https://doi.org/10.1007/s10924-019-01535-7
ORIGINAL PAPER
Cu(II) Sorption Performance of Novel Chitosan/Ter‑(vinyl
pivalate‑maleic anhydride‑N‑tert‑butylacrylamide) Microcapsules
Ahmet Okudan1 · Busra Ebru Ataoglu1 · Onur Sengoz1 · Gulsin Arslan2
Published online: 29 July 2019
© Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
In this study, first, the ter-polymerization reaction between vinyl pivalate (VP), maleic anhydride (MA), and N-tert-butyl-acrylamide (NTBA) was done in inert atmosphere (N2). FT-IR and 1HNMR spectroscopy was applied to study the
chemi-cal composition of the obtained ter-polymers. MA content of the ter-polymers was determined by following the chemichemi-cal titration method. Second, novel chitosan/ter-(vinyl-pivalate-maleic-anhydride-N-tert-butylacrylamide) microcapsules were synthesized. In microcapsule production, chitosan polymer served as a matrix for acrylamide ter-polymers with four dif-ferent molar ratios. The microcapsules were characterized by FT-IR and SEM analyses. Cu(II) sorption efficiency of the microcapsules were tested at different pH levels, temperature, sorbent dosage, and metal ion concentration. Comparison with blank chitosan microbeads revealed that incorporation of acrylamide ter-polymers into the cross-linked chitosan matrix increased the metal sorption. Sorption capacities of the sorbents were recorded; blank chitosan microbeads: 67.03 mg g−1,
and chitosan/acrylamide ter-polymer microcapsules: in range of 75.39–98.64 mg g−1. The findings demonstrated chitosan/
acrylamide ter-polymer microcapsules can be utilized in sorption of Cu(II) ions in water treatment
Keyword Adsorption · Acrylamide · Ter-polymer · Microcapsule · Water treatment
Introduction
Many polymers having functional moieties have been largely produced and used for their macromolecular properties as well as their properties of functional groups [1]. Anhydride and acrylamide groups containing polymers has received considerable recent attention because of their varied appli-cations. These functional groups make it possible to modify them for specific applications [2]. The polymers with anhy-dride and acrylamide groups are used as flocculants in reme-diation of industrial wastewater and as coating material in production of microcapsules [3]. Also, these amphiphilic polymers bearing active negatively and positively charged groups exhibit both temperature and pH sensitivity, and they can be utilized in biological applications requiring physi-ologically active macromolecular systems [4]. Moreover,
ter-(vinyl pivalate-maleic anhydride-N-tert-butylacrylamide) is one of the priorities of twenty-first century to make cer-tain of clean water supply; for this reason, the developing advanced technologies for water treatment is of crucial [5]. It has been already acknowledged that adsorption is an efficient method in water treatment [6]. Low-cost and eco-friendly nature of adsorption make it a preferred tool [7] for removal of contaminants. This technique also eliminates formation of sludge and permits the use of various material as adsor-bent such as activated carbon, natural or synthetic polymers, resins, fly ash, gels, oxides, silicates, clays, zeolites and agricultural or industrial waste materials. Recently, design-ing low-cost but efficient sorbents from renewable sources such as chitosan, cellulose or alginate has captured many researchers’ attention [8].
Chitosan or deacetylated derivative of chitin is a renewa-ble biopolymer that exhibits unique features such as biocom-patibility, biodegradability, non-toxicity and high affinity for metal cations or anions. The supremacy of this functional polysaccharide has already been acknowledged in many fields such as biotechnology, food industry, pharmaceutical and water treatment. It is one of the most desirable biopoly-mers in a lot of applications [9].
* Ahmet Okudan okudan1@gmail.com
1 Department of Chemistry, Faculty of Science, Selcuk
University, 42075 Konya, Turkey
2 Department of Biochemistry, Faculty of Science, Selcuk
So far, chitosan, particularly through functional –OH and –NH2 groups, has been modified by chemically, and a lot of derivatives have been synthesized via reaction of tosylation, alkylation, carboxylation or Schiff bases [10]. Amino groups on the chitosan polymer chains make it sol-uble in acidic medium. These groups also give chitosan cationic character [11]. Hydroxyl and amino groups plays key roles in the interaction of chitosan with cationic spe-cies [11–13].
To enhance its efficiency and performance in water treat-ment, chitosan polymer should be functionalized and its mechanical strength should be developed. Polyacrylamide is a biocompatible polymer and its gels have been used as vehicles for drug delivery [14]. Acrylamide hydrogels with chitosan has been also examined in metal sorption [15, 16] and dye elimination [17] studies. In the literature on chitosan composite materials for metal ion sorption, there is no report on ter-(vinyl pivalate-maleic anhydride-N-tert-butylacryla-mide) blended chitosan composite microcapsules for metal ion sorption. It seems that more studies are needed in this field to improve the knowledge in production of chitosan composite microcapsules.
This present study aimed to prepare novel chitosan micro-capsules with ter-(vinyl pivalate-maleic anhydride-N-tert-butylacrylamide) [ter-(VP-MA-NTBA)] polymer and to investigate their potential in Cu(II) ion sorption. This study also established the optimum molar ratio of N-tert-butyl-acrylamide in ter-(VP-MA-NTBA) polymer for production of microcapsules with chitosan.
Experimental
MaterialsChitosan (medium molecular weight, deacetylation degree; 75–85%) was obtained from Aldrich. Glutaraldehyde (25% v:v), Cu(NO3)2.3H2O, KCl, sodium hydroxide, hydrochloric
acid, nitric acid, acetic acid and methanol were purchased from Merck. Vinyl pivalate (VP) (99%) was purchased from Aldrich. An alumina column was used to remove the inhibitor from VP and then the degassing procedure was achieved by N2. Maleic anhydride (MA) (Fluka) was
puri-fied by recrystallization from anhydrous benzene and by sublimation in vacuum (melting point: 52.8 °C). N-tert-butylacrylamide(NTBA) (Aldrich) was recrystallized in warm dry benzene. 2,2′-Azoisobutyronitrile (AIBN) (Fluka) was twice recrystallized from methanol: (melting point: 102.5 °C). p-Dioxane (Merck) was refluxed over sodium and distilled from aluminium lithium hydride. The middle frac-tion was used. All other reagents were used without further purification.
Preparation of Cu(II) Solutions
Stock solutions of Cu(II) (100 mg L−1) were prepared by
dissolving copper(II) nitrate trihydrate in distilled water and 1 mL of concentrated HNO3 (65%) solution. Then,
the final solution volume was completed to 1000 mL. The stock solutions were kept at + 4 °C.
Synthesis of the VP‑MA‑NTBA Ter‑polymer
For the synthesis of the VP-MA-NTBA Ter-polymer, vari-ous molar ratios of VP, MA and NTBA were dissolved in p-dioxane in around-bottom flask equipped with a magnetic stir bar and in a thermostated silicon oil bath at 65 ± 0.1 °C and a nitrogen inlet. The polymerization was continued for approximately 20 h. After the polymeriza-tions were consummated, the VP-MA-NTBA Ter-polymers were separated from the reaction mixture by precipitation in diethyl ether at room temperature. Then, the products synthesized were redissolved in p-dioxane and reprecipi-tated by diethyl ether and washed a few times with diethyl ether. The insoluble ter-polymers were removed by filtra-tion and make dried in a vacuum oven at 50 °C for at least 2 days.
Synthesis of Microcapsules
Chitosan powder (3.0 g) was dissolved in acetic solution (150 mL, 2% v:v) and stirred for overnight to ensure a complete dissolution. 1.500 g of ter(VP-MA-NTBA) was pulverised in an agate mortar and then put into the chi-tosan solution. Chichi-tosan-ter(VP-MA-NTBA) blend was homogenised by stirring for 3 h and subsequently the blend was transferred into a burette. The chitosan-ter-(VP-MA-NTBA) blend was dropped into 600 mL methanol, 400 mL water and NaOH (120 g) solution. The microcapsules were kept in this solution overnight and then collected by filtration and washed tho roughly with water to neutral pH. Cross-linking of the microcapsules was achieved in a solution of glutaraldehyde (9 mL) and methanol (90 mL). The microcapsules in the reaction medium were heated for 6 h at 70 °C under reflux. The cross-linked microcapsules were filtrated out and make dried at 25 °C.
By following the same method, four types of sorbent were prepared and named as follows; blank: (cross-linked chitosan beads without ter-(VP-MA-NTBA) polymer particles), 1: (chitosan/ter-(VP(1)MA(2)NTBA(0.5)), the numbers given in the brackets denote the molar ratios), 2: (chitosan/ter-(VP(1)MA(2)NTBA(1)), 3: (chitosan/ter-(VP(1)MA(2) NTBA(3)) and 4: (chitosan/ter-(VP(1)MA(2)NTBA(5)).
Instrumentation
Scanning electron microscopy (SEM) (ZEISS EVO-LS10) was used to study the surface texture of chitosan/acrylamide ter-polymer microcapsules. Infrared spectra of the microcap-sules and ter-polymer were recorded with Fourier transform infrared spectroscopy (FT-IR) (Perkin Elmer 100 FT-IR, ATR). 1H NMR spectra were recorded on a Varian 400 NMR
spectrometer. Cu(II) content of the solutions was determined by a flame atomic absorption spectrometer (Contra AA 300/ Analytic Jena). Cu(II) sorption studies were conducted using a shaker (Nuve EN 500 Model) at 200 rpm.
Characterization of the VP‑MA‑NTBA Ter‑polymer
The ter-polymer [ter(VP-MA-NTBA)] compositions were found by chemical method (acid number for an hydride units).
1H NMR and FT-IR analysis provided supporting evidence for
the structural analysis. Intrinsic viscosities were determined by an Ubbelohde viscometer (Ildam, Turkey).
Acid numbers (AN) of the anhydride-containing ter-poly-mers were determined by standard titration method [18].
FT-IR spectrum was used to determine the ter-polymer composition using either solid films or mull samples in the range of wave numbers 4000–600 cm−1 (30 scans at 4 cm−1
resolution). In preparation of the mulls, the polymer was mixed with KBr powder and then pressed into transparent pellets. Solid films were obtained through solution casting the polymer onto a KBr pellet and then the solvent was evaporated.
1H NMR spectra of VP-MA-NTBA ter-polymer were
recorded on a Varian 400 NMR spectrometer (solvent: DMSO-d6 at 50 °C). For 1H NMR spectroscopy, the samples (about
20 mg of the products) were dissolved in deuterated chloro-form (1 mL). The internal reference was tetra methyl silane.
For viscosimetry measurements, acetone solution of the samples was prepared with a molarity range of 0.1–1.0 g/dL. The time flow of the solutions and solvents was measured by Ubbelohde type viscosimeter that was put in a thermostatic water bath at 25 ± 0.1 °C. Specific viscosity (ηsp) and relative
viscosity (ηr) were calculated [19, 20].
where to is flow time of solvent, t is flow time of solution.
(1) AN( mgKOH g ) = mLKOH× NKOH × 56.11 g polymer (2) 𝜂r= t to (3) 𝜂sp= 𝜂r− 1 (4) [𝜂] = 1.414 C (𝜂sp− ln 𝜂r) 1∕2
where [η] is intrinsic viscosity, C is the molarity of the solution.
Cu(II) Sorption Studies
The optimum adsorbent dosage, Cu(II) solution concentra-tion, solution pH and temperature for Cu(II) sorption were determined by a batchwise sorption system. Cu(II) solutions (2.5–12.5 mg L−1) were prepared from the 100 mg L−1 of
stock solution and pH adjustment was done with dilute solu-tions of hydrochloric acid (0.1 M) and sodium hydroxide (0.1 M). The microcapsules were collected from the solu-tion by using a filter paper; and Cu(II) content of the filtrate was determined by the flame atomic absorption spectrom-eter. Following equation was employed for the calculation of Cu(II) sorption capacity of the chitosan/acrylamide ter-polymer microcapsules (qe; in mg g−1):
where C0 and Ce denote the initial and equilibrium copper ion concentration (mg L−1); V is the volume of the metal
solution (in L) and W (in g) is the mass of the chitosan/ acrylamide ter-polymer microcapsules.
Adsorbent Dosage Studies
Adsorbent dosage studies were conducted for 0.0050; 0.0100; 0.0500; 0.1000 and 0.1500 g of chitosan/acrylamide ter-polymer microcapsules in 25 mL of Cu(II) (10 mg L−1,
pH: 5.56) solution at 25 °C. The bottles were agitated on the shaker for 4 h.
Cu(II) Ion Concentration Studies
To reveal the effect of the metal ion concentration, sorption studies were conducted for 2.5, 5.0, 7.5, 10.0 and 12.5 mg L−1 of Cu(II) solution at 25 °C. 0.1000 g of
chitosan/acryla-mide ter-polymer microcapsules were added into the metal solutions (25 mL, pH: 5.56) and agitated on the shaker at 200 rpm. The data were applied to two widely used isotherm models; Freundlich and Langmuir isotherm models.
pH Effect Studies
The effect of metal solution pH on the Cu(II) uptake was studied in the pH range of 2.00–5.00. 25 mL of Cu(II) (10 mg L−1 at 25 °C) solutions with 0.1000 g of chitosan/
acrylamide ter-polymer microcapsules were agitated on the shaker for 4 h.
(5)
Temperature Effect Studies
Cu(II) sorption studies were done at 25, 35 and 45 °C. The sorption studies were carried out in 25 mL Cu(II) solution (10 mg L−1, pH 5.56). 0.1000 g of chitosan/acrylamide
ter-polymer microcapsules were added into the metal solutions and agitated on the shaker for 4 h. Thermodynamic param-eters were obtained from the analysis of the experimental data.
Results and Discussion
Structural Analysis of the VP‑MA‑NTBA Ter‑polymer
The synthesis of the ter-polymer of VP, MA, and NTBA was done using AIBN as an initiator at 65 °C in a different monomer ratio (VP/MA/NTBA). The polymerization reac-tion is illustrated in Fig. 1. The influence of various param-eters, such as the feed mol ratio (VP/MA/NTBA), amount of AIBN, reaction temperature, and reaction time, were determined. Also, Table 1 gives the intrinsic viscosities [η],
conversion, and acid numbers (AN), for the VP-MA-NTBA ter-polymer.
As presented in Table 1, the optimum conditions were recorded as follows; feed mol ratio of VP/MA/NTBA is 0.01/0.02/0.005 (mol/mol), the amount of AIBN is 3.5 × 10−4 mol, at 65 °C and time of reflux is 20 h. A
sub-stantial increase in conversion was observed when increas-ing the amount of AIBN initiator. However, the results indi-cate that a lower amount of initiator is required to improve the viscosity. Decreasing the reaction temperature reduced the ter-polymerization conversion and the viscosity factor. Decreasing the reaction time markedly reduced the conver-sion of ter-polymerization. Under optimum reaction condi-tions, the acid number was enhanced to 592 mg KOH/g (Run 7, Table 1).
Surface Characterisation of the Chitosan/Arylamide Ter‑polymer Microcapsules
Surface characterisation of the chitosan/acrylamide ter-pol-ymer microcapsules was performed by using FT-IR spec-troscopy results and SEM images. FT-IR spectra analysis Fig. 1 Polymerization reaction
of VP-MA-NTBA
Table 1 The effect on ter-polymerization of reaction conditions
*The optimum values of the reaction is presented in run 7 Run Feed mol ratio
VP/MA/NTBA (mol)
AIBN (mol%) Tem-perature (°C)
Time (h) Acid number
(mg KOH/g)) [η] (g/dL) Conversion (%) 1 0.01/0.02/0.005 3.5 × 10–4 65 24 588 0.129 50.6 2 0.01/0.02/0.005 2.8 × 10–4 65 20 576 0.117 37.8 3 0.01/0.02/0.005 3.5 × 10–4 65 12 496 0.089 27.4 4 0.01/0.02/0.005 4.0 × 10–4 65 20 590 0.101 51.8 5 0.01/0.02/0.005 3.5 × 10–4 75 20 582 0.125 50.4 6 0.01/0.02/0.005 3.5 × 10–4 55 20 585 0.115 45.6 7* 0.01/0.02/0.005 3.5 × 10–4 65 20 592 0.127 50.8 8 0.01/0.02/0.01 3.5 × 10–4 65 20 560 0.118 47.7 9 0.01/0.02/0.02 3.5 × 10–4 65 20 551 0.113 40.2 10 0.01/0.02/0.03 3.5 × 10–4 65 20 530 0.109 34.7 11 0.01/0.02/0.05 3.5 × 10–4 65 20 512 0.103 28.1
and SEM images revealed that the incorporation of ter-(VP-MA-NTBA) polymer particles into the chitosan matrix was performed successfully.
FT‑IR Spectra Analysis
The FT-IR spectra of VP, MA, NTBA, and Poly(VP-MA-NTBA) are presented in Fig. 2a. In the VP-MA-NTBA ter-polymer spectra, the characteristic absorption bands were observed at 1670 cm−1 (n C=O, amide I), at
1537 cm−1 (N–H bending, amide II), at about 1265 cm−1
(trans-amide III), 3400–3100 cm−1 broad band for NH
secondary amide, and the band assigned to the tertiary butyl groups [–C(CH3)3] at 1222 cm−1 [21-23]. The
bands in the region of 2860‒2976 cm−1 can be attributed
to –CH stretching vibrations of –CH3 and –CH2 groups
VP, 1736 cm−1 corresponding to C=O, and the band at
1150 cm−1 to the stretching of the –C–O–C group [24, 25].
The MA spectrum display characteristic absorption bands at 1856–1782 cm−1 corresponding to C=O stretching of
anhydride group and 1170 cm−1 broad C–O–C anhydride
[4].
In the FT-IR spectra of the chitosan/acrylamide ter-polymer microcapsules, the bands in the region of 2976–2860 cm−1 are ascribed to the stretching of CH
3 and
CH2 groups of vinyl pivalate and the band at 1150 cm−1
to the stretching of –C–O–C– group Fig. 2b. The bands at 1850–1773 cm−1 denoting symmetric and asymmetric
stretching of carbonyl groups present in the rings of maleic anhydride were not observed. The disappearance of the bands after the interaction with chitosan can be attributed to the modification of the carbonyl groups. It was already Fig. 2 a Fragments of FT-IR
spectra of the VP (a), MA (b), NTBA (c) and poly(VP-MA-NTBA) (d). b FT-IR spectra of the chitosan/acrylamide ter-polymer microcapsules Blank, 1, 2, 3 and 4
reported in the literature that in the spectrum of N-tert-butylacrylamide, bands at 3358 cm−1 (secondary amide,
–NH), 2872 cm−1 (C–H stretching of CH
2 and CH3 groups),
1651 cm−1 (stretching of amide carbonyl, –HN–C=O),
and 1566 cm−1 (Amide II, –N–H). The stronger bands at
1651 cm−1 were observed as the ratio of amide increased
in the microcapsules. These observations demonstrated the incorporation of the ter(VP-MA-NTBA) polymer par-ticles into the chitosan matrix. The broad bands in the region of 3700‒3000 cm−1 can be assigned to the O–H and
N–H groups stretching and the bands in between 3000 and 2850 cm−1 can be corresponded to C–H group stretching of
chitosan.
1H NMR Analysis
1HNMR analysis revealed the chemical composition of the
chitosan/acrylamide ter-polymer microcapsules. Character-istic peaks were in 1H NMR spectra in DMSO-d
6 at 50 °C,
ppm: (1) 9H, CH3 1.1 (3) 2H, CH2 1.5 (6) 1H, CH 5.3 for VP
unit [26]; (5) 2H, CH 3.4 for maleic unit; (2) 9H, CH3 1.2 (3) 2H, CH2 1.5 (4) 1H, CH 2.3, and (7) 1H, NH 7.7 for NTBA
unit in Fig. 3 can be identified in the 1H NMR spectra of
the Poly(VP-MA-NTBA) [4]. Figure 3 shows the 1H NMR
spectra of the VP-MA-NTBA ter-polymer.
Scanning Electron Microscopy Images
The SEM images exhibited that blank chitosan beads had smooth and compact surface morphology (Fig. 4). On the
other hand, the chitosan/acrylamide ter-polymer micro-capsules had rough surfaces with randomly distributed pores, indicating the incorporation of ter-(VP-MA-NTBA) polymer particles into the chitosan matrix. As depicted in Fig. 4, the microcapsules were not spherical. In the micro-capsule preparation procedure, chitosan-polymer mixture was dropped into the coagulation solution. Chitosan-pol-ymer blend solution was dense and this led to longer drop time. Therefore, chitosan-polymer microbeads did not have spherical shapes as expected.
Cu(II) Sorption Studies
Effect of Sorbent Dosage on the Sorption of Cu(II)
As presented in Fig. 5, Cu(II) sorption for 5 types of sor-bents reached the plateau at around 0.1000 g for all types of sorbents; further increments in the sorbent dosage did not contribute to the sorption efficiency. At lower dosages, as the amount of chitosan/acrylamide ter-polymer micro-capsules or chitosan microbeads increased, so did the surface area of the sorbents [27]. This, in turn, enhanced Cu(II) sorption to some extent. This phenomenon was also observed in previous studies on interaction of Ni(II), Cd(II) and Cu(II) ions with poly(maleic anhydride-alt-acrylic acid) and the effect Cu(II) ions on poly (N-vinyl-2-pyrrolidone-co-N-isopropylacrylamide) [28, 29]. As seen in the studies on the polymers with anhydride, the drastic increase in the metal sorption at initial phase can Fig. 3 1H NMR spectra of
poly(VP-co-MA-co-NTBA) in DMSO- d 6 at 50 °C
be attributed to the protonation of anhydride groups and the formation of the corresponding ion-exchange medium [30–32].
Effect of pH on the Sorption of Cu(II)
As presented in Fig. 6, Cu(II) sorption capacity of the chi-tosan/acrylamide ter-polymer microcapsules were highly dependent on pH of the metal solution. In acidic medium, the metal cations on the chitosan desorbed, whereas at
more alkaline conditions precipitation of the metal ions as hydroxides occurred. The microcapsules that designated as two exhibited highest sorption capacity at pH 3.18 of all the microcapsules.
Maleic anhydride groups found in ter-(VP-MA-NTBA) polymer can hydrolysis easily; Cu(II) ion sorption occurs through ion-exchange mechanism between the H+ ions
formed through hydrolyses of maleic anhydride groups exchange and the metal ions [30-32]. In the presence of alka-line earth metal ions, anhydride groups of the polymer form Fig. 4 SEM images of Blank, 1,
weak complexes with these ions, but with transition metal ions, Cu(II), anhydride groups form stronger complexes [32]. Ionic potential, q/r (q: ionic charge, r: ionic radius) of Cu(II) ions also plays a role in sorption of Cu(II) ions. Cu(II) ions have small ionic radius and can strongly interact with the chelating groups on the sorbent surface [32].
Biosorption of Cu(II) ions can also occur via surface adsorption, chemical adsorption or complex formation [33]. Higher Cu(II) sorption at high pHs can be ascribed partially to the formation of Cu(OH)+ and Cu(OH)
2. On the other
hand, competition of Cu(II) and H+ ions occurs in acidic
medium; there occurs an electrostatic repulsion between the positively charged surface and the metal cations. Therefore, adsorption in the exchangeable sites weakens and metal ion sorption proceeds through some non- electrostatic forces [29]. In water treatment, one of the key parameters is the pH, and pH range of 4.0–6.0 is the most suitable for treatment. In the Cu(II) sorption experiments by chitosan/acrylamide ter-polymer microcapsules, the maximum sorption efficiency was achieved at pH ~ 5.0, indicating potential performance of the microcapsules in water treatment.
Effect of Metal Ion Initial Concentration on the Sorption of Cu(II)
Cu(II) sorption equilibrium data were evaluated by using Freundlich (Freundlich, 1906) and Langmuir (Langmuir, 1918) isotherm models. The Freundlich isotherm constants were obtained by plotting log qe against log Ce using the
linearized form of the model:
where qe is the mass of Cu(II) ions sorbed by the sorbent (in mg g−1); C
e is the equilibrium metal ion concentration (in
mg L−1); and K
F (mg g−1) and n are the Freundlich constants
denoting the adsorption capacity and the intensity.
The Langmuir isotherm model is expressed in linear form as the following. The adsorption capacity Cm(in mg g−1) and
the energy of adsorption KL (in L mg−1) are calculated from
the slope and intercept of the plot of Ce/qe versus Ce:
where qe is the mass of Cu(II) ions sorbed by the sorbent (in
mg g−1); C
e is the equilibrium concentration of the metal
ions (in mg L−1). Freundlich and Langmuir isotherm models
constants and regression coefficients obtained for the Cu(II) sorption by chitosan/acrylamide ter-polymer microcapsules and blank chitosan beads are given in Table 2.
In solutions with lower metal concentration, metal ion sorption by chitosan occurs through chemisorption, but physisorption is more dominant in the concentrated metal solutions; and sorption data fits better to the Langmuir model. Previous studies reported a stoichiometric relation between the metal ions and the protons released in systems of metal–ligand complexes at stable pH [34, 35]. In this study, blank chitosan beads sorption data fitted to the both isotherm models, whereas all the microcapsules exhibited better fit to the Langmuir model. Cu(II) sorption capaci-ties of the microcapsules and blank chitosan microbeads are listed in Table 3.
(6) log qe= log KF+ (1∕n) log Ce
(7) Ce∕qe= Ce∕Cm+ 1(CmKL) 0 20 40 60 80 100 120 -0.03 0.00 0.03 0.06 0.09 0.12 0.15 0.18 Sorpon (% ) Amount of adsorbent, g Blank 1 2 3 4
Fig. 5 Amount of adsorbent on the sorption of Cu(II) by microcap-sules (initial concentration of Cu(II): 10 mg L−1; pH of the solution:
5.56; temperature: 25 ± 1 °C; shaking speed: 200 rpm)
0 20 40 60 80 100 120 2 3 4 5 6 Sorpon (% ) pH Blank 1 2 3 4
Fig. 6 Effect of pH on the sorption of Cu(II) by microcapsules (initial concentration of Cu(II): 10 mg L−1; contact time: 240 min; amount
of microcapsule: 0.1000 g; temperature: 25 ± 1 °C; shaking speed: 200 rpm)
Effect of Temperature on the Sorption of Cu(II)
Cu(II) sorption studies were repeated at 25, 35 and 45 °C to better understand the effect of temperature on the sorp-tion process. Cu(II) sorpsorp-tion decreased as the temperature increased, this can be explained by the decreasing equilib-rium constants of metal sorption reactions. In the system of Cu(II)-chitosan/acrylamide ter-polymer microcapsules, as the temperature of the system increased, so did cation hydrolysis and electrostatic repulsion between the surface and the metal ions. Enthalpy (∆H°) and entropy (∆Sº) changes in the sorption system were obtained from the plot of logarithm of the distribution coefficient values (KD)
against 1/T. Change in Gibbs free energy (∆G°) was deter-mined by employing the van’t Hoff equation:
where R is universal gas constant (8.314 J mol−1 K−1) and
T is the temperature (K). Table 4 lists the results of Cu(II) sorption thermodynamics parameters. Negative ∆H° values for the sorption of Cu(II) ions were recorded for all the sor-bents, showing that the sorption was exothermic in nature.
(8) logKD= (ΔS◦∕2.303R)−(ΔH◦∕2.303RT)
(9) ΔG◦ = ΔH◦−TΔS◦
∆G° values were found to be negative, indicating that the sorption of Cu(II) ions by chitosan/acrylamide ter-polymer microcapsules was spontaneous. Negative ∆S° value exhib-ited that randomness decreased at the solid/liquid interface in the sorption system.
Conclusions
In this study, Poly(Vp-co-MA-co-NTBA)s are synthesized via the radical polymerization method in the selected condi-tions. The compositions of prepared ter-polymers using a wide range of monomer feed were determined via FT-IR spectra and 1H NMR analysis to evaluate some structural peculiarities
of synthesized ter-polymers. Additionally, four types chitosan microcapsules with
ter-(vinyl-pivalate-maleic-anhydride-N-tert-butylacrylamide) polymer were synthesized and used
in Cu(II) sorption from aqueous solutions. The effects of sorb-ent dosage, pH, initial metal ion concsorb-entration and tempera-ture were discussed. The sorption performance of the micro-capsules was compared to that of blank chitosan microbeads without synthetic polymer. Particles of ter-(VP-MA-NTBA) in the cross-linked chitosan matrix enhanced the affinity of the microcapsules for Cu(II) ions; blank: 67.03, type 1: 98.64, type 2: 75.39, type 3: 95.59 and type 4: 97.52 mg g−1. The
microcapsules with lowest mol ratio of N-tert-butylacryla-mide exhibited the highest sorption capacity for Cu(II) ions. The study revealed that Cu(II) sorption by all the adsorbents was exothermic and spontaneous. The findings demonstrated that chitosan can be used as a natural polymer matrix for microcapsule production with synthetic polymer, ter(VP-MA-NTBA). Chitosan/acrylamide ter-polymer microcapsules has Table 2 Parameters of
Freundlich and Langmuir isotherms for sorption of Cu(II) ion on microcapsules
Cu(II) Freundlich izoterm Langmuir izoterm
k n R2 Q 0 b R2 Blank 5.794 1.550 0.993 0.603 14.360 0.996 1 1.256 22.727 0.835 1.515 15,151.52 0.999 2 1.959 3.546 0.975 0.984 109.3610 0.999 3 1.291 23.809 0.520 1.524 15,243.902 0.999 4 1.019 11.765 0.920 1.302 13,020.830 0.995
Table 3 Cu(II) sorption capacities of the microcapsules and blank chitosan microbeads Adsorpsiyon Kapasitesi Qs, mg/g Blank 1 2 3 4 Cu(II) 67.03 98.64 75.39 95.59 97.52 Table 4 Thermodynamic parameters for the adsorption of Cu(II) on microcapsules Cu(II) ΔHo (J mol−1) ΔS o (J K−1 mol−1) ΔG o (J mol−1) T = 298 K T = 308 K T = 318 K Blank − 6011.55 − 14.24 − 1765.76 − 1623.35 − 1480.95 1 − 25.47 − 49.69 − 10,652.10 − 10,155.20 − 9658.32 2 − 4026.85 − 4.14 − 2793.70 − 2752.34 − 2710.98 3 − 14,945.10 − 24.61 − 7609.02 − 7362.97 − 7116.91 4 − 21.85 − 43.79 − 8791.49 − 8353.57 − 7915.65
a potential in Cu(II) sorption and these composite microcap-sules can be tested in sorption of other metal ions.
Acknowledgements This study presented was supported by Selcuk University Research Foundation (Project Number BAP-13201026). Compliance with Ethical Standards
Conflict of interest The authors declare no conflict of interest.
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