Evolution of CO₂ during birnessite-induced oxidation of ¹⁴C-labeled catechol

Phenolic compounds undergo several transformation processes in soil and water (i.e., partial degradation, mineralization, and polymerization), many of which have been attributed primarily to biological activity. Resets from previous work indicate that naturally occurring Mn oxides are also capable of oxidizing phenolic compounds. In the present study, ¹⁴C-labeled catechol was reacted with birnessite (manganese oxide) in aqueous suspension at pH 4. The mass of catechol-derived C in solid, solution, and gas phases was quantified as a function of time. Between 5 and 16% of the total catechol C was liberated as CO₂ from oxidation and abiotic ring cleavage under various conditions. Most of the ¹⁴C (55-83%) was incorporated into the solid phase in the form of stable organic reaction products whereas solution phase ¹⁴C concentrations increased from 16 to 39% with a doubling of total catechol added. Polymerization and CO₂ evolution appear to be competitive pathways in the transformation of catechol since their relative importance was strongly dependent on initial birnessite-catechol reaction conditions. Solid phase Fourier transform infrared (FTIR) spectra are consistent with the presence of phenolic, quinone, and aromatic ring cleavage products. Carbon dioxide release appears to be limited by availability of reactive birnessite surface sites and it is diminished in the presence of polymerized reaction products.