Pradip M.
Nandanwar
a,
D.
Saravanan
b,
Pankaj
Bakshe
a and
Ravin M.
Jugade
*a
aDepartment of Chemistry, RTM Nagpur University, Nagpur, 440033, India. E-mail: ravinj2001@yahoo.co.in
bNational College, Tiruchirapalli, Tamilnadu, India
First published on 26th May 2022
In this work, we synthesized a chitosan-activated carbon composite (Cs–C) using sodium tripolyphosphate (STTP) as a crosslinker. The Cs–C was characterized through Fourier transform infrared, X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, and the pH point of zero charge for the physicochemical, structural, and morphological analyses. The material was subjected to the adsorption of Remazol Brilliant Blue R (RBBR) dye. The effects of the solution pH, adsorbent dose, contact time, temperature, and dye concentration were examined. The equilibrium data was described well by the Langmuir isotherm model with a coefficient of determination (R2) value of 0.9994 and chi-square value of 6.94. At near-neutral pH, with a contact time of only 60 min at room temperature, the material showed an extremely high adsorption capacity of 540.3 mg g−1 which is much higher in contrast to all the previously reported materials. The kinetics of uptake was well-described by the pseudo-second-order model chemisorption model. Evaluation of the thermodynamic parameters reflected the spontaneous, endothermic and entropy-driven nature of the adsorption.
Reactive dyes are used for dyeing cellulosic fibres in textile industries to a large extent, and these are usually characterized by azo bonds. The colour of azo dyes is due to a chromophoric effect of the azo group. First, the dyes are treated with cellulose and then are made to interact with the fibrous material of the cellulose.8 The reaction occurs by the formation of a covalent bond between the dye molecule and the fibre. Adsorption, electrochemical oxidation, biological treatment, coagulation, physicochemical flocculation, precipitation, ozonation, and other comprehensive technologies have been applied and critically reviewed recently.9–13 Conventional wastewater treatment methods have a low removal efficiency.14 The adsorption process was found to be one of the effective techniques among several chemical, bacteriological, catalytic, and physical methods that have been successfully employed for the removal of colours from waste water due to its low cost, ease of operation, greater efficiency, and non-generation of toxic materials.15
Several adsorbents have been tested for their applicability in dye adsorption from effluents, such as activated sludge,16 alum,17 coal ash,18 silicates,19 acetyl acetone,20 and graphene-based nanomaterials.21 The ability of chitosan to remove contaminants from effluents is highly appreciated due to its diverse functionality, including amino and hydroxyl functional groups in the molecule for forming various interactions, including hydrogen bonding, electrostatic interaction, and van der Waals forces of attraction.22 However, the insufficient strength and stability in acidic medium hinder the greater application of chitosan.23 In order to resolve these issues and to improve the physicochemical properties of chitosan, its modifications using various crosslinkers as well as by composite formation with mechanically and thermally stable materials are the most obvious pathways. Composites of chitosan with activated charcoal can be applied to improve the surface structure, morphology, and surface-assimilative property of chitosan biopolymers.24 Adding a crosslinking agent enhances the chemical stability and leads to microstructure improvement for its improved adsorption performance.25 Several studies have been carried out on the adsorption of water pollutants using chitosan-activated charcoal composites involving the removal of dyes in the recent years.26–28
Recently, we reported a composite of cellulose fabricated by the co-precipitation of Fe and Al in the cellulose material for the detoxification of various reactive dyes with high adsorption capacities.29 In this study, we used a hybrid approach involving crosslinking chitosan with sodium tripolyphosphate (STTP) for imprioved mechanical strength and for entrapping activated carbon through the enhanced surface area and porosity, leading to a supersorbent with excellent confiscation capability for RBBR dye.
Trial runs were performed to compare the adsorption efficiencies of unmodified and sequentially modified adsorbents for the RBBR dye. For this, 50 mg L−1 dye solution was equilibrated for 60 min with 100 mg of unmodified chitosan, activated charcoal, STTP crosslinked chitosan, and Cs–C composite in different flasks. The solution phase concentrations in each flask were determined after filtration and the adsorption efficiency was calculated for each of them.
The pHPZC of Cs–C was determined by a previously reported method32 in order to establish the surface charge on the adsorbent. For this, 50 mL 0.1 M NaCl solutions with varying initial pH from 2.0 to 9.0 were taken in a series of conical flasks. These solutions were stirred with 100 mg of Cs–C for 24 h. The final pH of the supernatant solutions were measured. A graph was plotted of the change in pH as a function of the initial pH, with the point it intersects the x-axis called the pH point of zero charge.
For studying the effect of the initial solution pH on the adsorption efficiency, a series of dye solutions of 100 mg L−1 were prepared and their pH was varied from 4.0 to 9.0. To each of the systems, 100 mg of Cs–C adsorbent was administered and equilibrated for 30 min. After that, the systems were filtered and the absorbance values were obtained.
The kinetics of adsorption was studied by equilibrating 100 mg L−1 dye solution with 25, 50, and 100 mg of Cs–C from 5 to 150 min. The residual solution phase dye concentration was determined after filtration.
In order to study the effect of the initial dye concentration, various dye concentrations from 20 to 400 mg L−1 were equilibrated with 25, 50, and 100 mg Cs–C for 60 min and then the dye concentration in the solution was determined.
The Cs–C dose was increased from 25 to 300 mg. Four dye concentrations of 50, 100, 150, and 200 mg L−1 were stirred with various doses of Cs–C for the optimized contact time of 60 min and then their final concentrations were determined spectrophotometrically.
To study the effect of temperature as part of the evaluation of the thermodynamics parameters, the temperature was varied from 298 to 333 K. The quantity of adsorption of RBBR upon Cs–C was investigated at an initial RBBR concentration of 100 mg L−1 using a 25 mL volume and adsorbent dose of 100 mg.
The crosslinking could be further confirmed by EDX studies of Cs–C (Fig. 1b). The EDX spectrum showed the important peaks for phosphorous and sodium along with those of carbon and oxygen, indicating the incorporation of tripolyphosphate moieties in the chitosan framework.
The SEM micrographs of Cs–C (Fig. 1c) revealed that the surface was non-uniform and heterogenous. This heterogeneity led to an overall enhancement in the surface area and hence in the adsorption efficiency.
XRD analysis was carried out to determine the crystalline and/or amorphous nature of Cs–C, as shown in Fig. 1d.36 The diffractogram clearly showed the characteristic peaks of chitosan at 2θ = 10.5° and 20.1°, corresponding to the (020) and (110) planes. The slightly crystalline nature of the chitosan matrix could be attributed to the intermolecular and intramolecular hydrogen bonding. The XRD pattern of Cs–C showed additional peaks at 2θ = 44.5° and 64.9°, which were typical for the tripolyphosphate crosslinked chitosan.37,38
The TGA curve of Cs–C. (Fig. 1e) showed a small weight loss of about 15% up to 150 °C associated with an endothermic crest in the DTA curve corresponding to the loss of moisture. A second major weight loss of about 50–55% occurred between 330 °C and 500 °C and was associated with an exothermic peak in the DTA curve indicating the degradation of the polymeric organic framework.39
The pore dimensions and surface area of Cs–C were determined by N2 adsorption–desorption isotherms, as depicted in Fig. 1f, while the pore-size distribution is shown in Fig. 1g. The surface area of the unmodified chitosan was found to be 0.65 m2 g−1 which was found to be enhanced to 190 m2 g−1 in the Cs–C composite. These values were obtained by BET isotherm. As Cs–C was a porous material, its pore properties were established through the BJH method using the nitrogen adsorption–desorption curve. The pore volume was found to be 1.388 m3 g−1, showing the highly porous nature of the material in contrast to native chitosan, which is completely non-porous in nature. The mean pore radius of 1.459 nm was an indication of the microporous nature of the composite. As shown in Fig. 1f, the N2 physisorption isotherm could be assigned as a type IV hysteresis on the basis of IUPAC classification. This indicated the presence of micropores in the Cs–C structure.40
Activated charcoal (AC) has a large surface area and microporous structure. The incorporation of AC in the chitosan matrix led to an enhancement of the adsorption of anionic RBBR dye molecules on the Cs–C surface because of the high diffusion of RBBR dye molecules through the micropores of Cs–C.41 The hysteresis loop (Fig. 1g) was clear evidence of the presence of micro- and mesopores in the composite material. This observation was consistent with the flake-like appearance of the surface of the material as obtained in the SEM micrographs. The shift in the desorption curve towards the left was due to the cavitation-driven desertion.42,43 The high adsorption capacity of the Cs–C composite towards RBBR could be prominently attributed to the high surface area, large pore volume, and micro- and mesoporous nature of the adsorbent.
The effects of various variables in the adsorption experiments are presented in Fig. 3. The pH at which the surface charge on Cs–C was zero, i.e. the pHpzc was found to be 7.8, representing that the material exhibited a near-neutral pHpzc (Fig. 3a), meaning that the Cs–C surface would be distinctly positive below pH 7.8 and negative above pH 7.8. It was observed that there was no significant effect of pH on the removal efficiency of dye from pH 4 to 7 (Fig. 3b). In this pH range, the surface charge on Cs–C was positive, indicating a strong attractive interaction with the anionic dye molecules. In acidic pH, the –NH2 group of chitosan was protonated as –NH3+ while the RBBR dye has an –SO3− group, which would be responsible for the interaction in the acidic range.44 Strong electrostatic forces between the adsorbent and adsorbate were responsible for the high adsorption capacity of Cs–C towards RBBR dye. However, with the increase in pH above 7.8, the adsorption efficiency was found to be reduced at pH 8 and 9. This was quite obvious and could be related to the surface charge of the adsorbent. Hence, the original solution with pH 6.0 was used in all the studies.
The increase in the adsorption duration from 5 min to 150 min showed a rapid increase in adsorption in the beginning. This could be attributed to the available surface for the adsorbent molecules in the beginning. As the surface got covered with the dye, the percentage adsorption reached almost saturation in 60 min. After 60 min, there was a negligible hike in removal efficiency, and so the contact time of 60 min was considered optimum for RBBR dye (Fig. 3c). This was quite obvious as the maximum number of sites were available on the Cs–C surface in the beginning, but over time the sites get occupied by dye molecules, and equilibrium is reached in about 60 min.45
The increase in RBBR concentration led to a reduction in the percentage adsorption but increase in the value of qe (Fig. 3d). As the concentration of incoming RBBR molecules increased, the relative availability of active adsorption sites decreased, thus lowering the removal percentage. However, the gathering of the excess dye on the adsorbent surface enhanced its adsorption capacity in terms of mg of dye adsorbed per gram of adsorbate material. As the adsorbent dose was increased, the availability of the surface increased and so the accumulation per unit mass (qe) went on decreasing with the increase in adsorbent dose for the same concentration. With this context, an initial dye concentration of 100 mg L−1 was fixed for the further studies.
As the Cs–C amount was increased from 25 mg to 300 mg, the availability of a greater number of active adsorption sites led to an enhancement in the adsorption percentage (Fig. 3e). For 100 mg L−1 solution, an adsorbent dose of 100 mg was found to give more than 90% adsorption, and so this dose was selected as the optimum dose.
It was observed that the adsorption was favoured by rise in temperature, indicating the endothermic nature of the adsorption process (Fig. 3f). The simultaneous effect of two parameters is depicted in Fig. 3g and h. These graphs clearly indicate that a longer adsorption period, higher adsorbent dose, and lower dye concentration led to a greater percentage adsorption.
It can be clearly seen from Table 1 and Fig. 4 that the Langmuir model fitted best with the experimental data, with a coefficient of determination value very close to 1 and a smaller value of χ2, indicating monolayer adsorption on the homogeneous surface of the adsorbent. The maximum monolayer adsorption capacity obtained from the Langmuir model was found to be 540.3 mg g−1, which showed the excellent efficiency of the material. RL < 1 and 1 < n < 10 were indications of the feasible adsorption of the dye on the Cs–C surface.
Studies | Model | Parameter | Observation |
---|---|---|---|
Kinetics studies | PFO | K 1 (min−1) | 0.0456 |
q e cal (mg g−1) | 62.01 | ||
q e exp (mg g−1) | 24.67 | ||
R 2 | 0.965 | ||
PSO | K 2 (mg g−1 min−1) | 0.0050 | |
q e cal (mg g−1) | 26.41 | ||
q e exp (mg g−1) | 24.67 | ||
R 2 | 0.999 | ||
Intraparticle diffusion | K int (mg g−1 min −1/2) | 1.351 | |
Intercept (C) | 11.03 | ||
R 2 | 0.772 | ||
Isotherm models | Langmuir | q m (mg g−1) | 540.3 |
b (L g−1) | 0.478 | ||
R L | 0.040 | ||
R 2 | 0.9994 | ||
χ 2 | 6.94 | ||
Freundlich | K F (mg L−1/n g−1 L−1) | 10.418 | |
n | 5.216 | ||
R 2 | 0.9140 | ||
χ 2 | 69.37 |
Thermodynamic parameters | Temperature | ΔG (kJ mol−1) | ΔH (kJ mol−1) | ΔS (J mol−1 K−1) |
---|---|---|---|---|
298 K | −5.932 | 14.186 | 67.125 | |
313 K | −6.697 | |||
323 K | −7.290 | |||
333 K | −8.383 |
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Fig. 4 (a) Langmuir, (b) Freundlich, (c) Ce–qe plot, (d) PFO, (e) PSO, and (f) intraparticle diffusion. |
Pseudo-first-order (PFO), pseudo-second-order (PSO), and intraparticle diffusion models were applied under optimized operating conditions. The models were applied in a time-dependent study. As can be observed form Table 1 and Fig. 4, the PSO model was the best fitted model for the kinetics of adsorption. This was clear from the correlation coefficient (R2) values. The calculated qe values from the PSO model were in agreement with experimental value of qe, thereby suggesting that the adsorption of RBBR dye on the Cs–C surface involved chemical interactions, such as electrostatic attraction between the negative charge of RBBR dye and positive charge available on the Cs–C surface.49 The Weber Morris intraparticle diffusion model showed that the adsorption process was diffusion controlled in the beginning with a zero intercept and linear relation between qt and t1/2. However, with time, the process led to saturation and the boundary layer played an important role in the non-zero intercept of the graph. The overall intercept value of 11.03 indicated that the overall process was not just controlled by the diffusion.46
The equilibrium constant K was established at different temperatures from 298 to 333 K taking the ratio of concentration of dye in the adsorbed phase to that in the solution phase. Using the equation ΔG° = −RTln
K, the corresponding values of ΔG were calculated, while the values of ΔH and ΔS were calculated from the intercept and slope of the vant Hoff plot of lnK as a function of 1/T. It was interesting to note that the adsorption process was endothermic in nature, but spontaneous over the entire temperature range and resulted in an increase in randomness. This showed the entropy-driven nature of the process (Table 1).
Adsorbent | q m (mg g−1) | Ref. |
---|---|---|
(DTMA) bromide modified bentonite | 206.58 | 50 |
Magnesium oxide | 250 | 51 |
Lignocellulosic waste | 75.19 | 52 |
Activated clay | 400 | 53 |
Co-electrospun nanofibres | 61.2 | 54 |
Lemna minor | 9.45 | 55 |
Fly Ash | 47.86 | 56 |
Metal hydroxide waste sludge | 91 | 57 |
Mesoporous activated carbon | 35.5 | 58 |
Fe–Al doped cellulose (FADC) | 95.62 | 59 |
Activated charcoal from leaves of Thuja orientalis | 170 | 60 |
Fomes fomentarius | 90 | 61 |
Sewage sludge biochar | 126.59 | 62 |
Yarrowia lipolytica Biomass | 103 | 63 |
Cs–C | 540.3 | Present work |
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