A simple method for preparing novel biochar with Fe-N bonds and porous structure: Towards phosphate adsorption

In this study, a magnetic iron-nitrogen co-doped adsorbent (Fe/N-KBC) with a high specific surface area was successfully prepared through the simple co-pyrolysis of municipal sludge and modifiers (potassium ferrate, urea, and potassium chloride). The activation effect of K⁺ ions effectively enhanced the pore structure, while co-doping with iron and nitrogen facilitated the formation of Fe-N bonds, thereby improving both the adsorption rate and selectivity for phosphate. It exhibited a magnetic saturation value of 16.37 emu/g, enabling convenient magnetic separation. Even in the presence of coexisting ions, the phosphate removal efficiency remained above 81.62 %. Moreover, the Fe-N bonds significantly suppressed iron leaching, with the maximum Fe ion concentration measured at only 0.019 mg/L, ensuring excellent adsorption stability. Compared to other adsorbents analyzed in this study, Fe/N-KBC demonstrated both the highest adsorption capacity and the fastest adsorption kinetics. The adsorbent maintained full regenerability and could be reused for at least four cycles with minimal performance loss. In a continuous flow column test, Fe/N-KBC effectively treated approximately 590 bed volumes (BV) of phosphate-containing wastewater. The main adsorption mechanism was attributed to Fe-N coordination bonding, accompanied by electrostatic attraction, π-π interactions, hydrogen bonding and pore filling. Hydroponic experiments using mung beans confirmed the low biotoxicity of adsorbents, and the saturated material could be directly repurposed as a phosphate fertilizer. Overall, this work proposes a promising strategy for the simultaneous regulation of adsorption rate and selectivity and offers a facile adsorbent material for efficient phosphorus recovery and resource reutilization.