Sustainable process to produce activated carbon from Kraft lignin impregnated with H3PO4 using microwave pyrolysis

Abstract

Currently, the demand for activated carbon (AC) has been growing thanks to its adsorption capacity to remove liquid and gaseous pollutants. Lignin is a biomass derived from an abundant renewable resource, rich in carbon, with potential applications in modern society. This work seeks to add greater value for the lignin, transforming it in AC and also to demonstrate the microwave pyrolysis effectiveness. ACs were obtained from Kraft lignin treated with different proportions of phosphoric acid in two different periods of time, using microwave pyrolysis in oxidizing atmosphere. The results of Fourier transform infrared spectroscopy and X-ray diffraction showed that the conversion of lignin to carbon material occurred successfully. The morphology of lignin, observed by scanning electron microscopy, showed a globular appearance and different diameters, which was converted into AC with very porous structures. Adsorption/desorption of N2 (BET) analyses showed that the AC presented high surface areas, between 635 and 1055 m2g-1, with the presence of micro and mesopores. The highest acid:lignin weight ratio favored higher values of AC surface area and pore volume. The maximum adsorption capacity of prepared AC was tested with methylene blue dye (MB) and amoxicillin (AMOX), and values of 140 and 220 mgg−1 were found, respectively. In the case of MB, the adsorption mechanism adjusted better to the Langmuir model and the AMOX to the Freundlich model.