Abstract

The adsorption of Cd(II) from aqueous solution by synthesized zeolite NaX from coal gangue was investigated in a batch adsorption system. The studies include both equilibrium adsorption isotherms and kinetics. Different isotherm models were examined and the adsorption isotherm could be best represented with Langmuir. The adsorption kinetic experimental data were found to be better fitted with the pseudo-second-order kinetic model. An intra-particle diffusion model was employed to investigate the adsorption mechanism. The results showed that the intra-particle diffusion step was not the only rate limiting step. According to the Langmuir equation, the maximum adsorption capacity was 38.61 mg/g, suggesting that zeolite NaX synthesized from coal gangue can be used as a potential green alternative for the removal of Cd(II) from aqueous solution.

INTRODUCTION

Heavy metals resulting from industrial applications, e.g. battery, textile, and paper industry etc., have caused great damage to water resources. Cadmium, one of the most toxic metals, is attracting much attention due to its harmful effects such as kidney damage, anemia, lung cancer, etc. (Jamali et al. 2009). Hence, the development of an effective way to remove metal from wastewater is urgently required. Among available methods, the most commonly used methods are chemical precipitation, ion exchange, adsorption and reverse osmosis (Hui et al. 2005), while adsorption is viewed as the most effective method. Cadmium adsorption has been widely studied using willow root (Chen et al. 2013), modified sodium alginate (Yang et al. 2013), red mud (Kalkan et al. 2013), activated carbon (Venkatesan & Senthilnathan 2013), and zeolite (Izidoro et al. 2013), etc. According to our knowledge, no attempt was made to remove Cd(II) from aqueous solution using zeolite synthesized from coal gangue.

In this study, coal gangue, a solid waste, was used as an alternative low-cost precursor to synthesize zeolite NaX, aiming to remove Cd(II) from aqueous solution. The detailed synthesis process and method of zeolite NaX using coal gangue will be reported in another paper. The object of the present work was to determine the sorption characteristics of Cd(II) onto zeolite NaX synthesized from coal gangue through the investigation of kinetic and isotherm perspectives.

MATERIALS AND METHODS

Adsorbent and solution

Synthesized zeolite NaX from coal gangue (ZXCG) was used as adsorbent to remove Cd(II) from aqueous solution in the study. The surface area of ZXCG ws 557.05 m2/g and its total pore volume and micro-pore volume were 0.217 and 0.198 cm3/g, respectively. ZXCG has a relatively narrow pore distribution within the micro-pore range (<2 nm). Its pore size is larger than the hydrated ionic radius of Cd(II) (0.426 nm) (Nightingale 1959), suggesting a possibility to remove Cd(II) from aqueous solution using ZXCG as adsorbent.

Cd(NO3)2·4H2O was purchased from TianJin Chemistry Plant, China. Stock Cd(II) solution (1,000 mg/L) was prepared by dissolving accurate amounts of Cd(NO3)2·4H2O in distilled water. NaOH or HCl was used to adjust the initial pH. All the chemical reagents were of analytic grade.

Batch equilibrium study

Adsorbent dose, contact time, initial Cd(II) concentration and initial pH of solution were investigated through batch experiments in a 150 mL volume glass bottle. The mixture was agitated by a magnetic stirrer at a fixed speed of 300 rpm for 120 min. The equilibrium mixture was filtered with a 0.45 μm filter membrane, and the residual Cd(II) concentration in the solution was measured by ICP-AES. The amount of Cd(II) adsorbed at equilibrium, qe (mg/g), was determined by the following equation:  
formula
(1)
where C0 and Ce represent the initial and equilibrium Cd(II) concentration in the aqueous solution (mg/L), respectively, V is the solution volume (L) and M is the adsorbent dose (g). To ascertain the accuracy of the data, triple experiments were conducted and the average value was employed.

The effect of adsorbent dose on Cd(II) adsorption was investigated by adding different amounts of ZXCG (0.05–0.5 g) into 100 mL 100 mg/L Cd(II) solution. The mixture (at natural pH) was then shaken at room temperature (27 °C) until equilibrium.

The effect of initial pH of solution on Cd(II) removal was investigated by adding 0.2 g ZXCG to 100 mL 100 mg/L initial Cd(II) solution at various pH (2.0–6.0) and then stirred at room temperature, 0.1 M NaOH or 0.1 M HCl was employed to adjust the pH.

The effect of initial Cd(II) concentration on the adsorption result was investigated by adding 0.2 ZXCG to 100 mL solution with different initial Cd(II) concentrations (10, 20, 50, 100, 200 mg/L) which was then stirred at room temperature.

Batch kinetic study

The kinetic studies were conducted in a 150 mL volume glass bottle at room temperature, 0.2 g ZXCG was added to different initial Cd(II) solutions (10, 20, 50, 100, 200 mg/L) and the pH of the solution was adjusted to 4.0 with 0.1 M HCl or 0.1 M NaOH. Cd(II) solution was extracted at the given time (0, 5, 10, 20, 40, 60, 80, 100, 120 min) and Cd(II) concentration was measured. The total extraction amount was no more than 5% of the stock ion solution. The extraction solution was filtered with 0.45 μm filter membrane and then diluted into a 25 mL volumetric flask. The amount of Cd(II) adsorbed on ZXCG was determined by the following equation:  
formula
(2)
where qt (mg/g) is the amount of Cd(II) adsorbed on ZXCG at the extracting time t (0, 5, 10, 20, 40, 60, 80, 100, 120 min) and Ct is the Cd(II) concentration (mg/L) in the aqueous solution at the extracting time t.

RESULTS AND DISCUSSION

Effect of adsorbent dose on Cd(II) removal

The relationship between the ZXCG dose and Cd(II) uptake indicated that the removal percentage of Cd(II) increases initially with the increasing ZXCG dose and then remains stable when the adsorbent dose was 0.2 g. This shows that the total active sites of 0.2 g ZXCG were completely occupied by Cd(II), and an extra increase of adsorbent would not improve the removal percentage greatly. The optimum ZXCG dose was therefore fixed at 0.2 g.

Effect of pH on Cd(II) removal

The effect of initial solution pH on Cd(II) removal was investigated and the results are illustrated in Figure 1. It can be found that the equilibrium amount of Cd(II) adsorbed onto ZXCG increased with an increase in pH value and reached the maximum when pH was 5. Beyond that point, qe begins to decrease. The calculated precipitation pH of Cd(II) (Co = 100 mg/L) is 4.7, therefore, in the subsequent investigations, experiments were performed at a solution pH value of 4 to avoid possible hydroxide precipitation.

Figure 1

Effect of pH on adsorption Cd(II) onto ZXCG.

Figure 1

Effect of pH on adsorption Cd(II) onto ZXCG.

Effect of initial Cd(II) concentration and contact time

The effects of initial Cd(II) concentration and contact time on Cd(II) removal are displayed in Figure 2. It can be seen clearly that the adsorption rate is quite fast in the higher initial Cd(II) concentration. On the other hand, Cd(II) is absorbed onto ZXCG at the initial adsorption stage more quickly (almost at the beginning of the 30 min for all the concentration cases). This is because the larger difference between the Cd(II) content in the solution and that on the surface of ZXCG, the stronger drive force is achieved; as the adsorption goes on, more Cd(II) is adsorbed onto the ZXCG surface, so the drive force turns to be smaller, resulting in a stable stage.

Figure 2

Relationship between contact time and adsorbed Cd(II) at different initial Cd(II) ion concentrations (adsorption conditions: ZXCG dosage: 0.2 g, agitation speed: 300 rpm, pH 4.0, temperature: 27 °C).

Figure 2

Relationship between contact time and adsorbed Cd(II) at different initial Cd(II) ion concentrations (adsorption conditions: ZXCG dosage: 0.2 g, agitation speed: 300 rpm, pH 4.0, temperature: 27 °C).

Adsorption isotherm

To predict the adsorption mechanism and determine the maximum adsorption capacity, it is essential to investigate the adsorption isotherm. In the study, three linear isotherm models (Langmuir, Freundlich and Tempkin) were employed. The Langmuir model is suitable to describe the monolayer homogeneous process (Hameed 2009), while the Freundlich model is used to describe multiple adsorption processes that take place on a heterogeneous surface (Allen et al. 2004). The Tempkin model suggests that the adsorption heat of all the molecules in the layer would decrease linearly with coverage due to the interactions between the adsorbents (Thamilarasu & Karunakaran 2013). The linear forms of the Langmuir, Freundlich and Tempkin models are given as follows, respectively:  
formula
(3)
 
formula
(4)
 
formula
(5)
where KL, KF and β are the Langmuir, Freundlich and Tempkin constants, respectively; qm is the complete monolayer adsorption value; 1/n is the adsorption intensity and α is the equilibrium binding constant (L/mg).

Isotherm model parameters and correlation coefficients are shown in Table 1. According to the correlation coefficient values, the fitting outcome is Langmuir (R2 = 0.985) > Tempkin (R2 = 0.937) > Freundlich (R2 = 0.887). The correlation coefficient of the Langmuir model is greater than 0.95, showing that the Cd(II) adsorption process could best be described by this model, which can be clearly proved by Figure 3. Based on the Langmuir equation, the maximum Cd(II) monolayer adsorption capacity is 38.61 mg/g. Comparisons of the Cd(II) maximum adsorption value reported in the literature (Tangjuank et al. 2009; El-Said et al. 2010; Kannan & Veemaraj 2010; Sen et al. 2010; Chen et al. 2013; Kalkan et al. 2013) are presented in Table 2. It can be found that the ZXCG showed a higher affinity to the cadmium over other adsorbents. This could be ascribed to the larger surface area and its microporous structure. Of course, the interaction between zeolite and water and ions in aqueous solution can cause a change in the crystal structure and porosity of zeolite (Zhu et al. 2010, 2013, 2017), which can influence the adsorption capacity of zeolite.

Table 1

Isotherm parameters for Cd(II) adsorption

Isotherm model Parameters 
Langmuir KL (L/mg) 0.047 
 qm (mg/g) 38.61 
 R2 0.985 
Freundlich KF (L/mg) 0.052 
 1/n 0.98 
 R2 0.887 
Tempkin α (L/mg) 4.50 
 β (mg–127.79 
 R2 0.937 
Isotherm model Parameters 
Langmuir KL (L/mg) 0.047 
 qm (mg/g) 38.61 
 R2 0.985 
Freundlich KF (L/mg) 0.052 
 1/n 0.98 
 R2 0.887 
Tempkin α (L/mg) 4.50 
 β (mg–127.79 
 R2 0.937 
Table 2

Comparison of Cd(II) maximum adsorption value of different adsorbents

Adsorbent Adsorption capacity (mg/g) Reference 
Castor seed hull 5.8 Sen et al. (2010)  
Rice husk ash 6.57 El-Said et al. (2010)  
Cashew nut shell 14.29 Tangjuank et al. (2009)  
Jack fruit seed carbon 0.66 Kannan & Veemaraj (2010)  
Willow root 1.28 Chen et al. (2013)  
Bacterial modified red mud 83.03 Kalkan et al. (2013)  
Coal gangue based zeolite NaX 38.61 This study 
Adsorbent Adsorption capacity (mg/g) Reference 
Castor seed hull 5.8 Sen et al. (2010)  
Rice husk ash 6.57 El-Said et al. (2010)  
Cashew nut shell 14.29 Tangjuank et al. (2009)  
Jack fruit seed carbon 0.66 Kannan & Veemaraj (2010)  
Willow root 1.28 Chen et al. (2013)  
Bacterial modified red mud 83.03 Kalkan et al. (2013)  
Coal gangue based zeolite NaX 38.61 This study 
Figure 3

Linear adsorption isotherm model (a) Langmuir; (b) Freundlich; (c) Tempkin.

Figure 3

Linear adsorption isotherm model (a) Langmuir; (b) Freundlich; (c) Tempkin.

Adsorption kinetics

For the certain Cd(II) concentration (100 mg/L) and ZXCG dose (0.2 g), contact time is essential since it reveals the adsorption kinetic. The Cd(II) adsorption rate was characterized by two different kinetics models (pseudo-first-order, pseudo-second-order) which can be expressed as follows.

The pseudo-first-order model  
formula
(6)
The pseudo-second-order model  
formula
(7)
where K1 (min–1) and K2 (g/(mg min)) represent the pseudo-first order adsorption rate constant and the pseudo-second-order rate constant, respectively.

The values of the adsorption rate constant and correlation coefficient are listed in Table 3. Obviously, the pseudo-second-order plot seems to cover all the experimental data points while some points do not lie in the pseudo-first-order plot (Figure 4). Moreover, the correlation coefficients for the pseudo-second-order are much higher than that of the pseudo-first-order. Besides, the experimental qe for the pseudo-second-order model seems to highly agree with the calculated qe, indicating that it is more suitable to describe the adsorption process. It also indicates that such an adsorption is controlled by chemisorption (Hameed 2009).

Table 3

Sorption kinetic parameters (ZXCG dosage: 0.2 g, pH = 4)

Conc(mg/L) qe, exp (mg/g) Pseudo-first-order kinetic model
 
Pseudo-second-order kinetic model
 
qe, cal (mg/g) K1 (min–1R2 qe, cal (mg/g) K2 (g/(mg min)) R2 
10 4.5 0.13 0.025 0.352 4.5 11.86 1.0 
20 9.47 0.15 0.019 0.554 9.46 1.67 1.0 
50 24.18 0.24 0.014 0.248 24.16 0.66 1.0 
100 48.72 6.93 0.013 0.483 48.69 0.0048 1.0 
200 92.2 5.52 0.0086 0.597 93.2 0.015 0.9999 
Conc(mg/L) qe, exp (mg/g) Pseudo-first-order kinetic model
 
Pseudo-second-order kinetic model
 
qe, cal (mg/g) K1 (min–1R2 qe, cal (mg/g) K2 (g/(mg min)) R2 
10 4.5 0.13 0.025 0.352 4.5 11.86 1.0 
20 9.47 0.15 0.019 0.554 9.46 1.67 1.0 
50 24.18 0.24 0.014 0.248 24.16 0.66 1.0 
100 48.72 6.93 0.013 0.483 48.69 0.0048 1.0 
200 92.2 5.52 0.0086 0.597 93.2 0.015 0.9999 

qe, cal represents the amount of metal ions adsorbed on the zeolite NaX by modeling calculation at equilibrium.

qe, exp represents the experimental amount of metal ions adsorbed on the zeolite NaX.

Figure 4

Pseudo-first-order (a) and Pseudo-second-order kinetic (b) model plots for the Cd(II) adsorption onto ZXCG.

Figure 4

Pseudo-first-order (a) and Pseudo-second-order kinetic (b) model plots for the Cd(II) adsorption onto ZXCG.

Sorption mechanism

Kinetic studies fail to figure out the diffusion mechanism and rate determining step, and thus the intra-particle diffusion model was chosen to be tested. The intra-particle diffusion model can be expressed as:  
formula
(8)
where Kid ((mg/g min–1/2)) is the intra-particle diffusion rate constant, qt, the amount of metal ions adsorbed at time t and C (mg/g), a constant proportional to the thickness of boundary layer (Ravichandran & Arivoli 2013).

The amounts of Cd(II) adsorbed versus t1/2 for varied Cd(II) concentrations are displayed in Figure 5. It shows that the adsorption involved more than one model. Research (Khaled et al. 2009) found that there are four steps throughout an adsorption process: (1) bulk diffusion; (2) film diffusion; (3) pore diffusion or intra diffusion; and (4) surface adsorption. The first step could be ignored since the Cd(II) is enough and the adsorption rate stays high at the beginning. Figure 5 shows three stages while the second one is the gradual adsorption which controls the rate. Beyond that, in the third stage, namely the equilibrium process, diffusion remains low because of the relatively low Cd(II) concentration.

Figure 5

Intra particle diffusion model for Cd(II) adsorption.

Figure 5

Intra particle diffusion model for Cd(II) adsorption.

The values of Kid and C, calculated from the second portion, are listed in Table 4. As can be seen from Figure 5, there is occasionally a linear region, but in not the whole process. Additionally, the plot does pass through the origin. It demonstrated that the intra diffusion was not the only control step and others may be involved.

Table 4

Intra particle diffusion model parameters for the Cd(II) adsorption

C0 (mg/L) Kid C R2 
10 0.85 3.29 0.91 
20 1.13 14.4 0.93 
50 1.00 23.8 0.93 
100 0.77 31.5 0.99 
200 0.34 42.1 0.99 
C0 (mg/L) Kid C R2 
10 0.85 3.29 0.91 
20 1.13 14.4 0.93 
50 1.00 23.8 0.93 
100 0.77 31.5 0.99 
200 0.34 42.1 0.99 

Stability studies

Stability studies are useful to map out the adsorption nature and its recyclability. After the adsorption process, ZXCG was separated and dried. Then it was employed to do repeated Cd(II) adsorption experiments three times. The results were then characterized with X-ray diffraction (XRD) and scanning electron microscopy (SEM). From Figure 6 it can be clearly observed that the structure of ZXCG remains undestroyed, even though the surface suffered corrosion (Figure 7) when compared with raw ZXCG. The research shows that coal gangue based zeolite NaX has a high stability, even when used several times.

Figure 6

XRD pattern of raw and recycled ZXCG.

Figure 6

XRD pattern of raw and recycled ZXCG.

Figure 7

SEM graph of raw (a) and recycled (b) ZXCG.

Figure 7

SEM graph of raw (a) and recycled (b) ZXCG.

CONCLUSIONS

The zeolite NaX synthesized from coal gangue is proved to be effective for Cd(II) adsorption from aqueous solutions. Batch adsorption data were fitted with Langmuir, Freundlich and Tempkin isotherm models and it turned out that the process could be best described by the Langmuir equation. The kinetic was well determined by a pseudo-second-order model which suggests that the adsorption was chemisorption controlled. According to the intra-particle diffusion model fitting result, the rate was not only controlled by the intra-particle diffusion step and some other step may have been involved. The maximum equilibrium adsorption capacity for Cd(II) was found to be 38.61 mg/g, indicating that zeolite NaX synthesized from coal gangue can be a promising green adsorbent for removing Cd(II) from aqueous solutions.

ACKNOWLEDGEMENTS

This research was supported by the National Natural Science Foundation of China through Grant 41271510, the Research and Development Project of Science and Technology of Shaanxi Province and the Fundamental Research Funds for the Central University through Grants GK201601009. Fuqiang Zhao participated in sampling work at the early stage of the project. Sincere gratitude is given to the editors and the reviewers for their insightful suggestions and critical reviews of the manuscript.

AUTHOR DISCLOSURE STATEMENT

No competing financial interests exist.

REFERENCES

REFERENCES
Allen
,
S. J.
,
Mckay
,
G.
&
Porter
,
J. F.
2004
Adsorption isotherm models for basic dye adsorption by peat in single and binary component systems
.
J. Colloid Interface Sci.
280
(
2
),
322
333
.
Chen
,
G.
,
Liu
,
Y.
,
Wang
,
R.
,
Zhang
,
J.
&
Owens
,
G.
2013
Cadmium adsorption by willow root: the role of cell walls and their subfractions
.
Environ. Sci. Pollut. Res.
20
(
8
),
5665
5672
.
El-Said
,
A. G.
,
Badawy
,
N. A.
&
Garamon
,
S. E.
2010
Adsorption of cadmium (II) and mercury (II) onto natural adsorbent rice husk ash (RHA) from aqueous solutions: study in single and binary system
.
J. Am. Sci.
6
(
12
),
400
409
.
Jamali
,
H. A.
,
Mahvi
,
A. H.
&
Nazmara
,
S.
2009
Removal of cadmium from aqueous solutions by hazel nut shell
.
World Appl. Sci. J.
5
,
16
20
.
Kalkan
,
E.
,
Nadaroglu
,
H.
,
Dikbaş
,
N.
,
Taşgın
,
E.
&
Çelebi
,
N.
2013
Bacteria-modified red mud for adsorption of cadmium ions from aqueous solutions
.
Pol. J. Environ. Stud.
22
(
2
),
417
429
.
Kannan
,
N.
&
Veemaraj
,
T.
2010
Batch adsorption dynamics and equilibrium studies for the removal of cadmium (II) ions from aqueous solution using jack fruit seed and commercial activated carbons – a comparative study
.
Electron. J. Environ. Agric. Food Chem.
9
(
2
),
327
336
.
Khaled
,
A.
,
El Nemr
,
A.
,
El-Sikaily
,
A.
&
Abdelwahab
,
O.
2009
Treatment of artificial textile dye effluent containing Direct Yellow 12 by orange peel carbon
.
Desalination
238
(
1–3
),
210
232
.
Ravichandran
,
T.
&
Arivoli
,
S.
2013
Adsorption of Rhodamine-B from aqueous solution by activated calcite powder studies on equilibrium isotherm, kinetics and thermodynamics
.
Int. J. Pollut. Abate. Technol.
2
(
1
),
6
12
.
Sen
,
T. K.
,
Mohammod
,
M.
,
Maitra
,
S.
&
Dutta
,
B. K.
2010
Removal of cadmium from aqueous solution using castor seed hull: a kinetic and equilibrium study
.
Clean. Soil Air Water
38
(
9
),
850
858
.
Tangjuank
,
S.
,
Insuk
,
N.
,
Tontrakoon
,
J.
&
Udeye
,
V.
2009
Adsorption of lead (II) and cadmium (II) ions from aqueous solutions by adsorption on activated carbon prepared from cashew nut shells
.
Proc. World Acad. Sci. Eng. Technol.
52
,
110
116
.
Venkatesan
,
G.
&
Senthilnathan
,
U.
2013
Adsorption batch studies on the removal of cadmium using wood of Derris Indica based activated carbon
.
Res. J. Chem. Environ.
17
(
5
),
19
24
.
Yang
,
Z.
,
Wang
,
X.
,
Ma
,
C.
,
Lv
,
L.
,
Wei
,
C.
&
Min
,
B. G
, .
2013
Kinetics and thermodynamics of Cd(II) sorption onto sodium alginate/poly (vinyl alcohol)/hydroxyapatite composite fiber
.
Mater. Sci. Forum
743–744
,
480
485
.
Zhu
,
B.
,
Doherty
,
C. M.
,
Hu
,
X.
,
Hill
,
A. J.
,
Zou
,
L.
,
Lin
,
Y. S.
&
Duke
,
M.
2013
Designing hierarchical porous features of ZSM-5 zeolites via Si/Al ratio and their dynamic behavior in seawater ion complexes
.
Microporous Mesoporous Mater.
173
,
78
85
.
Zhu
,
B.
,
Hu
,
X.
,
Shin
,
J.-W.
,
Moon
,
I.-S.
,
Muraki
,
Y.
,
Morris
,
G.
,
Gray
,
S.
&
Duke
,
M.
2017
A method for defect repair of MFI-type zeolite membranes by multivalent ion infiltration
.
Microporous Mesoporous Mater.
237
,
140
150
.