Mechanisms of Water Interaction with Pore Systems of Hydrochar and Pyrochar from Poplar Forestry Waste

The aim of this study was to understand the water−surface interactions of two chars obtained by gasification (pyrochar) and hydrothermal carbonization (hydrochar) of a poplar biomass. The two samples revealed different chemical compositions as evidenced by solid state 13 C NMR spectroscopy. In fact, hydrochar resulted in a lignin-like material still containing oxygenated functionalities. Pyrochar was a polyaromatic system in which no heteronuclei were detected. After saturation with water, hydrochar and pyrochar were analyzed by fastfield cycling (FFC) NMR relaxometry. Results showed that water movement in hydrochar was mainly confined in very small pores. Conversely, water movement in pyrochar led to the conclusion that a larger number of transitional and very large pores were present. These results were confirmed by porosity evaluation derived from gas adsorption. Variable-temperature FFC NMR experiments confirmed a slow-motion regime due to a preferential diffusion of water on the solid surface. Conversely, the higher number of large pores in pyrochar allowed slow movement only up to 50°C. As the temperature was raised to 80°C, water interactions with the pore surface became weaker, thereby allowing a three-dimensional water exchange with the bulk liquid. This paper has shown that pore size distribution was more important than chemical composition in affecting water movement in two chemically different charred systems.