where KL (L mg−1) is the Langmuir constant related to the free adsorption energy and qmax (mg g−1) is the maximum adsorption capacity. KF [mg g−1 (L mg−1)1/n] and n are the Freundlich constants related to the adsorption capacity and adsorption intensity, respectively. Parameters of Langmuir and Freundlich models were calculated by plotting the Ce/qe versus Ce and ln qe versus ln Ce plots, respectively. As shown in Fig. 4, both of the two models can well fit the equilibrium data with the coefficients (R2) higher than 0.95, and the maximum adsorption capacity calculated from Langmuir model is 263.9 mg g−1. The Freundlich model is able to provide a better fit (R2 = 0.990) to the experimental data than the Langmuir model (R2 = 0.956), indicating the Pb adsorption onto CMTAB is not a strict monolayer adsorption process. Similar phenomena were also observed in other relevant study [35].