Addressing the dual challenges of waste disposal and the phosphorus resource crisis, the development of cost-effective and sustainable adsorbents for phosphorus recovery through the utilization of concentrated seawater and agricultural waste is of great significance. Herein, an effective strategy was presented for synthesizing Ca/Mg biochar composites by integrating concentrated seawater and agricultural waste through a co-precipitation and low temperature carbonization (400 °C), aimed at phosphorus recovery and release. The introduction of 3.0 mol/L KOH during preparation increased the specific surface area, optimized the pore structure, and promoted the co-precipitation of Ca/Mg from concentrated seawater into the biomass framework, providing more active sites. Characterization results confirmed the mesoporous structure and the presence of surface-loaded metal species in all three Ca/Mg-modified biochar composites. The adsorption of phosphorus by the Ca/Mg biochar composites produced from three types of agricultural waste was well-described by the pseudo-second-order kinetic and Langmuir isotherm models, recording a maximum adsorption capacity of 138.192 mg/g. This suggested that the biochar preparation strategy possessed considerable universality, yielding biochar with consistent physicochemical properties and phosphorus adsorption performance. The characterization and analysis further reveal that the primary adsorption mechanisms include electrostatic attraction, chemical precipitation, and ligand exchange. Significantly, excessive Ca/Mg in individual biochar composites group (CBC-K) inhibited Mung beans germination and growth, whereas phosphorus-saturated Ca/Mg biochar composites group (P-CBC-K) still promoted crops growth due to the formation of calcium- and magnesium-containing phosphorus compounds, resulting in a 2.64-fold increase in fresh biomass compared to CBC-K group. This study enables the simultaneous phosphorus removal and recovery, along with the harmless treatment of concentrated seawater and agricultural waste, thus achieving a multi-fold resource utilization.
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