Adsorption of Cd²⁺ and Cu²⁺ ions from aqueous solutions by a cross-linked polysulfonate–carboxylate resin

2.1 Physical methods

An Elemental Analyzer Series II (Perkin Elmer: Model 2400) was used to carry out elemental analysis. IR spectra were obtained using a Perkin Elmer 16F PC FTIR spectrometer. ¹H and ¹³C spectra were measured in a JEOL LA 500 MHz spectrometer with the residual proton resonance of the D₂O (at δ 4.65 ppm) and dioxane carbon resonance (at δ 67.4 ppm) as internal standards, respectively. An Atomic absorption spectroscopy (AAS) model iCE 3000 series (Thermo Scientific) was used to carry out the metal analysis. A TESCAN LYRA 3 (Czech Republic) instrument having an energy-dispersive X-ray spectroscopy (EDX) detector (model: X-Max, Oxford) was used to obtain the scanning electron microscopy (SEM) images. A thermal analyzer (STA 429, Netzsch, Germany) was employed to carry out thermogravimetric analysis (TGA) using a temperature range 20–800 °C (10 °C/min) and air flowing at a rate of 100 mL/min.

2.2 Materials

t-Butylhydroperoxide (TBHP) (70% in water), diallylamine and 1,3-propanesultone from Fluka Chemie AG (Buchs, Switzerland) were utilized.

2.3 Zwitterionic ester 1 and acid 2

The monomer (1) and the corresponding acid (2) were synthesized as reported in Haladu and Ali, (2013).

2.4 1,1,4,4-tetraallylpiperazine-1,4-diium chloride (4)

Monomer 4, a cross-linker, was prepared as described in Ali et al., (1996).

2.5 Copolymerization of 2 and 4

The monomer 2 (13.9 g, 50 mmol) and cross-linker 4 (1.3 g, 4.05 mmol) were dissolved in deionized water followed by the addition of TBHP (375 mg). The mixture in a closed flask was stirred for 24 h at 85 °C. After several hours, the magnetic stir-bar became static. The gel-like polymer was soaked with water followed by hot acetone to obtain resin 5 which was dried in vacuo at 70 °C (8.75 g, 62.2%).

2.6 Transformation of CPZ 5 to cross-linked polyzwitterion/anion (CPZA) 6

CPZ 5 (7.35 g, 26.1 mmol) mixed with sodium hydroxide (1.36 g, 34 mmol) in water (33 ml) was stirred for 24 h. After adding excess methanol, resin 6 was filtered and then dried in vacuo at 65 °C (6.8 g, 87%). At 280 °C, initial decomposition of the resin was shown by TGA.

2.7 Adsorption experiments

CPZA 6 (50 mg) was mixed with aqueous Cd(NO₃)₂ (1 mg Cd L⁻¹) solution (20 mL) at different pH and stirred for 24 h. The amount of Cd²⁺ remained in the filtrate was measured by AAS. Using Eq. (1), the adsorption capacity ( $q_{{\mathit{Cd}}^{2+}}$ ) in mg g⁻¹ was calculated as follows:

Evaluation of the adsorption isotherm was accomplished in the Cd²⁺ concentration range 200 μg L⁻¹ (i.e. ppb) to 1000 μg L⁻¹ for 24 h at 294 K. The adsorption kinetic experiments were carried out using Cd(NO₃)₂ solution (1 mg Cd²⁺ L⁻¹) at different times at a pH of 3 at 294, 308 and 328 K. The data obtained were used to calculate k₂ (the pseudo-second-order rate constant) of the adsorption process and its energy of activation (E_a). The thermodynamic parameters, ΔG, ΔH and ΔS were determined using the q_e and C_e values. In the same way, the adsorption of Cu²⁺ ions using Cu(NO₃)₂ solution was conducted.

2.8 FTIR spectroscopy

FTIR spectra of the loaded and unloaded resins have been recorded. About 30 mg of the unloaded resin in 0.1 M Cd(NO₃)₂ solution (20 mL) at pH of 3 was stirred for 4 h, filtered, and dried under reduced pressure to a constant weight.

3.1 Synthesis of CPZA 6

The present resin 6 (Scheme 1) has been prepared by cyclopolymerization reaction. The cyclopolymerization involving various N,N-diallylammonium monomers has generated various water-soluble cationic polyelectrolytes (Butler, 1992). Recently, a resin having aminophosphonate groups has been synthesized via cyclopolymerization and evaluated for its efficacy in the removal of toxic metal ions (Al Hamouz and Ali, 2012).

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Scheme 1

Synthesis of monomer and cross-linked polymers.

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A mixture of monomer 2 (92.5 mol%), and cross-linker 4 (7.5 mol%) in water was cyclocopolymerized using TBHP as an initiator giving CPZ 5 (Scheme 1). Upon treatment of the CPZ 5 with excess NaOH, CPZA 6 was formed. A mole ratio of ≈93:7 for monomer 2/cross-linker 4 as determined by elemental analysis of CPZ 5 was similar to the feed ratio.

Three major weight losses were revealed in the TGA curve of CPZA 6 (Fig. 1). The first slow weight loss of 3.0% resulted from the release of water trapped in the cross-linked polymer. The first sharp loss of 22.4% resulted from the release of SO₂, the second loss of 28.9% and the third loss of 7.7% were associated with the respective decarboxylation of the carboxylates and the degradation of the nitrogenated organic fraction (Martinez-tapia et al., 2000). At 800 °C, the residual mass was found to be 38%.

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Figure 1

TGA curve of CPZA 6.

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3.2 Characterization of synthesized materials using FTIR

The IR spectrum of CPZ 5 shows strong absorption bands at 1197 and 1041, which are attributed to the sulfonate groups (Ali et al., 2003). The absorption at 1734 cm⁻¹ indicates the presence of C⚌O stretch of COOH (Al-Muallem et al., 2002b) (Fig. 2a).

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Figure 2

IR spectra of (a) cross-linked CPZ 5, (b) CPZA 6, (c) Cd²⁺ loaded CPZA 6, (d) Cu²⁺ loaded CPZA 6.

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The C⚌O (of COOH) stretching absorption was missing in the unloaded resin CPZA 6; however, the absorptions at 1408 and 1626 cm⁻¹ were attributed to the symmetric and antisymmetric stretching of COO⁻ (Fig. 2b). The new band that appears at 1384 cm⁻¹ (Fig. 2c and d) was assigned to the NO₃⁻ group due to the adsorption process being carried out using copper and cadmium nitrates (Sahni et al., 1985). Thus, the resin can also act as an anion exchanger due to the presence of this new band. The increased intensity and broadness of the SO₃⁻ and C⚌O peaks of the resin loaded with Cu²⁺ and Cd²⁺ (Fig. 2c and d) indicated the adsorption of the metal ions (Kolodynska et al., 2009).

3.3 Adsorption kinetics

The kinetics for the removal of Cu²⁺ and Cd²⁺on CPZA 6 is presented in Figs. 3d and 4d, which depicts the changes in adsorption capacity with adsorption time at different temperatures. The adsorption process was relatively fast as it reached the equilibrium within 40 min. At higher temperatures, the adsorption capacities increased indicating larger swelling allowing more ions to be diffused and adsorbed on CPZA 6.

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Figure 3

The adsorption of Cd²⁺ (1.00 ppm) on CPZA 6 at pH 3 fitted to (a) Lagergren first-order kinetic model; (b) Lagergren second-order kinetic model; (c) intraparticle diffusion model; and (d) adsorption kinetic curve.

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Figure 4

The adsorption of Cu²⁺ (1.00 ppm) on CPZA 6 at pH 3 fitted to (a) Lagergren first-order kinetic model for; (b) Lagergren second-order kinetic model; (c) intraparticle diffusion model; and (d) adsorption kinetic curve.

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3.3.3

3.3.3 Adsorption activation energy

The kinetic rate constants (k₂) model was used to calculate the adsorption energy of activation (Table 1) using the Arrhenius equation (Eq. (4)):

(4)

$$\ln k_2=-\frac{E_a}{2.303\mathit{RT}}+\text{constant}$$ where k₂ is the rate constant (g mg⁻¹ h), and R, E_a and T represent the gas constant (8.314 J mol⁻¹ K), energy of activation (kJ mol⁻¹) and temperature (K), respectively. A straight line plot of lnk₂ versus 1/T gives a slope of −E_a/R (Fig. 5 and Table 1). The energies of activation for the adsorption of Cd²⁺ and Cu²⁺ were found to be 25.5 kJ mol⁻¹ and 37.3 kJ mol⁻¹ respectively.

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Figure 5

The Arrhenius plot for the metal adsorptions.

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3.4 Effect of pH on the adsorption

The pH effect (in the range 3–6) on the adsorption of Cd²⁺ and Cu²⁺ on CPZA 6 was studied at 21 °C and a fixed concentration (1 mg L⁻¹) for 24 h. The solution pH was maintained using a buffer of acetic acid/sodium acetate. The adsorption of metal ions with the variation of pH is displayed in Fig. 6a. The best pH was observed to be 3; the adsorption capacity was found to increase with further increase in the pH, after the minimum observed at pH 5. Carboxyl and sulfonate groups may capture Cd²⁺ and Cu²⁺ ions by chelation (Scheme 2). The respective pK_a values of SO₃H and CO₂H functionalities in CPZA 6 are known to be −2.1 and +2.5 (Al-Muallem et al., 2002b). As such the repeating unit A is expected to be dissociated to zwitterionic form B even at lower pH values. At pH 3 or even at 2, the acid-base equilibrium, as represented by the equilibrium B ⇌ C will involve a certain portion of CO₂⁻ (Zhao et al., 2011) while the SO₃H will exist in the conjugate base form of SO₃⁻. Lower adsorption of Cd²⁺ and Cu²⁺ at pH < 3 could be attributed to the competition of H⁺ ions with Cd²⁺ and Cu²⁺ ions for the exchange sites in the adsorbent to give metal-ion complex D. Above pH 3 the involvement of zwitterionic/anionic form D makes the adsorption faster and more effective. Note that the presentation of the metal complex in the cyclic form may not be the preferred chelation mode; the two ligands attached to the divalent metal ions may as well come from two different repeat units. At lower pH values species such as monovalent M(Cl)⁺ may also participate in the adsorption process (Li et al., 2010; Duong et al., 2006). The metal hydroxides, which are formed owing to the hydrolysis of the metal ions at higher pH values, can compete with the resin for the uptake the metal ion (Haghseresht and Lu, 1998). The optimal pH of 3 was chosen for the adsorption studies.

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Figure 6

(a) The pH versus adsorption capacity of CPZA for1.00 ppm Cd²⁺ and Cu²⁺ solutions for 24 h; (b) The initial concentration of Cd²⁺ versus adsorption capacity of CPZA 6 at pH 3 for 24 h at 21 °C; (c) Temkin isotherm for Cd²⁺ adsorption on CPZA 6; (d) Freundlich isotherm for Cd²⁺ adsorption on CPZA 6.

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Scheme 2

Metal–resin complex.

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3.5 Adsorption isotherms

The adsorption capacity of CPZA 6 increases as the initial concentration of Cu²⁺ and Cd²⁺ ions increases, Figs. 6b and 7a. The mechanism of the adsorption was investigated by the analysis of the data using Freundlich, Langmuir and Temkin isotherm models. The Langmuir isotherm model (Vijayaraghavan et al., 2006), based on a monolayer adsorption process on homogeneous surfaces where a negligible interaction occurs between the adsorbed molecules, can be expressed by Eq. (7) as follows:

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Figure 7

(a) The initial concentration of Cu²⁺ versus adsorption capacity of CPZA 6 at pH 3 for 24 h at 21 °C; (b) Temkin isotherm for Cu²⁺ adsorption on CPZA 6; (c) Freundlich isotherm for Cu²⁺ adsorption on CPZA 6.

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However, Freundlich isotherm model (as expressed by Eqs. (9) and (10) assumes that a heterogeneous surface with uniform energy is involved with a multilayer non-ideal adsorption:

The Temkin isotherm equation assumes a linear decrease in the heat of adsorption of all the molecules in a layer with the increase in the surface coverage owing to the adsorbent–adsorbate interactions. The Temkin isotherm (Liu et al., 2011) can be expressed as Eqs (10)–(12) as follows:

3.6 Adsorption thermodynamics

The thermodynamic parameters of the adsorption were also determined. The endothermic nature of the adsorption was ascertained by the increase in the adsorption capacity as the temperature increases. A log (q_e/C_e) versus 1/T plot is shown in Fig. 8. Vant-Hoff equation (Eq. (12)) was employed to calculate the thermodynamic parameters ΔS, ΔH and ΔG that are tabulated in Table 4 (Ramesh et al., 2007). The negative values of ΔG confirmed the spontaneity of the exchange process.

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Figure 8

Vant-Hoff plot.

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3.7 SEM and EDX images for CPZA 6 unloaded and loaded with copper and cadmium ions

SEM has been used to examine both the unloaded and loaded resins. For this purpose, the unloaded resins were stirred in 0.1 M Cu(NO₃)₂ and 0.1 M Cd(NO₃)₂ at a pH of 3 for 24 h. The resins were filtered, and dried in vacuo to constant weights. Both resins were then sputter-coated for 4 min with a thin film of carbon.

The SEM image of the unloaded CPZ 5 and CPZA 6 (Fig. 9a and b) reveals that the surface morphology of CPZ 5 is tighter due to the H-bonding among the COOH groups, while the open morphology of CPZA 6 confirms the presence of Na⁺ due to the repulsion between the negative charges of the CO₂⁻. The corresponding EDX analysis shows that the composition was similar to the proposed one in Scheme 1.

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Figure 9

SEM and EDX images for (a) Unloaded CPZ 5 and (b) Unloaded CPZA 6.

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Due to the change in morphology of the SEM images (Figs. 9 and 10) from cracked to smooth, it indicates that the adsorption of cadmium and copper ions had occurred on the resin. Likewise, the displacement of the sodium ions in CPZA 6 by cadmium and copper ions, Fig 10a and b confirmed the adsorption of the metal ions.

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Figure 10

SEM and EDX images for (a) Cd²⁺ loaded CPZA 6 and (b) Cu²⁺ loaded CPZA 6.

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3.8 Desorption experiment

Efficient use of any adsorbent demands recycling and reuse of the adsorbent. For this purpose, an experiment was performed as described under Section 2.7 by mixing CPZA 6 (50 mg) with aqueous Cd(NO₃)₂ (or Cu(NO₃)₂) (1 mg metal ion L⁻¹) solution (20 mL) for 24 h. The loaded resin was then recovered by centrifuging, soaked briefly with water (5 mL) and centrifuged again. The recovered resin was dried and treated with 0.1 M HNO₃ (20 mL) for 24 h for the desorption experiment. The amount of Cd²⁺ (or Cu²⁺) ions desorbed in the filtrate was determined; the efficiency of the desorption process was calculated by the ratio of desorbed amount of Cd²⁺ (or Cu²⁺) ions to the amount of adsorbed Cd²⁺ or Cu²⁺ ions (i.e., $q_{{\mathit{Cd}}^{2+}}$ or $q_{{\mathit{Cu}}^{2+}}$ ) in the resin.

The efficiencies of the desorption process, determined to be 87% and 89% for Cd²⁺ and Cu²⁺ ions, respectively, certify that the novel cross-linked polymer used in this work is a promising candidate as an adsorbent for separation and recovery of metal ions from an aqueous solution.