Enhancement of Reactive Black 5 adsorption uptake from aqueous solutions using onion skin biochar under near infrared radiation illumination

Reactive Black 5 (RB5), an azo dye, is released in large quantities in the water systems of developing countries, affecting many ecosystems and humans. This study explored converting onion skin, an agricultural waste product, into biochar (onion skin biochars, OSBs) through pyrolysis at 500°C to 700 °C (OSB500–OSB700) under nitrogen flow and comprehensively characterized using Fourier transform infrared spectroscopy, scanning electron microscopy, zeta potential analysis, and Brunauer–Emmett–Teller surface area measurements. Among these, OSB700 exhibited the highest adsorption capacity for RB5 (92.0%), significantly outperforming OSB500 (33.9%) and OSB600 (35.6%). The removal efficiency was optimized under acidic conditions (pH 3) due to favorable zeta potential interactions. When combined with near-infrared radiation (NIR) photothermal heating at 5 W cm 2-1 , OSB700 demonstrated enhanced RB5 adsorption (81.09% removal at 1000 ppm RB5) and a photothermal-induced rise in solution temperature from 27.1°C to 70°C within 7 minutes. The cellular toxicity results indicated that OSB700 possesses a high level of biocompatibility. Even at a concentration of 100 μg mL −1 , OSB700 did not induce cytotoxicity in human vascular endothelial EA.hy926 cells. Moreover, RB5-induced cytotoxicity in EA.hy926 cells (37.5% viability at 1000 ppm) was significantly alleviated to 91.9% viability after treatment with NIR-exposed OSB700. These results suggest that OSBs has considerable potential as a low-cost sorbent for removing RB5 from aqueous phases. When OSB700 was combined with NIR irradiation, it displayed enhanced RB5 adsorption activity and reduced RB5-induced cytotoxicity.