The equilibrium adsorption of diazo dye C.I. Acid Red 97 (AR97) from aqueous solutions onto Yemen natural clay has been studied as a
function of temperature and particle size range. The equilibrium data were correlated using Langmuir, Fruendlich, Temkin, Redlich-Peterson,
and Sips isotherm models. Thermodynamic parameters such as standard enthalpy (ΔHº), standard entropy (ΔSº) and standard Gibbs free
energy (ΔGº) were calculated. It was found that the adsorption capacity of clay for AR97 decreases with increasing temperature and particle
size range. The results show that Fruendlich and Sips isotherm models best fit the experimental data over the whole concentration range. The maximum adsorption capacity for Yemen natural clay was 196.7 (mg.g-1). The value of ΔHº was -20.5 (kJ.g-mol−1) indicating that the
adsorption of AR79 onto Yemen natural clay is characterized by physical adsorption. ΔGº values obtained were all negatives indicating a
spontaneous adsorption process.
Keywords: Diazo dye, Adsorption, Yemen Natural Clay, Langmuir, Fruendlich, Temkin, Redlich-Peterson, isotherm models