Photochemical Formation of Hydroxyl Radical from Effluent Organic Matter: Role of Composition

The photochemical formation of hydroxyl radical (HO^•) from effluent organic matter (EfOM) depends upon the chemical properties of this heterogeneous mixture. In this study, two EfOM samples collected from wastewater treatment plants (WWTP A and B) were fractionated by both hydrophobicity (bulk and non-humic) and apparent molecular weight (AMW). The apparent quantum yield for HO^• formation (Φ_HO^•) and the maximum fluorescence quantum yield (Φ_F) were subsequently measured for each subfraction. The formation rates of HO^• (considering only the hydrogen-peroxide-independent pathways) for the bulk waters were 4.8 × 10^–10 and 9.6 × 10^–11 M s^–1 for WWTP A and B, respectively. For the AMW fractions, the values of Φ_HO^• increased as the AMW of the material decreased. For the WWTP A sample, the Φ_HO^• increased from 2.54 × 10^–4 (bulk water) to 6.29 × 10^–4 for the <1 kDa fraction, and for the WWTP B sample, the value of Φ_HO^• increased from 6.50 × 10^–5 for bulk water to 3.45 × 10^–4 for the <1 kDa fraction. In the case of fluorescence, the values of Φ_F ranged from 2.37 × 10^–4 (bulk water) to 3.48 × 10^–4 (<1 kDa fraction) for WWTP A and 3.19 × 10^–4 (bulk water) to 5.75 × 10^–4 (<1 kDa fraction) for WWTP B. There was a linear correlation between Φ_HO^• and Φ_F, suggesting that different photophysical processes occur in the chemical components of the fractions. Understanding the formation of HO^• from EfOM is essential for understanding wastewater-impacted aquatic systems because these results influence the photochemical degradation and mineralization of trace organic contaminants