36.2.2 |
Freundlich Adsorption Isotherm
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It is an empirical relation between the amount of an adsorbate adsorbed per unit weight (x/m, mg g-1) of adsorbent and the adsorbate equilibrium concentration (Ce, moles L-1) in the fluid. The relation is given below |
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where, K and n are Freundlich coefficients. Here,
x = weight of adsorbate adsorbed on m unit weight of adsorbent and
Ce= equilibrium concentration of adsorbate.
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From the equation, we get |
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log(x/m) = logK + n log Ce |
(36.2.11) |
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The coefficients K and n can be determined from the intercept and slope of a plot of log(x/m) versus logCe. |
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From the appearance of the type I isotherm (Figure 36.1 A), it is seen that for low values of concentration, the amount adsorbed (x/m) is nearly proportional to Ce, whereas for large values it is nearly constant (or proportional to Ceo). So, it is reasonable that for intermediate values of Ce, x/m should be proportional to some power of Ce lying between 0 and 1. This is the motivation behind the empirical Freundlich adsorption isotherm.
Freundlich adsorption isotherm may be verified by performing a simple experiment for the adsorption of oxalic acid on charcoal. Supposing m gram of charcoal is added in 50 mL solution of oxalic acid of concentration Ci. After adsorption is established, the equilibrium concentration of oxalic acid was determined as Cf. Hence, amount of oxalic acid adsorbed per unit weight of charcoal, |
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x/m (mg g-1) = (Ci – Cf) x 63 x 50 |
(36.2.12) |
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In a series of such experiments with different initial concentrations of oxalic acid, values of x/m (mg g-1) are determined. A plot of log x/m versus Cf is made and Freundlich coefficients K and n are determined. |
