In this chapter, results obtained for the Ni (II) adsorption of synthetic aqueous solutions have been presented for pineapple and bamboo stem waste based biosorbents. Section 3.2 presents surface characterization results for the adsorbents. Sections 3.3 and 3.4 details upon batch adsorption characteristics followed by model fitness to measured adsorption equilibrium data for both adsorbents. Sections 3.5 and 3.6 addresses kinetic data and thermodynamic model parameters respectively. Section 3.7 presents cost analysis data for the prepared adsorbent followed with summary in section 3.8.
3.1 Introduction
This chapter addresses the development of low cost adsorbents developed using pineapple and bamboo stem waste which were used for the removal of Ni (II) from aqueous solutions. The primal role of phosphoric acid activation for the surface area enhancement of the adsorbent has been considered. Adsorbents characterization studies involved Fourier transform infrared (FTIR) spectral analysis, Brummer Emmett Teller (BET) adsorption and Laser Particle Size Analysis (LPSA). Further, for both the adsorbents, Ni (II) adsorption experiments were carried out with various combinations of solution concentrations (50-300 mg/L), adsorbent dosage (2 g/L), contact time (30-300 minutes) and pH (2-10). Equilibrium and kinetic modelling of adsorption process have been addressed using various isotherms. The next section elaborates with respect to adsorbent characterization.
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88 3.2 Adsorbent Characterization
Fig. 3.2(a) and 3.2(b) present the obtained FTIR spectra for PS and BSAC respectively. As shown in the figure, several functional groups exist on both adsorbents. For the BSAC adsorbent before nickel adsorption, peaks were observed at specific wavelengths of 2939.3, 2897.2, 1716.4, 1651.8, 1519.3, 1508.1, 1338.4, 1165, 1056.5 and 898.6 cm-1. However, after adsorption on BSAC, FTIR spectral peaks were observed at 2924.6, 2850.8, 1720.4, 1654.8, 1045.3 and 902.9 cm-1. Similarly, for PS adsorbent, spectral peaks were observed at 2931.2, 2908.6, 2357, 1728.2, 1600.91, 1369.3 and 1246.01 cm-1 for the raw adsorbent and these shifted to 2920.2, 2368.5, 1735.9, 1627.8, 1365.3 and 1242.1 cm-1, respectively, after Ni (II) adsorption. The spectral shifts indicate that there were binding processes taking place on the surface of both BSAC and PS adsorbents. Specific functional group interactions can be analyzed to indicate C≡N (2939 cm-1), stretching vibration of C=O (1651.8 cm-1), –CH3
4000 3000 2000 1000
20 40 60 80
100 PS
PS-Ni(II)
% Transmittance
Wave number cm-1
3332 2931
2357
1728 1516
1246
1369 1600
1161
3387 2920
2368
1735
16271365 1242
1053
(a)
4000 3000 2000 1000
70 80 90
100 BSAC-Ni(II)
% Transmittance
Wave number cm-1
BSAC
3332 2939
2897
1716 1651
1519 1508 1338 1165
1056 898 3745
3410 2920
2850
1720 1654 1045
902
(b)
Fig. 3.1: FTIR spectra of fresh and Ni (II) adsorbed (a) PS and (b) BSAC adsorbents.
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wagging (at 1365 cm-1) and C–O stretching vibration (at 1056 cm-1) (Lalhruaitluanga et al., 2011; Hameed et al., 2009; McKay et al., 2008).
Affinity towards Ni (II) ions and hence their adsorption to the adsorbent surface can be analyzed through the evaluated functional group related band shifts. For the C≡N, lone pair of electrons exists on N and partial negative charge is also prevalent on both C and N (depending on the direction in which the bond shift occurs). Thereby, the prevalent negative charge induces affinity towards the positively charged Ni (II) ion or its complex. Similarly, for the C=O, the lone pair of electrons present on the O and the partial negative charge present on either of the atoms will induce affinity towards the positively charged metal ion species. For the C-O, the lone pair of electrons on O might induce the affinity of Ni (II) ion to the adsorbent surface. The extent of reversible chemisorption or physisorption could not be confirmed by the FTIR spectra due to the inability to provide quantitative information with respect to the actively prevalent bonds and their distributions. Nonetheless, the FTIR spectral analysis has been useful to confirm upon the possibilities of strong irreversible chemisorption which can be further confirmed through desorption studies.
The BET surface area values were evaluated as 11.47 m2/g and 116 m2/g for PS and BSAC, respectively. This indicated that the chemical treatment of phosphoric acid contributed towards larger adsorption capacity of BSAC (Lalhruaitluanga et al., 2011) and chemical treatment contributed towards enhancement of surface area which could reduce the optimal dosage and enhance adsorptive efficacy of the developed adsorbent.
The particle size analysis of the prepared adsorbents indicated an average particle size of 300 and 78 µm for PS and BSAC respectively for the adsorbents screened using various mesh sizes ranging from 80-230. The lower size of the prepared adsorbents promising due to the
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90
reason that particle size strongly influences metal uptake and removal efficiency. Using the pH drift method, the PZC values of BSAC and PS have been obtained as 4.5 and 4.08 respectively. This indicates that for the adsorbents and for the solution pH lower than these PZC values, adsorption shall not be significant and this is evident from the results presented later in section 3.3.2 (Fig. 3.3). Also, the inability to have higher adsorption at higher pH (above pH of 5) is indicative towards the fact that complex chemistry is involved with the binding of Ni metal ions to the adsorbent surface and based on PZC and pH study, the optimal pH value corresponds to the value slightly above the PZC value for both the adsorbents (Singh et al., 2013; Alhawas et al., 2013). Thus, PZC being the lower limit of adsorption has also been confirmed by both PS and BSAC adsorbents for Ni (II) adsorption from aqueous solutions.
3.3 Adsorption characteristics
For both BSAC and PS, based on hierarchial selection of various operating parameters, the adsorbent performance studies were conducted. Firstly, the optimality of the contact time had been targeted by considering a fixed choice of adsorbent dosage (0.02 g/L), pH (5-6) and initial Ni (II) solution concentration of 50 mg/L. Eventually, the corresponding contact times for the adsorbents were fixed and the adsorption studies were conducted for variant pH (2-10) and fixed choice of adsorbent dosage (0.02 g/L), Ni (II) solution concentrations (50 mg/L) and optimized contact times (obtained from the first set of adsorption experiments). These studies identified optimal pH as 5. Finally, for the fixed choice of pH (obtained from second set of adsorption experiments), contact time, Ni (II) solution concentration (50 mg/L) and the adsorbent dosages were varied (0.02-0.1 g/L) to evaluate the adsorption performance of PS and BSAC. For all cases, adsorbent performance was evaluated in terms of metal uptake
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(mg/g) and removal % which were determined using Eq. (2.1-2.2). Finally, the performance of PS and BSAC were studied for various solution concentrations (50-300 mg/L), by choosing all other parameters from results obtained with above mentioned preliminary adsorption studies. Further details with respect to the results obtained from the three sets of adsorption experiments are presented below.
3.3.1 Effect of contact time
Fig. 3.2 presents the variation in % adsorption and metal uptake (mg/g) with contact time for BSAC and PS. Corresponding choice of other parameters include adsorbent dosage, pH and Ni (II) concentration as 0.02 g/L, 5-6 and 50 mg/L respectively. It can be observed that the minimum time required to achieve equilibrium (i.e., maximum adsorption) is 300 and 90
0 50 100 150 200 250 300 350 400 40
50 60 70