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

Sugi (Cryptomeria japonica D. Don) wood powder was carbonized at varying te mperatures by a one-step process up to 1000 degreesC and a two-step process using wood charcoal as the raw material up to 1600 degreesC. This study wa s conducted to evaluate the adsorptive properties of wood charcoal and disc uss the mechanism of its adsorptive function in relation to the physical an d anatomical characteristics of wood after carbonization. Anatomical charac teristics 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 microscop e (HRTEM). The largest weight losses were observed at the highest temperatu res, in both the one-step and two-step processes but leveled off above 800 degreesC. Shrinkages in the tangential, radial, and longitudinal directions increased with carbonization temperature, peaking at 1000 degreesC. Direct observations by ESEM showed distinct shrinkage at around 400 degreesC. 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 fo rmation of micropores. Adsorption was found to follow the Langmuir isotherm model. With the one-step carbonization process, the iodine adsorption capa cities of the carbonized wood powders increased with increasing carbonizati on temperature, peaking at 800 degreesC, but decreased at higher temperatur es. The wood powder carbonized at 1000 degreesC with the two-step process s howed the highest capacity, but further heating up to 1400 degreesC drastic ally decreased the adsorption. The shrinkage of cells was related to the in creases and decreases in its specific surface area. Specific surface area a nd total pore volume were evidently related to the adsorptive properties.