Understanding the Operating Mode of Fe0/Fe-Sulfide/H2O Systems for Water Treatment

Abstract

The general suitability of water treatment systems involving metallic iron (Fe0) is well-established. Various attempts have been made to improve the efficiency of conventional Fe0 systems. One promising approach combines granular Fe0 and an iron sulfide mineral to form Fe0/Fe-sulfide/H2O systems. An improved understanding of the fundamental principles by which such systems operate is still needed. Through a systematic analysis of possible reactions and the probability of their occurrence, this study establishes that sulfide minerals primarily sustain iron corrosion by lowering the pH of the system. Thus, chemical reduction mediated by FeII species (indirect reduction) is a plausible explanation for the documented reductive transformations. Such a mechanism is consistent with the nature and distribution of reported reaction products. While considering the mass balance of iron, it appears that lowering the pH value increases Fe0 dissolution, and thus subsequent precipitation of hydroxides. This precipitation reaction is coupled with the occlusion of contaminants (co-precipitation or irreversible adsorption). The extent to which individual sulfides impact the efficiency of the tested systems depends on their intrinsic reactivities and the operational conditions (e.g., sulfide dosage, particle size, experimental duration). Future research directions, including the extension of Fe0/Fe-sulfide/H2O systems to drinking water filters and (domestic) wastewater treatment using the multi-soil-layering method are highlighted.

Validating the Efficiency of the FeS2 Method for Elucidating the Mechanisms of Contaminant Removal Using Fe0/H2O Systems

“…These results were achieved with aged FeS 2 , and the final pH values varied between 6.4 and 7.6 for all investigated systems (variations of only 1.2 pH units). With pristine FeS 2 , the window of pH variation would have been larger and the experimental duration longer for similar observations [14]. Noubactep et al [43] Figure 3b summarizes the results of changes of the iron concentrations as a function of the experimental duration.…”

“…After day 5, there is a progressive increase of the pH value with the increasing time. This trend is justified by iron corrosion determining the pH value [14]. Note that, in a Fe 0 /FeS 2 /H 2 O system under oxic conditions, the pH value is determined by two concurrent processes: (i) FeS 2 oxidation lowering the pH and (ii) iron corrosion increasing the pH.…”

“…Thus, immersed Fe 0 in situ generates contaminant scavengers. Moreover, iron hydroxide precipitation occurs in the presence of trace amounts of contaminants, which are simply occluded via coprecipitation [14,18,19,81,82]. Iron hydroxide precipitation on FeS2 corresponds to the…”

“…The initial pH was 6.8 ± 0.2; data are not sorted. It is hoped that, in establishing that contaminant removals in Fe 0 /H 2 O systems are due to flocculation (adsorption and coprecipitation) ( Figure 6) [89], the results presented herein will redirect the research for the design of the next generation of Fe 0 -based remediation systems, accounting for the ion-selective nature of the systems [14,84,[89][90][91][92]. The view that Fe 0 is a reducing agent [15,93,94] should be immediately abandoned [85].…”

“…Metallic iron (Fe 0 ), commonly termed as zero-valent iron (ZVI), is widely used for decentralized safe drinking water provision [1][2][3] and environmental remediation [4][5][6]. Its suitability to remove a large array of biological and chemical pollutants from aqueous solutions has been demonstrated over the past three decades [7][8][9][10][11][12][13][14]. However, Fe 0 is mostly considered as a reducing agent (E 0 = −0.44 V for the redox couple Fe II /Fe 0 ) for reducible species [4,12,15].…”