The aim of this study is to investigate the oxalic acid activation of bentonites containing different types of smectites, analyse their surface modification as a function of acid concentration and create good quality bleaching earths. In particular, two different bentonite samples (one containing aluminum and one containing ferruginous smectite), after being characterized through XRD, XRF and FT-IR analysis, are treated with oxalic acid at a concentration of 0.5, 0.7 and 1 M. Their structural modifications after treatment are observed through FT-IR spectra and surface area and porosity measurement (using the BET equation and the BJH method, respectively) combined with the determination of the main structural metals’ extraction from them (using an atomic adsorption spectrometer). The results showed that the ferruginous smectite is more susceptible to oxalic acid activation compared to the aluminum smectite, and all the final products have developed extra porosity in their structure while retaining the structure of smectite (even at 0.5 M acid concentration). The activated samples were used as bleaching earths in soybean oil, and the results proved that Lovibond yellow and red colours as well as the chlorophyll of oil (measured spectrophotometrically) were reduced to the values set by the specifications.
Acid activation is a chemical modification of minerals and mainly smectites, in order to enhance their surface properties and reinforce their adsorption capacity for specific ions. Acid-activated clays and, more specifically, inorganic acid-activated calcium montmorillonites are mainly used in the bleaching process of vegetable oils, which are holding third place in the world food consumption after cereals and rice, in order to remove chlorophyll and other undesirable substances from the oils. This study proposes the use of oxalic acid for the acid activation of smectites, in order to modify their surface characteristics and create high-quality bleaching earths. In particular, aluminium and ferruginous smectites are treated with oxalic acid concentrations of 0.5, 0.7 and 1 M, and the structural modifications in the materials after treatment are observed through FTIR analysis, combined with the determination of the main structural metals’ extraction from the smectites. The results showed that ferruginous smectite is more susceptible to oxalic acid activation. The final product has developed vacant spaces in its structure to absorb other ions or molecules, although it retains the structure of smectite. Oxalic acid concentration of 0.5 M is sufficient for achieving this result. The materials produced through the oxalic acid activation are also compared with inorganic acid-activated smectites originating from the same deposits.
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