Adsorption capacities and related characteristics of wood charcoals carbonized using a one-step or two-step process

Abstract

Sugi (Cryptomeria japonlca D. Don) wood powder was carbonized at varying temperatures by a onestep process up to 1000‡C and a two-step process using wood charcoal as the raw material up to 1600‡C. This study was conducted to evaluate the adsorptive properties of wood charcoal and discuss the mechanism of its adsorptive function in relation to the physical and anatomical characteristics of wood after carbonization. Anatomical characteristics of carbonized wood materials were directly observed under heating using an environmental scanning electron microscope (ESEM); the cell wall structures were analyzed by high-resolution transmission electron microscope (HRTEM). The largest weight losses were observed at the highest temperatures, in both the one-step and twostep processes but leveled off above 800‡C. Shrinkages in the tangential, radial, and longitudinal directions increased with carbonization temperature, peaking at 1000‡C. Direct observations by ESEM showed distinct shrinkage at around 400‡C. The first trial observations by HRTEM on the changes in the ultrastructure of cell walls of wood charcoals were done, and it was assumed to affect the formation of micropores. Adsorption was found to follow the Langmuir isotherm model. With the one-step carbonization process, the iodine adsorption capacities of the carbonized wood powders increased with increasing carbonization temperature, peaking at 800‡C, but decreased at higher temperatures. The wood powder carbonized at 1000‡C with the two-step process showed the highest capacity, but further heating up to 1400‡C drastically decreased the adsorption. The shrinkage of cells was related to the increases and decreases in its specific surface area. Specific surface area and total pore volume were evidently related to the adsorptive properties.