluation of Fabricated Membranes Using Synthetic AMD
Synthetic AMD was used as feed solution in these experiments [
18]. Synthetic AMD was used as feed to avoid competition of desired contaminates with undesired species present in real industrial AMD. Suitable amount of metal sulphate salts (
Table 2) were dissolved and agitated at 200 rpm for 30 min in 1000 mL of deionized water to ensure complete dissolution and 0.1 M sulphuric acid was used to adjust the pH to 3.2 using 0.1 M sulphuric acid. To ensure consistent quality of the synthetic AMD, the AMD solution was prepared and used on the same day without storage.
Membrane performance was conducted on a laboratory-scale Dead-end filtration setup consisting of a holding cell (300 mL volume) and effective filtration area of 14.6 cm
2. Nitrogen gas was used to achieve the desired pressure. The membranes were pre-pressed and compacted with deionized water to ensure complete immersion of water. Pure water flux (J, L/m
2·h) of the membranes was determined by permeating deionized water through the membrane at ambient temperature to obtain the original flux of the membranes. Pure water flux (J, L/m
2·h) was determined by direct measurement of membrane permeate volume using the following Equation (5):
where V (Liters) is the volume of permeated water, A (m
2) is the effective membrane area and t (h) is the filtration time. To minimize errors, water flux and rejection experiments were carried out three times and average values were reported.
The filtrates were collected and analyzed for metal content using Atomic Absorption Spectroscopy (Thermo scientific ICE 3000 from ThermoFischer Scientific, Waltham, USA).
Table 3 depicts operating conditions of the AAS. Sulphates were determined using Uv-vis spectrophotomer (PG instruments T60 from Alma Park, Leicestershire, United Kingdom) following the Environmental Protection Agency method (EPA method 3754). Samples for sulphates analysis were conditioned with a conditioning solution prepares as follows: 100 mL 95% ethanol was mixed with 30 mL of HCl and 75 g NaCl in a 500 mL flask. Then glycerol was added to the mixture. For sulphates analysis, 1 mL of the filtrates and 5 mL of the conditioning solution were transferred mixed on a magnetic stirrer. Thereafter, a spoonful of BaCl
2 was added to the mix and stirring continued for an additional 5 min. After stirring, the solution was placed into a cuvette for 4 min at 30 s interval to obtain the turbidity of the solution. A calibration curve was prepared by appropriate dilution of 100 ppm Na
2SO
4 bulk solution.
Rejection was determined with the following Equation (6):
where R is the percentage rejection and
and
(mg/L) are feed and permeate concentrations, respectively.