Dual-stage electrochemical system enabling reagent-free nickel-ammonia decomplexation and resource recovery

Nickel-ammonia chelated wastewater generated by electroplating and battery manufacturing is difficult to treat using conventional precipitation-based methods that require high chemical input and offer limited resource recovery. Here, we report a reagent-free dual-stage electrochemical membrane reactor that resolves the intrinsic pH-speciation mismatch between nickel removal and ammonia stripping. In the cathodic chambers, in situ generation of hydroxide ions dissociates nickel-ammonia complexes, enabling nickel recovery through electrodeposition and hydroxide precipitation, while anodically generated protons drive ammonia capture across a gas-permeable membrane. The dual-stage configuration independently optimized the pH conditions required for nickel and ammonia removal, reducing effluent Ni and NH₄⁺-N concentrations to below 0.1 and 25 mg L^-1, respectively, while producing reusable metallic Ni, β-Ni(OH)₂, and ammonia solution. Life cycle assessment shows that although the dual-stage electrochemical process requires higher primary energy input than chemical precipitation, selective recovery of nickel and ammonia substantially lowers overall environmental damage. This work demonstrates a compact and sustainable electrochemical pathway for nickel-ammonia decomplexation and resource recovery, providing a circular-economy solution for metal-finishing and battery wastewater treatment.