The roles of diffuse double layer development and ion exchange in cation adsorption onto bentonite in aqueous system

This study investigates the coupled roles of diffuse double layer (DDL) development and ion exchange in cation adsorption onto Na-bentonite. Batch adsorption tests and free swell tests were conducted over a wide range of initial CaCl 2 molarities (0–1000 mmol/L), and both Ca 2+ adsorption and Na + release were quantified. The ion exchange dominated the Ca 2+ adsorption when the equilibrium bulk Ca 2+ molarity ( C e ) was below 1 mmol/L, whereas the majority of Ca 2+ adsorption was associated with DDL development. The measured adsorbed molality ( q eCa , mmol/g) exhibited a non-monotonic trend with respect to C e . When q eCa was normalized to adsorbed molarity ( C in , mmol/L) by accounting for the experimentally evaluated volume of DDL domain, the resulting molar adsorption isotherm exhibited a Langmuir-type correlation, indicating that the adsorption equilibrium was dependent on Langmuir equilibrium constant K L . This result emphasized that the DDL constituted a dynamic spatial domain governing cation adsorption process, while the adsorption equilibrium was governed by the correlation between C e and C in . This framework provides a physically consistent interpretation of DDL development and ion exchange to the cation adsorption onto bentonite, thereby advancing the mechanistic understanding of clay-water interactions relevant to bentonite-based barrier systems in geoenvironmental engineering. • The diffuse double layer (DDL) is the spatial domain of cation adsorption onto bentonite. • Ion exchange predominantly contributes to cation adsorption under dilute bulk cation conditions. • The adsorption equilibrium is jointly governed by DDL development and ion exchange.