Overcoming Fe0/Cr(VI) redox-induced low electron transfer efficiency under neutral pH by iron-based dual active sites-mediated hydrogen atom activation

Abstract

Efficiently removing hexavalent chromium (Cr(VI)) using iron-based adsorbents under neutral pH conditions continues to pose a challenge. In this study, we constructed nano zero-valent iron (nZVI) supported by carboxylated multi-walled carbon nanotubes (OCNT) composites with iron-based dual active sites through the nitrilotriacetic acid modification (FOC-NA). The acid dissolution experiment, high-resolution transmission electron microscopy (HRTEM), thermogravimetric analyzer (TGA), X-ray photoelectron spectroscopy (XPS), and Raman analysis proved these active sites consisted of iron-carboxylate complex (−COOFe(II)) and OCNT-confined nZVI (Fe0-in-OCNT). The existence of iron-based dual active sites enhanced electron transfer efficiency in Cr(VI) removal under neutral pH conditions. This leaded to a superior adsorption capacity for Cr(VI), as evidenced by a maximum adsorption capacity of 141.29 mg·g−1 at a pH of 6.5. Meanwhile, the formation of a Cr-containing product (−COOFe-O-Cr) on the surface of FOC-NA substantially hindered the re-release of chromium into the environment. Furthermore, this study proposed the significant involvement of active hydrogen atoms (H*) in the reduction of Cr(VI), as confirmed by quenching experiments and analyses using electron spin resonance (EPR), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The formation pathway and energy barrier of H* and the −COOFe-O-Cr complex were analyzed by density functional theory (DFT) calculations. This study provided insights for the rational design of iron-based adsorbents with dual active sites to facilitate the activation of H* for Cr(VI) reduction under neutral pH conditions.