BACKGROUNDTwo types of bleaching earths used in the production of green diesel, namely, sepiolite and palygorskite, were characterized by various physico-chemical methods, such as nitrogen adsorption, Hg porosimetry, scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray analysis, Fourier transfom infrared spectroscopy, pH of the clay slurry, thermogravimetrical analysis (TGA) and X-ray diffraction analysis. The characteristics of fresh, spent and extracted bleaching earths were compared in the current study.RESULTSThe results showed that particle sizes of the extracted adsorbents have decreased. Furthermore, some leaching of elements was confirmed by elemental analysis. A decrease in specific surface area of the adsorbents was observed thus compromising the economical feasibility of the reuse of these clays. TGA results showed that the oil content in spent sepiolite and palygorskite, which could be extracted was 35% and 12%, respectively. Extraction efficiencies were affected by the specific surface area of the clay.CONCLUSIONSStructural characterization of extracted bleaching earths showed that some changes occurred for sepiolite and palygorskite during bleaching and extraction in addition to decrease in their particle sizes. The specific surface areas of the extracted bleaching earths decreased substantially compared with those determined for fresh bleaching earths. This result indicates that reuse of spent, extracted bleaching earths might not be economically feasible.