Electrochemical Selective Removal of Oxyanions in a Ferrocene-Doped Metal–Organic Framework

Metal-organic frameworks (MOF) are a class of crystalline, porous materials possessing well-defined channels that have widespread applications across the sustainable landscape. Analogous to zeolites, these materials are well-suited for adsorption processes targeting environmental contaminants. Herein, a zirconium MOF, UiO-66, was functionalized with ferrocene for the selective removal of oxyanion contaminants, specifically NO₃^-, SO₄^2-, and PO₄^3-. Electrochemical oxidation of the embedded ferrocene pendants induces preferential adsorption of these oxyanions, even in the presence of Cl^- in a 10-fold excess. Anion selectivity strongly favoring PO₄^3- (<i>S</i>_oxy/comp = 3.80) was observed following an adsorption trend of PO₄^3- > SO₄^2- > NO₃^- > (10-fold)Cl^- in multi-ion solution mixtures. The underlying mechanisms responsible for ion selectivity were elucidated by performing <i>ex situ</i> X-ray photoelectron spectroscopy (XPS) on the heterogeneous electrode surface postadsorption and by calculating the electronic structure of various adsorption configurations. It was eventually shown that oxyanion selectivity stemmed from strong ion association with a positively charged pore interior due to the spatial distribution of charge by oxygen constituents. While ferrocenium provided the impetus for ion migration-diffusion, it was the formation of stable complexes with zirconium nodes that ultimately contributed to selective adsorption of oxyanions.