Caffeine removal using activated biochar from açaí seed (Euterpe oleracea Mart): Experimental study and description of adsorbate properties using Density Functional Theory (DFT)

Alternatives for the use of açaí seed have been investigated to remove caffeine from aqueous solutions. The pyrolytic process was conducted under two temperatures and heating rates (400 °C, 10 °C min−1 and 600 °C, 50 °C min−1), with a vacuum pump coupled operating at 20 kPa and the biochar was activated with K2CO3, resulting in CA-SA 400/10 and CA-SA 600/50. Density, moisture, volatile and ash content were evaluated for biomass, as well as their activated carbons were characterized through elemental, thermogravimetric (TGA) analysis, N2 adsorption and desorption isotherms (BET/BJH), infra red spectroscopy with Fourier transform (FTIR), X-ray diffraction (DRX), scanning electron microscopy (SEM), dispersive energy spectroscopy (EDS) and by the pHZPC method. CA-SA 400/10 and CA-SA 600/50 showed high values of specific area (1150.4 and 929.3 m². g−1), average similar pore diameter (2.55 and 2.66 nm) and a mixture in the structure between amorphous phase and graphitic. The adsorptive study showed that the pseudo-second order model adjusted better to the kinetic data and that intraparticle diffusion is not the only step that limits the process. Quantum chemical parameters were evaluated for caffeine, based on the Density Functional Theory (DFT) calculations, in order to understand its chemical reactivity at the atomic level. The results showed that the use of activated charcoal from açaí seed is a promising material for removing caffeine, and emerges as an innovation for using the tailings.