Effects of activating agent and pyrolysis temperature on the pore structure and adsorption performance of sweet potato vine-based activated carbon

Abstract

China produces a large amount of sweet potato vines annually that require processing. In this study, sweet potato vine was used as a precursor to prepare activated carbons with two chemical activating agents (ZnCl₂ and K₂CO₃). The obtained activated carbons were characterized by N₂ adsorption/desorption isotherms, scanning electron microscopy (SEM), and Fourier transform infrared (FTIR) spectroscopy. The synthesized activated carbons were subsequently applied for the adsorption of methylene blue (MB). The effects of the activating agent and pyrolysis temperature on the properties and adsorption capacity of the activated carbons were investigated. The activated carbon prepared with K₂CO₃ at 800 °C (CK800) exhibits the highest surface area (1523.9 m² g⁻¹) and total pore volume (0.7247 cm³ g⁻¹), and is composed of micropores and mesopores. In contrast, the sample activated with ZnCl₂ at 500 °C (CZn500) shows a surface area of 1347.2 m² g⁻¹ and is dominated by mesopores with a mesopore volume of 1.0953 cm³ g⁻¹. Both activated carbons exhibited high adsorption capacities for methylene blue. Their adsorption isotherms followed the Langmuir model, with maximum adsorption capacities reaching 775.2 mg g⁻¹ (CK800) and 400 mg g⁻¹ (CZn500), respectively. The adsorption kinetics were consistent with the pseudo-second-order kinetic model. This is primarily attributed to their large BET surface area and high total pore volume. Simultaneously, mechanisms such as electrostatic attraction, π–π stacking, hydrophobic interaction, and hydrogen bonding also make important contributions to the materials' high adsorption performance. The prepared materials demonstrate that sweet potato vine can serve as a biomass feedstock for the production of activated carbon. Different preparation methods can impart distinct functionalities and physicochemical properties to the activated carbon. By selecting appropriate activating agents and pyrolysis temperatures, it is possible to successfully produce sweet potato vine-based activated carbon with high BET specific surface area, large pore volume, and excellent methylene blue adsorption performance.