From human urine to vanadium recovery: Ammonia-mediated precipitation via a urease-integrated membrane contactor

Vanadium is an important strategic metal used in various industrial sectors. Its growing demand has increased the need for sustainable recovery technologies. Conventional vanadium recovery commonly relies on ammonium-salt-based precipitation. However, this approach depends on externally supplied ammonium salts and generates nitrogen-rich secondary wastewater. In this study, a urease-integrated membrane contactor (MC) process was proposed to use urine-derived nitrogen as an alternative ammonia source for vanadium precipitation. Human urine contains a high concentration of total nitrogen, but a large fraction exists as urea rather than membrane-transferable free molecular ammonia (NH₃). The integrated urease step hydrolyzed urea into ammoniacal nitrogen, thereby increasing the pool of NH₃ available for membrane transport. Urease treatment increased the feed-side ammonium nitrogen (NH₄⁺-N) concentration from 1,584.75 ± 259.04 mg/L to 11,109.21 ± 776.39 mg/L. This conversion substantially enhanced MC operation, increasing the feed-side NH₄⁺-N removal amount by 6.62-fold compared with that before urease treatment. The transported NH₃ increased the stripping-solution pH from 0.91 to 7.37 within 24 h and was protonated to NH₄⁺, which participated in vanadium precipitation. As a result, 81.79% of the initial dissolved vanadium was removed without the addition of external ammonium salts. The recovered precipitate was identified as an ammonium polyvanadate (APV)-type product through solid-phase characterization. These findings demonstrate a waste-to-resource strategy that converts urine-derived nitrogen into a functional reagent for vanadium recovery while reducing reliance on externally supplied ammonium salts.