Chars are ubiquitous in soil as a result of historical fires and have also attracted interest as soil additives in agricultural and environmental management (“biochar”). The pore and surface properties of chars depend on the precursor material and pyrolysis conditions. A potentially critical condition is the ambient O2 concentration. This study was conducted to understand the effects of thermal air oxidation of biomass chars experienced during formation or production on their adsorptive properties towards various compounds. Post-pyrolysis air oxidation (PPAO) of anoxically-prepared wood and pecan shell chars at 400 oC for a few minutes enhanced the mass-normalized adsorption at pH ~7.4 of all test compounds, especially the weak acids and bases. Both general and specific effects were identified. The general effect results from ‘reaming’ of pores by the oxidative removal of pore wall matter and/or tarry pyrolysis deposits, enlarging pores and creating new surface area. The specific effect results from creation of new acidic functionality that provides sites for formation of very strong, charge-assisted hydrogen bonds (CAHB) with solutes having comparable pKa. The CAHB hypothesis was supported by competition experiments and the finding that weak acid anion adsorption increased with surface carboxyl content, despite electrostatic repulsion from the growing negative charge. The results provide insight into the effects of air oxidation on pollutant retention.

Thermal air oxidation during biomass char formation and its effects on adsorption of organic compounds

253rd National Meeting (2017)

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