Effects of pyrolysis temperature on the photooxidation of water-soluble fraction of wheat straw biochar based on 21 ​T FT-ICR mass spectrometry

Biochar, formed through the pyrolysis or burning of organic wastes, has a complex chemical composition influenced by feedstock, pyrolysis temperature, and reaction conditions. Water-soluble, dissolved black carbon species released from biochar comprise one of the most photoreactive organic matter fractions. Photodegradation of these water-soluble species from wheat straw biochar, produced at different pyrolysis temperatures in laboratory microcosms, resulted in noticeable compositional differences. This study characterized water-soluble transformation products formed through the photodegradation of wheat straw biochar pyrolyzed at 300, 400, 500, or 600°C by electrospray ionization 21 ​T Fourier transform ion cyclotron resonance mass spectrometry (21T FT-ICR MS). We also evaluated global trends in the toxicity of these water-soluble fractions using MicroTox™ to assess the impacts of pyrolysis temperature. Additionally, we examined biochar surface morphology after photodegradation and observed minimal change after irradiation for 48 ​h, though the total yield of water-soluble biochar species varied with pyrolysis temperature. Trends in toxicity observed from MicroTox® analysis reveal that water-soluble photoproducts from biochar produced at 300°C and 900°C are nearly three times as toxic compared to dark controls. The ultrahigh resolving power of 21T FT-ICR MS allows for the separation of tens of thousands of highly oxidized, low-molecular-weight (<1 ​kDa) species, showing that photoproducts span a wider range of H/C and O/C ratios compared to their dark analogs. This study highlights the impacts of photodegradation on the molecular composition of water-soluble biochar species and underscores the influence of pyrolysis temperature on the quantity and composition of dissolved organic species.