Boosting the adsorption activity of single-atom iron on biochar via oxygen-coordination engineering: the critical role of π-complexation.

While metal-loaded biochar shows promise for pollutant adsorption, the regulatory role of O-coordination on the intrinsic activity of metal sites remains poorly understood. Herein, DFT and experiments were combined to investigate the adsorption of bisphenol A (BPA) on O-coordinated Fe single-atom biochar. It was determined that increasing O-coordination number at Fe sites altered the adsorption mechanism, with 3- and 4-fold O-coordination markedly strengthening adsorption and favoring π-complexation. Guided by this, an Fe single-atom biochar (Fe-O/BC) with an ultrahigh specific surface area of 1676 m2·g-1 was synthesized, and its O-coordination number was characterized approximately 3.7. Notably, Fe-O/BC exhibited a remarkable BPA adsorption capacity of 683.3 mg·g-1, which was approximately 70 % higher than that of the metal-free control (O/BC) and exceeded that of most reported biochar-based adsorbents. Moreover, Fe-O/BC maintained high efficiency over a wide pH range (3-9) and exhibited good tolerance to cations, anions, and real water matrices, indicating its strong adaptability to environmental conditions. Pore filling and π-complexation were identified as the main adsorption mechanisms. Consequently, Fe-O/BC exhibited excellent adsorption performance toward most other aromatic organics. In-depth analysis of electron transfer indicated that O-coordination promoted d-π* back-donation by acting as an electron donor or "electron bridge", thereby enhancing the π-complexation activity of the Fe sites. This work provides new insights into how O-coordination regulates the adsorption activity of Fe sites and enables the development of advanced biochar-based adsorbents for pollutant control.