Highly Selective Recovery of Phosphorus from Wastewater via Capacitive Deionization Enabled by Ferrocene-polyaniline-Functionalized Carbon Nanotube Electrodes

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Abstract

While capacitive deionization (CDI) is a promising technology for the recovery of nutrients from wastewater, a selective recovery of phosphate from the wastewater containing high concentrations of competing ions is still a huge challenge. Herein, we reported a ferrocene-polyaniline-functionalized carbon nanotube (Fc-PANI/CNT) electrode prepared through amidation reaction and chemical oxidation polymerization, aiming for a highly selective recovery of phosphorus from wastewater. The Fc-PANI/CNT electrode with a unique structure and high conductivity could efficiently adsorb phosphate ions from complex synthetic wastewater with a nearly 100% selectivity, mainly because the integration of ferrocene and an amide bond in Fc-PANI resulted in an enhanced charge transfer (Faradaic reactions) and a strong hydrogen bonding interaction with phosphate ions in its oxidized state. Density functional theory calculations showed that the binding energies of the oxidized Fc-PANI with HPO₄2- and H₂PO₄- were much greater than those of the oxidized Fc-PANI with other competing anions. The affinity of Fc-PANI/CNTs with phosphate can be controlled electrochemically based on the synergetic effects of Faradaic reactions and hydrogen bonding, enabling a selective recovery of phosphate through charging/discharging cycles. The phosphate adsorption capacity reached up to 35 mg PO₄3- g-1 in a NaCl/Na₂SO₄/NaNO₃/NaH₂PO₄ complex mixture at 1.2 V, outperforming most of the other reported CDI systems. The Fc-PANI/CNT electrode also exhibited a decent regeneration ability and durability during repeated CDI tests, demonstrating a great potential for the application of selective recovery and enrichment of phosphate from wastewater.

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