Solar-activated persulfate removal of bisphenol S from water: Kinetics, mechanism, transformation products, toxicity...

Bisphenols are often detected in environmental samples and are implicated as endocrine-disrupting compounds. This study examines the photodegradation of bisphenol S (BPS), a common replacement of bisphenol A, using a solar-activated persulfate (solar/PS) system at batch and pilot scale. The photodegradation of BPS followed pseudo-first-order decay; the observed kinetic constant increased with oxidant concentration up to 0.19 min^-1 at 500 μg L^-1 and decreased as the initial BPS concentration increased (0.0091 min^-1 at 3 mg L^-1). The system's efficiency was approximately three times lower under alkaline conditions, while the apparent kinetic constant significantly decreased in secondary effluent. Interestingly, complete BPS removal was achieved in the pilot reactor under natural sunlight at approximately 20 kJ L^-1. Based on scavenging experiments and electron paramagnetic resonance (EPR) spectroscopy, the degradation was driven by the synergistic action of sulfate radicals (SO₄^•-), singlet oxygen (¹O₂), and hydroxyl radicals (HO^•). Five transformation products (TPs) were identified by LC/TOF-MS, and a plausible pathway is proposed. According to the ECOSAR model, some TPs exhibited higher predicted toxicity than the parent compound for certain indicators, suggesting the need for longer treatment times or more intensive conditions. Seed-based assays showed inhibition by BPS alone, while solar/PS treatment generally attenuated the toxicity of the mixture of BPS and TPs. Nevertheless, residual, time-dependent inhibition persisted in some species, underscoring species-specific risks and the need for multi-bioassay evaluation before reuse.