Zn2+ Ion Adsorption from Aqueous Solution Using Montmorillonite

Article pubs.acs.org/jced

Zn2+ Ion Adsorption from Aqueous Solution Using Montmorillonite Clay Impregnated with Tri‑n‑octylamine Hasan Uslu,*,†,‡ Dipaloy Datta,§ and Hisham S. Bamufleh‡ Iṡ tanbul Esenyurt University, Faculty of Health Sciences, Esenyurt/Iṡ tanbul, 34510, Turkey Chemical and Materials Engineering Department, Faculty of Engineering, King Abdulaziz University, Jeddah, Saudi Arabia § Malaviya National Institute of Technology (MNIT), Department of Chemical Engineering, Jaipur, Rajasthan 302017, India † ‡

ABSTRACT: Montmorillonite (Mt) clay was used to remove zinc (Zn2+) ion from the aqueous solution. This clay material was modified (Mt-TOA) by impregnating trin-octylamine (TOA), a tertiary amine. The experiments on equilibrium and kinetics were done to analyze the effect of adsorbent amount (w, 0.05 to 0.3 g for Mt, 0.01 to 0.06 g for Mt-TOA), initial Zn2+ ion concentration (C0, 15 mg·L−1 to 35 mg· L−1), pH (1 to 9), and contact time (t, 0 to 100 min) on the efficacy of both adsorbents. With a greater amount of adsorbent, the intake capacity of Mt and MtTOA for Zn2+ ion removal was found to lower but there was an increase in the separation efficiency. The optimum amount of Mt and Mt-TOA was found to be 0.1 and 0.01 g, respectively. The pH of the aqueous solution could be maintained at 7 to achieve a better adsorption of Zn2+ ion. In the kinetic experiments, after 90 min, the separation efficiency of Zn2+ ion from aqueous solution reached to a value of 86.68% with Mt (0.1 g) and 84.56% with Mt-TOA (0.01 g). Modeling of the equilibrium and kinetic data were done by using the Langmuir, Freundlich, Temkin, and Dubinin− Radushkevich, and by using the pseudo-first-order, pseudo-second-order, and intraparticle diffusion models, respectively, and the best fitted model is presented.

1. INTRODUCTION

water as prescribed by the World Health Organization (WHO) is 5 mg/L.4 As the environmental norms are becoming more stringent nowadays, the heavy metal ions are listed as priority pollutants and have become a threat to the environment. So it becomes necessary to remove these toxic heavy metals from the wastewater to protect the people and the environment. Treatment techniques such as chemical precipitation, ionexchange, adsorption, membrane filtration, electrochemical treatment technologies, etc. are available to separate these heavy metals from wastewater streams. Heavy metal ions removal via adsorption is a very attractive technique because of its simplicity and reversibility. Therefore, improvement of new adsorbents for the removal of heavy metals from the wastewater stream receives increased attention day by day.5−7 In the present work, natural and low cost clay, montmorillonite was used to remove zinc (Zn2+) ion from the wastewater aqueous stream generated from industrial effluents. The natural montmorillonite was also modified by using a tertiary amine named tri-n-octylamine (TOA). The parameters such as Zn2+ ion concentration, amount of montmorillonite, pH, and contact time that influence the adsorption capacity of the adsorbent were studied, and the optimum conditions were obtained. Essential adsorption isotherm models such as the

Pollution of the aquatic environment by heavy metals has become one of the most severe environmental problems nowadays. The treatment of wastewater containing heavy metals is of special concern due to their recalcitrance and persistence in the environment.1 These heavy metals are elements with atomic weights between 63.5 and 200.6, and a specific gravity greater than 5. With the speedy development of several industries such as metal plating facilities, fertilizer industries, mining operations, tanneries, paper industries, batteries, and pesticides, etc., the effluents containing heavy metals are directly or indirectly discarded into the aqueous environment.2 Zinc is an important trace element necessary for human health. It controls several biochemical processes in the body, and is vital for the physiological functions of living tissues. However, excessive intake of zinc can result in eminent health problems, such as skin irritations, stomach cramps, nausea, vomiting, and anemia.3 Zinc may be present in the wastewater effluents from the galvanizing plants, acid mine drainage, as a leachate from galvanized structures and natural ores, and from the municipal wastewater treatment plant effluents. This heavy metal is nonbiodegradable and moves through the food chain which causes bioaccumulation. Therefore, there has been a growing interest to remove zinc from wastewaters, and to keep its level at 100−500 mg/day to avoid human toxicity. The recommended upper concentration limit of zinc in drinking © XXXX American Chemical Society

Received: March 13, 2017 Accepted: June 26, 2017

A

DOI: 10.1021/acs.jced.7b00254 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Equilibrium and Kinetic Models model

1. Langmuir

equation

9

1 ln Ce n

2. Freundlich

ln qe = ln KF +

3. Temkin

qe = qm ln(K T) + qm ln(Ce)

4. Dubinin−Radushkevich (D-R)

ref

Equilibrium 1 1 1 = + qe (KLqm)Ce qm

10 11

2

Q e = Q m exp(− KDR ·ε )ε = RT ln(1 + 1/Ce) Kinetic q ⎞ − t ⎟⎟ = − k1t qm ⎠

12

5. pseudo-first-order (PFO)

⎛ ln⎜⎜1 ⎝

6. pseudo-second-order (PSO)

t 1 1 = + qt qmt k 2qm2

13,15

7. intraparticle diffusion

qt = k int 0.5 + c

16,17

13−15

adsorbent were kept inside the flask. The samples were shaken for 2 h (determined by preliminary tests) until the equilibrium was achieved. The adsorbent amount was taken from 0.05 mg to 0.30 mg for Mt, and from 0.01 mg to 0.06 mg for Mt-TOA. To obtain adsorption isotherms, equilibrium experiments were also performed by changing the initial concentration of Zn2+ ion (15 mg·L−1 to 35 mg·L−1) with 0.1 g of Mt and 0.01 g of Mt-TOA at 298 K. To determine the adsorption kinetics, experiments were conducted in batch mode in a 100 mL volume conical flask with optimum adsorbent amount (Mt = 0.1 g and Mt-TOA = 0.01 g), and 25 mg·L−1 concentration of Zn2+ ion in the aqueous solution. These flasks were shaken at 120 rpm at 298 K. At a particular time, a sample was taken to analyze the amount of Zn 2+ ion remaining in the aqueous phase. Zn2+ ion concentration was found by using Perking Elmer atomic absorption spectrophotometer at equilibrium. From the mass balance the amount of Zn2+ ion transferred to the solid phase was calculated. The amount of metal ions adsorbed per unit mass of the adsorbent was found by using eq 1.

Langmuir, Freundlich, Temkin, and Dubinin−Radushkevich (D-R) were applied to explain interaction of Zn2+ ion with both adsorbents at equilibrium. Also, the experimental kinetic data were validated using three kinetic models (pseudo-first-order, pseudo-second-order, and intraparticle diffusion models). With the help of error analysis, the best isotherm and kinetic models fitting the experimental data were proposed.

2. REAGENTS AND EXPERIMENTAL PROCEDURE 2.1. Reagents. The study on the adsorption characteristics of Zn2+ ion was studied by using montmorillonite (procured from Resadiye/Tokat, Turkey) consisting of a soft phyllosilicate group of minerals present in its microscopic crystal form with the elemental composition of [Al1.47Fe0.29Mg0.23][Al0.076Si3.29]O10(OH)2. Distilled water was used to prepare the aqueous solutions of Zn2+ ion by dissolving zinc sulfate (ZnSO4; SigmaAldrich, purity: 99 wt %). Tri-n-octylamine (TOA, purity: 98 wt %) was procured from Sigma-Aldrich. All materials were of analytical grade used without any further purification. 2.2. Impregnation of TOA into Montmorillonite. The clay montmorillonite (Mt) was ground by using a laboratory type ball-mill (FRITSCH, Pulverisette 5 model), and then sieved by using ASTM sieves of 100 mesh size. The undersized particles of the montmorillonite were separated and used as adsorbent. The impregnation of TOA into montmorillonite was done according to the methods described by Ahmad et al.8 A 2.2 mL aliquot of 1 N HCI and 10 mL of TOA were mixed with 800 mL of distilled water maintained at 353 K. Then, montmorillonite of the desired size was dispersed in 600 mL of hot water maintained at 353 K. The amine solution and Mtsuspension were then kept in a magnetic stirred cell for 1 h to achieve modified montmorillonite (Mt-TOA). Mt-TOA thus prepared was separated from solution by filtration, and then the solid particles were washed many times with hot water (353 K) to get rid of all chloride ions present in the solution. Any presence of chloride ion was checked by titrating with silver nitrate. Then, the white solid of Mt-TOA was vacuum ovendried at 353 K for 24 h. The characterization of modified clay was also done, and that data are presented somewhere else.6−8 2.3. Experimental Procedure. The equilibrium experiments on adsorption were conducted in a 100 mL Erlenmeyer flask kept on a thermostatic shaker at 298 K. Twenty-five milliliters of aqueous solution of Zn2+ and constant dosage of

qe or qt =

⎛ C0 − Ce ⎞ ⎛ C0 − Ct ⎞ ⎜ ⎟ × V or ⎜ ⎟ × V ⎝ w ⎠ ⎝ w ⎠

(1)

where, C0 (mg·L−1) = initial metal ion concentration, Ce (mg· L−1) = metal ion concentration at equilibrium, qe (mg·g−1) = adsorption capacity of adsorbent at equilibrium, qt (mg·g−1) = adsorption capacity of adsorbent at time, t, V = volume of aqueous solution in L, w = mass of adsorbent in g. The amount of metal ion removed was determined by using eq 2. ⎛ C − Ce ⎞ % removal = ⎜ 0 ⎟ × 100 ⎝ C0 ⎠

⎛ C − Ct ⎞ or ⎜ 0 ⎟ × 100 ⎝ Ct ⎠ (2)

3. MATHEMATICAL MODELS The experimental data were validated using four different isotherm (Langmuir, Freundlich, the Temkin, and Dubinin− Radushkevich), and three different kinetic (pseudo-first-order = PFO, pseudo-second-order = PSO, and intraparticle diffusion = IPD) models. B



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Table 2. Effect of Amount of Mt and Mt-TOA on the Adsorption of Zn2+ Ion (Co = 25 mg·L−1) at Temperatures T of 298, 303, and 308 K adsorbent

adsorbent amount (g)

Ce (mg·L−1) (298 K)

qe (mg·g−1) (298 K)

% removal (298 K)

Ce (mg·L−1) (303 K)

qe (mg·g−1) (303 K)

% removal (303 K)

Ce (mg·L−1) (308 K)

qe (mg·g−1) (308 K)

% removal (308 K)

0.05 0.10 0.15 0.20 0.25 0.30 0.01 0.02 0.03 0.04 0.05 0.06

3.61 3.33 3.04 2.71 2.62 2.34 3.86 3.51 3.22 2.95 2.75 2.53

10.70 5.42 3.66 2.79 2.24 1.89 52.85 26.86 18.15 13.78 11.13 9.36

85.56 86.68 87.84 89.16 89.52 90.64 84.56 85.96 87.12 88.20 89.00 89.88

3.64 3.35 3.07 2.73 2.64 2.37 3.90 3.55 3.25 2.99 2.79 2.56

10.68 5.41 3.65 2.78 2.23 1.88 52.75 26.81 18.12 13.75 11.10 9.35

85.44 86.60 87.72 89.08 89.44 90.52 84.40 85.80 87.00 88.04 88.84 89.76

3.66 3.37 3.10 2.75 2.66 2.39 3.93 3.59 3.29 3.02 2.83 2.60

10.67 5.40 3.65 2.78 2.23 1.88 52.67 26.76 18.09 13.73 11.08 9.33

85.36 86.52 87.60 89.00 89.36 90.44 84.28 85.64 86.84 87.92 88.68 89.60

Mt

Mt-TOA

of % APE shows the fitting of predicted values of adsorption capacity with the experimental ones.

3.1. Adsorption Equilibrium and Kinetics. An equilibrium study of an adsorption process provides very significant and fundamental information which is required to evaluate the capacity or affinity of an adsorbent used. The study is also important as it helps in selecting a suitable adsorbent. Therefore, the results obtained for the adsorption of Zn2+ ion were analyzed by four different thermodynamic equilibrium models as given in Table 1. The kinetic models are used to describe or predict the behavior of solute uptake by adsorbent in a continuous process. The adsorbate concentration inside the solid phase is a function of both spatial coordinate inside the particle and time. But the lumped kinetic models are much simpler and easier to use because they illustrate how the spatially averaged solid phase concentration, qt changes with time. The PFO and PSO rate equations fall in this category. The driving force for adsorption is the difference between the average solid phase concentration, qt and the equilibrium concentration, qe of the adsorbate, and the adsorption rate is proportional to this driving force in the PFO model or to the square of this driving force in the PSO model. Therefore, the time dynamics of adsorption process were studied to understand how the mechanisms such as the chemical reaction, mass transfer and diffusion, and the residence time of adsorbate are controlling the overall process. Experimental results obtained were fitted to three kinetic models, namely, pseudo-first-order, pseudo-second-order, and intraparticle diffusion models (Table 1). The aim was to find the suitable kinetic model that best describes the experimental data, and hence the process of adsorption of Zn2+ ion by using Mt and Mt-TOA as adsorbents. 3.2. Model Analysis. In the adsorption equilibrium and kinetic studies, an error function is necessary in order to determine the fitness of the applied model to the experimental values. The values of linear correlation coefficient (r) are calculated by using the following equation: r=

1−

%APE =

⎛ |q

∑ ⎜⎜ i=1

− qcal| ⎞ ⎟ ⎟ qexp ⎠

exp

⎝

(4)

4. RESULTS AND DISCUSSION Batch adsorption experiments were carried out for the removal of Zn2+ ions from aqueous solution using montmorillonite (Mt) and its modified form by tri-n-octylamine (Mt-TOA). Experimental parameters such as adsorbent dose (w, g), pH, initial concentration of Zn2+ ion (C0, mg·L−1) and contact time (t, min) are important to design an adsorption system for the removal of Zn2+ ion. The effect of these parameters on the adsorption capacity of both adsorbents is presented in the present study. 4.1. Adsorption Mechanism. The adsorption mechanism on the surface of montmorillonite mainly comprises the ion exchange process which will include reaction and fixation of Zn2+ ions. The exchangeable cation present on the surface of the modified clay may be of two types: one that is naturally present cations such as Na, K, Mg, and Ca, and the second is the H+ ion that is incorporated due to the impregnation of TOA. Initially, TOA will ionize in an acid medium, and then the cation (Na+ ion) present in montmorillonite (Mt) clay exchanges Cl− ion to form the modified clay. This may be used to remove heavy metals (Cu2+, Zn2+, Cr6+, etc.) as shown in the following equation. Modification of clay (Uslu 2016)6 N(C8H17)3 + HCl → N(C8H17)3 ·HCl → (C8H17)3 NH+Cl−

(5)

MtNa + + (C8H17)3 NH+Cl− → MtNH(C8H17)3 + NaCl

∑ (qcal − qaexp)2 ∑ (qaexp − qexp)2

N

100 N

Interaction of Zn

(3)

2+

(6)

with Mt-TOA

2MtNH(C8H17)3 + Zn 2 +

where, qcal and qexp refer to the theoretical and experimental capacities (mg·g−1), respectively, and qaexp is the average experimental capacity. Also an average percentage error (% APE) is calculated which minimizes the fractional error distribution across the entire concentration range.18 The values

→ MtN(C8H17)3 Zn + 2H+

Interaction of Zn nZn C

2+

2+

with Mt (Bradl, 2004)

(7) 19

+ 2Na Mt → 2Na + + nZn 2 +Mt +

(8)



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Figure 1. Linear fittings of isotherms at equilibrium for the removal of Zn2+ ion. (a) Langmuir; (b) Freundlich; (c) Temkin; (d) Dubinin− Radushkevich at a temperature T of 298 K: ▲, Mt-TOA (0.01 g); ●, Mt (0.1 g).

where n is an exchangeable cation valence, and M is exchangeable cation (Na+) 4.2. Effect of Mt and Mt-TOA Amount (w, g) on Zn2+ Adsorption. The effect of adsorbent dosage was analyzed by changing the Mt and Mt-TOA from 0.05 to 0.3 g, and from 0.01 to 0.06 g, respectively. The corresponding equilibrium results are shown in Table 2. The concentration of Zn2+ ion was taken to be 25 mg·L−1 and an equilibrium temperature of 298 K was fixed. The results revealed that the removal efficiencies of zinc ion increased gradually with increasing the amounts of Mt (85.56% to 90.64%) and Mt-TOA (84.56% to 89.88%). With the increase in the adsorbent dosage, the availability of surface area and active sites on the surface of adsorbents increases making the transfer of solute zinc molecule to reach to the adsorption sites easy. The highest adsorption capacity was achieved (10.70 mg·g−1 for Mt and 52.85 mg·g−1 Mt-TOA) with 0.1 g of Mt, and with 0.05 g of Mt-TOA. This shows the impregnation of TOA had increased the adsorption capacity of montmorillonite. 4.3. Effect of Initial Zn2+ Ion Concentration (C0, mg· −1 L ). The concentration of Zn2+ ion in the water phase was changed from 15 mg·L−1 to 35 mg·L−1 at 298 K and with 0.1 g of Mt, and 0.01 g of Mt-TOA. The isotherms at equilibrium are shown in Figure 1 for Mt and Mt-TOA. The increase in initial concentration of Zn2+ ion increased Zn2+ ion adsorption by Mt (from 3.41 mg·L−1 to 7.29 mg·L−1) and Mt-TOA (33.08 mg· L−1 to 70.18 mg·L−1) but on the other hand, the removal efficiency was found to decrease from 90.93% to 83.26% with 0.1 g of Mt, and 88.20% to 80.20% with 0.01 g of Mt-TOA.

This is because the active sites get saturated with an increase in the zinc ion concentration. The prepared montmorillonite modified with tri-n-octylamine for the removal of Zn2+ from aqueous solution is compared with other adsorbents in terms of the maximum adsorption capacity (qm) of different adsorbent clays as reported in Table 3. From these data, it is clear that the maximum adsorption capacity of the prepared Mt-TOA is much higher than most of other adsorbents. Table 3. Comparison of Maximum Adsorption Capacity (qm) of Different Adsorbent Clays for Zn2+ adsorbent Mt-TOA bentonite kaolin bofe bentonite clay calcareous claya lateritic clay natural bentonite natural bentonite bentonite kaolin raw bentonite alkaline Cabentonite

maximum adsorption capacity, qm, (mg·g−1)

ref

24.35 10.75 3.7 4.395 434.783 5.60 52.91 80.64 9.12 3.05 73.5 149

present study 20 20 21 22 23 24 25 26 26 27 27

a

Collected from the ConiacianEarly Campanian outcroppings of the Gafsa area (south of Tunisia).

D



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4.4. Effect of pH. The aqueous solution pH is related to the adsorption mechanism and reflects the various adsorbent− adsorbate physicochemical interactions. In the present study, the effect of pH was analyzed by changing the pH in the range of 1 to 9. The results are shown in Table 4. The removal Table 4. Effect of pH on Zn2+ Ion (Co = 25 mg·L−1) Adsorption with Mt (0.1 g) and Mt-TOA (0.01 g) at a Temperature T of 298 K adsorbent Mt

Mt-TOA

pH

Ce (mg·L−1)

qe (mg·g−1)

% removal

1 3 5 7 9 1 3 5 7 9

25.00 24.51 9.63 3.02 4.32 25.00 24.66 9.89 3.62 4.86

0.00 0.12 3.84 5.50 5.17 0.00 0.85 37.78 53.45 50.35

0.00 1.96 61.48 87.92 82.72 0.00 1.36 60.44 85.52 80.56

efficiency of Zn2+ ions was observed almost nil at low pH values, but gradually increased with an increase in the value of solution pH. The highest removal efficiency of Zn2+ ion was 87.92% with Mt and 85.52% with Mt-TOA at pH of 7. The experiments by Arias et al. (2009)28 and Mishara & Patel (2009)26 showed that the removal efficiency of zinc increased on kaolin clay with pH increasing. A similar trend was also observed by Sen and Gomez (2011)29 in the adsorption of zinc on natural bentonite. With a decrease in the pH, the adsorption of metallic ions on solid bentonite decreases according to Abollino et al. (2003)30 and Zhang et al. (2011).27 At low values of pH, H+ ions present in the aqueous phase compete with the heavy metals for the adsorption sites present on the clay. In addition, Si−O− and Al−O clustering are less deprotonated and hardly form complexes with divalent and trivalent ions. Zhang et al. (2011)27 showed that zinc removal by use of bentonite as adsorbent increases with pH increasing from 1 to 7. By increasing the pH of aqueous solution, the number of negative charges present on the adsorbent surface will increase which will facilitate more adsorption of Zn2+ ions. At pH > 7, Zn2+ ions may be removed mostly by precipitation as Zn(OH)2. 4.5. Effect of Contact Time (t, min). The time required for the adsorbate to interact with the adsorbent is a critical phenomena. Hence, it is important to study the effect of contact time on the removal efficiency of zinc ion. Figure 2 shows the effect of contact time on the adsorption of zinc ion onto Mt and Mt-TOA from aqueous solutions. The adsorption of zinc increased significantly up to the first 60 min. The steep increase in the removal efficiency in the initial stages indicated that there are sufficient accessible sites available for the adsorption to take place. When the contact time reaches to 90 min, the adsorption of zinc ion remains constant (86.68% with 0.1 g of Mt and 84.56% with 0.01 g of Mt-TOA). 4.6. Comparison of Models. 4.6.1. Equilibrium Models. The equilibrium data on the adsorption of Zn2+ ions using Mt and Mt-TOA as adsorbents were fitted with the Langmuir, Freundlich, Temkin, and Dubinin−Radushkevich equations. The values of model parameters with R2 and %APE are presented in Table 5. It can be seen that a better fit of equilibrium data were found with the Freundlich (r = 0.9996

Figure 2. Kinetic data for the removal of zinc ion (C0 = 25 g·L−1) by (a) Mt (0.1 g) and (b) Mt-TOA (0.01 g) at a temperature T = 298 K. Symbols: ■, experimental data; solid line, PFO; ---, PSO; ···, IPD.

and %APE = 0.5679 for Mt, and r = 0.9982 and %APE = 1.0613 for Mt-TOA). The surface of the montmorillonite clay are covered with a diversity of active chemical sites, because of which the adsorption process demonstrates as a nonhomogeneous and multilayer type of phenomena occurring on the surface of the adsorbent clay with a nonuniform distribution of heat of adsorption. Therefore, the Freundlich isotherm model validates the data of adsorption more closely. Also, the equilibrium parameter, RL5−7 (= 1 ) was also determined 1 + C0KL

to know the feasibility of the isotherm. The removal of Zn2+ ion on these adsorbents was found to be favorable (RL = 0.0872 to 0.1824 for Mt, and = 0.1151 to 0.2328 for Mt-TOA) as per the Langmuir equation. 4.6.2. Kinetic Models. Three different kinetic models (PFO, PSO, and IPD) were used to describe the kinetic behavior of the adsorption process. The values of rate constants along with other parameters, linear correlation coefficient (r) and %APE obtained from fitting are listed in Table 5. The IPD (r = 0.9236 E


qm (mg·g ) k1 (min−1) R2 SD r % APE

−1

parameter

Mt

11.158 0.2989 0.9848 0.0231 0.9936 3.0419

parameter

qm (mg·g−1) KL (L·mg−1) R2 SD r % APE

Langmuir

value

115.210 0.2197 0.9960 0.0013 0.9987 1.2947

Mt-TOA

4.4768 0.0553 0.9356 0.7827 0.8360 36.6447

Mt

PFO

value parameter

42.0640 0.0436 0.9232 0.6784 0.8304 56.5464

Mt-TOA qm (mg·g ) k2 (g·g−1·min−1) R2 SD r % APE

−1

Mt 2.8993 1.9348 0.9995 0.0079 0.9996 0.5679

parameter

KF [(mg·g−1) (L·g−1)1/n] n R2 SD r % APE

Freundlich

Mt-TOA

parameter

8.7390 2.9860 × 10−3 0.9323 0.7300 0.8945 32.5277

Mt

PSO value

24.3502 qm (mg·g−1) 1.8049 KT (L·mg−1) 0.9972 R2 0.0184 SD 0.9982 r 1.0613 % APE Kinetic Model Parameters

value

Equilibrium Model Parameters value

95.5110 1.8613 × 10−4 0.9099 0.0690 0.9096 51.4814

Mt-TOA

2.6180 2.5238 0.9843 0.2214 0.9921 3.3168

Mt

Temkin

parameter

kin c R2 SD r % APE

Mt

IPD value

0.3774 65.608 0.8973 0,2841 0.9263 3.9547

Mt

D-R

0.8728 −1.8570 0.9672 0.3448 0.9236 12.7487

KDR (mol2 /kJ2) qm(mg·g−1) R2 SD r % APE

parameter

27.2807 1.8195 0.9945 1.2599 0.9972 1.9330

Mt-TOA

Table 5. Equilibrium Isotherms Model Parameters for the Adsorption of Zn2+ Ion with Mt and Mt-TOA at a Temperature T of 298 K

0.2142 6.575 0.8607 0.2988 0.9175 3.2547

Mt-TOA

9.1064 −25.2265 0.9855 2.3675 0.9525 8.4401

Mt-TOA

value

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Figure 3. Predicted rates of adsorption of Zn2+ by the IPD model. Symbols: +, Mt; ∗, Mt-TOA.

and %APE = 12.7487 for Mt, and r = 0.9525 and %APE = 8.4401 for Mt-TOA) yielded a good fit to the kinetic data with both adsorbents. Also, the initial rates of adsorption by the PSO model (r0 = k2qm2) were calculated to be 0.228 mg·g−1·min−1 with Mt, and 1.698 mg·g−1·min−1 with Mt-TOA. This initial rate of adsorption was observed to increase by a factor of almost 8 when Mt-TOA was used as the adsorbent. The predicted rates of adsorption by the IPD model for both the adsorbents are plotted in Figure 3. A higher rate of adsorption was found with Mt-TOA. Therefore, an improved version of montmorillonite clay is a better alternative for the removal of Zn2+ ion from the streams of wastewater. From the analysis of Figure 4, a linear trend was observed at initial stages of adsorption which corresponds to a faster uptake

adsorption is mainly controlled due to the preferential adsorption of adsorbate inside the micropores.31,32

5. CONCLUSIONS Montmorillonite, a natural clay material and its modified version were used for the separation of Zn2+ ion from industrial wastewater. The montmorillonite which was chemically modified by tri-n-octylamine showed good efficiency for removing the Zn2+ ion from aqueous solutions by adsorption. A better removal of Zn2+ was found at an optimum amount of Mt and Mt-TOA to be 4 g·L−1 and 0.04 g·L−1, respectively. Zn2+ ions were adsorbed onto the surface of adsorbents at a pH value of 7. The data of equilibrium adsorption were analyzed by using three isotherm models and the data fitted well to the Langmuir isotherm. The maximum adsorption capacities for the separation of Zn2+ ion from aqueous solution were found (predicted by the Langmuir model) to be 11.15 mg·g−1 and 115.21 mg·g−1 with Mt and Mt-TOA, respectively. Three kinetic models were used to explain the kinetic behavior of the adsorption. Among them, the intraparticle diffusion model was able to describe the rates of adsorption process.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hasan Uslu: 0000-0002-4985-7246 Dipaloy Datta: 0000-0002-2048-9064 Notes

The authors declare no competing financial interest.

■

Figure 4. Plots of qt versus t1/2 for the adsorption of zinc ion (C0 = 25 g·L−1) by Mt-TOA (0.01 g) at a temperature T of 298 K.

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of Zn2+ ions on Mt-TOA. Also, the line did not pass through the origin which makes it worth mentioning that uptake is dominated by film diffusion than intraparticle diffusion. In the next stage, the adsorption of zinc ions increased indicating diffusion of adsorbate molecules into the micropores with wider pore width within the adsorbent. In the third step, the diffusion remains almost constant as the pores were filled with adsorbate. Therefore, it may be said that in the intraparticle diffusion, the G



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Zn2+ Ion Adsorption from Aqueous Solution Using Montmorillonite

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