The Sips isotherm is derived from the limiting behavior of the Langmuir and Freundlich isotherms. The model is valid for localized adsorption without adsorbate-adsorbate interactions [15]. When approaches a low value, the Sips isotherm effectively reduces to Freundlich, while at high , it predicts the Langmuir monolayer sorption characteristic. The Sips linear equation model is expressed as
where (1/mg) and (mg/g) are the Sips equilibrium constant and maximum adsorption capacity values obtained from the slope and the intercept of the plot (Figure 4). The Sips isotherm equation is characterized by the dimensionless heterogeneity factor, , which can also be employed to describe the system’s heterogeneity when is between 0 and 1. When , the Sips equation reduces to the Langmuir equation and it implies a homogeneous adsorption process