Developing the Ascorbic Acid Test: A Candidate Standard Tool for Characterizing the Intrinsic Reactivity of Metallic Iron for Water Remediation

Granular metallic iron (gFe⁰) materials have been widely used for eliminating a wide range of pollutants from aqueous solutions over the past three decades. However, the intrinsic reactivity of gFe⁰ is rarely evaluated and existing methods for such evaluations have not been standardized. The aim of the present study was to develop a simple spectrophotometric method to characterize the intrinsic reactivity of gFe⁰ based on the extent of iron dissolution in an ascorbic acid (AA—0.002 M or 2 mM) solution. A modification of the ethylenediaminetetraacetic acid method (EDTA method) is suggested for this purpose. Being an excellent chelating agent for Fe^II and a reducing agent for Fe^III, AA sustains the oxidative dissolution of Fe⁰ and the reductive dissolution of Fe^III oxides from gFe⁰ specimens. In other words, Fe⁰ dissolution to Fe^II ions is promoted while the further oxidation to Fe^III ions is blocked. Thus, unlike the EDTA method that promotes Fe⁰ oxidation to Fe^III ions, the AA method promotes only the formation of Fe^II species, despite the presence of dissolved O₂. The AA test is more accurate than the EDTA test and is considerably less expensive. Eight selected gFe⁰ specimens (ZVI1 through ZVI8) with established diversity in intrinsic reactivity were tested in parallel batch experiments (for 6 days) and three of these specimens (ZVI1, ZVI3, ZVI5) were further tested for iron leaching in column experiments (for 150 days). Results confirmed the better suitability (e.g., accuracy in assessing Fe⁰ dissolution) of the AA test relative to the EDTA test as a powerful screening tool to select materials for various field applications. Thus, the AA test should be routinely used to characterize and rationalize the selection of gFe⁰ in individual studies.