A statistical approach to optimize the activated carbon production from Kraft lignin based on conventional and microwave processes

Activated carbon (AC) from Kraft lignin (KL) has attracted the attention of researchers due to abundance and carbon-rich chemical structure of this biomass. In this study, ACs were prepared from KL using conventional and microwave pyrolysis, both in oxidizing atmosphere, supported by a statistical design of experiments (2(3)). The influence of different parameters (proportion of H3PO4 used in the lignin impregnation, time, temperature, and the microwave power) on the surface area of ACs was investigated. The results showed that the proportion of acid is the variable with greater statistical importance on the surface area of ACs, in both processes. The optimized ACs (C4 and M4) were obtained when the KF samples were treated with the lignin:acid ratio of 1:2. These samples showed surface areas of 1150 and 1030 m(2) g(-1), respectively. C4 presented higher structural ordering, larger micropore volume, and higher acidity. Meantime, M4 was more efficient in the methylene blue removal (130 mgg(-1), 42%) compared to C4 (80 mgg(-1), 29%). The data were better adjusted to the Langmuir isothermal model, indicating that the adsorption is monolayer, and the pore distribution is influenced by this adsorption process.