CO₂-driven ion exchange for ammonium recovery from source-separated urine

Nitrogen recovery from urine and CO₂ utilization are both vital for achieving a circular economy and mitigating climate change. Divided engineering solutions have been proposed to address either problem, but there is still a lack of integrated technologies to simultaneously tackle the two tasks. We demonstrated CO₂-driven ion exchange for nitrogen recovery (CIXNR) from urine and evaluated the process in Malawi. By comprehensively studying the ion exchange chemistry using a proton-form weak acid cation exchanger (WAC-H), we revealed the suppressed adsorption capacity caused by counterion releasing. Regulating aqueous pH optimized the WAC-H capacity, particularly, by the natural buffering capacity provided by bicarbonate in the hydrolyzed urine. CO₂ regeneration achieved over 75 % nitrogen recovery in a multi-cycle test with synthetic hydrolyzed urine. A higher CO₂ pressure resulted in faster regeneration kinetics due to a lower aqueous pH and higher proton concentration gradient. Our field outreach activity in Malawi indicated future research demand in adapting the system for resource-limited, rural, and no-electricity areas. We envision this study to inspire more integrated solutions to tackle both circular economy and climate actions and call for more field outreach activities to facilitate urine technology adoption in developing countries.