where qt and qe are the amount of Pb adsorbed (mg g−1) on CMTAB at time t (min) and equilibrium, respectively; K2 (g mg−1 min−1) is the corresponding adsorption rate constant. Parameters of the pseudo-second-order model were calculated from the intercept and slope of the t/qt versus t plot (Fig. 3b). The correlation coef- ficient (R2) is more than 0.99. The equilibrium adsorption capacity calculated by this model (qe,cal) is close to that obtained from experiments (qe,exp), confirming the validity of the pseudo-second-order model which bases on the hypothesis that chemical sorption is
rate-limiting step of adsorption process [32].