Role of nanomaterials in water treatment applications: a review.

There is great interest in iron oxides, especially in nanosized form, for both fundamental and practical reasons. Because of its polymorphism, iron(III) oxide (ferric oxide, Fe2O3) is one of the most interesting and potentially useful phases of the iron oxides. Each of the four different known crystalline Fe2O3 polymorphs (alpha-, beta-, gamma-, and epsilon-Fe2O3) has unique biochemical, magnetic, catalytic, and other properties that make it suitable for specific technical and biomedical applications. High temperature treatment is a key step in most syntheses of iron(III) oxides but often triggers polymorphous transformations that result in the formation of undesired mixtures of Fe2O3 polymorphs. It is therefore important to control the parameters that induce polymorphous transformations when seeking to prepare a given Fe2O3 polymorph as a single phase; identifying and understanding these parameters is a major challenge in the study of the polymorphism of solid compounds. This review discusses the depende...

Polymorphous Transformations of Nanometric Iron(III) Oxide: A Review

A promising route to increase the performance of hematite (alpha-Fe2O3) photoelectrodes for solar hydrogen production through water-splitting is to use an extremely thin layer of this visible light absorber on a nanostructured scaffold. However, the typically poor performance of ultrathin (ca. 20 nm) films of hematite has been the limiting factor in implementing this approach. Here, the surprising effect of a substrate pretreatment using tetraethoxysilicate (TEOS) is reported; it results in drastic improvements in the photoperformance of 12.5 nm thick films of hematite. These films exhibit a water oxidation photocurrent onset potential at 1.1V versus the reversible hydrogen electrode (vs. RHE) and a plateau current of 0.63 mA cm(-2) at 1.5 V vs. RHE under standard illumination conditions, representing the highest reported performance for ultrathin hematite films. In contrast, almost no photoactivity is observed for the photoanode with the same amount of hematite on an untreated substrate. A detailed study of the effects of the TEOS treatment shows that a monolayer of SiOx is formed, which acts to change the hematite nucleation and growth mechanism, increases its crystallinity, reduces the concentration of carrier trapping states of the ultrathin films, and suggests its further application to quantum-dot and extremely-thin-absorber (ETA)-type solar cells.

Controlling Photoactivity in Ultrathin Hematite Films for Solar Water-Splitting

The heterogeneous photocatalytic water purification process has gained wide attention due to its effectiveness in degrading and mineralizing the recalcitrant organic compounds as well as the possibility of utilizing the solar UV and visible light spectrum. This paper aims to review and summarize the recently published works in the field of photocatalytic oxidation of toxic organic compounds such as phenols and dyes, predominant in waste water effluent. In this review, the effects of various operating parameters on the photocatalytic degradation of phenols and dyes are presented. Recent findings suggested that different parameters, such as type of photocatalyst and composition, light intensity, initial substrate concentration, amount of catalyst, pH of the reaction medium, ionic components in water, solvent types, oxidizing agents/electron acceptors, mode of catalyst application, and calcinations temperature can play an important role on the photocatlytic degradation of organic compounds in water environment. Extensive research has focused on the enhancement of photocatalysis by modification of TiO2 employing metal, non-metal and ion doping. Recent advances in TiO2 photocatalysis for the degradation of various phenols and dyes are also highlighted in this review.

Advances in heterogeneous photocatalytic degradation of phenols and dyes in wastewater : a review

A high surface area K-promoted iron-based catalyst is prepared by thermal decomposition of ammonium glycolate complexes, followed by impregnation with an aqueous solution of potassium carbonate, drying and calcination. It is found that the duration of the calcination has a dramatic effect on the textural and structural properties of the obtained material, as well as on the iron oxidation state. Fast calcinations allow to synthetize materials isostructural with Fe3O4/γ-Fe2O3 that, after a carburization treatment with CO/H2 mixtures, are highly active in the CO2 hydrogenation to lower (C2–C4) olefins at mild process conditions (300 °C and 5 barg). Interestingly, the decrease of the operating pressure leads to a shift from the “hydrocarbon synthesis” regime to reverse water gas shift regime. The selectivity to lower olefins is instead maximised at mild process conditions due to the moderate chain growth probability and the slow rate of olefin secondary hydrogenations, which dominate at higher pressure and temperatures, respectively. The former process is activated by type II catalytic sites (iron carbides), while the latter occurs on type III catalytic sites (Fe0). Type I catalytic sites also exist (Fe3O4) and are responsible for the RWGS activity of the catalyst. Collected data suggest that CO is the primary product of CO2 hydrogenation, while hydrocarbons are formed via CO hydrogenation, following a Fischer-Tropsch type mechanism. Eventually, the performances of the prepared catalyst are compared with those of K-promoted reference model samples based on commercial α-Fe2O3 and Fe3O4 powders. It is found that, due to a better carburization and the higher surface area, the catalyst prepared by fast decomposition of ammonium glycolate complexes is more active than the reference materials in terms of both CO2 conversion and C2–C4 olefins selectivity.

CO2 hydrogenation to lower olefins on a high surface area K-promoted bulk Fe-catalyst

Magnetic iron oxides were prepared by precipitation of Fe(II) hydroxide using different precipitation agents: ammonia, benzylamine and sodium hydroxide, followed by oxidation with the oxygen dissolved in water. Thermal analysis, coupled with FTIR spectroscopy, has evidenced the formation of a mixture of magnetite and maghemite, with a higher content of magnetite in case of the powder synthesized with benzylamine. The stability of magnetite at oxidation by air during storage at room temperature and 60 °C was investigated by means of TG/DSC simultaneous thermal analysis, FTIR spectroscopy and X-ray diffractometry. Thermal analysis evidenced an exothermic process with mass gain in temperature range 100–190 °C, corresponding to magnetite oxidation process, but due to the superposition of other processes it could not offer quantitative information. FTIR spectroscopy has provided, especially through the first and second derivatives of FTIR spectra, the most valuable information regarding the evolution of magnetite to maghemite, due to their different characteristic bands. XRD technique has evidenced a slight shift of the main diffraction peaks at higher 2-theta values during the evolution of magnetite to maghemite. According to thermal analysis data, the powder synthesized with ammonia was completely oxidized after 15 days, while the other two powders, synthesized with benzylamine and sodium hydroxide, were completely oxidized after 110 days of keeping in air at room temperature. For a temperature of 60 °C, the oxidation was much faster; the oxidation process of the powder synthesized with benzylamine disappeared from TG/DSC curves after 1 day. All final powders were formed from nanoparticles with diameters up to 25 nm, with magnetic properties characteristic to nanometric maghemite.

Investigation of magnetite nanoparticles stability in air by thermal analysis and FTIR spectroscopy

Single and simultaneous adsorption of methyl orange and phenol onto magnetic iron oxide/carbon nanocomposites

Hybrid organic-inorganic materials, silica—ethylene glycol (EG), were prepared by the sol-gel process from mixtures of tetraetoxysilane (TEOS) and EG under acidic catalysis. The resulting hybrid material was studied through thermal analysis and FT-IR spectroscopy. This techniques evidenced the presence of EG in the silica matrix in three forms: free in the hybrid matrix pores, hydrogen bonded with the silanol groups and chemically bonded (through condensation) in the silica network. Through the thermal treatment of the hybrid matrix, we obtained a silica matrix, with specific surface of 360 m2/g.

Thermal and FT-IR study of the hybrid ethylene-glycol–silica matrix

Novel low temperature synthesis method for nanocrystalline zinc and magnesium chromites

This paper presents a study for the preparation of CoxFe3−xO4 (x = 0.02, 0.2, 0.5, 0.8, 1.0, 1.1, 1.5) nanoparticles, starting from metal nitrates: Co(NO3)2·6H2O, Fe(NO3)3·9H2O and ethylene glycol (C2H6O2). By heating the solutions metal nitrates-ethylene glycol, the redox reaction took place between the anion NO3− and OH–(CH2)2–OH with formation of carboxylate anions. The resulted carboxylate anions reacted with Co(II) and Fe(III) cations to form coordinative compounds which are precursors for cobalt ferrite. XRD and magnetic measurements have evidenced the formation of cobalt ferrite for all studied molar ratios. The average diameter of the cobalt ferrite crystallites was estimated from XRD data and showed values in the range 10–20 nm. The crystallites size depends on the annealing temperature. The magnetization of the synthesized samples depends on the molar ratio Co/Fe and on the annealing temperature.

Marcela Stoia

Papers

Investigation of magnetite nanoparticles stability in air by thermal analysis and FTIR spectroscopy

Single and simultaneous adsorption of methyl orange and phenol onto magnetic iron oxide/carbon nanocomposites

Thermal and FT-IR study of the hybrid ethylene-glycol–silica matrix

Novel low temperature synthesis method for nanocrystalline zinc and magnesium chromites

Preparation of CoxFe3−xO4 nanoparticles by thermal decomposition of some organo-metallic precursors