The disclosure relates to the field of lining materials which are used to isolate landfill sites in order to avoid pollution of surrounding soil by ions such as heavy metals. Further disclosed is a process to increase the ion adsorption capacity of clay or dredged sediment. More specifically, the disclosed is a process comprising: a) mixing the clay or dredged sediment with an anionic polymer, and b) subsequently drying the mixture, results in a lining material having superior ion adsorption characteristics.