Efficient removal of EDTA-complexed Cu(II) by a combined Fe(III)/UV/alkaline precipitation process: Performance and role of Fe(II).

Efficient removal of heavy metal-EDTA complexes from water remains a challenge because of their good solubility and chemical stability. Herein, we employed a proprietary process, i.e., the Fe(III) displacement/UV irradiation/alkaline precipitation (denoted as Fe(III)/UV/OH), to enable an efficient removal of Cu(II)-EDTA complex from 19.2 mg Cu(II)/L to <1 mg Cu(II)/L. The combined process includes Fe(III) replacement with the complexed Cu(II) to form Fe(III)-EDTA and release the free Cu(II), UV-mediated catalytic decarboxylation of EDTA to form amine ligands and reduction of Fe(III) to Fe(II), and the final removal of Cu(II) through precipitation. The in situ formed Fe(II) is crucial to the final Cu(II) removal because it tends to form stable complexes with amine ligands (EDTA and its decarboxylation products), thereby inhibiting their re-complexation with the released Cu(II) and facilitating the formation of copper precipitates. Consequently, the methods capable of prolonging the life of Fe(II), e.g., increasing the Fe(III) addition or direct addition of Fe(II) into the Cu(II)-EDTA solution and deoxygenating the solution, could dramatically enhance the final Cu(II) removal. We also optimized the operational conditions of the process at the initial Cu(II)-EDTA of 19.2 mg Cu(II)/L.