Synergistic remediation of aqueous Cd(ii) by sewage sludge biochar via P/Fe co-impregnation

Converting municipal sewage sludge into high-efficiency adsorbents represents a sustainable strategy for cadmium [Cd(ii)] remediation in acid mine drainage (AMD) and for solid-waste valorization. A novel phosphorus/iron co-modified sludge biochar (P-Fe@SBC) was synthesized via a combined FeCl₃-KH₂PO₄ impregnation and pyrolysis route. Modification improved the microstructure. The specific surface area of P-Fe@SBC increased to 137.915 m² g^-1, 7.4 times that of pristine biochar. Adsorption tests demonstrated outstanding Cd(ii) removal. Adsorption conformed to the Langmuir isotherm model and the pseudo-second-order kinetic model. The maximum removal capacity reached 328.95 mg g^-1, markedly exceeding that of singly modified biochars and pristine biochar. High selectivity was observed under complex ionic matrices (K⁺, Ca²⁺, Mg²⁺). Approximately 90% of the removal amount remained after five adsorption-desorption cycle, indicating high stability and strong regeneration potential. Mechanistic analyses indicated a synergistic removal network involving electrostatic attraction, chemical precipitation, inner-sphere surface complexation, cation-π interaction, and ion exchange. P-Fe@SBC represented a promising waste-derived material for "waste-to-treat-waste" remediation.