Temperature is an important factor to enhance the adsorption process as it affects the adsorption capacity by altering the molecular interactions and the solubility potential of the adsorbate. The influence of temperature on adsorption process in the range of temperature 20 ºC ~ 80 ºC is studied by keeping all other parameter in optimal condition. The percentage removal was found to increase from 67% to 97% and 58% to 93% respectively for As(III) and Cr(VI) for initial concentration of 10 mg/L at equilibrium temperature of 60 ºC. An insignificant decrease in removal percentage was observed above 60 ºC which may be due to change in concentration gradient due to excessive stress in the entire ...view middle of the document...
The positive value of entropy (ΔS) indicates the increase in randomness in the adsorption process.
The adsorption isotherm is usually used to evaluate the adsorption capacity of an adsorbent for an adsorbate. The linearized Langmuir adsorption isotherm and Freundlich isotherm equation is presented below (Mandal et al., 2010).
1/qe = 1/qobCe + 1/qo [Langmuir adsorption Isotherm equation] (12)
log qe = log Kf +1⁄ n (log Ce) [Freundlich adsorption isotherm equation] (13)
where qe is the amount of adsorbate adsorbed at equilibrium (mg/g), Ce is the equilibrium adsorbate concentration (mg/L), b is the binding energy constant and qo is the mono layer adsorption capacity (mg/g). Kf and 1/n are the constants representing adsorption capacity and adsorption intensity, respectively. The data and graphical plot obtained at different pH values are represented in Table 5 and Fig. 10
In order to understand the adsorption process, the dimensionless equilibrium parameter and Dubinin-Radushkevich isotherm parameter (Fig. S-5) was determined by the following equation (Kannan et al., 2001, Mandal et al., 2010)
r=1/(1+bC_o ) (14)
And the mean free energy was calculated by using equation (Mandal et al., 2010),
Where Co is the initial As(III) and Cr(VI) concentration, ε is the Polanyi potential equal to RT ln(1+1/Ce), qe is the amount of As(III) and Cr(VI) adsorbed per unit mass of adsorbent, qm is the theoretical adsorption capacity, Ce is the equilibrium concentration of As(III) and Cr(VI) and K is the constant and T is the temperature in kelvin. Values r<1 represents favourable adsorption. The obtained r value (Table 5) at different pH shows a favourable adsorption system. Fig S 5 shows the plot of ln qe against ε2, which was almost linear with correlation coefficient, R2 = 0.832 and 0.808 at pH 6.5 and 5.5 respectively for As(III) and Cr(VI). The value of K and qm were reported in the Table 5. The value of E (Free energy) is calculated by using equation 16 presented above. The value of E helps to understand the type of adsorption. It is reported that if the value is less than 8 kJ mol -1, then the adsorption is physical in nature, if the value is in between 8 and 16 kJ mol -1, then the adsorption is due to ion exchange and if the value is more than 16 kJ mol -1, then the adsorption is chemisorption (Mandal et al., 2010). The value found in the present study was 83 and 32 at pH 6.5 and 5.5 respectively for As(III) and Cr(VI). So, the adsorption can be best explained as chemisorption at the observed pH.
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