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

Granular metallic iron (gFe(0)) 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 gFe(0) 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 gFe(0) 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 Fe-0 and the reductive dissolution of Fe-III oxides from gFe(0) specimens. In other words, Fe-0 dissolution to Fe-II ions is promoted while the further oxidation to Fe-III ions is blocked. Thus, unlike the EDTA method that promotes Fe-0 oxidation to Fe-III ions, the AA method promotes only the formation of Fe-II species, despite the presence of dissolved O-2. The AA test is more accurate than the EDTA test and is considerably less expensive. Eight selected gFe(0) 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 Fe-0 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 gFe(0) in individual studies.