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

Abstract

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 FeII and a reducing agent for FeIII, AA sustains the oxidative dissolution of Fe0 and the reductive dissolution of FeIII oxides from gFe0 specimens. In other words, Fe0 dissolution to FeII ions is promoted while the further oxidation to FeIII ions is blocked. Thus, unlike the EDTA method that promotes Fe0 oxidation to FeIII ions, the AA method promotes only the formation of FeII 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.

Iron-fortified water: a new approach for reducing iron deficiency anemia in resource-constrained settings

“…The high reactivity of sponge iron is attributed to its higher porosity and the corresponding surface area in comparison to other materials. The same order of reactivity was reported in related works 58 , 76 . Another important feature from Table 4 is the fact that using twice the amount of Fe 0 (2.0 g for iPuTec) did not double the extent of Fe leaching.…”

“…The four tested herein were included, and depicted significant different reactivity. This result was recently confirmed using a newly developed test for Fe 0 screening: the ascorbic acid test 58 .…”

“…In fact, preliminary experiments (results not shown) have demonstrated that EDTA is a far better lixiviant than AA. The ability of AA to reduce Fe III EDTA is documented and used in analytical chemistry 29 , 58 .…”

“…The extent of Fe absorption thus depends on the intrinsic reactivity of the used Fe 0 in the human gastric fluid. The lack of characterization of the Fe 0 intrinsic reactivity seems to be a major shortcoming as many different Fe 0 types have been tested and used without appropriate quality control 58 . A proper quality control would characterize the relative Fe bioavailability from used Fe 0 powders.…”

“…1 ). Comparable approaches are efforts from our research group using two organic chelates (EDTA and 1,10-Phenanthrolin) to characterize the intrinsic reactivity of Fe 0 specimens 29 , 58 , 76 . Moreover, our research group has been routinely using a 0.1 M AA as a washing solution to free glassware from Fe III oxides after Fe 0 decontamination experiments.…”