HULIS Enhancement of Hydroxyl Radical Formation from Fe(II): Kinetics of Fulvic Acid–Fe(II) Complexes in the Presence of Lung Antioxidants

Oxidative stress mediated by reactive oxygen species (ROS) is a hypothesized mechanism for particulate-matter related health effects. Fe­(II) is a key player in ROS formation in surrogate lung fluid (SLF) containing antioxidants. Humic-like substances (HULIS) in particulate matter such as biomass burning aerosol chelate Fe­(II), but the effect on ROS formation in the presence of lung antioxidants is not known. We use Suwanee River Fulvic Acid (SRFA) as a surrogate for HULIS and investigate its effect on OH formation from Fe­(II). For the first time, a chemical kinetics model was developed to explain behavior of Fe­(II) and SRFA in SLF. Model and experimental results are used to find best-fit rate coefficients for key reactions. Modeling results indicate SRFA enhances Fe-mediated reduction of O₂ to O₂^– and destruction of H₂O₂ to OH to 5.1 ± 1.5 and (4.3 ± 1.4) × 10³ M^–1 s^–1 respectively. Best-fit rates for Citrate–Fe­(II) mediated O₂ to O₂^– and H₂O₂ to OH were 3.0 ± 0.7 and (4.2 ± 1.7) × 10³ M^–1 s^–1 respectively. The kinetics model agrees with both the experimental results and thermodynamic model calculations of chemical speciation for 0 and 5 μg/mL SRFA, but both models are less successful at predicting further enhancements to OH formation at higher SRFA Concentrations.