Adsorption of Ni(II) from Aqueous Solution by Polyaminated Crosslinked Ni(II)-Imprinted Chitosan Derivative Beads

Abstract

Polyaminated crosslinked Ni(II)-imprinted chitosan derivative [P-C-CTS (Ni)] beads were first synthesized by molecular imprinting, crosslink reactions, and polyamination methods. Structure, size, and shape, and doping level of the adsorbents were analyzed in detail by X-ray photoelectron spectroscopy data, scanning electron microscope images, and Fourier transform infrared spectra. Study of Ni(II) adsorption on the prepared P-C-CTS (Ni) beads gave evidence that chemisorption was the main sorption mechanism and adsorption kinetics were fitted with a pseudo-second-order model. The adsorption capacity of 2.746 mmol/g for Ni(II), which was 2.6 times higher compared with unpolyaminated beads, was achieved by using the prepared P-C-CTS (Ni) beads. According to the results of Ni(II) adsorption quantities with different Ni(II)/citrate ratios (1:2, 1:1, 2:1) and pH (2.0, 3.0, 4.0, 5.0, 6.0), the effects of the pH and citrate ligands on Ni(II) adsorption quantities and the micromechanism of adsorption were discussed. The adsorption capability of the prepared P-C-CTS (Ni) beads was stable up to eight adsorption–desorption–regeneration cycles.

Introduction

The heavy metals, as an important group of persistent toxic substances, have attracted a great deal of scientific and public attention during the past decades due to their occurrence in environmental waters and their potential adverse effects on the ecosystem (Luo et al., 2012; Saeedi et al., 2012). Nickel, one of the most common toxic heavy metals, occurred mainly as Ni(II) ions in wastewaters from mining, electroplating, battery and electronic industries (Cempel and Nikel, 2006). Owing to incomplete removal in industrial wastewater, nickel metals may enter into the environment and further pose a threat to human health, such as skin allergy, lung fibrosis, and allergic dermatitis, and increase incidence of cancers (Kasprzak et al., 2003; Brouwere et al., 2012). Thus, it is necessary to develop effective technologies for treating the wastewater that reduce nickel metal ion concentrations to environmentally acceptable levels. A number of methods for removing heavy metals from industrial effluents, such as chemical precipitation, coagulation, solvent extraction, electrodeposition, membrane separation, adsorption, and ion exchange, have been reported (Dizge et al., 2009; Fu and Wang, 2011). Of those, the adsorption method is quite popular recently due to its simplicity and high efficiency, especially when heavy metal exists at lower concentrations in an aqueous solution. Activated carbon was the most widely used adsorbent, but its high cost and loss during regeneration limit its applications (Ahn et al., 2009; Lo et al., 2012). Many industrial by-products and agricultural wastes, such as waste pomace of olive oil factory and cashew nut shells, have been recycled and developed as low-cost alternative adsorbents of activated carbon for Ni(II) removal from wastewater systems (Nuhoglu and Malkoc, 2009; Kumar et al., 2011). However, the stability in complicated industrial wastewaters of those materials restricts their usage.

Recently, chitin derivatives, as a kind of natural biopolymer adsorbents, have been received with considerable attention due to their advantages of low operating cost, high adsorption efficiency, easy regeneration, wide availability, and minimization of the volume of chemicals and sludge to be disposed off during the metal adsorption process (Gerente et al., 2007; Kolodynska, 2012). In addition, another attractive feature of chitin derivatives is that they possess a number of different functional groups, such as hydroxyls and amines, to which metal ions can bind either by chemical or by physical adsorption (Deans and Dixon, 1992). Chitosan (CTS), one of the most common derivatives of natural chitin biopolymers, has been employed for a broad variety of industrial and biomedical applications, especially as a chelating agent (Guibal et al., 1998). However, CTS cannot stand long use due to its poor mechanical stability and its biodegradability when used as adsorbents. Various physical and chemical modifications have been extensively studied to improve the chemical stability of bioadsorbents in acid media and in its resistance to biochemical and microbiological degradation (Wan Ngah et al., 2002). On the other hand, Yoshihide (1998) has reported that the adsorption capability for recovery of heavy metals from the municipal waste incineration plant was enhanced by highly polyaminated CTS beads. Moreover, molecular imprinting, a technique to create template-shaped cavities in polymer matrices with memory of the template molecules to be used in molecular recognition, has been recently applied for preparing the adsorbents with high affinity and selectivity (Xie et al., 2011). However, to the author's best knowledge, little information on the modification of CTS materials using the combined techniques of molecular imprinting, crosslinking, and polyamination for nickel removal from aqueous solutions has been reported up to date.

The aims of this study were to develop an inexpensive, stable, and effective CTS-based adsorbent using molecular imprinting, crosslink reactions, and polyamination methods, to characterize the prepared adsorbents using the Fourier transform infrared (FT-IR), scanning electron microscope (SEM), and X-ray photoelectron spectroscopy (XPS) techniques, to investigate the effects of various experimental parameters, such as solution pH, contact time, humic substance of citrate, and temperature, on the adsorption behaviors toward nickel(II) ions with the developed polyaminated crosslinked Ni(II)-imprinted CTS derivative [P-C-CTS (Ni)] adsorbents, and to elucidate the adsorption kinetics and the possible adsorption mechanism.

Materials and Methods

Chemicals and materials

CTS powder was purchased from Sanland Chemical Reagent Co., Ltd. Sodium hydroxide (NaOH), epichlorohydrin (ECH), and tetraethylenepentamine (TEPA) were obtained from Guangfu Finechemicals Co., Ltd. Acetic acid, H₂SO₄, and NiSO₄·6H₂O were supplied by Tianjin Chemistry Reagent Factory. Except where noted, all reagents were of the reagent grade and all solution preparations were made using double distilled deionized water.

Synthesis of P-C-CTS (Ni) adsorbents

Thirty grams of CTS powder and 3.35 g of NiSO₄·6H₂O were dissolved in 1 L of 4% acetic acid solution followed by stirring vigorously for 12 h. The CTS-Ni beads with diameters of 2.0 mm were formed by adding the CTS-Ni-acetic acid solution into the 10% NaOH solution drop by drop. The neutral CTS-Ni beads were rinsed with deionized water. The crosslinked CTS-Ni (C-CTS-Ni) beads were prepared by immerging 10.0 g of the CTS-Ni beads into 50 mL of aqueous solution containing 3% (v/v) of ECH at 80°C for 5 h. Then, the C-CTS-Ni beads were polyaminated in a mixture solution of 30 mL water and 30 mL TEPA at 80°C for 5 h. Finally, the Ni²⁺-imprinted CTS [P-C-CTS (Ni)] derivative beads were obtained by desorbing Ni²⁺ from the polyaminated C-CTS-Ni (P-C-CTS-Ni) beads in a dilute H₂SO₄ solution via oscillation.

Characterization methods

The XPS spectra were recorded using a Kratos Axis Ultra DLD spectrometer (Shimazu-Kratos) equipped with a monochromated Al-Kα x-ray source (hv=1486.6 eV), hybrid (magnetic/electrostatic) optics, as well as a multichannel plate and a delay line detector. The surface topographies of the adsorbent beads were characterized with a SS-550 SEM (Shimadzu) at an accelerating voltage of 10 kV. The FT-IR spectra were recorded by using a Nicolet MAGNA-560 FT-IR spectrometer (Thermo Fisher Scientific).

Adsorption–desorption–regeneration experiments

Five grams of CTS, CTS-Ni, C-CTS-Ni, P-C-CTS-Ni, and P-C-CTS (Ni) beads were separately oscillated in 25.0 mL of 50 mmol/L Ni²⁺ solution at room temperature for 24 h. The adsorbents were then removed by filtration through a membrane filter (0.45 μm; Millipore) and the Ni²⁺ in the filtrates was determined using butane dioxime spectrometry. A blank experiment was also performed under the same conditions. The adsorption capacity (Q) and moisture content (W) of the as-prepared beads were determined based on the following equations, respectively: \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland,xspace}\usepackage{amsmath,amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6} \begin{document} \begin{align*} { Q } ( { Q } _t ) = \frac { ( C_0 - C_e ) \times 0.025 } { m_0 \times ( 1 - W ) } \tag { 1 } \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland,xspace}\usepackage{amsmath,amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6} \begin{document} \begin{align*} W = 100 \times \frac { m_0 - m_1 } { m_0 } \tag { 2 } \end{align*} \end{document}

where Q (mmol/g) is the adsorption capacity of the as-prepared adsorbents, Q_t (mmol/g) is the adsorbed amount at t hour(s), W (%) is the moisture content of the adsorbents, C₀ (mmol/L) and C_e (mmol/L) are the initial and equilibrated Ni²⁺ concentrations, respectively. The m₀ and m₁ are the masses of the adsorbents before and after drying at 50°C, respectively. All experimental data were recorded based on the averages of triplicate measurements.

The adsorbed Ni²⁺ on the P-C-CTS (Ni) beads was desorbed by immerging the beads in 100 mL of 0.1 mol/L H₂SO₄ solution at room temperature for 24 h under the oscillation condition. The P-C-CTS (Ni) beads were then rinsed until no Ni²⁺ was detected in the rinse water. After placing in a NaOH solution (0.4 mol/L) for 4 h, the beads were rinsed and used for adsorption of Ni²⁺ in the next cycle. The adsorption–desorption procedure was repeated for eight cycles. The regeneration efficiency and reutilization capability of the adsorbents were investigated.

Effects of solution pH and citrate on nickel adsorption

The process of metal ions adsorption onto the adsorbents is significantly related with the metal and coexisting ligand species in solution (Deepatana and Valix, 2006). As we well know, the composition and pH of the aqueous solution have significant influence on the speciation of metal ions in solution. Hydrolysis of metal ions or chelation with other ligands present in solution may change the mechanism of metal ion adsorption onto the adsorbent beads. On the other hand, the H⁺ itself has significant effects on the adsorption behaviors of the adsorbents. In an industrial process, such as nickel plating, citrate acid was one of the most widely used ligands. Thus, investigation of the effects of pH and citrate on the Ni (II) adsorption onto the prepared adsorbents is necessary to reasonably elucidate the adsorption mechanism.

To study the effects of solution pH and citrate on the absorption of Ni²⁺ onto the P-C-CTS (Ni) beads, the absorbents were placed in solutions containing Ni(II) and citrate with the molar ratios of 1:1, 1:2, and 2:1, respectively. And at each molar ratio, a series of test aqueous solutions at pH 2.0, 3.0, 4.0, 5.0, and 6.0, were prepared. The pH was adjusted by the NaOH or H₂SO₄ diluted solution and measured by pH meter. After 24 h, the Ni(II) and citrate (designated as [Cit]) concentrations in solutions were determined, and the Ni(II) adsorption quantities onto the P-C-CTS (Ni) absorbents were calculated based on Equation (1).

Removal of Ni(II) in real waters using the P-C-CTS (Ni) adsorbents

To demonstrate the application of the prepared P-C-CTS-(Ni) beads in real water samples, the industrial wastewater samples containing Ni(II) were collected from two electroplating factories in Tianjin and the environmental water, including lake and river water, collected from the Nankai University. The adsorption experiments were carried out by monitoring the Ni(II) concentration change in 100 mL of collected wastewaters and spiked environmental waters with 20 mg/L of Ni(II) at room temperature since the Ni(II) concentrations were low in lake and river waters. After 24-h oscillation, the adsorbents were then removed by filtration through a membrane filter (0.45 μm; Millipore). The concentrations of Ni²⁺ in solution before and after the adsorption experiment were determined.

Results and Discussion

Adsorption capacities and moisture content

The moisture content and Ni²⁺ adsorption capacities of the as-prepared adsorbents are listed in Table 1. The order of the moisture content in the adsorbent beads (CTS>CTS-Ni>C-CTS-Ni>P-C-CTS-Ni), due to the increasing of the molecular weight, indicated that the crosslink reaction between CTS-Ni and ECH and the polyamination between C-CTS-Ni and TEPA had taken place under the current experimental conditions. Meanwhile, the increase of the moisture in P-C-CTS (Ni) beads was attributed to the decreased molecular weight after the Ni²⁺ desorption from the P-C-CTS-Ni beads. On the other hand, the Ni²⁺ adsorption capacity of the CTS-Ni was less compared with the CTS since the reactive amino groups (−H₂N:) are efficient sites for Ni²⁺ adsorption and some of the −H₂N: groups in CTS-Ni were occupied by Ni²⁺. The absolute decrease in the −H₂N: group number and the absolute increase in molecular weight contributed to the relative decrease in −H₂N: groups per unit mass of CTS-Ni. For the same reason, the number of −H₂N: groups per unit mass of C-CTS-Ni decreased as a result of the increase in molecular weight after the crosslink reaction. Therefore, the value of the Q_C-CTS-Ni was smaller compared with the Q_CTS-Ni. The results that the Q_P-C-CTS-Ni was evidently greater than the Q_C-CTS-Ni indicated that the Ni²⁺-imprinted technology played an important role in improving the adsorption capacity for the prepared P-C-CTS (Ni) adsorbents. The greater Q_P-C-CTS(Ni) value obtained with ψ of 25 mg/g than that with ψ of 5 mg/g illustrated that more Ni²⁺ content in CTS and more −H₂N: groups protected in the crosslink reaction.

Table 1.

Adsorption Capacities for Ni ²⁺ and Moisture Content of the Adsorbents

Adsorbents	CTS	CTS-Ni	C-CTS-Ni	P-C-CTS-Ni	P-C-CTS (Ni)
ψ^a (mg/g)		25	25	25	5^b	25
Moisture contents W (%)	94.7	92.1	86.6	80.3	82.6	82.8
Adsorption capacities Q (mmol/g)	2.47	1.95	1.07	2.58	2.62	2.75

The Ni²⁺ content in CTS was designated as ψ mg/g.

The 5 mg/g Ni²⁺/CTS was obtained by changing the mass of NiSO₄·6H₂O to 6.70 g.

P-C-CTS (Ni), polyaminated crosslinked Ni(II)-imprinted chitosan derivative.

The results of the Ni²⁺ adsorption capacities and moisture content of the as-prepared adsorbents confirmed that the new polyamination method and Ni²⁺-imprinted technology were suitable for P-C-CTS (Ni) synthesis. The maximum adsorption capacity of the P-C-CTS (Ni) was 2.75 mmol/g, which is 2.6 times higher compared with the unpolyaminated beads. Magnetic CTS microspheres and the crosslinked magnetic CTS-isatin Schiff's base resin were prepared for adsorption of Ni²⁺ (Zhou et al., 2009; Monier et al., 2010). The maximum adsorption capacities were 0.026 and 0.68 mmol/g, respectively, according to the best interpretation of the equilibrium data given by Langmuir isotherm. Compared to these previous studies, the polyaminated crosslinked Ni(II)-imprinted CTS derivative [P-C-CTS (Ni)] beads synthesized in the work possessed a higher Ni²⁺ adsorption capacity.

The P-C-CTS (Ni) beads prepared with ψ of 25 mg/g were used to carry out the following experiments, including the characterization and Ni²⁺ adsorption behaviors.

Characterization of the P-C-CTS (Ni) beads

The FT-IR spectra of CTS, CTS-Ni, C-CTS-Ni, and P-C-CTS (Ni) are shown in Fig. 1. The broad and strong peak at 3400–3200 cm⁻¹ represents the overlapping peaks of the O–H and N–H groups. The peak at 1071 cm⁻¹ in Fig. 1c is the characteristic peak of the C−O−C group. The stronger peaks at 2930 and 2880 cm⁻¹ correspond to C–H in the methyl and methylene groups. The peak at 1599 cm⁻¹ represents as the −H₂N: groups. The peak changes and shifts at 1599 and 3415 cm⁻¹ indicate that most of the −H₂N: groups have been chelated with the Ni²⁺ in CTS-Ni. The enhanced peak of the C−O−C groups at 1071 cm⁻¹ in C-CTS-Ni illustrates that the crosslink reaction occurred between CTS-Ni and ECH and the new groups (C−O−C) were introduced in C-CTS-Ni. Meanwhile, the peak at 3423 cm⁻¹ becomes narrower, indicating that the −OH groups in the galactose residues of CTS were involved in the crosslink reactions. Compared with the C-CTS-Ni spectra, the adsorption peak of C–N in the fatty amine appears at 1230–1030 cm⁻¹, as shown in Fig. 1d. It indicates that the TEPA molecules were introduced into the P-C-CTS (Ni) beads. The adsorption peak observed at 3423 cm⁻¹ in Fig. 1c was shifted back to 3415 cm⁻¹ in Fig. 1d indicating that the Ni²⁺ is no longer present in the P-C-CTS (Ni).

FIG. 1.

Fourier transform infrared spectra of adsorbents: (a) CTS; (b) CTS-Ni; (c) C-CTS-Ni; (d) P-C-CTS(Ni). P-C-CTS (Ni), polyaminated crosslinked Ni(II)-imprinted chitosan derivative.

The XPS spectra of NiSO₄·6H₂O, CTS, CTS-Ni, and P-C-CTS (Ni), as shown in Table 2, illustrated that the C_1s photoelectron spectrum peaks of CTS, CTS-Ni, and P-C-CTS (Ni) beads had no difference. The chemical state of the C element in these adsorbents did not change during the above-mentioned chemical reactions. All results of the XPS spectra indicated that a portion of Ni chelated with −H₂N: groups in CTS-Ni through the isolated electrons of the nitrogen atom. This chelation resulted in a decreased electron cloud thickness around the nitrogen atoms, which contributed to the higher shift in the nitrogen atom binding energy. Moreover, the increased electron cloud thickness around the nickel atoms contributed to the lower shift in binding energy. No nickel photoelectron spectrum peak was observed from 850 to 890 eV. It confirmed the absence of Ni²⁺ in P-C-CTS (Ni), which was concluded from the FT-IR data. Meanwhile, two N_1s photoelectron spectrum peaks in P-C-CTS (Ni) were observed at the binding energies of 399.2 and 400.2 eV. The former peak corresponds to the protected −H₂N: groups in CTS by Ni²⁺ in the crosslink reactions, and the corresponding spectrum peak shifted back to 399.2 eV after desorption of Ni²⁺. The latter peak corresponds to the free −H₂N: groups in CTS.

Table 2.

X-Ray Photoelectron Spectroscopy Data for NiSO₄ċ6H₂O, CTS, CTS-Ni, and P-C-CTS (Ni)

Samples	C_1s			N_1S		Ni_2p1/2		Ni_2p3/2
NiSO₄·6H₂O	—			—		876.0^a		857.8
CTS	286.7	286.2	284.6	399.2		—		—
CTS-Ni	286.7	286.2	284.7	399.2	400.9	872.9	876.0	855.4	857.8
P-C-CTS (Ni)	286.7	286.2	284.7	399.1	400.2	—		—

Unit: eV.

The prepared P-C-CTS (Ni) beads were dried at 50°C in vacuum until their weight kept stable. The SEM photographs of the appearance and internal cross section of P-C-CTS (Ni) are shown in Fig. 2. As illustrated in Fig. 2a, the dried P-C-CTS (Ni) beads are still spheroids with a uniform diameter of 1.0 mm. The mesoporous structures, which have significant effects on the adsorption performance of the adsorbents, were observed on the prepared P-C-CTS (Ni) beads, as presented in Fig. 2b. The data of the FT-IR, XPS, and SEM spectra indicated that the P-C-CTS (Ni) beads could be successfully prepared using the described procedures.

FIG. 2.

Scanning electron microscope photographs of the P-C-CTS(Ni): (a) appearances and (b) internal cross section.

Adsorption kinetics

The P-C-CTS (Ni) beads were immerged into 25.0 mL of 100.0 mg/L Ni²⁺ aqueous solution with oscillations at 30°C, 40°C, and 50°C, respectively. The adsorption kinetics was investigated by the time course study of the absorbed Ni²⁺ (Q_t), which was determined based on Equation (1), in P-C-CTS (Ni) beads at 5, 10, 20, 30, 40, 50, 60, 90, 120, 240, 360, and 480 min.

As shown in Fig. 3, the curves illustrated the process of Ni²⁺ adsorption in P-C-CTS (Ni) beads at different temperatures. The higher the temperature, faster the Ni²⁺ adsorption, and greater the adsorption capacity for the prepared P-C-CTS (Ni) beads. All results supported that the adsorption mechanism for Ni²⁺ on the P-C-CTS (Ni) beads is chemisorption. Several −H₂N: groups, such as those employed onto the surface of adsorbents via polyamination, supplied the active sites for the adsorption Ni²⁺ through chelation. If the Ni²⁺ monolayer adsorption saturation was presumably achieved at the temporary balance stage, three fitting lines were obtained (Fig. 4) based on the kinetic Equation (3): \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland,xspace}\usepackage{amsmath,amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6} \begin{document} \begin{align*}t / {Q}_t = t / {Q}_e + 1 / k{Q}_e \tag{3}\end{align*} \end{document}

FIG. 3.

Time course of adsorbed Ni(II) in P-C-CTS (Ni) beads at different temperatures. (a) Time from 0 min to 480 min; (b) time from 0 min to 90 min.

FIG. 4.

Kinetic curves of Ni(II) adsorption by P-C-CTS (Ni) beads at different temperatures.

where Q_t (mg/g) is the adsorption quantity at t hour, k is the adsorption rate, and Q_e (mg/g) is the adsorption capacity of the adsorbent. k and Q_e can be determined based on the slopes and intercepts of the lines.

The Arrhenius equation (k=Ae^-Eα/RT) was used to obtain lgk=−E_a/2.303 RT+lgA, although a nonlinear relationship exists between lgk and 1/T. This nonlinear relationship between lgk and 1/T indicates that effects of the temperature on the adsorption rates do not follow the Arrhenius equation. Pseudo-first-order, pseudo-second-order, and internal diffusion kinetic models were applied to interpret adsorption dynamics to investigate the mechanism of adsorption kinetics. These kinetic models are given as the following equations.

Pseudo-first-order kinetic model: \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland,xspace}\usepackage{amsmath,amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6} \begin{document} \begin{align*}{ \rm lg} ( {Q}_e - {Q}_t ) = { \rm lg}{Q}_e - k_1 t / 2.303 \tag{4}\end{align*} \end{document}

Pseudo-second-order kinetic model: \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland,xspace}\usepackage{amsmath,amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6} \begin{document} \begin{align*}t / {Q}_t = 1 / ( k_2 {Q}_e^2 ) + t / { \cal Q}_e \tag{5}\end{align*} \end{document}

Internal diffusion model: \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland,xspace}\usepackage{amsmath,amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6} \begin{document} \begin{align*}{Q}_t = k_i t^{0.5} \tag{6}\end{align*} \end{document}

where k₁ is the pseudo-first-order adsorption rate constant, k₂ is the pseudo-second-order adsorption rate constant, k_i is the internal diffusion rate constant, and r₁², r₂², and r₃² are the corresponding correlation coefficients. To fit the adsorption kinetics curve at 30°C to the pseudo-first-order, pseudo-second-order, and internal diffusion kinetic models, the kinetic parameters for Ni²⁺ adsorption on P-C-CTS (Ni) were k₁ (min⁻¹)=5.9×10⁻² (r₁²=0.9697), k₂ (g/[mg·min])=8.1×10⁻³ (r₂²=0.9968), and k_i (mg/g·min^−0.5)=6.8 (r₃²=0.7501). Based on the correlation coefficients, the adsorption behaviors of P-C-CTS (Ni) for Ni²⁺ fitted well with the pseudo-second-order kinetic model. It confirmed that chemisorption was the mechanism of Ni²⁺ adsorption onto the P-C-CTS (Ni) adsorbents.

To verify that chemisorption was the mechanism of Ni²⁺ adsorption onto the P-C-CTS (Ni) adsorbents, the thermodynamic behavior of the adsorption of Ni²⁺ though the change in free energy (ΔG°), enthalpy (ΔH°), and enthopy (ΔS°) were evaluated. As shown in Table 3, the positive values of ΔH° for Ni²⁺ suggest an endothermic nature of adsorption. This is also supported by the increase of adsorption capacity with temperature increasing. The positive value of ΔS° showed an increase in randomness at the solid/solution interface during the adsorption of Ni²⁺. Furthermore, the negative value of ΔG° at different temperatures indicates that the adsorption of Ni²⁺ is spontaneous. The observed decrease in negative values of ΔG° with increasing temperature implies that the adsorption becomes more favorable at a higher temperature. The slight change of the TΔS° value at all temperatures and |TΔS°|>|ΔH°| indicates that the adsorption process is dominated by entropic rather than enthalpic changes (Atia et al., 2006). The results also verified that chemisorption was the mechanism of Ni²⁺ adsorption onto the P-C-CTS (Ni) adsorbents.

Table 3.

Thermodynamic Parameters of P-C-CTS-(Ni) for Ni ²⁺ Adsorption

T/K	ΔG° (kJ/mol)	ΔH° (kJ/mol)	ΔS° (J/[K·mol])	TΔS° (kJ/mol)
303	−14.84			33.99
313	−15.96	19.15	112.17	35.11
323	−17.08			36.23

Effect of pH on Ni²⁺ adsorption onto P-C-CTS (Ni) adsorbents without citrate

The Ni²⁺ adsorption quantities onto the P-C-CTS (Ni) adsorbents at different pH are shown in Fig. 5. The Q_Ni increases from pH 2.0 to 5.0 in the absence of citrate in the solution since the −H₂N: groups on beads were occupied by high concentration of H⁺ instead of the Ni²⁺ to form −[H₂N:H]⁺ at lower pH. The chelation competition with H⁺ contributes a smaller Q_Ni at lower pH. With pH value increases, the concentration of H⁺ decreases leading to the weaker competition and the increase of Q_Ni. The saturated Ni²⁺ adsorption almost reaches at pH 5.0 and no change at pH 6.0. It indicates that the chelation between −H₂N: and Ni²⁺ is the mechanism for the Ni²⁺ adsorption in the absence of citrate in the solution.

FIG. 5.

Effect of pH on Ni(II) adsorption onto P-C-CTS (Ni) adsorbents without citrate.

Effect of citrate on Ni²⁺ adsorption onto P-C-CTS (Ni) adsorbents at different pH

The ratios of the adsorption quantities onto adsorbents between nickel and citrate (Q_Ni:Q_Cit) and the changes of the nickel concentration (ΔC_Ni) in solution before and after adsorption at different pH are listed in Table 4. The citrate ligands significantly affect the adsorption of Ni²⁺ onto the P-C-CTS (Ni) adsorbents in all the tested solutions at different pH. Both the Q_Ni:Q_Cit and ΔC_Ni increased with the increasing ratio of the nickel/citrate. The citrate ligands, especially when present in excess over the nickel metal ions, may decrease the adsorption capacity of the adsorbents due to the competition for the amine groups in adsorbents between the nickel species and citrates.

Table 4.

Ratios of Nickel and Citrate Adsorption Quantities and Changes of Nickel Concentrations at Different pH Levels with Specific Nickel/Citrate Molar Ratios

	Nickel/citrate (1:2)		Nickel/citrate (1:1)		Nickel/citrate (2:1)
pH	Q_Ni:Q_Cit	ΔC_Ni ^a	Q_Ni:Q_Cit	ΔC_Ni	Q_Ni:Q_Cit	ΔC_Ni
2.0	0:0.2	0.00	0:0	0.00	0:0	0.00
3.0	0.7:1	0.29	1:1	0.30	2.3:1	0.64
4.0	1:1	0.52	1.5:1	0.62	2.6:1	0.75
5.0	1:1	0.56	1.2:1	0.53	3.5:1	0.80
6.0	1:1	0.50	1:1	0.58	3.5:1	0.89

ΔC_Ni is the concentration change of Ni(II) in solutions before and after adsorption (mmol/L).

Removal efficiency of Ni(II) in real water samples using the P-C-CTS (Ni) adsorbents

As shown in Table 5, the proposed P-C-CTS (Ni) beads were successfully applied for removal of Ni(II) from electroplating wastewaters and fortified environmental waters. The removal efficiency was over 92%. Of those, the relative low removal efficiency from fortified lake and river waters may result from the more dissolved organic matters in natural waters than in wastewaters from an electroplating factory.

Table 5.

Removal Efficiency of Ni(II) in Collected Industrial Wastewaters and Fortified Environmental Waters by Prepared P-C-CTS-(Ni) Beads

	Before adsorption		After adsorption ^a
Water samples	pH	C_Ni(II) (mg/L)	C_Ni(II) (mg/L)	Removal efficiency (%)
Wastewater(I)	1.8	18.06	0.53	97.1
Wastewater(II)	2.1	35.15	0.88	97.5
Lake water	5.3	22.65	1.79	92.1
River water	5.2	21.36	1.01	95.3

The pH of water samples was adjusted to 4.0 using a NaOH or H₂SO₄ diluted solution.

Stability of the P-C-CTS (Ni) adsorbents

In this study, the stability of the prepared P-C-CTS (Ni) adsorbents was investigated by measuring the Ni²⁺ adsorption quantity in eight adsorption–desorption–regeneration cycles. The Q_Ni, which was determined based on Equation (1) and the m value was the weight of the fresh beads, decreased only 5% after eight adsorption–desorption–regeneration cycles, as shown in Fig. 6. On the other hand, the color and appearance of the P-C-CTS (Ni) beads have no apparent difference after eight regeneration and reusing cycles.

FIG. 6.

Ni(II) adsorption quantity in eight adsorption–desorption–regeneration cycles.

Conclusions

The stable P-C-CTS (Ni) beads were prepared by using the molecular imprinting technique, crosslink reactions, and polyamination method and successfully applied for the Ni(II) removal from real wastewater and environmental waters. Chemisorption was found as the main mechanism of Ni(II) adsorption on P-C-CTS (Ni) beads. The adsorption processes and adsorption quantities were significantly affected by the pH and the citrate ligands in solution. The adsorption capability of the P-C-CTS (Ni) beads was stable up to eight reuse cycles. The prepared P-C-CTS (Ni) beads were the promising absorbents for Ni(II) removal from the industrial wastewaters.

Footnotes

Acknowledgments

The authors gratefully acknowledge financial support by the Asia Research Center of Nankai University, the National Natural Science Foundation of China (NSFC 21207102; 51176077), and the Key Natural Science Foundation of Tianjin (12JCZDJC29300).

Author Disclosure Statement

No competing financial interests exist.

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Adsorption of Ni(II) from Aqueous Solution by Polyaminated Crosslinked Ni(II)-Imprinted Chitosan Derivative Beads

Abstract

Abstract

Introduction

Materials and Methods

Chemicals and materials

Synthesis of P-C-CTS (Ni) adsorbents

Characterization methods

Adsorption–desorption–regeneration experiments

Effects of solution pH and citrate on nickel adsorption

Removal of Ni(II) in real waters using the P-C-CTS (Ni) adsorbents

Results and Discussion

Adsorption capacities and moisture content

Characterization of the P-C-CTS (Ni) beads

Adsorption kinetics

Effect of pH on Ni2+ adsorption onto P-C-CTS (Ni) adsorbents without citrate

Effect of citrate on Ni2+ adsorption onto P-C-CTS (Ni) adsorbents at different pH

Removal efficiency of Ni(II) in real water samples using the P-C-CTS (Ni) adsorbents

Stability of the P-C-CTS (Ni) adsorbents

Conclusions

Footnotes

Acknowledgments

Author Disclosure Statement

References

Effect of pH on Ni²⁺ adsorption onto P-C-CTS (Ni) adsorbents without citrate

Effect of citrate on Ni²⁺ adsorption onto P-C-CTS (Ni) adsorbents at different pH