Indirect Contact Regeneration of Ion Exchangers: A New Concept for Advancing Electrified Nitrogen Recovery from Urine

The emerging use of ion exchange for resource recovery has repositioned spent regenerant from "waste" to the desired "product". This transition requires a high purity regenerant for pure product recovery (e.g., ammonium recovery from urine) because the long-lasting direct contact regeneration directly mixes the eluted target ions with the regenerant. Direct contact regeneration prevents using impure regenerants such as electrochemically produced acid (containing electrolyte salts) from making a pure product, hindering the advancement of electrified ammonium recovery via ion exchange. We demonstrated a new concept of indirect contact regeneration leveraging the "proton leakage" from an anion exchange membrane to achieve tandem proton separation and resin regeneration, which enabled the recovery of pure ammonium by an impure acid. A weak acid cation exchange resin enabled multicycles over 80% regeneration efficiency by the limited proton leakage from mild acids with pH 2-3. Investigating the effects of acid purity, pH, and resin dosage revealed the critical role of aqueous pH and the dynamic balance of proton leakage and consumption in governing regeneration efficiency and kinetics. Critical insights on salt types and target pH were provided to minimize the trade-off between electrochemical acid production energy and regeneration efficiency.