Ferrous Tartrate Dihydrate Shell on Microscale Zerovalent Iron Enables Sustained Fe(II) Activation for Enhanced Cr(VI) Removal

Abstract The surface iron oxide layer formed on microscale zerovalent iron (mZVI) hinders its electron transfer efficiency, thereby significantly decreasing its reduction performance. In this study, an innovative approach was developed through the application of a wet ball-milling technique, followed by surface engineering, to synthesize tartaric acid (TA)-modified mZVI. A mZVI sample modified with 1.2 mol/L TA demonstrated exceptional Cr(VI) removal performance, achieving a capacity of 79.41 mg/g, which is 9.39 times higher than that of pristine mZVI. Spectroscopic and thermogravimetric analyses confirmed, for the first time, the hydrogen bond-mediated formation of a ferrous tartrate dihydrate (C4H4FeO6·2H2O) layer on the mZVI surface. Density functional theory calculations further confirmed the formation of a stable five-membered chelate ring structure within this surface complex. The content of C4H4FeO6·2H2O in the mZVI-1.2 TA material was determined to be 16.85%. This surface-bound C4H4FeO6·2H2O complex enhances both the selectivity and electron transfer toward Cr(VI). Furthermore, the TA-modified mZVI facilitates sustained Fe(III)/Fe(II) redox cycling, thereby enabling the continuous regeneration of Fe(II). This study presents a novel and practical strategy for effective mZVI surface engineering, offering significant potential for groundwater remediation.