Facile synthesis of Fe-N-C materials from municipal sludge for enhanced Cr(VI) purification: Insight into mechanism and performance

Hexavalent chromium (Cr(VI)) contamination in water poses a significant threat to human health. In this work, Fe and N co-doped biochar (Fe-N-C) was fabricated from municipal sludge using urea as a nitrogen source via the acidification-one-step pyrolysis method. This approach overcame the low adsorption capacity of self-carbonated sludge and achieved high Cr(VI) removal efficiency. Batch experiments and characterization techniques were employed to investigate the synthesis mechanism of Fe-N-C and its Cr(VI) removal mechanisms. At a urea/sludge mass ratio of 0.5, Fe-N-C-0.5 demonstrated superior Cr(VI) adsorption capacity (205.92 mg g⁻¹), primarily governed by spontaneous monolayer chemisorption and intraparticle diffusion. The adsorption process was unaffected by coexisting ions, and it exhibited excellent regeneration stability as well as substantial potential for practical applications. In addition, Fe-N-C-0.5 exhibited high biosafety before and after Cr(VI) removal. Urea doping facilitated nitrogen incorporation and anchored free Fe³⁺, forming the Fe-N-C structure that promoted the transformation of Cr(VI) to Cr(III) through electrostatic adsorption, reduction, and cation-π interactions. These results suggest Fe-N-C is an effective adsorbent for Cr(VI)-contaminated wastewater treatment.