Iron–Magnesium Bimetallic Biochar Composites Derived from Conocarpus Waste for Enhanced Phosphate Removal: Insights into the Underlying Adsorption Mechanisms in Aqueous Solution

Phosphate enrichment of surface waters is a major cause of eutrophication, creating demand for low-cost materials that support phosphorus removal and recovery. In this study, Conocarpus waste biochar (CBC) was modified with iron (Fe-CBC), magnesium (Mg-CBC), and both metals (FeMg-CBC) to improve phosphate adsorption. FTIR, XRD, SEM, and elemental analyses confirmed clear changes in surface chemistry and mineral composition after modification. FeMg-CBC showed the strongest overall performance, with higher surface area, faster uptake, and greater adsorption capacity than the mono-metallic materials. Phosphate removal was strongly influenced by pH, adsorbent dosage, and contact time, with optimum performance near pH 6. Adsorption kinetics followed the pseudo-second-order model (R2 = 0.997), while equilibrium data were best described by the Langmuir–Freundlich model. FeMg-CBC achieved Langmuir and Sips capacities of 86.2 and 106.5 mg g−1, respectively. Its enhanced performance was attributed to combined electrostatic attraction, ligand exchange, Mg-phosphate precipitation, and possible interlayer anion exchange within the Fe-Mg layered phase. These findings highlight its potential for wastewater treatment and nutrient recycling.