C HAPTER 3
3.3 RESULTS AND DISCUSSION
3.3.1 Characterization of Activated Carbon
3.3.1.5 Point of Zero Charge (pH pzc ) Analysis
The change in solution pH also alters the charge of the adsorbent surface due to protonation and deprotanation reaction with excess H+ and OH‒ ions. Figure 3.5 shows the measured zeta potential as a function of solution pH for BSAC. The zeta potential results in Figure 3.5 reveals that the positive charge of the BSAC decreases with increase in solution pH. The pH in which the zeta value of BSAC is zero is calculated as point of zero charge (pHpzc) and hence the net charge on the BSAC at pHpzc is zero. In this analysis, pHpzc of BSAC was 3.23. After pHpzc, the surface of BSAC became negative charge and thus favoured for positive Sr2+, MB and RB dye and phenolics ion adsorption.
3.3.2 Effect of pH
The pH of the solution plays an important role to alter the net surface charges of activated carbon and the degree of ionization of the adsorbate (Liu et al., 2010;
Subramanyam and Das, 2009). The effect of pH on adsorption capacity of chromium onto BSAC is shown in Figure 3.6a. It was observed from Figure 3.6a that the adsorption of Cr6+ decreased with increase in initial pH from 2.0 to 9.0. The maximum removal (63.57 mg/g) occurred at initial pH 2.0 for BSAC. At lower pH the surface area of the adsorbent was more protonated and competitive negative ions (HCrO4–
and Cr2O72–
) adsorption occurred between positive surface and free chromate ion. Adsorption of Cr6+ at pH 2.0 shows the bind of the negatively charged chromium species (HCrO4-
) occurred through electrostatic attraction to the positively charged (due to more H+ ions) surface functional groups of the adsorbent (Uysal and Ar, 2007; Barrera et al., 2006). But in highly acidic medium (pH ≈ 1), H2CrO4 (neutral form) is the predominant species of Cr6+ as reported by Agrawal et al. (2008)
and Hamadi et al. (2001). Hence, at pH 1.0 adsorption capacity decreased due to the involvement of less number of HCrO4-
anions on the positive surface. At higher pH due to more OH- ions adsorbent surface carrying net negative charges (see zeta values of BSAC in Figure 3.5), which tend to repulse the metal anions (CrO42-
) (Amarasinghe and Williams, 2007). However, there is also some percentage adsorption at pH > 2.0 but the rate of adsorption is reduced. This might be due to the weakening of electrostatic force of attraction between the oppositely charged adsorbate and adsorbent or physisorption due to weak undirected Van der Waals forces of attraction (Mohanty et al., 2005; Baral et al., 2006). When reaction pH increased after 2.0, Cr3+ adsorption also enhanced due to gradual increasing the negative charge on adsorbent. But the adsorption-coupled reduction (explained in adsorption mechanism section) of Cr6+ to Cr3+ was less after pH 2.0 due to the less adsorption of HCrO4-. At pH ≈ 8.0 CrO42- is the predominant form and it was repulsed by high negative surface (Li et al., 2008). Hence, the solution pH 2.0 was chosen for remaining entire study for Cr6+ adsorption.
The pH of the Sr2+ solutions (100 mg/l) was varied in the range of 2.0-10.0 keeping all other parameters constant. It was observed from Figure 3.6b that the adsorption of metal increased with increase in initial pH from 2.0 to 6.7. The maximum of Sr2+ was observed at pH 6.7 (47.12 mg/g). At low pH, adsorption capacity of Sr2+ decreased due to the protonated surface of the BSAC (Dakiky and Khamis, 2002). As the solution pH of Sr2+ increased from 2.0 to 6.0, the availability of -OH- and other negative functional groups increased due to deprotanation for positive metal adsorption. At pH > 7, Sr2+ ions precipitate as insoluble hydroxides (Ngah and.
Fatinathan, 2010; Kandaha and Meunier, 2007). So, natural or blank pH of Sr2+
(between 6.2 ± 0.05) was used as optimum pH throughout the study.
It can be seen from the Figure 3.6c that there is no significant deviation in the adsorption capacity of phenol from pH 2-9 except for a slight decrease at pH 10. The adsorption capacity of phenol was stable from pH 2-9 except for a slight decrease at pH above 9.0 (Figure 3.6c), due to high repulsion of negative phenolate ion C6H5O– with negative cell wall functional groups and –OH– ions at pH more than 9, which result in decreased adsorption capacity of phenol. Moreover, apart from the electrostatic interaction, phenol having many other interaction with carbon surface
that are discussed in phenol mechanism section (Liu et al., 2010; Girods et al., 2009).
Therefore, the blank pH (6.2) of phenol was chosen as optimum pH for the entire study.
The adsorption of o-cresol by BSAC was studied at different pH with the initial concentration of 200 mg/l. The effect of pH on the adsorption of o-cresol by BSAC has been presented in Figure 3.6d. It is evident from Figure 3.6d that the removal of o-cresol by BSAC seemed to show an increased adsorption trend in the pH range of 2 to 6. However, when pH was made to exceed 6, there was a distinct decline in the adsorption of o-cresol from solution. At alkaline pH, significant reduction in o-cresol adsorption was observed due to the high electrostatic repulsion between the adsorbent and adsorbate (C7H8O-) at higher pH. The optimum pH for removal of o-cresol by BSAC was found as pH 6.0; hence, all remaining experiments were conducted at blank pH of 6.3.
In case of RB dye adsorption, the maximum adsorption capacity of RB dye was obtained at initial pH 3.5 (199.7 mg/g). There was a sharp increase in adsorption capacity of RB dye (157.2–199.7 mg/g) with increase in pH from 2 to 3.5 (Figure 3.6e) and after pH 3.5, the sorption capacity of RB dye decreased from 199.7 to 113.2 mg/g. At pH ≤ 3.5 RB molecules remain in monomeric form and thus dye molecule can easily enter into the pore structure of the BSAC to enhance the pore diffusion. At pH above 3.5, zwitterionic form of RB in the water might increased the aggregation of RB (bigger molecule) to form the dimer and trimer molecules and thus unable to enter into the micro porous structure of the carbon. Therefore, all further RB dye adsorption studies were carried out at optimum pH 3.5.
Figure 3.6f shows the effect of pH on sorption capacity of MB. It was observed from Figure 3.6f that the sorption of MB increased with increase in initial pH from 2.0 to 7.0. The maximum removal occurred at initial pH 7.0 by BSAC. The amount of dye adsorbed was increased from 94.6 to 193.8 mg/g on the BSAC as the pH was increased from 2 to 7 (Figure 3.6f). At lower pH, positively charged sites of BSAC do not favour for the adsorption of cationic dyes due to the electrostatic repulsion.
Adsorption of MB dye at 7.0 pH shows the binding of positively charged MB cations occurred through the electrostatic attraction of the negatively charged (due to more
OH− ions) functional groups of the adsorbent (Royer et al., 2009). A similar behaviour was observed for MB dye adsorption on different activated carbon reported by El- Qada et al. (2008). Also, owing to chemical activation, a number of oxygen- containing functional groups exist in the crystal lattice of the surface, mainly the electron-donors e.g., carbonyl and carboxyl (Anandkumar and Mandal, 2009). It is thus probable that electrostatic interaction occurs between the electron-donor groups of the BSAC and the positive charge of basic dye compound. However, at pH above 8.0, dye removal occurred due to the dissociation of the dye molecules. In this study, the best pH for adsorption of MB on BSAC was observed as 7.0. However, blank pH (6.2) of MB solution was chosen while studying the effect of all other system variables in the adsorption process.