Ion Exchange Behavior of the Febex Bentonite: 2. Batch Experiments and Geochemical Modeling
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ESCRIPTION Five g of FEBEX bentonite powder were placed inside dialysis bags and then located inside PFA reactors filled with 125 ml of water of a given chemical composition (granitic-type waters with variable ionic strength). The PFA reactors containing the water and the clay powder were heated to different temperatures (from room temperature up to 80ºC) for a period of time ranging from 1 day to 1 year. Water was renewed according to a prescribed schedule but not the clay, which remained in place for the whole extent of each test. After each water renewal, major cations, silica, total inorganic carbon (TIC), and pH were analyzed. At the end of each test, the exchange complex and CEC of the bentonite were measured. CONCEPTUAL MODEL The dry clay powder and water at the beginning of each experiment are out of chemical equilibrium. Therefore they will react. After each liquid renewal, the chemical composition of water and bentonite will have changed. So, each new water batch will interact with a bentonite slurry whose composition evolves with time, re-adapting to every water renewal while trying to attain chemical equilibrium. Given enough time, the slurry will eventually equilibrate with the renewal water. However, this picture is further complicated by some additional circumstances that need to be taken into account prior to any successful modeling. Firstly, the solid phase is not only smectite. There are a few other mineral phases that, at the time scale of our experiments, are amenable to mass transfer in our experiments. Some of them are readily soluble (e.g. calcite) while some others are not (dolomite, quartz, chalcedony, cristobalite) although in any case they dissolve faster than many rock-forming silicates. Therefore they need to be included in the modeling. Secondly, due to the short extent of some of the experiments (starting at one day), the kinetics of mineral dissolution/precipitation needs to be considered. That also applies for minerals that are usually assumed to be governed by chemical equilibrium (i.e. carbonates). As we will see later, kinetics of calcite dissolution play a major role in the interpretation of the carbonate-pH system. Third, exchange constants are function of the exchanger composition, as evidenced in our companion paper [2]. Therefore, to fully account for this effect, exchange isotherms need to be incorporated into the model. Finally, according to the analyzed time dependent evolution of sulfate and chloride, there is a significant increase in the values measured in the water for the shorter time experiments that is later cushioned after the firsts water renewals. This effect appears to be due to the presence of tiny salt crystals dispersed in the initial dry clay powder. These crystals will readily dissolve when exposed to water, thereby increasing the sulfate and chloride concentrations. By simple mass balance, an estimated amount of halite and gypsum have been considered to be precipitated in the starting dry bentonite powder.
EQ3/6 MODELING Due to the large amount of experim
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