Carbons prepared from coffee grounds by H3PO4 activation: Characterization and adsorption of methylene blue and Nylosan Red N-2RBL

Activated carbons were prepared by the pyrolysis of coffee grounds impregnated by phosphoric acid at 450 °C for different impregnation ratios: 30, 60, 120 and 180 wt.%. Materials were characterized for their surface chemistry by elemental analysis, “Boehm titrations”, point of zero charge measurements, Infrared spectroscopy, thermogravimetric analysis (TGA); as well as for their porous and morphological structure by Scanning Electron Microscopy (SEM) and nitrogen adsorption at 77 K. The impregnation ratio was found to govern the porous structure of the prepared activated carbons. Low impregnation ratios (<120 wt.%) led to essentially microporous and acidic activated carbons whereas high impregnation ratios (>120 wt.%) yielded to essentially mesoporous carbons with specific surface areas as high as 925 m2 g−1, pore volume as large as 0.7 cm3 g−1, and neutral surface. The activated carbons prepared from coffee grounds were compared to a commercial activated carbon (SBET ∼ 1400 m2 g−1) for their adsorption isotherms of methylene blue and “Nylosan Red N-2RBL”, a cationic and anionic (azo) dye respectively. The mesoporous structure of the material produced at 180 wt.% H3PO4 ratio was found to be appropriate for an efficient sorption of the latter azo dye.