C HAPTER 3
3.3 RESULTS AND DISCUSSION
3.3.9 Adsorption Mechanism
The porosity of activated carbon is not the only factor which contributes to Cr6+, Sr2+, phenol, o-cresol, RB and MB dye adsorption. Their chemical characteristics may also have a great influence on the adsorption process. Therefore, it is very important to identify the adsorbate binding mechanism onto BSAC that will ultimately guide for the rational design of the adsorption process.
3.3.9.1 Adsorption Mechanism of Cr6+ and Sr2+
The binding mechanism of Cr6+ is principally based on ionic equilibrium between Cr6+ and Cr3+ as well as surface complexation reactions with protonated sites. The main species distribution of Cr6+ at pH ≈ 2.0 reported by Barrera et al. (2006) and Fahim et al. (2006) are HCrO4– (≈ 80%) and Cr2O72– (≈ 20%). The reduction of Cr6+
to Cr(III) can be suggested by two possible mechanisms and as follows. In first mechanism, Cr6+ is directly reduced to Cr3+ by BSAC surface electron-donor groups and the reduced Cr3+ forms complexes with BSAC or remains in the surrounding test solution. However, Cr3+ is not adsorbed by BSAC at pH 2.0 due to the repulsion with protonated surface. But in the second mechanism, the adsorption-coupled reduction of Cr6+ to Cr3+ occurred with the cell wall functional groups of BSAC (Figure 3.17a). It has three steps; (i) the binding of anions (HCrO4−
and Cr2O72–
) to the positively charged groups present on the surface of the BSAC, (ii) the reduction of adsorbed Cr6+ to Cr3+ takes place by neighbouring electron-donor (C=O and –OCH3) groups of adsorbed sites (Figures 3.17a and 3.17b) and (iii) a fraction of surface reduced Cr3+ is released into the aqueous solution due to the electronic repulsion between the positively charged groups of BSAC and the surface bound Cr3+. At pH 2.0, the reduced chromium is in Cr3+ form due to adsorption couple reduction of Cr6+ and that cannot be adsorbed by positively charged surface of the BSAC (Li et al., 2008; Uysal and Ar, 2007). For this purpose, XPS analysis was employed to verify the oxidation state of the bounded chromium ions on BSAC. XPS spectra collected from the Cr2p orbital core region of BSAC indicated that the sharp peaks observed at 577.15
(Cr2p3/2) and 586.65 (Cr2p1/2) eV were corresponding to the Cr3+ oxidation state (Figure 3.17c). This result conforms that the chromium adsorbed on the surface of BSAC was mostly in trivalent form due to adsorption-coupled reduction. Therefore, it can be concluded that the reduction of Cr6+ to Cr3+ also occurred by adsorption- coupled reduction (indirect reduction) on the BSAC surface by adjacent electron- donor groups (C=O and –OCH3) of Cr6+ adsorbed protonated positive sites (Figure 3.17b). Beyond pH 2.0, the adsorption of Cr6+ decreases and thus the release of Cr3+
decreases steadily. Similar behaviour was reported by Dakiky et al. (2002) for both Cr6+ and Cr3+ removal for seven other plant based adsorbents.
In relation with the change enthalpy of Sr2+, adsorption of Sr2+ is mainly contributed by physisorption mechanism. The inter-molecular and intra-molecular binding mechanism can be suggested for Sr2+ adsorption with cellulosic and lignin functional groups of BSAC. Similar kind of behaviour is reported in elaborately for Sr2+
adsorption using SBS in chapter 4.
3.3.9.2 Adsorption Mechanism of Phenol and O-cresol
In relation with the change in enthalpy mentioned earlier, the present work indicates that the phenol and o-cresol adsorption on BSAS is largely controlled by physisorption (van der Waals interaction). Ku and Lee (2000) reported that the predominant species of phenol in aqueous solution pH below 4.0 is C6H5OH (molecular form) and C6H5O– (ionic form) is negligible, whereas at pH above 9.0 C6H5O– is predominant and C6H5OH is negligible. Similarly, at alkaline pH o-cresol is in C7H8O‒ form. The possible interactions between phenol and carbon surface in the wide pH range of 2-10 are proposed in the following steps:
• Hydrophobic effect and ion-dipole interaction: at acidic pH, the phenol adsorption with carbon surface is highly influenced by the hydrophobic interaction of the aromatic ring (since phenol molecule contains both hydrophilic and hydrophobic groups) with the hydrophobic surface of the BSAC and that retain on the BSAC surface or in the pores (Liu et al., 2010; Castilla, 2004). The ion-dipole binding is due the interaction between non polar group of the phenol with permanent dipole (≡S) of the BSAC (Anandkumar and Mandal, 2012; Ku and Lee, 2000).
6 5 6 5
S + C H OH SC H OH
≡ ≡ (3.2)
• Electron donor-acceptor interaction and hydrogen bonding: the electron donor groups of the BSAC (C=O, –OCH3) and the electron acceptor groups of aromatic phenolic ring (weak acid) may be involved in this surface complex formation (Ku and Lee, 2000). The hydrogen atom of aromatic ring act as a hydrogen donor to make the hydrogen bond with a relatively electronegative atom of oxygen in hydroxyl group and nitrogen in amino group (hydrogen bond acceptors) of BSAC (Figure 3.18a), which removes phenol from solution (Castilla, 2004).
• Electrostatic interaction: at pH 4-8 the less available C6H5O– species are binded with positively charged cell wall functional groups (≡S) such as –OH2+
and amino groups (–NH2+ and –NH+) through electrostatic interaction.
6 5 6 5
S + C H O − SC H O−
≡ ≡ (3.3) However, at pH above 9.0 the considerable reduction in phenol adsorption is due to the repulsion of C6H5O– ions with highly deprotonated surface of BSAC.
• π-π interactions: the interaction between the π electrons of the phenolic ring and the carbon graphene layers (Figure 3.18b) are making the phenol adsorption which might comprise the charge transfer and the dispersive force. Kawati and Tsutsu (1995) reported that the negative charge of deprotonated phenolate ion is distributed on the aromatic ring in combination with its π electrons, which results in the formation of widely distributed electron cloud and this electron cloud might involve in the charge transfer between the π electrons of aromatic ring and graphene layer of carbon (Figure 3.18b). This mechanism plays a vital role in phenol adsorption, as reported by other studies dealing with adsorption of phenols on other activated carbons.
In case of o-cresol adsorption, at alkaline pH the predominant ionic form o-cresol is C7H8O-. These net negative charged cresol ions are repelled by net negative charged surface functional groups of BSAC mainly hydroxyl and oxygen groups. But, at acidic pH the repulsion between the cationic methyl group of o-cresol and protonated surface of BSAC significantly reduces the adsorption capacity. At the neutral pH, positive methyl group of o-cresol is easily attached with negative surface of BSAC.
In general, phenol and o-cresol adsorption mechanism is complex and the exact binding correlation between the phenol and o-cresol with activated carbon cannot be drawn. Therefore, abovementioned possible factors are considered to be important in the adsorption of phenol and o-cresol on BSAC.
3.3.9.3 Adsorption Mechanism of RB and MB Dyes
The probable binding mechanism of RB dye on BSAC can be described as follows: at acid pH (2.0-3.0), the highly protonated (excess of H+ ions) positive sites (–OH2+
) of BSAC do not favour for cationic RB dye adsorption due to its electrostatic repulsion (Chuah et al., 2005). The existence of monomeric form of RB molecules at pH 3.5 enhances both pore diffusion and surface adsorption (Figure 3.19). The diameter of RB molecule is 1.6 nm (monomer) and it can easily enter into the micro pores of BSAC (refer Table 1, micro pore surface area is 337 m2/g) to increase the pore diffusion (Tsunomori and Ushiki, 1999). Therefore, maximum adsorption capacity was observed at pH 3.5. At pH more than 3.5, the zwitterionic form of RB dye (Figure 3.19) in solution might enhance the aggregation of RB dye molecule to form bigger molecules (dimer and trimer) and thus unable to enter into the micropores of BSAC. The aggregation of zwitterions is due to the electrostatic attraction between the carboxyl and xanthene groups (Figure 3.19) of RB dye. The aggregations of RB in water have been reported by Arbeloa and Ojeda (1982) and Mchedlov-Petrosyan and Kholin (2004). MB dye molecules have a positive net charge but BSAC surface carrying net negative charge at neutral and alkaline pH of the solution due to oxygen and hydroxyl groups. This opposite charge difference between dye molecules and BSAC surface makes the higher adsorption through strong electrostatic interaction.
Therefore, at alkaline pH surface diffusion only dominates the higher RB and MB dye adsorption on BSAC.