Photochemical production of hydroxyl radical from algal organic matter

Photochemical production of hydroxyl radical (·OH) from algal organic matter (AOM) collected from Lake Torrens in South Australia was examined using a sunlight simulator. The two AOM isolates featured lower molecular weight, lower chromophoric content, and lower SUVA₂₅₄ (0.7 and 0.9, L mgC^-1 m^-1) than the reference Suwannee River hydrophobic acid (SR-HPO), they had considerably higher apparent quantum yields (ϕ_NOM^OH, 3.03 × 10^-5 and 2.18 × 10^-5) than SR-HPO (0.84 × 10^-5). Fluorescence excitation-emission matrix (FEEM) showed that the major components in the AOM were aromatic protein-like and soluble microbial substances. Unique formulas of the two AOM isolates as compared to SR-HPO were revealed using FTICR-MS and classified into four areas, namely protein-like molecules with low O/C (H/C > 1.5, O/C: 0.2-0.4), lignin-derived moieties with low O/C (H/C:1.0-1.5, O/C: 0.1-0.3), protein-like molecules with high O/C (H/C > 1.5, O/C: 0.5-0.7), and carbohydrate derivatives (H/C > 1.5, O/C > 0.7). These unique AOM moieties characterised utilizing FEEM and FTICR-MS were tentatively postulated to contribute to the high ϕ_NOM^OH. To the best of our knowledge, this is the first study performed to both evaluate natural AOM as an efficient photosensitiser of ·OH and propose AOM moieties responsible for the high ϕ_NOM^OH.