Fourier transform infrared spectroscopy analysis
The FT-IR spectrum of SB, presenting the plot of percentage transmission versus wave number, is given in Fig. 1. The FT-IR spectrum shows that sugarcane bagasse has functional groups of standard polymer α-cellulose and coir pith-lignin. The FT-IR spectroscopic analysis indicates broad band observed at 3,424 cm–1 representing bonded –OH groups. The band observed at about 2,927 cm–1 could be assigned to the aliphatic C–H group. The peaks described in the region of 1,600–1,590 cm–1 represent skeletal vibrations of benzene ring. The peaks observed at 1,420–1,300 cm–1 are attributed to C = C–H in plane bending indicating several bands in cellulose and xylose. Vibrational bands around wave number 1,356 cm–1 may be due to –CH2 wagging and twisting indicates several bonds in deoxysugars complex. The peak observed at 1,121 cm–1 represents C–O–C stretching as appears in polysaccharides.
Scanning electron microscopic study
The SEM images of typical SB sample at different magnifications have been taken to analyse the morphology of the material more clearly. Micrographs (Fig. 2) show that SB particles have fibrous character. These images also reveal that SB samples have compacted layer character in which thin layers of cellulosic material are laying one on one. These fibres are of generally ~1.0 mm in length and of ~0.35 mm diameter. The second micrograph depicts that some very small particles of nearly 0.5–2.0 μm in size adhere to the surface of large size fibres. These small grains may also take part in the adsorption process.
Effect of initial concentration of dye on adsorption
The effects of EB and MB dye concentrations on adsorption process have been studied at neutral pH and equilibration time of 1 h. To study the effects of initial dye concentration, EB solution of concentration 10, 100, 200, 400 and 600 mg L−1 are used in conjunction with SB sample of 50, 200 and 400 mg. On the other hand, MB solution of concentration 100, 500 and 1000 mg L−1 are used in conjunction with SB doses of 100, 200 and 400 mg. From Fig. 3 it has been found that the adsorption percentage decreases with increase in concentration of the adsorbate while the amount of dye removed at equilibrium increases with increase in dye concentration in both the cases. This is so because the initial dye concentration provides the driving force to overcome the resistance to mass transfer of dye (EB and MB) between the aqueous and solid phase (Azhar et al. 2005).
Effect of pH
The pH value of the solution is an important factor that must be considered during adsorption studies. The range of pH has been adjusted between pH 1–9. Digital pH meter (Model LT-10) has been used for the pH measurements. The initial pH of the working solutions has adjusted by addition of dilute HCl and NaOH solution (Analytical grade, CDH and S.D.fine Pvt. Ltd). The effect of pH on the adsorption of EB and MB by SB is shown in Fig. 4. It has been found that adsorption increases with increase in pH. This may be due to the fact that at higher pH values the surface of adsorbent becomes negative which enhances the adsorption of positively charged MB cations through electrostatic force of attraction. Although EB is an anionic dye, it still shows maximum adsorption at high pH (7–9) which indicates that there is not much effect of hydroxide ion at these pH values. If we compare the %adsorption of EB corresponding to pH 7 and 9, then we find that amount of dye adsorbed on the adsorbent is nearly same (Zhang et al. 2011). This reveals that at neutral pH, EB shows maximum adsorption.
Effect of contact time
Batch adsorption studies have also been conducted at different contact times (10, 30, 60, 90, 120, 150, and 180 min) by taking initial concentration of dye 100 mg L–1 with 200 mg adsorbent dose of SB in 25 mL dye solution of EB as well as MB, at neutral pH and 308 K temperature. Effects of contact time on adsorption of EB and MB dyes onto SB are illustrated in Fig. 5. This data reveals that as the contact time increases, rate of adsorption first increases and then becomes almost constant. This is due to the aggregation of dye molecules with the increase in contact time, which makes it almost impossible to diffuse deeper into the adsorbent structure at higher energy sites. This aggregation negates the influence of contact time as the pores get filled up and start offering resistance to diffusion of aggregated dye molecules in the adsorbents (Mall et al. 2005). It has been seen that the equilibration time of 1 h is sufficient since maximum adsorption is attained during this period.
Effect of adsorbent dosage
The adsorbent dose is also one of the important parameters to optimize an adsorption system. The effect of adsorbent dose on the adsorption of dyes has been investigated by employing different doses of SB varying from 50 to 1,500 mg. These adsorption experiments are performed at neutral pH, 308 K, equilibration time 1 h and at different dye concentrations. Figure 6 demonstrates that the removal of dye increases with increase in adsorbent dosage. An increase in the adsorption with increase in adsorbent dosage can attribute to greater surface area and the availability of more adsorption surface sites (Sharma et al. 2010; Gupta et al. 2004b). It can be seen that for 10 mg L–1 EB solution, 50 mg of SB in 25 mL is sufficient for the adsorption studies as it shows 100% adsorption but for higher concentration dye solutions, higher dosage of SB is required (Fig. 6a). While in case of MB removal, a high dosage of SB (Fig. 6b) is required even for low-concentrated dye solution.
Effect of temperature
In order to optimize the system temperature for the maximum removal efficiency, experiments have been conducted at three different temperatures 308, 318 and 328 ± 0.1 K by varying dye concentration and keeping other parameters constant such as neutral pH, equilibration time 1 h, adsorbent dose of 200 mg. Experimental results concerning the effect of temperature on the adsorption of dyes at different initial dye concentration are demonstrated in Fig. 7. In case of EB adsorption on SB, the adsorption percentage increases when the temperature of solution is increased from 308 to 328 K which indicates that the process is endothermic in nature. Better adsorption at higher temperature may be either due to the acceleration of some originally slow adsorption steps or due to the retardation of the processes such as association of ions, aggregation of molecules, ion pairing and complex formation in the system because of thermal agitation. This can also be due to creation of new active sites on the adsorbent surface. Thus, an increase in temperature will reduce the electrostatic repulsion between the surface and the adsorbing species and this allow adsorption to occur more readily. This could also be due to enhanced mobility of dye molecules from solution to the sorbent surface (Sharma and Tomar 2011). Furthermore, increasing temperature may produce a swelling effect within the internal structure of SB enabling large dye to penetrate further. In case of MB, the adsorption percentage first increases when temperature rises from 308 to 318 K and then start decreasing with further increase in temperature indicating that the process is exothermic in nature. This decrease indicates that there are very weak binding forces between the dye molecule and adsorbent surface which may break at high temperature. This is because with increasing temperature the attractive forces between adsorbent surface and dye molecule are weakened and the adsorption decreases (>318 K). Increase in temperature may also cause the more pore expansion which may leads to leaching of MB molecules adsorbed on the SB (Khattri and Singh 2011).
Thermodynamic parameters
Thermodynamic parameters, i.e. free energy change (ΔG°), enthalpy change (ΔH°) and entropy change (ΔS°), vary with the thermodynamic equilibrium constant (Kd) (Fig. 8) and have been calculated using standard equations (Eqs. 3 and 5) mentioned above. The values of thermodynamic parameters for EB on SB are reported in Table 2. The negative values of ΔG° obtained in both the cases reveal that the adsorption of dye is thermodynamically feasible and spontaneous. Positive value of ∆Ho confirms the endothermic nature the adsorption process. Hence, on increasing temperature, degree of adsorption will increase. The numerical value of ∆Ho also predicts the physisorption behaviour of these adsorption processes. The positive values of ΔS° reflect the affinity of SB towards EB and also indicate that the randomness is increased at the solid/solution interface during the adsorption process (Acemioglu 2005; Yuan 2008; Ahmad et al. 2007; Bestani et al. 2008; Azhar et al. 2005).
Table 2 Thermodynamic parameters for the adsorption of EB on SB at five different concentrations Adsorption isotherms parameters
The adsorption isotherm studies provide information about the capacity of adsorbent to remove dyes under given set of conditions. Experimental results obtained for the adsorption of EB and MB dyes on agricultural waste SB at three different temperatures (308, 318 and 328 K) have been fitted to Langmuir isotherm model. The plot between Ce/qe and Ce yield straight lines (Fig. 9). The obtained R2 values confirm that the adsorption equilibrium data fitted well to the Langmuir model under the studied conditions. These isotherms are found to be linear over the entire studied concentration range with extremely high R2 values (Table 3). The R2 values suggest that the Langmuir isotherm provides a good model for the present adsorption systems. This indicates uniform adsorption and strong dye–adsorbent interactions over the surfaces of the adsorbent. Figs. 9a, b show the Langmuirian plots for EB and MB adsorption on SB, respectively. The values of the Langmuir constant (KL), the monolayer capacity of adsorbent (qm), and R2 values are listed in Table 3. The qm values reveal that adsorption capacity of SB for MB adsorption (1,000.0 mg g–1) is double in comparison to EB adsorption (500.0 mg g–1). High KL values also indicate the high affinity of SB for the adsorption of EB and MB.
Table 3 Adsorption isotherms constants for EB and MB adsorption on SB at three different temperatures The RL parameter is considered as a more reliable indicator of adsorption. There are four possibilities for RL values: (1) for favourable adsorption, 0 < RL < 1; (2) for unfavourable adsorption, RL > 1; (3) for linear adsorption, RL = 1; and (iv) for irreversible adsorption, RL = 0 (Senturk et al. 2009). In both the cases, the values of RL (Table 3) are found to be positive and less than unity indicating thereby a highly favourable adsorption in all cases.
Freundlich adsorption isotherm gives an expression encompassing the surface heterogeneity and the exponential distribution of active sites and their energies. For a favourable adsorption process, the value of 1/n should be less than 1 and higher than 0.1 (Senturk et al. 2009). The Freundlich constants KF and 1/n are determined from the slope and intercept of ln qe versus ln Ce plots (Fig. 10) and are tabulated in Table 3 along with the correlation coefficient values (R2). The high correlation coefficients (R2 = 0.98 − 0.99) reflect that the experimental data agree well with the Freundlich adsorption isotherm model. The values of 1/n are higher than 0.1 and smaller than 1, indicating a favourable adsorption of dyes on SB. According to the statistical theory of adsorption when the value of 1/n is less than unity, it also implies heterogeneous surface with minimum interactions between the adsorbed molecules. The KF values (Table 3) indicate that SB has good adsorption capacity for MB removal from aqueous system in comparison to EB. This isotherm does not predict any saturation of the adsorbent by the adsorbate thus infinite surface coverage is predicted mathematically indicating the multilayer sorption of the surface.