iron-modified biochar/tio₂ advanced oxidation process for enhanced degradation of norfloxacin and estradiol in water: efficiency optimization and ecotoxicity evolution of transformation products

This study aims to construct an advanced oxidation system using photocatalytically modified biochar to remove pharmaceutical residues (norfloxacin and estradiol) from water and evaluate the ecotoxicity of their degradation products. biochar was prepared from rice husks and modified through iron doping. an advanced oxidation system was established by combining the modified biochar with p25 tio₂ photocatalyst. degradation experiments were conducted in a 500 ml quartz reactor under mercury lamp illumination to investigate the effects of various reaction conditions (e.g., catalyst dosage, ph value) on pharmaceutical removal efficiency. additionally, the formation and toxicity of degradation products were assessed using lc-ms/ms and toxicity tests such as microtox®, chlorella growth inhibition rate, and acute toxicity to zebrafish. the degradation rate of the modified biochar group reached 89.73% within 60 minutes, significantly higher than 45.67% in the unmodified group (p < 0.001). photocatalytically modified biochar increased the surface reaction rate constant klh to 2.6661 min⁻¹. toxicity assessments showed that the ec50 value decreased initially during degradation and then gradually increased, indicating an initial rise in toxicity followed by a reduction (p < 0.05). the growth inhibition rate of chlorella increased from 15.32% to 47.89% over 12 to 48 hours (p < 0.001), demonstrating the ecotoxicity of the degradation products to algae. the lc50 value for zebrafish decreased continuously, suggesting that some degradation intermediates have cumulative toxicity to fish. photocatalytically modified biochar effectively removes pharmaceutical residues from water and significantly reduces the toxicity of most degradation products. however, the potential ecological risks posed by certain intermediate products require further control and optimization.