Fenton-like degradation of 2,4-dichlorophenol (2,4-DCP) in aqueous solution was investigated over Fe3O4 magnetic nanoparticles (MNPs) as catalyst. The obtained samples were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), nitrogen adsorption–desorption isotherms, and physical property measurement system (PPMS). The catalytic results showed that Fe3O4 MNPs presented good properties for the degradation and mineralization of 2,4-DCP, achieving complete decomposition of 2,4-DCP and 51% of TOC removal after 180 min at reaction conditions of H2O2 12 mM, Fe3O4 MNPs 1.0 g/L, 2,4-DCP 100 mg/L, pH 3.0 and T 30 °C. The effect of different reaction parameters such as initial pH, H2O2 dosage, Fe3O4 MNPs addition, initial concentration of 2,4-DCP and temperature on two-stage first-order kinetics of 2,4-DCP degradation was studied. A high utilization efficiency of H2O2 calculated as 73% was observed. According to the analyses of iron leaching, reactive oxidizing species and degradation intermediates, a possible mechanistic steps of 2,4-DCP degradation dominated by OH reactions (especially by free OH in the bulk solution) were proposed. Besides, stability and reusability of Fe3O4 MNPs were tested.

Fenton-like degradation of 2,4-dichlorophenol using Fe3O4 magnetic nanoparticles

A series of manganese oxides (MnO, MnO2, Mn2O3 and Mn3O4) were synthesized and tested in heterogeneous activation of peroxymonosulfate (PMS) for phenol degradation in aqueous solutions. Their properties were characterized by several techniques such as X-ray diffraction (XRD), thermogravimetric-differential thermal analysis (TG-DTA), scanning electron microscopy (SEM), and N2 adsorption/desorption isotherms. Catalytic activities of Mn oxides were found to be closely related to the chemical states of Mn. Mn2O3 is highly effective in heterogeneous activation of PMS to produce sulfate radicals for phenol degradation compared with other catalysts (MnO, MnO2, and Mn3O4). The activity shows an order of Mn2O3 > MnO > Mn3O4 > MnO2. Mn2O3 could completely remove phenol in 60 min at the conditions of 25 mg/L phenol, 0.4 g/L catalyst, 2 g/L PMS, and 25 °C. After heat regeneration, the activity could be fully recovered. A pseudo first order model would fit to phenol degradation kinetics and activation energy was obtained as 11.4 kJ/mol.

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Manganese oxides at different oxidation states for heterogeneous activation of peroxymonosulfate for phenol degradation in aqueous solutions

Heterogeneous catalytic treatment of synthetic dyes in aqueous media using Fenton and photo-assisted Fenton process

Iron is the fourth most common element by mass in the Earth’s crust and forms compounds in several oxidation states. Iron (hydr)oxides, some of which form inherently and exclusively in the nanometre-size range, are ubiquitous in nature and readily synthesized. These facts add up to render many Fe (hydr)oxides suitable as catalysts, and it is hardly surprising that numerous studies on the applications of Fe (hydr)oxides in catalysis have been published. Moreover, the abundant availability of a natural Fe source from rocks and soils at minimal cost makes the potential use of these as heterogeneous catalyst attractive.

Besides those Fe (hydr)oxides that are inherently nanocrystalline (ferrihydrite, Fe5HO8·4H2O, and feroxyhyte, δ′-FeOOH), magnetite (Fe3O4) is often used as a catalyst because it has a permanent magnetization and contains Fe in both the divalent and trivalent states. Hematite, goethite and lepidocrocite have also been used as catalysts in their pure forms, doped with other cations, and as composites with carbon, alumina and zeolites among others.

In this review we report on the use of synthetic and natural Fe (hydr)oxides as catalysts in environmental remediation procedures using an advanced oxidation process, more specifically the Fenton-like system, which is highly efficient in generating reactive species such as hydroxyl radicals, even at room temperature and under atmospheric pressure. The catalytic efficiency of Fe (hydr)oxides is strongly affected by factors such as the Fe oxidation state, surface area, isomorphic substitution of Fe by other cations, pH and temperature.

Iron oxide catalysts: Fenton and Fenton-like reactions - a review

S-doped α-Fe2O3 (α-Fe2O3/S) was synthesized with ferrous sulfate and Na2S2O3 via a hybrid hydrothermal-calcination treatment. The crystal phase, special surface area, morphology of the α-Fe2O3/S as well as the chemical state of the sulfur were studied by X-ray diffraction (XRD), scanning electro microscopy (SEM), Raman spectrum and X-ray photoelectron spectroscopy (XPS). The Fenton and photo-Fenton reactivities of the α-Fe2O3/S were tested by degrading acid orange 7 (AO7) and phenol. Although α-Fe2O3/S showed little Fenton reactivity in the dark, it had an excellent heterogeneous Fenton reaction under either UV or visible irradiation, while other α-Fe2O3s were still kept inactive. S element was found to dope into α-Fe2O3 in the forms of FeS or FeS2 in α-Fe2O3/S. The doped S element promotes the photo-Fenton reaction of α-Fe2O3/S via two roles: retarding the recombination of photogenerated charge carriers and promoting the electron transfer between the peroxide species and iron ions at the interface.

S-doped α-Fe2O3 as a highly active heterogeneous Fenton-like catalyst towards the degradation of acid orange 7 and phenol

Immobilized iron oxides on silica matrixes in fluidized bed reactors, including SiG1, SiG2, C1, and the commercial catalyst FeOOH, were used in the catalytic decomposition of H2O2 and the catalytic degradation of phenol. They were characterized using XRD, SEM, N2-sorption, and elucidation of the kinetics of dissolved iron by oxalic acid in dark surroundings. XRD patterns reveal that SiG1, SiG2, and C1 exhibit amorphous structures, and FeOOH exhibits the poor crystallinity of goethite. The SEM images reveal that the surfaces of all the iron oxides are smooth and that the iron oxides are aggregated by the iron oxide floc. The N2-sorption isotherm indicates that SiG1 and SiG2 are non-porous materials, and that C1 and FeOOH are typical type II and typical type IV materials, respectively. A kinetic model for iron dissolved by oxalic acid is established. The order of apparent first-order dissolution rate constants (kc) is SiG1 > SiG2 > FeOOH ∼ C1. The immobilized iron oxides, SiG1 and SiG2, are weakly bonded to the support (silica sand) in the presence of oxalic acid. The decomposition of H2O2 follows pseudo-first-order kinetics. The number of active sites for the decomposition of H2O2 is similar among all iron oxides at a particular kapp (1.8 × 10−3 min−1). There are no interactions between phenol and iron oxides in the absence of hydrogen peroxide at pH 4. SiG1 and SiG2 exhibit much higher catalytic activities in phenol degradation than either C1 or FeOOH. The reactivity of iron oxides in catalyzing the phenol degradation by H2O2 relates to the tendency of iron to be dissolved by oxalic acid. The intermediates of phenol degradation, such as catechol and oxalic acid, promote the dissolution of iron from SiG1 and SiG2 by reductive and non-reductive pathways and lower the pH values. The catalyses of SiG1 and SiG2 involve heterogeneous and homogeneous reactions.

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Comparison of catalytic decomposition of hydrogen peroxide and catalytic degradation of phenol by immobilized iron oxides

Kinetic study of an immobilized iron oxide for catalytic degradation of azo dye reactive black B with catalytic decomposition of hydrogen peroxide

A batch photo-fluidized bed reactor (photo-FBR) is designed for the mineralization of phenol and the minimization of iron species leached from an active immobilized iron oxide (denoted as SiG2). The hydroxyl radical ( OH), which is produced from the reaction of hydrogen peroxide (H 2 O 2 ) and SiG2 under irradiation by a 400-W Hg-vapor lamp, was confirmed by electron paramagnetic resonance signals of 5,5-DIMETHYL-1-pyrroline- N -oxide (DMPO)- OH (aqueous solution). The mineralization of phenol was more efficient in the presence of light. In general, a decrease of solution pH is commonly ascribed to the formation of carboxylic acids (acetic acid, oxalic acid, formic acid etc.) during the degradation of phenol. However, in this study, an interesting phenomenon was observed. The solution pH first decreased and then rose during the reaction in the presence of light. Also, in this case, the variation of solution pH reflected the degree of mineralization in this study. The approximately 98% mineralization of phenol led to fewer iron species being leached from SiG2 in the photo-FBR. A savings of more than 40% of the H 2 O 2 dosage required for the phenol mineralization process was confirmed. In a batch photo-FBR, the system loading reflects the ratio of catalyst weight to the reaction volume. About 96%, 86% and 11% of the total organic carbons were removed after 180 min, resulting in catalyst weight to reaction volume ratios of 30:1, 20:1 and 13:1, respectively. The activity of SiG2 did not decline after six reactions. The reactions for photo-assisted SiG2 in the presence of hydrogen peroxide and phenol were summarized.

Application of an active immobilized iron oxide with catalytic H2O2 for the mineralization of phenol in a batch photo-fluidized bed reactor

The separation of anion and cation exchange resins has two advantages: (1) simplifying further treatment such as wet oxidation process, and (2) reducing further shielding, storage and disposal costs of radioactive wastes. In this paper two methods including the resin bead fluidization with fluid in a column and immersion into a solution with proper density to separate the resins were investigated. When the up-flowrate was operated between 40 and 80 cm/min, approximately 80% of anion exchange resin was retrieved and the immersion method using a 16–20 wt% of NaOH solution resulted in the anion and cation exchange resins being easily separated although with the most of Cs and partial of Co and Sr ions/radionuclides on the resins exchanged by Na from the solution. After separation the anion exchange resin from radioactive spent resins retrieved from the decommissioning activities in Taiwan Research Reactor, the volume of radioactive wastes with higher radiation and long half-life radionuclides could be reduced. 

Chun-Ping Huang

Papers

Comparison of catalytic decomposition of hydrogen peroxide and catalytic degradation of phenol by immobilized iron oxides

Kinetic study of an immobilized iron oxide for catalytic degradation of azo dye reactive black B with catalytic decomposition of hydrogen peroxide

Application of an active immobilized iron oxide with catalytic H2O2 for the mineralization of phenol in a batch photo-fluidized bed reactor

Separation anion and cation exchange resins using fluidization and adjusted NaOH solution density methods