thermodynamics of adsorption of fluoride, bromide and iodide …psrcentre.org/images/extraimages/46...

Abstract— A Thermodynamics studies of the anions such as

Fluoride, Bromide and Iodide removal from contaminated water was studied using activated zeolite as adsorbent. The Langmuir adsorption isotherm model provided a good fit to the experimental data; this finding allowed the calculation of the thermodynamic parameters such as the changes in Gibbs free energy (ΔGo), enthalpy (ΔHo) and entropy (ΔSo) at temperatures of 293K, 303K, 313K and 323K, respectively. In the realistic system, the anions adsorption process proved to be spontaneous and exothermic. It was noticed that an increase in temperature resulted in higher anions loading per unit weight of the adsorbent.

Keywords—Thermodynamics; adsorption; activated zeolite; fluoride; bromide; iodide

I. INTRODUCTION HE anions such as fluoride (F-), bromide (Br-) and iodide (I-) were selected from this study because of their harmful

effects on human health when present in high concentrations. To remove these toxic anions from water, the adsorbent must have the anion exchange properties. Natural zeolites, due to their large areas, high cation exchange capacities, favorable hydraulic characteristics, and low cost, are a major class of material being considered for adsorption. Because zeolites are crystalline alumino-silicates with the structure based on tetrahedral SiO4 and AlO4 units, connected by shared oxygen atoms, they are one of the synthetics inorganic cation-exchangers [1]. This kind of three-dimensional structure has small pores where the exchangeable ions are located and the ion exchange reactions take place. Zeolites possess a net negative structure charge resulting from isomorphic substitution of cations in the crystal lattice. This permanent negative charge results in the favorable ion-exchange selectivity of zeolites for certain cations. This negative charge also causes natural zeolites to have little or no affinity for anions. Because of this property, the zeolites cannot remove anions but fortunately the modification of zeolites by the cationic surfactant can enhance

J. Kabuba is with the Centre of Renewable Energy and Water, Department of Chemical Engineering, Vaal University of Technology, Private bag X021, Vanderbijlpark 1900, South Africa (phone: +27-16-950-9887; Fax: +27-16-950-9796; e-mail: johnka@ vut.ac.za).

A. Mulaba-Bafubiandi is with the Minerals Processing and Technology Research Centre, Department of Metallurgy, School of Mining, Metallurgy and Chemical Engineering, Faculty of Engineering and the Built Environment, University of Johannesburg, PO BOX 526, Wits 20150, South Africa, Tel. +27-11-559-6215; Fax: +27-11-559-6491. (e-mail: amulaba @uj.ac.za).

the capacity of zeolites to remove anions such as F−, Br−and I- from aqueous solution. Hexadecyltrimethylammonium (HDTMA) is a tetra substituted ammonium cation with permanently charged pentavalent nitrogen and a long straight alkyl chain (C16), which imparts a high degree of hydrophobicity. With quaternary amines ions HDTMA has been used to activate the anionic sites [2]. Recently, the utilization of natural zeolites, particularly clinoptilolite modified with cationic surfactant (SMZ) with the purpose to remove multiple types of contaminants from water was studied by many researchers especially from Li and Bowman groups [3], [4], [5] and [6]. Recent study by Campos showed that the modification of mordenite by ethylhexadecyldimethyl ammonium (EHDDMA) and hexadecyltrimethyl ammonium (HDTMA) can remove hexavalent chromium [2]. Although a number of investigations have been conducted on the modification of zeolites by the cationic surfactant to enhance the capability of zeolites to remove anions in water, some species anions need to be given a particular focus for investigation. In this work, the adsorption of F−, Br−and I- from aqueous solutions onto zeolites obtained by modification of clinoptilolite with hexadecyltrimethyl ammonium (HDTMA) ions was studied. The aim of this work was to investigate the thermodynamic study on the removal while the distribution of removed species in the adsorbent contaminated water systems is elaborated using the adsorption isotherms.

II. MATERIAL AND METHODS The zeolite used in this study was sourced from the Vulture

Creek, KwaZulu-Natal Province of South Africa. The zeolite was washed several times with distilled water to remove surface impurities and was dried at 100oC for 24 h. The sample was crushed and milled into powder with average particle sizes of approximately 2 mm. The crushed sample was then activated with 0.5M hexadecyltrimthylammonium (HDTMA) at 25°C for 8 h. The mixture (zeolite with surfactant solution) was centrifuged, and the supernatant solution was decanted for measurement. The solid was then rinsed with deionized water several times, filtered, air dried, and stored in plastic containers before they were used for adsorption. The HDTMA-activated zeolite sample and non-activated zeolite samples were subjected to SEM in order to determine the effect of chemical conditioning on the sample. The solutions were assayed using atomic adsorption spectroscopy (AAS), (Model Varian Spectra (20/20)).

John Kabuba, and Antoine Mulaba-Bafubiandi

Thermodynamics of Adsorption of Fluoride, Bromide and Iodide ions by an activated zeolite

T

International Conference on Chemical, Mining and Metallurgical Engineering (CMME'2013) Nov. 27-28, 2013 Johannesburg (South Africa)

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Batch experiments were conducted to determine the effect operating temperature, initial concentration and contact time on ion-exchange process of clinoptilolite as ion-exchanger material. The effect of pH was carried out in pH range of 2-10 and the effect of various operating temperatures ranging from 20oC (293K), 30oC (303K), 40oC (313K) and 50oC (323K) was investigated. Temperature adjustments were conducted in the same orbital shaker. Initial concentrations of synthetic solution were analyzed by ASS and the samples were taken at predetermined time intervals (10, 20, 30, 40, 50 and 60 min.) on the percentage of anion by the clinoptilolite. In each case, 10 g of the activated zeolite was mixed with one liter of the synthetic solution and held in a closed polyethylene flask at 90oC for 24 h. Each experiment was performed in duplicate to observe the reproducibility and the mean value used for each set of values. The uptake efficiency (%) was calculated as follows:

% Uptake = (Co- Cf) x100/Co (1)

Where Co (mg/l) is the initial concentration and Cf (mg/l) the final concentration.

The thermodynamic parameters for ion-exchange process were studied for the trial of ion-exchange at 25 mg/l of anions at 25oC and pH 7.

III. RESULTS AND DISCUSSION

A. Effect of activation The SEM images in Fig. 2 shows a more open structure in

the form of larger agglomerates. Moreover most particles have lost their initial layered shape and converted into irregular shapes of the HDTMA-activated zeolite’s surface compared to Fig. 1 which present the original zeolite. This could explain why HDTMA -activated zeolite performed better than the original zeolite. There were some morphological changes where the clinoptilolite was observed to exhibit a more open and somewhat softened structure brought about by HDTMA activation on zeolite structure when comparing the two SEM images. HDTMA is a tetrasubstituted ammonium cation with permentantly charged pentavalent nitrogen and a long straight alkyl chain (C16), which imparts a high degree of hydrophobicity. The quaternary amines (HDTMA) can substantially enhance the removal of anion from water.

Fig. 1 SEM of Original zeolite

Fig. 2 SEM of HDTMA-activated zeolite

The chemical composition of the HDTMA-activated zeolite was checked for metal ions Na, Ca, Al, and Si using energy dispersive spectrometry. A revealing feature in the synthesis of zeolite is the strong correlation between Si/Al ratio of the resulting crystals, the nature of the cation used and medium of synthesis. The flexibility in the synthesis to produce desired composition of zeolite represent a critical step in the improvement of the material for many environmental process in which Si/Al ratio is the key for maximizing performance. The chemical composition of the prepared material is given in Fig. 3. It is observed that composition of the prepared material is quite close to activated zeolite.

Fig. 3 Energy dispersive spectrometry (EDS) of HDTMA-activated

zeolite

B. Adsorption isotherm study

B.1 Langmuir isotherm

The Langmuir model represented by the following linear equation:

maxmax

1QKQ

CqC

L

e

e

e += (2)

Where Ce (mg/l) is the equilibrium concentration of anion in the solution and KL (l/mg) is the Langmuir constant related to the ion-exchange capacity.

The “qe” in the Langmuir equation is the solids loading corresponding to complete but monolayer coverage. In a specific adsorption system at any particular temperature, Qmax (mg/g) is the maximum loading that can be achieved by


222

increasing the equilibrium concentration, which practically means increasing the initial concentration. Consider the following form of the Langmuir isotherm equation:

111

max +=

eL

e

CKQq

By increasing the equilibrium concentration Ce,

001

max

=⇒=Qq

CKe

eL

This means that the equilibrium

loading (qe) will eventually become equal to Qmax. It is obvious that this capacity is a characteristic parameter of the adsorption system of a given temperature. This model allowed the determination of the adsorption energy of a solute as the corresponding change in Gibbs free energy (∆Go). The parameters Qmax and KL deduced from the Eq. (2) were determined from the slope and intercept

[plot )( ee

e CfqC

= ] as shown in Fig. 4-6 and are listed in

Table I. The ∆Go values suggest the following order of affinity of the zeolite for the anions studied, namely I- < F- < Br- which is in agreement with the results published by Maji et al., [7]. The Langmuir isotherm fairly good fit to the experimental results obtained with as indicated in Table I.

Fig. 4 Ce/qe vs. Ce for the adsorption of F-

Fig. 5 Ce/qe vs. Ce for the adsorption of Br-

Fig. 6 Ce/qe vs. Ce for the adsorption of I-

B.2 Freundlich isotherm

The Freundlich model is linearized as follows:

)(log1loglog eLe Cn

Kq += (3)

Where KF and n (dimensionless) are Freundlich isotherm constants determined from the nonlinear regression. The values of the parameter n are representative of both the adsorption intensity and the surface heterogeneity which is related to the affinity or binding strength. Despite the parameter n lacking physical meaning it is generally admitted

that 110 <<n

indicates favourable adsorption, while 1/n = 1

characterizes a linear adsorption phenomenon. The KF constant may be considered as a rough indication of adsorption capacity expressed in mg/g or as a sorption affinity parameter. Considering Ce and qe expressed in mg/l and mg/g, respectively, KF (dimensionless). Thus, when 1/ n = 1 (linear adsorption), when KF is expressed in l/g and can be related to an adsorption energy, and when 1/ n → 0, then KF → qe and KF can then be roughly expressed in mg/g. The Freundlich constants n and KF appear to incorporate all the physicochemical factors influencing the adsorption process. The high 1/ n values are associated with low KF values and a high fitting of the model with the experimental values (0.900 < R2 < 0.970). Thus the Freundlich multilayer sorption model appears to describe the lower energy processes relatively well.

The parameters n and KF deduced from the linear regression were determined from the slope and intercept [plot

)(log)log( ee Cfq = ] as shown in Fig. 6-8 and the values are listed in Table I.


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TABLE I ADSORPTION ISOTHERM MODELS CONSTANTS _______________________________________________________

Anion Temperature KL Qmax R2

__________________________________________________

F- 293 3.145 0.242 0.997 303 3.497 0.199 0.996 313 4.115 0.160 0.986 323 4.329 0.133 0.975 Br- 293 3.115 0.232 0.999 303 3.436 0.192 0.994 313 4.032 0.154 0.985 323 4.255 0.129 0.970 I- 293 3.067 0.253 0.996 303 3.424 0.197 0.987 313 4.016 0.151 0.977 323 4.219 0.125 0.973 _________________________________________________ Anion Temperature KF n R2

______________________________________________ F- 293 1.881 0.664 0.972 303 1.818 0.621 0.955 313 1.803 0.567 0.930 323 1.815 0.531 0.924 Br- 293 1.815 0.654 0.978 303 1.812 0.618 0.954 313 1.818 0.551 0.928 323 1.801 0.521 0.903 I- 293 1.812 0.644 0.966 303 1.818 0.610 0.936 313 1.808 0.554 0.906 323 1.815 0.531 0.900 ______________________________________________ Anion Temperature Qm KDR R2

______________________________________________ F- 303 0.026 1.810 0.911 313 0.024 1.700 0.890 323 0.023 1.590 0.882 333 0.021 1.485 0.809 Br- 303 0.024 1.560 0.882 313 0.022 1.480 0.876 323 0.021 1.390 0.850 333 0.200 1.285 0.845 I- 303 0.020 1.400 0.874 313 0.018 1.360 0.852 323 0.017 1.240 0.848 333 0.015 1.200 0.832 _______________________________________________________________

Fig. 7 log qe vs. log Ce for the adsorption of F-

Fig. 8 log qe vs. log Ce for the adsorption of Br-

Fig. 9 log qe vs. log Ce for the adsorption of I-

B. 3 Dubinin-Radushkevich (D-R)

2lnln εRDm KQQ −−= (4)

Where ε (Polanyi Potential, J/mol), Q is the amount of anion adsorbed (mg/g) and Qm is the adsorption capacity (mg/g). D-R isotherm constants values RDK − and mQ from Eq. (3) were calculated from the slope and intercept [plot

)()ln( 2εfQ = ] as shown in Fig. 10-12 and the values were also listed in Table I. The value of the regression (R2) obtained showed that this model provided a fairly good fit to the experimental data compared to the Langmuir and Freundlish isotherm. This suggests that micropore volume filling is the best representation of the sorption processes involved in the studies ion exchange processes.


224

Fig. 10 ln Q vs. ε2 for the adsorption of F-

Fig. 11 ln Q vs. ε2 for the adsorption of Br-

Fig. 12 ln Q vs. ε2 for the adsorption of I-

The value of the correlation coefficients (R2) is higher than the other two isotherm values presented in Table I and indicated that at each temperature, the Langmuir equation represents the best fit of experimental data. Freundlich and D-

R isotherm were used but the value of R2 seemed to be less than the Langmuir isotherm. The distribution of the adsorbent between the surface of the adsorbent and the solution at equilibrium at a given temperature has been described by Langmuir. The performance of the adsorbent can be evaluated in a more systematic and quantitative way through adsorption isotherm analysis. To describe the mechanism of sorption and to draw up the affinity constants, the experimental results were fitted to the linear form of Langmuir Eq. (1), the adsorption data obtained were subjected to Langmuir adsorption isotherms. Langmuir displays good application of anions sorption, the following isotherm is that of fluoride, bromide and iodide ions with a regression coefficient. Langmuir isotherms showed that the selectivity sequence of the tested anions was F-, Br- and I- according to their regression coefficients. This was confirmed by the position of the three anions on the periodic table. Higher anion removal efficiencies were obtained at lower pH values than at higher pH values. This is because at lower pH there is a low concentration of competing anions as compared to high pH values where there is a large concentration of competing anions. These competing anions lead to low efficiencies due to the fact that ion selectivity is based on valence and ion salvation of the anion. Thus F- will be absorbed first followed by Br- and I-. Lower synthetic solution concentrations resulted in higher removal efficiencies of anions as compared to high synthetic solution concentration. This is due to the fact that at lower concentrations particles travel faster than that at higher concentrations and also lesser amount of anions compete for the same clinoptilolite pores. The values of LK (constant

related to the energy of adsorption) and maxQ at different temperatures are reported in Table I. As the temperature increases the dehydration of active sites (-N+H2/-N+H3) increases giving a strong anion interaction (higher LK

values). The lower LK values may be related to the steric and electronic effects of methyl group substituent on positive nitrogen active sites. These effects hinder the interaction of anions by steric hindrance and lowering the positive charge on nitrogen by hyper conjugation of the methyl group [8]. The essential features of the Langmuir adsorption isotherm can be expressed in terms of a dimensionless constant called the separation factor ( LR ) which is defined by the following equation [8]:

oLL CK

R+

=1

1 (5)

Where oC is the initial concentration of the anions in the

solution. The values of LR indicate whether the nature of the

adsorption to be irreversible ( LR = 0), favourable

( 10 << LR ) or unfavourable ( LR = 1) [9]. A value of LR

< 1 implies that the adsorption of anions on zeolite from an aqueous solution is favourable under the conditions used in this study.


225

C. Thermodynamic parameters

In any adsorption procedure, both energy and entropy considerations should be taken into account in order to determine which process will take place spontaneously. The values of the thermodynamic parameters are the actual indicators for the practical application of a process [19]. The amounts of anions adsorbed at equilibrium at varying temperatures 293K, 303K, 313K and 323K respectively were examined to obtain thermodynamic parameters for the adsorption system. Because LK is the Langmuir constant and its dependence on temperature and can be used to predict thermodynamics parameters such as changes in Gibbs free energy (ΔGo) enthalpy (ΔHo) and entropy (ΔSo) associated with the adsorption process, these parameters were determined by the following equation:

∆Go = −RT ln KL (6)

Where ∆G◦ is the standard free energy change (kJ/mol), T is

the temperature in Kelvin and R is the universal gas constant (8.314 J mol−1 K−1). The sorption distribution coefficient KL for the sorption reaction was determined from the slope of the plot ln (qe/Ce) against Ce at different temperatures and extrapolating to zero Ce according to the method suggested by Khan and Singh [20]. The sorption distribution coefficient may be expressed in terms of ∆Ho and ∆So as a function of temperature and the Thermodynamics parameters of the adsorption were calculated using Van’t Hoff equation [15]:

RS

RTHK

oo

L∆

+∆

−=ln (7)

Where ∆Ho is the standard enthalpy change (kJ/mol) and ∆So is standard entropy change (kJ/mol K). The values of ∆Ho and ∆So can be obtained from the slope and intercept of a plot of ln KL against 1/T as shown in Fig. 13. The thermodynamic behavior of the adsorption of anions (F-, Br-. I-) onto clinoptilolite was evaluated and the thermodynamic parameters are given in the Table II. The negative values of ΔGo obtained at all temperature levels studied revealed the fact that the adsorption process was spontaneous with clinoptilolite showing affinity for anions. However, the negative value of ΔGo increased with an increase in temperature, indicating that the spontaneous nature of the exchange is proportional to temperature. Positive values of ΔHo indicate the endothermic nature of the adsorption process. ΔGo decreases as the temperature increases which indicates that anion sorption is less favoured at higher temperatures. This may again be attributed to the increased steric effect of the methyl group due to their increased vibrational motion with increasing temperature [9]. These thermodynamic features of the anions towards the adsorption on zeolite confirm the suggestion that the structural characteristics of the anions play an effective role in the strength of interaction. The positive value of ΔSo confirms the increased randomness of the solid-solution interface during the adsorption of anions onto clinoptilolite adsorbent.

TABLE II

THERMODYNAMIC PARAMETERS ON ION EXCHANGE AT DIFFERENT TEMPERATURES

_______________________________________________________________

Anion Temperature KL ∆Ho ∆Ho ∆Ho

__________________________________________________

F- 293 3.145 2.789 303 3.497 6.246 0.0089 3.047 313 4.115 3.446 323 4.329 3.571 Br- 293 3.115 2.767 303 3.436 6.201 0.0090 3.006 313 4.032 3.395 323 4.255 3.527 I- 293 3.067 2.730 303 3.424 5.652 0.0085 2.998 313 4.016 3.306 323 4.219 3.505 _______________________________________________________________

Fig. 13 ln KL vs. 1/T for the adsorption of anions (Br-, F-, I-)

IV. CONCLUSION This paper demonstrates that clinoptilolite is an effective

adsorbent and can be successfully used as an adsorbing agent for the removal of anions (Br-, F- and I-) from aqueous solutions. Regression coefficients R2 were found to be higher than 0.98, revealing the best fit for the adsorption data by the Langmuir isotherm model for the three anions. From the results obtained the fluoride ion was exchanged faster than the bromide and the iodide ions. This is due to organic chemistry Br–, F– and I– behave as a congeneric series of Lobe-HOMO Lewis bases. The anion adsorption equilibrium states for HDTMA-zeolites can be reached via ion-exchange processes and Van der Waals forces. It also suggests that anion exchange and electrostatic interaction are probably the main mechanisms that govern the anion adsorption. The thermodynamic parameters, ΔHo, ΔSo, and ΔGo values, of adsorption (F-, Br- and I-) onto clinoptilolite show the exothermic heat of adsorption, favoured at higher temperatures. The positive value of ΔSo revealed an increase in randomness of the solid- solution interface during the adsorption of (Br-, F- and I-).


226

ACKNOWLEDGMENT Author wishes to thank the University of Johannesburg for

the experimental results.

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[8] A. M. Donia, A. A., Atia, D. H. Mabrook, D. H., “ Fast kinetic and efficient removal of As (V) from aqueous solution using anion exchange resins”, J. Hazard Mater., vol. 191, 2011, pp.1.

[9] W. Qian, X., Lin, X., Zhou, X., Chen, J., Xiong, J., Bai ,H.,Ying “Studies of equilibrium, kinetics simulation and thermodynamics of cAMP adsorption onto an anion-exchnage resin”. Chemical Engineering Journal, vol.165, 2010, pp. 907-915.

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