Synthesis of an eco-friendly calcium-rich biochar: Unveiling the effect of pyrolysis temperature on its properties, phosphorus recovery and involved mechanisms

Calcium-modified biochars have attracted considerable attention as promising materials for phosphorus (P) recovery from aqueous effluents. In the current study, marble waste was used as a sustainable alternative to conventional calcium reagents (e.g., CaCl 2 ) and co-pyrolyzed with date palm wastes and poultry manure at 700°C and 900°C to produce two Ca-rich biochars, designated as B-700 and B-900. Physico-chemical characterization revealed that increasing the pyrolysis temperature promoted the extensive formation and deposition of calcium-based nanoparticles on the biochar matrix, resulting in significant improvements in structural, textural and surface chemistry properties. For example, the BET surface area and total pore volume of B-900 were 14.4 and 2.5 times greater, respectively, than those of B-700. Consequently, B-900 exhibited excellent P recovery over a wide experimental conditions, achieving a maximum adsorption ability of 146.1 mg g -1 , nearly twice that of B-700 (74.5 mg g -1 ). Comprehensive characterization before and after P recovery demonstrated that P immobilization by B-900 occurred through surface complexation with O-H groups, and predominately, through precipitation as hydroxyapatite (Ca 5 (PO 4 ) 3 OH). In contrast, the lower abundance of calcium oxides on the surface of B-700, favored the formation of brushite (CaHPO 4 ). Besides complexation electrostatic attraction and hydrogen bonding also contributed to P recovery by B-700. The obtained results demonstrated that the co-pyrolysis of organic feedstocks mixed with marble wastes, particularly at temperatures around 900°C, provides an effective route for producing environmentally-friendly calcium-rich biochars with excellent P recovery performance. The resulting P-loaded biochars can be directly valorized in agriculture as slow-release organic fertilizer, highlighting the potential of this approach to integrate wastewater treatment, nutrient recovery, waste valorization, and circular economy principles.