Removal of Nickel(II) from Aqueous Solutions Using Synthesized Î²

Article pubs.acs.org/IECR

Removal of Nickel(II) from Aqueous Solutions Using Synthesized β‑Zeolite and Its Ethylenediamine Derivative Peng Liu, Ni Yuan, Wei Xiong, Hanyu Wu, Duoqiang Pan,* and Wangsuo Wu* Radiochemistry Laboratory, Lanzhou University, Lanzhou 730000, China Key Laboratory of Special Function Materials and Structure Design, Ministry of Education, Lanzhou 730000, China ABSTRACT: β-Zeolite was synthesized and modified with ethylenediamine (EDA). The synthesized materials were characterized and used for the removal of nickel(II) from aqueous solutions. The influences of the pH, contact time, and temperature on nickel(II) adsorption onto synthesized β-zeolite and β-zeolite modified with ethylenediamine (β-zeolite−EDA) were studied by a batch technique, and X-ray photoelectron spectroscopy was employed to analyze the experimental data and identify the formation of coordination bonds between the adsorbents and nickel(II). The dynamic process showed that the adsorption of nickel(II) onto β-zeolite and βzeolite−EDA matched the pseudo-second-order kinetics model, and the adsorption of nickel(II) was significantly dependent on the pH values. Through simulation of the adsorption isotherms by the Langmuir, Freundlich, and Dubinin−Radushkevich models, it could be seen that the adsorption patterns of nickel(II) onto β-zeolite and β-zeolite−EDA were mainly controlled by surface complexation and the adsorption processes were endothermic and spontaneous. The modification of β-zeolite−EDA improved the adsorption capacity of nickel(II) significantly; this shows a novel material for the removal of nickel(II) from the water environment for industrialized application.

1. INTRODUCTION

Zeolites are a family of porous inorganic aluminosilicate materials.23 These materials have a 12-ring framework that consists of SiO4 and AlO4 tetrahedra.24 Because of their high surface area, high thermal stability, and ecofriendly nature, zeolites have long been utilized in various applications ranging from catalysts in petroleum refining to carbon dioxide capture, lasers, gas sensing, drug delivery, and gas separation.25 Because both inorganic and organic chemistry advance with the progressive efforts in zeolite synthesis, new modified zeolites have been increasingly discovered via various synthesis strategies.26−28 Nevertheless, for applications under commercially relevant operating conditions, a type of thermally stable and inexpensive microporous material is much needed.

The rapid development of industrialization has led to the production of large amounts of wastewater and increased disposal of heavy metals into the environment.1 Over the past decades, nickel pollution in water can be attributed to different industrial applications like dyeing operations, mining, galvanization, smelting, battery production, metal finishing, and alloying. The toxicity of nickel may result in dermatitis, headache, nausea, cardiovascular and kidney diseases, and even carcinogenesis for human.2 From an environmental protection point of view, it is of great need to develop effective and environmentally friendly methods to remove and recover nickel from aqueous solutions. Several treatment processes such as chemical precipitation,3 membrane dialysis,4 nanozerovalent iron,5 solvent extraction,6 ion exchange,7 flotation,7 nanofiltration,8 ultrafiltration,9 adsorption,10,11 etc., have been developed for the preconcentration or separation of nickel(II). Among these processes, adsorption was proven to be an effective and convenient method.12 One of the key issues in the adsorption process is choosing a suitable adsorbent. Various substances, such as proteins,13 carbon materials,14 anion-exchange resins,15 modified metal oxides,16 clay minerals,17 wood ash,18 lignite,19 porous materials20 and fibers,21 have been investigated as adsorbents for the rapid and effective removal of heavy metals from aqueous media. Among the various adsorbents, zeolites attract much attention because of their high thermal and radiation stability and high exchange capacity.22 © XXXX American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Materials. Nickel(II) nitrate hexahydrate [Ni(NO3)2· 6H2O], tetraethylammonium hydroxide (TEAOH; 20 wt % in water), and ethylenediamine (EDA) were purchased from Aladdin Reagent Co. Ltd. (Shanghai, China). A nickel(II) stock solution (1.13 × 10−3 mol/L) was made from Ni(NO3)2·6H2O. All other chemicals used were of analytical grade and were used without further purification. All solutions and suspensions were prepared with deionized water (18 MΩ/cm). Received: Revised: Accepted: Published: A

December 11, 2016 March 3, 2017 March 5, 2017 March 5, 2017 DOI: 10.1021/acs.iecr.6b04784 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 1. Characterization images. SEM images of (A) β-zeolite and (B) β-zeolite−EDA. (C) XRD patterns of (a) β-zeolite and (b) β-zeolite−EDA. (D) Thermal curves of (a) β-zeolite and (b) β-zeolite−EDA.

2.2. Preparation of β-Zeolite and β-Zeolite Modified with Ethylenediamine (β-Zeolite−EDA). β-Zeolite was synthesized by a hydrothermal method. Briefly, a mixture containing 60 g of TEAOH, 1.113 g of NaAlO2, 0.45 g of KCl, and 0.075g of NaOH was stirred until it became transparent, and then 16.30 g of aerosil was added into the mixture. The resulting homogeneous sol was transferred into a Teflon-lined stainless-steel autoclave and heated to 170 °C statically for 40 h. After the autoclave was quenched, the content was filtered and then washed with deionized water three times. After drying at 96 °C overnight, the product was calcined at 540 °C for 20 h.29 To prepare an EDA-functionalized adsorbent, 1.5 g of βzeolite was mixed with 30 mL of an EDA solution in a 100 mL reaction flask. The mixture was refluxed at 120 °C for 40 h. The reaction mixture was poured into cold water, and the solid was filtered and washed with a 0.1 M NaCl solution until the filtrate was free from amine. The remaining NaCl was washed with water and methanol. The functionalized adsorbents were dried at 70 °C and designated as β-zeolite−EDA.30 2.3. Batch Experiments. The method of investigations for nickel(II) adsorption was studied by a batch technique. The batch adsorption experiments were carried out in a series of polyethylene centrifuge tubes. A certain amount of β-zeolite or β-zeolite−EDA and calculated amounts of a NaNO3 solution and a nickel(II) stock solution were slowly spiked into tubes, and deionized water was used to maintain a total volume of 6.0 mL. The extremely small volume of a HCl or NaOH solution was added to the system to adjust the pH to a desired value. After a batch of samples was shaken in a constant-temperature shaker at 25 ± 0.5 °C for 3 days, the samples were centrifuged at 10000 rpm for 30 min. A certain volume of supernatant was removed to measure the aqueous concentration of nickel(II) by using a spectrophotometer at a wavelength of 530 nm using

dimethylglyoxime. The wavelength of 530 nm was determined with an UV−vis spectrophotometer; the bright-red precipitate generated by dimethylglyoxime and nickel(II) has the biggest peak at 530 nm. The adsorption percentage of nickel(II) on βzeolite or β-zeolite−EDA was calculated from the difference between the initial and equilibrium concentrations. Adsorption isotherms were obtained with an initial concentration range of nickel(II) from 1.13 × 10−5 to 1.13 × 10−3 mol/L. All of the experimental data were the averages of duplicate or triplicate experiments, and the relative errors of the data were less than 5%. The adsorption capacity of the adsorbent and removal efficiency of nickel(II) were calculated by the following equation: sorption =

C0 − Ce × 100 C0

(1)

Sorption is the removal efficiency of nickel(II) in percentage (%); C0 and Ce are the initial and equilibrium nickel(II) concentrations, respectively.

3. RESULTS AND DISCUSSION 3.1. Characterization. For the study of the morphology of the samples, a scanning electron microscopy (SEM; Hitachi S4800) technique was employed (Figure 1A,B). The images reveal the morphological differences between the zeolite and its derivative. For β-zeolite, some aggregated spherical shape and some flakes are observed. After modification, the zeolite surface is more aggregated, and a large number of flakes with severely crumpled structures arise. The morphological changes of the samples may be attributed to the change of the particle surface charge after modification. B

DOI: 10.1021/acs.iecr.6b04784 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article


Figure 2. Effect of the pH on nickel(II) adsorption onto β-zeolite and β-zeolite−EDA. S/L = 0.75 g/L, I = 0.01 mol/L NaNO3, T = 25 °C, and [nickel(II)] = 1.13 × 10−4 mol/L.

The X-ray diffraction (XRD) patterns (X’Pert Pro Panalytical) of the parent and modified zeolite are shown in Figure 1C. For the scanning range from 4° to 60°, two characteristic peaks (2θ = 7.7° and 22.7°) indicate that the solid products have β crystals. The location of diffraction lines that remained unchanged indicates that the structure of β-zeolite was well preserved after modification of EDA. Nevertheless, the intensity of the diffraction lines decreased after EDA was added, which agrees with the results of Xia et al. and Xu et al.29,31 In the thermal curves of β-zeolite (Figure 1D), a single continuous weight loss step from room temperature to 110 °C is observed and is attributed to the desorption of water molecules. In the profiles of the modified samples, three distinct weight loss steps at 100, 280, and 520 °C are recorded, and the second and third peaks are attributed to the volatilization and degradation of EDA.32 3.2. Effect of the pH. The solution pH plays an important role in controlling the removal performance of adsorbent materials. Figure 2 indicates that the adsorption percentage of β-zeolite−EDA is obviously higher than that of β-zeolite. It is conceivable that at low pH there is an excess of H3O+ ions in solution and competition exists between both positively charged hydrogen ions and nickel(II) for the available adsorption sites of the β-zeolite or β-zeolite−EDA surface, but the amino groups (−NH2) of β-zeolite−EDA would be in a protonated cationic form (−NH3+) to a higher extent in an acidic solution, which results in more nickel(II) ions in solution being adsorbed on the surface of the sorbent.33 At pH > 8.0, the solubility of nickel(II) decreased, resulting in their precipitation as hydroxide with no significant increase in the percentage of adsorption. When nickel(II) adsorption is compared upon grinding of local clay,34 palygorskite clay35 and sodium rectorite,36 the results of this work are consistent with the results of the references. 3.3. Effect of the Contact Time and Adsorption Kinetics. The effect of the contact time on the adsorption of nickel(II) onto β-zeolite and β-zeolite−EDA at pH = 6.70 and 7.50 is shown in Figure 3A. The adsorption percentage increased with increasing time, until a maximum was achieved. Some other authors received the same results for nickel(II) removal from aqueous solutions.37,38

Figure 3. Kinetics of nickel(II) adsorption onto β-zeolite and βzeolite−EDA. (A) Effect of the contact time. (B) Pseudo-second-order adsorption kinetics plot. (C) Weber−Morris adsorption kinetics plot: (a) β-zeolite, pH = 6.70; (b) β-zeolite−EDA, pH = 7.50; (c) β-zeolite, pH = 6.70; (d) β-zeolite−EDA, pH = 7.50. S/L = 0.75 g/L, I = 0.01 mol/L NaNO3, T = 25 °C, and [nickel(II)] = 1.13 × 10−4 mol/L.

Knowledge of the adsorption kinetics can be used not only to predict the rate at which nickel(II) is adsorbed on β-zeolite and β-zeolite−EDA but also to elucidate the adsorption mechanism C


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Industrial & Engineering Chemistry Research Table 1. Kinetic Parameters of Pseudo-First-Order and Pseudo-Second-Order Models pseudo-first-order

pseudo-second-order

pH

sample

qe (mg/g)

qe1(mg/g)

R12

qe2(mg/g)

R22

6.70 6.70 7.50 7.50

β-zeolite β-zeolite−EDA β-zeolite β-zeolite−EDA

0.00489 0.00759 0.00697 0.00967

0.004580 0.007378 0.006591 0.009432

0.7143 0.7112 0.8765 0.7871

0.004882 0.007547 0.006961 0.009623

0.9998 0.9999 0.9999 0.9999

Table 2. Kinetic Parameters of the Weber−Morris Model pH

sample

6.70

β-zeolite

6.70

β-zeolite−EDA

7.50

β-zeolite

7.50

β-zeolite−EDA

R12

parameters −5

Ka1 = 1.66 × 10 Ka2 = 4.06 × 10−6 Ka3 = 4.51 × 10−7 Kb1 = 1.37 × 10−5 Kb2 = 2.45 × 10−6 Kb3 = 4.20 × 10−7 Kc1 = 4.62 × 10−5 Kc2 = 4.30 × 10−6 Kc3 = 6.44 × 10−7 Kd1 = 1.66 × 10−5 Kd2 = 3.14 × 10−6 Kd3 = 4.48 × 10−7

−5

Ca1 = 6.70 × 10 Ca2 = 7.84 × 10−5 Ca3 = 9.27 × 10−5 Cb1 = 1.29 × 10−4 Cb2 = 1.42 × 10−4 Cb3 = 1.47 × 10−4 Cc1 = 7.36 × 10−5 Cc2 = 1.18 × 10−4 Cc3 = 1.32 × 10−4 Cd1 = 1.67 × 10−4 Cd2 = 1.80 × 10−4 Cd3 = 1.88 × 10−4

0.9547 0.9549 0.5792 0.9247 0.9316 0.7266 0.9575 0.9344 0.7344 0.9547 0.8564 0.6931

underlying adsorption through valuable data. Three models were employed to study the kinetics process of nickel(II) adsorption on β-zeolite and β-zeolite−EDA. First, the adsorption kinetics is expressed as the pseudo-firstorder equation.39 It gives a linearized data plot in the form K 1 1 1 = 1 + qt qe t qe (2)

Figure 5. Effect of the adsorbent dosage on nickel(II) adsorption onto β-zeolite and β-zeolite−EDA. I = 0.01 mol/L NaNO3, T = 25 °C, and [nickel(II)] = 1.13 × 10−4 mol/L.

t 1 t = + 2 qt qe K 2qe

where qt (mg/g) and qe (mg/g) represent the amounts of nickel(II) adsorbed at time t (h) and equilibrium, respectively, and K1 is the adsorption rate constant (h−1). The theoretical qe1 values were significantly smaller than the experimental qe values, suggesting that the pseudo-first-order equation was not applicable to the present systems.40 Based on this concept, the adsorption kinetics was further analyzed using the pseudo-second-order equation.41 The pseudo-second-order kinetics equation linear expression is as follows:

(3)

where qe (mg/g) is the equilibrium adsorption amount and qt is the adsorption amount of nickel(II) at time t. The parameter K2 (g/mg·h) represents the second-order rate constant of the kinetics model. One can see that the calculated R22 is much closer to unity than R12; this result suggests that the adsorption processes of nickel(II) onto β-zeolite and β-zeolite−EDA could be well described using the pseudo-second-order model (Figure 3B), which assumes that chemical adsorption rather than physical adsorption is the main adsorption mechanism.41,42 The

Figure 4. Effect of the ionic strength on nickel(II) adsorption onto β-zeolite and β-zeolite−EDA: (A) pH = 6.70; (B) pH = 7.50. S/L = 0.75 g/L, T = 25 °C, and [nickel(II)] = 1.13 × 10−4 mol/L. D


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0.9592 0.9910 0.9949 0.9971 0.9682 0.9725 0.9762 0.9816 10 104 104 104 104 104 104 104

× × × × × × × × 1.2514 1.7081 1.6842 2.3004 1.4233 2.2017 2.0928 2.6333 10 10−4 10−4 10−4 10−4 10−4 10−4 10−4 0.9488 0.9851 0.9903 0.9706 0.9575 0.9593 0.9637 0.9690 0.0025 0.0010 0.0010 0.0006 0.0022 0.0007 0.0007 0.0006 0.9987 0.9992 0.9989 0.9994 0.9984 0.9997 0.9998 0.9995

2.9077 5.0955 4.9633 8.1967 3.1997 7.2955 6.6450 9.8328

R

qt = Kt 1/2 + C

10 104 104 105 104 104 104 105

× × × × × × × × 1.6609 4.5086 4.1747 1.0986 2.0980 9.0163 7.3942 1.8843 7.50

6.70 318

7.50

6.70 298

(4)

According to the equation, the plot would be linear and K is the rate constant and C is the intercept of the line. The multilinear plots in Figure 3C indicate that more than one mechanism might control the adsorption process of nickel(II).44 The adsorption processes could be constituted with three stages: initial, second, and equilibrium. The initial steep-sloped stage was attributed to external surface adsorption or instantaneous adsorption of the most available adsorbing sites on the zeolite surfaces, while the second gentle-sloped portion stage can be ascribed to gradual adsorption. This revealed that the intraparticle diffusion is rate-controlled and the intraparticle diffusion rate could be obtained from the slope of the gentlesloped portion. At the beginning, nickel(II) was adsorbed by the exterior surface of zeolites, until the exterior surface reached saturation and nickel(II) entered into the zeolites along the pores within the particles and started the adsorption processes in the interior surfaces of the particles. With nickel(II) diffusing

β-zeolite β-zeolite−EDA β-zeolite β-zeolite−EDA β-zeolite β-zeolite−EDA β-zeolite β-zeolite−EDA

RL

0.3758 0.1815 0.1932 0.0834 0.3228 0.0998 0.1191 0.0504 1.7375 2.0836 2.0673 2.3358 1.8948 2.1658 2.1181 2.5712

× × × × × × × ×

10 10−4 10−4 10−4 10−4 10−4 10−4 10−4

results of using the two equations to fit the experimental data are shown in Table 1. Finally, the empirically functional relation Weber−Morris plot was used to describe the adsorption process. The Weber− Morris model can be described as43

−4

qmax (mol/g) Ka (L/mol) pH T (K)

sample

Figure 6. Effect of the temperature on nickel(II) adsorption onto βzeolite and β-zeolite−EDA: (A) pH = 6.70; (B) pH = 7.50. S/L = 0.75 g/L and I = 0.01 mol/L NaNO3.

4

R

KF (mol/g)

n

3.1930 1.7138 1.7627 9.4488 2.4681 1.0315 1.1416 7.2104

× × × × × × × ×

10 10−9 10−9 10−10 10−9 10−9 10−9 10−10

5.3807 4.0547 4.0684 3.4330 5.3981 3.5168 3.5887 3.6818

× × × × × × × ×

−4

qm(mol/g)

−9

K (mol /kg )

2 2 2 2

Freundlich model Langmuir model

Table 3. Comparison of the Langmuir, Freundlich, and D−R Adsorption Constants of Nickel(II) onto β-Zeolite and β-Zeolite−EDA

D−R model

E (kJ/mol)

4

R2


E


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Table 4. Linear Fit of ln Kd versus Ce and Thermodynamic Parameters for Nickel(II) Adsorption onto β-Zeolite and β-Zeolite− EDA ln Kd= ACe + B

thermodynamic data

pH

T (K)

sample

A

B

R

ΔG° (kJ/mol)

ΔH° (kJ/mol)

ΔS° (J/mol·K)

6.70 6.70 6.70 6.70 7.50 7.50 7.50 7.50

298 298 318 318 298 298 318 318

β-zeolite β-zeolite−EDA β-zeolite β-zeolite−EDA β-zeolite β-zeolite−EDA β-zeolite β-zeolite−EDA

−4843.67 −7457.68 −5404.58 −9144.07 −7296.61 −10731.78 −8580.00 −13287.95

7.50 8.32 7.75 8.70 8.27 9.01 8.57 9.48

0.9903 0.9645 0.9878 0.9537 0.9648 0.9331 0.9598 0.9123

−18.58 −20.61 −20.49 −23.00 −20.49 −22.32 −22.66 −25.06

9.85 14.97 9.85 14.97 11.82 18.51 11.82 18.51

95.40 119.41

108.41 137.04

increase in the amount of β-zeolite or β-zeolite−EDA. This is possibly because of the increased adsorptive surface area and the availability of more active binding sites on the surface of the adsorbent with an increase in the adsorbent dosage. This result is similar to other previous adsorption of nickel(II) results.49,50 3.6. Effect of the Temperature and Thermodynamic Estimation. It was necessary to propose a suitable model to gain a better understanding of the mechanism. To optimize the adsorption of nickel(II) from aqueous solutions, Langmuir, Freundlich, and Dubinin-Radushkevich (D−R) adsorption isotherms were applied to calculate the relative adsorption data. The three models were used most commonly by other researchers to describe the adsorption characteristics of sorbent in water and wastewater treatment, and the relative parameters are listed in Table.3. The Langmuir model is one of the most widely used models for modeling equilibrium data, and the relative isotherm is valid for monolayer adsorption onto a surface containing a finite number of identical sites. This process could be described by the following form:51

in the pores of β-zeolite and β-zeolite−EDA, the increasing diffusion resistance caused a decrease of the diffusion rate until the diffusion processes reached equilibrium. The corresponding parameters calculated are listed in Table 2, where the rate constants (K1, K2, and K3) can be attributed to the adsorption stages of the exterior surface, interior surface, and equilibrium, respectively. Also, the rate constants of β-zeolite were higher than those of β-zeolite−EDA before equilibrium. This indicates that modification of β-zeolite made the macropores on zeolite surface become smaller or partially blocked, which caused nickel(II) to be more difficult to diffuse and transport into pores on β-zeolite−EDA than on β-zeolite. The increasing values of C (μg/g) indicated that nickel(II) adsorption became less influenced by the boundary layer thickness.45 Additionally, the equilibrium times of nickel(II) adsorption on β-zeolite and β-zeolite−EDA were 12 and 16 h at their experimental conditions, respectively. 3.4. Effect of the Ionic Strength. The ionic strength is another important factor affecting the interaction, especially the adsorption behavior of pollutants in the environment. The influence of the ionic strength on nickel(II) adsorption is illustrated in Figure 4. The adsorption of nickel(II) onto βzeolite decreases with increasing NaNO3 concentration, which suggests that Na+ weakly affects nickel(II) adsorption. By contrast, the adsorption of nickel(II) onto β-zeolite−EDA is independent of the ionic strength. As is well-known, the innersphere complexes suggested in the adsorption of nickel(II) should not be greatly affected by the ionic strength because of the strength of their adsorption affinity, and outer-sphere complexes should be more susceptible to the variation of the ionic strength because of the coupling effects of ion competition.46 Consequently, the present results show that nickel(II) adsorption onto β-zeolite was controlled by the outer-sphere complexes, but nickel(II) adsorption onto βzeolite−EDA was controlled by the inner-sphere complexes. Fan et al. investigated the sorption of nickel(II) onto sodium attapulgite and found that the adsorption was dependent on the ionic strength.47 Yang et al. studied the adsorption of nickel(II) on bentonite and also found that the adsorption of nickel(II) was mainly dominated by ion exchange or outer-sphere surface complexation at low pH values and by inner-sphere surface complexation at high pH values.48 3.5. Effect of the Adsorbent Dosage. The adsorbent dosage is also an important parameter because it determines the adsorption capacity of an adsorbent for a given initial concentration. The effect of the adsorbent dosage on the adsorption of nickel(II) by β-zeolite and β-zeolite−EDA is shown in Figure 5. It is apparent from Figure 5 that there was an increase in the percentage removal of nickel(II) with an

Ce 1 + CeK a = q K aqmax

(5)

where qmax (mol/g) is the capacity of adsorption at saturation and Ka (L/mol) shows the adsorption coefficient that is related to the energy of adsorption. The Langmuir isotherm could be calculated using the following equation: Ka =

1 1 − RLC0 C0

(6)

where C0 (mol/L) is the concentration of the initial metal in solution. The value of RL indicates a good sorbent for the sorbate if it is between 0 and 1. The Freundlich expression is an empirical equation that describes adsorption onto the heterogeneous surface.52 The Freundlich isotherm model stipulates that the ratio of adsorbed solute to aqueous solute is a function of the solution. It also allows for several different kinds of adsorption sites on the solid and represents the adsorption data at low intermediate concentrations on the heterogeneous surfaces. The linear equation could be presented as n ln q = n ln KF + ln Ce

(7)

where KF (mol/g) is the adsorption capacity. It indicates a favorable adsorption when the value of n is between 1 and 10. Because the D−R isotherm assumes neither a homogeneous surface nor a constant adsorption potential, it is more ordinary than the Langmuir isotherm. Also, the linear form of the D−R equation is53 F


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Figure 7. O 1s spectra of β-zeolite before adsorption [A(0)] and after adsorption [A(1) and A(2)]. O 1s and N 1s spectra of β-zeolite−EDA before adsorption [B(0) and C(0)] and after adsorption [B(1), C(1), B(2), and C(2)]. For A(1), B(1), and C(1), pH = 6.70; for A(2), B(2), and C(2), pH = 7.60.

⎛ 1 + Ce ⎞ ln qm = ln q + KR2T 2 ln 2⎜ ⎟ ⎝ Ce ⎠

The magnitude of E is widely used for estimating the type of adsorption reaction. The adsorption may be effected by physical forces if E < 8 kJ/mol; if the value of E is between 8 and 16 kJ/mol, the adsorption is governed by chemical ion exchange; if E > 16 kJ/mol, it could be supposed that particle diffusion may affect the adsorption.54 It could be concluded from the constants that the Langmuir models fitted the experimental data better. This fact indicated almost complete monolayer coverage of the adsorbent particles. The adsorption capacity qm values were quite different from the adsorption capacity qmax values at the Langmuir region. This difference has been reported in other works,55,56 which might

(8)

where K is the constant that is related to the adsorption energy, qm (mol/g) is the theoretical saturation capacity, R (kJ/mol·K) is the gas constant, and T (K) is the temperature. The free energy of adsorption E (kJ/mol) is defined as when one molecule of ion is transferred from infinity in a solution to the surface of the solid, the free energy changed. It could be calculated from the value of K using the following equation:

E=

1 2K

(9) G


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oxygen and nitrogen atoms decreased because the oxygen and nitrogen atoms easily lost electrons, which migrated to nickel(II).59 Therefore, the XPS spectra provided evidence that complexes between the nickel(II) and oxygen atoms of βzeolite and the oxygen and nitrogen atoms of β-zeolite−EDA can form coordination bonds with nickel(II).

be attributed to the different assumptions considered in the formulation of the isotherms. The influence of the temperature on the adsorption is shown in Figure 6. The adsorption of nickel(II) is strongly dependent on the temperature, indicating that the adsorption of nickel(II) onto β-zeolite and β-zeolite−EDA is more favorable at higher temperature. The thermodynamic parameters of nickel(II) adsorption can be calculated from the temperature-dependent adsorption isotherms. The free energy change (ΔG0) can be expressed as ΔG° = −RT ln K 0

4. CONCLUSION In this study, the sorption of nickel(II) onto β-zeolite and its EDA derivative was investigated. The kinetics process of nickel(II) adsorption onto β-zeolite and β-zeolite−EDA could be satisfactorily described by the pseudo-second-order kinetics model. Thermodynamic studies showed that the adsorption processes were endothermic and spontaneous. The experimental data were well fitted by the Langmuir isotherm. XPS analysis considered the formation of different complexes at different experimental conditions. The maximum adsorption capacity of β-zeolite was about 4.97 × 10−5−1.35 × 10−4 mol/g from pH 6.50 to 8.00 at room temperature, and after modification, the capacity of β-zeolite was about 6.67 × 10−4−1.44 × 10−4 mol/g at the same condition; EDA modification of β-zeolites significantly improved the adsorption capacity of nickel(II), which could provide a feasible method for industrial application.

(10)

where R is the ideal gas constant (8.314 J/mol·K), T is the temperature (K), and K0 is the adsorption equilibrium constant. The equilibrium partition coefficient (Kd) can be calculated as follows: q Kd = e Ce (11) where qe and Ce represent the adsorbed and aqueous nickel(II) concentration, respectively. The value of ln K0 can be extrapolated by plotting ln Kd versus Ce when Ce is close to zero.57 The standard entropy changes (ΔS°) can be presented as ⎛ ∂ΔG° ⎞ ⎟ ΔS° = −⎜ ⎝ ∂T ⎠ p

■

Corresponding Authors

(12)

*E-mail: [email protected] (D.P.). *E-mail: [email protected] (W.W.).

The average standard enthalpy changes (ΔH°) are calculated using the following equation: ΔH ° = ΔG° + T ΔS°

AUTHOR INFORMATION

ORCID

(13)

Peng Liu: 0000-0003-3761-7226

The relative parameters are listed in Table 4. Negative values of ΔG° indicate that the adsorption processes were spontaneous, and the gradually decreased values of ΔG° with increasing temperature imply a greater driving force for the adsorption processes. Positive values of ΔH° indicate that the adsorption of nickel(II) onto β-zeolite and β-zeolite−EDA is endothermic. The positive entropy change (ΔS°) reflects the affinity of β-zeolite and β-zeolite−EDA for nickel(II) in aqueous solutions and possibly suggests some structural changes in the adsorbents.46.58 3.7. XPS Study. To assess the adsorption mechanism of nickel(II), XPS spectral analysis was performed. Figure 7 shows the XPS spectra of O 1s for β-zeolite before and after nickel(II) adsorption and the XPS spectra of O 1s and N 1s for β-zeolite− EDA before and after nickel(II) adsorption. As shown in Figure 7, the XPS spectra of O 1s before and after adsorption show significant changes; the peaks at 532.06 and 532.85 eV of the oxygen atom shifted to 532.12 and 532.91 eV at pH = 6.70 and to 532.16 and 532.94 eV at pH = 7.50, respectively. This phenomenon can be attributed to the oxygen atoms of β-zeolite from the complexes with nickel(II), which result in a decrease of the electron cloud density of the oxygen atom and an increase of the binding energy. From a comparison of the binding energy of oxygen and nitrogen elements of β-zeolite− EDA before and after nickel(II) adsorption, the peaks at 531.70 and 532.48 eV of the oxygen atom shifted to 532.06 and 532.89 eV at pH = 6.70 and to 531.98 and 532.83 eV at pH = 7.50, respectively; the peaks at 399.51 and 401.6 eV of the nitrogen atom moved to 399.50.11 and 401.61 eV at pH = 6.70 and to 399.68 and 401.66 eV at pH = 7.50, respectively. This may be explained by the fact that the electronic density around the

Notes

The authors declare no competing financial interest.

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Removal of Nickel(II) from Aqueous Solutions Using Synthesized Î²

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