May I ask, the difference of theoretical values and experimental values for Pb adsorption capacity has something to do with isolation of functional group?
Adsorption of metal ions is often associated with complexation between functional groups and metals. Therefore, polyvalent metals, as Pb2+, bind to two or more functional groups, and if these groups are distant, the adsorption capacity may be lower.
Just like Carlos Eduardo Rodrigues has said, in the presence of complexation which could occur within the system, the experimental value would be found to be lower than the theoretical value