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An Efficient Cost-Effective Removal of Ca2+, Mg2+, and Cu2+ Ions from Aqueous Medium Using Chlorosodalite Synthesized from Coal Fly Ash Ashok V. Borhade* and Sanjay R. Kankrej Research Center, Department of Chemistry, H.P.T. Arts and R.Y.K. Science College, Nashik 422005, India S Supporting Information *

ABSTRACT: Fly ash is one of the coal combustion solid wastes which causes environmental problems around the world. In this context for the first time, this paper reports the conversion of coal fly ash (CFA) into aluminosilicate chlorosodalite, Na8[AlSiO4]6Cl2. The synthesized chlorosodalite was analyzed by X-ray diffraction, Fourier transform infrared, scanning electron microscopy, and Brunauer− Emmett−Teller surface area measurements. The synthesized aluminosilicate chlorosodalite material was tested for potential applications for the removal of calcium, magnesium, and copper from their aqueous medium. The catch method was successfully used to study the effect of the initial concentration of metal ions, adsorbent dose, and contact time. The equilibrium data obtained were fitted by the Langmuir, Freundlich, Temkin, and Dubinin−Radushkevich isotherm models and showed the affinity order Cu2+ > Ca2+ > Mg2+. The well-known thermodynamic aspects such as the variation in Gibbs free energy (ΔG), entropy (ΔS), and enthalpy (ΔH) were also evaluated. Further the kinetic parameters including the rate constant and the order of sorption process were also determined.

1. INTRODUCTION In India, the Nashik thermal power station is one of the important power plants, situated at Eklahara village, near Nashik, Maharashtra. This power plant mainly works on Australian coal as a fuel for the generation of electricity. During the combustion, class-F “coal fly ash” (CFA) is obtained as a waste product which contains heavy metals that causes soil and air pollution. This raw CFA has a heterogeneous mixture of Al2O3, SiO2, Fe2O3, and traces of CaO. Therefore, the disposal of a voluminous amount of CFA is a serious problem. Hence, in this study we have undertaken the eco-friendly synthesis of aluminosilicate chlorosodalite using waste CFA for the efficient removal of heavy divalent metal ions. Chlorosodalite is more advantageous than any other adsorbent because it shows adsorption as well as the sorption capacity. The sodalite skeleton has alternate SiO4 and AlO4 tetrahedral structures,1,2 forming a β-cage due to four- and six-membered rings. However, the synthesis of artificial zeolites from pure chemicals is not economical. Such cost may be reduced by using coal fly ash. Different types of adsorbents like peanut shells,3 fruit peels,4 animal bones,5 mesoporous silica,6 activated carbon,7 chelating ligand,8 soya cake,9 and rice husk10 are also used by many workers for heavy metal ion removal from their aqueous medium. But synthetic zeolites have a better sorption ability than other adsorbents. It seems that there is still a need to study the thermodynamic aspects including ΔH, ΔG, ΔS, and K (equilibrium constant) as well as kinetics. This will give better evidence for the mechanism of sorption of metal ions © XXXX American Chemical Society

from aqueous medium, so that this chlorosodalite may be efficiently used to make water free from Ca2+, Mg2+, and Cu2+. Both calcium and magnesium metal ions are essential for strong and healthy bones and for metabolic processes of human body to some extent. The lower and higher doses needed for human body are 40−80 ppm calcium and 20−30 ppm magnesium, as defined by the World Health Organization. However, calcium and magnesium salts are water hardeners; the disadvantages of hard water are well-known. An increased intake of calcium and magnesium may cause health hazards. Bowel problems (diarrhea) may be caused due to the high concentration of magnesium and its salt. A high concentration of calcium and magnesium may cause cardiovascular disease. Some studies show an increased tendency of eczema among the children.11 On the other hand, for the consumption of copper, 0.9 mg/day for an adult of 70 kg weight is the tolerable limit (0.013 mg/kg/day); however, if the consumption of copper is more than the limit, one may suffer from nausea, vomiting, or stomach cramps. In the first instance, a high level of copper may cause damage to the kidney and liver.12,13 Reports in the literature reveal that it is necessary to investigate a new potential cost-effective material for adsorption. Hence, the present work was undertaken to synthesize aluminosilicate chlorosodalite for the efficient removal of Ca2+ , Mg2+ , and Cu 2+ ions. The present Received: July 5, 2016 Accepted: January 6, 2017

A

DOI: 10.1021/acs.jced.6b00588 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Table 1. Chemical Composition of Raw Coal Fly Ash constituents

Na2O

Al2O3

SiO2

K2O

CaO

Fe2O3

MgO

other LOI

weight/%

0.23

29.03

55.00

1.38

2.52

7.36

0.80

3.68

investigation aims to utilize waste coal fly ash for environmental purposes. In this work chlorosodalite was synthesized from coal fly ash under various experimental conditions. Further this work examines the applicability of chlorosodalite for the thermodynamics of Ca, Mg, and Cu adsorption. This study enables us to verify the Langmuir, Freundlich, Temkin, and Dubinin−Radushkevich adsorption isotherms along with various kinetic models.

furnace for 2 h and used for further characterization and its application for removal of Ca2+, Mg2+, and Cu2+ ions. 2.3. Chlorosodalite Characterization. The synthesized product Na8[AlSiO4]6Cl2 obtained by the above method was analyzed by FT-IR, XRD, SEM, and BET surface area. FT-IR spectroscopic study was performed on Shimadzu 8400: S FT-IR spectrophotometer within the range of 4000 to 400 cm−1 by the potassium bromide method. XRD analysis was carried out to identify different phases in raw CFA and synthesized aluminosilicate chlorosodalite (Philips PW-1710 instrument operating at 25 kV and 25 mA using CuKα radiation with wavelength λ = 0.154 nm). The surface morphology was studied by SEM, JEOL-JEM-6360, a model equipment JEOLJEC-560 autocation facilitated with auto carbon coater method and Brunauer−Emmett−Teller (BET) surface area was determined by Autosorb-1 NOVA 1200 model. 2.4. Batch Sorption Experiment. To verify different adsorption isotherms, batch sorption experiments of individual metal ions were studied. Different experimental variables, namely, the effect of contact time, metal ion concentration, dose, and pH were investigated in detail and optimized as follows. 2.4.1. Contact Time Variation. In the present investigation the extent of sorption of Ca2+, Mg2+, and Cu2+ ions onto chlorosodalite was studied for contact time ranging from 1 min to 4 h 25 mL of 1 × 10−2 M CaCl2 solution was taken in a series of glass stoppered bottles. Optimum pH was maintained by adding 5 mL pH-7 buffer solution in each bottle. Then 0.250 g of chlorosodalite was added in each bottle. These bottles were kept in the oven at desired constant temperature. These bottles were shaken manually at frequent intervals and then filtered through Whatmann filter paper at various time intervals. The filtrate was titrated against 1 × 10−2 M EDTA with Eriochrome black T-indicator in the presence of pH-10 buffer solution. Similar concentrations, quantities, pH, and procedure was used for magnesium ion solution. However, in the case of copper ion solution, 5 mL of pH-5 buffer solution was added in each bottle, and filtrate was titrated against 1 × 10−2 M EDTA with murexide indicator. 2.4.2. Metal Ion Concentration Variation. Various concentrations of metal ion solution were obtained from the stock solution by appropriate dilutions using double-distilled water. Twenty-five milliliter solution of different concentrations was taken in a series of glass stoppered bottles; the optimum pH (7 in case of Ca2+ and Mg2+ and 5 in case of Cu2+) was maintained by adding 5 mL of buffer solution in each bottle along with 0.250 g of chlorosodalite. These bottles were kept in the oven for 3 h at the desired constant temperature and shaken manually at frequent time intervals. After 3 h the solutions were filtered, and filtrates thus obtained were estimated by the same method as discussed in 2.4.1. 2.4.3. Effect of Dose Variation. 100 mL solutions of 100 ppm concentration of metal ion were taken in various glass stoppered bottles. Optimum pH (for Ca and Mg pH = 7 and for Cu pH = 5) was maintained by using 5 mL of buffer solution as discussed in 2.4.1. Different amounts (0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1 gm) of adsorbent were added in a series of bottles. These bottles were kept in the oven for 3 h at

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. AR grade chemicals were used during the synthesis of aluminosilicates and chlorosodalite and were used as is. Sodium hydroxide, NaOH, with ≥98.5 wt % purity, sodium chloride, NaCl, purity of ≥99.9 wt %, magnesium chloride, MgCl2, purity of ≥99.0 wt %, calcium chloride, CaCl2, purity of ≥97.0 wt %, and copper chloride, CuCl2, purity of ≥98.5 wt % were obtained from Sigma-Aldrich. A sample of CFA was collected from Eklahara Thermal Power Plant, Nashik (India). 2.2. Chlorosodalite Synthesis. The raw CFA used for synthesis of chlorosodalite was analyzed by inductively coupled plasma (ICP) spectroscopy for quantitative determination of its composition (Table 1). Results in Table 1 confirm that [SiO2/Al2O3] ratio ≈2; hence CFA can be used as a raw material for the synthesis of aluminosilicate chlorosodalite. Moreover, to improve the chemical composition, raw CFA is sieved, and Fe2O3 is removed by magnetic separation. The raw CFA was then cleaned with distilled H2O and kept in the oven at 120 °C (24 h) to remove water if any. Then 15 g of this CFA was further mixed with 15 g of NaOH, and it was heated at 550 °C (2 h). The obtained material was then cooled, milled, and mixed with 150 mL of distilled water, and then 90 g of sodium chloride (1.54 mol) was added. This reaction mixture was taken into Teflon lined steel autoclave and placed in the oven at 100 °C (144 h). The crystalline product obtained was further filtered and washed repeatedly to remove excess NaOH and NaCl (Figure 1). The white crystalline product was dried at 120 °C in the oven for 24 h and heated at 550 °C in a

Figure 1. Schematic diagram for the synthesis of aluminosilicate chlorosodalite from coal fly ash by a hydrothermal process. B



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between the solid and liquid phases are not depend on metal ion concentration. When 1/n is less than one, then the process of adsorption is normal, and if 1/n is greater than one, then the process is cooperative adsorption.19 3.1.3. Temkin Isotherm. This model considers that the heat of adsorption decreases linearly for all molecules present in the layer.20,21 Mathematically, it is

the desired constant temperature and shaken manually at frequent time intervals. After 3 h the filtration was carried out through Whatmann filter paper. These filtrates obtained were analyzed by the same method as discussed in 2.4.1. 2.3.4. Effect of pH. The pH effect was investigated by taking 25 mL of 1 × 10−2 M CaCl2 solution in a series of stoppered bottles. Five milliliter buffer solutions of different pH (pH = 1, 3, 5, 7, 9.2, 11) were added in a series of bottles. Then 0.250 g of chlorosodalite was added in each bottle. The bottles were kept in the oven at desired constant temperature. The bottles were shaken manually at frequent time intervals and then filtered after 3 h through Whatmann filter paper. Thus, filtrate obtained was titrated against 1 × 10−2 M EDTA with Eriochrome black T-indicator in the presence of pH-10 buffer solution. Similar concentrations, quantities, pH, and procedure was used for magnesium ion solution. However; the titration was performed between 10 mL of copper ion solution and 1 × 10−2 M EDTA with a murexide indicator. At equilibrium the amount adsorbed, Qe (mg·g−1), was evaluated by Vanderborght and Van Griekenm equations.14

(5)

if B = RT /bT

(6)

Q e = B ln A T + B ln Ce

(7)

(8) −1

In above equation, Qe is the amount adsorbed (mg·g ), Qs the isotherm saturation capacity (mg·g−1) (theoretical), Kad the Dubinin−Radushkevich isotherm constant (mol2·k−1·J−2), and ε the potential energy; it can be calculated as, ε = RT ln[1 + (1/Ce)]

(9)

If ln Qe is plotted versus ε , a linear plot is obtained with Kad as a slope and ln Qs as an intercept. The mean free energy, E, is related to the Kad; the Dubinin− Radushkevich isotherm constant as 2

(1)

E = [1/ 2K ad ]

In the above equation, Qe (mg·g ) is the quantity of metal ion adsorbed at equilibrium, Ce (mg·L−1) the concentration of metal ions, Q0 (mg·g−1) the monolayer coverage capacity (max.), and KL (L·mg−1) the Langmuir isotherm constant. The plot between 1/Qe versus 1/Ce is a straight line in which (1/ KLQ0) gives the slope, and 1/Q0 indicates the intercept. The important feature of this isotherm is the separation factor RL as a dimensionless entity and is given by17

(10)

The low value of E, mean free path energy, indicates the physisorption process. 3.2. Thermodynamic Studies. To determine the thermodynamic feasibility, ΔG, ΔH, and ΔS, following equations are used.

(2)

ΔG = −RT ln K a

(11)

ΔG = ΔH − T ΔS

(12)

Thus,

where C0 is the concentration before adsorption and RL is the value indicated by the feasibility of adsorption process. If RL is in between zero and one, equal to one and greater than one, then adsorption is favorable, linear, and unfavorable, respectively, whereas if it is zero then adsorption is not reversible. 3.1.2. Freundlich Adsorption Isotherm. For the adsorption of organic as well as inorganic compounds in aqueous medium, the Freundlich adsorption isotherm is very much favorable and as is given as log Q e = log K f + (1/n)log Ce

Q e = (RT /bT)ln A T + (RT /bT)ln Ce

ln Q e = ln Q s − K adε 2

−1

RL = 1/1 + (1 + KLC0)

(4)

In above equations, AT is the Temkin isotherm equilibrium binding constant (L·g−1), bT the Temkin isotherm constant, and B gives the heat of sorption (J·mol−1). The slope obtained is equal to B (J·mol−1). The smaller value of B indicates physical sorption, and the larger value shows chemical sorption. 3.1.4. Dubinin−Radushkevich Isotherm. The adsorption mechanism is well-studied by this isotherm.22,23 The linear expression for this adsorption isotherm is

3. DATA ANALYSIS 3.1. Adsorption Isotherms. The present study gives emphasis on the verification of different isotherm models for aluminosilicate chlorosodalite as an adsorbent. This study will enable us to understand which isotherm model illustrates the mechanism of adsorption and sorption by using chlorosodalite. 3.1.1. Langmuir Adsorption Isotherm. An empirical equation derived by Langmuir15 was used to study the extent of adsorption with the concentration of adsorbate at a particular temperature. This isotherm also postulates16 that the energy during adsorption is uniform and also the definite number of equivalent adsorption sites. The expression is 1 1 1 = + Qe Q0 KLQ 0Ce

Q e = (RT /bT)ln(A TCe)

ln K a = −(ΔH /RT ) + (ΔS /R )

(13)

Where, K a = Ca /Ce (distribution coefficient)

(14)

where Ca is the metal ion concentration adsorbed by absorbent (mg·g−1) and Ce is the metal ion concentration (mg·L−1) at equilibrium. 3.3. Kinetic Studies. The present study aims to use chlorosodalite as an adsorbent to verify different kinetic models for the removal of Ca2+, Mg2+, and Cu2+ ions from water. The kinetic data were analyzed by using following models. Pseudo-first-order:24

(3)

−1

In above equation Kf (mg·g ) is the Freundlich constant, and (1/n) gives the strength of adsorption.18 The plot between log Qe and log Ce is a straight line with the slope, (1/n), and intercept, log Kf. When n is equal to one, then distribution

log(Q e − Q t ) = (log Q e) − k1t /2.303

Pseudo-second-order: C

(15)

25


Journal of Chemical & Engineering Data (t /Q t ) = (1/k 2Q e 2) + (1/Q e)t

Intraparticle diffusion:

(16)

26

Q t = ktt 1/2 + C

Bangham’s model:

Article

(17)

27

log(C0/C0 − mQ t ) = log(k bm /2.303V ) + α log(t ) (18)

where k1 and k2 are the pseudo-first and second order rate constants (min−1 and g·mg−1·min−1), kt is the intraparticle diffusion rate constant (mg·g−1·min−1/2), V is the volume (mL), and α ( 0 and ΔS > 0. 4.7. Kinetics Study. In order to examine the adsorption of Ca2+, Mg2+, and Cu2+ onto hydrothermally synthesized chlorosodalite, the different kinetic models including pseudofirst, second-order, intraparticle diffusion model, and Bangham’s models were used (eq 15, 16, 17, and 18 and Figure 14). This shows the applicability of kinetic models for Ca2+, Mg2+, and Cu2+ and the experimental data obtained by these models are presented in Table 6. It has been observed from the data that R2, the correlation coefficient calculated using pseudo-second-order model, was found to be larger 0.9927 ± 0.0058 than those observed for pseudo-first-order model 0.7111 ± 0.1451. Despite the Qe calculated and experimental values evaluated using pseudo-firstorder model does not match with each other. On the other

the adsorption of Ca2+, Mg2+, and Cu2+ are found to be 0.06973, 0.061144, and 0.07314 kJ·mol−1, respectively. The positive values of ΔS confirm the increase in entropy because of adsorption. This observation is accounted on the basis of redistribution of energy between chlorosodalite (adsorbent) and metal ions (adsorbate). Comparing the adsorption of metal ions before and after, near the chlorosodalite surface, it is found that the metal ions are more ordered before adsorption than after adsorption. The translational energy associated with molecules increases and hence the greater is the adsorption. The increase in these energies are responsible to produce a +ve value of ΔS, and hence at the solid−solution interface during the adsorption process the disorder increases.44 Thus, H



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Table 2. Langmuir and Freundlich Adsorption Isotherm Parameters for the Removal of Ca2+, Mg2+, and Cu2+ by Chlorosodalite, Metal Ion M2+, Temperaturea T, Maximum Adsorption Amount Q0, Slope m, Intercept y, Langmuir Constant KL, Correlation Factorb R2, Adsorption Intensity n, and Freundlich Isotherm Constant Kf Langmuir M

T (K)

m

y

KL

Q0 (mg·g )

R

Ca

2+

299.15 323.15 348.15 303.15 323.15 348.15 298.15 323.15 348.15

1.9539 1.5809 1.1119 3.7721 3.0700 1.9176 2.2772 2.6012 0.7401

0.0546 0.0408 0.0203 0.1633 0.1012 0.0310 0.0598 0.0239 0.0151

0.0279 0.0258 0.0182 0.0432 0.0329 0.0161 0.0262 0.0091 0.0204

18.3150 24.5098 49.2610 6.12369 9.88142 32.2580 16.7224 41.8410 66.2251

0.9587 0.9739 0.9618 0.9479 0.9337 0.9715 0.9906 0.9721 0.9752

Mg2+

Cu2+

a

Freundlich −1

2+

2

RL

m

y

n

Kf (mg·g−1)

*R2

0.2085 0.2183 0.2613 0.1579 0.1888 0.2765 0.2161 0.3426 0.2475

0.5454 0.5930 0.6279 0.3092 0.3819 0.6635 0.5520 0.7870 0.4765

0.1015 0.1434 0.2934 0.0932 0.1357 0.0354 0.0352 0.2140 0.6639

1.8335 1.6863 1.5926 3.2341 2.6184 1.5071 1.8115 1.2706 2.0986

1.2632 1.3912 1.9651 1.2393 1.3667 1.0849 1.0844 0.6109 4.6121

0.9444 0.9597 0.9538 0.9463 0.8965 0.9705 0.9818 0.9779 0.8551

Standard uncertainty u is u(T) = 0.5 K. bStandard uncertainty u is u(R2) = 0.0158 and u(*R2) = 0.0391.

Table 3. Maximum Adsorption Capacities of Ca2+, Mg2+, and Cu2+ on Various Low Cost Adsorbents Q0 (mg·g−1) Adsorbent

Ca

activated Chilean zeolite natural zeolite manganese oxide coated zeolite sugar cane bagasse modified with tartaric acid green tomato husk chemically modified cellulose sugar cane bagasse rise husk kaolinite palm shell activated carbon charge soils shells of rice (RS) aluminosilicate chlorosodalite

2+

Mg2+

Cu2+

0.77 0.25 1.12 14.72

15.60

49.26

reference 28 28 28 29

6.76 13.50 23.50 3.87 0.44

32.25

1.58 1.00−2.42 2.95 66.22

30 31 31 32 33 34 35 36 present work

Figure 13. Plot of ln Kc vs 1/T for the adsorption of Ca2+, Mg2+, and Cu2+ on chlorosodalite to determine ΔH (enthalpy change) and ΔS (entropy change), 1/T, ln Kc.

including film or external diffusion, sorption, and adsorption onto pore surface. By using the diffusion model, intraparticle diffusion resistance affecting the adsorption process is explored by Srivastava et al.45 and Weber et al.46 Equation 17 represents the intraparticle diffusion model; it shows that if a straight line graph is obtained between Qt versus t0.5 it implies that the process is controlled only by intraparticle diffusion model. kt is the rate constant for intraparticle diffusion (mg·g−1·min−1/2), and C denotes the intercept. The larger C (boundary layer

hand the Qe calculated and experimental values in case of pseudo-second-order model are in good agreement with each other. Thus, the adsorption of Ca2+, Mg2+, and Cu2+ onto chlorosodalite followed the pseudo-second-order model. The transport of adsorbate from its aqueous medium to the surface of chlorosodalite can be controlled by different steps

Table 4. Temkin and Dubinin−Radushkevich Adsorption Isotherm Parameters for the Removal of Ca2+, Mg2+, and Cu2+ by Chlorosodalite, Metal Ion M2+, Temperaturea T, Constant Related to Heat of Sorption B, Temkin Isotherm Equilibrium Binding Constant AT, Temkin Isotherm Constant BT, Dubinin−Radushkevich Isotherm Constant Kad, Intercept y, Theoretical Isotherm Saturation Capacity Qs, and Correlation Factorb R2 Temkin T (K)

B (J·mol−1)

y

AT (L·g−1)

BT

R2

Ca2+

299.15 323.15 348.15 303.15 323.15 348.15 298.15 323.15 348.15

4.0867 5.5113 9.4217 1.3043 2.2432 6.5390 4.0379 7.2722 8.3615

−5.5725 −8.0389 −14.615 −0.9321 −2.6487 −11.787 −6.2809 −15.096 −4.2537

0.2557 0.2325 0.2119 0.4893 0.3070 0.1648 0.2110 0.1254 0.6012

608.5920 487.4837 307.2183 1932.369 1197.695 442.6547 613.8882 369.4438 346.1722

0.9718 0.9754 0.9926 0.9458 0.8987 0.9858 0.9842 0.9637 0.7451

Mg2+

Cu2+

a

Dubinin−Radushkevich

M2+

Kad (mol2·kJ−2) −3.00 −2.00 −2.00 −6.00 −6.00 −3.00 −3.00 −5.00 −1.00

× × × × × × × × ×

10−5 10−5 10−5 10−5 10−5 10−6 10−5 10−5 10−5

y

E (kJ·mol−1)

Qs (mg·g−1)

*R2

2.3447 2.5545 3.1028 1.5999 1.9900 2.6684 2.2772 2.6900 3.3281

0.1290 0.1580 0.1580 0.0913 0.0913 0.4080 0.1290 0.1000 0.2240

10.4301 12.8648 22.2601 4.95253 7.31553 14.4168 9.74934 14.7316 27.8853

0.9537 0.9161 0.9716 0.9183 0.9339 0.9767 0.8853 0.9283 0.8470

Standard uncertainty u is u(T) = 0.5 K. bStandard uncertainty u is u(R2) = 0.0740 and u(*R2) = 0.0387. I



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Table 5. Thermodynamic Parameters of the Present Study Temperature Ta (299.15, 323.15, and 348.15 K), Metal Ion M2+, Gibbs Free Energy ΔG, Distribution Coefficient Ka, Enthalpy Change ΔH, and Entropy Change ΔS M2+ Ca

2+

Mg2+

Cu2+

a

temperature (K)

Ka

ΔG (kJ·mol−1)

ΔH (kJ·mol−1)

ΔS (kJ·mol−1)

299.15 323.15 348.15 303.15 323.15 348.15 302.15 323.15 348.15

1.5405 2.9837 4.6916 0.8949 1.2436 2.3540 0.9087 1.2967 2.9653

−1.0747 −2.9370 −4.4743 0.2798 −0.5858 −2.4781 0.2402 −0.6981 −3.1463

19.7274

0.0697

18.9326

0.0611

22.5317

0.0731

Standard uncertainty u is u(T) = 0.5 K.

thickness) implies the greater effect of the boundary layer.47 But, when multilinear plots are obtained by existing data then the sorption process48 may involves two or multisteps. Figure 14c shows the plot of Qt versus t0.5 for the adsorption of Ca2+, Mg2+, and Cu2+ onto chlorosodalite at room temperature; all of the three curves show three linear portions, showing the sorption process mainly considers three steps, metal ion diffusion from bulk of solution to the external surface of chlorosodalite, then diffusion into the pores of chlorosodalite, and adsorption inside the void. It has been reported by various co-workers49,50 that the plot of Qt against t0.5 shows that, apart from intraparticle diffusion, there may be another process which may control the process of adsorption. This is because the plot obtained shows a certain value of intercept. Equation 18 is Bangham’s equation which is employed for adsorption of Ca2+, Mg2+, and Cu2+ ions on chlorosodalite, and it confirms whether the pore diffusion process is a unique step which controls the adsorption process. Table 6 shows kinetic and correlation coefficient parameters which are evaluated by using Bangham’s equation. Further, the data obtained furnish that it does not fit well to Bangham’s model. 4.8. Effect of Temperature. The effect of temperature on the time dependence of the adsorption process for Ca2+, Mg2+, and Cu2+ onto chlorosodalite was studied at different temperatures (room temperature, 50 and 75 °C) by batch contact time experiments. The uptake of all of the metal increases with temperature at the same adsorption time. Thus, the adsorption rate for Ca2+, Mg2+, and Cu2+ almost increased with temperature and hence increased in sorbent kinetic energy. It leads to a rise in collision frequency between chlorosodalite and Ca2+, Mg2+, and Cu2+ ions and thus increases metal ion adsorption on the surface of the sorbent. Second, as the temperature increases, the mobility of metal ions increases which increases the uptake of metal ions.

5. CONCLUSION The high content of Si and Al in coal fly ash makes it successfully possible to use it as a source material for the synthesis of aluminosilicate chlorosodalite. The hydrothermal synthesis of chlorosodalite required significantly less energy and reagent than traditional hydrothermal synthesis of chlorosodalite from pure reagents. The surface area obtained by the BET method is found to be 16.42 m2·g−1, and the pore volume by the BJH method is 0.0154 cc·g−1. This is a maiden report of this kind of batch sorption study as a function of temperature for aluminosilicate chlorosodalite with respect to Langmuir, Freundlich, Temkin, and Dubinin−

Figure 14. Different kinetic models for the adsorption of Ca2+, Mg2+, and Cu2+ on chlorosodalite at room temperatures: (a) pseudo-first order, (b) pseudo-second order, (c) intraparticle diffusion model, and (d) Bangham’s model. J


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Standard uncertainty u is u(T) = 0.5 K. bStandard uncertainty for pseudo-first order u is u(R2) = 0.0102, for pseudo-second order, u(R2) = 0.0066, for Bangham’s model, u(R2) = 0.1063, and for intraparticle diffusion model u(R2) = 0.1297.

Radushkevich adsorption isotherms, out of which Langmuir’s isotherm shows the highest regression value; hence it fit best to this model. The Q0 (mg·g−1) values for the metal ions Ca2+, Mg2+, and Cu2+ are found higher than any other adsorbent reported earlier by various researchers. The thermodynamic studies show negative ΔG, and hence the process is spontaneous in nature; therefore, it is feasible. The smaller positive value of ΔH shows that the adsorption process is endothermic and is of physical in nature. Further, the small and positive value of ΔS confirms the increase in entropy as a result of adsorption and also shows the feasibility of adsorption. Kinetic studies shows that the sorption process is pseudosecond order.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.6b00588. Experimental data for the Figures 6−8 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: (+91) 9421831839. E-mail: ashokborhade2007@ yahoo.co.in. ORCID

Ashok V. Borhade: 0000-0002-4909-9254 Funding

Authors are thankful to BCUD, S.P. Pune University, Pune for financial support. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors are thankful to HPT Arts and RYK Science College and Bhonsala Military College, Nashik-422 005 for providing necessary facilities.

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REFERENCES

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a

0.8553 0.6243 0.9289 0.0666 0.0384 4.4203 299.15 303.15 302.15 Ca2+ Mg2+ Cu2+

3.09 2.06 2.27

0.0640 0.0730 0.1437

77.980 58.275 64.518

0.8610 0.8565 0.8801

3.09 2.06 2.27

3.246 2.069 2.344

0.0284 0.0578 0.0248

0.9999 0.9956 0.9842

0.2310 0.2308 0.2300

0.9423 0.7389 0.9816

R2

intraparticle diffusion model

Kt (mg·g−1·min−1/2) R2

Bangham’s model

Kb (L·g−1) R2 k2 (g·mg−1·min−1)

pseudo-second order

Qe(cal.) (mg·g−1) Qe(expt) (mg·g−1) R2 k1 (min−1)

pseudo-first order

Qe(cal.) (mg·g−1) Qe(expt) (mg·g−1) T (K) M2+

Table 6. Kinetic Parameters Calculated by Applying Pseudo-first-order, Pseudo-second-order, and Intraparticle Diffusion Model and Bangham’s Equation for the Adsorption of Ca2+, Mg2+, and Cu2+ onto Chlorosodalite, Metal Ion M2+, Temperaturea T, Amount Adsorbed at Equilibrium Qe, Pseudo-first Order Rate Constant k1, Pseudo-second Order Rate Constant k2, Bangham’s Rate Constant kb, Intraparticle Diffusion Rate Constant kt, and Correlation Factorb R2


K



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