Preparation of chemically activated carbon from waste biomass by single-stage and two-stage pyrolysis

Agricultural waste biomass is often used to produce activated carbon with high specific surface area, high degree of porosity and excellent sorption properties. Many researchers used chemical activation in connection with single-stage or two-stage pyrolysis. Therefore, the aim of this work was to compare both methods. The first method covered direct chemical activation of biomass with a single pyrolytic step. The second procedure consisted of two-stage pyrolysis (after first pyrolysis the char was chemically activated and pyrolysed secondly). Significant differences in morphology, textural properties, organic structure and crystallinity of prepared activated carbons were observed. Activated carbons were tested as a sorbent of Cd2+ from water. Textural analysis showed that the volume of micropores in sample after two-stage pyrolysis was more than four times higher in comparison with single-stage. The value of external surface area related with presence of mesopores was similar for both activated carbons (Sext(single-stage) = 48.1 m2/g; Sext(two-stage) = 50.3 m2/g). The study of ACs sorption capacity was performed as exemplified by cadmium. The experiments evaluated using Langmuir and Freundlich models showed that AC prepared by two-stage pyrolysis has more than three times higher sorption capacity, i.e. 107.5 mg Cd/g comparing to single-stage one, where the sorption capacity of 32.8 mg Cd/g was attained. Thus, loading of environment by cadmium and its uptake by various sorbents are also discussed and compared.