Ion exchange reactions of major inorganic cations (H+, Na+, Ca2+, Mg2+ and K+) on beidellite: Experimental results and new thermodynamic database. Toward a better prediction of contaminant mobility in natural environments

To our knowledge, no thermodynamic database is available in the literature concerning ion-exchange reactions occurring in low-charge smectite with tetrahedral charge (beidellite). The lack of this information makes it difficult to predict the mobility of contaminants in environments where beidellite and major cations, which act as competitors with contaminants for sorption on the clay phase, are present. The present study proposes a multi-site ion exchange model able to describe experimental data obtained for H+ and the four major cations (Na+, Ca2+, Mg2+ and K+) found in natural waters interacting with a <0.3μm size fraction of Na-beidellite. The nature of the sites involved in the sorption processes is assessed using qualitative structural data. Moreover, the effect of the charge location in the smectite on the selectivity coefficient values is discussed by comparison with the results reported in the literature for smectite characterized by octahedral charge (montmorillonite). The new thermodynamic database proposed in this study is based on the same total sorption site density and distribution of sites regardless of the cations investigated. This database is valid for a large range of physico-chemical conditions: a [1–7] pH range, a total normality higher than 5×10−3mol/L corresponding to a flocculated state for water/clay systems, and when sorption of ions pairs can be neglected. Note that this study provides evidence that a thermodynamic database describing ion exchange reactions between H+ and the four major cations of natural water for smectite cannot be valid irrespective of the total normality of the aqueous phase because smectite can form different phases (e.g., floc, gel) when immersed in aqueous system. The model proposed can be easily included in reactive transport software and is applied in the present study to predict the evolution of beidellite surface composition toward major inorganic cations in the context of mining site remediation. Finally, the new thermodynamic database proposed could be used for a better understanding of ion-exchange reactions occurring in many environments where beidellite is present, from soils to smectite-bearing sedimentary formations.