Fast field cycling NMR relaxometry characterization of biochars obtained from an industrial thermochemical process

Biochar has unique properties which make it a powerful tool to increase soil fertility and to contribute to the decrease of the amount of atmospheric carbon dioxide through the mechanisms of C sequestration in soils. Chemical and physical biochar characteristics depend upon the technique used for its production and the biomass nature. For this reason, biochar characterization is very important in order to address its use either for agricultural or environmental purposes. Three different biochars obtained from an industrial gasification process were selected in order to establish their chemical and physical peculiarities for a possible use in agronomical practices. They were obtained by charring residues from the wine-making industry (marc) and from poplar and conifer forests. Routine analyses such as pH measurements, elemental composition, and ash and metal contents were performed together with the evaluation of the cross-polarization magic angle spinning (CPMAS) C-13 NMR spectra of all the biochar samples. Finally, relaxometry properties of water-saturated biochars were retrieved in order to obtain information on pore size distribution. All the biochars revealed basic pH values due to their large content of alkaline metals. The quality of CPMAS C-13 NMR spectra, which showed the typical signal pattern for charred systems, was not affected by the presence of paramagnetic centers. Although paramagnetism was negligible for the acquisition of solid state spectra, it was effective in some of the relaxometry experiments. For this reason, no useful information could be retrieved about water dynamics in marc char. Conversely, both relaxograms and nuclear magnetic resonance dispersion profiles of poplar and conifer chars indicated that poplar char is richer in small-sized pores, while larger pores appear to be characteristic for the conifer char. This study showed the potential of relaxometry in revealing chemical-physical information on industrially produced biochar. This knowledge is of paramount importance to properly direct biochar agronomical uses.