Struvite Biomineralization and Phosphorus Recovery by Immobilized Bacillus velezensis Using Seawater as the Magnesium Source

Seawater is rich in Mg2+ and can serve as an alternative magnesium source for struvite biomineralization, facilitating phosphorus recovery from aqueous systems. However, the complex ionic conditions can limit the stable metabolism and reuse of free microorganisms. In this study, Bacillus velezensis LB002 was immobilized to establish a reusable struvite biomineralization system using seawater as the magnesium source. Its mineralization performance, reusability, and the resource utilization potential of the recovered mineral were evaluated. Immobilized LB002 continuously metabolized nitrogen-containing organic compounds, promoted NH4+ accumulation and alkalization, and induced the formation of struvite. In the initial cycle, total phosphorus (TP) removal, phosphorus recovery, and struvite yield reached 98.26%, 54.72%, and 1.43 g/L, respectively. After three consecutive reuse cycles, these values remained at 95.84%, 50.06%, and 1.31 g/L, indicating stable biomineralization performance during repeated operation. Partial breakage of the immobilized bacterial microspheres occurred after the third cycle, while mineralization efficiency remained high, suggesting that carrier mechanical stability may limit longer-term operation. The recovered struvite exhibited a mesoporous structure with a specific surface area of 168.71 m2/g and a maximum Cd2+ adsorption capacity of 446.43 mg/g. Overall, the results demonstrate the feasibility of repeated phosphorus mineralization and recovery using reusable immobilized bacterial microspheres and seawater as the magnesium source under controlled laboratory conditions.