LITERATURE REVIEW
2.4 Multi-component kinetic studies
studied using sorption kinetic models based on mechanism. Generally the adsorption includes 4 basic steps as follows: (1) adsorbate transfer from bulk solution to boundary film, (2) adsorbate transfer from the boundary film to surface of adsorbent (external mass transfer step), (3) adsorbate transfer from the adsorbent surface to intraparticle active site or binding site (intraparticlediffusion step) and (4) adsorption of the adsorbate on the active or binding sites of adsorbent. Steps 1 and 4 generally occur rapidly and not considered as the rate limiting steps while the slower steps 2 or 3 or both are mainly considered as rate limiting steps.
Weber and Morris intra-diffusion sorption kinetic model: Weber and Morris sorption kinetic model (Weber & Morris, 1963) can be expressed as:
t K I
q= + WM× (2.18)
where KWM is Weber and Morris intra-particle diffusion rate
min0.5
g
mg , and I is intercept of vertical axis (mg/g). The determination of the two parameters can be achieved by plotting q vs. t . The linear region of the curve is selected for fitting with the model using linear regression analysis where the slope and intercept represented KWM and I respectively. The y- axis interception (adsorption capacity axis) in the plot of q vs. t could be employed to examine the relative significance of the two transport mechanisms of the solute, i.e. intra- particle diffusion and external mass transfer. If I = 0, the intra-particle diffusion is considered as the rate limiting step, while, at I > 0, both external mass transfer and intra-particle diffusion were considered as rate limiting step (Kalavathy et al., 2005).
and R. arrhizus, with the Cr(VI) ions present as the single metal and in the presence of Fe(III) ions was carried out. Similar study was carried out for Fe(III) metal ion. The initial biosorption rates of Cr(VI) ions on R. arrhizus were significantly reduced by Fe(III) ions. The presence of Fe(III) ions at low concentrations did not appreciably affect the instantaneous uptake of Cr(VI) ions by C. vulgaris. In the single Cr(VI) situation, for R. arrhizus a maximum initial adsorption rate of Cr(VI) ions at an initial Cr(VI) concentration of 150 mg/L was measured as 8.43 mg/g.min. Maximum initial adsorption rates of Cr(VI) ions on C.
vulgaris were obtained at initial metal ion concentrations in the range 150 to 250 mg/L, and determined to be 4.62 and 4.98 mg/g.min respectively. When 50 mg/L of Fe(III) was added to biosorption media containing 150 mg/L of Cr(VI), the initial adsorption rates of Cr(VI) ions on R. arrhizus and C. vulgaris decreased to 4.42 and 4.44 mg/g.min respectively, compared with the single ion situation. The inhibitory effects of Fe(III) ions on the initial biosorption rates of Cr(VI) ions on C. vulgaris increased with increasing initial Fe(III) concentrations, e.g. the adsorption rate of Cr(VI) ions on C. vulgaris decreased to 3.46 mg/g.min on increasing the initial Fe(III) concentration to 247.5 mg/L. Since the initial biosorption rates decreased with increasing concentration of the other metal ion, the combined action of Cr(VI) and Fe(III) ions on C. vulgaris and R. arrhizus was found to be antagonistic. However, this decrease in adsorption rates for metal ions could be attributed to different concentration gradient existed in mono- and binary-metal ion systems for fixed number of adsorption sites. Since the study had been carried out on ‘mg’ basis in binary metal systems and it is well known that metal ions adsorption take place on mole or equivalent basis, the moles or equivalents of 1 mg/L concentration of one particular metal ion is not equal to 1 mg/L of other metal ions i.e., the concentration gradient posed by Cr(VI) and Fe(III) metal ions will be different due to their varying molecular weights and valency.
Further, no information has been provided about the variation in values of pH during the study, which again cannot help to elucidate the removal process exactly i.e., due to only adsorption or combined effect of precipitation and adsorption. Similar study was carried out for the simultaneous biosorption of Cu(II) and Zn(II) on Rhizopus arrihizus (Sağ et al., 1998b). The effects of the presence of Cu(II) and Zn(II) ions together on the biosorption of Cu(II) and Zn(II) ions was investigated in terms of initial rates of biosorption.
Simultaneous adsorption of Cu(II), Pb(II) and Cd(II) onto an iminodiacetic acid (IDA) chelating resin was investigated from mono and binary metal systems (Li et al., 2011). The kinetics were carried out using fixed initial concentrations of 0.5, 1.0 and 2.0 mmol/L for the
concentration indicating competition amongst metal ions for adsorption sites. However, total metal uptake capacities obtained from binary systems investigated were quite higher compared to the same from mono systems. The difference in metal uptake capacities from mono- and binary-metal systems could be attributed to increased total metal ion concentration initially present in the solution in case of binary systems, thereby increasing the driving force
concentration gradient, compared to mono systems for the fixed number of adsorption sites available. The kinetics of metal uptakes should have also been investigated by maintaining a fixed total initial metal ions concentration in the solution thereby keeping the driving force unchanged in mono- and binary-metal systems. Further, this study do not present the variation in solution pH during binary systems kinetic studies, which again cannot help to elucidate the removal process exactly i.e., due to only adsorption or combined effect of precipitation and adsorption.The competitive adsorption kinetic study had been carried out for removal of Cd(II), Ni(II) and Zn(II) onto iron rich tourmaline (Liu et al., 2013). In mono-metal ion systems, initial concentrations for each metal ion was 100 mg/L while in binary-metal ion systems total metal ion concentration on mass basis was 200 mg/L (100 mg/L for each metal in the mixture). The study was carried out at an initial pH value of 4, temperature of 25 oC and tourmaline dose of 6 g/L (Liu et al., 2013). It was observed that in the Cd(II)+Zn(II) and Cd(II)+Ni(II) binary-metal systems, Cd(II) ions were always favourably adsorbed on tourmaline relative to Zn(II) and Ni(II) ions. Within first 5 min., the uptake capacity for Cd(II) was measured as 5.12 and 8.50 mg/g in the presence of Zn(II) and Ni(II) ions, respectively. At equilibrium time of 1440 min., maximum uptake capacities for Cd(II) were measured to be approximately 5.45 and 8.77 mg/g from Cd(II)+Zn(II) and Cd(II)+Ni(II) binary-metal systems respectively. The observed uptake capacities for Cd(II), Zn(II) and Ni(II) from binary-metal systems were less compared to their respective capacities from mono-metal systems. This decrease in adsorption capacities for Cd(II) metal ions could be attributed to different concentration gradient existed in mono- and binary-metal ion systems for fixed number of adsorption sites. Since the study had been carried out on ‘mg’ basis in binary-metal systems and it is well known that metal ions adsorption take place on mole or
is not equal to 1 mg/L of other metal ions i.e., the concentration gradient posed by metal ions will be different due to their varying molecular weight and valence. Further, no information has been provided about the variation in values of pH during the study, which again cannot help to elucidate the removal process exactly i.e., due to only adsorption or combined effect of precipitation and adsorption. Similar kind of work has been carried out to investigate the kinetics of Pb(II) and Zn(II) metal uptake from multi solute systems on activated carbon prepared from Van apple pulp (VAAC) in which uptake of both the metal from multi solute systems decreased compared to mono solute systems due to different total initial metal ions concentration provided in mono and multi solute systems for fixed number of adsorption sites (Depci et al., 2012).