Insight into the urine progression and DOM transformation mechanism for sustainable nitrogen-water synergistic recovery in space stations.

Urine represents a critical water resource for closing the water and nitrogen loop in life support systems during manned space exploration. This work focuses on the impacts of pretreatment agents on urine progression and dissolved organic matter (DOM) transformation during long-term storage, with a perspective towards technology innovation for nitrogen-water synergistic recovery from urine in space. Urea hydrolysis, driven by microbial activity, was identified as the primary driver of urine variation, initiating rapid changes within 8 h of storage. The time point of significant DOM variation was prior to urea hydrolysis, and fresh humic-like substance from recent biological activity, which were dominated the variations of DOM. Meanwhile, the increasing NH4+-N and pH can resulted in precipitation, such as MgNH4PO4, which showed potential risks to the urine transport and treatment devices. Both H2SO4 and CrO3 showed excellent urine stabilization performance with urea hydrolysis rates of 0.10∼0.17 within 336 h. However, there have divergent DOM transformation pathways and sediment components: H2SO4 influenced the fresh DOM by microbial inhibition through acidic environment, and uric acid and CaSO4 were the main sediments; CrO3 could enhance the degradation of protein-like substances by strong oxidation, and generate Cr-containing dense sediments. Therefore, the selection criteria for technologies and pretreatment agents on urine recovery are proposed, which encourage the integration of membrane separation and biosynthesis. This work reveals the dynamic variation and the mechanism of pretreatment agents during urine storage, which offers the basis for innovation in urine recovery for deep space exploration.