Non-equilibrium adsorption in post-illumination dark-stage tetracycline removal over Ni(OH)2-modified Bi2WO6 photocatalysts

Non-equilibrium adsorption contributes to post-illumination TC removal. • Bi 2 WO 6 /Ni(OH) 2 shows enhanced adsorption and photocatalytic kinetics. • Charge storage is unlikely to dominate the dark-stage behavior. • Acid extraction reveals adsorption-related surface retention of TC. • Light-off timing supports adsorption relaxation after photocatalysis. The photocatalytic removal of antibiotic pollutants has been widely investigated for water purification, where continued pollutant decay after light irradiation (“dark-stage removal”) represents an intriguing post-illumination phenomenon in photocatalytic systems. Although charge storage provides an important explanation for post-illumination activity, adsorption relaxation after light-off may also contribute to dark-stage removal. In this work, a Ni(OH) 2 -modified Bi 2 WO 6 system was employed as a representative model to investigate non-equilibrium adsorption in photocatalytic dark-stage tetracycline removal. The photocatalytic performance under visible light was first evaluated to establish a high-activity reaction system. Subsequently, a combination of acid extraction, solution–surface distribution analysis, and charge probing experiments was used to clarify the contributions of adsorption redistribution and charge storage. The results demonstrate that charge storage is negligible and unlikely to dominate the dark-stage behavior in the Bi 2 WO 6 /Ni(OH) 2 system. Instead, the apparent dark-stage removal is primarily governed by a non-equilibrium adsorption process, arising from the imbalance between rapid photocatalytic consumption and adsorption–desorption dynamics during illumination. After light-off, the system gradually relaxes toward equilibrium, leading to continued redistribution of tetracycline from the solution to the catalyst surface. This work reveals a photocatalysis-induced non-equilibrium adsorption pathway for dark-stage removal, providing a complementary perspective for understanding post-illumination behavior in photocatalytic systems. Download: Download high-res image (181KB) Download: Download full-size image