Enhanced phenol adsorption on Fe-N co-doped biochar: Experimental and simulation insights into the formation mechanism

Phenol is a typical toxic and persistent pollutant in wastewater, requiring efficient and recyclable adsorbents for its removal. In this study, Fe–N co-doped biochar was synthesized through four preparation routes combining KOH activation and FeCl 3 -NH 3 modification, and its phenol adsorption behavior and formation mechanism were systematically investigated. The biochar exhibited specific surface areas of 400–900 m 2 /g and N contents of 1.85–3.92 at.%, enabling the formation of Fe 2 O 3 , Fe 3 O 4 , and Fe-N x species. Among the samples, the biochar prepared at 800 °C via activation and soaking–drying achieved the highest phenol adsorption capacity (263.09 mg/g). Adsorption followed the Freundlich isotherm and pseudo-second-order kinetic model, indicating multilayer chemical adsorption. Cycling experiments revealed that Fe 2 O 3 and Fe 3 O 4 were easily detached, whereas Fe-N x remained stable, maintaining 76 % adsorption capacity after five cycles. DFT calculations further confirmed the strong anchoring of Fe ions by N-doped micropores and the thermodynamic stability of Fe-N x structures. This work demonstrates a simple and effective approach for preparing stable Fe-N biochar and provides mechanistic insight into its enhanced phenol adsorption.