Alteration of cameroonian clays under acid treatment. Comparison with industrial adsorbents

Abstract

In view to evaluate an alternative local cheaper source for vegetable oil refiners, three cameroonian clays containing swelling (smectites) and non-swelling (kaolinite) clay minerals were treated with sulphuric acid solutions at concentrations of 1, 2, 4 and 8 N at 80 °C for 2 h. Their alteration and resulting products were characterized by XRD, FT-IR-spectroscopy, chemical analyses, along with TEM-EDS, nitrogen adsorption isotherms at 77 K, and compared with industrial bleaching earths. Acid treatment of Cameroonian clays leads to the classically observed cristallochemical and textural modifications. The plot of TEM-EDS data in Al 2O 3/SiO 2 and Fe 2O 3/Al 2O 3 diagrams indicate that upon acid treatment, clay particles principally move towards the SiO 2 pole, consistent with the formation of an amorphous compound mainly siliceous as the result of acid activation. Concomitantly, a progressive decrease in cation exchange capacity (CEC) was observed with increasing sulphuric acid concentrations while both specific surface area and mesoporosity of the studied materials increase. Elemental analyses, X-ray and spectroscopy data reveal that acid activation affects both the octahedral and the tetrahedral sheets. The analysis of their reactivity shows that clays with high amounts of smectite are more sensitive to acid leaching than those with relative large kaolinite contents. Further, upon acid treatment, most trace and rare earth elements are leached, and the amount of toxic elements (e.g. Cd, Pb, Ni, Cu, As, Zn) in the by-products is low (< 50 mg/kg). Mineralogy, chemical analyses and textural properties of the activated products match the specifications of industrial adsorbents. Therefore, activated Cameroonian clays are expected to perform well in respect with decolorization of vegetable oils.