One-Step Fe-Doped Cyanobacterial Biochar: 4-Nitrophenol Reduction Boosted by Synergistic Adsorption and Redox Cycling

Nitrophenols are toxic and recalcitrant organic pollutants, posing a major challenge for wastewater treatment. Herein, a Fe-doped cyanobacterial biochar catalyst (Fe 3 /C 1 -800) was fabricated via a one-step impregnation pyrolysis strategy using lake-harvested cyanobacterial biomass as the carbon matrix and FeCl 3 ∙6H 2 O as both the in-situ activator and Fe source realizing the 'waste-to-treat-waste' concept. When applied to the catalytic reduction of 4-nitrophenol (4-NP), the Fe 3 /C 1 -800/NaBH 4 system achieved nearly complete reduction of 4-NP within 5   min, with a high turnover frequency (TOF) of 0.67   mmol·g -1 ·min -1 , surpassing most reported transition-metal catalysts and approaching noble-metal activity. The catalyst exhibited high efficiency over a wide pH range (3-11) and temperature range (15-45 ℃), resisted interference from Cl - and NO 3 - , and retain 93.7% of its activity after four consecutive cycles. In real water matrices (Lake Taihu water and secondary effluent from a sewage treatment plant), the 4-NP removal efficiency reached 95.4% and 88.1%, respectively. Mechanistic investigations revealed a synergistic effect between the catalyst’s large specific surface area (802.16 m 2 ·g -1 ) for 4-NP adsorption and the Fe 2+ /Fe 3+ redox cycle for accelerated electron transfer. This synergy promotes BH₄⁻ dissociation to generate active hydrogen species, enabling the selective reduction of 4-NP to 4-aminophenol (4-AP) with 97% selectivity. Furthermore, Fe 3 /C 1 -800 displayed broad-spectrum catalytic activity toward diverse nitrophenol contaminants (e.g., o -nitrophenol, p -nitrotoluene) with conversion rates >90%. This work not only offers a low-cost, eco-friendly pathway for cyanobacterial valorization but also offers a novel design principle of high-performance biochar-based catalysts, advancing the development of sustainable reduction systems for refractory organic wastewater.