Theoretical Framework for Photodegradation Kinetics of Persistent Organic Pollutants in Nigerian Aquatic Environments Using Solar Simulator Studies

Persistent organic pollutants including organochlorine pesticides, polycyclic aromatic hydrocarbons, polybrominated diphenyl ethers, and per- and polyfluoroalkyl substances are accumulating in Nigerian aquatic environments, yet the photodegradation kinetics of these compounds under tropical solar radiation conditions relevant to Nigerian water bodies have not been described through a systematic theoretical framework. This study established an original theoretical kinetic framework for the photodegradation of persistent organic pollutants in aquatic environments under Nigerian tropical solar irradiance conditions, integrating direct photolysis, indirect photolysis via reactive oxygen species, and sensitised photolysis pathways. A solar simulator apparatus calibrated to reproduce the solar spectrum and irradiance characteristic of southern Nigeria was used to conduct controlled photodegradation experiments with twelve target persistent organic pollutants in natural water matrices collected from the Bight of Benin coastal zone and the upper Niger River. Rate constants for direct photolysis, hydroxyl radical reaction, singlet oxygen reaction, and triplet dissolved organic matter reactions were individually determined using selective quenching and actinometry methods. An original kinetic model incorporating the interaction effects of dissolved organic matter composition, pH, temperature, and salinity on indirect photolysis pathways was developed and parameterised using the experimental dataset. Model validation using an independent test dataset confirmed predictive accuracy within a factor of 2 for 89 percent of compounds tested. Benzo[a]pyrene photodegradation in river water was dominated by triplet dissolved organic matter sensitised pathways accounting for 76 percent of transformation, whereas per- and polyfluoroalkyl substances showed negligible photolysis under all tested conditions. This framework provides an original predictive tool for estimating persistent organic pollutant fate in Nigerian aquatic environments under climate-change-modified solar conditions.

Keywords: photodegradation kinetics, persistent organic pollutants, reactive oxygen species, aquatic photochemistry, Nigeria