Vaterite calcium carbonate for the adsorption of Congo red from aqueous solutions

Journal of Environmental Chemical Engineering 2 (2014) 2156–2161

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Vaterite calcium carbonate for the adsorption of Congo red from aqueous solutions Kai Yin Chong, Chin Hua Chia *, Sarani Zakaria, Mohd Shaiful Sajab Materials Science Program, School of Applied Physics, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, Bangi, Selangor 43600, Malaysia

A R T I C L E I N F O

A B S T R A C T

Article history: Received 6 May 2014 Received in revised form 16 September 2014 Accepted 22 September 2014

The present study describes the evaluation of vaterite calcium carbonate (CaCO3) for the adsorption of Congo red (CR) from aqueous solution. Vaterite CaCO3 is produced via a precipitation method assisted by ethylene glycol (EG), which acts as stabilizer to prevent transformation of vaterite to others polymorphs of CaCO3. The crystal structure of vaterite CaCO3 formed was proven by SEM, XRD and FTIR. The highest zeta potential of the vaterite CaCO3 was 15.2 mV, which was obtained at pH 5. Batch adsorption studies illustrated that the adsorption of CR to the vaterite CaCO3 was dependent on different parameters, such as pH, initial dye concentration and temperature. The adsorption isotherm data were highly consistent with Langmuir isotherm model, while the adsorption kinetics data fitted better to the pseudo-second-order model. The maximum adsorption capacity of the CaCO3, 32.60 mg/g, was obtained from the adsorption kinetic experiment under adsorption conditions, pH 5, adsorbent dose 0.2 g, adsorbate concentration 100 mg/L and temperature 25
C. The adsorption of CR on the CaCO3 was proven to be endothermic and non-spontaneous in nature. ã 2014 Elsevier Ltd. All rights reserved.

Keywords: Adsorption Calcium carbonate Congo red Ethylene glycol Vaterite

Introduction Most dyes used in industries are of synthetic chemical compounds with complex aromatic structures [1]. Synthetic dyes are widely used in various industries, including textile, rubber, paper, plastic, cosmetic, paint, leather, food, and pharmaceutical [2]. Azo dyes are popular and widely used in textile industry [3]. They can cause environmental pollution problems by forming hazardous aromatic amines through metabolic processes in plants and animals. They can cause carcinogenic and mutagenic effects in animal even at low concentration [4]. Azo dye can also cause coloration on the surface of water, which would block the penetration of sunlight and decrease oxygen levels in water. Moreover, azo dyes are difficult to be degraded via biodegradation and photocatalysis due to their complex and stable aromatic molecular structures [3]. There are various treatment processes for the removal of dyes from wastewater, such as adsorption, coagulation, oxidation, membranes separation, etc. [5]. Amongst, adsorption is one of the effective, simple and economical methods to remove different

* Corresponding author. Tel.: +603 8921 5473; fax: +603 8921 3777. E-mail addresses: [email protected], [email protected] (C.H. Chia). http://dx.doi.org/10.1016/j.jece.2014.09.017 2213-3437/ ã 2014 Elsevier Ltd. All rights reserved.

types of pollutants from wastewater. There are various types of inorganic particles which have been used as adsorbent for the removal of dyes and heavy metal ions from aqueous solutions, such as MnO2 [6] and kaolinite [7] used for removal methylene blue (MB), Ni(OH)2 and NiO for the removal of Congo red (CR) [8]. CaCO3 is one of the most abundant minerals on Earth. It has been used as filler in many consumer products, such as paint, plastic, rubber, paper, printing ink, toothpaste, cosmetics and food [9], due to its low cost, excellent physical and chemical characteristics, biocompatibility, biodegradability, etc. [10]. Besides, CaCO3 has been used to remove fluoride [11] and heavy metals from water [12]. There are three types of anhydrous crystalline polymorphs of CaCO3, i.e., calcite, aragonite, and vaterite [9]. Vaterite is a less stable polymorph which can be transformed easily into a thermodynamically stable form of calcite [13]. However, the excellent properties of vaterite, including high specific surface area, high dispersivity, and low specific gravity, make it widely used in many applications compared to the two others [14]. In this study, vaterite CaCO3 is produced via wet precipitation method using ethylene glycol (EG) as stabilizer. The produced CaCO3 was used as adsorbent to remove CR from aqueous solution. The effects of different parameters, such as pH, temperature and dye concentration, on the adsorption performance of the CaCO3 were investigated.

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Materials and methods

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20 mg of CaCO3 powder was weighted and added into the deionized water for each measurement.

Chemicals Adsorption studies of CR on CaCO3 Calcium chloride (96.0%, CaCl2) and sodium carbonate (99.5%, Na2CO3) were purchased from Sigma–Aldrich (Japan). Ethylene glycol (EG, 99.5%, Mw = 62.07 g/mol) was purchased from Systerm (Malaysia). Congo red (pure grade, C32H22N6O6S2  2Na, Mw = 696.7) was purchased from Acros (USA). All chemicals were of analytical grade and used without further purification. Hydrochloric acid (37%, HCl) was purchased from Merck (Germany) while sodium hydroxide (NaOH, 99.0%, Mw = 40.0 g/mol) was purchased from R&M Chemical (UK). Measurements of CR concentration Concentration of CR in aqueous solution was analyzed using a UV–vis spectrophotometer (Jenway 7315 Spectrophotometer). Standard solutions of CR with different concentrations, ranging from 10 to 100 mg/L, were scanned at a lmax of 500 nm and the absorbance intensity was used to prepare a calibration curve according to Beer–Lambert’s law, as shown in Fig. S1. This calibration curve was used to determine the concentration of CR after adsorption experiment. Supplementry material related to this article found, in the online version, at http://dx.doi.org/10.1016/j.jece.2014.09.017. Synthesis of vaterite CaCO3 A EG solution (20 vol%) was prepared using deionized water. Then, 0.5 M CaCl2 and 0.5 M Na2CO3 were prepared using the EG solution. Both solutions were mixed in a beaker and stirred at room temperature (20
C) by using a magnetic stirrer for 60 min. The precipitated CaCO3 was filtered, washed with deionized water, and dried in a freeze dryer (ScanVac CoolSafe). Characterizations of the CaCO3 Micrograph of the CaCO3 sample was taken by a variable pressure scanning electron microscope (VPSEM LEO 1450VP) at 30 keV. The crystalline phase and structure of the synthesized CaCO3 were detected by an X-ray diffractometer (XRD) Bruker D8Advance. Fourier transform infra-red spectroscopy (FT-IR) analysis was performed on a PerkinElmer Spectrum 400 FT-IR NIR. Zeta potential, z, of the CaCO3 at different pHs was analyzed using a Malvern Zetasizer Nano ZS. Deionized water with different pHs (5, 7, and 9) was prepared using 0.1 M HCl and 0.1 M NaOH. About

Effect of pH on adsorption Adsorption experiments were conducted to investigate the effect of initial pH (pH0 = 3, 5 and 7) of the CR solution on the adsorption capacity of the CaCO3. All the experiments were carried out at 20
C using 0.2 g CaCO3 in 100 mL CR (100 mg/L). The pH of the CR solution was adjusted using 0.1 M HCl and 0.1 M NaOH. The mixture was left to stir at 300 rpm using an overhead stirrer for 3 h to achieve adsorption equilibrium. The concentration of CR was measured using the UV–vis spectrophotometer at lmax of 500 nm. The adsorption capacity, qe (mg/g), of the CaCO3 was calculated using Eq. (1): qe ¼

C0  Ce V m

(1)

where C0 and Ce are the initial and equilibrium concentration of CR (mg/L), respectively. V is the volume of the CR solution (L) and m is the mass of the CaCO3 (g). Batch adsorption kinetics To study the adsorption kinetics of CR on the CaCO3, the mixture of CaCO3 and CR was stirred at 300 rpm using the overhead stirrer. A 0.2 mL aliquot sample was withdrawn at various time intervals, i.e., 10, 20, 30, 45, 60, 90, 120, 150 and 180 min. The aliquot was centrifuged at 12,000 rpm for 10 min. The concentration of CR adsorbed at time t,qt (mg/g), was calculated using Eq. (2): qt ¼

C0  Ct V m

(2)

where Ct is the concentration of CR at time, t (mg/L). Adsorption isotherms Adsorption isotherm experiments were conducted using 30 mg of CaCO3 and 15 mL of CR solution with different initial CR concentrations, i.e., 25, 50, 100, 200 and 300 mg/L, in glass vials at different temperatures (25, 40 and 60
C) for 4 h using a water bath shaker. The mixture was then centrifuged at 12,000 rpm for 10 min, and the concentration of CR at the adsorption equilibrium was analyzed by the UV–vis spectrophotometer. Results and discussion Characterization of CaCO3 SEM Fig. 1 shows a SEM image of the produced CaCO3. As can be seen that the CaCO3 is in spherical shape with an average diameter

Fig. 1. SEM image of vaterite CaCO3 synthesized in the presence of EG. The inset is the vaterite particle at 10 k magnification.

Fig. 2. XRD pattern of vaterite CaCO3–EG.

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Fig. 5. Possible electrostatic interactions between CR and CaCO3.

Fig. 3. FTIR spectrum of vaterite CaCO3–EG.

2.7  0.4 mm. Spherical CaCO3 is known to be vaterite [9]. There is no other polymorph of CaCO3 observed, such as calcite and aragonite. The favor of the formation of vaterite may due to the presence of hydroxyl groups on EG which serve as sites to interact with the nuclei of vaterite. This further promotes aggregation of the vaterite to change the surface energy of vaterite, preventing it to be transformed into calcite and aragonite [15]. XRD and FTIR XRD pattern of the produced CaCO3 confirmed the lattice structure of vaterite phase (see Fig. 2). The Bragg reflection peaks for the vaterite crystal at 2u = 20.9, 24.9, 27.1, 29.4, 32.8
, 43.8
, 50.0
and 55.8
are corresponding to facets (hkl) of (0 0 4), (11 0), (11 2), (1 0 4), (1 1 4), (3 0 0), (118) and (2 2 4), crystal planes, respectively [16]. Peaks corresponding to calcite and aragonite were not observed in the XRD pattern, suggesting the effective stabilization on the vaterite by EG. This is also consistent with the SEM image shown in Fig. 2. The structure of the vaterite CaCO3 was also further confirmed by FTIR analysis, as shown in Fig. 3. The strong absorption bands at 744.8, 849.6, 872.6 and 1088.8 cm1 are the evidence of the formation of the vaterite polymorph [15]. The peaks located from 700–1400 cm1 can be attributed to the CO bonding of CaCO3 [17]. The absence of absorption bands of calcite and aragonite has further confirmed that the sample is purely vaterite, which is in agreement with the XRD result.

pH lower than 5, due to the solubility of CaCO3 [18]. The z of the vaterite CaCO3 decreased with the increase of pH, resulting in the decrease of adsorption capacity of the CaCO3. This can be attributed to the greater competition between the OH  ions and CR molecules to be adsorbed on the CaCO3. The greater surface charges of the CaCO3 at low pH enhanced the electrostatic attraction between the negatively charged CR and positively charged CaCO3. The possible adsorption mechanism and interaction between CR molecule and the CaCO3 are shown in Fig. 5. Adsorption kinetic studies Adsorption kinetics experiments were conducted to study the adsorption mechanism of CR on the CaCO3 at different concentrations of CR. Pseudo-first-order and pseudo-second-order models were used to fit the experimental data. The pseudofirst-order kinetic model [19] is as follow: logðqe  qt Þ ¼ logqe 

k1 t 2:303

(3)

where qe and qt are the amount of dye adsorbed on the CaCO3 (mg/g) at equilibrium and time t, respectively. k1 is the rate constant of pseudo-first-order (min1). The slope and intercept of plots of ln (qe  qt) versus t were used to determine k1 and qe. Pseudo-second-order model can be expressed as follow [20]: t 1 t ¼ þ qt k2 q2e qe

(4)

Zeta potential and effect of pH on adsorption of CR on vaterite CaCO3 Fig. 4 presents the effect of pH on the zeta potential (z) and adsorption capacity of the CR to vaterite. The zeta potential results revealed that the CaCO3 is in positive charge at pH 5–9. Measurement of z of the CaCO3 is difficult to be conducted at

where k2 (g mg1/min) is the rate constant of pseudo-second-order model. The slope and intercept of plots of t/qt versus t were used to calculate k2 and qe. The linear plot of log (qe  qt) versus t of pseudo-first-order (which not shown here), provides a poor R2 value, ranging from 0.19 to 0.81. Besides, the calculated qe is lower than that of the qexp. This indicated that the pseudo-first-order model had limited applicability in interpreting the adsorption kinetics of CR on the vaterite CaCO3. The adsorption data were then analyzed using the pseudo-second-order kinetic model. The plots of t/qt versus t are shown in Fig. 6. The fitting of the adsorption data with the pseudo-second-order kinetics model for various initial CR

Fig. 4. Zeta potential (mV) of vaterite CaCO3 (left) and effect of initial pH of CR on vaterite CaCO3 (right) at different pH.

Fig. 6. Pseudo-second-order kinetics model for the adsorption of CR on vaterite CaCO3 (20
C, pH 7 and 100 mL of the CR solution, 0.2 g CaCO3).

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Table 1 Percentage of CR removal and pseudo-second-order kinetics model of CR adsorption on vaterite CaCO3. Initial CR concentration (mg/L)

25 50 100 200 300

Percentage of CR removal (%)

79.93 75.39 69.27 35.78 21.21

Experiment, qexp, (mg/g) 6.712 16.581 35.131 36.160 33.011

concentrations is presented in Table 1. In contrast to the pseudofirst-order model, the pseudo-second-order fitted the experimental data well, with exceptionally high correlation coefficients (R2 > 0.99) at all CR concentrations. In addition, the experimental adsorption capacity, qexp, is comparatively closer with the theoretical values, qe. This suggested that the rate determining step of the adsorption process might involve valency forces by sharing or exchanging of electrons between the adsorbent and the adsorbate [21]. The pseudo-second-order rate constant (k2) decreased when the CR concentration increased. This can be attributed to the higher competition of the dye molecules for accessing the surface active sites of the adsorbent at higher CR concentration [22]. Adsorption isotherms In this study, Langmuir and Freundlich models were used to interpret the adsorption tests data of CR on the vaterite CaCO3 at three different temperatures (25, 40 and 60
C). Langmuir theory presumes that adsorption is limited to the formation of monolayer coverage of adsorbate on a homogeneous adsorbent surface [23], where all adsorption sites are identical and energetically equivalent [24]. Langmuir equation is expressed as Eq. (5) and the linear form is given as Eq. (6): qe ¼

Ce ¼ qe

Q 0 bC e 1 þ bC e

(5)



 1 Ce þ Q 0b Q0

(6)

where Q0 is the maximum adsorption capacity of the adsorbent (mg/g), and b is the Langmuir constant related to the energy of the adsorption process (L/mg). The Q0 and b were calculated from the slope and intercept of straight lines obtained from plots between Ce/qe versus Ce (Fig. 7(a)). The essential characteristics of Langmuir isotherm can be expressed by a dimensionless constant, the separation factor (RL) that is defined by the following equation:

Pseudo-first-order

Pseudo-second-order

qe (mg/g)

k1 (g/mg min)

R2

qe (mg/g)

k2 (g/mg min)

R2

1.766 1.651 8.262 3.930 2.373

0.005 0.002 0.016 0.017 0.007

0.156 0.022 0.684 0.440 0.159

6.757 16.667 35.336 36.364 33.784

3.269 0.0373 0.0097 0.0187 0.0096

0.997 0.999 0.999 0.999 0.997

RL ¼

1 ð1 þ bC m Þ

(7)

where b is the Langmuir constant and Cm is the initial of dye concentration. RL indicates the nature of adsorption which, unfavorable RL > 1; linear RL = 1; favorable 0 < RL < 1; irreversible RL = 0 [25]. RL values calculated for different initial dye concentrations in this study were well within the defined range of 0.403, 0.390, and 0.365 for 25, 40 and 60
C, respectively (Table 2). This suggested that the adsorption of CR onto the vaterite CaCO3 is a favorable process under the conditions used for the experiments. The Freundlich model is based on the assumption that multilayer adsorption occurs on a heterogeneous adsorption surface containing unequally available sites of different adsorption energies [26]. Freundlich equation is written as Eq. (8) and the linearized form is expressed as Eq. (9): qe ¼ K F C e1=n

(8)

1 logqe ¼ logK F þ ðlogC e Þ n

(9)

where qe is the amount of the dye adsorbed per unit of adsorbent at equilibrium time (mg/g), Ce is equilibrium concentration of dye in solution (mg/L). KF is the Freundlich constant ((mg/g) (L/mg)1/n) and 1/n is the heterogeneity factor, which were calculated from the interception and slope of the Freundlich plots, respectively (Fig. 7 (b)). The calculated values of the Langmuir and Freundlich’s parameters are given in Table 2. The adsorption of CR on CaCO3 showed higher R2 for the Langmuir than the Freundlich model, suggesting that the adsorption of CR onto the homogeneous adsorbent surface of vaterite CaCO3 is with equal adsorption activation energy [6]. Adsorption thermodynamics The thermodynamic parameters, such as Gibbs free energy change (DG
), standard enthalpy (DH
) and standard entropy (DS
), were also studied to understand better the effect of

Fig. 7. (a) Langmuir plot for the adsorption of CR on vaterite CaCO3 at different temperature, (b) Freundlich plot for the adsorption of CR on vaterite CaCO3 at different temperature.

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Table 2 Adsorption isotherm constants for adsorption of CR on vaterite CaCO3 at different temperatures. Temperature (
C)

Langmuir isotherm

25 40 60

Freundlich isotherm 2

qm (mg/g)

b (L/mg)

R

RL

KF ((mg/g) (L/mg)1/n)

n

R2

12.392 14.493 16.529

0.0593 0.0626 0.0696

0.985 0.989 0.994

0.403 0.390 0.065

6.290 6.051 6.497

9.681 6.897 5.797

0.804 0.959 0.868

Table 3 Thermodynamic parameters for the adsorption of CR on vaterite CaCO3. T (K)

KL (L/mg)

DG
(kJ/mol)

DH
(kJ/mol)

DS
(J/mol)

R2

298 313 333

0.05925 0.06264 0.06962

7.002 7.209 7.377

3.827 3.827 3.827

10.708 10.708 10.708

0.9841 0.9841 0.9841

temperature on the adsorption process. The DG
of the adsorption process was calculated from the equilibrium constant (KL) using following equation:

DG
¼ RTlnK L

(10)

while DH
and DS
were estimated from van't Hoff equation [27]: lnðK L Þ ¼

DS
R



DH


(11)

RT

KL is the adsorption equilibrium constant, which is the ratio of concentration of dye adsorbed to the concentration of dye in solution at equilibrium. It represents the capability of the adsorbent to retain adsorbate and also the extent of its movement in the solution phase. Plot of ln KL versus 1/T should give a linear line, where the value of DH
(kJ/mol) and DS
(J/mol K) can be calculated from the slope and intercept of the plot, respectively. DG
(kJ/mol) was then calculated from the obtained DH
and DS
[28]. The obtained thermodynamic parameters are summarized in Table 3. The increase of KL and adsorption capacity of the vaterite CaCO3 with temperature revealed that the adsorption is an endothermic process. Higher temperature increased the mobility of the dye molecules and the number of active sites for the adsorption. The calculated DH
value, 3.827 kJ/mol, further confirms the endothermic nature of the adsorption of CR on the vaterite CaCO3, where dye uptake increased with increasing temperature [29]. Besides, the DH
is in the range of 0–20.0 kJ/mol, suggesting that the adsorption of CR on vaterite CaCO3 was mainly taken place by physisorption [30]. The positive DG
values obtained reveal that the adsorption process is non-spontaneous and the influence of entropy is more dominant than enthalpy [31]. The negative value of DS
suggested

Table 4 Comparison of adsorption capacity of different adsorbents toward CR. Adsorbent

Adsorption capacity (mg/g)

36.2 B-cyclodextrin (B-CD)-based polymers Waste red mud 4.05 Montmorillonite 12.70 Australian 7.27 kaolin 5.18 Cashew nut shell 16.6 Vaterite CaCO3

Reference

[4]

[32] [33] [28] [34] This study

the decrease in randomness at the solid/solution interface and no significant changes occurred in the internal structure of the adsorbent throughout the adsorption process [28]. Comparison of the adsorption performance of various adsorbents toward CR is presented in Table 4. Although the performance of the vaterite CaCO3 produced in this study is lower than that of some other adsorbents. However, the simplicity and low cost of the preparation method of CaCO3 could be one of the advantages for this adsorbent to be used in industrial scale applications.

Conclusion The results reported in this study show that vaterite CaCO3 produced has good capability to adsorb CR due to his high positive surface charge. From the study of the influence of the physico– chemical parameters on the adsorption kinetics, it appeared that, at lower pH the amount adsorbed was high due to the greater surface charges of the CaCO3. The kinetics and isotherms studies were best described by the pseudo-second-order kinetics and Langmuir isotherm models respectively. The adsorption process is non-spontaneous and endothermic. The value of DH
obtained confirms that the main adsorption mechanism between CR molecules and Vaterite CaCO3 was physical adsorption. Ultimately, this study shows that vaterite CaCO3 has high positive surface charge. Therefore, it can be used for the removal of organic micropollutants (dyes for example) in aqueous solution.

Acknowledgments The authors would like to acknowledge the financial support given by UKM University Research Grant DIP-2014-013. Chong acknowledges the Ministry of Education (MOE) for the disbursement of MyMaster scholarship.

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