tailieunhanh - Thermodynamics Interaction Studies Solids, Liquids and Gases Part 5

Tham khảo tài liệu 'thermodynamics interaction studies solids, liquids and gases part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 230 Thermodynamics - Interaction Studies - Solids Liquids and Gases down to an extremely low value before adsorbate molecules would desorb from the surface. The Freundlich equation is very popularly used in the description of adsorption of organics from aqueous streams onto activated carbon. It is also applicable in gas phase systems having heterogeneous surfaces provided the range of pressure is not too wide as this isotherm equation does not have a proper Henry law behavior at low pressure and it does not have a finite limit when pressure is sufficiently high. Therefore it is generally valid in the narrow range of the adsorption data. Parameters of the Freundlich equation can be found by plotting loglO CM versus loglO P Fig. 8. Plots of the Freundlich isotherm versus P Po 1 l gio cA logicK logic p n 124 which yields a straight line with a slope of 1 n and an intercept of loglO K . Temperature dependence of K and n The parameters K and n of the Freundlich equation 122 are dependent on temperature. Their dependence on temperature is complex and one should not extrapolate them outside their range of validity. The system of co adsorption on charcoal has temperaturedependent n such that its inverse is proportional to temperature. This exponent was found to approach unity as the temperature increases. This however is taken as a specific trend rather than a general rule. To derive the temperature dependence of K and n we resort to an approach developed by Urano et al. 1981 . They assumed that a solid surface is composed of sites having a distribution in surface adsorption potential which is defined as _ p A R Tin 2- 8 p 125 The adsorption potential A is the work energy required to bring molecules in the gas phase of pressure p to a condensed state of vapor pressure Po. This means that sites associated with this potential A will have a potential to condense molecules from the gas phase of pressure p If the adsorption potential of the gas Thermodynamics of .