Structural, inorganic, and adsorptive properties of hydrochars obtained by hydrothermal carbonization of coffee waste

Abstract

The hydrothermal carbonization process is a suitable process for the conversion of potentially harmful lignocellulosic waste into hydrochars. Defective coffee beans were the precursor raw material for hydrochar synthesis. Reactions were performed in a high-pressure reactor at 150, 200, and 250 °C, in autogenous pressure, for 40 min. Hydrochars were recovered by filtration and characterized by energy dispersive X-ray fluorescence spectroscopy, UV–Vis spectrophotometry, attenuated total reflection Fourier-transform infrared spectroscopy, differential thermal analysis, and scanning electron microscopy. Methylene blue adsorption tests were performed and analyzed by Langmuir and Freundlich adsorption isotherms. Adsorption mechanisms were investigated by computational calculations at DFT level. Results suggest that hydrochars from defective coffee beans can be applied as technological resources in the agronomic and environmental fields due to their inorganic composition, mainly to high magnesium content, the structural characteristics of porosity, biodegradation control, soil carbon-fixation and adsorption capacity. Important adsorption processes are caused by the development of oxygenated functional groups on the hydrochar surface.