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 Brunauer- Emmett-Teller surface area of the biochar shows little change with increasing pyrolytic temperature. The bark biochar pyrolyzed at 200 degrees C (BC200) outperformed those biochar produced 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. Especially high mesopore content structure within D-BC200 was superior to other demineralized biochar samples. Fourier-transform infrared spectroscopy analysis demonstrated that only the D-BC200 had comparable content of surface functional groups with the raw bark. Boehm titration and X-ray photoelectron spectroscopy 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 q(max) on the as-prepared BC200 and D-BC200 achieved 199.5 and 237.8 mg/g, respectively. Thermodynamic analysis demonstrates that the adsorption process was spontaneous and endothermic. We measure a higher adsorption enthalpy for D-BC200 which indicates that the process was more sensitive to the change of reaction temperature.