Photometric hydroxyl radical scavenging analysis of standard natural organic matter isolates

Abstract

Hydroxyl radical (˙OH) scavenging reaction rate constants of standard natural organic matter (NOM) isolates (k_˙OH,NOM) were measured with a rapid background scavenging method, to expand the dataset of published k_˙OH,NOM values. The proposed method relies on ˙OH generation with a simple UV/H₂O₂ AOP-based system. The associated decay of a ˙OH probe compound is monitored with a field-deployable spectrophotometer and k_˙OH,NOM is determined through competition kinetics. The resulting k_˙OH,NOM values for the six NOM standard isolates ranged from 1.02 (±0.10) × 10⁸ M_C⁻¹ s⁻¹ for Suwannee River Fulvic Acid I Standard to 2.03 (±0.12) × 10⁸ M_C⁻¹ s⁻¹ for Pony Lake Fulvic Acid Reference NOM, which is within the range reported with more elaborate and time-consuming k_˙OH,NOM methods. A slight correlation between nitrogen content and scavenging rate constant was evident while no significant correlation between k_˙OH,NOM and atomic composition, carbon structure, weight-average molecular weight, UV absorbance (SUVA₂₅₄), or fluorescence index (FI) was observed. Overall, the results demonstrate that k_˙OH,NOM can be rapidly assessed in NOM isolate samples. The results suggest that this type of rapid field-deployable spectrophotometric method may minimize the need for expensive and time-consuming background scavenging methods, and for models that predict k_˙OH,NOM based on other NOM characteristics.