A Conceivable Technique of Measuring Sorption Coefficients in Intact Rock Using an Electrical Potential Gradient as the
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$&RQFHLYDEOH7HFKQLTXHRI0HDVXULQJ6RUSWLRQ&RHIILFLHQWVLQ,QWDFW5RFN8VLQJDQ (OHFWULFDO3RWHQWLDO*UDGLHQWDVWKH'ULYLQJ)RUFHIRU0LJUDWLRQ Martin Löfgren and Ivars Neretnieks Chemical Engineering and Technology, Royal Institute of Technology, Teknikringen 26, 100 44 Stockholm, Sweden $%675$&7 Sorption coefficients are traditionally obtained in batch experiments if the sorbent is strongly or intermediately sorbing. In a batch experiment the rock is crushed and this could increase the surface area as well as induce new and fresh surfaces. Therefore there is some concern whether sorption coefficients obtained in batch experiments represents those of intact rock. Performing sorption experiment by diffusion in intact rock with intermediately and strongly sorbing species seems impossible in practice due to extremely long experimental times. In this paper the possibility of increasing the migration rate in the rock by two of three orders, thus enabling KD measurements of intermediately sorbing species in intact rock, is discussed. The increase in migration rate has already been achieved successfully in so called through electromigration experiments using non-sorbing species. Here a potential gradient acts as the main driving force. In our experiments the migration rate was increased 320 times by using a potential drop of only 9 volts. ,1752'8&7,21 Sorption coefficients are traditionally obtained in batch experiments if the sorbent is strongly or intermediately sorbing and in batch or diffusion experiments if the sorbent is weakly sorbing. In batch experiments the rock sample is crushed. The reason intact rock is not used is that the apparent diffusivity of the sorbent in intact rock is very small and it would take an unreasonable long time to transport the sorbent to the sorption sites by diffusion. If contacting crushed rock with a solution containing the sorbent under stirring, convection and diffusion will carry the sorbent to the mineral surfaces. Before contacting the crushed rock with the sorbent it is normally sieved into different fractions ranging from less than 100 µm up to a few millimetres. When crushing the rock, especially into finer particles, new surfaces are created. As the contact area is increased the sorption coefficient increases with decreasing particle size [1,2]. The new surfaces have not previously been in contact with the pore water and may therefore interact differently with the sorbent. In addition crushing and sieving may lead to a differentiation in mineral composition for different size fractions, as more brittle minerals tend to end up in the small size fractions [2]. Because of these reasons, it is questionable if batch experiments give sorption coefficients representative for intact rock. In diffusion experiments the sorbent is either allowed to diffuses through or into an intact rock sample [3]. If using intact rock fewer new surfaces are induced in preparing the sample. This method is limited to weakly sorbing species with relatively high apparent diffusivities, as the sorbe
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