Caffeine removal from water using rice husk residue biochar: characterization, kinetics and sorption isotherms

Caffeine is one of the most consumed substances in the world. It’s inappropriate disposal in water resources pollutes the environment. Sorption is one of the most efficient water treatment processes to remove this kind of pollutant, and there is a wide range of materials available as sorbents, with emphasis on materials from waste biomass. The aim of this work was to use rice husk residue biochar, without undergoing any modification process, to remove caffeine from water by sorption process. The sorbent used was characterized by scanning electron microscopy, Fourier-transform infrared spectroscopy, point of zero charge (pH_PZC), thermogravimetric analysis and nitrogen physisorption at 77 K. The sorbent presented a specific surface area of 272 m² g⁻¹. Four different conditions varying temperature and sorbent dosage were performed aiming sorption kinetics. The profiles show fast mass transfer of caffeine to biochar and the variation on sorbent dosage analyzed presented statistical difference, while temperature variation did not affect the sorption capacity. The rate of sorption was analyzed by three kinetics models: (i) pseudo-first order, (ii) pseudo-second order and (iii) Elovich. To evaluate the mechanism of sorption, four different isotherm models were fitted to the experimental data, and Redlich-Peterson best fitted for 40 °C, Langmuir for 30 °C and Sips for 20 °C. The maximum sorption capacity was 47.21, 40.66 and 38.43 mg g⁻¹ for 20, 30 and 40 °C respectively, confirming that rice husk residue biochar could be an excellent alternative for depollution of waters contaminated with emerging pollutants such as caffeine.