Physicochemical Changes in Biomass Chars by Thermal Oxidation or Ambient Weathering and Their Impacts on Sorption of a Hydrophobic and a Cationic Compound

This study examined conditions that mimic oxidative processes of biomass chars during formation and weathering in the environment. A maple char prepared at the single heat treatment temperature of 500 degrees C for 2 h was exposed to different thermal oxidation conditions or accelerated oxidative aging conditions prior to sorption of naphthalene or the dication paraquat. Strong chemical oxidation (SCO) was included for comparison. Thermal oxidation caused micropore reaming, with ambient oxidation and SCO much less so. All oxidative treatments incorporated O, acidity, and cation exchange capacity (CEC). Thermal incorporation of O was a function of headspace O-2 concentration and reached a maximum at 350 degrees C due to the opposing process of burn-off. The CEC was linearly correlated with O/C, but the positive intercept together with nuclear magnetic resonance data signifies that, compared to O groups derived by anoxic pyrolysis, O acquired through oxidation by thermal or ambient routes contributes more to the CEC. Thermal oxidation increased the naphthalene sorption coefficient, the characteristic energy of sorption, and the uptake rate due to pore reaming. By contrast, ambient oxidation (and SCO) suppressed naphthalene sorption by creating a more hydrophilic surface. Paraquat sorption capacity was predicted by an equation that includes a CEC2 term due to bidentate interaction with pairs of charges, predominating over monodentate interaction, plus a term for the capacity of naphthalene as a reference representing nonspecific driving forces.

Automated summary

This study examined the oxidative processes of biomass chars during formation and weathering, finding that different oxidative treatments induced varying effects on properties, such as thermal oxidation increasing the sorption coefficient of naphthalene, while ambient oxidation hindered its sorption.