Designing metallic iron based water filters: Light from methylene blue discoloration.

Available water filtration systems containing metallic iron (Fe(0) filters) are pragmatically designed. There is a lack of sound design criteria to exploit the full potential of Fe(0) filters. A science-based design relies on valuable information on processes within a Fe(0) filter, including chemical reactions, hydrodynamics and their relation to the performance of the filter. The aim of this study was to establish a simple method to evaluate the initial performance of Fe(0) filters. The differential adsorptive affinity of methylene blue (MB) onto sand and iron oxide is exploited to characterize the evolution of a Fe(0)/sand system using the pure sand system as operational reference. Five systems were investigated for more than 70 days: pure sand, pure Fe(0), Fe(0)/sand, Fe(0)/pumice and Fe(0)/sand/pumice. Individual systems were characterized by the extent of changes in pH value, iron breakthrough, MB breakthrough and hydraulic conductivity. Results showed that for MB discoloration (i) pure sand was the most efficient system, (ii) hybrid systems were more sustainable than the pure Fe(0) system, and (iii) the pores of used pumice are poorly interconnected. Characterizing the initial reactivity of Fe(0) filters using MB discoloration has introduced a powerful tool for the exploration of various aspects of filter design.

Comment on "Reductive dechlorination of γ-hexachloro-cyclohexane using Fe-Pd bimetallic nanoparticles" by Nagpal et al. [J. Hazard. Mater. 175 (2010) 680-687].

The author used a recent article on lindane (γ-hexachloro-cyclohexane) reductive dechlorination by Fe/Pd bimetallics to point out that many other of published works in several journals do not conform to the state-of-the-art knowledge on the mechanism of aqueous contaminant removal by metallic iron (e.g. in Fe(0)/H(2)O systems). It is the author's view that the contribution of adsorbed Fe(II) to the process of contaminant reduction has been neglected while discussing the entire process of contaminant reduction in the presence of bimetallics.

Designing laboratory metallic iron columns for better result comparability.

Despite the amount of data available on investigating the process of aqueous contaminant removal by metallic iron (Fe(0)), there is still a significant amount of uncertainty surrounding the design of Fe(0) beds for laboratory testing to determine the suitability of Fe(0) materials for field applications. Available data were obtained under various operating conditions (e.g., column characteristics, Fe(0) characteristics, contaminant characteristics, oxygen availability, solution pH) and are hardly comparable to each other. The volumetric expansive nature of iron corrosion has been univocally reported as major drawback for Fe(0) beds. Mixing Fe(0) with inert materials has been discussed as an efficient tool to improve sustainability of Fe(0) beds. This paper discusses some problems associated with the design of Fe(0) beds and proposes a general approach for the characterization of Fe(0) beds. Each Fe(0) column should be characterized by its initial porosity, the composition of the steady phase and the volumetric proportion of individual materials. Used materials should be characterized by their density, porosity, and particle size. This work has introduced simple and reliable mathematical equations for column design, which include the normalisation of raw experimental data prior to any data treatment.

Nano-scale metallic iron for the treatment of solutions containing multiple inorganic contaminants.

Although contaminant removal from water using zero-valent iron nanoparticles (INP) has been investigated for a wide array of chemical pollutants, the majority of studies to date have only examined the reaction of INP in simple single-contaminant systems. Such systems fail to reproduce the complexity of environmental waters and consequently fail as environmental analogues due to numerous competitive reactions not being considered. Consequently there is a high demand for multi-elemental and site-specific studies to advance the design of INP treatment infrastructure. Here INP are investigated using batch reactor systems over a range of pH for the treatment of water containing multi-element contaminants specifically U, Cu, Cr and Mo, selected to provide site-specific analogues for leachants collected from the Lisava mine, near Oravita in South West Romania. Concurrently, a U-only solution was also analysed as a single-system for comparison. Results confirmed the suitability of nano-Fe(0) as a highly efficient reactive material for the aqueous removal of Cr(IV), Cu(II) and U(VI) over a range of pH applicable to environmental waters. Insufficient Mo(VI) removal was observed at pH >5.7, suggesting that further studies were necessary to successfully deploy INP for the treatment of geochemically complex mine water effluents. Results also indicated that uranium removal in the multi-element system was less than for the comparator containing only uranium.

On nanoscale metallic iron for groundwater remediation.

This communication challenges the concept that nanoscale metallic iron (nano-Fe(0)) is a strong reducing agents for contaminant reductive transformation. It is shown that the inherent relationship between contaminant removal and Fe(0) oxidative dissolution which is conventionally attributed to contaminant reduction by nano-Fe(0) (direct reduction) could equally be attributed to contaminant removal by adsorption and co-precipitation. For reducible contaminants, indirect reduction by adsorbed Fe(II) or adsorbed H produced by corroding iron (indirect reduction) is even a more probable reaction path. As a result, the contaminant removal efficiency is strongly dependent on the extent of iron corrosion which is larger for nano-Fe(0) than for micro-Fe(0) in the short term. However, because of the increased reactivity, nano-Fe(0) will deplete in the short term. No more source of reducing agents (Fe(II), H and H(2)) will be available in the system. Therefore, the efficiency of nano-Fe(0) as a reducing agent for environmental remediation is yet to be demonstrated.

Elemental metals for environmental remediation: learning from cementation process.

The further development of Fe(0)-based remediation technology depends on the profound understanding of the mechanisms involved in the process of aqueous contaminant removal. The view that adsorption and co-precipitation are the fundamental contaminant removal mechanisms is currently facing a harsh scepticism. Results from electrochemical cementation are used to bring new insights in the process of contaminant removal in Fe(0)/H(2)O systems. The common feature of hydrometallurgical cementation and metal-based remediation is the heterogeneous nature of the processes which inevitably occurs in the presence of a surface scale. The major difference between both processes is that the surface of remediation metals is covered by layers of own oxide(s) while the surface of the reducing metal in covered by porous layers of the cemented metal. The porous cemented metal is necessarily electronic conductive and favours further dissolution of the reducing metal. For the remediation metal, neither a porous layer nor a conductive layer could be warrant. Therefore, the continuation of the remediation process depends on the long-term porosity of oxide scales on the metal surfaces. These considerations rationalized the superiority of Fe(0) as remediation agent compared to thermodynamically more favourable Al(0) and Zn(0). The validity of the adsorption/co-precipitation concept is corroborated.

Comment on "Reductive dechlorination of gamma-hexachloro-cyclohexane using Fe-Pd bimetallic nanoparticles" by Nagpal et al. [J. Hazard. Mater. 175 (2010) 680-687].

The author used a recent article on lindane reductive dechlorination by Fe/Pd bimetallics to complain that dozen of published works in several journals are not conform to the state-of-the-art knowledge on the mechanism of aqueous contaminant removal by metallic iron (e.g. in Fe(0)/H(2)O systems). It is shown that the contribution of adsorbed Fe(II) to the process of contaminant reduction has been neglected while discussing the whole process of contaminant reduction in the presence of bimetallics.

Comments on "Decontamination of solutions containing Cu(II) and ligands tartrate, glycine and quardol using metallic iron" [J. Hazard. Mater. (175 (2010) 452-459)].

This letter presents ways for an improved discussion of the data provided in a recent article on aqueous removal of Cu(II) complexes from aqueous solutions using metallic iron (Fe(0)) by Gyliene and his co-workers. It is shown that the authors have furnished another brilliant validation of the concept that adsorption onto iron corrosion products and co-precipitation with iron corrosion products are the fundamental mechanisms of dissolved contaminant removal in Fe(0)/H(2)O systems.

Metallic iron for environmental remediation: learning from electrocoagulation.

The interpretation of processes yielding aqueous contaminant removal in the presence of elemental iron (e.g. in Fe(0)/H(2)O systems) is subject to numerous complications. Reductive transformations by Fe(0) and its primary corrosion products (Fe(II) and H/H(2)) as well as adsorption onto and co-precipitation with secondary and tertiary iron corrosion products (iron hydroxides, oxyhydroxides, and mixed valence Fe(II)/Fe(III) green rusts) are considered the main removal mechanisms on a case-to-case basis. Recent progress involving adsorption and co-precipitation as fundamental contaminant removal mechanisms have faced a certain scepticism. This work shows that results from electrocoagulation (EC), using iron as sacrificial electrode, support the adsorption/co-precipitation concept. It is reiterated that despite a century of commercial use of EC, the scientific understanding of the complex chemical and physical processes involved is still incomplete.

Exploring the influence of operational parameters on the reactivity of elemental iron materials.

In an attempt to characterize material intrinsic reactivity, iron dissolution from elemental iron materials (Fe(0)) was investigated under various experimental conditions in batch tests. Dissolution experiments were performed in a dilute solution of ethylenediaminetetraacetate (Na(2)-EDTA - 2mM). The dissolution kinetics of 18 Fe(0) materials were investigated. The effects of individual operational parameters were assessed using selected materials. The effects of available reactive sites [Fe(0) particle size (

Characterizing the effects of shaking intensity on the kinetics of metallic iron dissolution in EDTA.

Despite 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.

Evaluation of the effects of shaking intensity on the process of methylene blue discoloration by metallic iron.

The term mixing (shaking, stirring, agitating) is confusing because it is used to describe mass transfer in systems involving species dissolution, species dispersion and particle suspension. Each of these mechanisms requires different flow characteristics in order to take place with maximum efficiency. This work was performed to characterize the effects of shaking intensity on the process of aqueous discoloration of methylene blue (MB) by metallic iron (Fe(0)). The extent of MB discoloration by three different materials in five different systems and under shaking intensities varying from 0 to 300 min(-1) was directly compared. Investigated materials were scrap iron (Fe(0)), granular activated carbon (GAC), and deep sea manganese nodules (MnO(2)). The experiments were performed in essay tubes containing 22 mL of the MB solution (12 mg/L or 0.037 mM). The essay tubes contained either: (i) no reactive material (blank), (ii) 0-9.0 g/L of each reactive material (systems I, II and III), or (iii) 5g/L Fe(0) and 0 to 9.0g/L GAC or MnO(2) (systems IV and V). The essay tubes were immobilized on a support frame and shaken for 0.8-5 days. Non-shaken experiments lasted for duration up to 50 days. Results show increased MB discoloration with increasing shaking intensities below 50 min(-1), a plateau between 50 and 150 min(-1), and a sharp increase of MB discoloration at shaking intensities >or=200 min(-1). At 300 min(-1), increased MB discoloration was visibly accompanied by suspension of dissolution products of Fe(0)/MnO(2) and suspension of GAC fines. The results suggest that, shaking intensities aiming at facilitating contaminant mass transfer to the Fe(0) surface should not exceed 50 min(-1).

An analysis of the evolution of reactive species in Fe0/H2O systems.

Aqueous contaminant removal in the presence of metallic iron (e.g. in Fe(0)/H(2)O systems) is characterized by the large diversity of removing agents. This paper analyses the synergistic effect of adsorption, co-precipitation and reduction on the process contaminant removal in Fe(0)/H(2)O systems on the basis of simple theoretical calculations. The system evolution is characterized by the percent Fe(0) consumption. The results showed that contaminant reduction by Fe(0) is likely to significantly contribute to the removal process only in the earliest stage of Fe(0) immersion. With increasing reaction time, contaminant removal is a complex interplay of adsorption onto iron corrosion products, co-precipitation or sequestration in the matrix of iron corrosion products and reduction by Fe(0), Fe(II) or H(2)/H. The results also suggested that in real world Fe(0)/H(2)O systems, any inflowing contaminant can be regarded as foreign species in a domain of precipitating iron hydroxides. Therefore, current experimental protocols with high contaminant to Fe(0) ratios should be revisited. Possible optimising of experimental conditions is suggested.