Ion Exchange Behavior of the Febex Bentonite. 1. Na/K, Na/Mg and Na/Ca Experimental Exchange Isotherms
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nd to derive thermodynamic data useful to model the ion exchange reactions taking place among bentonite and alkaline and alkaline−earth cations. EXPERIMENTS AND RESULTS Several tons of bentonite were sampled from a quarry located in Cortijo de Archidona in Serrata de Níjar (Cabo de Gata, SE Spain) in order to perform the different FEBEX experiments. From hereafter this material will be called the FEBEX bentonite. The origin of the bentonite rock is associated with hydrothermal alteration processes taking place over tuffaceous volcanic rocks [1-5]. After being sampled from the quarry the bentonite followed a homogenization process according to the protocol described in ENRESA [6] and kept stored in big-bags under atmospheric moisture conditions. Subset samples taken from the big bags were distributed to laboratories participating in the FEBEX project. One of these subsets was used in the present study. The bulk bentonite sample was dry-ground in an agate mortar, mechanically rehomogenized, and dried to 60°C in a conventional oven. Before the experiment the clay was kept stored in polyethylene bottles at room temperature and humidity. Mineralogical analyses of the samples were carried out by X-ray diffraction (XRD). Several XRD patterns were recorded on powder specimens and on oriented and glycolated (< 2 µm size fraction) mounts using a Philips PW1730 diffractometer with CuKα radiation, Ni filter, and a graphite monochromator. Semiquantitative analysis based on the relative corrected areas of selected peaks from the X-ray diffractograms yielded an average mineralogical distribution of 96% of dioctahedral smectite, 1.5% cristobalite, 1% quartz, 1% calcite, and trace amounts of Kfeldspar. XRD of the oriented and glycolated mounts showed that it was actually a R=0 mixedlayer illite/smectite (I/S), with 15% illite layers. Wet chemical analyses of the bulk bentonite were performed following the method described by Shapiro [7]. The cation exchange capacity, CEC, and exchangeable cations were determined separately according to the procedures of the Soil Conservation Service [8], consisting on saturation with NH4+ at pH 7 and subsequent displacement of NH4+ with a NaCl solution. The resulting CEC of the FEBEX sample is 102 ± 3 meq/100 g of dry clay. After correction for the presence of non-smectitic mineral phases identified by XRD, the structural formula of the mixed-layer I/S is calculated: Na0.179K0.201Mg0.124Ca0.157 [Al2.854Fe0.356Mg0.885] [Si7.656Al0.344] O20 (OH)4 Cation exchange data indicate that only 0.016 K+ ions per unit cell are amenable for ion exchange. The remaining 0.185 ions corresponding to the illite layers that do not participate in exchange reactions. To perform the exchange experiments, the FEBEX bentonite was homoionized to a Nasaturated form. To do that, a bentonite suspension was prepared by adding 50 g of ground bentonite to 500 mL of a 0.5 M NaCl solution. The suspension was periodically hand-shaken. After 24 h contact, the supernatant was decanted and separated from the clay slurry. A new batch of
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