ADSORPTION-ISOTHERM COMPARISON AND MODELI NG IN LIQUID-PHASE ONTO ACTIVATED CARBON

Activated carbon is used in many processes for the treatment of aqueou s solutions or the cleaning of industrial waste water. If we want to k now the dimension of a reactor, using an adsorbant material, it is nec essary to provide the capacity and rate of adsorption. The study of th e adsorption equilibrium allows the estimation of the material capacit y to adsorb various molecules. The representation of adsorption isothe rm onto activated carbon can be based on models with two, three or eve n more parameters. Nevertheless, when solute concentrations vary over many orders of magnitude (e.g. 10(-4) to 20 mmol.l(-1)) (Snoeyink et a t, 1969; Jossens et nl., 1978), the equations with two parameters cann ot match the experimental results. Meanwhile, the equations with three parameters (Redlich and Peterson, 1959; Jossens er nl., 1978) match t he experimental results better (Radke and Prausnitz, 1972; Mathews and Su, 1983). The aim of this work is to compare the different adsorptio n isotherms in solution For aromatic compounds (nitrobenzene, benzalde hyde, nitro-4-phenol, 4-cresol, phenol and aniline) onto activated car bon Picactif NC60. Four representations of models with 2 or 3 paramete rs are discussed. The models with two parameters (Langmuir, 1915; Freu ndlich and Heller, 1939) fit experimental data over a narrow solute co ncentration range (Figs 3 and 4). For the Langmuir equation, the linea r method has an effect on the estimation of isotherm parameters and si mulation precision (Tables 1 and 2). The values of q(m) obtained by li near method (I) are always lower than the values obtained by linear me thod (ir). On the other hand, the values of parameter b obtained by li near method (I) are higher than the values obtained by linear method ( II). The method (I) leads to a higher absolute, standard deviation and a lower relative standard deviation, than those obtained by linear me thod (II). Therefore, linear methods (I) and (II) are unsuitable becau se neither of them takes the measurement range into account. For a wid e concentration range, models with three parameters (Redlich-Peterson and Jossens-Myers) correlate better to experimental results (Figs 5 an d 6). Since if is based on the values of standard deviation (Table 2) for the studied molecules, the equation of Jossen-Myers is slightly be tter than the Redlich-Peterson one. According to these isotherm equati ons and calculated results (Table 3), we can give an average order of adsorbability for the studied compounds: (nitrobenzene > benzaldehyde > nitro-4-phenol > 4-cresol > phenol > aniline).