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

Oxidized hydrothermal biochar was prepared by hydrothermal carbonization of Spartina alterniflora biomass (240°C for 4h) and subsequent oxidization (240°C for 10min) under air. Oxidized hydrochar achieved a Fe(III) reducing capacity of 2.15mmol/g at pH 2.0 with 120h, 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 Fe(3+) reduction. A Fenton-like process was established with Fe(2+) replaced by oxidized hydrochar and tested for methylene blue (MB) decoloration. Oxidized hydrochar achieved a MB decolorization (200mg/L, pH 7.0) rate of 99.21% within 3h 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.