On the role of surface functional groups in enhancing methylene blue adsorption by low-temperature biochar derived from Platanus orientalis bark

In this study the Platanus orientalis bark biochar prepared by pyrolysis under different temperatures was used for adsorptive removal of a dye methylene blue. Our results show that the biochar adsorption capacity decreases with increasing pyrolysis temperature. However, the BET surface area of the biochar shows little change with increasing pyrolytic temperature. The bark biochar pyrolyzed at 200°C (BC200) outperformed those biochar proodued under other pyrolytic temperatures. For both the as-prepared and demineralized BC200 (D-BC200), Elovich kinetic model described the adsorption process better, indicating a chemisorption process in nature. While our biochar typically display a mesoporous structure, the D-BC200 is observed to have a micropore distribution structure with finer pore sizes .
FTIR analysis demonstrated that only the D-BC200 had comparable content of surface functional groups with the raw bark. Boehm titration and XPS analysis indicated the abundant existence of carboxyl acid/ester group. Surface functional groups play a key role in contaminant removal. By Langmuir isotherm model, the qmax on the as-prepared BC200 and D-BC200 achieved 199.5 and 237.8 mg/g, respectively. Thermodynamic analysis demonstrated that the adsorption process was spontaneous and endothermic. We measure a higher adsorption enthalpy for D-BC200 which indicate that the process was more sensitive to the change of reaction temperature.