COUPLED DIFFUSION OF AQUEOUS-ELECTROLYTES ACROSS BENTONITE CLAY MEMBRANE

In soil, coupling between various transport processes occurring simult aneously often leads to a flow of one kind from a non-conjugate force. Such coupled movement of salt and water in soils and clays is of inte rest from theoretical viewpoint and also from practical considerations . The present study reports coupled isothermal diffusion encountered i n dilute aqueous solutions of HCl, H2SO4, NaCl, Na2SO4, KCl, K2SO4, Mg Cl2, MgSO4 and CaCl2 across a bentonite clay membrane, in hydrogen and calcium forms, at 35 and 40-degrees. For the purpose, a specially des igned osmotic cell (made of Perspex) was used in which the clay-membra ne was sandwiched between two solution chambers. The transport data we re fitted to the formulations based on irreversible thermodynamics to obtain the reflection coefficient (sigma) for the given membrane-trans port systems. The latter provides a measure of the extent of 'demixing ' of solute and solvent in the membrane phase, and is thus expected to lead to information on solute-solvent interactions in the membrane. F or diffusion of aqueous acids across H-bentonite, the sigma values wer e small, indicating near complete coupling between the solute and the solvent flows. This might be attributed to the absence of any exchange reaction in the membrane phase during the passage of the acids. The i sothermal transports of other aqueous solutions across H-bentonite wer e associated with large sigma values, obviously due to an exchange rea ction in the clay phase. However, among the chlorides studied, the sig ma values were the highest for diffusion of CaCl2 across Ca-bentonite membrane. This is attributed to the difficulty of Na+ and K+ ions to r eplace Ca2+ from the bentonite phase, the latter having a higher bondi ng energy. Such a trend may also be explained in terms of a superimpos ed mobility effect of the hydrated cations, and also the ion-pair form ation effect in the clay.