where R (8.314 J/mol K) is the gas

where R (8.314 J/mol K) is the gas constant, T (K)
absolute temperature and KL
(L/mg) is the Langmuir
isotherm constant. The value enthalpy (DHo
) and
entropy (DSo
) were calculated from the slope and
intercept of the plot of ln KL
vs 1/T. The results were
listed in Table 6. The positive values of Gibbs free
energy (DGo
) at all temperatures showed that the
removal process was non-spontaneous. The decrease
of the Gibbs free energy (DGo
) change from 3.89 to
1.05 with increase temperature from 25 o
C to 85 o
C
indicated that the presence of an energy barrier at low
temperature (Wang et al., 2013). The value of DGo
become more negative with the increase of temperature,
which indicates that the reaction is more favorable at
high temperatures. The positive DHo
values obtained
indicated that the sorption process was endothermic
in nature. Generally, the enthalpy change (DHo
) for