Improving abiotic reducing ability of hydrothermal biochar by low temperature oxidation under air

• 240 °C oxidization under air improving hydrochar reducing ability towards Fe(III).
• Carbonyl group supposed to be responsible for Fe3+ transformation to Fe2+.
• Reducing ability be easily restored by further air-oxidization at 240 °C.
• Oxidized hydrochar + H2O2 show potential on organic degradation.

Oxidized hydrothermal biochar was prepared by hydrothermal carbonization of Spartina alterniflora biomass (240 °C for 4 h) and subsequent oxidization (240 °C for 10 min) under air. Oxidized hydrochar achieved a Fe(III) reducing capacity of 2.15 mmol/g at pH 2.0 with 120 h, which is 1.2 times higher than un-oxidized hydrochar. Low temperature oxidization increases the contents of carboxyl and carbonyl groups on hydrochar surface. It is supposed that carboxyl groups provide bonding sites for soluble Fe species and carbonyl groups are responsible for Fe3+ reduction. A Fenton-like process was established with Fe2+ replaced by oxidized hydrochar and tested for methylene blue (MB) decoloration. Oxidized hydrochar achieved a MB decolorization (200 mg/L, pH 7.0) rate of 99.21% within 3 h and demonstrates prominent prevail over H2O2 absent control test. This study reveals low temperature oxidization is an effective way to improve and restore abiotic reducing ability of hydrochar.