The photosensitivity of EOM from diatom and its mediation of the photodegradation of

Algal extracellular organic matter (EOM) can promote the photodegradation of p- phenylenediamines (PPDs), yet its effects exhibit highly compound-specific. To investigate this specificity, this study examined the role of diatom’s EOM in the photodegradation of three representative PPDs, including N-(1,3-dimethylbutyl)-N’-phenyl-p-phenylenediamine (6PPD), N-isopropyl-N’-phenyl-p- phenylenediamine (IPPD), and N,N’-diphenyl-p-phenylenediamine (DPPD). Results indicated that EOM exhibited photosensitising enhancement for IPPD and 6PPD degradation. IPPD showed the most pronounced response to EOM, with its rate constant increasing from 0.00233 min-1 to 0.00345 min-1, representing a 48.1% increase. The rate constant for 6PPD increased from 0.00437 min-1 to 0.00502 min-1, corresponding to a 14.9% enhancement. However, the degradation of DPPD was inhibited, with its rate constant decreasing from 9.89 × 10-4 min-1 to 7.12 × 10-4 min-1, with a reduction of 28.0%. This inhibition is attributed to the fully aromatic structure of DPPD, which may facilitate a photoprotective interaction with EOM. These findings indicate that the direction of EOM’s effect (promotion or inhibition of photodegradation) is determined by the molecular structure of the specific PPD. This study revealed the dual role of diatom EOM in the photochemical fate of PPDs, providing new insights into their environmental behaviour in aquatic system.