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Modeling the Permeability Loss of Metallic Iron Water Filtration Systems
No full textOver the past 30 years the literature has burgeoned with in situ approaches for groundwater remediation. Of the methods currently available, the use of metallic iron (Fe0) in permeable reactive barrier (PRB) systems is one of the most commonly applied. Despite such interest, an increasing amount of experimental and field observations have reported inconsistent Fe0 barrier operation compared to contemporary theory. In the current work, a critical review of the physical chemistry of aqueous Fe0 corrosion in porous media is presented. Subsequent implications for the design of Fe0 filtration systems are modeled. The results suggest that: (i) for the pH range of natural waters (andgt;4.5), the high volumetric expansion of Fe0 during oxidation and precipitation dictates that Fe0 should be mixed with a non-expansive material; (ii) naturally occurring solute precipitates have a negligible impact on permeability loss compared to Fe0 expansive corrosion; and (iii) the proliferation of H2 metabolizing bacteria may contribute to alleviate permeability loss. As a consequence, it is suggested that more emphasis must be placed on future work with regard to considering the Fe0 PRB system as a physical (size-exclusion) water filter device. © 2012 WILEY-VCH Verlag GmbH andamp; Co. 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Review : The fundamental mechanism of aqueous contaminant removal by metallic iron
Get PDFContaminant co-precipitation with continuously generated and transformed iron corrosion products has received relatively little attention in comparison to other possible removal mechanisms (adsorption, oxidation, precipitation) in Fe0/H2O systems at near neutral pH values. A primary reason for this is that the use of elemental iron (Fe0) in environmental remediation is based on the thermodynamic-founded premise that reducible contaminants are potentially reduced while Fe0 is oxidised. However, co-precipitation portends to be of fundamental importance for the process of contaminant removal in Fe0/H2O systems, as the successful removal of bacteria, viruses and non reducible organic (e.g. methylene blue, triazoles) and inorganic (e.g. Zn) compounds has been reported. This later consideration has led to a search for the reasons why the importance of co-precipitation has almost been overlooked for more than a decade. Three major reasons have been identified: the improper consideration of the huge literature of iron corrosion by pioneer works, yielding to propagation of misconceptions in the iron technology literature; the improper consideration of available results from other branches of environmental science (e.g. CO2 corrosion, electrocoagulation using Fe0 electrodes, Fe or Mn geochemistry); and the use of inappropriate experimental procedures (in particular, mixing operations). The present paper demonstrates that contaminant co-precipitation with iron corrosion products is the fundamental mechanism of contaminant removal in Fe0/H2O systems. Therefore, the ‘iron technology’ as a whole is to be revisited as the ‘know-why’ of contaminant removal is yet to be properly addressed.Keywords: adsorption, co-precipitation, iron corrosion, iron oxhydroxides, zerovalent iro
Characterizing the effects of shaking intensity on the kinetics of metallic iron dissolution in EDTA.
No full textDespite two decades of intensive laboratory investigations, several aspects of contaminant removal from aqueous solutions by elemental iron materials (e.g., in Fe(0)/H2O systems) are not really understood. One of the main reasons for this is the lack of a unified procedure for conducting batch removal experiments. This study gives a qualitative and semi-quantitative characterization of the effect of the mixing intensity on the oxidative dissolution of iron from two Fe(0)-materials (materials A and B) in a diluted aqueous ethylenediaminetetraacetic solution (2 mM EDTA). Material A (fillings) was a scrap iron and material B (spherical) a commercial material. The Fe(0)/H2O/EDTA systems were shaken on a rotational shaker at shaking intensities between 0 and 250 min(-1) and the time dependence evolution of the iron concentration was recorded. The systems were characterized by the initial iron dissolution rate (k(EDTA)). The results showed an increased rate of iron dissolution with increasing shaking intensity for both materials. The increased corrosion through shaking was also evidenced through the characterization of the effects of pre-shaking time on k(EDTA) from material A. Altogether, the results disprove the popular assumption that mixing batch experiments is a tool to limit or eliminate diffusion as dominant transport process of contaminant to the Fe(0) surface
The fundamental mechanism of aqueous contaminant removal by metallic iron
No full textContaminant co-precipitation with continuously generated and transformed iron corrosion products has received relatively little attention in comparison to other possible removal mechanisms (adsorption, oxidation, precipitation) in Fe(0)/H(2)O systems at near neutral pH values. A primary reason for this is that the use of elemental iron (Fe(0)) in environmental remediation is based on the thermodynamic-founded premise that reducible contaminants are potentially reduced while Fe(0) is oxidised. However, co-precipitation portends to be of fundamental importance for the process of contaminant removal in Fe(0)/H(2)O systems, as the successful removal of bacteria, viruses and non reducible organic (e.g. methylene blue, triazoles) and inorganic (e.g. Zn) compounds has been reported. This later consideration has led to a search for the reasons why the importance of co-precipitation has almost been overlooked for more than a decade. Three major reasons have been identified: the improper consideration of the huge literature of iron corrosion by pioneer works, yielding to propagation of misconceptions in the iron technology literature; the improper consideration of available results from other branches of environmental science (e.g. CO(2) corrosion, electrocoagulation using Fe(0) electrodes, Fe or Mn geochemistry); and the use of inappropriate experimental procedures (in particular, mixing operations). The present paper demonstrates that contaminant co-precipitation with iron corrosion products is the fundamental mechanism of contaminant removal in Fe(0)/H(2)O systems. Therefore, the 'iron technology' as a whole is to be revisited as the 'know-why' of contaminant removal is yet to be properly addressed.Deutsche Forschungsgemeinschaft (DFG) [626/2-2
Review : The fundamental mechanism of aqueous contaminant removal by metallic iron
No full textContaminant co-precipitation with continuously generated and transformed iron corrosion products has received relatively little attention in comparison to other possible removal mechanisms (adsorption, oxidation, precipitation) in Fe0/H2O systems at near neutral pH values. A primary reason for this is that the use of elemental iron (Fe0) in environmental remediation is based on the thermodynamic-founded premise that reducible contaminants are potentially reduced while Fe0 is oxidised. However, co-precipitation portends to be of fundamental importance for the process of contaminant removal in Fe0/H2O systems, as the successful removal of bacteria, viruses and non reducible organic (e.g. methylene blue, triazoles) and inorganic (e.g. Zn) compounds has been reported. This later consideration has led to a search for the reasons why the importance of co-precipitation has almost been overlooked for more than a decade. Three major reasons have been identified: the improper consideration of the huge literature of iron corrosion by pioneer works, yielding to propagation of misconceptions in the iron technology literature; the improper consideration of available results from other branches of environmental science (e.g. CO2 corrosion, electrocoagulation using Fe0 electrodes, Fe or Mn geochemistry); and the use of inappropriate experimental procedures (in particular, mixing operations). The present paper demonstrates that contaminant co-precipitation with iron corrosion products is the fundamental mechanism of contaminant removal in Fe0/H2O systems. Therefore, the ‘iron technology’ as a whole is to be revisited as the ‘know-why’ of contaminant removal is yet to be properly addressed
Metallic iron for environmental remediation: learning from the Becher process.
No full textMetallic iron (Fe(0)) is a moderately reducing agent that has been reported to be capable of reducing many environmental contaminants. Reduction by Fe(0) used for environmental remediation is a well-known process to organic chemists, corrosion scientists and hydrometallurgists. However, considering Fe(0) as a reducing agent for contaminants has faced considerable scepticism because of the universal role of oxide layers on Fe(0) in the process of electron transfer at the Fe(0)/oxide/water interface. This communication shows how progress achieved in developing the Becher process in hydrometallurgy could accelerate the comprehension of processes in Fe(0)/H(2)O systems for environmental remediation. The Becher process is an industrial process for the manufacture of synthetic rutile (TiO(2)) by selectively removing metallic iron (Fe(0)) from reduced ilmenite (RI). This process involves an aqueous oxygen leaching step at near neutral pH. Oxygen leaching suffers from serious limitations imposed by limited mass transport rates of dissolved oxygen across the matrix of iron oxides from initial Fe(0) oxidation. In a Fe(0)/H(2)O system pre-formed oxide layers similarly act as physical barrier limiting the transport of dissolved species (including contaminants and O(2)) to the Fe(0)/H(2)O interface. Instead of this universal role of oxide layers on Fe(0), improper conceptual models have been developed to rationalize electron transfer mechanisms at the Fe(0)/oxide/water interface
Characterizing the discoloration of methylene blue in Fe0/H2O systems.
No full textMethylene blue (MB) was used as a model molecule to characterize the aqueous reactivity of metallic iron in Fe(0)/H(2)O 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(2)O(3), Fe(3)O(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(2)O systems is reiterated
The fundamental mechanism of aqueous contaminant removal by metallic iron
No full textContaminant co-precipitation with continuously generated and transformed iron corrosion products has received relatively little attention in comparison to other possible removal mechanisms (adsorption, oxidation, precipitation) in Fe(0)/H(2)O systems at near neutral pH values. A primary reason for this is that the use of elemental iron (Fe(0)) in environmental remediation is based on the thermodynamic-founded premise that reducible contaminants are potentially reduced while Fe(0) is oxidised. However, co-precipitation portends to be of fundamental importance for the process of contaminant removal in Fe(0)/H(2)O systems, as the successful removal of bacteria, viruses and non reducible organic (e.g. methylene blue, triazoles) and inorganic (e.g. Zn) compounds has been reported. This later consideration has led to a search for the reasons why the importance of co-precipitation has almost been overlooked for more than a decade. Three major reasons have been identified: the improper consideration of the huge literature of iron corrosion by pioneer works, yielding to propagation of misconceptions in the iron technology literature; the improper consideration of available results from other branches of environmental science (e.g. CO(2) corrosion, electrocoagulation using Fe(0) electrodes, Fe or Mn geochemistry); and the use of inappropriate experimental procedures (in particular, mixing operations). The present paper demonstrates that contaminant co-precipitation with iron corrosion products is the fundamental mechanism of contaminant removal in Fe(0)/H(2)O systems. Therefore, the 'iron technology' as a whole is to be revisited as the 'know-why' of contaminant removal is yet to be properly addressed.Deutsche Forschungsgemeinschaft (DFG) [626/2-2
A critical review on the process of contaminant removal in Fe0-H2O systems.
No full textA central aspect of the contaminant removal by elemental iron materials (Fe0 or Fe0 materials) is that reduction reactions are mediated by the iron surface (direct reduction). This premise was introduced by the pioneers of the reactive wall technology and is widely accepted by the scientific community. In the meantime enough evidence has been provided to suggest that contaminant reduction through primary corrosion products (secondary reductants) does indeed occur (indirect reduction). It was shown for decades that iron corrosion in the pH range of natural waters (4-9) inevitably yields an obstructive oxide film of corrosion products at the metal surface (oxide film). Therefore, contaminant adsorption on to corrosion products and contaminant co-precipitation with corrosion products inevitably occurs. For adsorbed and coprecipitated contaminants to be directly reduced the oxide film should be electronic conductive. This study argues through a literature review a series of points which ultimately lead to the conclusion that, if any quantitative contaminant reduction occurs in the presence of Fe0 materials, it takes place within the matrix of corrosion products and is not necessarily a direct reduction. It is concluded that Fe0 materials act both as source of corrosion products for contaminant adsorption/coprecipitation and as a generator of FeII and H2 (H) for possible catalytic contaminant reduction
Metallic iron for safe drinking water worldwide
No full textA new concept for household and large-scale safe drinking water production is presented. Raw water is successively filtered through a series of sand and iron filters. Sand filters mostly remove suspended particles (media filtration) and iron filters remove anions, cations, micro-pollutants, natural organic matter, and micro-organisms including pathogens (reactive filtration). Accordingly, treatment steps conventionally achieved with flocculation, sedimentation, rapid sand filtration, activated carbon filtration, and disinfection are achieved in the new concept in only two steps. To prevent bed clogging, Fe0 is mixed with inert materials, yielding Fe0/sand filters. Efficient water treatment in Fe0/sand filters has been extensively investigated during the past two decades. Two different contexts are particularly important in this regard: (i) underground permeable reactive barriers, and (ii) household water filters. In these studies, the process of aqueous iron corrosion in a packed bed was proven very efficient for unspecific aqueous contaminant removal. Been based on a chemical process (iron corrosion), efficient water treatment in Fe0 beds is necessarily coupled with a slow flow rate. Therefore, for large communities several filters should work in parallel to produce enough water for storage and distribution. It appears that water filtration through Fe0/sand filters is an efficient, affordable, an flexible technology for the whole world
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