A statistical approach to study the valorization process of spent coffee ground

Abstract

This study focused on identifying the best conditions and studying the effects of the operational variables on the integral valorization process of used coffee ground. The valorization approach was defined in two successive steps: oil extraction and using the remaining residue to prepare activated carbon. For this purpose, a set of experiments was designed to test the following four variables in the oil extraction stage: (A) drying time (50 °C/12 h and 105 °C/6 h), (B) relative polarity of hexane and ethanol (0.009 and 0.654 a.u.), (C) solvent ratio (10 and 12 mL/g), and (D) extraction time (30 and 60 min). The best conditions from the corresponding experiments (50 °C/12 h, using ethanol as a solvent in a 12:1 solvent ratio (v/v) with 30 min of extraction time) resulted in the extraction of 0.4 g of oil per gram of spent coffee ground. This value is, to the best of our knowledge, the highest reported thus far and is a result combining of the coffee origin and the significant interactions found for the tested parameters. For the stage of activated carbon preparation, the (E) activating agent ratio (1, 2, and 3 gAA/gS) and the (F) carbonization temperature (450, 550, and 700 °C) were considered independent variables. The corresponding results indicate that 1 gAA/gS and 450 °C are the best conditions, resulting in a sorption capacity of 29 mg/g, which is similar to the value obtained for commercial activated carbon. Physicochemical analysis on the other hand, showed that extracted coffee oil is characterized by a high content of free fatty acids; thus, an additional purification process could be considered to obtain a precursor for biodiesel. Activated carbon was found to have an elemental composition similar to that of vegetal activated carbon. The adsorption kinetics of the activated carbon obtained from the spent coffee residues are characterized by a pseudo-second-order model that suggests chemisorption-controlled phenomena.