Rochel M. Lago

Activated carbon / iron oxide magnetic composites for the adsorption of contaminants in water

Abstract In this work the adsorption features of activated carbon and the magnetic properties of iron oxides were combined in a composite to produce magnetic adsorbents. These magnetic particles can be used as adsorbent for a wide range of contaminants in water and can subsequently be removed from the medium by a simple magnetic procedure. Activated carbon/iron oxide magnetic composites were prepared with weight ratios of 2:1, 1.5:1 and 1:1 and characterized by powder XRD, TG, magnetization measurements, chemical analyses, TPR, N2 adsorption–desorption isotherms, Mossbauer spectroscopy and SEM. The results suggest that the main magnetic phase present is maghemite (γ-Fe2O3) with small amounts of magnetite (Fe3O4). Magnetization enhancement can be produced by treatment with H2 at 600 °C to reduce maghemite to magnetite. N2 adsorption measurements showed that the presence of iron oxides did not significantly affect the surface area or the pore structure of the activated carbon. The adsorption isotherms of volatile organic compounds such as chloroform, phenol, chlorobenzene and drimaren red dye from aqueous solution onto the composites also showed that the presence of iron oxide did not affect the adsorption capacity of the activated carbon.

Clay–iron oxide magnetic composites for the adsorption of contaminants in water

Abstract In this work, the adsorption features of clays with the magnetic properties of iron oxides have been combined in a composite to produce a magnetic adsorbent. These magnetic composites can be used as adsorbent for contaminants in water and can be subsequently removed from the medium by a simple magnetic process. The bentonite–iron oxide magnetic composites have been prepared with weight ratios of 2:1, 1.5:1, and 1:1 and characterized by powder X-ray diffraction (XRD), thermogravimetric analysis (TG), magnetization measurements, chemical analyses, temperature-programmed reduction (TPR), N 2 adsorption–desorption isotherms, Mossbauer spectroscopy, and scanning electron microscopy (SEM). The results suggest that the main magnetic phase present is maghemite (γ-Fe 2 O 3 ). A magnetization enhancement can be produced by treatment with H 2 at 600 °C to reduce maghemite to magnetite. Nitrogen adsorption isotherms showed that the surface area and microporosity increased from 7 m 2 g −1 ( V micropores =0.003 cm 3 g −1 ) for the pure bentonite to 55 m 2 g −1 ( V micropores =0.009 cm 3 g −1 ) for the composite clay/iron oxide (2:1). The adsorption isotherms of metal ions Ni 2+ , Cu 2+ , Cd 2+ , and Zn 2+ from aqueous solution onto the composites also showed that the presence of iron oxide produced an increase on the adsorption capacity of the bentonite.

Hydrogen peroxide decomposition over Ln1−xAxMnO3 (Ln = La or Nd and A = K or Sr) perovskites

The following perovskites La1−xKxMnO3 (x=0–0.15), La1−xSrxMnO3 (x=0–0.3), Nd1−xSrxMnO3 (x=0–0.4) and Nd1−xKxMnO3 (x=0–0.15) have been prepared by the freeze-drying method. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy spectra (XPS) analyses showed the presence of the single crystalline perovskite phases with an enrichment of the surface with potassium for the Ln1−xKxMnO3 samples. δ-Oxygen non-stoichiometries of the perovskites were studied by chemical analyses, temperature programmed reduction (TPR) and oxygen desorption (TPD) experiments. The results obtained suggest that the catalytic H2O2 decomposition over these perovskites is not determined by the surface K, Ln and Mn concentration nor by the Mn4+/Mn3+ surface ratio. On the other hand, a good correlation between the catalytic H2O2 decomposition and the δ-oxygen non-stoichiometries was found, especially for La perovskites. A suprafacial mechanism where the oxygen vacancies participate in the H2O2 decomposition is suggested.

Application of Fenton’s reagent to regenerate activated carbon saturated with organochloro compounds

In this work Fentons reagent was used to treat activated carbon saturated with organochloro compounds for the oxidation of the adsorbed contaminants and the regeneration of the carbon adsorbent. Activated carbon containing adsorbed chlorinated model substrates such as chlorobenzene, tetrachloroethylene, chloroform or 1,2-dichloropropane was treated with Fentons reagent at room temperature resulting in a rapid consumption of the organochloro compounds. Thermogravimetric, infrared, BET surface area and MIMS adsorption studies showed that Fentons treatment has no significant effect on the physico-chemical properties of the activated carbon. The used carbon adsorbents can be efficiently regenerated and recycled with no loss of its adsorption capacity even after five consecutive treatments with Fentons reagent.

Controlled reduction of red mud waste to produce active systems for environmental applications: Heterogeneous Fenton reaction and reduction of Cr(VI)

In this work, controlled reduction of red mud with H(2) was used to produce active systems for two different environmental applications, i.e. the heterogeneous Fenton reaction and the reduction of Cr(VI). Mössbauer, powder X-ray diffraction, thermal analyses and scanning electron microscopy analyses showed that at different temperatures, i.e. 300, 400, 500 and 600 degrees C, H(2) reduces red mud to different phases, mainly Fe(3)O(4), Fe(0)/Fe(3)O(4) and Fe(0). These Fe phases are dispersed on Al, Si and Ti oxides present in the red mud and show high reactivity towards two environmental applications, i.e. the heterogeneous Fenton reaction and the reduction of Cr(VI). Reduction with H(2) at 400 degrees C showed the best results for the oxidation of the model dye methylene blue with H(2)O(2) at neutral pH due to the presence of the composite Fe(0)/Fe(3)O(4). The reduced red mud at 500-600 degrees C produced Fe(0) highly active for the reduction of Cr(VI) in aqueous medium. Another feature of these red mud based system is that after deactivation due to extensive use they can be completely regenerated by simple treatment with H(2).

Use of activated carbon as a reactive support to produce highly active-regenerable Fe-based reduction system for environmental remediation

Composites based on iron supported on high surface area activated carbon were prepared and characterized with (57)Fe Mössbauer spectroscopy, X-ray diffraction, saturation magnetization measurements and temperature-programmed reduction. Upon thermal treatment, the supported iron oxides react with carbon to yield reduced chemical species, i.e. Fe(3)O(4) and Fe(0). This so produced composite was found to be highly efficient in two environmental applications: (i) degradation of textile dye and (ii) reduction of Cr(VI) in aqueous medium. Sequential reuses evidenced a progressive chemical deactivation of the composites due to a corresponding oxidation of the reactive species. Even after being virtually deactivated, the initial chemical reducing ability of the composites can be regenerated by heating at 800 degrees C under N(2) atmosphere, and then reused for several consecutive times.

Iron: a versatile element to produce materials for environmental applications

Iron is a versatile element forming several phases with different oxidation states and structures, such as Feo, FeO, Fe3O4, γ-Fe2O3, α-Fe2O3 and FeOOH. All these phases have unique physicochemical properties which can be used for different applications. In this work, it is described the use of different iron compounds, synthetic and also from natural and waste sources, in environmental and technological applications. Two main research areas are described. The first one is related to strategies to increase the reactivity of Fe phases, mainly by the formation of Feo/iron oxide composites and by the introduction of new metals in the iron oxide structure to promote new surface reactions. The second area is the use of the magnetic properties of some iron phases to produce versatile magnetic materials with focus in adsorption, catalysis and emulsions.

Magnetic amphiphilic composites based on carbon nanotubes and nanofibers grown on an inorganic matrix: effect on water-oil interfaces

Novos compositos magneticos anfifilicos foram preparados pelo crescimento de nanotubos e nanofibras de carbono contendo particulas magneticas atraves de deposicao quimica de vapor (CVD), utilizando etanol como fonte de carbono e lama vermelha (RM, subproduto do processo Bayer de producao de alumina) como suporte e catalisador. Monitoramento da reacao CVD a temperatura programada (TPCVD), difracao de raios X (XRD), espectroscopia Mossbauer, espectroscopia de energia dispersiva (EDS), espectroscopia Raman, termogravimetria (TG/DTA), analise elementar (CHN), determinacao de area superficial (BET), microscopia eletronica de varredura (SEM) e de transmissao (TEM) e medidas magneticas mostraram que etanol reduz ions de ferro na RM para formar fases magneticas, por exemplo Fe 3 O 4 e Fe 0 , e depositos de carbono (5-42 wt.%) na forma de nanotubos e nanofibras. A combinacao de nanoestruturas hidrofobicas de carbono com oxidos hidrofilicos de Al, Si e Ti presentes na lama vermelha produziu materiais anfifilicos com excelente interacao com a interface agua-oleo. Misturas de oleo de soja ou de decalina com agua (completamente imisciveis) foram emulsificadas facilmente na presenca dos compositos anfifilicos. Quando os compositos foram adicionados a uma emulsao agua-biodiesel estavel, as particulas anfifilicas difundiram-se para a interface agua- oleo. As particulas do composito foram atraidas por imas e carregaram com elas as gotas de oleo, levando a completa desemulsificacao e separacao entre biodiesel e agua. New magnetic amphiphilic composites were prepared by the catalytic carbon vapor deposition (CVD) growth of carbon nanotubes and nanofibers using ethanol as carbon source and red mud waste (RM, a by-product of the Bayer process of alumina production) as catalyst and support. Temperature-programmed CVD (TPCVD), analyses by X-ray diffractometry (XRD), Mossbauer spectroscopy, energy dispersive X-ray spectroscopy (EDS), Raman spectroscopy, thermogravimetry (TG/DTA), elemental analysis (CHN), superficial area determination (BET), scanning (SEM) and transmission (TEM) electron microscopies and magnetic measurements showed that ethanol reduces the iron ions in the red mud to form magnetic phases, e.g., Fe 3 O 4 and Fe 0 , and carbon deposits (5-42 wt.%), particularly nanotubes and nanofibers. The combination of the hydrophobic carbon nanostructures with the hydrophilic Al, Si and Ti oxides present in the RM produced amphiphilic materials with excellent interaction with the water-oil interface. Soybean oil or decalin mixtures with water (completely immiscible) were easily emulsified in the presence of the amphiphilic composites. When the composites were added to stable biodiesel-water emulsions, the amphiphilic particles diffused to the interface oil-water. These composite particles were attracted by a magnet, carrying the oil droplets with them and leading to the complete demulsification and separation of the biodiesel from the water.

On‐line monitoring by membrane introduction mass spectrometry of chlorination of organics in water. Mechanistic and kinetic aspects of chloroform formation

Chloroform formation during the chlorination of simple organic molecules modeling humic substances, such as phenol and di- and trihydroxybenzenes, was studied by on-line membrane introduction mass spectrometry (MIMS). Under the reaction conditions employed, chloroform was rapidly formed from 1,3-dihydroxybenzene, 1, 4-dihydroxybenzene, phenol and 1,2,3-trihydroxybenzene with yields of 17, 13, 7 and 5%, respectively. With the exception of aniline, which afforded a 17% chloroform yield, non-phenolic compounds, such as nitrobenzene, chlorobenzene, toluene, benzene and cyclohexanol, furnished low yields. Mechanistic studies showed that phenol is chlorinated consecutively and produces initially chlorophenol. It is suggested that chloroform might be formed mainly from chlorinated 3, 5-cyclohexadienone-type intermediates. MIMS was also used to determine the reaction rates and to study the kinetics of the chlorination. A good Hammett linear correlation for an electrophilic substitution mechanism was found for the compounds C(6)H(5)X (X = NH(2), OH, CH(3), H, Cl and NO(2)).

The effect of Mn substitution on the catalytic properties of ferrites

Fe3-xMnxO4 (x=0.00; 0.21; 0.26 and 0.53) spinels have been characterized by Mossbauer, XRD, TG, DSC and TPR experiments. XRD and Mossbauer data corroborate the Mn incorporation in the spinel lattice. These Mn containing ferrites have been tested as catalysts for two different reactions: the gas phase CO oxidation and the aqueous oxidation of organochloro contaminants with H2O2. During CO oxidation, it was observed that the presence of Mn strongly affects the phases transformation Fe3O4→ψ-Fe2O3 (maghemite)→-Fe2O3 (hematite). The Mn-substituted magnetite series shows the highest catalytic for CO oxidation at low temperatures but during the reaction it is oxidised to maghemite which is less active. At higher temperatures, maghemite is converted to hematite, specially the Mn-containing samples, resulting in a strong increase in CO conversion. These ferrites were also active for the oxidation of chlorobenzene in water with H2O2 being the catalytic activity strongly increased by Mn-containing catalysts. The reaction mechanism apparently involves a Fenton-like radical chemical pathway.