Highly efficient removal of aqueous phosphate via iron-manganese fabricated biochar: Performance and mechanism

Biochar (BC) has emerged as a potential solution to phosphate removal from wastewater primarily resulting from global overuse of fertilizers. Further modification by embedment of iron (Fe)-manganese (Mn) oxides on BC can enhance phosphate removal; however, the modification method serves as a vital factor underlying distinctive removal performances and mechanisms, which have yet been systematically examined. Herein, two Fe-Mn modified BC, Fe/MnBC (comprised of Fe₃O₄ and MnO₂) and Fe-MnBC (comprised of MnFe₂O₄), were comprehensively investigated for gaining insights into the unsolved perspectives. The results indicated that Fe-MnBC exhibited a markedly greater maximum phosphate adsorption capacity of 135.88 mg g^-1 than that of Fe/MnBC with 17.93 mg g^-1. The comparative results based on microstructure and spectroscopic analyses suggested that different Fe and Mn oxides were successfully loaded, which played a distinctive role in phosphate removal. Further characterizations unveiled that the key mechanisms for phosphate removal by Fe/MnBC are inner-sphere complexation and precipitation, while electrostatic interaction and outer-sphere complexation are the dominant mechanisms underlying the notable performance of Fe-MnBC. The delicately designed Fe-MnBC with special structure and property also enabled a superior regeneration capacity, which presented a promisingly high phosphate removal efficacy of over 81.34% after five cycles. These results enhance comprehension regarding the impact of biochar modification techniques on phosphate removal, offering positive indications for the remediation of excessive phosphate and other pollutant-containing water through feasible design and green chemicals.