High-efficiency nickel recovery from spent electroless nickel plating solution: effective degradation of high-concentration nickel complexes to form a nickel ferrite nanomaterial via Fe₃O₄ catalytic oxidation

The widespread use of electroless nickel plating inevitably leads to substantial discharge of spent electroless nickel plating solution (SENPS), which poses a serious threat to the environment. It remains highly challenging to efficiently treat SENPS, as it mostly contains organic nickel complexes that have high stability and low biodegradability. In this work, we verified that nickel mostly existed as nickel lactate and Ni-EDTA in SENPS, and then developed a highly effective hydrothermal method to oxidatively degrade these complexes in an alkaline environment containing Fe3O4, to ultimately recover Ni in the form of NiFe2O4 as a valuable nanomaterial available for use in electrodes and catalysts. Under optimal conditions, this method could attain a remarkable Ni removal efficiency of 99.99%, and the residual Ni content was only 0.98 mg L-1 from an initial concentration of 12 448.8 mg L-1. Mechanistic investigations revealed that in an alkaline environment, nickel lactate was transformed by OH- into Ni(OH)(3)(-) and lactate, and Ni-EDTA was converted to the much less stable NiOHEDTA(3-). Hydroxyl radicals generated by the reaction between active oxygen groups and Fe3O4 could then degrade the NiOHEDTA(3-) complex to release Ni2+, and NiFe2O4 was formed after Ni2+ entered the octahedral lattice of Fe3O4 and replaced Fe2+. This work presents a novel approach for treating SENPS, which serves as an example of recovering heavy metals from their organic complexes to create useful nanomaterial resources.