RACNTs and CNTs' isotherms are shown in Fig. 5 and, as it can be seen, the CNTs presented a higher adsorptive capacity compared to the RACNTs. Probably the BSA molecules can bind some CNTs' absorptive sites, decreasing the adsorptive capacity. Nevertheless, the detectability of the method with RACNTs is still adequate for the Pb2þ determination in human blood serum samples. The experimental data of the isotherms for CNTs and RACNTs were assessed according to both the Langmuir and Freundlich linear models. The best adjusts were obtained for Langmuir (r40.99, for both CNT and RACNTs, see Table 1). These results suggest that the adsorbent contains homogeneously distributed sites, which retain only one Pbþ2 ion per site. Moreover, all sites must have similar adsorption energy .