Porous Ca-rich Ca-Mn bimetallic oxides for enhanced phosphate adsorption

Effective removal of phosphate from wastewater is crucial for environmental protection and resource recovery. Calcium (Ca)-based materials (e.g., CaO or Ca(OH) 2 ) show promise for phosphorus capture but suffer from poor stability and a narrow pH application range. Herein, a series of porous Ca-rich Ca Mn bimetallic oxides (CCMO- x ) were synthesized via a solvothermal method with additional Ca incorporation. Their structure and adsorption properties were optimized by varying the Ca/Mn ratios. The compound with the highest Ca/Mn ratio of 5.5 (CCMO-5.5) achieves a maximum phosphate adsorption capacity ( Q ₑ) of 179.8 mg·g −1 , approximately 9 times that of the Ca-poor CCMO-0.5 (19.8 mg·g −1 ). Kinetic and isotherm analyses suggest that phosphate adsorption on CCMO-5.5 involves chemisorption-related surface interactions and monolayer-dominated adsorption, accompanied by multiple mass-transfer processes. Furthermore, CCMO-5.5 exhibits excellent phosphate adsorption across a wide pH range (3–12, Q ₑ = 198–160 mg·g −1 ). Mechanism studies reveal that multiple chemical processes including surface protonation, electrostatic interaction, ligand exchange, inner-sphere complexation reaction, and precipitation occurred during adsorption, leading to the formation of a stable Ca 5 (PO 4 ) 3 (OH) phase. This study demonstrates a viable strategy for constructing Ca-rich Ca Mn oxides as high-performance phosphate adsorbents toward phosphorus removal and recovery from wastewater.