Abstract
There is growing interest in using pyrite minerals (FeS2) to enhance the efficiency of metallic iron (Fe0) for water treatment (Fe0/H2O systems). This approach contradicts the thermodynamic predicting suppression of FeS2 oxidation by Fe0 addition. Available results are rooted in time series correlations between aqueous and solid phases based on data collected under various operational conditions. Herein, the methylene blue method (MB method) is used to clarify the controversy. The MB method exploits the differential adsorptive affinity of MB onto sand and sand coated with iron corrosion products to assess the extent of Fe0 corrosion in Fe0/H2O systems. The effects of the addition of various amounts of FeS2 to a Fe0/sand mixture (FeS2 method) on MB discoloration were characterized in parallel quiescent batch experiments for up to 71 d (pH0 = 6.8). Pristine and aged FeS2 specimens were used. Parallel experiments with methyl orange (MO) and reactive red 120 (RR120) enabled a better discussion of the achieved results. The results clearly showed that FeS2 induces a pH shift and delays Fe precipitation and sand coating. Pristine FeS2 induced a pH shift to values lower than 4.5, but no quantitative MB discoloration occurred after 45 d. Aged FeS2 could not significantly shift the pH value (final pH ≥ 6.4) but improved the MB discoloration. The used systematic sequence of experiments demonstrated that adsorption and coprecipitation are the fundamental mechanisms of contaminant removal in Fe0/H2O systems. This research has clarified the reason why a FeS2 addition enhances the efficiency of Fe0 environmental remediation.
Highlights
Metallic iron (Fe0 ), 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]
In establishing that contaminant removals in Fe0/H2O systems are due to flocculation (Figure 6) [89], the results presented will redirect the research for the design of the generation of Fe0-based remediation systems, accounting for
The concept that contaminant removal in Fe0 /H2 O systems is caused by the precipitation of solid iron corrosion products (FeCPs) is consistent with many experimental observations
Summary
Metallic iron (Fe0 ), 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]. Fe0 is mostly considered as a reducing agent (E0 = −0.44 V for the redox couple FeII /Fe0 ) for reducible species [4,12,15] This reductionist but wrong view has been motivated by the failure to consider the inherent features of aqueous iron corrosion, which results in the shortcomings of the Fe0 remediation technology [9]. These features include: (i) the increase of the pH value, coupled with subsequent shielding of the Fe0 surface by an oxide scale (reactivity loss), Processes 2020, 8, 1162; doi:10.3390/pr8091162 www.mdpi.com/journal/processes.
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