Phosphorus recovery from simulated anaerobic fermentation supernatant through vivianite crystallization: the role of aluminium and its implications for the synthesis of LiFePO4

Recovering phosphorus (P) from wastewater benefits both the environment and the field of clean energy. Vivianite is a key precursor for clean energy lithium batteries, making it a highly valuable and promising product in wastewater P recovery. As a widely used cathode material in lithium-ion batteries, LiFePO 4 is valued for its excellent thermal stability, safety, and long cycle life, making it a promising material for sustainable energy applications. However, the large amount of aluminium (Al) present in the recirculated sludge can be released into the supernatant during the anaerobic digestion process, which affects the recovery of vivianite and further impacts the performance of LiFePO 4 . This study investigated the effect of Al on vivianite crystallization throughout the P recovery process and explored the direct synthesis of LiFePO 4 using Al-containing vivianite. Varying initial Al concentrations were examined to understand their influence on the crystallization kinetics, purity, and morphology of vivianite. At low Al concentrations (1–50 mg/L), Al formed amorphous aluminium phosphate that attached to vivianite crystals via electrostatic interactions and surface adsorption. At high Al concentrations (100–300 mg/L), Al predominantly entered the solid phase through co-precipitation, significantly inhibiting vivianite formation. LiFePO 4 /C cathode materials were synthesized using the recovered Al-containing vivianite. The results revealed that Al existed as inert substances within LiFePO 4 /C, reducing its crystallinity and specific discharge capacity. These findings elucidate how Al affects vivianite crystallization during P recovery and provide theoretical insights for converting Al-containing vivianite into value-added LiFePO 4 .