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Adsorption of Mordant Red 73 dye on acid activated bentonite: Kinetics and thermodynamic study
Accepted Manuscript Adsorption of Mordant Red 73 dye on acid activated bentonite: Kinetics and thermodynamic study
Syed Hassan Javed, Abdul Zahir, Atif Khan, Shabana Afzal, Muhammad Mansha PII: DOI: Reference:
S0167-7322(17)35001-8 https://doi.org/10.1016/j.molliq.2018.01.100 MOLLIQ 8554
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
20 October 2017 23 December 2017 18 January 2018
Please cite this article as: Syed Hassan Javed, Abdul Zahir, Atif Khan, Shabana Afzal, Muhammad Mansha , Adsorption of Mordant Red 73 dye on acid activated bentonite: Kinetics and thermodynamic study. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Molliq(2017), https://doi.org/ 10.1016/j.molliq.2018.01.100
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ACCEPTED MANUSCRIPT Adsorption of Mordant Red 73 dye on acid activated bentonite: Kinetics and Thermodynamic study
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Syed Hassan Javed1, Abdul Zahir2,*, Atif Khan1 Shabana Afzal3, Muhammad Mansha4 1 Department of Chemical Engineering: University of Engineering and Technology Lahore, Pakistan 2 National Textile Research Center: National Textile University Faisalabad, Pakistan 3 Department of Basic Sciences and Huminites, University of Engineering and Technology Multan, Pakistan 4 Pakistan Space and Upper Atmosphere Research Commission (SUPARCO), Karachi, Pakistan
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Email address
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Syed Hassan Javed
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[email protected]
Abdul Zahir (corresponding author)
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[email protected]
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Atif Khan
Shabana Afzal
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[email protected]
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[email protected]
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Muhammad Mansha
[email protected]
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Abstract:
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Waste-water from the textile industry is one of the major sources of contamination, causing serious health problems. The present study involves activation of bentonite clay using different acids and investigates the adsorption efficiency of acid activated bentonite clay for the removal of Mordant Red 73 from textile wastewater. Change in any functionality before and after activation was evaluated by FTIR analysis. Whereas, the surface morphological properties were observer by Scanning electron microscope. EDS analysis confirmed the leaching of Ca+2 and Mg+2 as a result of acid activation. Effects of different parameters i.e. effect of adsorbent dosage, contact time, pH and temperature and their effect on adsorption capacity were studied. Thermodynamic parameters explored the endothermic nature of adsorption. Pseudo-second order model was found to fit best for the equilibrium adsorption data. Keywords: Dye adsorption, Acid activation, Surface morphology, Zeta Potential, Textile waste-water
Bentonite was activated with different organic and inorganic acids. Acid Activated bentonite is a potential adsorbent for textile wastewater treatment. Acid activation leached out the Ca+2 and Mg+2 and made the surface more porous and raptured. The adsorption equilibrium data was well followed by pseudo second order kinetic equation.
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Highlights
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1. Introduction
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Waste water from textile industries containing hazardous contaminants is one of the major sources of pollutants causing contamination of water resources [1,2]. Due to large number of textile industries in various areas, the effluent dyes are contaminating water resources and creating a water pollution on massive scale [3,4]. Even a low concentration of dye in water have devastating effects on aesthetics as well as for the photosynthesis [5]. Considering these problems, it is important to control these dye effluents especially for the protection of underground drinking water. There are different classes of dyes present in textile waste water. Among these classes AZO dye is the most common and found in almost in all kind of textile waste water. Various techniques have been developed for reducing the dye contents in waste water. Among different methodologies, dye removal through adsorption is the most effective and economical method [6–8].
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Many researchers have reported different adsorbents for the removal of dyes from the contaminated wastewater [9]. However, activated carbon has been used as a conventional adsorbent for the removal of dyes due its high surface area. Despite of its high surface area, the regeneration of activated carbon is quite expensive technique. As regeneration results in 10-15% loss of adsorbent [10–12]. Alternative of conventional and expensive adsorbents is naturally existing low cost and nontoxic adsorbents. Bentonite clay is a low-cost and naturally occurring adsorbent. This adsorbent is gaining attentional due to its photochemical properties. Bentonite is categorized as a class of clay mineral which is made up of two consecutive silicate layers and its unit cell consists of two tetrahedral sheets bonded to either side of octahedral sheet. The low valency negative charge is due to the isomorphs replacement of Al+3 for Si+4 in tetrahedral layer and Mg+2 for Al+3 in octahedral layer. The negative charge is neutralized by the presence of replacement of cations i.e. Ca+2, Na+ and K+. Moreover, the adsorption capacity of bentonite clay in low because of the hydrophilic nature of bentonite due to the presence of mineral surfaces. Different techniques have been developed to enhance the adsorption 2
ACCEPTED MANUSCRIPT capacity of bentonite for the adsorption of dyes i.e. sodium activation, baking, and magnesium chloride modification [13–15]. Among various activation techniques the most effective method of surface activation and increasing the specific surface area is by acidic treatment [16].
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Based on the literature review, no significant data is available which compares adsorption capacity of dye on three different type of acid activated bentonite. The present study targets the activation of bentonite clay by different acids i.e. Phosphoric acid, Acetic acid and Oxalic acid. The activated adsorbents were characterized by FTIR, XRD, EDS, Zeta Potential and SEM to identify the change in any functionality, crystallinity, composition, surface charge and surface morphology respectively. Dependency of adsorption on different parameters were investigated such as effect of adsorbent dosage, contact time, pH and temperature. The experimental equilibrium adsorption data were tested against Pseudo-first order and Pseudo-Second order kinetic model. Furthermore, to understand the nature of adsorption thermodynamic parameters were calculated. 2. Materials and methods
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2.1. Materials
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Textile Waste water was collected from local textile industry (Al-Rehmat Textiles. Pvt. ltd) and was used in experiments without further treatment. Mordant Red 73, Phosphoric acid, Acetic acid and Oxalic acid (GR grade) were purchased from of Sigma-Merck (Germany). Bentonite clay was purchased from PubChem. All the reagents were used with any further treatment.
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2.2. Preparation of adsorbent
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Bentonite was activated with three different acids by adding 8g of bentonite in 50 ml of 1M solution of each acid i.e. Phosphoric acid, Acetic acid, Citric acid and Oxalic acid. Solutions were allowed to agitate in an orbital shaker at 60oC for 24 hours. The adsorbents were then filtered by Whatman® filter paper (mesh-41). The filtered adsorbents were then washed several times with distilled water and the pH of the adsorbents were reduced to 6-7. The adsorbents were dried in an oven (WiseCube, Korea) at 70oC for 12 hours. The Acid activated bentonite was stored in an air tight container for further use. These acids activated bentonite adsorbents were abbreviated as AAB-P, AAB-A, AAB-C and AAB-O.
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Waste water analysis and concentration measurement.
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10 ml of textile waste water was taken in quart cuvette and was analyzed in UV-VIS spectrophotometer. Three different dyes, having different λmax was detected i.e. 492 nm, 595 nm and 531 nm which corresponds to Mordant Red 73 (MR73), Remazol Brilliant Blue R (RBBR) and Remazol Red 3B (RR3B). Mordant red 73 (Azo dye) was chosen for the rest of the batch experiments. To calculate the concentration of MR73 after each experiment, a calibration curve of Mordant Red 73 was first prepared. Seven different concentrations of pure dye were prepared and the absorbance was recorded using UV-Vis spectrometer (Spectronic Camspe, UK) at λmax 492 nm. The calibration checks were carried out in triplicate. Then, the maximum absorbance at λmax 492 nm was plotted against each dye concentration. From this plot, the concentration of the dye was calculated. 2.3. Batch Adsorption Study Batch experiments were performed to evaluate the adsorption efficiency of raw and acid activated bentonite. 100ml of textile waste water was taken in 250mL Erlenmeyer flask. 0.5g of adsorbent was added in the flask and was placed in an orbital shaker for 5 hours at 30 °C. The solution was then filtered by Whatman filter 3
ACCEPTED MANUSCRIPT paper and the filtrate was analyzed in double beam UV-VIS spectrometer at λmax 492nm to determine the concentration of dye in the solution. Different parameters were studies i.e. Adsorbent dosage, contact time and temperature; to study the adsorption behavior of adsorbent under various specific conditions.
(2)
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The percentage removal of dye and dye uptake capacity after each experiment was calculated by using the following equations
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Where, “ the amount of dye adsorbed at equilibrium (mg/g), is the initial concentration of dye in solution (mg/L), is the final equilibrium concentration (mg/L) after batch adsorption, “V” is the volume of the dye solution in each experiment (L) and w is the dry weight of the adsorbent.
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Characterization
3.1 Characterization
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3.1.1 FTIR analysis
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3. Results and discussion
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FTIR analysis was carried out by JASCO-4100 from 4000 to 650 cm-1 in transmittance mode with 32 scans at the resolution of 4cm-1. About 2-3 mg of material was well crushed in mortal pistle with 50 mg of KBr and was hydraulically pressed to obtained homogeneous pallet. Pallet was kept in a desiccator for 10 min prior to the analysis. The surface morphology of adsorbents was analyzed by Scanning Electron Microscope (FEIQuanta 200, Czech Republic). The elemental analysis of raw and activated bentonite was done by Energy dispersive x-ray spectroscope (Oxford-INCA x-act). XRD diffractograms were recorded using PAN Analytical Xpert Pro (Netherland) equipped with Cu KR radiation and was operated at 45KV and 40 mA at scan speed of 1 /min over a range of 10-70 2 . The surface charge of the adsorbents was measured by ZetaSizer Nano NS (UK). The zeta potential of the adsorbents was measured at different pH ranging 2-10. The pH of the suspension was adjusted using 0.1M HCl and NaOH solutions. Suspensions were sonicated for about 15 min prior to analysis.
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FTIR analysis of raw and acid activated bentonite are shown in figure 1. For RB the characteristic peaks at 3277 cm-1 and 1034 cm-1 corresponds to the joint stretching and bending vibrations of O-H, associated with the water molecules which get adsorbed on the surface of RB and the O-H of octahedral Al+3 cations. The sharp peak at 1034 cm-1 is attributed to the tetrahedral Si-O stretching. Whereas, weak absorption band at 915 cm-1 exhibits the stretching vibrations of Al-Mg-OH. Moreover, the band at 791 cm-1 confirms the presence of quartz in the RB [17,18]. After acid activation with three different acids, all three activated adsorbents show some prominent changes in the IR spectrum. The decrease in intensity and increase in peak broadening of O-H band at 3290 cm-1 is due to the removal of metallic cations i.e. Mg+2 and Al+3. The reduction in intensity of absorption peak at 777 in AAB-A IR spectra also depict the leaching of Mg+2 and Al+3 from the bentonite as a result of acid activation. However, the intensity of Al-Mg-OH stretching vibrations are further decreased in AAB-O and AAB-P as can be seen in figure 1 [19].
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Figure 1: FTIR analysis of Raw and acid Activated bentonite
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3.2 SEM and EDS analysis:
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Figure 2 shows surface texture of bentonite before and after activation. The surface of raw bentonite is rough and uneven. whereas, after acid activation of bentonite, the surface texture of the adsorbent is dramatically affected. The surface of AAB-A became more dense and rough. However, activation by oxalic acid did not show any prominent effect on the surface morphology of the adsorbent as shown in figure 2. Whereas, the surface of AAB-P is rough, raptured and porous. This change in morphological properties of adsorbent is due to the removal of metals from the surface of the bentonite which makes it more porous and rough.
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The elemental composition of the adsorbent was analyzed by Energy Dispersive X-rays spectrometer. The Raw bentonite have traces of magnesium, silicon, Aluminum and iron which tend to decrease after acid activation. The elemental composition of raw bentonite, AAB-P, AAB-O and AAB-A is shown in table 1 bellow
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Table: 1 Elemental composition of raw and activated bentonite Element C O Mg Al Si Ca Fe
Bentonite wt.% 11.84 45.25 1.81 7.94 23.21 4.46 5.50
AAB-O Wt. % 10.41 50.62 1.41 7.20 21.44 3.24 5.74
AAB-A Wt. % 7.99 50.12 1.72 7.71 21.21 5.61 5.64
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AAB-P Wt. % 12.27 50.99 0 6.61 26.93 0 3.20
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Figure 2: SEM and EDS analysis (a) bentonite, (b) AAB-P, (c) AAB-A and (d) AAB-O at 4000X
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ACCEPTED MANUSCRIPT XRD analysis
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The diffractogram of raw and activated bentonite is shown in figure 3. As the figure illustrate that the raw bentonite is mainly composed of montmorillonite, quartz, Feldspars and calcite. All the constituents other than montmorillonite are present as impurity. The intensity of peak at 22o is decreased in the diffractogram of AAB-O (Oxalic acid activated bentonite), which corresponds to Feldspars. However, the peaks intensity of montmorillonite is also decreased at 20o and 62o as can be seen in figure 3. This decrease in intensity is due to the leaching of Aluminum during the acid activation which can also be confirmed by the EDS analysis. Moreover, the XRD analysis of AAB-P shows some considerable changes in the diffraction pattern i.e. decrease in peak intensity and increase in peak broadening at 20o. Peak broadening at 20o ascribed the amorphous crystals of montmorillonite which is due to the destruction of crystallinity during the acid activation. Absence of peak at 35o indicates that there is no residual calcite and all the calcium content has been leached out during the activation process. Since, acetic acid is a weak acid and has less ability to leach out the metallic impurities when compared with other acids. The XRD pattern of AAB-A shows the presence of montmorillonite, quartz, Feldspars and calcite and exhibits no change in crystal structure of any of the component of the adsorbent.
Figure 3: XRD analysis of raw and Acid Activated bentonite Zeta potential Zeta potential of raw and acid activated bentonite as function of pH is presented in figure 4. The point that cut off the horizontal line depicts the pHpzc at which an adsorbent possesses a net neutral charge. As can be seen from the figure that RB has net neutral charge at pH 2. At this pH value, the total positive charge on RB is equal to total negative charge. Moreover, at pH 2 the RB particles will remain stationary under 7
ACCEPTED MANUSCRIPT the influence of an electric field thus exhibits zero zeta potential. Above pH 2 the surface of the RB is negatively charge and therefore, it will favor adsorption of cationic species and at pH pHzpc there will be excess of H+ ions that makes the surface positively charged which is advantageous for the adsorption of anionic dye [20].
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Furthermore, with the leaching of metallic content after the acidic activation, the surface of the adsorbent became more negatively charged as can be seen in this figure. Due to the scarcity of positive charge on the surface of acid activated bentonite adsorbent, the line does not intersect the horizontal line up to pH 2.
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Figure 4: Zeta Potential of RB, AAB-P, AAB-O and AAB-A
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Effect of adsorbent dosage
Figure 5 shows the effect of adsorbent dosage on percentage removal of MR73. The adsorption of dye increases with an increase in adsorbent dosage. This high uptake is because of more number of active site available for adsorption. Moreover, acid activated bentonite showed higher adsorption efficiency than the raw bentonite. This higher adsorption capacity is attributed to the leaching of metals as a result of acid activation which creates vacant spaces behind that favors the adsorption of MR73. However, AAB-P possesses more pores and rough surface than rest of the adsorbents thus provides more dimensions to the dye molecules to get adsorbed on the surface, therefore, exhibited highest adsorption capacity for the removal of MR73 from textile wastewater.
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Figure 5: Effect of adsorbent dosage (Temperature 30oC, Dye conc. 149 mg/L, pH 7)
Effect of pH
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pH of the solution is an important factor to consider while investigating the adsorption behavior of dye on an adsorbent. Figure 6 illustrates the effect of pH on the adsorption of MR73 on raw and acid activated bentonite. RB shows higher removal of dye at lower pH. In basic solution, at pH>7 there will be force of repulsion between anionic ions of the dye molecules, hydroxyl ions and with the negatively charged adsorbent surface because of deprotonation of hydroxyl sites on the bentonite surface. Furthermore, both negatively charged species i.e. anionic ions of the dye molecules and hydroxyl ions will compete for the same active sites of adsorbent, thus inhibits the adsorption of MR73 on adsorbent surface [15,21,22]. The mechanism for decrease in adsorption capacity with increase in pH can be explained as follow + H2O →
→
+
+
→ H2O +
→ repulsion (no adsorption)
Where is the bentonite surface hydroxyl sites, associated with Al-OH and Si-OH. However, acid activated bentonite adsorbents do not exhibit any significant change in adsorption with decrease in pH as can be seen in figure 6 This trend of removal may be explained on the basis of Zeta potential. With the removal of metallic impurities, the surface of adsorbent became more negatively charge as confirmed by the Zeta potential analysis, thus, there was no optimum pH value that makes the surface 9
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positively charge. Therefore, acid activated bentonite adsorbents show no prominent effect on adsorption with change in pH.
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Thermodynamic parameters
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Figure 6: Effect of pH (Temperature 30oC, Dye conc. 149 mg/L)
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Figure 7 shows the effect of temperature on the adsorption of Mordant Red 73 dye from wastewater on acid activated and raw bentonite. The adsorption of Mordant Red 73 on acid activated and raw bentonite is endothermic process as the adsorption increases with increase in temperature. Higher adsorption at high temperature is ascribed to the kinetic energy of the dye molecules which increases with increase in temperature and make dye molecules more susceptible to get adsorbed on the surface of adsorbent. Whereas, the higher adsorption capacity of AAB-P is due to the leaching of metals which created pores and surface roughness on the surface of the AAB-P. Thus, exhibits higher adsorption capacity. Thermodynamic parameters i.e. enthalpy, entropy and Gibbs free energy are crucial to conclude the feasibility of the adsorption process. These parameters predict whether the process is spontaneous or not, endothermic or exothermic nature of adsorption and the magnitude of the changes onto the adsorbent surface [23]. The thermodynamic parameters were calculated using the following equations [24]. (3) (4) 10
ACCEPTED MANUSCRIPT (5) Where, Kc is the constant of equilibrium, is the amount of dye adsorbed on the surface of adsorbent at equilibrium and is the amount of dye in the solution at equilibrium, T is the temperature of solution (K), R is universal gas constant. The thermodynamic parameters and were determined by the slope and intercept of Van’t Hoff plot i.e. lnKc vs T-1. The calculated parameters are summarized in table 2.
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The negative values of for all adsorbent under studied temperature range (20-50 °C) depicts the feasibility and spontaneous nature of adsorption and the decrease in energy with increase in temperature shows that the adsorption is more spontaneous at high temperature. Moreover, the value of Gibbs free energy of the adsorption process indicates that the adsorption of MR73 on raw and acid activated bentonite is physical adsorption. The positive value of ∆S° reflects reduction in order of the solid/solution interface. Further, the positive value of enthalpy suggests that the adsorption is endothermic. Thus, higher temperature shows increase in adsorption as can be seen in figure 7.
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Figure 7: (a) Effect of temperature and (b) Van’t Hoff plot
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ACCEPTED MANUSCRIPT Table 2 : Thermodynamic parameters calculated for the adsorption of MR73 on raw and activated bentonite Sample
T(°C)
20 30 40 50
-0.62 -0.28 0.00 0.32
20 30 40 50
1.44 2.23 3.55 4.88
20 30 40 50
0.89 1.38 1.94 3.00
20 30 40 50
-0.16 0.12 0.58 0.80
0.99
∆G° (kJ/mol)
∆H° (kJ/mol)
∆S° (KJ/K mol)
23.8
0.08
0.36 -0.44 -1.24 -2.04
32.6
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-1.03 -2.13 -3.23 -4.33
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-0.07 -0.97 -1.87 -2.77
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-5.49 -6.79 -8.09 -9.39
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R2
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Adsorption Kinetics
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Figure 8(a) summarizes the change in percentage removal of MR73 as a function of time for raw and acid activated bentonite. It can be seen in figure 8(a) that the time required to achieve an equilibrium for RB is about 90 min and no more considerable adsorption is observed after equilibrium time. Furthermore, acid activation of bentonite creates additional cavities and dimensions. Therefore, acid activated bentonite takes more time to obtain equilibrium in a system. Kinetic study of adsorption system is important to evaluate the rate adsorbent which infect is required to determine the efficiency of adsorbent and also provide sound knowledge about mechanism of adsorption [25]. In order to determine the rate controlling step as well as the kinetic mechanism associated with adsorption process, two kinetic models were applied on the experimental data. The
pseudo
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(6) Where qe is the adsorption capacity of adsorbent (mg/g) at equilibrium, qt is the adsorption capacity of adsorbent at time “t”, is the rate constant of Lagergren model (pseudo first order model). The kinetic parameters are calculated by the slope and intercept of “ln (qe-qt)” versus “t” plot. 13
ACCEPTED MANUSCRIPT The most common linear form of Pseudo-second order model is given below in equation. (7) Whereas,
is the Pseudo-second order rate constant. The equilibrium adsorption capacity and Pseudo-
second order rate constant can be determined by the slope and intercept of “t verses
” plot.
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The kinetic parameters of adsorption for raw and acid activated bentonite are summarized in table 3. The Pseudo-first order kinetic model does not fit the experimental data as can be seen in figure 8(a). Moreover, the calculated equilibrium adsorption capacity qe shows more deviation from the experimental data with poor regression coefficient (R2) i.e. 0.84, 0.95, 0.94 and 0.88 for RB, AAB-A, AAB-O and AAB-P respectively. Based on regression coefficient (R2), Pseudo-second order model seems to satisfy the experimental data best as can be seen in figure 8(b). Furthermore, the calculated equilibrium adsorption capacity qe is close to the experimental equilibrium adsorption, therefore Pseudo-second order model is more appropriate to explain the kinetics of adsorption.
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Pseudo-first order qe R2
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0.018
6.17
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Table 3: Kinetic parameters of adsorption
(mg/g)
0.0053
14.5
0.99
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0.0040
17.5
0.99
11.9
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0.0023
18.9
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14.6
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23.6
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Pseudo-second order qe R2
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Figure 8: (a) Effect of contact time and (b) Pseudo-Second order Model Fitting
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The current scientific study proved that the acid activated bentonite is a potential adsorbent for the removal of dye from industrial textile waste water. Various parameters i.e. adsorbent dosage, pH, temperature and contact time has showed direct impact on the adsorption capacity of the adsorbent. SEM and EDS analysis confirmed the change in surface morphology and adsorbent composition after acid activation. Thermodynamic study revealed that the adsorption of MR73 was physical adsorption. While the thermodynamic parameters suggest the feasibility of the adsorption process. Kinetic model was best followed by Pseudo-Second order Model. The proposed results confirmed the promising ability of activated bentonite for the removal of hazardous dyes from waste water.
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Acknowledgements
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The authors would like to acknowledge the Department of Chemical Engineering, University of Engineering and technology Lahore for financial and technical support.
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Syed Hassan Javed, Abdul Zahir, Atif Khan, Shabana Afzal, Muhammad Mansha PII: DOI: Reference:
S0167-7322(17)35001-8 https://doi.org/10.1016/j.molliq.2018.01.100 MOLLIQ 8554
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
20 October 2017 23 December 2017 18 January 2018
Please cite this article as: Syed Hassan Javed, Abdul Zahir, Atif Khan, Shabana Afzal, Muhammad Mansha , Adsorption of Mordant Red 73 dye on acid activated bentonite: Kinetics and thermodynamic study. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Molliq(2017), https://doi.org/ 10.1016/j.molliq.2018.01.100
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ACCEPTED MANUSCRIPT Adsorption of Mordant Red 73 dye on acid activated bentonite: Kinetics and Thermodynamic study
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Syed Hassan Javed1, Abdul Zahir2,*, Atif Khan1 Shabana Afzal3, Muhammad Mansha4 1 Department of Chemical Engineering: University of Engineering and Technology Lahore, Pakistan 2 National Textile Research Center: National Textile University Faisalabad, Pakistan 3 Department of Basic Sciences and Huminites, University of Engineering and Technology Multan, Pakistan 4 Pakistan Space and Upper Atmosphere Research Commission (SUPARCO), Karachi, Pakistan
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Email address
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Syed Hassan Javed
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[email protected]
Abdul Zahir (corresponding author)
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[email protected]
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Atif Khan
Shabana Afzal
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[email protected]
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[email protected]
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Muhammad Mansha
[email protected]
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Abstract:
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Waste-water from the textile industry is one of the major sources of contamination, causing serious health problems. The present study involves activation of bentonite clay using different acids and investigates the adsorption efficiency of acid activated bentonite clay for the removal of Mordant Red 73 from textile wastewater. Change in any functionality before and after activation was evaluated by FTIR analysis. Whereas, the surface morphological properties were observer by Scanning electron microscope. EDS analysis confirmed the leaching of Ca+2 and Mg+2 as a result of acid activation. Effects of different parameters i.e. effect of adsorbent dosage, contact time, pH and temperature and their effect on adsorption capacity were studied. Thermodynamic parameters explored the endothermic nature of adsorption. Pseudo-second order model was found to fit best for the equilibrium adsorption data. Keywords: Dye adsorption, Acid activation, Surface morphology, Zeta Potential, Textile waste-water
Bentonite was activated with different organic and inorganic acids. Acid Activated bentonite is a potential adsorbent for textile wastewater treatment. Acid activation leached out the Ca+2 and Mg+2 and made the surface more porous and raptured. The adsorption equilibrium data was well followed by pseudo second order kinetic equation.
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Highlights
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1. Introduction
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Waste water from textile industries containing hazardous contaminants is one of the major sources of pollutants causing contamination of water resources [1,2]. Due to large number of textile industries in various areas, the effluent dyes are contaminating water resources and creating a water pollution on massive scale [3,4]. Even a low concentration of dye in water have devastating effects on aesthetics as well as for the photosynthesis [5]. Considering these problems, it is important to control these dye effluents especially for the protection of underground drinking water. There are different classes of dyes present in textile waste water. Among these classes AZO dye is the most common and found in almost in all kind of textile waste water. Various techniques have been developed for reducing the dye contents in waste water. Among different methodologies, dye removal through adsorption is the most effective and economical method [6–8].
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Many researchers have reported different adsorbents for the removal of dyes from the contaminated wastewater [9]. However, activated carbon has been used as a conventional adsorbent for the removal of dyes due its high surface area. Despite of its high surface area, the regeneration of activated carbon is quite expensive technique. As regeneration results in 10-15% loss of adsorbent [10–12]. Alternative of conventional and expensive adsorbents is naturally existing low cost and nontoxic adsorbents. Bentonite clay is a low-cost and naturally occurring adsorbent. This adsorbent is gaining attentional due to its photochemical properties. Bentonite is categorized as a class of clay mineral which is made up of two consecutive silicate layers and its unit cell consists of two tetrahedral sheets bonded to either side of octahedral sheet. The low valency negative charge is due to the isomorphs replacement of Al+3 for Si+4 in tetrahedral layer and Mg+2 for Al+3 in octahedral layer. The negative charge is neutralized by the presence of replacement of cations i.e. Ca+2, Na+ and K+. Moreover, the adsorption capacity of bentonite clay in low because of the hydrophilic nature of bentonite due to the presence of mineral surfaces. Different techniques have been developed to enhance the adsorption 2
ACCEPTED MANUSCRIPT capacity of bentonite for the adsorption of dyes i.e. sodium activation, baking, and magnesium chloride modification [13–15]. Among various activation techniques the most effective method of surface activation and increasing the specific surface area is by acidic treatment [16].
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Based on the literature review, no significant data is available which compares adsorption capacity of dye on three different type of acid activated bentonite. The present study targets the activation of bentonite clay by different acids i.e. Phosphoric acid, Acetic acid and Oxalic acid. The activated adsorbents were characterized by FTIR, XRD, EDS, Zeta Potential and SEM to identify the change in any functionality, crystallinity, composition, surface charge and surface morphology respectively. Dependency of adsorption on different parameters were investigated such as effect of adsorbent dosage, contact time, pH and temperature. The experimental equilibrium adsorption data were tested against Pseudo-first order and Pseudo-Second order kinetic model. Furthermore, to understand the nature of adsorption thermodynamic parameters were calculated. 2. Materials and methods
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2.1. Materials
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Textile Waste water was collected from local textile industry (Al-Rehmat Textiles. Pvt. ltd) and was used in experiments without further treatment. Mordant Red 73, Phosphoric acid, Acetic acid and Oxalic acid (GR grade) were purchased from of Sigma-Merck (Germany). Bentonite clay was purchased from PubChem. All the reagents were used with any further treatment.
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2.2. Preparation of adsorbent
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Bentonite was activated with three different acids by adding 8g of bentonite in 50 ml of 1M solution of each acid i.e. Phosphoric acid, Acetic acid, Citric acid and Oxalic acid. Solutions were allowed to agitate in an orbital shaker at 60oC for 24 hours. The adsorbents were then filtered by Whatman® filter paper (mesh-41). The filtered adsorbents were then washed several times with distilled water and the pH of the adsorbents were reduced to 6-7. The adsorbents were dried in an oven (WiseCube, Korea) at 70oC for 12 hours. The Acid activated bentonite was stored in an air tight container for further use. These acids activated bentonite adsorbents were abbreviated as AAB-P, AAB-A, AAB-C and AAB-O.
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Waste water analysis and concentration measurement.
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10 ml of textile waste water was taken in quart cuvette and was analyzed in UV-VIS spectrophotometer. Three different dyes, having different λmax was detected i.e. 492 nm, 595 nm and 531 nm which corresponds to Mordant Red 73 (MR73), Remazol Brilliant Blue R (RBBR) and Remazol Red 3B (RR3B). Mordant red 73 (Azo dye) was chosen for the rest of the batch experiments. To calculate the concentration of MR73 after each experiment, a calibration curve of Mordant Red 73 was first prepared. Seven different concentrations of pure dye were prepared and the absorbance was recorded using UV-Vis spectrometer (Spectronic Camspe, UK) at λmax 492 nm. The calibration checks were carried out in triplicate. Then, the maximum absorbance at λmax 492 nm was plotted against each dye concentration. From this plot, the concentration of the dye was calculated. 2.3. Batch Adsorption Study Batch experiments were performed to evaluate the adsorption efficiency of raw and acid activated bentonite. 100ml of textile waste water was taken in 250mL Erlenmeyer flask. 0.5g of adsorbent was added in the flask and was placed in an orbital shaker for 5 hours at 30 °C. The solution was then filtered by Whatman filter 3
ACCEPTED MANUSCRIPT paper and the filtrate was analyzed in double beam UV-VIS spectrometer at λmax 492nm to determine the concentration of dye in the solution. Different parameters were studies i.e. Adsorbent dosage, contact time and temperature; to study the adsorption behavior of adsorbent under various specific conditions.
(2)
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The percentage removal of dye and dye uptake capacity after each experiment was calculated by using the following equations
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Where, “ the amount of dye adsorbed at equilibrium (mg/g), is the initial concentration of dye in solution (mg/L), is the final equilibrium concentration (mg/L) after batch adsorption, “V” is the volume of the dye solution in each experiment (L) and w is the dry weight of the adsorbent.
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Characterization
3.1 Characterization
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3.1.1 FTIR analysis
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3. Results and discussion
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FTIR analysis was carried out by JASCO-4100 from 4000 to 650 cm-1 in transmittance mode with 32 scans at the resolution of 4cm-1. About 2-3 mg of material was well crushed in mortal pistle with 50 mg of KBr and was hydraulically pressed to obtained homogeneous pallet. Pallet was kept in a desiccator for 10 min prior to the analysis. The surface morphology of adsorbents was analyzed by Scanning Electron Microscope (FEIQuanta 200, Czech Republic). The elemental analysis of raw and activated bentonite was done by Energy dispersive x-ray spectroscope (Oxford-INCA x-act). XRD diffractograms were recorded using PAN Analytical Xpert Pro (Netherland) equipped with Cu KR radiation and was operated at 45KV and 40 mA at scan speed of 1 /min over a range of 10-70 2 . The surface charge of the adsorbents was measured by ZetaSizer Nano NS (UK). The zeta potential of the adsorbents was measured at different pH ranging 2-10. The pH of the suspension was adjusted using 0.1M HCl and NaOH solutions. Suspensions were sonicated for about 15 min prior to analysis.
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FTIR analysis of raw and acid activated bentonite are shown in figure 1. For RB the characteristic peaks at 3277 cm-1 and 1034 cm-1 corresponds to the joint stretching and bending vibrations of O-H, associated with the water molecules which get adsorbed on the surface of RB and the O-H of octahedral Al+3 cations. The sharp peak at 1034 cm-1 is attributed to the tetrahedral Si-O stretching. Whereas, weak absorption band at 915 cm-1 exhibits the stretching vibrations of Al-Mg-OH. Moreover, the band at 791 cm-1 confirms the presence of quartz in the RB [17,18]. After acid activation with three different acids, all three activated adsorbents show some prominent changes in the IR spectrum. The decrease in intensity and increase in peak broadening of O-H band at 3290 cm-1 is due to the removal of metallic cations i.e. Mg+2 and Al+3. The reduction in intensity of absorption peak at 777 in AAB-A IR spectra also depict the leaching of Mg+2 and Al+3 from the bentonite as a result of acid activation. However, the intensity of Al-Mg-OH stretching vibrations are further decreased in AAB-O and AAB-P as can be seen in figure 1 [19].
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Figure 1: FTIR analysis of Raw and acid Activated bentonite
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3.2 SEM and EDS analysis:
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Figure 2 shows surface texture of bentonite before and after activation. The surface of raw bentonite is rough and uneven. whereas, after acid activation of bentonite, the surface texture of the adsorbent is dramatically affected. The surface of AAB-A became more dense and rough. However, activation by oxalic acid did not show any prominent effect on the surface morphology of the adsorbent as shown in figure 2. Whereas, the surface of AAB-P is rough, raptured and porous. This change in morphological properties of adsorbent is due to the removal of metals from the surface of the bentonite which makes it more porous and rough.
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The elemental composition of the adsorbent was analyzed by Energy Dispersive X-rays spectrometer. The Raw bentonite have traces of magnesium, silicon, Aluminum and iron which tend to decrease after acid activation. The elemental composition of raw bentonite, AAB-P, AAB-O and AAB-A is shown in table 1 bellow
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Table: 1 Elemental composition of raw and activated bentonite Element C O Mg Al Si Ca Fe
Bentonite wt.% 11.84 45.25 1.81 7.94 23.21 4.46 5.50
AAB-O Wt. % 10.41 50.62 1.41 7.20 21.44 3.24 5.74
AAB-A Wt. % 7.99 50.12 1.72 7.71 21.21 5.61 5.64
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AAB-P Wt. % 12.27 50.99 0 6.61 26.93 0 3.20
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Figure 2: SEM and EDS analysis (a) bentonite, (b) AAB-P, (c) AAB-A and (d) AAB-O at 4000X
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The diffractogram of raw and activated bentonite is shown in figure 3. As the figure illustrate that the raw bentonite is mainly composed of montmorillonite, quartz, Feldspars and calcite. All the constituents other than montmorillonite are present as impurity. The intensity of peak at 22o is decreased in the diffractogram of AAB-O (Oxalic acid activated bentonite), which corresponds to Feldspars. However, the peaks intensity of montmorillonite is also decreased at 20o and 62o as can be seen in figure 3. This decrease in intensity is due to the leaching of Aluminum during the acid activation which can also be confirmed by the EDS analysis. Moreover, the XRD analysis of AAB-P shows some considerable changes in the diffraction pattern i.e. decrease in peak intensity and increase in peak broadening at 20o. Peak broadening at 20o ascribed the amorphous crystals of montmorillonite which is due to the destruction of crystallinity during the acid activation. Absence of peak at 35o indicates that there is no residual calcite and all the calcium content has been leached out during the activation process. Since, acetic acid is a weak acid and has less ability to leach out the metallic impurities when compared with other acids. The XRD pattern of AAB-A shows the presence of montmorillonite, quartz, Feldspars and calcite and exhibits no change in crystal structure of any of the component of the adsorbent.
Figure 3: XRD analysis of raw and Acid Activated bentonite Zeta potential Zeta potential of raw and acid activated bentonite as function of pH is presented in figure 4. The point that cut off the horizontal line depicts the pHpzc at which an adsorbent possesses a net neutral charge. As can be seen from the figure that RB has net neutral charge at pH 2. At this pH value, the total positive charge on RB is equal to total negative charge. Moreover, at pH 2 the RB particles will remain stationary under 7
ACCEPTED MANUSCRIPT the influence of an electric field thus exhibits zero zeta potential. Above pH 2 the surface of the RB is negatively charge and therefore, it will favor adsorption of cationic species and at pH pHzpc there will be excess of H+ ions that makes the surface positively charged which is advantageous for the adsorption of anionic dye [20].
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Furthermore, with the leaching of metallic content after the acidic activation, the surface of the adsorbent became more negatively charged as can be seen in this figure. Due to the scarcity of positive charge on the surface of acid activated bentonite adsorbent, the line does not intersect the horizontal line up to pH 2.
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Figure 4: Zeta Potential of RB, AAB-P, AAB-O and AAB-A
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Effect of adsorbent dosage
Figure 5 shows the effect of adsorbent dosage on percentage removal of MR73. The adsorption of dye increases with an increase in adsorbent dosage. This high uptake is because of more number of active site available for adsorption. Moreover, acid activated bentonite showed higher adsorption efficiency than the raw bentonite. This higher adsorption capacity is attributed to the leaching of metals as a result of acid activation which creates vacant spaces behind that favors the adsorption of MR73. However, AAB-P possesses more pores and rough surface than rest of the adsorbents thus provides more dimensions to the dye molecules to get adsorbed on the surface, therefore, exhibited highest adsorption capacity for the removal of MR73 from textile wastewater.
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Figure 5: Effect of adsorbent dosage (Temperature 30oC, Dye conc. 149 mg/L, pH 7)
Effect of pH
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pH of the solution is an important factor to consider while investigating the adsorption behavior of dye on an adsorbent. Figure 6 illustrates the effect of pH on the adsorption of MR73 on raw and acid activated bentonite. RB shows higher removal of dye at lower pH. In basic solution, at pH>7 there will be force of repulsion between anionic ions of the dye molecules, hydroxyl ions and with the negatively charged adsorbent surface because of deprotonation of hydroxyl sites on the bentonite surface. Furthermore, both negatively charged species i.e. anionic ions of the dye molecules and hydroxyl ions will compete for the same active sites of adsorbent, thus inhibits the adsorption of MR73 on adsorbent surface [15,21,22]. The mechanism for decrease in adsorption capacity with increase in pH can be explained as follow + H2O →
→
+
+
→ H2O +
→ repulsion (no adsorption)
Where is the bentonite surface hydroxyl sites, associated with Al-OH and Si-OH. However, acid activated bentonite adsorbents do not exhibit any significant change in adsorption with decrease in pH as can be seen in figure 6 This trend of removal may be explained on the basis of Zeta potential. With the removal of metallic impurities, the surface of adsorbent became more negatively charge as confirmed by the Zeta potential analysis, thus, there was no optimum pH value that makes the surface 9
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positively charge. Therefore, acid activated bentonite adsorbents show no prominent effect on adsorption with change in pH.
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Thermodynamic parameters
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Figure 6: Effect of pH (Temperature 30oC, Dye conc. 149 mg/L)
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Figure 7 shows the effect of temperature on the adsorption of Mordant Red 73 dye from wastewater on acid activated and raw bentonite. The adsorption of Mordant Red 73 on acid activated and raw bentonite is endothermic process as the adsorption increases with increase in temperature. Higher adsorption at high temperature is ascribed to the kinetic energy of the dye molecules which increases with increase in temperature and make dye molecules more susceptible to get adsorbed on the surface of adsorbent. Whereas, the higher adsorption capacity of AAB-P is due to the leaching of metals which created pores and surface roughness on the surface of the AAB-P. Thus, exhibits higher adsorption capacity. Thermodynamic parameters i.e. enthalpy, entropy and Gibbs free energy are crucial to conclude the feasibility of the adsorption process. These parameters predict whether the process is spontaneous or not, endothermic or exothermic nature of adsorption and the magnitude of the changes onto the adsorbent surface [23]. The thermodynamic parameters were calculated using the following equations [24]. (3) (4) 10
ACCEPTED MANUSCRIPT (5) Where, Kc is the constant of equilibrium, is the amount of dye adsorbed on the surface of adsorbent at equilibrium and is the amount of dye in the solution at equilibrium, T is the temperature of solution (K), R is universal gas constant. The thermodynamic parameters and were determined by the slope and intercept of Van’t Hoff plot i.e. lnKc vs T-1. The calculated parameters are summarized in table 2.
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The negative values of for all adsorbent under studied temperature range (20-50 °C) depicts the feasibility and spontaneous nature of adsorption and the decrease in energy with increase in temperature shows that the adsorption is more spontaneous at high temperature. Moreover, the value of Gibbs free energy of the adsorption process indicates that the adsorption of MR73 on raw and acid activated bentonite is physical adsorption. The positive value of ∆S° reflects reduction in order of the solid/solution interface. Further, the positive value of enthalpy suggests that the adsorption is endothermic. Thus, higher temperature shows increase in adsorption as can be seen in figure 7.
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Figure 7: (a) Effect of temperature and (b) Van’t Hoff plot
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T(°C)
20 30 40 50
-0.62 -0.28 0.00 0.32
20 30 40 50
1.44 2.23 3.55 4.88
20 30 40 50
0.89 1.38 1.94 3.00
20 30 40 50
-0.16 0.12 0.58 0.80
0.99
∆G° (kJ/mol)
∆H° (kJ/mol)
∆S° (KJ/K mol)
23.8
0.08
0.36 -0.44 -1.24 -2.04
32.6
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-1.03 -2.13 -3.23 -4.33
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-0.07 -0.97 -1.87 -2.77
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R2
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Adsorption Kinetics
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Figure 8(a) summarizes the change in percentage removal of MR73 as a function of time for raw and acid activated bentonite. It can be seen in figure 8(a) that the time required to achieve an equilibrium for RB is about 90 min and no more considerable adsorption is observed after equilibrium time. Furthermore, acid activation of bentonite creates additional cavities and dimensions. Therefore, acid activated bentonite takes more time to obtain equilibrium in a system. Kinetic study of adsorption system is important to evaluate the rate adsorbent which infect is required to determine the efficiency of adsorbent and also provide sound knowledge about mechanism of adsorption [25]. In order to determine the rate controlling step as well as the kinetic mechanism associated with adsorption process, two kinetic models were applied on the experimental data. The
pseudo
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(6) Where qe is the adsorption capacity of adsorbent (mg/g) at equilibrium, qt is the adsorption capacity of adsorbent at time “t”, is the rate constant of Lagergren model (pseudo first order model). The kinetic parameters are calculated by the slope and intercept of “ln (qe-qt)” versus “t” plot. 13
ACCEPTED MANUSCRIPT The most common linear form of Pseudo-second order model is given below in equation. (7) Whereas,
is the Pseudo-second order rate constant. The equilibrium adsorption capacity and Pseudo-
second order rate constant can be determined by the slope and intercept of “t verses
” plot.
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The kinetic parameters of adsorption for raw and acid activated bentonite are summarized in table 3. The Pseudo-first order kinetic model does not fit the experimental data as can be seen in figure 8(a). Moreover, the calculated equilibrium adsorption capacity qe shows more deviation from the experimental data with poor regression coefficient (R2) i.e. 0.84, 0.95, 0.94 and 0.88 for RB, AAB-A, AAB-O and AAB-P respectively. Based on regression coefficient (R2), Pseudo-second order model seems to satisfy the experimental data best as can be seen in figure 8(b). Furthermore, the calculated equilibrium adsorption capacity qe is close to the experimental equilibrium adsorption, therefore Pseudo-second order model is more appropriate to explain the kinetics of adsorption.
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Table 3: Kinetic parameters of adsorption
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Figure 8: (a) Effect of contact time and (b) Pseudo-Second order Model Fitting
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The current scientific study proved that the acid activated bentonite is a potential adsorbent for the removal of dye from industrial textile waste water. Various parameters i.e. adsorbent dosage, pH, temperature and contact time has showed direct impact on the adsorption capacity of the adsorbent. SEM and EDS analysis confirmed the change in surface morphology and adsorbent composition after acid activation. Thermodynamic study revealed that the adsorption of MR73 was physical adsorption. While the thermodynamic parameters suggest the feasibility of the adsorption process. Kinetic model was best followed by Pseudo-Second order Model. The proposed results confirmed the promising ability of activated bentonite for the removal of hazardous dyes from waste water.
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Acknowledgements
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The authors would like to acknowledge the Department of Chemical Engineering, University of Engineering and technology Lahore for financial and technical support.
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