The limitations of applying zero-valent iron technology in contaminants sequestration and the corresponding countermeasures: the development in zero-valent iron technology in the last two decades (1994-2014).

Rapid decolorization of azo dye methyl orange in aqueous solution by nanoscale zerovalent iron particles

Zero-valent iron (ZVI) nanoparticles tend to agglomerate, resulting in a significant loss in reactivity. To address this issue, synthesized bentonite-supported nanoscale zero-valent iron (B-nZVI) was used to remove azo dye methyl orange (MO) in aqueous solution. Batch experiments show that various parameters, such as pH, initial concentration of MO, dosage, and temperature, were affected by the removal of MO. Scanning electron microscopy (SEM) confirmed that B-nZVI increased their reactivity and a decrease occurred in the aggregation of iron nanoparticles for the presence of bentonite (B). Using B-nZVI, 79.46% of MO was removed, whereas only 40.03% when using nZVI after reacting for 10 min with an initial MO concentration of 100 mg/L (pH = 6.5). Furthermore, after B-nZVI reacted to MO, XRD indicated that iron oxides were formed. FTIR showed that no new bands appeared, and UV–vis demonstrated that the absorption peak of MO was degraded. Kinetics studies showed that the degradation of MO fitted well to the pseudo first-order model. A degradation mechanism is proposed, including the following: oxidation of iron, adsorption of MO to B-nZVI, formation of Fe(II)–dye complex, and cleavage of azo bond. Finally, the removal rate of MO from actual wastewater was 99.75% when utilizing B-nZVI.

Removal of methyl orange from aqueous solution using bentonite-supported nanoscale zero-valent iron

A 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 co‐precipitated 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 l...

A critical review on the process of contaminant removal in fe0–h2o systems

The degradation of Orange G, a monoazo dye, in aqueous solutions was investigated using Fe-Ni bimetallic nanoparticles. Transmission electron microscopy (TEM) of as-synthesized nanoparticles showed the presence of spherical particles having a size of 20–40 nm. X-ray photoelectron spectroscopy (XPS) did not detect the presence of nickel on the nanoparticle surface, which suggested a uniform distribution of both metals inside the particle core. Batch experiments with a minimum nanocatalyst loading of 3 g/L showed complete dye degradation after 10 min of reaction time. The degradation efficiency was linearly dependent on the initial dye concentration, pH of the solution and total Fe-Ni catalyst concentration. The efficiency increased with increasing Fe-Ni concentration and decreasing pH of the solution, but decreased with an increase in the dye concentration. The degradation rate followed first order reaction kinetics with respect to the dye concentration. High performance liquid chromatography–mass spectrometry (HPLC–MS) analysis of the degradation products revealed that the degradation mechanism proceeds through a reductive cleavage of the azo linkage resulting in the formation of aniline and surface-adsorbed naphthol amine derivatives. The latter are subsequently hydroxylated through an oxidative process.

Iron-nickel bimetallic nanoparticles for reductive degradation of azo dye Orange G in aqueous solution

While decomposition of water pollutants in the presence of metallic iron can be strongly influenced by the nature and structure of the iron surface layer, the composition and structure of the layer produced and transformed in the decomposition process, have been meagerly investigated. The studies presented here establish strong relationships between the composition and structure of the iron oxidized surface layer and the kinetics and reaction pathways of orange II decomposition. The most striking observation is a dramatic difference between dye decomposition at pH 3 and 4. Orange decomposition at pH 2 and 3 is a very fast process with pseudo-first-order kinetics, with a surface normalized rate constant kSA = 0.18 L/m2 min at pH 3 and 30 °C. Whereas at pH 4 and 5 the rate is lower with pseudo-zero-order kinetics, with normalized rate constant kSA = 1.4 × 10−5 mol/m2 min at pH 5 and 30 °C. At pH 3 the iron surface is covered by a polymeric Fe(OH)2 mixed with FeO very thin layer whose thickness remains almost constant with reaction time. There is a slow formation of an additional surface product with akaganeite-like structure. At pH 3 almost all oxidized iron is detected in solution, whereas at pH 5 almost total oxidized iron is cumulated on iron surface in the form of a lepidocrocite, γ-FeOOH, layer. The thickness of the layer increases continuously with time. The quantitative evaluation of the produced surface lepidocrocite and its surface distribution were performed by means of infrared reflection spectroscopy and spectral simulation methods. At higher temperature 40–50 °C, other surface products such as goethite, α-FeOOH, and feroxyhite, β-FeOOH, are also observed. Decomposition of orange is a multi-step process, at pH 3 the orange molecule is at first adsorbed on the very thin iron oxidized layers through SO3 group and then undergoes reduction. Discoloration of orange II in aerobic solution takes place by reduction of the single bondNdouble bond; length as m-dashNsingle bond bond at the iron surface. The major intermediate is 1-amino-2-naphtol, which undergoes further decomposition without forming any aromatic species. The previously suggested sulfanilic acid as intermediate was not detected in solution. At pH 3 orange reduction and reduction of intermediates are governed by the combination of an electron transfer reaction, with the thin oxide surface layer as a mediator, and the catalytic hydrogenation reaction. At pH 4 and 5 continuous growing of lepidocrocite surface layer demonstrates the importance of the layer as a mediator in the electron transfer reaction. The layer shows a good conductivity, which results from adsorption and absorption of iron ions in the surface structure. It is observed that the decomposition reaction becomes significant at open circuit potential (OCP) below −120 mV (SHE). At pH 3 this condition is fulfilled almost immediately after introduction of iron to aqueous solution, whereas at pH 4 and 5 the OCP of iron decreases very slowly. Iron surface layer composition and structure can be modified by an addition of Fe2+ to solution, which increases the dye decomposition rate. The performed observations make the treatment of waste water in the presence of metallic iron a promising environmental solution.

Role of iron surface oxidation layers in decomposition of azo-dye water pollutants in weak acidic solutions

Although oxidation/passivation of iron in basic solution has been extensively investigated, there is very little information on iron corrosion in weak acidic solutions. In this work, iron surface composition and structure, produced in aerobic aqueous solutions ranging from pH 2 to 5, were determined in detail by the use of infrared external reflection spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy. The most striking observation is that at pH 2 and 3 almost all oxidized iron is dissolved in solution, whereas at pH 4 and 5 the product of iron oxidation is deposited on the iron surface in the form of lepidocrocite, gamma-FeOOH. Detailed iron surface and solution analyses allow the proposition of the following overall oxidation reactions: [EQUATION: SEE TEXT]. At pH 2 and 3, only a very thin surface layer consisting of FeO and Fe(OH)2 with polymeric structure is observed on the iron surface. The amounts of these surface species remain almost constant (2-5 nm) from the first minutes to a few hours of reaction, if pH is kept constant. Nevertheless, with time the akaganeite-like, beta-FeOOH structure is also detected. At pH 4 and 5, the amount of lepidocrocite deposited on the iron surface increases with reaction time. Detailed quantitative evaluation of the lepidocrocite produced at pH 5 and its surface distribution on iron was performed based on the comparison of infrared spectroscopic data with spectral simulation results of assumed surface structures. At pH 4 and 5 and a temperature of 40-50 degrees C, in addition to a very large amount of lepidocrocite other oxy-hydroxide surface species such as goethite (alpha-FeOOH) and feroxyhite (delta-FeOOH), were identified. Addition of Cl- ions to solution at 10(-3) M concentration at pH 5 increases the oxidation rate of iron by about 50%, and lepidocrocite remains the only oxidation product. Similarly, an addition of Fe2+ ions to solution at pH 5 very strongly enhances lepidocrocite formation as well as its conductivity. The latter finding is important for the possible application of metallic iron as a catalyst in redox reactions, for example, for decomposition of difficult-to-biodegrade water pollutants.

Gonzalo Montes Atenas

Papers

Role of iron surface oxidation layers in decomposition of azo-dye water pollutants in weak acidic solutions

Composition and structure of iron oxidation surface layers produced in weak acidic solutions.

Remarkable influence of surface composition and structure of oxidized iron layer on orange I decomposition mechanisms.