Chars are ubiquitous in the environment and release significant amounts of redox-active pyrogenic dissolved organic matter (pyDOM). Yet, the redox properties of pyDOM remain poorly characterized. This work provides a systematic assessment of the quantity and redox properties of pyDOM released at circumneutral pH from a total of 14 chars pyrolyzed from wood and grass feedstocks from 200 to 700 °C. The amount of released pyDOM decreased with increasing pyrolysis temperature of chars, reflecting the increasing degree of condensation and decreasing char polarity. Using flow-injection analysis coupled to electrochemical detection, we demonstrated that electron-donating capacities (EDCpyDOM; up to 6.5 mmole-·gC-1) were higher than electron-accepting capacities (EACpyDOM; up to 1.2 mmole-·gC-1) for all pyDOM specimens. The optical properties and low metal contents of the pyDOM implicate phenols and quinones as the major redox-active moieties. Oxidation of a selected pyDOM by the oxidative enzyme laccase resulted in a 1.57 mmole-·gC-1 decrease in EDCpyDOM and a 0.25 mmole-·gC-1 increase in EACpyDOM, demonstrating a largely irreversible oxidation of presumably phenolic moieties. Non-mediated electrochemical reduction of the same pyDOM resulted in a 0.17 mmole-·gC-1 increase in EDCpyDOM and a 0.24 mmole-·gC-1 decrease in EACpyDOM, consistent with the largely reversible reduction of quinone moieties. Our results imply that pyDOM is an important dissolved redox-active phase in the environment and requires consideration in assessing and modeling biogeochemical redox processes and pollutant redox transformations, particularly in char-rich environments.
Keywords: electrochemistry; electron transfer; electron-accepting and -donating capacities; flow-injection analysis; phenols; quinones.