Linearized expression gives the goodness of the model, but non
linear adjustment is preferred in order to the reliability of the
parameters [49]. Fig. 5 depicts the adsorption isotherms for pollutant
removal. Experimental points are placed onto the predicted
path for Langmuir adsorption hypothesis, according to a very high
regression level. At a first glance, the three isotherms present the
typical shape of L-class in the specific classification made by Giles
et al. [50]: a concave initial region followed by a saturation zone
in the last part of the curve. This is usual in those systems where
the adsorption becomes more difficult as the pollutant concentration
increases. Table 2 shows each regression parameter, such as
kl1, kl2 and r2 for each case. As can be seen, CTAB adsorption is
more efficient as maximum q is obtained. This is evident from the
values of the Langmuir constants. On the contrary, Zn2+ presents
lower adsorption level. These differences can be due to the molecular
size (higher in the case of CTAB) and to the cationic character
of the three model compounds, which probably affect to the sorption
mechanism [51]. Linear adjustment is also shown in Fig. 6,
where the linearized expression for the three adsorption systems
are presented.