Adsorption kinetics and isotherms for the removal of rhodamine B dye and Pb+2 ions from aqueous solutions by a hybrid ion-exchanger

Saruchi; Vaneet Kumar

doi:10.1016/j.arabjc.2016.11.009

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Original article

12 (

); 316-329

doi:

10.1016/j.arabjc.2016.11.009

Adsorption kinetics and isotherms for the removal of rhodamine B dye and Pb⁺² ions from aqueous solutions by a hybrid ion-exchanger

Saruchi^{^a,⁎}, Vaneet Kumar^{^b}

a DAV University, Jalandhar, Punjab, India

b CT Group of Institutions, Jalandhar, India

⁎Corresponding author. suruchinitj15@gmail.com (Saruchi),

Received: 2016-3-21, Accepted: 2016-11-17, Published: 2016-03-01

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

The capacity of synthesized hybrid ion exchanger was found for removal of rhodamine B dye and Pb⁺² ions from aqueous solution. The effect of different reaction conditions such as sample dosage, pH, ionic strength, contact time and concentration on removal of rhodamine B dye and lead ions was studied. Experimental results of pH and ionic strength showed that hydrophobic-hydrophobic interactions might be the dominant force of attraction for the rhodamine B and lead ions onto synthesized hybrid exchanger system. Adsorption equilibrium isotherms were analyzed by Langmuir, Freundlich, Dubinin-Radushkevich, Tempkin and Redlich-Peterson and Sips models. Langmuir model was found to show the best fit for experimental data and the maximum adsorption capacity was found to be 82.3 and 182.7 mg g⁻¹ for lead ions and 76.4 and 156.8 mg g⁻¹ for rhodamine B at 20 and 50 °C, respectively. The experimental kinetic of the data was analyzed using pseudo-first-order, pseudo-second-order and Weber-Morris intra-particle kinetic models and the results showed that pseudo-second-order kinetic model described the ion exchange kinetics accurately for rhodamine B and lead ions. Thermodynamic activation parameters such as ΔG^∗, ΔS^∗ and ΔH^∗ were also calculated.

Keywords

Rhodamine B

Dye

Lead ion

Adsorption

Hybrid ion exchanger

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1 Introduction

Synthetic dye industries grow vigorously day by day and caused the natural downfall of the natural dye textile industry due to cost effectiveness and efficient synthesis process, which cope up the high demands of the industries. These synthetic dyes are not limited to use as coloring agents for textile, paper, pharmaceutical, food, leather and cosmetic industries, but these are also used as disinfectant in aquaculture and as preservative for animals feed (Chenga et al., 2011; Crini and Badot, 2008; Trung et al., 2003; Paulino et al., 2006 ). These synthetic dyes are photo-chemically and chemically resistant i.e. these are highly stable. Many of these dyes are toxic and some are carcinogenic in nature. These dyes are directly discharged to the environment and lead to severe ecological consequences (Abdel-Halim, 2013; Panic et al., 2013; Said et al., 2010 ). So environment protections become the utmost important with the industry development. Industry waste water treatment is prior requirement for saving the natural requirement (Maheria and Chudasama, 2007; Chand et al., 2011).

There are nearly about 100,000 commercially available dyes and annually nearly about 7 × 10⁵ tonnes of dyes were produced and about 2% of the dyes produced were directly discharged into the aqueous effluents. These are the common pollutants in water and are frequently found in trace quantities in industrial wastewater effluents (Thakkar and Chudasama, 2009; Barhon et al., 2009; Fil et al., 2012 ). For the increased stringent restriction on the organic content of industrial effluents and highly solubility of these dyes, it is necessary to eliminate dyes from wastewater before discharged. These dyes cause serious hazard to aquatic organisms. Dowex HCR-S a synthetic resin is used to remove different heavy metals such as cadmium (Cd²⁺), nickel (Ni²⁺) and zinc (Zn²⁺) from aqueous solutions and found that adsorption rate increased with increase in the initial concentration of metal ions, stirring speed and temperature. Adsorption kinetics through different isotherm models such as Langmuir, Freundlich, Temkin, Elovich, Khan, Sips, Toth, Radke-Praunstrzki, and Koble-Corrigan were also studied (Nabi and Khan, 2006; Borissova et al., 1993; Kooh et al., 2016 ).

Ion exchange is a process in which soluble materials when come in contact with an electrolyte solution, take up the stoichiometrically ions of positive or negative charge and release other ions of like charge from the exchanger to the solution phase. The interest of organic-inorganic based hybrid ion exchanger increased day by day as these materials can be used under unfavorable conditions. These are resistant toward heat and ionizing radiation and are highly stable under different reaction conditions. Synthetic inorganic ion exchangers are iron (III) antimonosilicate, zirconium iodooxalate antimony (III) tungstovanadate, zirconium (IV) iodotungstate, titanium molybdophosphate, stannic silicomolybdate, etc. Heteropolyacid salt based on the above said materials has better properties and high ion exchange capacity as compared to the simple salts of metals (Birlik et al., 2006; Shakir et al., 2010; Jain et al., 2007 ).

It is clear from the literature survey that a very little work has been carried out on Gum tragacanth based ion exchanger. So in present work Gum tragacanth- zirconium (IV) tungstoiodophosphate based ion exchanger was synthesized, which is chemically and thermally stable and has high ion exchange capacity. The present work also concerned with the evaluation of synthesized sample for the removal of lead (Pb⁺²) ion and rhodamine B dye.

Heavy metal and dye pollution has become the one of the most serious environmental problems due to their recalcitrance and persistence behavior in the environment. Lead a heavy metal is the main cause of damage of central nervous system. It can also affect our liver, kidney, reproductive system, basic cellular processes and brain function, so its removal from the environment is required. Rhodamine B dye belongs to the class of xanthene and if swallowed it is harmful to human beings and animals. It causes irritation to skin, eyes and respiratory tract. It is a chronic neurotoxin and is carcinogenic to humans and animals. As both Pb⁺² ion and rhodamine B dye have very harmful affect on human being and animals, so researchers all over the world are concerned for the removal of the Pb⁺² ion and dye from the industrial wastewater before it discharges into the environment (Shakir et al., 2010; Fenglian and Wang, 2011).

2 Experimental

2.1

2.1 Materials

Gum tragacanth (SD Fine Chemical Pvt. Ltd.), acrylamide, glutaraldehyde, ascorbic acid, potassium persulphate (Merck), Zirconium (IV) oxychloride (ZrOCl₂·8H₂O), sodium tungstate, potassium iodate, orthophosphoric acid (Qualigens).

2.2

2.2 Synthesis of hybrid ion exchanger

Solution of 0.1 M ZrOCl₂·8H₂O, 0.5 M sodium tungstate, potassium iodate and 1 M orthophosphoric acid in known volume ratios was taken in a reaction flask. Gum tragacanth (1.0 g) and ascorbic acid-potassium persulphate were added in the reaction flask with continuous stirring followed by the addition of glutaraldehyde (0.5305 × 10⁻³ mol L⁻) and acrylamide (0.35 × 10⁻³ mol L⁻¹) with thoroughly stirring to attain homogeneity, it was then kept in hot air oven at 50 °C for 2 h, and after cooling to room temperature it was washed with acetone and was dried in hot air oven at 60 °C (Kaith et al., 2012; Saruchi et al., 2014). Synthesized sample was kept in 1 M HNO₃ solution for 24 h to convert the product into the H⁺ and then dried. The resulting ion exchanger was crushed into the particle of mesh size around 50–60. Ion exchanger in different volume ratios of the mixture (Table 1) was prepared to get the ion exchanger with higher ion exchange capacity (IEC) (Kaith et al., 2015).

Table 1 Different volume ratios of the mixture taken for the synthesis of ion exchanger and its ion exchange capacity.

Molar concentration (M)				Volume ratio	pH	Ion exchange capacity (meq/g)
Zr	WO₄²⁻	IO³⁻	PO₄³⁻	Volume ratio	pH	Ion exchange capacity (meq/g)
0.1	0.5	0.5	1	1:1:1:1	1	1.8
0.1	0.5	0.5	1	1:2:1:1	1	1.6
0.1	0.5	0.5	1	1:1:3:1	1	2.8
0.1	0.5	0.5	1	2:1:1:1	1	2.2
0.1	0.5	0.5	1	1:1:2:1	1	2.6

2.3

2.3 Physicochemical properties of ion-exchanger

2.3.1

2.3.1 Ion-Exchange capacity (IEC)

Ion exchange capacity of the synthesized hybrid ion exchanger was determined by the column process. Ion exchanger in H⁺ form (0.5 g) was packed in a glass column and washed with demineralized water in such a way to maintain the flow rate 20 drops per minute, so that, if any acid remained on the particles can be remove, the process is continue until it becomes acid free. pH was checked using pH paper. 1 M KCl solution (100 ml) was passed through the column maintaining the flow rate 20 drops per minute. The effluent was collected and then titrated against the standard solution of NaOH to determine the total H⁺ ions released. The ion exchange capacity of the synthesized hybrid ion exchangers was calculated using the equation given below (Ulker Asli et al., 2014): $IEC = \frac{Volume of titrant used \times volume of effluent released \times NaOH Concentration}{KCl or NaCl concentration}$

2.3.2

2.3.2 Effect of temperature on IEC of synthesized ion exchanger

Thermal stability of the synthesized hybrid ion exchanger was performed at a heating rate of 10 °C min^–1. Temperature effect on the ion-exchange capacity of the synthesized hybrid ion exchanger was examined by heating at different temperatures in a muffle furnace for 1 h and the ion-exchange capacity for K⁺ was evaluated by the column process as described in Section 2.3.1 after cooling it to room temperature.

2.3.3

2.3.3 pH titration of hybrid ion exchanger

pH titration was performed by the batch process using Topp and Pepper method (Topp and Pepper, 1949). 0.5 g of the synthesized hybrid ion exchanger was taken in a conical flasks (250 mL) followed by addition of equimolar solution of alkali metal chlorides (KCl, NaCl) and their hydroxides (KOH, NaOH) in different volume ratios by maintaining the final volume up to 50 mL, so as to maintain the ionic strength constant. The pH of each solution was measured using pH strip and plotted against milliequivalents of OH ions (Gonzalez-Pradas et al., 2005).

2.3.4

2.3.4 Chemical stability

Chemical stability of the synthesized hybrid ion exchanger was studied by adding the synthesized hybrid ion exchanger in different mineral acids and organic acid. 0.5 g of synthesized ion exchanger was equilibrated with different solutions such as 1 M HCl, 2 M HCl, 1 M HNO₃ and 2 M HNO₃ for 24 h at room temperature. The left out material was filtered and washed by distilled water. After removal of excess acid or base, it was dried in hot air oven at 50 °C. The IEC of the synthesized material was then determined by column method as described in Section 2.3.1.

2.3.5

2.3.5 Distribution studies of synthesized ion exchanger

Distribution studies of synthesized ion exchanger for three metal ions were carried out by batch process. The metal ion solutions were treated with the known amount of the synthesized ion exchanger. Solutions were kept on shaking in a shaker for 6 h to maintain equilibrium. Then solutions were kept at room temperature for a definite interval of time. A definite volume of the solution was pipetted out into a conical flask and titrated against EDTA solution. The metal ion solutions without synthesized ion exchanger were also titrated against EDTA solution and it was referred as reference. The distribution coefficient (K_d) for synthesized hybrid ion exchanger for metal ions was calculated using the equation given below (Huang et al., 2007; Alakhras et al., 2005):

(1)

K_{d} = \frac{(1 - F)}{F} \times \frac{V}{W}

where I and F are initial and final metal ion concentrations, respectively. V is the volume of solution and W is the weight of the ion exchanger.

2.4

2.4 Dye removal studies

Removal of rhodamine B dye and lead ion (pb⁺²) was studied through batch experiment. The concentration of the rhodamine B dye and Pb⁺² was varied from 10 to 100 ppm. Synthesized hybrid ion exchanger sample was taken in the 100 mL of known concentration of rhodamine B dye and Pb⁺² ion solution. Effect of physiological pH, dye concentration, temperature and synthesized sample dosage on dye and Pb⁺² ion removal was studied. The amount of dye removed from per unit mass of the synthesized sample (q_t) was calculated using the given equations (Huang et al., 2007; Alakhras et al., 2005):

(2)

q_{t} = \frac{C_{o} - C_{t}}{M} V

where C_o and C_t are the concentration of dye at the initial stage and after time t, V is the volume of the dye solution; M is the mass of the synthesized material.

(3)

% R = \frac{(C_{o} - C_{e})}{C_{o}} \times 100

where C_o and C_e are the initial and equilibrium concentration of dye; %R is the percentage removal of dye.

2.4.1

2.4.1 Error analysis

The experimental data of the best represented kinetics and adsorption isotherm models were determined by the coefficient of determination value i.e. R². The predicted qe (qe, cal) values were generated using the formulae of various kinetics or isotherm models. Both the data predicted as well as experimental were fitted into the equations of diverse error analysis functions and the results having smallest value indicate the least error (Tsai and Juang, 2000).

The equations of the four types of error analysis are as follows: $Sum of the absolute error (EABS) : Σ_{i = 1}^{N} [q_{e, \exp} - q_{e, cal}] .$ $Chi – square test (χ^{2}) : Σ_{i = 1}^{N} {\frac{(q_{e, \exp} - q_{e, cal})}{q_{e, \exp}}}^{2}$ $Average relative error (ARE) : \frac{100}{n} Σ_{i = 1}^{N} [\frac{q_{e, \exp} - q_{e, cal}}{q_{e, \exp}}]$ $Marquardt ’ spercent standard deviation (MPSD) : 100 \sqrt{\frac{1}{N - P} Σ_{i}^{N} {(\frac{q_{e, \exp} - q_{e, cal}}{q_{e, \exp}})}^{2}}$

3 Results and discussion

3.1

3.1 Ion exchange capacity (IEC)

The different volume ratios for the synthesis of ion exchanger and their IEC are given in Table 1. Increased volume of IO³⁻ resulted in increase in IEC. The synthesized sample by refluxing for 12 h at 1 pH showed maximum IEC 2.8 meq/g dry exchanger at the volume ratio of 1:1:3:1. All the reaction parameters for the synthesis of ion exchanger were optimized with respect to IEC, as the maximum IEC was found at the volume ratio of 1:1:3:1. Further all the reactions were carried out using this volume ratio.

3.2

3.2 Characterization

3.2.1

3.2.1 Fourier transform infrared spectroscopy (FT-IR)

IR spectra of synthesized ion exchanger and dye treated ion exchanger were studied and the spectrum of synthesized ion exchanger showed peaks at 3418 cm⁻¹ (O-H stretching), 2926 cm⁻¹ (C-H stretching of CH₃) and 1720 cm⁻¹ (C-O stretching of ester), symmetrical stretching vibrations of (PO₄)³⁻ appeared at 1021–1165 cm⁻¹, spectrum also showed strong band in the region of 815–668 cm^–1 indicating the presence of iodate and metal oxide, and peak at 515 cm¹⁻ represents the Zr-O stretching vibrations were observed (Fig. 1a). The peaks were shifted which suggested that synthesized ion exchanger interacts with the rhodamine B dye (Fig. 1b).

FTIR spectra of (a) synthesized ion exchanger and (b) dye treated ion exchanger.

3.2.2

3.2.2 Energy-dispersive spectroscopy (EDS)

Energy-dispersive spectroscopy of the Gum tragacanth and synthesized ion exchanger was studied. It is quite clear from the EDS results that the main constituents of Gum tragacanth are carbon and hydrogen only (Fig. 2a). EDS spectra of the synthesized ion exchanger clearly showed the presence of zirconium (Zr), tungsten (W) and phosphorous (P) in the synthesized ion exchanger (Fig. 2b).

EDS analysis of (a) Gum tragacanth and (b) synthesized ion exchanger.

3.2.3

3.2.3 Scanning Electron Microscopy (SEM)

SEMs of Gum tragacanth, synthesized ion exchanger and dye treated ion exchanger were studied. SEM images clearly exhibited the differences in the surface morphology of the Gum tragacanth (Fig. 3a), synthesized ion exchanger (3b), dye treated ion exchanger (3c) and Pb⁺² ion treated ion exchanger (Fig. 4b). Surface morphology of Gum tragacanth showed homogenous surface, while smaller granules and folded structure were found in case of synthesized hybrid ion exchangers. SEM of the synthesized ion exchanger showed folded structure, which may provide a large surface area to interact with the rhodamine B dye and Pb⁺² ion. This folded structure is quite clear from Fig. 3(b). There is little bit modification in the surface morphology of the rhodamine B dye and Pb⁺² ion treated ion exchanger. Thus, it can be concluded that small change occurred in the surface morphology before and after treatment.

SEM images of (a) Gum tragacanth; (b) synthesized ion exchanger; (c) dye treated ion exchanger and (d) Pb+2 ion treated ion exchanger.

XRDs of (a) Gum tragacanth and (b) synthesized ion exchanger.

3.2.4

3.2.4 X-ray diffraction

The XRD results indicate that Gum tragacanth was less crystalline than the synthesized ion exchanger with least coherence value. Anisotropy decreases with increase in coherence length. Synthesized ion exchanger (Fig. 4b) showed maximum intensity peak corresponds to 2θ = 20.1485^o (L = 0.12426 Å) which is higher than that of Gum tragacanth (L = 0.0816 Å) (Fig. 4a). This indicated that the synthesized ion exchanger has higher crystallinity as compared to Gum tragacanth.

3.3

3.3 Evaluation of various properties of ion-exchanger

3.3.1

3.3.1 Effect of temperature on ion exchange capacity

The ion exchange capacity of synthesized ion exchangers was studied at different temperatures. It was found that initially it was increasing with increase in temperature, but further increase in temperature decreases the ion exchange capacity of the synthesized ion exchanger. The effect of increase in temperature on ion exchange capacity of the synthesized ion exchanger is given in Table 2. At very high temperature more than 100 °C ion exchange capacity of the synthesized ion exchangers decreases, which may be due to the fact that at very high temperature physical denaturation of network at both molecular and macroscopic levels takes place.

Table 2 Effect of temperature on ion exchange capacity of synthesized ion exchangers.

Ion exchange capacity (meq/g)
Temperature	50 °C	100 °C	200 °C	300 °C	400 °C	500 °C
Ion exchanger capacity	2.8	2.8	2.2	1.70	1.20	0.80

3.3.2

3.3.2 pH titrations

pH titration curve of synthesized ion exchanger was prepared and is given in Fig. 5. Initially pH of the ion- exchanger was low (∼2.9), but it was increased with increase in KCl/KOH and NaCl/NaOH. It appears to be strong cation exchanger as indicated by a low pH (∼2.9) of the solution. As the volume of NaOH increased, more OH⁻ ions are consumed suggesting the increase in the rate of ion exchange in basic medium due to the removal of H⁺ ions from the external solution (Fig. 5). All the experiments were carried out in triplicate to maintain accuracy as well as the reproducibility of the results. The pH of the solution moves toward neutrality as per the following equation: $H^{+} + {OH}^{-} \to H_{2} O$

Effect of pH titration curve for synthesized ion exchanger.

3.3.3

3.3.3 Chemical stability

Chemical effects on ion exchange capacity of the synthesized ion exchanger were studied and it was found that synthesized ion exchanger was disintegrated when dipped in 2 M HCl, 2 M HNO_3, 1 M HCl, and 1 M HNO₃ and there is decrease in weight of the synthesized ion exchanger. The effect of different mineral acids on ion exchange capacity of synthesized ion exchanger is given in Table 3. It is clear from the results that ion exchange capacity increases, which may be due to disintegration of the ion-exchanger under different acidic conditions as the weight loss was observed (Bayramoglu et al., 2007).

Table 3 Effect of acid and base on ion exchange capacity of synthesized ion exchanger.

Solutions	Initial wt. (g)	Final wt. (g)	Ion exchange capacity (meq/g)
1 M HCl	0.50	0.1003	2.2
2 M HCl	0.50	0.0924	2.4
1 M HNO₃	0.50	0.0864	2.6
2 M HNO₃	0.50	0.0643	2.8
2 M NaOH	0.50	Disintegrates completely	–
2 M NaCl	0.50	Disintegrates completely	–

3.4

3.4 Distribution studies

The distribution studies of synthesized ion exchanger showed high value of K_d for Pb²⁺; thus, it showed that synthesized ion exchanger was highly selective for Pb²⁺. The selectivity of the sample toward Pb²⁺ may be due to the size of this cation just matches with the size of the cavity in the respective exchanger matrix and this leads to the formation of stronger M—O bond; hence, it is preferred over the other cations which don’t have appropriate size as that of the exchanger matrix. The study revealed that the synthesized ion exchanger in DMW showed high preference in the following order: Pb²⁺> Cu²⁺> K⁺>, so the exchanger is lead selective. It is also reported that zirconium (IV) tungstoiodophosphate based ion exchanger has a very high selectivity toward Pb²⁺ ions over the other ions (Zhang et al., 2009). The results for distribution studies of synthesized ion exchanger are summarized in Table 4.

Table 4 Distribution studies of synthesized ion exchanger.

Metal ion (Nitrate)	K_d (mL/g)
Cu(II)	134.42
Pb(II)	356.98
K(II)	72.4

3.5

3.5 Dye removal from aqueous solution

The synthesized sample was evaluated for the removal of rhodamine B dye from aqueous solution. Different parameters such as dye concentration, pH, temperature and synthesized sample doses were optimized in order to get the maximum dye removal from aqueous solution.

3.5.1

3.5.1 Effect of synthesized sample dosage on dye removal

The effect of sample dosage on removal of rhodamine B dye and Pb⁺² ions was studied in the range of 1–5 g/L. The dye and Pb⁺² ions removal using different dosages of synthesized sample are presented in Fig. 6. It is quite clear from the results that percentage dye and Pb⁺² ion removal was increased with increase in the dosage level. The maximum dye and Pb⁺² ion removal was 97.8% and 98.5%, respectively, with 5 g/L of the sample dose. It was due to the fact that in the presence of higher adsorbent dose leads to increase in surface area for the adsorption of dye and Pb⁺² ions and thus increases the removal of rhodamine B and Pb⁺² ions from the solution or in simple words this can also explain that larger dosage size provides large active sites for the removal of dye and Pb⁺² ions (Liu et al., 2010; Ahmadi et al., 2010).

Effect of synthesized sample dosage on rhodamine B dye and Pb+2 ion removal.

3.5.2

3.5.2 Effect of pH

The effect of pH on dye and Pb⁺² ions removal was investigated, and for this experiment it was carried out at different pH conditions (4, 7.7 and 9) as given in Fig. 7. It is clear from the results that efficiency of synthesized ion exchanger for dye and Pb⁺² ions removal is highly affected by the solution pH. When the pH of the solution was less than pKa₂ (7.7) the dye and Pb⁺² removal was found to be 90.3 and 95.6% respectively, and if the value is higher than pKa₂ at pH 10 the dye and Pb⁺² removal was sharply decreased and was found to be 21 and 24.8% respectively (Choi et al., 2008). Dye and Pb⁺² ion removal was found to be 68 and 72.4% respectively in acidic condition i.e. at pH 4. The above results showed that pH has great influence on the removal of dye and Pb⁺² ions from the synthesized ion exchanger. The point of zero discharge is represented as pH_pzc. pH_pzc is defined as the point at which the net charge on the adsorbent surface is zero. pH_zac value of the synthesized ion exchanger was found at pH 9. pH was monitored using a paper pH strip.

Effect of pH on rhodamine B dye and Pb+2 ion removal.

3.5.3

3.5.3 Effect of dye concentration

Effect of initial dye concentration on percentage dye and Pb⁺² ion removal was also investigated by varying the dye and Pb⁺² ions concentration from 10 to 80 ppm. The results obtained showed that percentage dye and Pb⁺² ion removal was found to be maximum (96 and 98.2%, respectively) using 10 ppm dye (Fig. 8a) (Annadurai and Krishnan, 1997).

Effect of initial concentration on (a) dye and Pb+2 ion removal; adsorption of (b) Pb+2 ion and (c) rhodamine B dye through adsorbent at different time intervals.

3.5.4

3.5.4 Effect of contact time and kinetics modeling

Optimization of contact time is essential for the adsorption studies, to ensure complete equilibrium between the dye and Pb⁺² ions-adsorbent systems. Effect of contact time on Pb⁺² ions and dye removal is given in Fig. 8b and c, respectively. It was found from the results that dye and Pb⁺² ion removal from the synthesized ion exchanger was fast during the first 30 min., followed by the slow removal of the dye beyond 30 min and finally equilibrium was attained after 180 min. This may be due to the fact that in the initial stage there is high availability of the adsorption sites, but with the passage of time there is depletion of the adsorption sites.

3.6

3.6 Thermodynamics studies and activation energy

The thermodynamics studies give the information about the effect of temperature change on adsorption process. Temperature reliance of adsorption process was obtained using different thermodynamic parameters such as Gibbs function change (ΔG⁰), standard enthalpy change (ΔH⁰) and standard entropy change (ΔS⁰).

ΔG⁰ obtained from the following equation:

(4)

Δ G^{0} = - RT \ln K_{d}

where K_d is thermodynamic equilibrium constant and it can be calculated from the equilibrium dye adsorption (q_e) and equilibrium dye concentration (C_e).

(5)

K_{d} = \frac{q_{e}}{C_{e}}

Effect of temperature on thermodynamic constant can be calculated using the equation given below:

(6)

\frac{d \ln K_{d}}{dT} = \frac{Δ H^{0}}{{RT}^{2}}

By integrating and rearranging the above equation:

(7)

\ln K_{d} = - \frac{Δ H^{0}}{RT} + \frac{Δ S^{0}}{R}

The value of ΔH⁰ and ΔS⁰ was obtained from the slope and intercept of the linear plot of ln K_d vs 1/T (Fig. 9). The results are given in Table 5. The K_d value was taken at different temperatures at different concentrations. It is quite clear from the results that dye removal was gradually decreased with increase in temperature which indicated exothermic nature of dye removal. The positive value of ΔG⁰ at all the temperatures indicated that dye removal process is not spontaneous. The value of ΔG⁰ decreases with decrease in temperature indicated that dye removal was higher at low temperature. The negative value of ΔH° confirmed that dye removal process is exothermic in nature.

The plot of ln kd vs. 1/T for evaluation of standard enthalpy and entropy change.

Table 5 Thermodynamic analysis at different temperatures.

Temperature (°C)	ln kd	ΔG^o (kJ mol⁻¹)	ΔH^o (kJ mol⁻¹)	ΔS^o (kJ mol⁻¹) × 10⁻²	R²
20	-0.56	1.32	-4.32	-1.81	0.991
30	-0.65	1.48			0.993
40	-0.69	1.47			0.990
50	-0.78	1.69			0.984
60	-0.79	1.74			0.989
70	-0.85	1.62			0.992

Where K_d = thermodynamics equilibrium constant; ΔG^o= Gibbs function; ΔH^o= Standard enthalpy change and ΔS^o= Standard entropy change.

3.7

3.7 Dye removal kinetics

Removal of metal ion and dye through synthesized sample depends upon the interaction between the metal ion and dye on the surface of the synthesized sample. The metal ion and dye removal through synthesized sample was evaluated in terms of the metal ion and dye removal kinetics by measuring removal of metal ion and dye at different times. Pseudo-first and second order and Weber-Morris intra-particle diffusion models were used for characterizing the kinetics data for removal of metal ion and dye (Asgari et al., 2012; Dogan et al., 2004). The linearized pseudo-first-order equation is expressed as follows:

(8)

\log (q_{e} - q_{t}) = \log q_{e} - \frac{t}{2.303} k_{1}

where q_t and q_e is the amount of metal ion and dye adsorbed at time t and equilibrium, respectively. k₁ is the pseudo-first-order rate constant (min⁻¹).

Pseudo-second-order equation is expressed as follows:

(9)

q_{i} = \frac{q_{e}^{2} k_{2} t}{1 + k_{2} q_{e} t}

(10)

q_{t} = k_{p} t^{1 / 2} + c

K_p is the rate constant for intra particle diffusion and K₂ (g mg⁻¹ min⁻¹) is the rate constant of second order. Kinetic parameters such as q_t, k₂, k_p and c were determined by directly fitting the value of q_t and t in the above equation.

The different parameters of the kinetics models are summarized in Table 6. The R² is one of the most important determinants and this represents which experimental data best fitted the model (Dogan et al., 2004; Chieng et al., 2015; Hamdaoui and Naffrechoux, 2007 ). It is quite clear from the results that the value of R² for pseudo-second-order (0.992–0.995 for Pb⁺² ion and 0.987–0.994 for dye) is higher than that obtained from the pseudo-first-order (0.968–o.972 for Pb⁺² ions and 0.854–0.872 for Dye) (Table 6); therefore, based on the value of R², it can be concluded that the pseudo-second-order model is best fitted to the experimental data.

Table 6 Pseudo-first-order, pseudo-second-order kinetics and intra particle diffusion constants.

Concentration (ppm)		25	50	75
Pseudo-first-order
K1 (min⁻¹)	Pb + 2	0.0141	0.0152	0.0121
K1 (min⁻¹)	Rhodamine B	0.0138	0.0146	0.0115
qecal (mg/g)	Pb + 2	52.8	68.4	72.8
qecal (mg/g)	Rhodamine B	38.6	51.7	42.8
R²	Pb⁺²	0.968	0.967	0.972
R²	Rhodamine B	0.864	0.854	0.872
$χ^{2}$	Pb⁺²	216	268	287
$χ^{2}$	Rhodamine B	172	161	187

Pseudo-second-order
k₂ (g mg⁻¹ h⁻¹)	Pb⁺²	0.0024	0.0019	0.0012
k₂ (g mg⁻¹ h⁻¹)	Rhodamine B	0.0029	0.0032	0.0036
q_ecal (mg/g)	Pb⁺²	76.4	72.8	75.4
q_ecal (mg/g)	Rhodamine B	67.8	69.4	72.6
R²	Pb⁺²	0.995	0.992	0.994
R²	Rhodamine B	0.987	0.99	0.992
$χ^{2}$	Pb⁺²	12	14	11
$χ^{2}$	Rhodamine B	11	9	8

Weber-Morris Intra-particle diffusion
K_p (mg g⁻¹ h^−1/2)	Pb⁺²	2.48	4.37	5.68
K_p (mg g⁻¹ h^−1/2)	Rhodamine B	4.32	5.78	6.4
C	Pb⁺²	8.3	10.9	12.6
C	Rhodamine B	10.6	12.3	14.3
R²	Pb⁺²	0.987	0.975	0.983
R²	Rhodamine B	0.974	0.982	0.961

Weber-Morris intra-particle diffusion model was used to describe the diffusion mechanism and is expressed as follows: $q_{t} = k_{3} t^{1 / 2} + C$ where k₃ and C are the intra-particle diffusion rate constant (mg g⁻¹ min^−1/2) and intercept, respectively. k³ and C were determined using the linear plot of q_t vs t^1/2. If the diffusion mechanism is controlled by intra-particle diffusion then the intercept C should cross the origin. As per the Weber-Morris model, but as shown in Fig. 10 and Table 6 all the intercepts are nonzero i.e. not crossing the origin; thus, it showed that intra particle diffusion is not rate limiting.

3.8

3.8 Adsorption Isotherm, kinetics and activation parameters

3.8.1

3.8.1 Adsorption isotherms

Adsorption isotherm gave the invaluable curve, which describes the phenomena prevailing the retention of the substance from the aquatic environment to a solid phase at constant pH and temperature. Adsorption isotherm is important as it describes the interaction between adsorbate and adsorbent. Adsorption isotherms help to find out the relationship between amount of adsorbate adsorbed on the adsorbent and the left out concentration of adsorbate in liquid at the time of equilibrium. There are lots of equations that are often used to describe the experimental isotherms and developed isotherm models. The capacity of the synthesized hybrid ion exchanger for the Pb⁺² ion and rhodamine B dye can be determined by measuring equilibrium isotherms.

There are a wide variety of equilibrium isotherm models, and some of them such as Langmuir, Freundlich, Redlich-Peterson, Dubinin-Radushkevich, Temkin, Sips and Flory-Huggins (two/three parameter isotherms model) are used for present study.

3.8.2

3.8.2 Two parameter isotherms

3.8.2.1

3.8.2.1 Langmuir isotherm model

Langmuir isotherm model generally describes gas-solid phase adsorption. This empirical model assumes monolayer adsorption, where adsorption can only occur at definite localized sites. Derivation of Langmuir isotherm represents the homogenous adsorption, where each molecule possesses constant enthalpies and sorption activation energy. It is characterized by plateau, an equilibrium saturation point. It can be represented as below (Zhang et al., 2009; Chiou and Li, 2002; Langmuir, 1918 ):

(12)

Q_{e} = \frac{Q_{\max} K_{L} C_{e}}{1 / K_{L} C_{e}}

where C_e is the dye concentration at equilibrium. Langmuir constant (dm³/g) is represented as K_L. Q_max is the monolayer capacity of the synthesized sample (mg/g). The constants K_L and Q_max were calculated from the intercept and slope of the linear plot of 1/Q_e and 1/C_e (Fig. 11) and the results are given in Table 7.

Langmuir isotherm 1/Qe vs. 1/Ce (a) Pb+2 ion and (b) Rhoadamine B dye.

Table 7 Parameters of isotherm models on adsorption of rhodamine B and Pb⁺² using synthesized ion exchanger.

Langmuir isotherm
Temperature (°C)	k_L (L mg⁻¹)		q_{max cal} (mg g⁻¹)		R²
Temperature (°C)	Pb⁺²	Rhodamine B	Pb⁺²	Rhodamine B	Pb⁺²	Rhodamine B
25	0.861	0.856	1.38	1.32	0.998	0.996
50	0.956	0.973	1.19	1.23	0.997	0.992
75	0.831	0.846	1.41	1.42	0.985	0.981

Freundlich isotherm
Temperature (°C)	k_F (mg g⁻¹)		N		R²
Temperature (°C)	Pb⁺²	Rhodamine B	Pb⁺²	Rhodamine B	Pb⁺²	Rhodamine B
25	1.139	1.117	1.21	1.29	0.989	0.975
50	1.312	1.248	1.39	1.18	0.991	0.986
75	1.423	1.392	1.47	1.21	0.996	0.994

Dubinin-Radushkevich isotherm
Temperature (°C)	k_D (mol² kJ⁻²)		E (kJ mol⁻¹)		R²
Temperature (°C)	Pb⁺²	Rhodamine B	Pb⁺²	Rhodamine B	Pb⁺²	Rhodamine B
25	2.752	2.654	0.389	0.287	0.82	0.72
50	3.481	3.564	0.367	0.265	0.71	0.69
75	3.978	3.871	0.373	0.318	0.79	0.61

Tempkin isotherm
Temperature (°C)	b (kJ mol⁻¹)		k_T (L g⁻¹)		R²
Temperature (°C)	Pb⁺²	Rhodamine B	Pb⁺²	Rhodamine B	Pb⁺²	Rhodamine B
25	99.4	86.7	0.564	0.469	0.97	0.89
50	87.2	81.2	0.623	0.572	0.91	0.86
75	79.8	72.6	0.435	0.387	0.95	0.83

Redlich-Peterson isotherm
Temperature (°C)	a_R (L mg⁻¹)		B		R²
Temperature (°C)	Pb⁺²	Rhodamine B	Pb⁺²	Rhodamine B	Pb⁺²	Rhodamine B
25	0.78	0.69	0.74	0.65	0.97	0.92
50	0.62	0.61	0.86	0.84	0.98	0.93
75	0.96	0.76	0.89	0.82	0.94	0.91

Sips isotherm
Temperature (°C)	Ks (L mg⁻¹)		q_m (mg g⁻¹)		R²
Temperature (°C)	Pb⁺²	Rhodamine B	Pb⁺²	Rhodamine B	Pb⁺²	Rhodamine B
25	0.056	0.048	87.9	81.4	0.89	0.87
50	0.064	0.056	108.4	98.2	0.95	0.89
75	0.018	0.016	96.8	89.2	0.92	0.86

The shape of the Langmuir isotherm was calculated from the dimensionless constant called separation factor R_L (Zhang et al., 2009; Kinniburgh, 1986; Gupta et al., 2013; Mittal et al., 2016 ).

A dimensionless constant, known as separation factor R_L defined by Webber and Chakkravorti was used to calculate the shape of the Langmuir isotherm (Zhang et al., 2009; Kinniburgh, 1986; Gupta et al., 2013; Mittal et al., 2016 ).

(13)

R_{L} = \frac{1}{1 + K_{L} C_{i}}

where C_i is the initial dye concentration, and K_L is the Langmuir constant. The value of R_L indicates the type of isotherm. If the value of R_L = 1, it indicates linear isotherm, if R_L = 0, it indicates isotherm to be irreversible, if the value lies (0 < R_L < 1) it indicates favorable isotherm and if R_L > 1 it is unfavorable.

3.8.2.2

3.8.2.2 Freundlich isotherm model

Freundlich model is known to describe the non-ideal and reversible adsorption, which was not restricted to the formation of monolayer i.e. it is applied to multilayer adsorption over the heterogenous surface. The slope ranges between 0 and 1 is the measure of adsorption intensity or surface heterogeneity, and if the value is close to zero the surface is more heterogenous. The value below unity implies the chemosorption process, but if the value of 1/n (adsorption intensity) is above 1, then it is the indication of the cooperative process. It is described using the equation given below:

(14)

Q_{e} = K_{F} C_{e}^{1 / n}

where K_F (mg/g) is the adsorption capacity. The value of 1/n indicates the type of isotherm. If the value of 1/n lies in between (0 < 1/n < 1) it showed that isotherm is favorable, if 1/n = 0, it indicates isotherm is irreversible and if 1/n > 1, it is unfavorable. The value of n and K_F was taken from the slope and intercept of the plot log q_e vs log C_e and is given in Table 7.

3.8.2.3

3.8.2.3 Dubinin-Radushkevich (D-R) isotherm model

Dubinin-Radushkevich (D-R) isotherm is generally applied to express the adsorption mechanism with a Gaussian energy distribution on a heterogenous surface. This model is successfully fitted high solute activities at different concentration ranges. This approach was implied to distinguish the chemical and physical adsorption of metal ions. This isotherm is temperature dependent and assumes that there is no homogenous surface on the adsorbent and the equation is expressed as follows: $\ln q_{e} = \ln q_{m} - k_{d} ε^{2}$ $ε = RT \ln [1 + \frac{1}{c_{e}}]$ where k_D and ɛ are D-R constant (mol² kJ⁻²) and D-R isotherm constant, respectively; R and T are the gas (8.314 × 10⁻³ kJ mol⁻¹ K⁻¹) and temperature (K) constant, respectively and q_m is the saturation capacity (mg g⁻¹). The parameter was obtained by the linear plots of ln q_e vs. ɛ².

The mean free energy, E (kJ mol⁻¹) is obtained from k_D and is expressed as follows: $E = \frac{1}{\sqrt{2 k_{D}}}$

3.8.2.4

3.8.2.4 Tempkin isotherm model

Tempkin isotherm model is excellent for predicting the gas phase equilibrium. This model takes into account the interaction between adsorbate and adsorbent. This model assumed that a uniform distribution of binding energy occurred. The Tempkin equation is given below: $q_{e} = \frac{RT}{b} \ln k_{T} C_{e}$ where b and k_T are the Tempkin isotherm constant (kJ mol⁻¹) and equilibrium binding constant, respectively. The parameter was obtained by the linear plots of ln q_e vs. ln C_e.

3.8.3

3.8.3 Three parameter isotherms

3.8.3.1

3.8.3.1 Redlich–peterson (R–P) isotherm model

This is a hybrid isotherm model, which features both Langmuir and Freundlich isotherms and incorporates three parameters into an empirical equation. It has a linear dependence on exponential function in the denominator and concentration in the numerator to represent adsorption equilibrium over a wide concentration range. This isotherm model can be applied to both homogenous and heterogeneous systems due to its versatility and is expressed as follows: $q_{e} = \frac{k_{R} C_{e}}{1 + a_{R} C_{e}^{β}}$ where k_R (L g⁻¹) and a_R (L mg⁻¹) are the R-P constants, while β is the R-P exponent. The parameter of R-P isotherm was obtained by the linear plots of ln [(k_R(C_e/q_e) − 1)] vs. ln C_e.

3.8.3.2

3.8.3.2 Sips isotherm model

This model is combined form of Langmuir and Freundlich expression used for predicting the heterogenous adsorption system and overcoming the drawback associated with Freundlich isotherm model of continuing increase in the adsorbed amount with increase in concentration. Sips equation is similar to the Freundlich equation, but it has a finite limit when the concentration is sufficiently high. $q_{e} = \frac{q_{ms} K_{S} C_{e}^{ms}}{1 + K_{S} C_{e}^{ms}} .$ where C_e is the equilibrium concentration of the adsorbate (mg L⁻¹), q_ms and K_S are the Sips maximum adsorption capacity (mg g⁻¹) and Sips equilibrium constant (L mg⁻¹), respectively. ms is the Sips model exponent.

The parameters of the Langmuir, D-R, Freundlich, Tempkin, Sips and R-P isotherm models were obtained by the linear plots of C_e/q_e vs. C_e, ln q_e vs. ɛ², ln q_e vs. ln C_e, q_e vs. ln C_e, ln [q_e/(q_m − q_e)] vs. ln C_e and ln [(k_R(C_e/q_e) − 1)], respectively and are summarized in Table 7. The value of the R² of all the isotherm parameters was compared and found that the Langmuir model has the highest value of R². Error function of all the parameters was compared and found that Langmuir model has lowest overall experimental error. Thus, it can be concluded from the foregone discussion that Langmuir model best represents the experimental data and is best suited model for the present work. The Langmuir model indicated the homogenous distribution of adsorption sites on the adsorption’s surface and this means a single layer of the dye molecules was formed on the surface. The R_L value comes between 0 and 1, indicated that adsorption process is favorable. The worst fitted model was D-R, which indicates the unsuitability to describe the adsorption characteristics.

4 Conclusion

The use of hybrid ion exchanger for the removal of Pb⁺² ion and rhodamine B dye from aqueous solution was studied in batch experiments. Rapid attainment of adsorption equilibrium and high adsorption capacity values were the significant features. Adsorption of rhodamine B dye on synthesized sample followed the Langmuir and Freundlich isotherms. The kinetics of adsorption followed a pseudo-second order rate equation. Two-parameter Langmuir and Freundlich isotherms were the best-fitting models for the experimental data by the nonlinear method. An increase in temperature resulted in decrease in rhodamine B removal, suggesting that the adsorption process was exothermic. The kinetics of adsorption followed a pseudo-second order rate equation. An overall selectivity for rhodamine B dye was observed showing that synthesized hybrid ion exchanger can be effectively used to remove rhodamine B dye from aqueous solutions. Thus, synthesized hybrid ion exchanger was found to be an efficient and cost effective adsorbent for treatment of industrial wastewater containing dyes.

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Show Sections

[1] Abdel-Halim E.S., . Preparation of starch/poly(N,N-Diethylaminoethyl methacrylate) hydrogel and its use in dye removal from aqueous solutions. React. Funct. Polym.. 2013;73:1531-1536.
[Google Scholar]

[2] Ahmadi S.J., Noori-Kalkhoran O., Shirvani-Arani S., . Synthesis and characterization of new ion-imprinted polymer for separation and preconcentration of uranyl (UO₂²⁺) ions. J. Hazard. Mater.. 2010;175:193-197.
[Google Scholar]

[3] Alakhras F.A., Abu D.K., Mubarak M.S., . Synthesis and chelating properties of some poly(amidoxime-hydroxamic acid) resins toward some trivalent lanthanide metal ions. J. Appl. Polym. Sci.. 2005;97:691-696.
[Google Scholar]

[4] Annadurai G., Krishnan M.R.V., . Adsorption of acid dye from aqueous solution by chitin: batch kinetic studies. Indian J. Chem. Technol.. 1997;4:213-222.
[Google Scholar]

[5] Asgari G., Roshani B., Ghanizadeh G., . The investigation of kinetic and isotherm of fluoride adsorption onto functionalize pumice stone. J. Hazard. Mater.. 2012;17(218):123-132.
[Google Scholar]

[6] Barhon Z., Albizane A., Azzi M., Saffaj N., Bennazha J., Unssi Y.S.A., . Effect of silver activation of zirconium phosphate on methylene blue adsorption. J. Appl. Sci. Res.. 2009;5(7):893-904.
[Google Scholar]

[7] Bayramoglu G., Arica M.Y., Bektas S., . Removal of Cd(II), Hg(II) and Pb(II) ions from aqueous solution using p(HEMA/chitosan) membranes. J. Appl. Polym. Sci.. 2007;106:169-177.
[Google Scholar]

[8] Birlik E., Buyuktiryaki S., Ersoz, Denizli A., Say R., . Selective separation of thorium using ion imprinted chitosan-phthalate particles via solid phase extraction. Sep. Sci. Technol.. 2006;41:3109-3121.
[Google Scholar]

[9] Borissova R., Debouki A., Nikolov T., . Trimetric determination of phosphate and monofluorophosphate in tooth paste. Fresenius J. Anal. Chem.. 1993;347:63-66.
[Google Scholar]

[10] Chand S., Arti S., Chahal C.V., . Synthesis, characterization and ion exchange properties of a new ion exchange material: bismuth (iii) iodophosphate. Recent Res. Sci. Technol.. 2011;3(6):1-8.
[Google Scholar]

[11] Chenga R., Xiangb B., Li Y., Zhanga M., . Application of dithiocarbamate-modified starch for dyes removal from aqueous solutions. J. Hazard. Mater.. 2011;188:254-260.
[Google Scholar]

[12] Chieng H.L., Lim L.B.L., Priyantha N., . Sorption characteristics of peat from Brunei Darussalam for the removal of rhodamine B dye from aqueous solution: adsorption isotherms, thermodynamics, kinetics and regeneration studies. Desalination Water Treat.. 2015;55:664-677.
[Google Scholar]

[13] Chiou M.S., Li H.Y., . Equilibrium and kinetic modeling of adsorption of reactive dye on cross-linked chitosan beads. J. Hazard.. 2002;93:233-248.
[Google Scholar]

[14] Choi K.J., Kim S.G., Kim S.H., . Removal of antibiotics by coagulation and granular activated carbon filtration. J. Hazard. Mater.. 2008;151(38):29-41.
[Google Scholar]

[15] Crini G., Badot P.M., . Application of chitosan, a natural aminopolysaccharide, for dye removal from aqueous solutions by adsorption processes using batch studies: a review of recent literature. Prog. Polym. Sci.. 2008;33:399-447.
[Google Scholar]

[16] Dogan M., Alkan M., Turkyilmaz A., Ozdemir Y., . Kinetics and mechanism of removal of methylene blue by adsorption onto perlite. J. Hazard. Mater.. 2004;109:141-148.
[Google Scholar]

[17] Fenglian F., Wang Q., . Removal of heavy metal ions from wastewaters: a review. J. Environ. Manage.. 2011;92:407-418.
[Google Scholar]

[18] Fil B.A., Boncukcuoglu R., Yilmaz A.E., Bayar S., . Adsorption of Ni(II) on ion exchange resin: kinetics, equilibrium and thermodynamic studies. Korean J. Chem. Eng.. 2012;29:1232-1238.
[Google Scholar]

[19] Gonzalez-Pradas E., Socias-Viciana M., Urena-Amate M.D., Cantos-Molina A., Villafranca-Sanchez M., . Adsorption of chloridazon from aqueous solution on heat and acid treated sepiolites. Water Res.. 2005;39:1849-1857.
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