Abstract
Methylene blue (MB) was used as a model molecule to characterize the aqueous reactivity of metallic iron in Fe 0/H 2O systems. Likely discoloration mechanisms under used experimental conditions are: (i) adsorption onto Fe 0 and Fe 0 corrosion products (CP), (ii) co-precipitation with in situ generated iron CP, (iii) reduction to colorless leukomethylene blue (LMB). MB mineralization (oxidation to CO 2) is not expected. The kinetics of MB discoloration by Fe 0, Fe 2O 3, Fe 3O 4, MnO 2, and granular activated carbon were investigated in assay tubes under mechanically non-disturbed conditions. The evolution of MB discoloration was monitored spectrophotometrically. The effect of availability of CP, Fe 0 source, shaking rate, initial pH value, and chemical properties of the solution were studied. The results present evidence supporting co-precipitation of MB with in situ generated iron CP as main discoloration mechanism. Under high shaking intensities (>150 min −1), increased CP generation yields a brownish solution which disturbed MB determination, showing that a too high shear stress induced the suspension of in situ generated corrosion products. The present study clearly demonstrates that comparing results from various sources is difficult even when the results are achieved under seemingly similar conditions. The appeal for an unified experimental procedure for the investigation of processes in Fe 0/H 2O systems is reiterated.
Highlights
Permeable reactive barriers using elemental iron -based alloys (Fe 0-based alloys widely termed as zerovalent iron) as a reactive medium have been proven to be an efficient and affordable technology for removing i norganics and organics species from groundwater [1 -7]
Despite the low adsorptivity exhibited by Methylene blue (MB) towards Fe 0, Fe 2O 3 and Fe 3O 4, under the experimental conditions, MB was quantitatively discolored as F e0 corrosion proceeded
The extent of MB discoloration was insignificant in experiments in which the availability of in situ generated corrosion products was delayed
Summary
Permeable reactive barriers using elemental iron -based alloys (Fe 0-based alloys widely termed as zerovalent iron) as a reactive medium have been proven to be an efficient and affordable technology for removing i norganics and organics species from groundwater [1 -7]. The well -established premise that contaminant removal results from the low electrode potential of the redox couple Fe II/Fe0 (E 0 = -0.44 V) can not explain why redox-insensitive species are quantitatively removed [11,12]. Understanding the nature of primary processes yielding to contaminant removal in Fe importance for advancing technological applications. A survey of the electrode potentials of the redox couples relevant for the discussion in this study [Fe II(aq)/Fe0, Fe III(aq)/FeII (aq), Fe III(s)/FeII(s), MnO 2/Mn2+, O 2/HO -, and MB +/LMB (Eq 1 to Eq 6)]. The electrode potential of Eq 3 to 6 shows that Fe dissolved O2 and MnO2 may re-oxidize colorless LMB to blue MB +.
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