Poly(Norbornene-Diphenothiazine) for Electrochemical Capture and Release of Chromium and Arsenic Oxyanions from Water

Drinking water contamination by heavy metals, particularly chromium and arsenic oxyanions, is a severe challenge threatening humanity's sustainable development. Electrochemically mediated water purification is gaining attention due to its high uptake, rapid kinetics, modularity, and facile regeneration. Here, we designed a composite electrode by combining a redox-active/Faradaic polymer, poly(norbornene-diphenothiazine) (PNP₂), with carbon nanotubes (CNTs) – PNP₂-CNT. The PNP₂-CNT demonstrated exceptional pseudocapacitance behavior, resulting in significantly accelerated adsorption rates for dichromate (Cr(VI); 0.008 g mg⁻¹ min⁻¹) and arsenite (As(III); 0.03 g mg⁻¹ min⁻¹), surpassing reported materials by a margin of 3–200 times, while demonstrating a high adsorption capacity, 666.3 and 612.4 mg g⁻¹, respectively. Furthermore, it effectively converted As(III) to the less toxic arsenate (As(V)) during adsorption and Cr(VI) to the less toxic chromium (Cr(III)) during desorption. This PNP₂-CNT system also showed significantly lower energy consumption, only 0.17% of the CNT control system. This study demonstrated for the first time the use of PNP₂ redox-active polymers in the separation and conversion process, meeting the six criteria of high uptake, rapid kinetics, selectivity, stability, recyclability, and energy efficiency. This achievement expands the scope of advanced materials that address environmental concerns and make an impact by generating energy- and cost-effective water purification.