An easily overlooked aspect of hydrothermal charcoal: The effect of drying method on structure and properties

Abstract

Hydrothermal carbon (HC) demonstrates significant potential in heavy metal removal due to its low production cost and high adsorption efficiency. However, the impact of drying methods on HC performance is often overlooked. This study employed six types of agricultural waste as precursors to prepare HC using four different drying methods and evaluated their performance through Cd²⁺ adsorption experiments. Compared to natural drying, vacuum high temperature oven drying significantly increased the C-O functional group content by 25.29 %, optimized the pore structure, thereby enhancing the specific surface area and promoting mass transfer, diffusion, and adsorption behavior. In contrast, high temperature oven drying had a lesser degree of damage to the pore structure, while vacuum freeze drying led to the collapse of pores due to mechanical forces during ice crystal formation and sublimation, reducing the specific surface area to 2.15 m²/g. Additionally, the study comprehensively investigated other mechanisms such as complexation, co-precipitation, kinetics, and theoretical calculations. The practicality of this HC was assessed through seven cycles of adsorption-desorption. This study provides important scientific basis for the application of hydrothermal carbon in heavy metal removal, emphasizes the key role of drying methods, and points to a more energy-saving and environmentally friendly realization path.