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Kinetics and thermodynamics of Cd(II) adsorption onto pyrite and synthetic iron sulphide
Separation and Purification Technology 51 (2006) 240–246
Kinetics and thermodynamics of Cd(II) adsorption onto pyrite and synthetic iron sulphide Mehmet Erdem ∗ , Arzu Ozverdi Department of Environmental Engineering, Fırat University, 23279 Elazı˘g, Turkey Received 26 December 2005; received in revised form 1 February 2006; accepted 2 February 2006
Abstract In this paper, the Cd(II) adsorption abilities of pyrite and synthetic iron sulphide (SIS) were studied. Experiments were carried out as a function of pH, Cd(II) concentration, contact time and temperature. Maximum adsorption yields for pyrite and SIS were determined to be 34.2% and 65.8% under the conditions of initial Cd(II) concentration of 100 mg/l, pH 5.4, contact time of 120 min and adsorbent dosage of 20 g/l, respectively. The adsorption data fitted both the Langmuir and Freundlich adsorption models. Adsorption capacities of SIS and pyrite at 25 ◦ C were found to be 3.05 and 2.08 mg Cd(II)/g, respectively. The first-order and pseudo second-order rate expressions were applied to experimental data and it was determined that the adsorption process followed the first-order kinetic model. In addition, activation energy values and some thermodynamic parameters such as G◦ , H◦ and S◦ for the cadmium adsorption processes were calculated from the isotherm and kinetic data. The adsorption of Cd(II) on to SIS and pyrite was found to be endothermic and spontaneous. © 2006 Elsevier B.V. All rights reserved. Keywords: Cadmium; Adsorption; Kinetics; Isotherms; Pyrite; Iron sulphide
1. Introduction Heavy metal pollution in wastewaters is an extremely important environmental problem. The main sources of heavy metals for wastewaters are mining, metal industries and some other industrial areas using metals and metal salts. Lead, chromium, cadmium, copper, zinc and mercury are among the most frequently encountered metal contaminants [1]. Heavy metals are extremely toxic and threaten to the living by joining the food chain. When they release into the waters, most of them are strongly retained and their adverse effects can last for a long time. Thus, it is important to apply an effective treatment method to wastewaters polluted with heavy metals. In order to remove the toxic metal ions from wastewater, current methods are chemical precipitation, ion exchange, solvent extraction, adsorption and reverse osmosis techniques [2]. Chemical precipitation, especially as metal hydroxide or sulphide, is widely practiced, having the advantages of simplicity and inexpensive chemicals. However, it is not effective to reduce heavy metal concentration to low level required by water qual-
∗
Corresponding author. Tel.: +90 424 2370000; fax: +90 424 2415526. E-mail address: [email protected] (M. Erdem).
1383-5866/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2006.02.004
ity standards [3] and generation of a voluminous toxic waste sludge is a major problem encountered. Therefore, in the last few decades, adsorption process has received much concern and become an alternative to conventional precipitation technique, especially for wastewaters that contain low concentrations of metals and complex forming substances [2]. Activated carbon is the most widely used adsorbent in the wastewater treatment. Owing to high-cost of activated carbon, usage of the low-cost adsorbents such as agricultural wastes [4], metallurgical slags [5,6], fly ashes [7,8] and various minerals have been investigated [9–17]. In order to remove cadmium from aqueous solutions by minerals such as perlite [18], low-grade phosphate [19], clinoptilolite [20], goethite and aluminum oxide [21], montmorrilonite and kaolinite [22], akaganeite-type nanocrystals [23], a lot of investigations have been done. Currently, a few studies on the usage of pyrite as an adsorbent have been reported. But, these studies have been generally focused on the removal of molybdate and tetrathiomolybdate and influence of cadmium sorption on FeS2 oxidation [24,25]. Pyrite and synthetic iron sulphide have not been systematically investigated and evaluated for the metal adsorption purposes. That is, any data have been reported for the adsorption of Cd(II) onto pyrite, one of the abundant minerals in the nature, and synthetic iron sulphide. Starting from this point, in this study, it has been investigated Cd(II) adsorption
M. Erdem, A. Ozverdi / Separation and Purification Technology 51 (2006) 240–246
Nomenclature A b C0 Ce EA G◦ H◦ kad kF m n Q◦ qe q r R R2 S◦ t T x
frequency factor Langmuir constant (l/mg) initial Cd(II) concentration (mg/l) equilibrium Cd(II) concentration (mg/l) activation energy (kJ/gmol) Gibbs free energy change (kJ/gmol) enthalpy change (kJ/gmol) adsorption rate constant (min−1 ) Freundlich constant (mg/g) amount of adsorbent (g) Freundlich constant maximum adsorption capacity (mg/g) amount of Cd(II) adsorbed per g of adsorbent at equilibrium (mg/g) amount of Cd(II) adsorbed per g of adsorbent at any time (mg/g) dimensionless equilibrium parameter universal gas constant (8.314 J/gmol K) correlation coefficient entropy change (kJ/gmol) time (min) temperature (K) amount of adsorbed Cd(II) (mg)
241
separately. The visible impurities were removed by hand from pyrite sample and then the samples were ground and sieved. The fractionated materials in the particle size of <53 m were dried at 60 ◦ C for 6 h, and then they were stored in a tightly closed jar throughout the study. Characterizations of the samples were explained in our previous studies [26,27], however, results of the chemical and mineralogical compositions of the samples determined in previous study mentioned above are given in Table 1. In order to prepare the experimental solutions, stock solution of Cd(II) (10 g/l) was prepared by dissolving its nitrate salt in distilled water. The working solutions were prepared by diluting the stock solution with distilled water. Taking into consideration common concentration values of cadmium in actual wastewaters [2,28], 100 mg Cd(II)/l has been selected as the suitable initial metal concentration value. pH adjustments were made by using HNO3 and NaOH solutions in various concentrations.
properties of pyrite and synthetic iron sulphide. Langmuir and Freundlich adsorption isotherms have also been tested at various temperatures and adsorption kinetics and some thermodynamic parameters have been determined. 2. Experimental The pyrite sample, collected from pyrite-bed, Keban-Elazı˘g (Turkey), and synthetic iron sulphide (SIS), catalogue number of which is Merck-3908, were used as adsorbents in this study. The samples in gross particles (pyrite) and pellets (SIS) were crushed
Fig. 1. Effect of pH on the adsorption of Cd(II) by SIS and pyrite [initial Cd(II) concentration: 100 mg/l; SIS dosage: 20 g/l; pyrite dosage: 20 g/l; contact time: 120 min; temperature: 25 ◦ C].
Table 1 Chemical and mineralogical compositions of the pyrite and SIS Chemical compositions
Mineralogical compositions
Constituents
w/w
Minerals
Formula
Pyrite
Fe S Al Ca Mg Si Cu Co Ni Mn Zn
41.45% 36.43% 2.20% 3.58% 1.18% 5.47% 3700 mg/kg 300 mg/kg 200 mg/kg 700 mg/kg 350 mg/kg
Pyrite Quartz Calcite Dolomite Natrosilite Brussite Chamosite
FeS2 SiO2 CaCO3 CaMg(CO3 )2 Na2 Si2 O5 CaHPO4 ·2H2 O (Fe,Al,Mg)6 (Si,Al)4 O10 (OH)8
SIS
Fe S
61.03% 32.82%
Troilite Iron Wustite
FeS Fe FeO (minor amount)
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All the chemicals used in the study were of analytical reagent grade. Adsorption experiments were carried out by shaking 2 g/l of pyrite and 2 g/l of SIS with 100 ml aqueous solutions of Cd(II) in series of batch reactors at a constant speed of 200 rpm in an orbital shaker. Samples were withdrawn after predetermined contact time interval and filtered through 0.45 m filter paper. Final pH of the filtrates was measured by a pH meter. The filtrates were acidified with 1 ml of HNO3 solution to prevent the precipitation of metal ions and then they were analyzed to determine residual concentration of Cd(II). The concentration of metals in the solutions was determined by ATI-Unicam 929 atomic absorption spectrophotometer using flame atomization technique. pH measurements were done by a pH meter (Orion SA720). Standard solutions were prepared by using analytical chemicals. All dilutions were made by distilled water.
its sulphide (Eq. (2)) and consequently cadmium removal yield increases. But, the mechanism of Cd(II) removal from aqueous solution in this situation is not adsorption, it is completely chemical precipitation:
3. Results and discussion
The data of adsorption at different temperature indicated that the Cd(II) adsorption yield onto both adsorbents increased by the increasing temperature (Fig. 2). This increase by the increasing temperature indicates that the adsorption process is endothermic and higher temperature favours Cd(II) removal by adsorption onto SIS and pyrite. The removal efficiency of Cd(II) by adsorption on SIS and pyrite also increased with contact time and it attained a maximum value at 120 min except for 45 ◦ C for SIS.
3.1. Effect of pH Since pH is an important parameter affecting metal adsorption, the effect of the initial pH on Cd(II) removal by either pyrite or SIS was investigated at the pH range of 3.5–6 by taking into account precipitation pH value of Cd(II) [29]. The results obtained depending on equilibrium pH are given in Fig. 1. In the studies carried out with SIS, it has been determined that the adsorption yield of Cd(II) at equilibrium pH of 3.7 (initial pH of 2) is 100% and they decrease with the increasing pH up to 4.1 and then they increase at low rate again. When taking into account that cadmium ions have positive charge, it can be stated that the cadmium ions can be more effective adsorbed at the high pH values. On the contrary, the Cd(II) adsorption yield in the presence of SIS was rather high at pH below 4.1. This situation is associated with H2 S generation in acidic media (Eq. (1)). H2 S is known as an efficient reagent for metal precipitation in the form of metal sulphides. Therefore, due to generation of H2 S in solutions having pH below 4.1, Cd(II) precipitates as
FeS(s) + 2H+(aq) → H2 S(g) + Fe2+ (aq)
(1)
+ Cd2+ (aq) + H2 S(g) →↓ CdS(s) + 2H(aq)
(2)
The Cd(II) adsorption yields for pyrite increased with the increasing pH up to 5.9. At equilibrium pHs of 5.4 and 5.9 for both adsorbents, the Cd(II) adsorption yields were close to each other. For that reason, it can be stated that the optimum pH is 5.4. This pH value corresponding initial pH of 6 is in agree with the results of some earlier studies [18–20], thus, and subsequent experiments were carried out at this pH. 3.2. Effect of temperature
3.3. Adsorption kinetics In order to determine kinetic parameters and explain to the mechanism of the adsorption processes, lots of researchers have used first and pseudo second-order rate expressions [18,30,31]. Similarly, in order to determine the rate constants (kad ) of Cd(II) adsorption onto SIS and pyrite, some kinetic analyses were made at various temperatures depending on contact time by using following first-order rate expression of Lagergren and pseudo second-order rate expression: ln(qe − q) = ln qe − kad t
(3)
Fig. 2. Effects of temperature and contact time on the Cd(II) adsorption by SIS and pyrite [initial Cd(II) concentration: 100 mg/l; SIS dosage: 20 g/l; pyrite dosage: 20 g/l; pH 6].
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243
Fig. 3. Lagergren plot for the adsorption of Cd(II) onto SIS and pyrite [initial Cd(II) concentration: 100 mg/l; adsorbent dosage: 20 g/l; pH 6].
1 1 = + kad t Ce C0
(4)
Straight lines will be obtained of the left-hand sites of Eqs. (3) and (4) versus t suggest the applicability of these kinetic models. Data obtained in this study were fitted to the first-order rate expression of Lagergren (Fig. 3). The correlation coefficient values (R2 ) for first-order rate expression were found greater than 0.97 for all temperatures studied. The values of kad at 25, 35 and 45 ◦ C for both adsorbents were calculated to be 0.0298, 0.0396 and 0.1295 min−1 for SIS and 0.0308, 0.0347 and 0.0419 min−1 for pyrite, respectively, from the slopes of the straight lines in figure. The increase in rate constants depending on temperature shows that the rate-limiting step is surface adsorption and the process is endothermic. The Arrhenius Equation expresses the first-order rate constant of the adsorption reaction as a function of temperature and the activation energy of the process is calculated from this equation. The magnitude of activation energy may give an idea about the type of adsorption. There are two types of adsorption, physical and chemical. In the physical adsorption, the equilibrium is rapidly attained and reversible. Since the effective forces for this type of adsorption are weak, its activation energy value is low. On the contrary, chemical adsorption has higher activation energy
value (between 8.4 and 83.7 kJ/mol) and it is specific. Also, the rate in chemical adsorption varies with increasing temperature [32]. In order to determine adsorption type of Cd(II) adsorption onto SIS and pyrite, Arrhenius Equation was used. For this purpose, ln kad values were plotted versus 1/T and activation energy values were calculated from the slope of the line obtained (Fig. 4). Activation energy values for the adsorption process which SIS and pyrite were used as an adsorbent were calculated to be 57.47 and 12.09 kJ/gmol, respectively. These values suggest that chemical forces govern the adsorption process. In order to predict whether the adsorption is favourable or unfavourable, the dimensionless equilibrium parameter was determined by the following equation: r=
1 1 + bC0
(5)
where C0 is the initial Cd(II) concentration (mg/l) and b is the Langmuir isotherm constant. Value of r < 1 represents the favourable adsorption and value greater than one represents unfavourable adsorption. The values of r for Cd(II) adsorption were found to be 0.027, 0.003 and 0.002 for SIS and 0.287, 0.284 and 0.269 for pyrite at 25, 35 and 45 ◦ C, respectively. These values indicate that the both Cd(II) adsorption processes are highly favourable.
Fig. 4. Arrhenius plots for the Cd(II) adsorption onto SIS and pyrite.
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Fig. 5. Langmuir adsorption isotherms for Cd(II) adsorption onto SIS and pyrite.
3.4. Adsorption isotherms Adsorption isotherms are commonly used for describing adsorption equilibrium for wastewater treatment. Langmuir and Freundlich adsorption isotherms are classical models to describe the equilibrium between metal ions adsorbed onto adsorbent and metal ions in solution at a constant temperature. These models were also applied in this study. For this purpose, the experimental data obtained under the various concentrations and temperatures were plotted in linearized forms of Langmuir and Freundlich adsorption isotherms (Eqs. (6) and (7), respectively): Ce 1 Ce (6) = + ◦ ◦ x/m bQ Q x ln = ln kf + n ln Ce (7) m where Ce is equilibrium concentration (mg/l), x/m the amount adsorbed at equilibrium (mg/g), Q◦ , b, kf and n are isotherm constants. Q◦ and kf are defined as adsorption maxima or adsorption capacity (mg/g) for Langmuir and Freundlich isotherms, respectively.
Langmuir and Freundlich adsorption isotherms of Cd(II) on SIS and pyrite are shown in Figs. 5 and 6. The isotherm constants and correlation coefficients calculated at different temperatures are also tabulated in Tables 2 and 3. Experimental data fitted to the both isotherms. But, the correlation coefficients of Langmuir adsorption isotherm showed that the Langmuir isotherm yielded the best fitted to experimental data. As seen from the table, Langmuir adsorption capacity values increased by the temperature. The other Langmuir parameter b shows a similar trend. This situation confirms the finding that chemical forces govern the adsorption process. Similar result was found by Kandah, who has investigated zinc and cadmium adsorption on low-grade phosphate [19]. In addition, Mathialagan and Viraraghavan have reported that the adsorption data of the cadmium from aqueous solutions onto perlite fits to Freundlich isotherm [18]. It has been determined that the Cd(II) adsorption capacity (Q◦ ) of SIS is higher than that of the pyrite. For example, while maximum adsorption capacity for SIS was 9.47 mg adsorbed Cd(II)/g SIS, it for pyrite was 3.43 mg adsorbed Cd(II)/g pyrite at 45 ◦ C. Therefore, it can be concluded that the SIS for Cd(II) adsorption is more appropriate than the pyrite. In the earlier
Fig. 6. Freundlich adsorption isotherms for Cd(II) adsorption onto SIS and pyrite.
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245
Table 2 Langmuir constants and correlation coefficients of Cd(II) adsorption onto SIS and pyrite Temperature (◦ C)
25 35 45
SIS
Pyrite
b (l/mg)
Q◦
0.3596 2.8011 4.1247
3.05 8.40 9.47
(mg/g)
R2
b (l/mg)
Q◦ (mg/g)
R2
0.998 1 0.998
0.0248 0.0252 0.0271
2.08 2.78 3.43
0.971 0.926 0.975
Table 3 Freundlich constants and correlation coefficients of Cd(II) adsorption onto SIS and pyrite Temperature (◦ C)
25 35 45
SIS
Pyrite
kf
1/n
R2
1.2433 7.1836 7.4529
0.1765 0.0375 0.0559
0.974 0.983 0.940
kf
1/n
R2
0.5351 0.5188 0.5575
0.2351 0.2946 0.318
0.932 0.935 0.956
Table 4 Thermodynamic parameters for the adsorption of Cd(II) onto SIS and pyrite Temperature (◦ C)
25 35 45
SIS
Pyrite
−G◦ (kJ/gmol)
S◦ (kJ/gmol K)
−G◦ (kJ/gmol)
S◦ (kJ/gmol K)
26.279 32.418 34.493
0.413 0.419 0.413
19.654 20.354 21.208
0.0775 0.0773 0.0776
studies related to Cd(II) adsorption, it has been determined that the adsorption capacities of perlite [18], low-grade phosphate [19], clinoptilolite [20], goethite and aluminium oxide [21], montmorrilonite and kaolinite [22] are 0.64, 7.54, 7.41, 72, 31, 0.72, 0.32 mg/g, respectively. When taking into consideration Cd(II) adsorption capacities of the adsorbents mentioned above and the values obtained from the present study to be 3.05 mg/g SIS and 2.08 mg/g pyrite (at 25 ◦ C), it can be seen that the Cd(II) adsorption efficiencies of the minerals mentioned decrease in the order of goethite > aluminium oxide > low-grade phosphate > clinoptilolite > SIS > pyrite > montmorrilonite > perlite > kaolinite. Although a significant number of low-cost mineral adsorbents have been studied, the higher adsorption capacities for cadmium have been reported in the biosorption researches. For instance; the Cd(II) adsorption capacities of S. platensis [33], marine macroalgae [34], aerobic granules [35], marine alga Laminaria japonica [36] and C. vulgaris [37] in these research have been reported to be 98.04, 64–95, 566, 146.12 and 85.3 mg Cd(II)/g biomass, respectively. According to these results, it can be stated that the applicability to the real wastewater of mineral adsorbents mentioned above is probably difficult.
standard enthalpy (H◦ ) and entropy (S◦ ), which can be calculated from the following relationships: ln
1 G◦ = b RT
ln b = ln b0 −
(8) H ◦ RT
G◦ = H ◦ − T S ◦
(9) (10)
where b is Langmuir constant which is related with the energy of adsorption, b0 a constant, R the ideal gas constant and T is temperature (K) [38]. The standard enthalpy changes (H◦ ) of the Cd(II) adsorption onto SIS and pyrite were determined to be 96.783 and 3.468 kJ/gmol from the ln 1/b versus 1/T, respectively. The positive values of H◦ suggest the endothermic nature of adsorption. The Gibbs’ free energy (G◦ ) and entropy values (S◦ ) for the adsorption process were calculated from Eqs. (8) and (10) and tabulated in Table 4. The negative Gibbs’ free energy values confirm that the adsorption is spontaneous. The increase in free energy change with the rise in temperature shows an increase in feasibility of adsorption at higher temperatures. 4. Conclusions
3.5. Adsorption thermodynamics The increasing adsorption efficiency with increasing temperature can be explained on the basis of some thermodynamic parameters such as the changes in Gibbs free energy (G◦ ),
In this study, the Cd(II) adsorption abilities of synthetic iron sulphide and pyrite were tested by using equilibrium, kinetic and thermodynamic aspects. The results indicated that the adsorption capacities of the adsorbents were changed depending on pH,
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contact time and temperature. The optimum equilibrium pH and equilibrium time were determined to be 5.4 and 120 min, respectively. The kinetic studies indicated that the adsorption of Cd(II) on pyrite and SIS followed first-order kinetic model. The first-order rate constants for Cd(II) adsorption on pyrite and SIS were found to be 2.98 × 10−2 and 3.08 × 10−2 min−1 at 25 ◦ C, respectively, and it increases with the temperature. The activation energy values of the process suggest that the chemical forces govern the adsorption process. Experimental data fitted to the Langmuir and Freundlich adsorption isotherms. That the adsorption capacities increase with the temperature and positive enthalpy values of the process show that the adsorption process is endothermic. Standard Gibbs free energy values showed that the adsorption process was spontaneous. Taking into consideration present findings, it can be stated that the SIS and pyrite are mineral based adsorbent having low Cd(II) adsorption capacity. However, these minerals, generate H2 S in acidic solution, can be evaluated as a precipitation reagent for Cd(II) removal from the wastewaters having low pH (pH <3) such as acidic metal plating wastewater.
[10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
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Kinetics and thermodynamics of Cd(II) adsorption onto pyrite and synthetic iron sulphide Mehmet Erdem ∗ , Arzu Ozverdi Department of Environmental Engineering, Fırat University, 23279 Elazı˘g, Turkey Received 26 December 2005; received in revised form 1 February 2006; accepted 2 February 2006
Abstract In this paper, the Cd(II) adsorption abilities of pyrite and synthetic iron sulphide (SIS) were studied. Experiments were carried out as a function of pH, Cd(II) concentration, contact time and temperature. Maximum adsorption yields for pyrite and SIS were determined to be 34.2% and 65.8% under the conditions of initial Cd(II) concentration of 100 mg/l, pH 5.4, contact time of 120 min and adsorbent dosage of 20 g/l, respectively. The adsorption data fitted both the Langmuir and Freundlich adsorption models. Adsorption capacities of SIS and pyrite at 25 ◦ C were found to be 3.05 and 2.08 mg Cd(II)/g, respectively. The first-order and pseudo second-order rate expressions were applied to experimental data and it was determined that the adsorption process followed the first-order kinetic model. In addition, activation energy values and some thermodynamic parameters such as G◦ , H◦ and S◦ for the cadmium adsorption processes were calculated from the isotherm and kinetic data. The adsorption of Cd(II) on to SIS and pyrite was found to be endothermic and spontaneous. © 2006 Elsevier B.V. All rights reserved. Keywords: Cadmium; Adsorption; Kinetics; Isotherms; Pyrite; Iron sulphide
1. Introduction Heavy metal pollution in wastewaters is an extremely important environmental problem. The main sources of heavy metals for wastewaters are mining, metal industries and some other industrial areas using metals and metal salts. Lead, chromium, cadmium, copper, zinc and mercury are among the most frequently encountered metal contaminants [1]. Heavy metals are extremely toxic and threaten to the living by joining the food chain. When they release into the waters, most of them are strongly retained and their adverse effects can last for a long time. Thus, it is important to apply an effective treatment method to wastewaters polluted with heavy metals. In order to remove the toxic metal ions from wastewater, current methods are chemical precipitation, ion exchange, solvent extraction, adsorption and reverse osmosis techniques [2]. Chemical precipitation, especially as metal hydroxide or sulphide, is widely practiced, having the advantages of simplicity and inexpensive chemicals. However, it is not effective to reduce heavy metal concentration to low level required by water qual-
∗
Corresponding author. Tel.: +90 424 2370000; fax: +90 424 2415526. E-mail address: [email protected] (M. Erdem).
1383-5866/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2006.02.004
ity standards [3] and generation of a voluminous toxic waste sludge is a major problem encountered. Therefore, in the last few decades, adsorption process has received much concern and become an alternative to conventional precipitation technique, especially for wastewaters that contain low concentrations of metals and complex forming substances [2]. Activated carbon is the most widely used adsorbent in the wastewater treatment. Owing to high-cost of activated carbon, usage of the low-cost adsorbents such as agricultural wastes [4], metallurgical slags [5,6], fly ashes [7,8] and various minerals have been investigated [9–17]. In order to remove cadmium from aqueous solutions by minerals such as perlite [18], low-grade phosphate [19], clinoptilolite [20], goethite and aluminum oxide [21], montmorrilonite and kaolinite [22], akaganeite-type nanocrystals [23], a lot of investigations have been done. Currently, a few studies on the usage of pyrite as an adsorbent have been reported. But, these studies have been generally focused on the removal of molybdate and tetrathiomolybdate and influence of cadmium sorption on FeS2 oxidation [24,25]. Pyrite and synthetic iron sulphide have not been systematically investigated and evaluated for the metal adsorption purposes. That is, any data have been reported for the adsorption of Cd(II) onto pyrite, one of the abundant minerals in the nature, and synthetic iron sulphide. Starting from this point, in this study, it has been investigated Cd(II) adsorption
M. Erdem, A. Ozverdi / Separation and Purification Technology 51 (2006) 240–246
Nomenclature A b C0 Ce EA G◦ H◦ kad kF m n Q◦ qe q r R R2 S◦ t T x
frequency factor Langmuir constant (l/mg) initial Cd(II) concentration (mg/l) equilibrium Cd(II) concentration (mg/l) activation energy (kJ/gmol) Gibbs free energy change (kJ/gmol) enthalpy change (kJ/gmol) adsorption rate constant (min−1 ) Freundlich constant (mg/g) amount of adsorbent (g) Freundlich constant maximum adsorption capacity (mg/g) amount of Cd(II) adsorbed per g of adsorbent at equilibrium (mg/g) amount of Cd(II) adsorbed per g of adsorbent at any time (mg/g) dimensionless equilibrium parameter universal gas constant (8.314 J/gmol K) correlation coefficient entropy change (kJ/gmol) time (min) temperature (K) amount of adsorbed Cd(II) (mg)
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separately. The visible impurities were removed by hand from pyrite sample and then the samples were ground and sieved. The fractionated materials in the particle size of <53 m were dried at 60 ◦ C for 6 h, and then they were stored in a tightly closed jar throughout the study. Characterizations of the samples were explained in our previous studies [26,27], however, results of the chemical and mineralogical compositions of the samples determined in previous study mentioned above are given in Table 1. In order to prepare the experimental solutions, stock solution of Cd(II) (10 g/l) was prepared by dissolving its nitrate salt in distilled water. The working solutions were prepared by diluting the stock solution with distilled water. Taking into consideration common concentration values of cadmium in actual wastewaters [2,28], 100 mg Cd(II)/l has been selected as the suitable initial metal concentration value. pH adjustments were made by using HNO3 and NaOH solutions in various concentrations.
properties of pyrite and synthetic iron sulphide. Langmuir and Freundlich adsorption isotherms have also been tested at various temperatures and adsorption kinetics and some thermodynamic parameters have been determined. 2. Experimental The pyrite sample, collected from pyrite-bed, Keban-Elazı˘g (Turkey), and synthetic iron sulphide (SIS), catalogue number of which is Merck-3908, were used as adsorbents in this study. The samples in gross particles (pyrite) and pellets (SIS) were crushed
Fig. 1. Effect of pH on the adsorption of Cd(II) by SIS and pyrite [initial Cd(II) concentration: 100 mg/l; SIS dosage: 20 g/l; pyrite dosage: 20 g/l; contact time: 120 min; temperature: 25 ◦ C].
Table 1 Chemical and mineralogical compositions of the pyrite and SIS Chemical compositions
Mineralogical compositions
Constituents
w/w
Minerals
Formula
Pyrite
Fe S Al Ca Mg Si Cu Co Ni Mn Zn
41.45% 36.43% 2.20% 3.58% 1.18% 5.47% 3700 mg/kg 300 mg/kg 200 mg/kg 700 mg/kg 350 mg/kg
Pyrite Quartz Calcite Dolomite Natrosilite Brussite Chamosite
FeS2 SiO2 CaCO3 CaMg(CO3 )2 Na2 Si2 O5 CaHPO4 ·2H2 O (Fe,Al,Mg)6 (Si,Al)4 O10 (OH)8
SIS
Fe S
61.03% 32.82%
Troilite Iron Wustite
FeS Fe FeO (minor amount)
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All the chemicals used in the study were of analytical reagent grade. Adsorption experiments were carried out by shaking 2 g/l of pyrite and 2 g/l of SIS with 100 ml aqueous solutions of Cd(II) in series of batch reactors at a constant speed of 200 rpm in an orbital shaker. Samples were withdrawn after predetermined contact time interval and filtered through 0.45 m filter paper. Final pH of the filtrates was measured by a pH meter. The filtrates were acidified with 1 ml of HNO3 solution to prevent the precipitation of metal ions and then they were analyzed to determine residual concentration of Cd(II). The concentration of metals in the solutions was determined by ATI-Unicam 929 atomic absorption spectrophotometer using flame atomization technique. pH measurements were done by a pH meter (Orion SA720). Standard solutions were prepared by using analytical chemicals. All dilutions were made by distilled water.
its sulphide (Eq. (2)) and consequently cadmium removal yield increases. But, the mechanism of Cd(II) removal from aqueous solution in this situation is not adsorption, it is completely chemical precipitation:
3. Results and discussion
The data of adsorption at different temperature indicated that the Cd(II) adsorption yield onto both adsorbents increased by the increasing temperature (Fig. 2). This increase by the increasing temperature indicates that the adsorption process is endothermic and higher temperature favours Cd(II) removal by adsorption onto SIS and pyrite. The removal efficiency of Cd(II) by adsorption on SIS and pyrite also increased with contact time and it attained a maximum value at 120 min except for 45 ◦ C for SIS.
3.1. Effect of pH Since pH is an important parameter affecting metal adsorption, the effect of the initial pH on Cd(II) removal by either pyrite or SIS was investigated at the pH range of 3.5–6 by taking into account precipitation pH value of Cd(II) [29]. The results obtained depending on equilibrium pH are given in Fig. 1. In the studies carried out with SIS, it has been determined that the adsorption yield of Cd(II) at equilibrium pH of 3.7 (initial pH of 2) is 100% and they decrease with the increasing pH up to 4.1 and then they increase at low rate again. When taking into account that cadmium ions have positive charge, it can be stated that the cadmium ions can be more effective adsorbed at the high pH values. On the contrary, the Cd(II) adsorption yield in the presence of SIS was rather high at pH below 4.1. This situation is associated with H2 S generation in acidic media (Eq. (1)). H2 S is known as an efficient reagent for metal precipitation in the form of metal sulphides. Therefore, due to generation of H2 S in solutions having pH below 4.1, Cd(II) precipitates as
FeS(s) + 2H+(aq) → H2 S(g) + Fe2+ (aq)
(1)
+ Cd2+ (aq) + H2 S(g) →↓ CdS(s) + 2H(aq)
(2)
The Cd(II) adsorption yields for pyrite increased with the increasing pH up to 5.9. At equilibrium pHs of 5.4 and 5.9 for both adsorbents, the Cd(II) adsorption yields were close to each other. For that reason, it can be stated that the optimum pH is 5.4. This pH value corresponding initial pH of 6 is in agree with the results of some earlier studies [18–20], thus, and subsequent experiments were carried out at this pH. 3.2. Effect of temperature
3.3. Adsorption kinetics In order to determine kinetic parameters and explain to the mechanism of the adsorption processes, lots of researchers have used first and pseudo second-order rate expressions [18,30,31]. Similarly, in order to determine the rate constants (kad ) of Cd(II) adsorption onto SIS and pyrite, some kinetic analyses were made at various temperatures depending on contact time by using following first-order rate expression of Lagergren and pseudo second-order rate expression: ln(qe − q) = ln qe − kad t
(3)
Fig. 2. Effects of temperature and contact time on the Cd(II) adsorption by SIS and pyrite [initial Cd(II) concentration: 100 mg/l; SIS dosage: 20 g/l; pyrite dosage: 20 g/l; pH 6].
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Fig. 3. Lagergren plot for the adsorption of Cd(II) onto SIS and pyrite [initial Cd(II) concentration: 100 mg/l; adsorbent dosage: 20 g/l; pH 6].
1 1 = + kad t Ce C0
(4)
Straight lines will be obtained of the left-hand sites of Eqs. (3) and (4) versus t suggest the applicability of these kinetic models. Data obtained in this study were fitted to the first-order rate expression of Lagergren (Fig. 3). The correlation coefficient values (R2 ) for first-order rate expression were found greater than 0.97 for all temperatures studied. The values of kad at 25, 35 and 45 ◦ C for both adsorbents were calculated to be 0.0298, 0.0396 and 0.1295 min−1 for SIS and 0.0308, 0.0347 and 0.0419 min−1 for pyrite, respectively, from the slopes of the straight lines in figure. The increase in rate constants depending on temperature shows that the rate-limiting step is surface adsorption and the process is endothermic. The Arrhenius Equation expresses the first-order rate constant of the adsorption reaction as a function of temperature and the activation energy of the process is calculated from this equation. The magnitude of activation energy may give an idea about the type of adsorption. There are two types of adsorption, physical and chemical. In the physical adsorption, the equilibrium is rapidly attained and reversible. Since the effective forces for this type of adsorption are weak, its activation energy value is low. On the contrary, chemical adsorption has higher activation energy
value (between 8.4 and 83.7 kJ/mol) and it is specific. Also, the rate in chemical adsorption varies with increasing temperature [32]. In order to determine adsorption type of Cd(II) adsorption onto SIS and pyrite, Arrhenius Equation was used. For this purpose, ln kad values were plotted versus 1/T and activation energy values were calculated from the slope of the line obtained (Fig. 4). Activation energy values for the adsorption process which SIS and pyrite were used as an adsorbent were calculated to be 57.47 and 12.09 kJ/gmol, respectively. These values suggest that chemical forces govern the adsorption process. In order to predict whether the adsorption is favourable or unfavourable, the dimensionless equilibrium parameter was determined by the following equation: r=
1 1 + bC0
(5)
where C0 is the initial Cd(II) concentration (mg/l) and b is the Langmuir isotherm constant. Value of r < 1 represents the favourable adsorption and value greater than one represents unfavourable adsorption. The values of r for Cd(II) adsorption were found to be 0.027, 0.003 and 0.002 for SIS and 0.287, 0.284 and 0.269 for pyrite at 25, 35 and 45 ◦ C, respectively. These values indicate that the both Cd(II) adsorption processes are highly favourable.
Fig. 4. Arrhenius plots for the Cd(II) adsorption onto SIS and pyrite.
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Fig. 5. Langmuir adsorption isotherms for Cd(II) adsorption onto SIS and pyrite.
3.4. Adsorption isotherms Adsorption isotherms are commonly used for describing adsorption equilibrium for wastewater treatment. Langmuir and Freundlich adsorption isotherms are classical models to describe the equilibrium between metal ions adsorbed onto adsorbent and metal ions in solution at a constant temperature. These models were also applied in this study. For this purpose, the experimental data obtained under the various concentrations and temperatures were plotted in linearized forms of Langmuir and Freundlich adsorption isotherms (Eqs. (6) and (7), respectively): Ce 1 Ce (6) = + ◦ ◦ x/m bQ Q x ln = ln kf + n ln Ce (7) m where Ce is equilibrium concentration (mg/l), x/m the amount adsorbed at equilibrium (mg/g), Q◦ , b, kf and n are isotherm constants. Q◦ and kf are defined as adsorption maxima or adsorption capacity (mg/g) for Langmuir and Freundlich isotherms, respectively.
Langmuir and Freundlich adsorption isotherms of Cd(II) on SIS and pyrite are shown in Figs. 5 and 6. The isotherm constants and correlation coefficients calculated at different temperatures are also tabulated in Tables 2 and 3. Experimental data fitted to the both isotherms. But, the correlation coefficients of Langmuir adsorption isotherm showed that the Langmuir isotherm yielded the best fitted to experimental data. As seen from the table, Langmuir adsorption capacity values increased by the temperature. The other Langmuir parameter b shows a similar trend. This situation confirms the finding that chemical forces govern the adsorption process. Similar result was found by Kandah, who has investigated zinc and cadmium adsorption on low-grade phosphate [19]. In addition, Mathialagan and Viraraghavan have reported that the adsorption data of the cadmium from aqueous solutions onto perlite fits to Freundlich isotherm [18]. It has been determined that the Cd(II) adsorption capacity (Q◦ ) of SIS is higher than that of the pyrite. For example, while maximum adsorption capacity for SIS was 9.47 mg adsorbed Cd(II)/g SIS, it for pyrite was 3.43 mg adsorbed Cd(II)/g pyrite at 45 ◦ C. Therefore, it can be concluded that the SIS for Cd(II) adsorption is more appropriate than the pyrite. In the earlier
Fig. 6. Freundlich adsorption isotherms for Cd(II) adsorption onto SIS and pyrite.
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Table 2 Langmuir constants and correlation coefficients of Cd(II) adsorption onto SIS and pyrite Temperature (◦ C)
25 35 45
SIS
Pyrite
b (l/mg)
Q◦
0.3596 2.8011 4.1247
3.05 8.40 9.47
(mg/g)
R2
b (l/mg)
Q◦ (mg/g)
R2
0.998 1 0.998
0.0248 0.0252 0.0271
2.08 2.78 3.43
0.971 0.926 0.975
Table 3 Freundlich constants and correlation coefficients of Cd(II) adsorption onto SIS and pyrite Temperature (◦ C)
25 35 45
SIS
Pyrite
kf
1/n
R2
1.2433 7.1836 7.4529
0.1765 0.0375 0.0559
0.974 0.983 0.940
kf
1/n
R2
0.5351 0.5188 0.5575
0.2351 0.2946 0.318
0.932 0.935 0.956
Table 4 Thermodynamic parameters for the adsorption of Cd(II) onto SIS and pyrite Temperature (◦ C)
25 35 45
SIS
Pyrite
−G◦ (kJ/gmol)
S◦ (kJ/gmol K)
−G◦ (kJ/gmol)
S◦ (kJ/gmol K)
26.279 32.418 34.493
0.413 0.419 0.413
19.654 20.354 21.208
0.0775 0.0773 0.0776
studies related to Cd(II) adsorption, it has been determined that the adsorption capacities of perlite [18], low-grade phosphate [19], clinoptilolite [20], goethite and aluminium oxide [21], montmorrilonite and kaolinite [22] are 0.64, 7.54, 7.41, 72, 31, 0.72, 0.32 mg/g, respectively. When taking into consideration Cd(II) adsorption capacities of the adsorbents mentioned above and the values obtained from the present study to be 3.05 mg/g SIS and 2.08 mg/g pyrite (at 25 ◦ C), it can be seen that the Cd(II) adsorption efficiencies of the minerals mentioned decrease in the order of goethite > aluminium oxide > low-grade phosphate > clinoptilolite > SIS > pyrite > montmorrilonite > perlite > kaolinite. Although a significant number of low-cost mineral adsorbents have been studied, the higher adsorption capacities for cadmium have been reported in the biosorption researches. For instance; the Cd(II) adsorption capacities of S. platensis [33], marine macroalgae [34], aerobic granules [35], marine alga Laminaria japonica [36] and C. vulgaris [37] in these research have been reported to be 98.04, 64–95, 566, 146.12 and 85.3 mg Cd(II)/g biomass, respectively. According to these results, it can be stated that the applicability to the real wastewater of mineral adsorbents mentioned above is probably difficult.
standard enthalpy (H◦ ) and entropy (S◦ ), which can be calculated from the following relationships: ln
1 G◦ = b RT
ln b = ln b0 −
(8) H ◦ RT
G◦ = H ◦ − T S ◦
(9) (10)
where b is Langmuir constant which is related with the energy of adsorption, b0 a constant, R the ideal gas constant and T is temperature (K) [38]. The standard enthalpy changes (H◦ ) of the Cd(II) adsorption onto SIS and pyrite were determined to be 96.783 and 3.468 kJ/gmol from the ln 1/b versus 1/T, respectively. The positive values of H◦ suggest the endothermic nature of adsorption. The Gibbs’ free energy (G◦ ) and entropy values (S◦ ) for the adsorption process were calculated from Eqs. (8) and (10) and tabulated in Table 4. The negative Gibbs’ free energy values confirm that the adsorption is spontaneous. The increase in free energy change with the rise in temperature shows an increase in feasibility of adsorption at higher temperatures. 4. Conclusions
3.5. Adsorption thermodynamics The increasing adsorption efficiency with increasing temperature can be explained on the basis of some thermodynamic parameters such as the changes in Gibbs free energy (G◦ ),
In this study, the Cd(II) adsorption abilities of synthetic iron sulphide and pyrite were tested by using equilibrium, kinetic and thermodynamic aspects. The results indicated that the adsorption capacities of the adsorbents were changed depending on pH,
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contact time and temperature. The optimum equilibrium pH and equilibrium time were determined to be 5.4 and 120 min, respectively. The kinetic studies indicated that the adsorption of Cd(II) on pyrite and SIS followed first-order kinetic model. The first-order rate constants for Cd(II) adsorption on pyrite and SIS were found to be 2.98 × 10−2 and 3.08 × 10−2 min−1 at 25 ◦ C, respectively, and it increases with the temperature. The activation energy values of the process suggest that the chemical forces govern the adsorption process. Experimental data fitted to the Langmuir and Freundlich adsorption isotherms. That the adsorption capacities increase with the temperature and positive enthalpy values of the process show that the adsorption process is endothermic. Standard Gibbs free energy values showed that the adsorption process was spontaneous. Taking into consideration present findings, it can be stated that the SIS and pyrite are mineral based adsorbent having low Cd(II) adsorption capacity. However, these minerals, generate H2 S in acidic solution, can be evaluated as a precipitation reagent for Cd(II) removal from the wastewaters having low pH (pH <3) such as acidic metal plating wastewater.
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