The key role of contact time in elucidating the mechanisms of enhanced decontamination by Fe0/MnO2/sand systems

Abstract

Metallic iron (Fe0) has shown outstanding performances for water decontamination and its efficiency has been improved by the presence of sand (Fe0/sand) and manganese oxide (Fe0/MnOx). In this study, a ternary Fe0/MnOx/sand system is characterized for its discoloration efficiency of methylene blue (MB) in quiescent batch studies for 7, 18, 25 and 47 days. The objective was to understand the fundamental mechanisms of water treatment in Fe0/H2O systems using MB as an operational tracer of reactivity. The premise was that, in the short term, both MnO2 and sand delay MB discoloration by avoiding the availability of free iron corrosion products (FeCPs). Results clearly demonstrate no monotonous increase in MB discoloration with increasing contact time. As a rule, the extent of MB discoloration is influenced by the diffusive transport of MB from the solution to the aggregates at the bottom of the vessels (test-tubes). The presence of MnOx and sand enabled the long-term generation of iron hydroxides for MB discoloration by adsorption and co-precipitation. Results clearly reveal the complexity of the Fe0/MnOx/sand system, while establishing that both MnOx and sand improve the efficiency of Fe0/H2O systems in the long-term. This study establishes the mechanisms of the promotion of water decontamination by amending Fe0-based systems with reactive MnOx.

Metallic Iron for Environmental Remediation: The Fallacy of the Electron Efficiency Concept

“…In biological systems, for example, a lag time is the time necessary to induce the synthesis of appropriate enzymes (Schreier and Reinhard, 1994). Such lag periods were repeatedly reported in the literature over the years (e.g., Huang et al, 1998;Noubactep et al, 2003;Hao et al, 2005;Cao et al, 2021a;Cao et al, 2021b;Cao et al, 2021c;Cao et al, 2021d). In Fe 0 /H 2 O systems, the lag time is indicative of the time required for the generation of FeCPs following Fe 0 immersion into a polluted water.…”

“…Contextualizing this to the Fe 0 literature, the highlighted mistake (error) has been propagated since the time the work of Khudenko (1991) was ignored. Despite several efforts by our group pointing out this conceptual mistake (Noubactep 2007, Noubactep, 2008Noubactep, 2010a;Noubactep, 2010b;Noubactep, 2014;Noubactep, 2016), the truth has been ignored for 1 decade already (Xiao et al, 2020a;Xiao et al, 2020b;Cao et al, 2020;Cao et al, 2021a;Cao et al, 2021b;Cao et al, 2021c;Cao et al, 2021d;Hu et al, 2021;Noubactep, 2022). However, an accurate fundamental understanding of processes governing contaminant removal is critical in the design and operation of Fe 0 -based systems.…”

“…However, little attention was paid to the role of primary iron corrosion products (e.g. Fe II , H/H 2 , Fe 3 O 4 , green rusts) as sole relevant source of electrons to mediate chemical reduction (Figure 1) (Jiao et al, 2009;Noubactep, 2010b;Noubactep, 2011;Cao et al, 2021a;Cao et al, 2021b). Another important observation of Schreier and Reinhard (1994) was that in experiments conducted at room temperature with Fe 0 , reductive transformations ceased abruptly after some 28 days.…”

“…Efforts towards more reliable experimental conditions were discussed and constantly actualized for example from 1999 to 2011 by The Interstate Technology and Regulatory Council (www.itrcweb.org) (ITRC, 2011). However, the key factor that the formation of oxide scales in the vicinity of Fe 0 should be favored has received little attention (Devlin and Allin, 2005;Noubactep, 2008;Cao et al, 2021b). This premise implies that only quiescent or very slow-mixed batch experiments would produce results relevant for the design of filters (Noubactep et al, 2009).…”

“…Moreover, the is no iron corrosion in dry (H 2 O free) chlorinated solvents (Rhodes and Carty, 1925;Archer, 1979). Fe 0 corrosion by H 2 O (including moisture) and no Fe 0 corrosion by dry RCl clearly indicates that more attention should have been paid to water as corroding agent in the Fe 0 remediation literature (Ghauch, 2015;Cao et al, 2021b;Noubactep, 2022).…”