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

Abstract

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.