Adsorption Mechanism of Microcrystalline Cellulose as Green Adsorbent for the Removal of Cationic Methylene Blue Dye.
Summary: The adsorption mechanism of pure cellulose is yet to be explored. Thus, in this study, the adsorption mechanism of Microcrystalline Cellulose (MCC), a polysaccharide which is renewable, low cost and non-toxic, was studied on the adsorption of model dye Methylene blue (MB). It was found that the main adsorption mechanism of MB on MCC was due to the electrostatic attraction between the positively charged MB dye and negatively charged MCC. Thus, physical adsorption was the dominant effect, since electrostatic attraction is categorized as physical adsorption. This was verified by Dubinin-Radushkevich isotherm, whereby mean free energy adsorption value was found to be 1), linear (RL=1), favorable (0<RL<1) and irreversible (RL=0).
Freundlich isotherm  is modeled based on the assumption that multilayer adsorption occurs on heterogeneous sites and surface of the adsorbent, whereby reversible adsorption occurred. Besides that, there is interaction between molecules adsorbed on neighboring sides. The linearized Freundlich Isotherm is derived as Eq. (5):
Temkin Isotherm is modelled based on the assumption that the decreased of heat of adsorption as a function of temperature is linear instead of logarithmic . Linearized Temkin Isotherm is derived based on Eq. (6):
where, KT represents the Temkin isotherm equilibrium binding constant and BT is Temkin isotherm constant associated with adsorbate- adsorbate interaction and adsorption heat DHT, whereby BT= -RT/ DHT , with R is the universal gas constant of 8.314 J/K/mol, and T is the temperature in kelvin scale . Therefore, Temkin isotherm is a function to temperature. Adsorption heat DHT is essential to determine whether MCC adsorbent behave as a physical adsorption or chemisorption .
On top of these three isotherm model, Dubinin-Radushkevich Isotherm is also essential to determine whether MCC adsorbent behave as a physical adsorption or chemisorption. This is because Dubinin-Radushkevich isotherm can be used for the estimation of apparent free energy and the characteristics of adsorption . Thus, Dubinin- Radushkevich Isotherm is also used to fit the equilibrium data. Linearized Dubinin-Radushkevich Isotherm is derived based on Eq. (7):
where, B denotes the Dubinin-Radushkevich isotherm constant associated with adsorption energy, Qm is the Dubinin-Radushkevich adsorption capacity and E represents the Polanyi potential, derived from Eq. (8):
The mean free energy adsorption Ea could be determine based on the Dubinin-Radushkevich isotherm constant B, derived in Eq. (9).
Based on Eq. (4), (5), (6) and (7), the corresponding characteristics parameters are tabulated in Qmax and KL could be obtained from the slope and the intercept respectively from the linearized Langmuir isotherm in Fig. 6, the equilibrium data fitted well in Langmuir Isotherm at all temperatures, with R2 ranging from 0.994-0.9977. Compared to Freundlich, Temkin and Dubinin- Radushkevich isotherms, Langmuir isotherms had the highest R2 values at all temperature. Thus, the adsorption of MB on MCC was based on monolayer adsorption at the homogenous surface of MCC, whereby adsorbed MB molecules did not interact with each other at neighboring adsorption sites. As shown in Fig. 5, the maximum adsorption capacity Qmax decreased with temperature, which is further discussed in Section 3.5. Table-1 also tabulates the separation parameter RL, which are shown to be within 0<RL<1, which confirmed a favorable adsorption process at all temperatures.
Kf and 1/n could be obtained from the intercept and slope of the Linearized Freundlich Isotherm respectively in Fig. 7. Kf, which is related to Freundlich adsorption capacities, was shown to decrease with temperature, and has a similar trend with Qmax from Langmuir Isotherm. As observed in Table-1, all the 1/n values ranged between 0.1 and 1 at all temperatures. This suggests that the adsorption of MB on MCC is favorable at all temperatures , supported by the RL values based on Langmuir Isotherm.
The equilibrium data did not fitted well in Temkin and Dubinin-Radushkevich isotherms modelling, as evident by their lower R2 values compared to Langmuir Isotherms. However, despite their low R2 values, Temkin and Dubinin- Radushkevich isotherms could give insights on the adsorption mechanisms , based on adsorption heat DHT and mean free energy adsorption Ea, respectively. As could be observed in Table-2, the adsorption heat DHT were -3.25 kJ/mol, -3.61 kJ/mol and -5.42 kJ/mol at 298 K, 308 K and 318 K respectively. The adsorption heat values at all the temperatures were higher than -20 kJ/mol; suggesting that physical adsorption is the dominant effect  behind the adsorption mechanism of MB on MCC. Similarly, the mean free energy adsorption Ea values at different temperatures as shown in Table-2, are <8 kJ/mol, suggesting that physical adsorption  is the dominant effect behind the adsorption mechanism of MB on MCC.
These verified the deduction on Section 3.3 that the adsorption of MB on MCC was mainly physical adsorption, which was attributed to rely on electrostatic attraction between MCC and MB.
Table-1: Isotherm Parameters values at different temperature.
Effect of Temperature on Equilibrium and Thermodynamics Studies
Thermodynamics studies were essential to study the spontaneous of the process with temperature, by taking into considerations into both energy and entropy. This was performed by using the Van Hoff's equation, which is derived below as Eq. (10):
whereby, Kc is the equilibrium constant, DH is the change in enthalpy, DS is the change in entropy and DG is the Gibbs free energy. A plot of Ln Kc vs 1/T was plotted (not shown) and the value of DH and DS was obtained from the intercept and slope of the plot respectively. These values were tabulated in Table-2.
Looking into results presented in Table-2, DG values at all temperatures were in negative values, which confirmed the spontaneous and feasibility of the adsorption of MB dye on MCC , thus verified the RL and 1/n values from Langmuir and Freundlich Isotherms respectively. Furthermore, the DG values at all the temperatures were within the range of - 20kJ/mol and 0 kJ/mol, further confirmed the deduction in Section 3.3 that physical adsorption was the dominant adsorption mechanism behind the adsorption of MB dye on MCC.
The increasing in DG values with temperature and the negative DS indicated that the adsorption was spontaneous at lower temperature due to the decrease in randomness occurs with increasing temperature . As a result, the adsorption was exothermic in nature, indicated by the negative DH values at all temperatures . The decreased in randomness with temperature resulting in the decrease in adsorption capacities at all dye initial concentration with temperature, as evident in the equilibrium curves in Fig. 5, and the decreased in Qmax and Kf values with temperatures from Langmuir and Freundlich Isotherms respectively.
Table-2: Thermodynamics Parameters.
Effect of Contact Time and Adsorption Kinetics
The effects of contact time on the adsorption of MB on the percentage at different temperatures are shown in Fig. 8, the adsorption occurred very rapidly, reaching equilibrium at 3 minutes, 1.5 minutes and 1 minute at 298 K, 308 K, and 318 K respectively. The maximum percentage removals and equilibrium adsorption capacities were achieved at 298 K, 308 K and 318 K are 90%, 85% and 80%, 1.44 mg/g, 1.36 mg/g and 1.28 mg/g respectively.
It should be highlighted that in comparisons to other adsorbents for the adsorption of MB, MCC had a significantly very fast adsorption. This include nanoparticles such as Carbon Nanotubes , Ilemite nanoparticles  and Titania nanoparticles , achieving equilibrium time at 60 minutes, 600 minutes and 90 minutes respectively, and wastes such as pyrolyzed petrified sediments, achieving equilibrium time in 480 minutes . The strong electrostatic attraction enable rapid filling of adsorption sites by MB molecules on the surface of MCC, resulting in a very rapid adsorption rate , whereby achieving percentage removal of 66%-74% within 0.5 minutes. However, beyond 0.5 minutes, most of the adsorption sites were occupied, which resulted in decrease in adsorption rate.
By 1-3 minutes, all the adsorption sites were occupied , whereby saturation and thus maximum percentage removal and equilibrium adsorption capacities had been achieved. Thus, beyond 3 minutes, there was no more further adsorption occurred.
Further insight to the adsorption rate could be conducted by evaluating the adsorption kinetics of MB on MCC, which could also determine the rate- determining steps, as well as the adsorption mechanism. Three kinetics models were analyzed, which consist of pseudo-first order, pseudo-second order and Elovorich models. Linearized Pseudo-first order model is defined below in Eq. (11):
where, Qt is the adsorption capacity at time t and k1 is the first order reaction rate constant. Linearized Pseudo-Second Order Model is defined below in Eq. (12):
whereby, k2 presents the second order reaction rate constant. Linearized Elovorich Kinetics Model is defined below in Eq. (13):
where, a denotes the initial adsorption rate and b represents the constant rate related to surface coverage and the activation energy for chemical adsorption. Normalized standard deviation (STD) is used to determine the most suitable model that describes the kinetic study of adsorption. Fig. 9 shows the linearized Pseudo-First Order at different temperatures. Clearly, at all the temperatures, the adsorptions of MB dye on MCC did not fit in the Pseudo-first order model at all, as evident with a low R2 values of 0.9851, 0.9743 and 0.9436 at 298 K, 308 K and 318 K respectively, as listed in Table-3. Furthermore, the STD between Qexp (experimental adsorption capacity at equilibrium) and Qcal (calculated adsorption capacity at equilibrium) were very large for all temperatures, which were 10%, 21% and 31% at 298 K, 308 K and 318 K, respectively.
The results obtained from Pseudo-first order model indicated that the adsorption of MB on MCC might follow Pseudo-second order. Fig. 10 shows the linearized Pseudo-second Order at different temperatures. Clearly, at all temperatures, the adsorption of MB on MCC did indeed fit very well in Pseudo-second order model. This was evident with very high R2 values of 0.9997-1 at all temperatures, as listed in Table-3. The STD between Qexp and Qcal were very low for all temperatures, which were 0.5%, 0.05% and 0.02% at 298 K, 308 K and 318 K respectively. Pseudo-second order model is defined based on the rate of adsorption is proportional to the square of the number of unoccupied sites on the adsorption sites on the adsorbate surface as well. adsorption mechanism relating to chemical adsorption . Fig. 11 shows the Linearized Elovorich model at different temperatures.
It was observed that the adsorption kinetics of MB adsorption of MCC fitted only moderately with Elovorich model, with R2 were determined to be 0.9863, 0.9876 and 0.9698 at 298 K, 308 K and 318 K respectively as tabulated in Table-3. Since it is only fitted moderately, it is concluded there is no chemical adsorption mechanism involved.
Characterization was done using FTIR, SEM and XRD and BET surface area, before and after adsorption to better understand and explain the results obtained.
Table-3: Kinetics Parameters.
Pseudo-first Order###Qexp (mg/g)###1.44###1.36###1.28
Pseudo-second order###Qexp (mg/g)###1.440###1.360###1.280
Specific surface area and porosity of MCC were determined using the Brunauer-Emmet-Teller (BET) method. As shown in Table-4, the specific surface area and the pore volume obtained were 1.32 m2/g and 0.02405 cm2/g respectively. The average pore width was 22.72 nm, suggesting that MCC is a mesoporous material . The low specific area of MCC resulted in a relatively low adsorption capacity of MCC on MB, which is 4.95 mg/g at 298 K. This was despite the fact that MCC could effectively remove MB with high percentage removal due to several adsorption mechanisms. This was because specific surface area correlates to the adsorption capacity and is directly proportional to the number of adsorption sites according to Brunauer-Emmet- Teller (BET) theory .
Table-4: BET data for MCC.
###Specific Surface Area###1.32 m2/g
###Pore Size###22.72 nm
###Pore Volume###0.02405 cm3/g
Smooth, crystalline surface was observed on the surface of pure MCC as presented in Fig. 12(a).
Meanwhile, Fig. 12(b) shows a sharp-edged, crystallite structure of MB powder, which is significantly different from the MCC surface. Fig. 12(c) is the FE-SEM image for MCC after adsorption of 12 minutes .Compared to Fig. 12 (a), rougher surface was observed for the MCC after adsorption at Fig. 12(c). Several sharp-edged crystalline structures, which indicate the MB particle, are observed to be attached on the surface of MCC due to the dominant electrostatic attraction between the oppositely charged MCC and MB, as verified based on the results of Temkin and Dubinin-Radushkevich Isotherms. The smooth surface of MCC could no longer be observed in Fig. 12(c), as it was saturated with MB particles. As a result, all the adsorption sites of MCC had been used up, and could not further remove the MB dye.
Fig. 13(a) shows the FTIR spectra of pure MCC before the adsorption process. The wavelength of 3436 cm-1 and 1642 cm-1 represented the stretching and bending of hydroxyl group respectively in the MCC structure . A peak was observed in 2917cm-1 due to the asymmetric stretching vibration of C-H in pyranoid ring, while the stretching of 1057-1033 cm-1 represent the C-O-C bond of the cyclic alcohol of cellulose . Fig. 13 (b) demonstrates the FTIR spectra for MB. A sharp peak could be seen at wavelength of 1601 cm-1 which represented the C=N- bond in the MB structure while the stretching at 1180-1147 cm-1 represented the N-C bond of the aromatic amines functional group in MB structure. There was no hydroxyl functional group exist in MB structure . Therefore, the hydroxyl peak appeared at 3423 cm-1 was attributed to the hydrates of the MB powder. The spectrum of MCC after adsorption, which is shown in Fig. 13(c), was very much similar to that of Fig. 13(a).
There was still a hydroxyl peak appeared at 3399 cm-1 which was due to the moisture of the environment. However higher intensities at 1642 cm-1 and 1168-1111 cm-1 were observed which are attributed to the overlapping with C=N- bond and N-C bond of the aromatic amines functional group in MB structure, respectively. This showed that MB has been anchored on the surface of the MCC. Comparing Fig. 13(a) and Fig. 13(c), there is neither additional nor disappearance of peaks observed. This verified that there was no chemical adsorption involved. Only electrostatic attraction or physical adsorption involved in the adsorption mechanism.
Adsorption Mechanism and Future Prospective
Results have shown that MCC had strong affinity towards MB dye, which led to the rapid adsorption rate of MB on MCC adsorbent. The strong affinity of MB on MCC was predominantly due to physical adsorption mechanism. As investigated in Section 3.3, the isoelectric point for MCC is at pH 2.8. Thus, the net charge of MCC at pH 2 was positive, due to high concentration of H+ ion. As a result, lowest dye percentage removal was obtained at this pH value due to electrostatic repulsion. Beyond pH 2.8, the net charge of MCC was negative, resulting in significant increase in cationic MB dye percentage removal from pH 2 to pH 3. Thus, the adsorption mechanism of MB on MCC was predominantly due to electrostatic attraction, which was categorized as physical adsorption. This was verified further by Temkin and Dubinin-Radushkevich isotherms and thermodynamics studies, whereby these studies shows that physical adsorption is the predominant adsorption mechanism.
Results from Elovorich kinetics model suggested that there is no chemical adsorption mechanism involved. This was verified by using FTIR on MCC after adsorption, where there was neither additional nor disappearance in peaks. As due to the strong electrostatic interactions between MCC and MB, equilibrium of adsorption occurred at a much faster rate than many other materials. These justified the potential of MCC as sustainable and green adsorbent.
Although this study have been investigated and justify the potential of MCC as sustainable and green adsorbent, there are still some challenges which have to be addressed in the future. The main challenge is the low adsorption capacity. Langmuir isotherm has shown that the maximum adsorption capacity is 4.95 mg/g, which is still significantly lower than other researched adsorbents on MB dye removal. These include carbon nanotube , natural palygorskite , and bentonite  which have 59.7 mg/g, 158.03 mg/g and 168.63 mg/g respectively. The low adsorption capacity was due to the low surface area (1.32 m2/g) of MCC, resulting in inefficient surface contact area between MCC and MB. Adsorption of water soluble dye is only adsorbed on the amorphous region of MCC as it is more hydrophilic than the crystalline region .
Therefore, since MCC has high crystallinity of 83%, there were lesser amorphous regions available for the adsorption of dye, which also contributes to the low adsorption capacity of MCC.
Suggested further improvements could be investigated in the future to increase the adsorption capacities. This includes the fabrication of MCC beads , which could significantly increase porosity, surface area, decrease crystallinity and improve access to internal sorption sites , which in turn, will increase the adsorption capacity of MCC. Similar bio-adsorbents like chitosan has the same problem of low surface area and high crystallinity in powder formed, which were finally solved by fabrication of chitosan bead . Fabrication of cellulose and MCC beads is a well-established method, whereby cellulose is generally dissolved in a mixture of alkaline with urea or thiourea solution, before it is regenerated as cellulose beads in acidic solution .
With a high surface area of between 336-470 m2/g, and porosity between 93-95% , pure cellulose beads are mostly used for drug encapsulation and delivery . However, there are very limited studies on the application of MB dye adsorption using MCC and cellulose beads, which could be explored in the future.
Highly porous, low density and high surface area of MCC or cellulose-based aerogels  could be also fabricated to overcome the challenge on the low adsorption capacity of MCC powder. The formation of aerogel started with the gelation and dissolution of cellulose, followed by solvent exchange with alcohol, and finally drying to form aerogel . The evaporation of alcohol in the drying steps resulting in several voids formation, subsequently highly porous structure consisting of a network of interconnected uniform cellulose is formed . This provides more adsorption sites for the adsorption of MB, and potentially enhances the adsorption capacity significantly. Cellulose-based aerogel is used as a removal of oil by modifying it into hydrophobic aerogel , as pure cellulose aerogel is hydrophilic. Its hydrophilicity thus enhances the interaction between pure cellulose and water soluble dye.
However, there are also very limited studies on the application of MB dye adsorption using MCC and cellulose-based aerogel, which could be further explored in the future.
This paper has studied the adsorption mechanisms of MCC, and its potential as green adsorbent. The adsorption equilibrium fitted well with Langmuir Isotherm, which indicated that monolayer adsorption at the homogenous surface of MCC, whereby adsorbed MB molecules do not interact with each other at neighboring adsorption sites. Temkin and Dubinin-Radushkevich isotherms and thermodynamics studies have shown that electrostatic attraction, which was categorized as physical adsorption, was the dominant adsorption mechanism between MCC and MB. It was due to the electrostatic attraction as adsorption mechanism of this adsorption process which resulted more rapid adsorption of MB dye, compared to a lot of materials, This showed the potential of MCC as the future of green and sustainable adsorbent, particularly for the adsorption of dye, although further improvement is still required to enhance the adsorption capacity.
The authors would like to gratefully acknowledge the Ministry of Education Malaysia (MOE) for providing research funding under the FRGS scheme grant number FRGS/2/2013 TK05/MUSM/03/1.
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|Author:||Tan, Kok Bing; Abdullah, Ahmad Zuhairi; Horri, Bahman Amini; Salamatinia, Babak|
|Publication:||Journal of the Chemical Society of Pakistan|
|Date:||Aug 31, 2016|
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