Characterization of the acid hydrolyzed NC and adsorbent nanocomposite
The resulted suspension after chemical treatment followed by ultrasonication contains the majority of particles ranging from 100 to 804 nm and the smaller particles were in the range of 1 to 100 nm. The results confirms that nanocellulose particles possess nano form after acid hydrolysis. The particle size distribution analysis showed that the obtained cellulose nanocrystals had an average size of 226 nm shown in Fig. 3.
The mechanical properties such as young’s modulus, Ultimate strength and elongation at break of acid hydrolyzed cellulose nanocrystals (CNCs) and M3 adsorbent nanocomposite were identified at room temperature. It was observed that acid hydrolyzed CNCs showed very low young’s modulus in comparison to M3 adsorbent. Furthermore, on crosslinking with MBA, the young’s modulus and yield strength improved comparatively, it showed the highest young’s modulus (509 ± 1.5 MPa), yield strength (2.05 ± 1.5 MPa) and largest elongation but at the same time showed upper yield point and lower yield points, shown in Fig. 4.
The XRD diffraction patterns of the acid hydrolyzed nanocrystals, sample M3 nanocomposite adsorbent before and after adsorption are shown in Fig. 5. Acid hydrolyzed nanocrystals showed crystalline peak at angles 2θ = 15.79o, 22.37 o and 34.57 o confirm cellulose I structure. The observed peaks may be due to the presence 110, 200 and 004 crystallographic planes which signify cellulose I structure (Khan et al. 2012). After adding chitosan and MBA new peaks originated at 2θ = 11.91 denoting the hydrated crystalline structure of the chitosan (Wang et al. 2005). The average crystallite size and crystallinity index (CI) was calculated using (Eqs. 1 and 2) is 1.1 nm, 1.3 nm, 1.6 nm and 72%, 14%, 12% respectively. It was observed that the crystallinity index of acid hydrolyzed NC was very high comparative to nanocomposite adsorbent before and after adsorption. It is may be due to the removal of lignin or amorphous content from the cellulose after chemical treatment which results in the occurrence of nanocrystalline plains (Mazlita et al. 2016). Increase in the crystallite size from 1.1 to 1.6 mm confirms the adhesion of MG dye molecules over the surface of adsorbent. This study also confirms that the resulted nanocellulose after acid hydrolysis possess high crystalline configuration and as the polymers added the crystalline behavior decline, hence CI decreases from 72 − 12%.
Further the surface morphology information of the nanocomposite was collected using the FESEM technique. Fig. 6 shows the surface morphology of acid hydrolyzed nanocrystals, M3 adsorbent nanocomposite before and after adsorption. The obtained SEM micrographs of acid hydrolyzed cellulose confirmed that the extraction of cellulose nanocrystal was successfully occurs, which showing rod like structure of crystalline nanocellulose (Fig. 6 (a)), the M3 adsorbent nanocomposite before adsorption contained porous matrix and sieved network due to the addition of chitosan and MBA which provides easy diffusion of the MG dye molecules into the adsorbent inner surface and on enlarging the micrograph dimension the porous structure of the adsorbent nanocomposite also confirms (Fig. 6 (b &c)). After the sorption phenomenon the voids spaces get occupied by the MG dye to obtain smooth surfaces shown in Fig. 6 (d), confirms the adsorption of MG dye molecules by forming bond with the adsorbent material.
The IR spectra for the adsorbent nanocomposite samples before treatment and after treatment represents shown in Fig. 7. Acid hydrolyzed nanocrystals bands are formed at 3332 cm− 1 showing O-H stretching due to intramolecular hydrogen bonding of cellulose molecules, 2909 cm− 1 showing C-H stretching, 1640 cm− 1 showing O-H bending due to adsorbed water molecules, 1419 cm− 1 showing CH2 scissoring behavior of cellulose molecules ,1314 cm-1 showing CH2 wagging behavior,1167cm− 1 showing C-O-C pyranose ring stretching vibration, 1021 cm− 1 showing C-O-C glycosidic bond present in cellulose molecules, 891 cm− 1 indicates the cellulosic β glycosidic bonding. Fortunati et al. (2013) showed that spectral band positions in range from 850–1500 cm− 1 is utmost important for the detection of crystalline behavior of the cellulose molecules after hydrolysis process.
After adsorbent nanocomposite formulation by using Chitosan and MBA (before adsorption) the IR spectral peaks shifted from 3332 cm− 1 and showed the prominent peaks at 3328 cm− 1 showing presence of intermolecular hydroxyl (-OH) bond as well as the presence of amide (-NH) group, 2919 cm− 1 (C-H stretching vibration bond), due to the formation of hydrogen bonding between cellulose nanocrystal and MBA molecules, C = O stretching bond at 1643 cm− 1 whereas presence of 1520 cm− 1 spectral band indicating N-H flexural vibration of MBA signifying the successful crosslinking and mixing of MBA with Chitosan respectively, 1319 cm− 1 (symmetric CH2 bending vibration bond), 1034 cm− 1 (C-O stretching), 898 cm− 1 (–CN stretching vibration).
After sorption phenomenon the 3328 cm− 1 was shifted to 3319 cm− 1, 2919 was shifted to 2907 cm− 1, 1643 was shifted to 1633 cm− 1, band at 1520 was disappeared, 1034 shifted to 1020 cm− 1, 898 cm− 1 was shifted to 887 cm− 1 (Reddy & Rao 2008). Changes in the position of the functional groups indicate the adsorption of MG dye onto nanocomposite (Fig. 7 (c)).
As per the data observed in IR spectra it confirms the successful linkage of cellulose nanocrystals between the Chitosan and MBA molecules. It is assumed that crosslinking may be occurs in between cellulose & chitosan molecules, between cellulose molecules itself and between chitosan molecules.
Contact angle was done to obtain wettability of the nanocomposite adsorbent, before analyzing the nanocomposite sample, plain nanocellulose material contact angle was measured and it was observed that the water absorption ability of the nanocomposite is very high as well as fast and not able to measure the static contact angle (Song et al. 2019).
For the M3 adsorbent nanocomposite the observed contact angle was found 84.9o, in dry condition. After soaking for 15 min, it retains the hydrophilic behavior by forming contact angle of 71.1o. Water absorbing behavior is also confirmed by swelling ratio study. Recorded contact angle of formulated nanocomposite confirms the hydrophilic behavior of the adsorbent used in the study. Observed images of contact angle are shown in Fig. 8.
During swelling study, the M3 adsorbent nanocomposite was observed under 24, 36 and 48 h and maximum swelling ratio of 1.6, 1.9 and 2.0 was observed, under 36 h incubation period it reaches equilibrium. Observed results showed that on adding cellulose nanocrystal water molecule enters as an unstructured element between the chitosan polymer chains making the nanocomposite hydrophilic in nature. The observed results may be explained by the affinity of cellulose nanocrystal towards water molecules, due to presence of hydroxyl (O–H) linkages (Xu et al. 2020).
Adsorption Study
For the adsorption of MG dye molecules batch study was done in the time interval of 10–120 min to evaluate the potential of M3 adsorbent nanocomposite of different dosage ranging from 0.1–0.7 g for the removal of different initial dye concentration i.e., 20, 40 and 60 mg/L.
Effect of contact time and initial dye concentration during adsorption
Contact time plays a vital role in adsorption process shows that on increasing contact time the removal rate of the MG dye increased to certain and then becomes constant. For the present study the sample was collected after every ten min by taking 2.0 ml of sample solution. The maximum removal rate was observed at 30 min and after this the adsorption rate lowers down to attain equilibrium state near at 90 min and after this very slight removal was observed due to saturation of the available binding sites (Bangyekan et al. 2006).
In addition, it was observed that on increasing the initial dye concentration from 20 to 60 mg/L during study, the ability of the nanocomposite gets exhausted over time, which means on increasing initial dye concentration the removal rate was decreased. Observed data showed that the dye uptake varied from 19.0 (± 1.0 SD)-42.0 (± 1.5 SD) mg/g in sample M3 adsorbent nanocomposite. Enhancement in the dye uptake suggested that it may be due to the presence of the high dye molecules in the solutions which get adsorbed onto the matrix surface. Whereas, the percent removal rate decreased from 94 (± 0.6 SD)-69 (± 1.5 SD) % for the M3 nanocomposite. The observed standard deviation ranges from ± 1 to ± 1.5 and ± 0.6 to ± 1.5 in case of dye uptake (mg g− 1) and removal rate (%) respectively, confirms the reproducibility of the triplicate samples taken for batch study and also SD values are more concentrated towards mean. On analyzing removal potential, it is confirming that availability of the active site for the adsorbate was highest when the initial concentration was low i.e., 20 mg/L but when it increases up to 60 mg/L, less availability of the active sites was observed. Therefore, this data showed that on increasing initial dye concentration the percent removal rate gets decreased suggested that when the dye molecules present in high quantity it required maximum number of active sites for the adsorption which ultimately develop a tough competition for the active site availability hence required more time duration for the removal of higher concentration dye solution.
Effect of dosage in adsorption
Different adsorbent nanocomposites dosage (0.1–0.7 g) was taken to analyze its effect towards dye removal percent and dye uptake. On increasing the dosage, the percentage dye removal of the initial dye concentration was increased, due to increased number of binding sites for the dye molecules. As per the observation shown in Fig. 9 (b) with the error bar, it is presented that M3 adsorbent nanocomposite which contained 5.0 g CNCs content, was able to remove dye very effectively for all the three initial concentration (20, 40 and 60 mg/L) (Deng et al. 2009). Above this 0.7 g dosage showed no significant results towards the removal of dye molecules due to availability of more active site which are unused during the sorption phenomenon.
Effect of pH on adsorption study
In the experimental study the adsorption behavior of the nanocomposite was observed in the pH range 4.0 to 9.0. Changes in the pH range enhance the adsorption process by affecting the degree of ionization, surface charge over the adsorbent surface. The effect of pH was studied by taking 100 mg of adsorbent (M3) in 50 mL of 60 mg/L dye solution by adjustment the pH value using 0.1 M NaOH solution. It was observed that a maximum dye removal was found when the solution pH value is less than 8 and after it becomes stable and formed a plateau (Fig. 10). So, pH 6.0 was selected as optimum pH value for MG dye adsorption onto nanocellulose composites. Due to the presence of chitosan at lower pH values it showed low adsorption of dye, it might be due to the presence of amino groups present in chitosan which gets protonated at low pH values due to repulsion of dye molecules in acidic dye solution. However, when the pH value showed basic nature, it showed high dye uptake due to negatively charged surface of chitosan.
Adsorption Isotherms
An adsorption isotherm articulates about the adsorption potential, binding potential, surface properties. An isotherm defines the equilibrium correlation between the adsorbate and the adsorbent at optimum conditions. Various theoretical models are used to analyze the adsorption behavior, but in this study, we have concentrated on Langmuir and Freundlich isotherms to observe the adsorption behaviour of the adsorbent M3 nanocomposite. In terms of adsorption behaviour Langmuir isotherm shows monolayer adsorption whereas Freundlich isotherm shows heterogeneous behaviour of adsorption. The Langmuir isotherm expressed by equation -
$$\frac{Ce}{Qe}=\frac{Ce}{Qm}+\frac{1}{KLQm}$$
7
And the Freundlich isotherm represented in equation -
$$Log Qe=Log Kf+\left(\frac{1}{n}\right)Log Ce$$
8
Where, Qe and Ce is the equilibrium dye concentration on adsorbent (mg g− 1) and in solution (mg L− 1) respectively. Qm (mg g− 1) denotes maximum adsorption capacity, KL is the Langmuir constant and Kf is the Freundlich constant.
The isotherms parameters shown in Table 4 explained that Freundlich isotherm and the best fitted because of the linear regression coefficient value (R2 = 0.992) which are higher than the Langmuir isotherm (R2 = 0.928), thus proving the heterogeneous layer adsorption of MG dye. Maximum adsorption of M3 nanocomposite was observed to be 51.12 mg g− 1 whereas the value of separation factor (RL) was less than 1.0 proved the occurrence of adsorption phenomenon for the removal of dye. The modeled isotherms used in the study are plotted in (Fig. 11 (a &b)).
Adsorption Kinetics
In this study kinetic curves are plotted using pseudo 1st order and 2nd order reactions, (Fig. 11 (c & d)) shows the correlation between the adsorption behaviour of the adsorbate and the active surface of the adsorbent. In the first 30 min maximum dye removal was observed and reached equilibrium at 90 min. This fast kinetic phenomenon indicates that due to the availability of active binding sites the rate of adsorption was high in first 30 min which was ultimately moves towards equilibrium due to the saturation of available active sites. Further to evaluate the kinetic parameters of dye adsorption onto M3 nanocomposite pseudo 1st and 2nd order equation were used (Eqs. 9 & 10).
Pseudo 1st order reaction
$$In\left(qe-qt\right)=In qe-k1t$$
9
Pseudo 2nd order reaction
$$\frac{t}{qt}=\frac{1}{k2qe2}+\frac{t}{qe}$$
10
Where, K1 and K2 are the pseudo 1st order and pseudo 2nd order rate constant respectively.
The calculated parameters for the kinetic study shown in Table 2 indicated that pseudo 2nd order reaction is the best fitted kinetic model for the adsorption phenomenon in the dye removal. Comparatively the pseudo 2nd order showed greater value of linear regression coefficient (R2 = 0.935) then pseudo 1st order (R2 = 0.519), which proved that the adsorption process occurred through chemisorption mechanism. The calculated value of qe (39.0 mg/g) and the experimental value of qe (42.0 mg/g) by pseudo 2nd order reaction show similarity in their values (Chowdhury & Saha 2010).
Table 2
Adsorption isotherms and kinetic study parameters
a. Adsorption Isotherms | Parameters | | Values |
| Condition- Initial Dye concentration 20–100 mg/L, Adsorbent - M3, Time duration 24h, pH 6.0 |
Langmuir Isotherms | qmax (mg/g) | | 51.125 |
KL (L/mg) | | 0.369 |
RL | | 0.0432 |
R2 | | 0.928 |
Freundlich Isotherms | Kf | | 15.805 |
1/n | | -0.347 |
R2 | | 0.992 |
b. Order of Reaction | Parameters | | Values |
| Conditions- Initial Dye concentration 60 mg/L, Adsorbent - M3, Time duration 10–120 min, pH 6.0 |
| qe Experimental (mg/g) | | 42.0 |
Pseudo 1st order | qe calculated (mg/g) | | 35.038 |
K1 (min− 1) | | -0.000065 |
R2 | | 0.519 |
Pseudo 2nd order | qe calculated (mg/g) | | 39.0 |
K2 (g mg− 1 min− 1) | | 196.975 |
R2 | | 0.935 |
Desorption/Regeneration study
Firstly, distilled water was used for the desorption purpose which only desorb maximum 10% of MG from the pre-adsorbed nanocomposite adsorbent. Figure 12 depicts that on altering the solvent with 100 mmol HCl concentration for the desorption. It was observed that the desorption rate increased by 40%, 80% and 90% in 12h, 24h and 36h respectively and the color of nanocomposite becomes lighter after 36h confirms the desorption phenomenon. Desorption may be taken place due to the breakdown the electrostatic interaction between the MG dye molecules and nanocomposite, results in releasing of dye molecules into the solution. This proves that the nanocomposite has regeneration potential could be used further for the removal of dye from the wastewater (Jiang et al. 2017).
Comparison of nanocellulose composite adsorbent with another adsorbent
Malachite green dye removal was previously done by various types of adsorbents made up from different sources. Table 3 showed the comparative removal of malachite green dye using different adsorbent. The present study by using nanocellulose composite showed maximum adsorption capacity of dye (42 mg g− 1). The ease of use, biodegradability, and nontoxic is some additional properties of the sugarcane bagasse used in the formulation of the nanocellulose composite adsorbent.
Table 3
Comparison of Various adsorbent for removal of Malachite Green dye
Adsorbent Used | Qe (mg g− 1) | References |
Activated charcoal | 0.179 | Iqbal & Ashiq 2007 |
Cellulose | 2.422 | (Sekhar et al. 2009) |
Bentonite | 7.72 | (Tahir & Rauf 2006) |
Arundo donax root carbon | 8.69 | (Zhang et al. 2008) |
Chemically modified rice husk | 17.76 | (Demirbas 2009) |
Iron humate | 19.2 | (Janos 2003) |
Hen feathers | 26.1 | (Hashem 2007) |
Caulerpa racemosa var. cylindracea | 26.57 | (Bekçi et al. 2009) |
Ricinus communis | 27.78 | (Santhi et al. 2010) |
Maize cob powder | 37.0 | (Sonawane & Shrivastava 2009) |
Nanocellulose composite | 42.0 | Present study |