UDK 678.7	ISSN 1580-2949
Professional article/Strokovni članek	MTAEC9, 48(5)791(2014)
PREPARATION AND APPLICATION OF POLYMER
INCLUSION MEMBRANES (PIMs) INCLUDING ALAMINE 336 FOR THE EXTRACTION OF METALS FROM AN AQUEOUS SOLUTION
PRIPRAVA IN UPORABA MEMBRANE IZ POLIMERA (PIM) IN ALAMINA 336 ZA LOČENJE KOVIN IZ VODNIH RAZTOPIN
Yasemin Yildiz1, Aynur Manzak1, Bü°ra Aydin1, Osman Tutkun2
1Department of Chemistry, Sakarya University, Sakarya, Turkey 2Beykent University, Department of Chemical Engineering, Engineering and Architecture Faculty, Istanbul, Turkey
manzak@sakarya.edu.tr
Prejem rokopisa - received: 2013-10-12; sprejem za objavo - accepted for publication: 2013-11-12
Polymer inclusion membranes (PIMs) present an attractive approach for the separation of metals from an aqueous solution. The present study is about the application of Alamine 336 as an ion carrier in PIMs. The separation of copper (II), cobalt (II), nickel (II) and cadmium (II) from aqueous solutions with polymer inclusion membranes was investigated. PIMs are formed by casting a solution containing a carrier (extractant), a plasticizer and a base polymer, such as cellulose tri-acetate (CTA) or poly(vinyl chloride) (PVC), to form a thin, flexible and stable film. Several important transport parameters such as the type and amount of the plasticizer, the type of the stripping solution, the thickness of the membrane, the pH of the acid in the donor phase and the concentration of the base in the acceptor phase are discussed. The membrane was characterized to obtain information regarding its composition using AFM, FT-IR and SEM.
Keywords: polymer inclusion membranes, plasticizer, extractant, thickness of membrane
Membrane, ki vsebujejo polimere (PIM), so zanimive za ločenje kovin iz vodnih raztopin. Prikazana je študija uporabe Alamina 336 kot nosilca ionov v PIM. Preiskovano je bilo ločenje bakra (II), kobalta (II), niklja (II) in kadmija (II) iz vodnih raztopin z membrano s polimeri. PIM je bila izdelana z ulivanjem raztopine z nosilcem (ekstraktantom), z mehčalcem in osnovo iz polimera, kot je celuloza-tri-acetat (CTA) ali polivinil klorid (PVC), da je nastala tanka, gibljiva plast. Razloženih je več pomembnih transportnih parametrov, kot so delež mehčalca, vrsta raztopine za snemanje, debelina membrane, pH kisline v donorski fazi in koncentracija baze v aceptorski fazi. Izvršene so bile preiskave z AFM, FT-IR in SEM, da bi dobili podatke o sestavi membrane.
Ključne besede: membrane s polimerom, mehčalec, ekstraktant, debelina membrane
1 INTRODUCTION
nuclear and harmful-metal waste remediation on an industrial scale. They consist of a polymer providing the
The separation of metals from sulphate and chloride	mechanical strength, a carrier molecule that effectively
media has been of practical interest to the researchers.	binds and transports the ions across the membrane, and a
Solvent extraction is a well-established technology used	plasticizer that provides elasticity and acts as the solvent,
for the production of metals from a relatively concen-	in which the carrier molecule can diffuse. PIMs are
trated feed. However, industrial diluent effluents pose an	formed by casting a solution containing a carrier (extrac-
important challenge as the solvent-extraction technique	tant), a plasticizer and a base polymer, such as cellulose
is not cost effective for the separation of metals from a	tri-acetate (CTA) or poly(vinyl chloride) (PVC), to form
dilute solution1.	a thin, flexible and stable film6.
Recently, the supported liquid membrane (SLM)	The choice of different constituents of the membrane
extraction has been emerging as an alternative to the	is crucial to ensure its separation efficiency, so it is
conventional solvent extraction due to its advantages	important to investigate the effect of different compo-
such as high selectivity, operational simplicity, low	nents on the extraction and transport of the target
solvent inventory, low energy consumption, zero effluent	species. Among the polymers used to form a gel-like
discharge, and a combination of extraction and stripping	network that entraps the carrier and plasticizer/modifier,
in a single unit23. Currently, considerable attention is fo-	poly(vinyl chloride) (PVC) and cellulose triacetate
cused upon polymer inclusion membranes (PIMs)4. Their	(CTA) are most frequently encountered7.
specific advantages are an effective carrier immobili-	Examples of such membranes are those containing
zation, easy preparation, versatility, stability, good che-	only PVC and Aliquate 336 that have been used success-
mical resistance and better mechanical properties than in	fully for the transport of both metallic (e.g., Cd (II) and
the case of SLM5. The large surface-area-to-volume ratio	Cu (II)8 and non-metallic (e.g., thiocyanate)9 ionic spe-
exhibited by PIMs gives them the potential to be used in	cies. Moreover, Konczyk et al.10 have used Aliquate 336
as a plasticizer in a PIM system containing D2EHPA as the carrier for the removal of Cr (III).
The present study focuses on the application of Ala-mine 336 as an ion carrier in PIMs and deals with the selective separation of Co, Cd, Ni and Cu ions from an acidic media into an NH4SCN aqueous solution. Amines were used to extract the metal ions. The amine extraction chemistry of thiocyanate complexes was investigated by Sanuki et al.11
2 EXPERIMENTAL WORK
2.1	Materials
All the reagents used were of analytical grade. Cellulose triacetate (CTA), 2-nitrophenyl pentyl ether (NPPE) and 2-nitrophenyl octyl ether (NPOE) were obtained from Fluka. Tributyl phosphate (TBP), dichlo-romethane, CoCla ■ 6H2O, NiSO4 ■ 6H2O, 3CdSO4 ■ 8H2O, CuSO4 ■ 5H2O, acetic acid, NaOH, ammonium, triethanolamine, NH4SCN and Alamine 336 were of analytical grade (Merck) and all the stock solutions were prepared by dissolving the salts in distilled water.
2.2	Preparation o^ PIMs
PIMs were prepared in accordance with the casting solution. CTA (480 mg) was dissolved in 70 mL of dichloromethane at room temperature. In the following step 0.1-0.5 mL of 2-NPPE was added into the solution. After stirring, the carrier (Alamine 336 and TBP) was added and the solution was stirred for 6 h to obtain a homogenous solution. The solvent of this mixed solution was allowed to slowly evaporate in a square glass container (24 cm x 24 cm). The organic solvent was allowed to evaporate overnight at room temperature. After the evaporation of the solvent, a few drops of cold and distilled water swirled on the top of the polymer film. Afterwards, the membrane was peeled out of the container. The average thickness of the membrane was determined as 25 pm with a digital micrometer (Salu Tron Combi-D3).
2.3	PIM transport experiment
The prepared polymeric film was sandwiched between two glass cells. The transport of metal ions across the PIM from the aqueous solutions was studied by using a two-compartment permeation cell made from Pyrex glass, having flat-sheet membranes with the 12.56 cm2 area (A), as shown schematically in Figure 1.
The volumes of both the aqueous feed and the strip phases were 250 mL. The feed solutions were prepared by adding cobalt, nickel, cadmium and copper salts to study the effect of the feed composition. Ammonium thiocyanate (NH4SCN) was added into the feed mixture to increase the selectivity of cobalt against nickel. 1 M acetic acid/1 M sodium acetate buffer was used to maintain the desired feed pH. A stripping solution containing
Figure 1: Schematic diagram of the experimental apparatus Slika 1: Shema naprave za preizkuse
1 M NH3 + 1 M TEA was selected as the stripping-phase mixture. The feed and stripping phases were mechanically stirred at the desired mixing speed of (20 ± 1) °C to avoid the concentration polarization conditions at the membrane interfaces and in the bulk of the solutions. During the PIM-transport experiments, the samples of the feed and strip phases (about 1 mL) were periodically removed for a determination of the metal concentration with ICP-OES.
3 RESULTS AND DISCUSSION
3.1 Plasticizer type and concentration
The nature of the plasticizer used to form the membrane is also a key parameter to consider. Plasticizers are organic compounds incorporating a hydrophobic alkyl backbone and one or several highly solvating polar groups. They are added to hard, stiff plastics to make them softer and more flexible. The softening action of the plasticizers, plasticization, is usually attributed to their ability to reduce the intermolecular attractive forces between the polymer chains. For this reason, it is anticipated that in PIMs the presence of these compounds may also influence the mobility of membrane components, the degree of interaction between different constituents of the membrane and the characteristics of the polymeric medium7. A low plasticizer concentration may cause more rigid and brittle membranes. So, it is not preferred4. The minimum plasticizer concentration varies widely depending on both the plasticizer and the base polymer. The influence of the plasticizer nature on the Cd2+, Co2+, Ni2+, and Cu2+ transport through PIMs with different plasticizers, i.e., 2-Nitrophenyl octyl ether (NPOE) and 2-nitrophenyl pentyl ether (NPPE) was tested. Copper was precipitated in the feed phase. Nickel was not transferred to the stripping solution. The cobalt and cadmium ions in the acidic feed solutions reacted with the excess NH4SCN, whereas in the case of nickel ions, they hardly formed a thiocyanate complex12,13.
The results obtained for the Cd2+ and Co2+ ion transport with different concentrations of the plasticizers
Figure 2: Effect of the NPPE concentration on the cadmium extraction (feed phase: 100 mg/L Co2+, 100 mg/L Ni2+, 100 mg/L Cd2+, 100 mg/L Cu2+; feed stirring speed: 1200 r/min; strip-phase stirring speed: 1200 r/min; strip solution: 1 M NH3 + 1 M TEA; complex reagent (NH4SCN): 0.5 mol/l; temp.: 20 °C; feed solution pH: 4) Slika 2: Vpliv koncentracije NPPE na ekstrakcijo kadmija (iz raztopine: 100 mg/L Co2+, 100 mg/L Ni2+, 100 mg/L Cd2+, 100 mg/L Cu2+; hitrost me{anja raztopine: 1200 r/min; hitrost me{anja v fazi traku: 1200 r/min; raztopina traku: 1 M NH3 + 1 M TEA; kompleksni reagent (NH4SCN): 0.5 mol/l; Temp.: 20 °C; raztopina pH: 4)
in the PIMs are shown in Figures 2 and 3. For NPPE, this concentration can be in the range of up to 0.2 mL (w = 27 %) (Figure 2). Above this upper limit the mass transport diminishes.
The results obtained for the Cd2+ and Co2+ ion transport with different types of plasticizers in the PIMs are shown in Figures 4 and 5. 2-Nitrophenyl pentyl ether (NPPE) is the most frequently used plasticizer in PIMs due to its high dielectric constant that enhances the
Figure 4: Effect of the plasticizer type on the cadmium transport (feed phase: 100 mg/L Co2+, 100 mg/L Ni2+, 100 mg/L Cd2+, 100 mg/L Cu2+; feed stirring speed: 1200 r/min; strip-phase stirring speed: 1200 r/min; strip solution: 1 M NH3 + 1 M TEA; complex reagent (NH4SCN): 0.5 mol/l; temp.: 20 °C; feed solution pH: 4) Slika 4: Vpliv vrste meh~alca na prenos kadmija (raztopina: 100 mg/L Co2+, 100 mg/L Ni2+, 100 mg/L Cd2+, 100 mg/L Cu2+; hitrost me{anja raztopine: 1200 r/min; hitrost me{anja v fazi traku: 1200 r/min; raztopina traku: 1 M NH3 + 1 M TEA; kompleksni reagent (NH4SCN): 0.5 mol/l; Temp.: 20 °C; raztopina pH: 4)
membrane permeability. A decrease in the permeability, together with an increase in the plasticizer content, is probably related to a reduction in the viscosity of the medium4.
The recovery factor (RF) of the metal ions from the feed phase into the stripping phase is given by: C, - C
RF =
Ci
■100%
(1)
where C is the metal-ion concentration in the feed phase at some given time and Ci is the initial metal-ion concentration in the feed phase. Recovery factors (RF) for different plasticizers are shown in Table 1.
Table 1: Effect of the plasticizer type on the cobalt and cadmium transport
Tabela 1: Vpliv vrste meh~alca na prenos kobalta in kadmija
Figure 3: Effect of the NPPE concentration on the cobalt extraction (feed phase: 100 mg/L Co2+, 100 mg/L Ni2+, 100 mg/L Cd2+, 100 mg/L Cu2+; feed stirring speed: 1200 r/min; strip-phase stirring speed: 1200 r/min; strip solution: 1 M NH3 + 1 M TEA; complex reagent (NH4SCN): 0.5 mol/l; temp.: 20 °C; feed solution pH: 4) Slika 3: Vpliv koncentracije NPPE na ekstrakcijo kobalta (raztopina: 100 mg/L Co2+, 100 mg/L Ni2+, 100 mg/L Cd2+, 100 mg/L Cu2+; hitrost me{anja raztopine: 1200 r/min; hitrost me{anja v fazi traku: 1200 r/min; raztopina traku: 1 M NH3 + 1 M TEA; kompleksni reagent (NH4SCN): 0.5 mol/l; temp.: 20 °C; raztopina pH: 4)
Figure 5: Effect of the plasticizer type on the cobalt transport (feed phase: 100 mg/L Co2+, 100 mg/L Ni2+, 100 mg/L Cd2+, 100 mg/L Cu2+; feed stirring speed: 1200 r/min; strip-phase stirring speed: 1200 r/min; strip solution: 1 M NH3 + 1 M TEA; complex reagent (NH4SCN): 0.5 mol/l; temp.: 20 °C; feed solution pH: 4) Slika 5: Vpliv vrste meh~alca na prenos kobalta (raztopina: 100 mg/L Co2+, 100 mg/L Ni2+, 100 mg/L Cd2+, 100 mg/L Cu2+; hitrost me{anja raztopine: 1200 r/min; hitrost me{anja v fazi traku: 1200 r/min; raztopina traku: 1 M NH3 + 1 M TEA; kompleksni reagent (NH4SCN): 0.5 mol/l; temp.: 20 °C; raztopina pH: 4)
NPOE	59	25
NPPE	81	46
3.2	Effe^^ of the stripping-solution type
In general, metallic ions extracted by amines can be stripped from the protonated amine with the removal of a proton using neutral or alkaline solutions. 1 M ammonia and 1 M triethanol amine solution mixtures were used as the reagents to strip and separate the cobalt and cadmium from the membrane phase to the aqueous phase.
3.3	Membrane characteristics
One important aspect of PIMs is the microstructure of the membrane materials, which determines the distribution of the carriers in the polymer matrix and ultimately affects the membrane transport efficiency. Consequently, a considerable research effort was devoted to clarifying this issue. While a variety of surface-characterization techniques were employed in these studies, scanning electron microscopy (SEM) and atomic force microscopy (AFM) were most frequently used. The results obtained from the SEM and AFM studies consistently indicate a remarkable influence of the polymeric composition on the membrane morphology.
The membrane was characterized to obtain information regarding its composition using AFM (Figure 6), SEM (Figure 7) and FT-IR (Figure 8).
The AFM technique was used to characterize the surface morphology of the prepared membranes. The AFM picture of the PIM formed with CTA + NPPE + TBP +
Figure 6: CTA + NPPE + Alamine 336 + TBP, AFM image Slika 6: AFM-posnetek CTA + NPPE + Alamin 336 + TBP
Figure 7: CTA + NPPE + Alamine 336 + TBP, SEM image Slika 7: SEM-posnetek CTA + NPPE + Alamin 336 + TBP
Figure 8: CTA + NPPE + Alamine 336 + TBP, FT-IR Slika 8: FT-IR-posnetek CTA + NPPE + Alamin 336 + TBP
Alamine 336 is shown in Figure 6. The surface morphology of the membrane shows a rough surface. These regions may have occurred because of either a different speed of the solvent vaporization14, 15 or the membrane having a porous structure where the pores were filled by NPPE or NPPE + Alamine 336 + TBP16,17.
Although both SEM and AFM techniques are versatile and can provide a good image of the membrane surface and, to some degree, of the membrane interior structure, to date, the studies employing these techniques have not been able to clearly elucidate the distribution of the carrier and the plasticizer within the membrane. Consequently, more advanced material-characterization techniques have been attempted.4
In order to investigate the absorption bands of the constituents of the membranes containing CTA + NPPE+ Alamine 336 + TBP, FTIR was performed as shown in Figure 8. The bands at 2986 cm-1 and 2936 cm-1 were attributed to the stretching vibration of C-H in -CH2 and -CH3. The absorption at 1750 cm-1 was assigned to the stretching vibration of C=O in CTA.
The fingerprint region of the spectra becomes complicated because of the P-O, C-O and C-N vibrations. These three vibrations are absorbed in the same region. For example, the peaks in the 1250 cm-1 and 1100 cm -1 region appear in both CTA and TBP and they overlap completely. The expected peaks in the spectra appear in almost the same region as in the case of pure components like CTA, Alamine 336 and TBP. This indicates that these four compounds do not form any new covalent interactions, but only secondary interactions like hydrogen bonding or electrostatic interactions18.
Consequently, the analysis and comparison of the obtained spectra revealed that all the membrane constituents remained as pure components inside the membra-ne17,18. The surface of the films shows a good uniformity and the absence of cracks indicates a good regularity of the membranes as shown in Figure 7.
3.4 Membrane thickness
The investigated membrane thickness was 20 ^m to 45 pm, shown in Figures 9 and 10. The best recovery factor (RF) was obtained with a thickness of 25 pm, with 81 % in the feed phase over 5 h as shown in Table 2. The
Figure 9: Effect of the membrane thickness on the cadmium transport (feed phase: 100 mg/L Co2+, 100 mg/L Ni2+, 100 mg/L Cd2+, 100 mg/L Cu2+; feed stirring speed: 1200 r/min; strip-phase stirring speed: 1200 r/min; strip solution: 1 M NH3 + 1 M TEA; complex reagent (NH4SCN): 0.5 mol/l; temp.: 20 °C; feed-solution pH: 4) Slika 9: Vpliv debeline membrane na prenos kadmija (raztopina: 100 mg/L Co2+, 100 mg/L Ni2+, 100 mg/L Cd2+, 100 mg/L Cu2+; hitrost me{anja raztopine: 1200 r/min; hitrost me{anja v fazi traku: 1200 r/min; raztopina traku: 1 M NH3 + 1 M TEA; kompleksni reagent (NH4SCN): 0.5 mol/l; temp.: 20 °C; raztopina pH: 4)
Table 2: Effect of the membrane thickness on the cobalt and cadmium transport
Tabela 2: Vpliv debeline membrane na prenos kobalta in kadmija
Membrane thickness (pm)	RF (Co)	RF (Cd)
20	39	15
25	81	46
45	54	15
optimum membrane thickness was 25 pm. As the membrane thickness increased, the extraction would decrease.
As shown in19 thinner membranes exhibiting high permeability are formed. However, the thinnest membranes only partly allow high permeability due to a decrease in the extractant content limiting the transport efficiency.
As shown in reference20 the flux decreased linearly with the membrane thickness. This is unambiguous evidence that the slow step in the transport process represents the migration through the membrane and not a decomplexation from the carrier.
4 CONCLUSIONS
With the use of Alamine 336 and TBP as the carriers, the competitive transport of metal ions shows the preferential selectivity order: Co (II) > Cd (II). The transport facilitated through the polymer inclusion membranes containing Alamine 336 and TBP was found to be an effective method for separation and recovery of cobalt (II) and cadmium (II) from aqueous solutions. Copper was precipitated in the feed phase. Nickel was not transferred to the stripping solution. The recovery factor for the cobalt ions was over 87 % over a period 6 h.
Figure 10: Effect of the membrane thickness on the cobalt transport (feed phase: 100 mg/L Co2+, 100 mg/L Ni2+, 100 mg/L Cd2+, 100 mg/l Cu2+; feed stirring speed: 1 200 r/min; strip-phase stirring speed: 1200 r/min; strip solution: 1 M NH3 + 1 M TEA; complex reagent (NH4SCN): 0.5 mol/l; temp.: 20 °C; feed solution pH: 4) Slika 10: Vpliv debeline membrane na prenos kobalta (raztopina: 100 mg/l Co2+, 100 mg/l Ni2+, 100 mg/l Cd2+, 100 mg/l Cu2+; hitrost me{anja raztopine: 1 200 r/min; hitrost me{anja v fazi traku: 1 200 r/min; raztopina traku: 1 M NH3 + 1 M TEA; kompleksni reagent (NH4SCN): 0.5 mol/l; temp.: 20 °C; raztopina pH: 4)
Acknowledgement
The financial support of this work, provided by the scientific research commission of Sakarya University (BAPK), Project No: 2010-02-04-025, is gratefully acknowledged.
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