Surface hydroxyl-driven =FeOFe(II)+ and =FeOH2+ generation on polyphenol-modified ZVI: Mechanistic insights into Cr(VI) and surface passivation layer removal

Abstract

Micron zero-valent iron (mZVI) has been widely employed for heavy metal remediation. However, its surface oxide layer severely restricts interfacial electron transfer, resulting in low decontamination efficiency. Herein, a tannic acid-modified mZVI (TA-ZVI) was developed using ball milling. The modified material exhibited excellent Cr(VI) removal efficiency (98.2% in 2 min) due to the abundant presence of surface hydroxyl groups. The results of electrochemical analysis demonstrated that the hydroxyl groups increased the electron density and conductivity of the material surface, accelerated the electron transfer, and led to the formation of low and uniform surface potential on the material surface. The hydroxylated interface facilitated in situ Fe(II) generation and release to the solution. The hydroxyl groups then complexed with Fe(II) in the solution to form =FeOFe(II)⁺, which facilitated protonic reactions and led to the formation of =FeOH₂⁺. Density functional theory (DFT) calculation proved that the reaction energy barrier of =FeOFe(II)⁺ for Cr(VI) was much smaller than that of Fe(II) in the solution. Moreover, the surface hydroxyl species (=FeOH and =FeOH₂⁺) directly participated in Cr(VI) removal through synergistic adsorption-reduction pathways, thereby contributing significantly to the overall removal. This work reveals the removal mechanism of Cr(VI) by surface-hydroxylated ZVI, which is of great significance for the treatment of heavy metal ion pollution.