Chitosan-iso-vanillin was prepared using a previously reported method [13Alakhras, F.; Al-Shahrani, H.; Al-Abbad, E.; Al-Rimawi, F.; Ouerfelli, N. Removal of Pb(II) metal ions from aqueous solutions using chitosan–vanillin derivatives chelating polymers. Pol. J. Environ. Stud., 2019, 28(3), 1523-1534.
[http://dx.doi.org/10.15244/pjoes/89545] ]. Briefly, 5.70 g of chitosan was refluxed with 9 mL of glacial acetic acid, 90 mL of methanol, and 13.69 g of iso-vanillin monomer. The resulting material was filtered and washed with ethanol and acetone. Subsequently, purification was carried out by Soxhlet extraction. Finally, the sorbent was dried overnight at 70°C.

MO powder was used without any additional purification (water-soluble anionic azo dye). The chemical formula of MO is C₁₄H₁₄N₃NaO₃S, and its structural formula is shown in Fig. (1). The stock dye solution was prepared by dissolving the appropriate mass of MO in purified water. Serial dilution was used to prepare dye concentrations of 5, 10, 15, 20, 30, 40, and 50 ppm. The purpose of these different concentrations was to obtain a calibration absorption curve versus concentration at the predetermined wavelength of 472 nm. The calibration curve was used to determine the exact concentrations of the unknown dye solutions.

Fig. (1) Structure of MO.

Batch equilibrium adsorption experiments were used to investigate dye removal. In these experiments, 25 mL of MO solution was placed in a 100 mL flask containing 0.05 g of the suspended sorbent at 30°C for 3 h. The pH range (3.0-10.0) was adjusted using 1 M HCl or 1 M NaOH. Sorption experiments were carried out for 5, 10, 20, 30, 60, 120, 240, 480, and 1440 min to evaluate the effect of time on dye removal. In addition, these tests were performed at different MO dye concentrations (50, 75, 100, 125, 150, 200, and 300 mg. L^-1) and at various temperatures (30, 50, and 70°C). The effect of the amount of chitosan-iso-vanillin polymer was also studied with respect to 0.01, 0.05, 0.1, 0.2, and 1 g of the dry adsorbent. After the experiments, the samples were filtered, and the residual MO concentration in solution was determined by UV-Vis spectrophotometry. At equilibrium, the concentration of the adsorbed MO ions, expressed by the equilibrium adsorption capacity (Q_e, mg. g^-1) was evaluated using Eq. (1),

(1)

where Q_e has units of milligrams of MO per gram of sorbent, C_o and C_e represent the initial and equilibrium MO concentration (mg.L^-1), V is the volume of MO solution (L), and W is the amount of chitosan-iso-vanillin (g).

From the adsorption experiments, the amount of MO removed over time (Q_t, mg. g^-1) was evaluated using Eq. (2),

(2)

C_t is the concentration of MO in the liquid (mg.L^-1) at time t.

After cellulose, chitosan is one of the most abundant biopolymers [14Ravi Kumar, M.N.V. A review of chitin and chitosan application. React. Funct. Polym., 2000, 46(1), 1-27.
[http://dx.doi.org/10.1016/S1381-5148(00)00038-9] ]. Because of its properties such as good biocompatibility, non-toxicity, ability to form a mechanical membrane, and antimicrobial activity, chitosan has been widely used in the food industry and for metal chelation from wastewater, water purification, fruit juice clarification and solid removal, edible film formation, and preservation of foods against microbial degradation [15Shahidi, F.; Kamil, J.; Jeon, Y.; Kim, S. Antioxidant role of chitosan in a cooked cod (GADUS MORHUA) model system. J. Food Lipids, 2002, 9(1), 57-64.
[http://dx.doi.org/10.1111/j.1745-4522.2002.tb00208.x] -18Shahidi, F.; Synowiecki, J. Isolation and characterization of nutrients and value-added products from snow crab (Chionoecetes opilio) and shrimp (Pandalus borealis) processing discards. J. Agric. Food Chem., 1991, 39(8), 1527-1532.
[http://dx.doi.org/10.1021/jf00008a032] ]. In addition, chitosan in the form of nanospheres or microspheres can also be used as a controlled delivery system for active agents. To delay the controlled release of the active agents, chitosan microcrystals usually require cross-linking, for which glyoxal, formaldehyde, glutaraldehyde, and other reactive binding agents can be used [19Ajit, P.R.; Namdev, B.S.; Sangamesh, A.P.; Tejraj, M.A. Novel interpenetrating polymer network microspheres of chitosan and methylcellulose for controlled release of theophylline. Carbohydr. Polym., 2007, 69(4), 678-687.
[http://dx.doi.org/10.1016/j.carbpol.2007.02.008] ].

Vanillin, obtained from the pods or bean of the tropical vanilla orchid, is often used as a flavoring and purifying agent for nutrients and is Generally Recognized as a Safe (GRAS) substance [20Karathanos, V.T.; Mourtzinos, I.; Yannakopoulou, K.; Andrikopoulos, N.K. Study of the solubility, antioxidant activity and structure of inclusion complex of vanillin with b-cyclodextrin. Food Chem., 2007, 101(2), 652-658.
[http://dx.doi.org/10.1016/j.foodchem.2006.01.053] ]. Apart from its flavor, vanillin also exhibits bioactivity [21Sinha, A.K.; Sharma, U.K.; Sharma, N. A comprehensive review on vanilla flavor: extraction, isolation and quantification of vanillin and others constituents. Int. J. Food Sci. Nutr., 2008, 59(4), 299-326.
[http://dx.doi.org/10.1080/09687630701539350] [PMID: 17886091] ]. Moreover, vanillin is also used for other purposes, such as in cosmetics and medicines. The aldehyde groups of vanillin and the amino groups of chitosan can undergo a base-catalyzed nucleophilic substitution reaction, forming imines, and continued polymerization results in a network structure that is useful for the controlled release applications [22Mourtzinos, I.; Konteles, S.; Kalogeropoulos, N.; Karathanos, V.T. Thermal oxidation of vanillin affects its antioxidant and antimicrobial properties. Food Chem., 2009, 114(3), 791-797.
[http://dx.doi.org/10.1016/j.foodchem.2008.10.014] ].

The condensation reaction between the carbonyl group of vanillin and primary amine of chitosan results in the formation of a Schiff base, i.e., an imine. In our study, the reaction of iso-vanillin with chitosan yielded a polymeric absorbent material, as shown in Fig. (2). The obtained biosorbent has been characterized previously [13Alakhras, F.; Al-Shahrani, H.; Al-Abbad, E.; Al-Rimawi, F.; Ouerfelli, N. Removal of Pb(II) metal ions from aqueous solutions using chitosan–vanillin derivatives chelating polymers. Pol. J. Environ. Stud., 2019, 28(3), 1523-1534.
[http://dx.doi.org/10.15244/pjoes/89545] ].

The effects of the solution pH on MO dye sorption from pH 3.0 to 10.0 are plotted in Fig. (3). The pH of the MO solution plays an important role in the adsorption process because it affects the dissociation of the functional groups on the sorbent active sites and consequently, the surface charge of the adsorbent. In addition, the pH affects the chemistry of the MO solution [23Huang, R.; Liu, Q.; Huo, J.; Yang, B. Adsorption of methyl orange onto protonated cross-linked chitosan. Arab. J. Chem., 2017, 10(1), 24-32.
[http://dx.doi.org/10.1016/j.arabjc.2013.05.017] ]. The adsorption of MO by chitosan-iso-vanillin decreased as the solution pH increased. The highest MO removal (96.8%) was recorded at pH 3 when the sorbent dose was 0.05 g. According to previous studies [24Chiou, M.S.; Li, H.Y. Equilibrium and kinetic modeling of adsorption of reactive dye on cross-linked chitosan beads. J. Hazard. Mater., 2002, 93(2), 233-248.
[http://dx.doi.org/10.1016/S0304-3894(02)00030-4] [PMID: 12117469] -26Iqbal, J.; Wattoo, F.H.; Wattoo, M.H.S.; Malik, R.; Tirmizi, S.A.; Imran, R.; Ghangro, A.B. Adsorption of acid yellow dye on flakes of chitosan prepared from fishery wastes. Arab. J. Chem., 2011, 4, 389-395.
[http://dx.doi.org/10.1016/j.arabjc.2010.07.007] ], the optimum pH range for sorption by chitosan-iso-vanillin is 3 - 6. As shown in Fig. (3), the sorbent showed a relatively stable adsorption capacity for MO over a wide range of pH levels, but, this process was the most effective at pH 3 - 6. These results suggest that in an aqueous solution, the MO dye dissociates, yielding negatively charged sulfonate groups and positively charged sodium ions, i.e., negatively charged MO ions. Second, the electrostatic attraction between the MO anion and sorbent surface results in adsorption. Notably, when the pH of the MO solution was increased, the adsorption decreased [27Deepika, R.; Venkateshprabhu, M.; Pandimdevi, M. Studies on the behaviour of reactive dyes onto the cross-linked chitosan using adsorption isotherms. Int. J. Environ. Sci., 2013, 4(3), 323-351.] because of the competition for adsorption sites between the OH^- ions in the basic solution and MO anions.

Fig. (2) Structure of the chitosan-iso-vanillin biosorbent.

Fig. (3) Effect of pH on MO removal by chitosan-iso-vanillin (MO = 100 ppm, T = 30°C, W = 0.05 g, and t = 3 h).

The influence of the initial MO concentration at pH 3 and at 30°C on the adsorption process is shown in Fig. (4). Increasing initial MO concentration led to an increase in the equilibrium adsorption capacity of chitosan-iso-vanillin for MO. The ratio of dye molecules to adsorption sites is low at low initial dye concentrations and, as a result, there are more sites for the adsorption of the dye [28Kyaw, T.T.; Wint, D.S.; Naing, K.M. Studies on the sorption behavior of dyes on cross-linked chitosan beads in acid medium. International Conference on Biomedical Engineering and Technology IPCBEE, 2011, 174-178.].

In addition, the initial dye concentration affects the rate of adsorption, which also affects the adsorption capacity of the sorbent [24Chiou, M.S.; Li, H.Y. Equilibrium and kinetic modeling of adsorption of reactive dye on cross-linked chitosan beads. J. Hazard. Mater., 2002, 93(2), 233-248.
[http://dx.doi.org/10.1016/S0304-3894(02)00030-4] [PMID: 12117469] ]. This is because of the increased concentration gradient with the increase in the initial dye concentration, which acts as the driving force for adsorption. In contrast, at very high initial dye concentrations, the number of available adsorption sites decreases with an increase in the initial concentrations of the dye molecules. As a result, the number of available sites where adsorption occurs is rapidly reduced, resulting in a corresponding reduction in dye removal [29El-Sayed, E.M.; Tamer, T.M.; Omer, A.M.; Mohy Eldin, M.S. Development of novel chitosan schiff base derivatives for cationic dye removal: methyl orange model. Desalination Water Treat., 2016, 57(47), 22632-22645.
[http://dx.doi.org/10.1080/19443994.2015.1136694] ]. In other words, in the case of low concentrations, the ratio of the initial number of dye molecules to the available adsorption sites is low, and more adsorption sites are available for the dye molecules, which increase the removal rate. However, at higher concentrations, the ratio of the initial number of available adsorption sites to the dye molecules is low; thus, the number of available adsorption sites is reduced, resulting in decreased dye removal. Protonation improved the adsorption of the anionic dye onto chitosan.

Fig. (4) Effect of initial MO concentration on the equilibrium adsorption capacity of chitosan-iso-vanillin (W= 0.05 g, pH = 3, T = 30°C, t = 3 h).

The linear plots of MO adsorption by chitosan-iso-vanillin were fitted to the Langmuir, Freundlich, and Temkin isotherms. These models relate the distribution of adsorbed molecules between solid and liquid phases at equilibrium. The Langmuir isotherm model is expressed in Eq. (3):

(3)

Here, Q₀ is the adsorption capacity (mg. g^-1), and K_L is the Langmuir constant (L. mg^-1), which is related to the energy of sorption. In the Langmuir model, an additional parameter, the dimensionless separation factor R_L given in Eq. (4), can be used to understand adsorption. If R_L=1, the adsorption relationship is linear; if R_L>1, adsorption is unfavorable; if 0 <R_L <1, adsorption is favorable; and if R_L = 0, adsorption is irreversible. The obtained value of R_L is listed in Table 1.

Table 1
Results of fitting the MO removal data to the Langmuir, Freundlich, and Temkin isotherms.

(4)

The data were also fitted to the Freundlich adsorption isotherm, which is suitable for modelling low concentrations and for heterogeneous surfaces; the Freundlich isotherm is given by Eq. (5):

(5)

The Freundlich constants K_F and n are related to the degree of adsorption and adsorption capacity, respectively, and can be calculated using the slope and intercept of the linear plot of log q_e vs. log C_e.

The Temkin model (Eq. (6)) assumes that the heat of adsorption decreases linearly with increasing coverage of the adsorbent surface because of the adsorbate-adsorbate interactions.

(6)

Here, the constant B = RT/b, where b is the Temkin constant related to the heat of sorption, T is the temperature (K), R is the gas constant (8.314 J/mol K), A is the Temkin equilibrium binding constant (L. mg^-1) which is related to the maximum binding energy (∆E = -∆H, i.e., the change in adsorption energy (J. mol^-1), and Q₀ is the adsorption (maximum) capacity [30Kundu, S.; Gupta, A.K. Arsenic adsorption onto iron oxide-coated cement (IOCC): regression analysis of equilibrium data with several isotherm models and their optimization. Chem. Eng., 2006, 122(1-2), 93-106.
[http://dx.doi.org/10.1016/j.cej.2006.06.002] , 31Hamdaoui, O.; Naffrechoux, E. Modeling of adsorption isotherms of phenol and chlorophenols onto granular activated carbon. Part I. Two-parameter models and equations allowing determination of thermodynamic parameters. J. Hazard. Mater., 2007, 147(1-2), 381-394.
[http://dx.doi.org/10.1016/j.jhazmat.2007.01.021] [PMID: 17276594] ]. A and B can be obtained from the intercept and the slope of the plot q_e against ln C_e, and these values are listed in Table 1. In this study, the fitting of the data to the Langmuir, Freundlich, and Temkin isotherms yielded a coefficient of variation values (R² = 0.9151, 0.9999, and 0.8886, respectively). The Langmuir coefficient (K_L) was found to be 0.049 and Q₀ to be 666.6 mg.g^-1. The Freundlich isotherm K_F, which reflects the adsorption capacity was 45.3 mg.g^-1. Further, the Freundlich isotherm yielded a better fit to the experimental data than the Langmuir or Temkin isotherms [32Saha, T.K.; Bhounil, N.C.; Karmaker, S.; Ahmed, M.G.; Ichikawa, H.; Fukumori, Y. Adsorption of methyl orange noto chitosan from aqueous solution. J. Water Resource Prot., 2010, 2, 898-906.
[http://dx.doi.org/10.4236/jwarp.2010.210107] ]. Table 2 lists Q₀ for the removal of MO from aqueous solution using several different polymeric sorbents at 25°C. In a previous study, self-assembled gels were prepared from Exfoliated Montmorillonite and Chitosan (EMCG) and used to remove MO in the presence of Methylene Blue (MB), where the adsorption capacity was 545 mg. g^-1 [33Kang, S.; Qin, L.; Zhao, Y.; Wang, W.; Zhang, T.; Yang, L.; Rao, F.; Song, S. Enhanced removal of methyl orange on exfoliated montmorillonite/chitosan gel in presence of methylene blue. Chemosphere, 2020, 238,124693.
[http://dx.doi.org/10.1016/j.chemosphere.2019.124693] [PMID: 31524627] ]. In another study, an environment-friendly sorbent was used as a cheap method to remove textile dyes, especially MO from wastewater, yielding a Q₀ of 44.8 mg. g^-1 [34Wong, V.L.; Tay, S.Y.; Lim, S.S. Enhanced removal of methyl orange from aqueous solution by chitosan-CaCl₂ beads. Mater. Sci. Eng. J., 2020, 736, 1-14.
[http://dx.doi.org/10.1088/1757-899X/736/2/022049] ]. For a system of protonated cross-linked chitosan in a batch system, a Q₀ of 89.29 mg. g^-1 was obtained [35Huang, R.; Liu, Q.; Huo, J.; Yang, B. Adsorption of methyl orange onto protonated cross- linked chitosan. Arab. J. Chem., 2017, 10, 24-32.
[http://dx.doi.org/10.1016/j.arabjc.2013.05.017] ], and the use of chitosan biomass yielded a Q₀ of 29 mg. g^-1 [36Allouche, F.; Yassaa, N.; Lonuici, H. Sorption of methyl orange from aqueous solution on chitosan biomass. Procedia Earth Planet. Sci., 2015, 15, 596-601.
[http://dx.doi.org/10.1016/j.proeps.2015.08.109] ]. Thus, the chitosan-iso-vanillin polymer sorbent developed in this study exhibits the highest Q₀ amongst all previously reported sorbents for MO at 25°C.

Table 2
Q_o (mg. g^-1) for MO on different sorbents at 25°C.

The effect of the adsorbent dosage on the adsorption of MO was investigated to determine the minimum sorbent dosage that enabled the maximum dye adsorption. The dose of the sorbent was increased from 0.01 to 0.2 g, and the effect on the equilibrium adsorption capacity for MO was determined, as listed in Table 3. At a sorbent dosage of 0.01 g, 93.64% of the MO was removed, which is the lowest value of the dosage tested. In contrast, 97.91% removal was obtained with sorbent dosages of 0.05 and 0.1 g. Thus, as shown by the data in Table 3 and Fig. (5), the MO removal rate increased with increasing the biosorbent’s dose until it reached a plateau at 0.2 g, where the removal rate was 98.18%. Despite the marked improvement in the rate of removal of the MO with increasing adsorbent dosage, the effect was not proportional, even though the number of sites available for adsorption increased. This can be attributed to the possible overlapping or aggregation of adsorption sites resulting in an increase in the diffusion path length.

Table 3
Effect of adsorption dosage of chitosan-iso-vanillin on MO removal at pH 3.

Fig. (5) Effect of biosorbent dose on MO removal (MO = 100 ppm, pH = 3, T = 30 °C, t = 3 h).

Another factor affecting the adsorption processes is the contact time between the sorbent and dye. The result of the kinetic analysis is shown in Fig. (6). Initially, the uptake of MO by chitosan-iso-vanillin was rapid because of the electrostatic attraction between the surface of the sorbent and the dye. Notably, with an increase in the initial MO concentration, the time required to reach equilibrium increased; hence, contact time is an important variable in adsorption processes [37Maleki, A.; Pajootan, E.; Hayati, B. Ethyl acrylate grafted chitosan for heavy metal removal from wastewater: Equilibrium, kinetic and thermodynamic studies. J. Taiwan Inst. Chem. Eng., 2015, 51, 127-134.
[http://dx.doi.org/10.1016/j.jtice.2015.01.004] ]. The kinetic data were fitted to the pseudo-first-order, as shown in Eqs. (7) and (8), respectively. These models have been shown to stimulate the dye removal by different sorbents well [8Alabbad, E. Estimation the sorption capacity of chemically modified chitosan toward cadmium ion in wastewater effluents. Orient. J. Chem., 2019, 35(2), 757-765.
[http://dx.doi.org/10.13005/ojc/350236] ].

Fig. (6) Effect of contact time on MO removal by chitosan-iso-vanillin (MO =100 ppm, pH = 3, T = 30 °C,W = 0.05 g).

(7)

(8)

The experimentally observed Q_e of 98.00 mg. L^-1, was reached in 180 min, reflecting the occupation of the remaining active sites on the adsorbent surface. As shown in Table 4, the coefficient of variation for fitting to the pseudo-second-order model ( = 0.9990) was greater than that for fitting to the pseudo-first-order ( = 0.9524). As shown in Fig. (6), the MO chitosan-iso-vanillin system rapidly reached equilibrium. Thus, the surface was rapidly covered with dye molecules, which blocked further adsorption of the dye molecules that remained in the solution. As a result, adsorption slowed down, resulting in the plateau at equilibrium adsorption. These results are consistent with those reported in Ref [38Gibbs, G.; Tobin, J.M.; Guibal, E. Sorption of acid green 25 on chitosan: influence of experimental parameters on uptake kinetics and sorption isotherms. J. Appl. Polym. Sci., 2003, 90(4), 1073-1080.
[http://dx.doi.org/10.1002/app.12761] ], in which a greater concentration required a longer time to reach equilibrium, implying that there is a direct relationship between these factors.

Table 4
Pseudo-first-order and pseudo-second-order kinetic parameters.

(9)

Here q_t is the mass of adsorbed MO at contact time t (min), k_id (mg. g^-1 min^-½) is the Weber-Morris intraparticle diffusion rate constant, and c is the boundary layer. Fig. (7). shows the linear plot of q_t versus t_1/2 for MO based on the intraparticle diffusion mechanism, which does not determine the rate-determining step, as shown by the correlation coefficient (R²) of 0.9870. In addition, adsorption occurs by both surface and intraparticle diffusion. In this study, the pseudo-second-order model was the most suitable kinetic model for the data. Therefore, direct chemisorption dominated the sorption kinetics, which is consistent with the findings of a study [39Plazinski, W.; Rudzinski, W.; Plazinska, A. Theoretical models of sorption kinetics including a surface reaction mechanism: A review. Adv. Colloid Interface Sci., 2009, 152(1-2), 2-13.
[http://dx.doi.org/10.1016/j.cis.2009.07.009] [PMID: 19735907] ]. This finding indicates that electron sharing between the adsorbates and adsorbent was the key factor driving adsorption [40Habiba, U.; Siddique, T.A.; Joo, T.C.; Salleh, A.; Ang, B.C.; Afifi, A.M. Synthesis of chitosan/polyvinyl alcohol/zeolite composite for removal of methyl orange, Congo red and chromium(VI) by flocculation/adsorption. Carbohydr. Polym., 2017, 157, 1568-1576.
[http://dx.doi.org/10.1016/j.carbpol.2016.11.037] [PMID: 27987870] ].

Fig. (7) Fitting of MO adsorption data to the Weber-Morris intraparticle diffusion model.

Fig. (8) Effect of temperature on MO removal by chitosan-iso-vanillin. (MO = 100 ppm, pH = 3, W = 0.05 g, t = 3 h).

The effect of temperature on MO adsorption by chitosan-iso-vanillin at pH 3 is shown in Fig. (8). The removal of MO was studied at 20, 30, 50, and 70°C. The adsorption process is affected by temperature, which affects the equilibrium capacity of the adsorbent for MO. With increasing temperature, MO adsorption decreased, indicating an exothermic adsorption reaction. Thus, the interaction between dye molecules and adsorption sites weakened with increasing temperature. In addition, a lower temperature led to a reduction in the number of available active sites for adsorption [41Theydan, S.K. Effect of Process Variables, Adsorption kinetics and equilibrium studies of hexavalent chromium removal from aqueous solution by date seeds and its activated carbon by ZnCl₂. Iraqi Academic Sci. J., 2018, 19(1), 1-12., 42Alqaragully, M.B. Removal of textile dyes (maxilon blue, and methyl orange) by date stones activated carbon. Int. J. Adv. Res. Chem. Sci., 2014, 1(1), 48-59.]. To determine if the adsorption process was endothermic or exothermic, the thermodynamic parameters were determined for the dye adsorbent systems, i.e., enthalpy (∆H), entropy (∆S), and free energy (∆G) changes, and the results are listed in Table 5. The increase in the absolute value of ΔG with increasing temperature indicates that elevated temperatures facilitated adsorption. The negative value of ΔH indicates that the adsorption process is exothermic. In other studies, these parameters have been used to identify the underlying adsorption mechanism [43Li, Q.; Yue, Q.Y.; Su, Y.; Gao, B.Y.; Sun, H.J. Equilibrium, thermodynamics and process design to minimize adsorbent amount for the adsorption of acid dyes onto cationic polymer-loaded bentonite. Chem. Eng., 2010, 158(3), 489-497.
[http://dx.doi.org/10.1016/j.cej.2010.01.033] ]. As shown in Table 5, at all temperatures, ΔG is negative, indicating that the adsorption process is spontaneous. In addition, an increase in the temperature increased the absolute value of ΔG and may facilitate the adsorption process. The positive ΔS values suggest increased disorder at the liquid-solid interface. Our findings are in agreement with earlier studies [44Kora, A.J.; Rastogi, L. Catalytic degradation of anthropogenic dye pollutants using palladium nanoparticles synthesized by gum olibanum, a glucuronoarabinogalactan biopolymer. Ind. Crops Prod., 2016, 81, 1-10.
[http://dx.doi.org/10.1016/j.indcrop.2015.11.055] ].

Table 5
Thermodynamic parameters for MO removal on chitosan-iso-vanillin at different temperatures.