concentration increases. The correlation coefficient of the isotherms
was N0.98 for both dyes, suggesting that adsorption entailed the
formation of a monolayer. By means of the Langmuir model, the
maximum adsorption capacity for each dye was determined:
4.50 mg g−1 for DR80 and 8.31 mg g−1 for RB21. Conditions for
adsorption are considered favorable if the RL dimensionless parameter
value (Separation Factor) is between 0bRLb1 [32]. Thus, the adsorption
process proved favorable for both dyes since this value ranged
between 0.030 and 0.471 for DR80 and between 0.156 and 0.826 for
RB21. The KL values were 0.16 L mg−1 and 0.03 L mg−1 for DR80 and
RB21, respectively. Larger KL values are indicative of a favorable
adsorption process. Thus, comparing the KL values, it was concluded
that the adsorption of DR80 was more favorable [33]. A possible
explanation for this behavior may be related to a greater number of
sulfonic groups present in the DR80 dye than in RB21. Hence, a DR80
molecule can interact with more adsorbent sites than an RB21
molecule and saturate the active sites of the adsorbent at a lower
concentration. Consequently, relatively more RB21 is adsorbed since it
takes up fewer sites per molecule