where: KL1 is the Langmuir constant at T1; R is the gas constant
(8.314 J/mol K). The apparent enthalpy of adsorption,(DHads), and entropy of adsorption, (DSads), were calculated from adsorption data at different temperatures using the Van’tHoff equation [33]:
where: KL is the Langmuir constant and T is the solution temperature(K). The magnitude of DHads and DSads was calculatedfrom the slope and y-intercept from the plot of ln KL vs 1/T, with these thermodynamic parameters being given in
Table 6.As shown in Figs. 7 and 8, the adsorption capacity of both dyes increased at higher temperatures, which indicates that adsorption of dyes in this system was an endothermic process. For both dyes, the surface coverage increased