Efficient recovery of phosphate from simulated urine by Mg/Fe bimetallic oxide modified biochar as a potential resource

The massive use of phosphate fertilizers in agriculture is costly and induces water pollution, calling for more sustainable phosphate sources in the context of the circular economy. Here we prepared a new adsorbent based on waste straw biochar modified with the Mg/Fe bimetallic oxide, namely the Mg/Fe biochar, to recover phosphate from the simulated urine as an possible phosphate fertilizer. About 90% phosphate was recovered from the simulated urine with a wide pH range of 3.0–9.0 and a maximum adsorption capacity of 206.2 mg/g, using 1 g/L of Mg/Fe modified biochar. A pseudo second-order kinetics and Sips model were proposed to fit the experimental data well, suggesting that the adsorption was controlled by physical and chemical processes, which is driven by electrostatic attraction, intra-particle diffusion, ion exchange and surface ligand exchange. Overall, the Mg/Fe biochar is renewable and can recover more than 70% of phosphate in the simulated urine after 5 cycles of reuse, which appears as a safe and efficient adsorbent to recycle phosphate from urine.