Nonthermal air plasma dehydration of hydrochar improves its carbon sequestration potential and dissolved organic...

Labile organic compounds are associated with high environmental risk and are common on hydrochar surfaces. However, a comprehensive re-evaluation of hydrochar properties after the removal of labile compounds has long been overlooked. This study confirms that air-based nonthermal plasma can successfully modify hydrochar properties and change hydrochar's environmental benefits. NMR and FTIR results indicate that, aliphatic and alkyl structures are more reactive, while aromatic structures are highly resistant to the hydrochar modification process, leading to increased carbon sequestration potential and decreased dissolved organic matter (DOM). Van Krevelen diagram results indicate that dehydration controls the hydrochar modification process and leads to a decrease in oxygen content and O/C atomic ratio in the hydrochar; this weakens the ability of the hydrochar to immobilize hydrophobic organic pollutants (such as triclosan) due to the decrease in O‑alkyl C species within the hydrochar. Most importantly, air-based nonthermal plasma changes the structures of hydrochar associated DOM, and high molecular weight (>351 Da), and high degree of unsaturation and oxidation in the modified-hydrochar DOM compounds is observed. This study is therefore considered to have important implications for the carbon cycle and sustainable application of hydrochar.