Modifier and pyrolysis optimization of algae-derived biochar enhances sulfamethoxazole removal via microalgae-biochar hybrids

• Microalgae-biochar hybrid system was established for efficient antibiotic removal. • BCFe-700 was optimum with positive charge, Fe crystallization and enhanced porosity. • SMX removal achieved 67.3 % by the hybrid system, surpassing microalgal treatments. • Physical mechanism of SMX removal was clarified. Antibiotic pollution poses severe environmental and health risks due to its persistence and mobility in aquatic systems. This study developed a novel microalgae-biochar hybrid system for enhanced sulfamethoxazole (SMX) removal by systematically investigating the synthesis of algae-derived biochar through a combination of different pyrolysis temperatures (300 °C, 500 °C, 700 °C) and chemical modifiers (KOH, NaH 2 PO 4 , FeCl 3 ·6H 2 O, CaCl 2 ). Results demonstrated that FeCl 3 ·6H 2 O-modified biochar pyrolyzed at 700 °C exhibited optimal properties: a positively charged surface (zeta potential: +6.34 mV), iron oxide crystallization, a looser structure, and enhanced porosity. Although microalgae growth was the best supported by CaCl 2 -modified biochar at 300 °C (biomass: 0.14 g/L), the hybrid system with FeCl 3 ·6H 2 O-modified biochar at 700 °C achieved the highest SMX removal efficiency of 67.32 %, significantly outperforming other biochars (9.53–43.79 %). Overall, the microalgae-biochar system facilitated SMX elimination through bio-material coupling effects. This work establishes FeCl 3 ·6H 2 O modification and high-temperature pyrolysis as key strategies for designing efficient microalgae-biochar hybrids toward sustainable antibiotic wastewater remediation and resource recovery.