Chemical and mineralogical characterization of a clay taken from the Moroccan Meseta and a study of the interaction between its fine fraction and methylene blue

The primary part of this study is concerned with a chemico-mineralogical characterization of a Lutetian sedimentary clay taken from the phosphateous basin of Ouled Abdoun (Moroccan Meseta). This investigation was carried out by using X-ray diffraction (XRD), thermal analyses (DTA and TG), infrared spectroscopy (IR) and analytical techniques. It was found that the clay consists of calcite (38 wt.%), dolomite (12 wt.%), quartz (8.4 wt.%) and goethite (1.2 wt.%) together with kaolinite and a mixture of smectites. A minor amount (<3 wt.%) of organic matter is also present. The clay fine fraction (<2 μm in size), which is characterized by a relative large cation-exchange capacity (CEC, 68 meq/100 g) and surface area (290 m2/g), has been saturated by Na+ ions and used for the retention of methylene blue (MB). In this purpose, the UV–VIS spectroscopy was used to follow the kinetics of the process, measure the adsorption isotherms and identify the nature of the derived MB species fixed to the fine fraction. It was shown that the kinetics is fast and mainly controlled by the intragranular diffusion. On the other hand, the isotherms allowed to show that the retention process obeys a modified Langmuir equation, which takes into account the adsorption and the cationic exchange processes. The adsorbed quantity represents about 40% of the wholly fixed amount. In this respect, the contribution of each process has been evaluated by using samples of Cu2+-saturated fine fraction. The nature of MB+ deriving species fixed to Na+-saturated fine fraction was found to be dependent on the MB loading. For the smallest loadings, MB+ ions are preferentially fixed to external acid sites, thus leading to the formation of MBH2+. As the loading increased, (MB+)3 developed while the protonated species disappeared, likely at the expense of the trimer form. It was also found that some amount of the monomer MB+ occurred exclusively on the external surface.