San copolymer membranes with ion exchangers for Cu(II) removal from synthetic wastewater by electrodialysis

J O U RN A L OF E N V I RO N ME N TA L SC IE N CE S 3 5 (2 0 1 5) 2 7– 3 7

Available online at www.sciencedirect.com

ScienceDirect www.journals.elsevier.com/journal-of-environmental-sciences

San copolymer membranes with ion exchangers for Cu(II) removal from synthetic wastewater by electrodialysis Simona Caprarescu 1 , Mihai Cosmin Corobea 2,⁎, Violeta Purcar 2 , Catalin Ilie Spataru 2 , Raluca Ianchis 2 , Gabriel Vasilievici 2 , Zina Vuluga 2 1. “Politehnica” University of Bucharest, Faculty of Applied Chemistry and Materials Science, Department of Inorganic Chemistry, Physical Chemistry and Electrochemistry, Calea Grivitei, no. 132, 010737, Bucharest, Romania 2. National Research and Development Institute for Chemistry and Petrochemistry — ICECHIM, Polymer Department, Splaiul Independentei, no. 202, 060021, Bucharest, Romania

AR TIC LE I N FO

ABS TR ACT

Article history:

Heterogeneous membranes were obtained by using styrene-acrylonitrile copolymer (SAN)

Received 3 November 2014

blends with low content of ion-exchanger particles (5 wt.%). The membranes obtained by phase

Revised 15 December 2014

inversion were used for the removal of copper ions from synthetic wastewater solutions by

Accepted 13 February 2015

electrodialytic separation. The electrodialysis was conducted in a three cell unit, without

Available online 27 May 2015

electrolyte recirculation. The process, under potentiostatic or galvanostatic control, was followed by pH and conductivity measurements in the solution. The electrodialytic performance,

Keywords:

evaluated in terms of extraction removal degree (rd) of copper ions, was better under

SAN copolymer

potentiostatic control then by the galvanostatic one and the highest (over 70%) was attained at

Membrane

8 V. The membrane efficiency at small ion-exchanger load was explained by the migration of

Ion exchange

resin particles toward the pores surface during the phase inversion. The prepared membranes

Copper ions

were characterized by various techniques i.e. optical microscopy, Fourier transform infrared

Electrodialysis

spectroscopy, scanning electron microscopy, thermogravimetric analysis and differential

Wastewater treatment

thermal analysis and contact angle measurements. © 2015 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.

Introduction Water contamination with cooper ions is still proven as a serious issue for the decontamination processes, in the recent years. Exceeding concentrations above 2 mg/L, from industrial wastewaters, increase drastically the toxicity, sometimes with irreversible damage on the environment (Barakat, 2011; Fu and Wang, 2011; Hashim et al., 2011; Peng et al., 2011; Vinodh et al., 2011). The electrochemical recovery of heavy metal ions (as the copper ones), from sea water and industrial wastewaters may

be recovered for recycling. Deionization processes such as electrodeionization (Arar et al., 2011, 2014a), reverse osmosis (Arar et al., 2014a) and electrodialysis (Rodrigues et al., 2008; Tzanetakis et al., 2003) are some major alternatives. The separation principle, the advantages and disadvantages can be considered as follows. The electrodeionization method is a combination between electrodialysis and ion exchange methods (Arar et al., 2011, 2014b). This method has already been applied for the removal of Cu2 +, Ni2 + and Cr6 + ions from the dilute solutions (Dzyazko, 2006; Dzyazko et al., 2008; Spoor et al., 2002; Mahmoud et al., 2007; Feng et al., 2007). But in these cases the process is

⁎ Corresponding author. E-mail: [email protected] (Corobea Mihai Cosmin).

http://dx.doi.org/10.1016/j.jes.2015.02.005 1001-0742 © 2015 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.

28

J O U RN A L OF E N V I RO N ME N TA L SC IE N CE S 3 5 (2 0 1 5) 27 – 3 7

complicated by the deposition of insoluble compounds on the ion exchange membrane. Therefore the reported removal of Cu2 + ions by electrodeionization starts usually from diluted initial solutions and uses high performance membranes like Nafion® (Mahmoud et al., 2007). The reverse osmosis process uses pressure to force a solution through a semi-permeable membrane that retains the solute on one side and allows the pure solvent to pass to the other side. The removal of heavy metals from synthetic wastewater (like copper and cadmium ions) was investigated by reverse osmosis (Qdais and Moussa, 2004). In this case high removal efficiency is available (98% and 99% for copper and cadmium, respectively), but reverse osmosis in contrast with normal osmosis requires high pressure for the separation. Therefore the major disadvantages of reverse osmosis are the high power consumption needed for the pressure pump and the restoration of the membranes. Due to the high pressure requirements (even 200 psig or more above the osmotic pressure) reverse osmosis is usually not applicable for concentrated solution (Fu and Wang, 2011). Electrodialysis is an electrochemical separation process, in which selective separation of certain ions from water solution is made. The phenomena consist in the migration of ions, across the membranes under the electric field influence. The electrodialytic cell contains at least one or more anionic and cationic membranes, alternating between two electrodes. The properties of the membrane used in the process are very important because these determine the degree of separation in electrodialysis (Nasef and Güven, 2012; Strathmann, 2010). The electrodialysis membrane requirements include good thermal and chemical stability, since the process can be run at elevated temperatures, in solution of very low or high pH values (Caprarescu et al., 2012). Electrodialysis has been extensively used in the electroplating, metal finishing and metallurgy in general, to remove and recycle metal ions such as copper, lead, zinc, silver, among other metals from plating bath rinse solutions or to remove inert electrolyte salts that build-up during plating (Barakat, 2011; Fu and Wang, 2011; Nasef and Güven, 2012; Sadrzadeh et al., 2008). Few reports are describing the use of styrene-acrylonitrile copolymer (SAN), alone as membrane materials. However in some combinations (with cellulose acetate, or butadiene) SAN copolymer have been proven as useful membrane material (in dialysis, ultrafiltration, enzyme-immobilization, molecular imprinting and pervaporation) (Joshi et al., 2001; Fritzsche, 1986; Radha et al., 2009, 2014; Silva et al., 2008; Murthy et al., 2012). From the best of our knowledge SAN was not yet exploited in electrodialysis, despite its optimal profile in terms of chemical resistance, thermal resistance, high strength, dimensional stability, processability and durability (Murthy et al., 2012). These high performances are completed by a certain versatility of polymer material processing, availability and relatively low cost (as supply material). Therefore, such a stable copolymer could bring benefits in electrodialysis separations in terms of costs and stability over a large range of pH (since the ion exchange resins alone are either brittle for the cation resins or too soft in the case of anion resins). The reduction of the ion exchange resin consumption in the separation process would be an advantage since SAN copolymer is several times cheaper.

The objective of this work was to investigate the feasibility of obtaining SAN-ion exchange membranes and their use in the removal of copper ions from a synthetic wastewater using low amounts of ion-exchangers (5 wt.%). The proposed approach emphasizes new cost effective solution in membranes for electrodialysis process for water purification. Electrodialysis experiments were operated both in potentiostatic and galvanostatic modes in order to compare them in view of percent extraction of copper ions. The membranes have been characterized using Optical Microscopy, Fourier transform infrared spectroscopy, scanning electron microscopy, contact angle measurements, thermogravimetric analysis and differential thermal analysis.

1. Experimental 1.1. Materials The chemical reagents used in these experiments were of analytical grade. In all experiments fresh distilled water was used for the preparation of aqueous synthetic electroplating wastewaters. The styrene-acrylonitrile copolymer (SAN) was a commercial product (Luran® 358N, from BASF PLASTICS, Ludwigshafen, Germany) and N, N-dimetylformamide (DMF) (Scharlab S.L., Barcelona, Spain; 99.5%). The ion exchange resins were the strong acidic cation exchanger Puropack PPC100 (PPC100) and the weak basic anion exchanger Purolite A100 (A100) both from Purolite, Romania. Their initial ionic forms were Na+ and Cl− respectively (Fig. 1). Copper sulfate pentahydrate (CuSO4·5H2O) (Chimopar, Romania) and sulfuric acid 98% (Merck) were used as received. A stock solution of Cu(II), containing 1 g Cu(II)/L, was prepared by dissolving both copper sulfate pentahydrate and sulfuric acid (molar ratio 1:1) in distilled water.

1.2. Membrane preparation In order to obtain the heterogeneous membrane a typical procedure was followed. A mixed matrix was prepared as follows. 8 wt.% SAN was purred in DMF under mechanical stirring (400 r/min at 40°C) until a clear solution was obtained. Then 5 wt.% ion exchanger resin, previously processed by ball milling up to a fine powder grain size of 2.5 μm was added to polymer. In order to obtain a uniform dispersion, the mix was stirred for several hours, then sonicated for a few minutes and left over night, in order to achieve the air removing and swelling of the polymer solution into the pores of the resin particles. The day after, the dispersion was again stirred for 1 hr and then used for casting the membranes. The dispersion was thus spread with a film applicator (0.45 mm) on a glass panel. Immediately after coating, this panel was immersed in a coagulation bath filled with distilled water at 22°C. Within a few minutes, the membrane detaches itself from the glass support, then it immersed again in a fresh water bath (the procedure of fresh water immersion was repeated 5 times) and finally left after night in water, in order to remove completely the DMF solvent. The membrane was kept

J O U RN A L OF E N V I RO N ME N TA L SC IE N CE S 3 5 (2 0 1 5) 2 7– 3 7

a

Cation exchange site

n

29

case of a potentiostatic control). A power supply (Protek, Germany) ensuring up to 30 V and up to 5 A was used. The solution conductivity was measured using a PIERRON (France) conductometer and the pH measurements were performed with a Hanna Instruments HI 8915 pH-meter using a combined glass electrode. The copper ions concentration was determined by titration as previously reported (Caprarescu et al., 2012).

1.4. Methods for characterization of membranes

b

n Anion exchange site

Fig. 1 – Idealized structure of acidic cation exchanger, Puropack PPC100 (a) basic anion exchanger, Purolite A100 (b) used for membrane obtaining.

permanently immersed at constant temperature in water until the measurements have begun. The thickness of wet membranes was between 0.012 and 0.023 cm. Before pouring the grain particles of ion exchange resins into the membranes, they were brought to the suitable ionic form, namely H+ for the cation exchanger and OH− for the anion one, by repeated immersion into concentrated aqueous solutions of 23% HCl respectively 25% NH3.

Fourier transform infrared spectroscopy (FT-IR) was done using a Bruker-Tensor 37 instrument. Samples were analyzed in attenuated total reflection (ATR) module with a Golden Gate diamond unit. The surface morphology of the SAN anion-exchange membranes was followed by an optical microscope (MC 5, IOR, Bucharest, Romania) with color high resolution video camera and scanning electron microscopy (SEM), using a S-2600N, HITACHI, Japan, electron microscope. All the membrane samples were dipped in liquid nitrogen and gold coated with a thin layer, before SEM analysis. Water contact angle (CA) was measured at room temperature using a Contact Angle Tensiometer (CAM 200, KSV Instruments, Helsinki, Finland). Static contact angle measurements were done on 2 cm × 2 cm specimens (droplet 10 μL, ± 2° CA error). Thermogravimetrical and differential thermal analysis of the membranes was performed by using a Du Pont TGA 2000 instrument (heating rate of 10°C/min in air).

2. Results and discussions 1.3. Three-compartment electrodialysis cell 2.1. Electrodialytic current and voltage time profiles Experimental tests were performed in a laboratory device for electrodialysis of own construction, composed of three detachable cylindrical compartments reinforced by gaskets, made of clear acrylic, separated by the two types of SAN/ion exchange heterogeneous membranes (anionic respectively cationic) with active areas of 23.316 cm2 and two working plan parallel electrodes (Fig. 2). The electrodes, both of pure copper (99.9%) and presenting the same effective area of 23.32 cm2 were disposed at a distance of 3.6 cm as extremities of the electrodialysis cell. Each compartment has an external diameter of 9.75 cm, inside diameter of 5.45 cm and thickness without silicone gaskets of 0.918 cm. The thickness of each compartment with silicone gaskets was 1.279 cm. The distance between the inner faces of the membranes was 1.2 cm. In the electrodialysis cell, SAN/anion-exchange membrane separates the central compartment from the anode, and in a similar manner the SAN/cation-exchange membrane was placed between the central compartment and the cathode. The liquid volume was 29.85 cm3 in each of the three compartments. All experiments were carried out during 90 min, at room temperature (22 ± 1°C) and without recirculation. The three compartments were filled with the same solution. Electrodialysis was carried out either applying an anode-cathode current of 0.05 A and of 0.1 A (current density 2.14 mA/cm2 respectively 4.29 mA/cm2) under galvanostatic control, or an anode-cathode voltage of 6 V and of 8 V, (in the

In acid medium, the following reactions occur at the electrodes during the electrodialysis of water containing copper ions: Anode : 2H2 O→O2 ðgÞ þ 4Hþ þ 4e−

ð1Þ

Fig. 2 – Three compartment electrodialysis laboratory unit representation.

30

J O U RN A L OF E N V I RO N ME N TA L SC IE N CE S 3 5 (2 0 1 5) 27 – 3 7

Cathode : 2Hþ þ 2e− →H2 ðgÞ

ð2Þ

Cu2þ þ 2e− →CuðsÞ

ð3Þ

Cation exchange membrane : 2 Hþ CER þ Cu2þ aq →2 Hþ aq þ Cu2þ aq CER

ð4Þ

Anion exchange membrane : 2 OH− AER þ SO4 2− aq →2 OH− aq þ SO4 2− aq CER

ð5Þ

Energy consumption also increased with applied voltage as expected. These results are in good agreement with the ones reported in literature (Güvenç and Karabacakolu, 2005; Kabay et al., 2002). Although the investigated membranes were not entirely made of ion exchangers, they behave in the electrodialysis likewise high-performance membranes. The behavior was similar like the ones reported in literature for example in the removal of Ag+ ions with Nafion 424 and Ionac MA-on-exchange membranes. Later, the current decreased rapidly with time, while the Ag+ ions are transferred from the dilute side to the concentrate side. On the other hand, during the operation, the initial concentration of Ag+ ions in the treated water decreased with time (Güvenç and Karabacakolu, 2005).

where the subscripts aq, CER and AER designate respectively the aqueous solution, the cation exchange resin and the anion exchange resin. In order to prevent the fall of conductance by accumulation of the gaseous products H2 and O2, one small hole on the top of each compartment was made to release gases from the electrodialysis cell.

2.1.2. Constant current operation In galvanostatic mode, a constant current was imposed between electrodes. The current flowing through the cell is kept constant all over the batch operation; the potential drop across the cell, E = I · R varies with time. In these studied conditions, when constant current was used (Fig. 3b), the voltage value gradually increases. The voltage increase occurs, because during the separation process, the ions migrate toward the electrodes and the conductivity of the water in the central compartment decreases. Therefore as the concentration of ions decreases, the resistance gradually increases as well. At higher current intensity at 0.1 A (i.e. under a current density of 43 A/m2), the cell voltage, was near two times greater than at 0.05 A (which corresponds to a current density ≅ 21 A/m2). Thus the electric resistance of cell remains fast independent of the current intensity, but shows an increase of 2.5 to 3 times during the process. Again heterogeneous SAN-ion exchange membranes showed similar results in the electrodialysis process in comparison with high performance membranes found in the existing literature (Güvenç and Karabacakolu, 2005; Chen et al., 2009; Dalla Costa et al., 2002; Tanaka, 2002; Cifuentes et al., 2009; Parulekar, 1998; Melnyk and Goncharuk, 2009). The experimental results confirmed that the potentiostatic control is safer then the galvanostatic control, by avoiding the

2.1.1. Constant voltage operation Cell voltage E (V) as the driving force of the electrodialysis determines the migration rate of Cu2+ ions across the membranes. In potentiostatic operation control, the current I (A) flowing through the electrodialysis cell vary with the time t (hr). The potential fall E between the cell electrodes is proportional to the cell resistance R, according to the relationship I = E/R. Fig. 3a shows the progress of the electrodialysis with the proposed membranes for two values of applied voltage. To note that in both cases (6 V and 8 V) the current gradually fall with time, since the concentration of Cu2+ ions in all three aqueous solutions decreased leading to an increase in the cell resistance. The explanation for the phenomena could be given from the ion migration rate (responsible for the high conductivity of the initial electrolyte solution). In contrast with the later experiment (Fig. 3b) the migration seems faster at constant voltage, then in constant current mode. Further the separation degree should be of higher aspect, which will be clarified later in the removal degree section. As reported in the literature, the amount of ions transported through the membrane is directly proportional to the electrical current or current density. If the electrical potential difference was increased, the current density will increase.

0.45

10

a

0.40

8

0.35 Cell voltage (V)

Current (A)

0.30 0.25 0.20 0.15 0.10

0

10

20

7 6 5 4 3 2

6V 8V

0.05 0.00

b

9

0.05 A 0.10 A

1 30

40 50 60 Time (min)

70

80

90

100

0

0

10

20

30

40

50 60 Time (min)

70

80

90

100

Fig. 3 – Time decrease of current intensity at constant voltage (a) and time increase of cell voltage at constant current (b).

31

14.26 7.49 2.44 2.16 14.26 6.06 4.68 2.04 14.26 7.65 2.78 1.22 14.26 7.34 2.84 3.94 1.60 2.60 3.15 2.57 1.60 3.33 2.32 2.47

0.1 A 0.05 A 8V 6V 0.1 A

pH

Galvanostatic control

Table 1 presents the pH values for the different solutions for the potentiostatic and galvanostatic control, measured after 1.5 hr, at room temperature. The solutions from Table 1 are acidic — all present low pH values in good agreement with the data reported in literature, hence H+ compete with Cu2 + at the cathode surface resulting in hydrogen embitterment in the copper deposits (Caprarescu et al., 2011; Kabay et al., 2002; Chen et al., 2009). The pH of the solution is not considered to affect the removal rate and there is no need to adjust the pH and hence to add additional reagents before and after treatment (Dalla Costa et al., 2002; Tanaka, 2002).

0.05 A

2.1.3. Final pH

Potentiostatic control

Conductivity (mS/cm)

concentration polarization phenomena (Güvenç and Karabacakolu, 2005; Chen et al., 2009; Dalla Costa et al., 2002; Tanaka, 2002) and development of high voltages at low final concentration in the final electrolyte. Electrodialysis process operated both in potentiostatic and galvanostatic modes for the treatment of metal finishing wastewater was described in literature using Nafion 450 and Selemion AMP. Results indicated that potentiostatic control could be safer to operate in the sense that it does not permit the development of high voltages across the stack when the ion concentration in the diluate is excessively low, although each species of metallic ions exhibited a different rate of extraction from the treated solution. This operation mode avoids the occurrence of concentration polarization and the problems related to it (Dalla Costa et al., 2002).

Galvanostatic control

J O U RN A L OF E N V I RO N ME N TA L SC IE N CE S 3 5 (2 0 1 5) 2 7– 3 7

rd ¼

C in;cat −C fi;cat  100% C in;cat

ð6Þ

where, Cin,cat (mol/dm3) and Cfi,cat (mol/dm3) are the initial, respectively the final concentration of the copper ion in the cathodic compartment.

1.60 3.31 3.24 5.15 1.60 3.25 3.03 2.17

8V 6V

Potentiostatic control Samples

Untreated wastewater Treated water from anodic compartment Treated water from central compartment Treated water from cathodic compartment

The laboratory electrodialysis cell performance was evaluated in terms of extraction removal degree (rd) of copper in the cathodic compartment, defined by:

0 1.5

2.1.5. Final removal degree and energy consumption

Time (hr)

The electrical conductance of the system is determined by the ohmic resistances of the ion-exchange membranes and by the concentrations of different ions in solutions. The final changes in solution conductivity for anodic, central and cathodic compartments are shown in Table 1. In both cases (potentiostatic and galvanostatic control) the conductivity decreased in all three compartments during the electrodialysis. The decrease was moderate in the anodic compartment and more important in the central cell compartment (indicating the removal of Cu2+ and SO2− 4 ions and in the cathodic compartment because of the copper electrodeposition reaction on the cathode Caprarescu et al., 2012; Dalla Costa et al., 2002). In three of the four experiments, the conductivity fall was the most pronounced for the cathodic compartment and in a single one (namely, at potentiostatic control under lower voltage) this decline was the most important for the central compartment.

Table 1 – Values of solution pH for the potentiostatic and galvanostatic control.

2.1.4. Final solution conductivity

32

J O U RN A L OF E N V I RO N ME N TA L SC IE N CE S 3 5 (2 0 1 5) 27 – 3 7

The specific energy consumption (E.C., kWh/m3) necessary in an electrodialysis process is given by Eq. (7): E:C: ¼

1  1000  V cat

Z

t¼t fin t¼0

Eðt Þ  Iðt Þ  dt

ð7Þ

where, E (V) is the applied potential, tfin (hr) is the final time and Vcat (m3) is the volume of the cathodic compartment.

2.2. Optimal conditions for electrodialysis The cell voltage applied in the electrodialysis is considered to be one of the most important operational parameters, which can affect radically the copper recovery and energy consumption. As can be seen from Table 2, rd values for copper ions increase from 64.69% to 70.31% with the values (6 and 8 V) of the current or voltage used in the electrochemical setup (Rodrigues et al., 2008; Caprarescu et al., 2012; Dzyazko, 2006; Spoor et al., 2002; Dzyazko et al., 2008). It was demonstrated that a high copper ion concentration (1 g/L Cu2 + ) can be removed in 90 min of the electrodialysis operation performed under two constant applied voltages at the room temperature. For the same operating time, the purification efficiency rd is a bit lower under galvanostatic control 55.63% and 65.00%; but the energy consumption is much better in the case of galvanostatic control. During the process study, no precipitations in membranes were observed; the solutions remain clear after measurements. The membranes were not damaged during the electrodialysis process and could be reused. The results obtained with heterogeneous SAN-ion exchange membranes were comparable with the existing literature on electrodialysis with high performance membranes. For example using Ionac MA3475 as anion, and respectively Nafion423 as cation exchange membranes, the reported removal degree was 52.6% by an energy consumption of 5 kWh/m3 at 20 V, in a period of 75 min, for an initial concentration of 100 mg Cu2+/L water. Our result for 0.05 A was very similar: a bit better on the removal (55.6%), for a bit higher energy consumption of 6.5 kWh/m3 (Öğütveren et al., 1997). In our setup, separations over 70% can be achieved in 90 min in comparison with other commercial products were 120 min are needed (Chang et al., 2010). Moreover the separation in our setup starts from a highly concentrated initial solution (twofold more than the one reported for similar products) (Chang et al., 2010). This shows that the SAN-ion exchange membrane used in this study does not

Table 2 – Final cell performance and energy consumption. E.C. (kWh/m3)

rd Potentiostatic control Galvanostatic control

6V 64.69% 0.05 A 55.63%

8V 70.31% 0.1 A 65.00%

6V 40 0.05 A 6.5

8V 61 0.1 A 25.3

Observations: cathodic compartment, t = 1.5 hr, Vcat = 2985 × 10−5 m3.

hinder the transport of counter ions despite the affordable operating costs.

2.3. Membrane characteristics 2.3.1. Optical microscopy The first inspection by optical microscopy revealed for SAN-ion exchangers membranes a highly texturized surface, in contrast with the apparent flat and homogeneous shell at macroscale (Fig. 4). Micropores were formed during membrane coagulation with quite near distribution of pore sizes for each membrane. However in the case of SAN-Purolite A100 the tendency of forming larger pores was higher than for the SAN-Puropack PPC 100 membrane. This aspect suggests a more polar character of the Purolite A100 in contrast with Puropack PPC 100, which favors the non-solvent (water) adsorption and increases the pores formation rate. Using the ion exchanger in the membrane formation could be an advantage for the process, which for highly nonpolar polymers is generally assisted by a surfactant (Voicu et al., 2014). Next to the pore size the distribution as well seems to be favorable to SAN-Puropack PPC 100.

2.3.2. FT-IR spectroscopy The chemical structure of the obtained membranes (initial membranes and after use membranes, at different operated conditions) was investigated by FT-IR spectroscopy (Fig. 5). The samples were examined before and after the exposure to electrodialysis process (galvanostatic and potentiostatic mode). FT-IR spectra showed the aromatic C–H stretching vibration at 3028, 3061 cm−1 coming from the styrene units, the phenyl ring stretching vibration C_C at 1603 cm−1, for the out-of-plane hydrogen bending vibration C–H, at the 760 cm−1 and 701 cm−1 (Xia et al., 2010; Cai et al., 2007). The C–H bending modes (two signals) are indicating the monosubstituted ring specific in polystyrene units in contrast with para substituted rings from Purolite A100 (Fig. 5a). Two peaks were observed at 2925 and 2856 cm− 1 corresponding to the C–H symmetric and asymmetric stretching vibration from CH2 aliphatic chain from both SAN copolymer and Purolite polymer chain. The vibration of C`N groups appeared at 2237 cm−1 indicating C`N stretching vibration belonging to acrylonitrile units from the SAN copolymer. The C–H bending from polystyrene units at 1451 cm−1, respectively 1495 cm−1 from acrylonitrile units. The bands absorption from 2362 cm−1 and 2320 cm−1 belongs very probable to CO2 absorption band (Choi et al., 2003; Zhang et al., 2009; Yu et al., 2003). CO2 adsorption occurs during sample manipulation in air, when used in experiments. Carbonation of the polymer materials is known especially when it comes for a large surface area (like a membrane) (Kazarian et al., 1996). The adsorption of CO2 was more pronounced when Purolite particles were used, since they have an even higher specific surface area (in comparison with neat SAN membrane). Supplementarily, we should consider that Purolite particles posses a higher polarity in comparison with the copolymer alone. The ion exchanger presence in the heterogeneous membrane was evidenced with bands similar to SAN copolymer

33

J O U RN A L OF E N V I RO N ME N TA L SC IE N CE S 3 5 (2 0 1 5) 2 7– 3 7

SAN-Purolite A100

SAN-Puropack PPC100

Fig. 4 – Optical microscope images for the surface morphology of SAN-Purolite A100 (a) and SAN-Puropack PPC100 (b). SAN: styrene-acrylonitrile copolymer.

0.14 0.12

0.12 ATR units

ATR units

0.10 0.08 0.06

0.02

0.16 0.14

900

0

400

SAN A100 blank SAN A100 6 V SAN A100 8 V

0.12 ATR units

0.08 0.06

2900 2400 1900 1400 Wavenumber (cm-1)

900

400

900

400

PPC100 powder SAN blank SAN PPC100 blank

d

0.08 0.06 0.04

0.04

0.02

0.02 0 3900

0.16

3400

0.10

0.10

0.18

3900

0.14

c

0.12

3400

2900

2400 1900 1400 Wavenumber (cm-1)

900

0 3900

400

0.16

e

SAN PPC100 blank SAN PPC100 0.05 A SAN PPC100 0.1 A

0.14

0.14

ATR units

0.1 0.08 0.06

0.06

0.02

0.02 2900

2400 1900 1400 Wavenumber (cm-1)

SAN PPC100 blank SAN PPC100 6 V SAN PPC100 8 V

f

0.08

0.04

3400

900

400

2900 2400 1900 1400 Wavenumber (cm-1)

0.10

0.04 0 3900

3400

0.12

0.12 ATR units

0.06

0.02 2900 2400 1900 1400 Wavenumber (cm-1)

SAN A100 blank SAN A100 0.05 A SAN A100 0.01 A

0.08

0.04

3400

b

0.10

0.04

0 3900

ATR units

0.14

A100 powder SAN blank SAN A100 blank

a

0 3900

3400

2900 2400 1900 1400 Wavenumber (cm-1)

900

400

Fig. 5 – FT-IR spectra of anion exchange (a) and cation exchange (b) membrane before electrodialysis and of its constituents; FT-IR spectra of SAN-anion (c) and SAN-cation (d) exchange membranes before and after electrodialysis at given current; FT-IR spectra of SAN-anion (e) and SAN-cation (f) exchange membranes before and after electrodialysis at given voltage. FT-IR: Fourier transform infrared spectroscopy; SAN: styrene-acrylonitrile copolymer.

34

J O U RN A L OF E N V I RO N ME N TA L SC IE N CE S 3 5 (2 0 1 5) 27 – 3 7

because of the same polystyrene segments (Fig. 5a, c, and e). Supplementarily, some specific bands appeared like C–N specific absorptions from the tertiary amine segments (found at 1256 cm− 1) in stretching mode. Other specific bands for the ion exchanger are indicated by the 854, 813, 760 and 701 cm−1 from C–H (phenyl ring) in out of plane bending mode. These absorptions are confirming the coexistence of mono and bisubstituted (ortho, metha, para) ring from the segments obtained with divinyl benzene for cross-linking and the ion exchanger groups from the active sites (Fig. 1) (Cai et al., 2007) Puropack PPC100 spectra (Fig. 5b, d, and f) shows the specific bands for polystyrene sulfonate cross-linked with divinyl benzene. The specific bands are showing small deviations from the ones observed in Purolite A100 since the functional groups are different (from tertiary amine, sulfonic acid in this case). The sulfonic group specific peaks can be found at 1038 cm−1, S_O in symmetric stretching mode and 1176 cm−1, S_O in asymmetric stretching mode. The para substituted position appears also in the 832 cm−1 (C–H from the aromatic ring, out of plane bending mode) (Xia et al., 2010; Cai et al., 2007). The FT-IR spectra in Fig. 5c, and e show aromatic C–H stretching vibrations at 2928 cm−1 and aliphatic C–H stretching at 2858 cm−1 (cross-linked polystyrene). In addition, the peaks at 2237 cm−1 and 1451 cm−1 indicate C`N stretching and C–H bending, respectively (Cai et al., 2007). The bands in all spectra from 2362 cm−1 and 2320 cm−1 are very probable from carbon dioxide absorption band. The peak observed at 1740 cm−1 can be attributed to C_O stretching vibration. A distinct band appears at 1236 cm−1 for SAN-Purolite A100 and 1237 cm−1 for SAN-Puropack PPC100 membranes. We believe that this band could indicate the separated ion complex perturbing the tertiary amine C–N bonds (initial at 1256 cm−1 in stretching mode) (Purolite A100 powder), respectively the S_O asymmetric stretching mode (initial 1176 cm−1) (Puropack PPC100). The FT-IR profiles of the membranes obtained from SAN with Purolite A100 or Puropack PPC100 confirmed the absence of strong chemical or physical interactions between membrane partners. This aspect was considered essential for the ion exchanger nature and sequentially for his ability to participate to the exchange process. Moreover in terms of accessibility a favorable placement of the ion exchanger particles toward membrane pores surface is very probable,

judging after FTIR profiles (measured on surface on ATR module) and by the ion exchanger polarity which shows affinity for the water phase. This behavior could be involved in the membrane casting section, in the particular case of the coagulation step occurring in the water phase.

2.3.3. TGA DTG The thermogravimetric analysis and differential thermal analysis (TGA DTG)–thermogravimetrical analysis and differential thermal analysis-profiles (Fig. 6) displayed the membrane thermal stability over a wide range of operation temperature (this is essential not only in electrodialysis but also for other membrane applications) (Xia et al., 2010; Voicu et al., 2014). The importance of the thermostability should be considered in electrochemical process since the local heat up in the membrane exceeds the temperatures from the water phase. Neat SAN copolymer membrane was found stable up to 263°C, meanwhile the SAN-ion exchanger membranes thermal stability increased to over 290°C. The ion-exchanger resin not only provided functionality from the separation point of view, but can also increase the final membrane thermal stability even at such low amounts (5 wt.%). Another advantage of the SAN-ion exchange heterogeneous membranes was found in the operational process; since the thermal stability of the operated membranes (Fig. 6) showed an increase in thermal stability (the degradation starts over 300°C). The cross-linked structure of the porous ion-exchanger could provide sites of adsorption of the SAN copolymer phase before degradation and can act as a certain barrier effect for heat diffusion and sorption of radicals during decomposition.

2.3.4. SEM The SEM pictures for the SAN-ion exchange membranes (Fig. 7) confirmed the operational performance since a large availability of the ion exchangers was found. The ion exchange particles were found in large amounts toward pore surfaces. This aspect can be explained by the migration of the polar resin particles toward water phase in the casting process. Macro and microspores were observed according to SEM images (Fig. 7), suggesting a possible involvement of the membrane morphology in the current vs. time deviation from an ideal curve. The pores offered a large surface area on

a

b

2.5

100

SAN A100 blank SAN PPC 100 8Va

SAN A100 Deriv (% / ºC)

Weight (%)

2

SAN PPC 100

80

SAN PPC100 8V 60

SAN blank SAN PPC 100 blank

SAN blank

SAN A100 8V

40

SAN A100 8V

1.5

1

0.5

20

0

0 0

100

200

300

400

Temperature (ºC)

500

600

700

0

100

200

300

400

500

600

700

Temperature (ºC)

Fig. 6 – Thermal stability by TGA DTG profiles (a) and by DTG profiles (b) of the membranes before and after operating. TGA: thermogravimetric analysis; DTG: differential thermal analysis.

35

J O U RN A L OF E N V I RO N ME N TA L SC IE N CE S 3 5 (2 0 1 5) 2 7– 3 7

which the ion exchanger presence was observed. This aspect explains also the good ion mobility and extraction values obtained on such a low amount of ion exchanger in the membrane. The ion-exchange particles are acting in a concerted manner with the solvent phase in relation with the polymer phase during membrane obtaining. The ion-exchange resin has a good affinity for the SAN matrix (based on polystyrene segments), but strongly favors water adsorption (as the DMF) because it needs the hydration on the strong polar functional groups. Moreover the ion-exchange particles are insoluble in the water phase despite the great affinity and the placement at the interface between water and SAN phase was again favored. Considering also the ion exchanger particles superficial embedding in the fast coagulating SAN phase the surface displacement should be again a favorable event. Ion exchange particle polar character is favoring besides pore formation in a concerted manner the general morphological profile and particular the active surface. SEM analyses confirmed the primary FT-IR data above, which highlighted the clear absorptions of the resin functional groups on material surface. These favorable events were the driven vectors for obtaining the above emphasized separation dynamics involved in the electrodialysis process at such low ion exchanger loads (5 wt.% to SAN phase) (Rodrigues et al., 2008; Arthanareeswaran and Starov, 2011).

2.3.5. Contact angle measurements Contact angle (CA) is a convenient way to assess the hydrophilic/hydrophobic properties and wettability of the membrane surface. The CA values (Fig. 8) showed that SAN– Purolite heterogeneous membranes are quite hydrophilic. CA measurements sustain once again the surface placement of the ion exchanger resin — with polar groups. Ion exchange particles are increasing the water contact angle in the analyzed membranes (Fig. 8). The resin particles induced a texturizing effect on the membrane surface as could be seen also in the morphology details above. The increase in contact angle was more pronounced after operating the membranes suggesting a certain decrease of the membrane reliability in time, involving

40 ×

120 ×

180 ×

2500 ×

a

the wettability if longer operation times or more cycles are intended. In the same time this drawback could be interesting in other applications which should involve lower polarity fluids (or organic phases). The highest obtained value (92° CA) was exactly on the edge of the hydrophilic and hydrophobic surfaces, which still offers a certain versatility of the membrane applications. This kind of versatility, next to the potential antibacterial effect of the copper ions trapped on the membrane surface could be the basis of reusing such membranes in biowaste separation (Rusen et al., 2014).

3. Conclusion The removal of copper ions from synthetic electroplating wastewater can be carried out effectively by applying electrodialysis with heterogeneous membranes, composed of styrene-acrylonitrile copolymer and small loads (5 wt.%) of ion-exchange resin particles: The process parameters evidenced a good current and ion mobility through the membrane, in the electrodialysis, since the transport through the pores allowed the gradual separation. If the duration of the electrodialysis is 90 min, one can operate either in potentiostatic control at 8 V, leading to a high extraction degree (over 70%); or preferably, under constant current conditions, as for example at a current density of 20 A/m2, when the extraction degree is a bit lower (52%) but the energetic consumption performs 10 times better — only 6 kWh/m3. The performances obtained at small loads of ion exchangers are close to those of the homogeneous ion exchange membranes (Chang et al., 2010). SAN copolymer was proven as an efficient matrix with high thermal stability (over 260°C). SAN heterogeneous membranes thermal stability showed even an increase after operation (degradation above 290°C). The operability of the membrane under these conditions was explained from the structural and morphological characteristic occurred as a consequence of ion exchanger placements on the pores surfaces. The small amounts of cooper trapped in the membrane ion exchange sites, the increase in contact angles after operating, next to a

40 ×

800 ×

1800 ×

2500 ×

b

Fig. 7 – SEM images of the SAN-anion exchange membrane (Puropack PPC100) (a) and SAN-cation exchange membrane (Purolite A100) (b) before experiments. SEM: scanning electron microscopy; SAN: styrene-acrylonitrile copolymer.

36

J O U RN A L OF E N V I RO N ME N TA L SC IE N CE S 3 5 (2 0 1 5) 27 – 3 7

Blank

6V

8V

0.05 A

0.1 A

Contact angle (°C)

120 100 80 60 40 20 0 SAN A100 SAN PPC100 Membrane type Fig. 8 – Surface modifications of the SAN-ion exchange membranes followed by contact angle. SAN: styrene-acrylonitrile copolymer.

the thermal stability profile could open new directions in reusing such membranes in biowaste applications (for example in antibacterial separations since cooper ions activity in such a direction is known).

Acknowledgment Prof. Florin Danes is kindly acknowledged for his valuable support in the manuscript editing. Special thanks to: Mrs. Mariana Andrei and Mr. Corneliu Andrei from “Politehnica” University of Bucharest for their logistic support.

REFERENCES

Arar, Ö., Yüksel, Ü., Kabay, N., Yüksel, M., 2011. Removal of Cu2+ ions by a micro-flow electrodeionization (EDI) system. Desalination 277 (1-3), 296–300. Arar, Ö., Yüksel, Ü., Kabay, N., Yüksel, M., 2014a. Various applications of electrodeionization (EDI) method for water treatment—a short review. Desalination 342, 16–22. Arar, Ö., Yüksel, Ü., Kabay, N., Yüksel, M., 2014b. Demineralization of geothermal water reverse osmosis (RO) permeate by electrodeionization (EDI) with mixed bed configuration. Desalination 342, 23–28. Arthanareeswaran, G., Starov, M.V., 2011. Effect of solvents on performance of polyethersulfone ultrafiltration membranes: investigation of metal ion separations. Desalination 267 (1), 57–63. Barakat, M.A., 2011. New trends in removing heavy metals from industrial wastewater. Arab. J. Chem. 4 (4), 361–377. Cai, Y.B., Hu, Y., Xiao, J.F., Song, L., Fan, W.C., Deng, H.X., et al., 2007. Morphology, thermal and mechanical properties of poly (styrene-acrylonitrile) (SAN)/clay nanocomposites from organic-modified montmorillonite. Polym.-Plast. Technol. Eng. 46 (5), 541–548. Caprarescu, S., Purcar, V., Vaireanu, D.I., 2012. Separation of copper ions from synthetically prepared electroplating wastewater at different operating conditions using electrodialysis. Sep. Sci. Technol. 47 (16), 2273–2280. Chang, J.H., Elli, S.A.V., Tung, C.H., Huang, W., 2010. Copper cation transport and scaling of ionic exchange membranes using electrodialysis under electroconvection conditions. J. Membr. Sci. 361 (1-2), 56–62.

Chen, S.S., Li, C.W., Hsu, H.D., Lee, P.C., Chang, Y.M., Yang, C.H., 2009. Concentration and purification of chromate from electroplating wastewater by two-stage electrodialysis processes. J. Hazard. Mater. 161 (2-3), 1075–1080. Choi, Y.S., Xu, M., Chung, I.J., 2003. Synthesis of exfoliated poly(styrene-co-acrylonitrile) copolymer/silicate nanocomposite by emulsion polymerization; monomer composition effect on morphology. Polymer 44 (22), 6989–6994. Cifuentes, L., García, I., Arriagada, P., Casas, J.M., 2009. The use of electrodialysis for metal separation and water recovery from CuSO4–H2SO4–Fe solutions. Sep. Purif. Technol. 68 (1), 105–108. Dalla Costa, R.F., Klein, W.C., Bernardes, A.M., Ferreira, J.Z., 2002. Evaluation of the electrodialysis process for the treatment of metal finishing wastewater. J. Braz. Chem. Soc. 13 (4), 540–547. Dzyazko, Y.S., 2006. Purification of a diluted solution containing nickel using electrodeionization. Desalination 198 (1-3), 47–55. Dzyazko, Y.S., Rozhdestvenskaya, L.M., Vasilyuk, S.L., Belyakov, V.N., Kabay, N., Yuksel, M., et al., 2008. Electro-deionization of Cr (VI)containing solution. Part I: chromium transport through granulated inorganic ion-exchanger. Chem. Eng. Commun. 196 (1-2), 3–21. Feng, X., Wu, Z.C., Chen, X.F., 2007. Removal of metal ions from electroplating effluent by EDI process and recycle of purified water. Sep. Purif. Technol. 57 (2), 257–263. Fritzsche, A.K., 1986. Effect of ionizing radiation on styrene/ acrylonitrile copolymer hollow fiber membranes. J. Appl. Polym. Sci. 32 (2), 3541–3550. Fu, F.L., Wang, Q., 2011. Removal of heavy metal ions from wastewaters: a review. J. Environ. Manag. 92 (3), 407–418. Güvenç, A., Karabacakolu, B., 2005. Use of electrodialysis to remove silver ions from model solutions and wastewater. Desalination 172 (1), 7–17. Hashim, M.A., Mukhopadhyay, S., Sahu, J.N., Sengupta, B., 2011. Remediation technologies for heavy metal contaminated groundwater. J. Environ. Manag. 92 (10), 2355–2388. Joshi, M., Mukherjee, A.K., Thakur, B.D., 2001. Development of a new styrene copolymer membrane for recycling of polyester fibre dyeing effluent. J. Membr. Sci. 189 (1), 23–40. Kabay, N., Demircioglu, M., Ersöz, E., Kurucaovali, I., 2002. Removal of calcium and magnesium hardness by electrodialysis. Desalination 149 (1-3), 343–349. Kazarian, S.G., Vincent, M.F., Bright, F.V., Liotta, C.L., Eckert, C.A., 1996. Specific intermolecular interaction of carbon dioxide with polymers. J. Am. Chem. Soc. 118 (7), 1729–1736. Mahmoud, A., Muhr, L., Grévillot, G., Lapicque, F., 2007. Experimental tests and modelling of an electrodeionization cell for the treatment of dilute copper solutions. Can. J. Chem. Eng. 85 (2), 171–179. Melnyk, L., Goncharuk, V., 2009. Electrodialysis of solutions containing Mn (II) ions. Desalination 241 (1-3), 49–56. Murthy, Z.V.P., Mungray, A.A., Singh, J., 2012. Preparation and characterization of acrylonitrile butadiene styrene and trifluoroacetylethyl cellulose blend nanofiltration membrane and performance in the separation of mercury. Proceedings of the World Congress on Engineering and Computer Science Vol. II. WCECS, San Francisco, USA. Nasef, M.M., Güven, O., 2012. Radiation-grafted copolymers for separation and purification purposes: status, challenges and future directions. Prog. Polym. Sci. 37 (12), 1597–1656. Öğütveren, Ü.B., Koparal, S., Özel, E., 1997. Electrodialysis for the removal of copper ions from wastewater. J. Environ. Sci. Health Part A: Environ. Sci. Eng. Toxic. 32 (3), 749–761. Parulekar, J.S., 1998. Optimal current and voltage trajectories for minimum energy consumption in batch electrodialysis. J. Membr. Sci. 148 (1), 91–103. Peng, C., Liu, Y., Bi, J., Xu, H., Ahmed, A.S., 2011. Recovery of copper and water from copper-electroplating wastewater by the combination process of electrolysis and electrodialysis. J. Hazard. Mater. 189 (3), 814–820.

J O U RN A L OF E N V I RO N ME N TA L SC IE N CE S 3 5 (2 0 1 5) 2 7– 3 7

Qdais, H.A., Moussa, H., 2004. Removal of heavy metals from wastewater by membrane processes: a comparative study. Desalination 164 (2), 105–110. Radha, K.S., Shobana, K.H., Mohan, D., Narayana, L.Y., 2009. Synthesis and characterization of styrene-acrylonitrile copolymer blend ultrafiltration membranes. Desalin. Water Treat. 12 (1-3), 114–126. Radha, K.S., Shobana, K.H., Tarun, M., Mohan, D., 2014. Studies on sulfonated styrene acrylonitrile and cellulose acetate blend ultrafiltration membranes. Desalin. Water Treat. 52 (1-3), 459–469. Rodrigues, M.A.S., Amado, F.D.R., Bischoff, M.R., Ferreira, C.A., Bernardes, A.M., Ferreira, J.Z., 2008. Transport of zinc complexes through an anion exchange membrane. Desalination 227 (1-3), 241–252. Rusen, E., Mocanu, A., Nistor, L.C., Dinescu, A., Călinescu, I., Mustăţea, G., et al., 2014. Design of antimicrobial membrane based on polymer colloids/multiwall carbon nanotubes hybrid material with silver nanoparticles. ACS Appl. Mater. Interfaces 6 (20), 17384–17393. Sadrzadeh, M., Mohammadi, T., Ivakpour, J., Kasiri, N., 2008. Separation of lead ions from wastewater using electrodialysis: comparing mathematical and neural network modeling. Chem. Eng. J. 144 (3), 431–441. Silva, A.L.A., Takase, I., Pereira, R.P., Rocco, A.M., 2008. Poly(styrene-co-acrylonitrile) based proton conductive membranes. Eur. Polym. J. 44 (5), 1462–1474. Spoor, P.B., Grabovska, L., Koene, L., Janssen, L.J.J., ter Veen, W.R., 2002. Pilot scale deionisation of a galvanic nickel solution using a hybrid ion-exchange/electrodialysis system. Chem. Eng. J. 89 (1-3), 193–202.

37

Strathmann, H., 2010. Electrodialysis, a mature technology with a multitude of new applications. Desalination 264 (3), 268–288. Tanaka, Y., 2002. Current density distribution, limiting current density and saturation current density in an ion-exchange membrane electrodialyzer. J. Membr. Sci. 210 (1), 65–75. Tzanetakis, N., Taama, W.M., Scott, K., Jachuck, R.J.J., Slade, R.S., Varcoe, J., 2003. Comparative performance of ion exchange membranes for electrodialysis of nickel and cobalt. Sep. Purif. Technol. 30 (2), 113–127. Vinodh, R., Padmavathi, R., Sangeetha, D., 2011. Separation of heavy metals from water samples using anion exchange polymers by adsorption process. Desalination 267 (2-3), 267–276. Voicu, S.I., Dobrica, A., Sava, S., Ivan, A., Naftanaila, L., 2014. Cationic surfactants-controlled geometry and dimensions of polymeric membrane pores. J. Optoelectron. Adv. Mater. 14 (11-12), 923–928. Xia, Q., Zhao, X.J., Chen, S.J., Ma, W.Z., Zhang, J., Wang, X.L., 2010. Effect of solution-blended poly(styrene-co-acrylonitrile) copolymer on crystallization of poly(vinylidene fluoride). Express Polym. Lett. 4 (5), 284–291. Yu, J.R., Yi, B.L., Xing, D.M., Liu, F.Q., Shao, Z.G., Fu, Y.Z., et al., 2003. Degradation mechanism of polystyrene sulfonic acid membrane and application of its composite membranes in fuel cells. Phys. Chem. Chem. Phys. 5 (3), 611–615. Zhang, L., Li, A.M., Wang, J.N., Lu, Y.F., Zhou, Y.D., 2009. A novel aminated polymeric adsorbent for removing refractory dissolved organic matter from landfill leachate treatment plant. J. Environ. Sci. 21 (8), 1089–1095.