Solid-State NMR Study of Hydrochars Produced from Hydrothermal Carbonization of Poultry Litter

Abstract

Poultry litter (PL) hydrochars obtained at different temperatures and charring times were characterized by solid-state ¹H, ¹³C and ³¹P nuclear magnetic resonance (NMR) spectroscopy. ¹³C NMR spectra obtained with cross polarization (CP) and magic-angle spinning evidenced the chemical and structural changes suffered by PL during its transformation into hydrochar; these changes were particularly dependent on the production temperature rather than the residence time. The hydrochars were essentially composed of aromatic and alkyl domains at the temperature of 250 °C. ³¹P NMR observations were conducted using single-pulse excitation (SPE) and CP sequences to distinguish between phosphorus far from protons and protonated phosphate species. Results showed that water-soluble phosphorus was the only form detected in hydrochars through the CP sequence. In contrast, the stable phosphorus species formed during hydrothermal carbonization (HTC) exhibited broad signals, detected exclusively using the SPE sequence. This indicates that unprotonated orthophosphates were the dominant form. These NMR results offer a deeper understanding of hydrochar formation from PL, shedding light on the chemical and structural changes caused by the HTC process at the atomic scale.