Advanced textural characterization of biogenic silica by nitrogen physisorption, positron annihilation lifetime spectroscopy and hyperpolarized 129Xe NMR spectroscopy

This work addresses the advanced textural characterization of biogenic silica obtained from different Si-accumulating biomasses and agricultural waste products like rice, oat and spelt husk as well as horsetail. These materials exhibit a non-uniform, disordered pore structure in a broad size range deviating from well-ordered “artificial” species of porous materials. For the first time in literature, this contribution determines and validates mainly the mesoporosity by means of nitrogen physisorption, positron annihilation lifetime spectroscopy (PALS) and hyperpolarized 129Xenon NMR (HP-129Xe NMR). Nitrogen physisorption and HP 129Xe NMR both were able to describe the pore structure in the mesopore range. In addition, the specific surface area and the mesopore volume were determined by nitrogen physisorption. PALS complemented the results by an assessment of microporosity in biogenic silica. It was shown that biogenic silica contains micropores (1.5 nm–1.9 nm), mesopores in a broad range and indications of macroporosity. In addition, the results led to the assumption that different plant origins and plant species evoke different textural properties and inter-connections between the pores in biogenic silica. This contribution demonstrates that only a combination of all techniques is viable to characterize the non-uniform and complex pore structure of biogenic silica of different origin.