Desenvolvimento de compósitos ZnC2O4/xerogel de carbono para degradação fotocatalítica de azul de metileno sob radiação solar

O desenvolvimento de novos compósitos oxalato de zinco (ZnC2O4)/xerogel de carbono para aplicações fotocatalíticas foi avaliado. O uso do xerogel de carbono é justificado por sua alta condutividade, que facilita a separação de cargas geradas na estrutura do oxalato de zinco. A eficiência fotocatalítica dos materiais foi avaliada pela decomposição do azul de metileno. Os materiais foram produzidos pela dispersão do oxalato de zinco em gel de carbono, durante sua policondensação em meio ácido. Os difratogramas de raios-X evidenciam a existência da fase β-ZnC2O4  no compósito desenvolvido. O material apresenta significativa absorção de radiação na faixa de comprimentos de onda visível. O compósito apresentou degradação completa do azul de metileno (10 mg L-1, 0.5 L) no sistema após 3 h sob luz solar simulada, com dosagem de 50 mg L-1 de catalisador, evidenciando o potencial uso desse material em processos de tratamento de efluentes. Palavras-chave: Oxalato de Zinco. Xerogel de carbono. Fotocatálise

Synthesis of hydrous niobium oxide nanoparticles by reverse microemulsion

Com o objetivo de gerar um material mais homogêneo quanto à forma e tamanho das partículas formadas (nanopartículas), o presente trabalho visa o estudo das variáveis de preparação do Nb2O5.nH2O via microemulsão inversa (ME). Por meio de um planejamento fatorial completo, estudou-se a influência da concentração do agente precursor, da ordem de adição das microemulsões e da razão água/surfatante (W) no tamanho das partículas formadas. Para análise comparativa, Nb2O5.nH2O também foi preparado pelos métodos da precipitação convencional (PC) e da precipitação em solução homogênea (PSH). Os materiais preparados foram caracterizados por difratometria de raios X (DRX), termogravimetria (TG), microscopia eletrônica de varredura (MEV) e de transmissão (MET), espectrômetria de energia dispersiva (EDS), espectroscopia de absorção no infravermelho por transformada de Fourier (FTIR) e análise de área superficial específica pelo método B.E.T. A adsorção de íons fosfato em Nb2O5.nH2O foi estudada. A melhor forma de preparação do Nb2O5.nH2O via ME foi a utilizada no experimento 2. As micrografias MEV confirmam que os materiais preparados pelo método ME possuem partículas esféricas menores que as dos materiais preparados pelos métodos PC e PSH. Dos modelos cinéticos estudados, o que melhor se ajustou aos dados experimentais de adsorção de íons fosfato nos materiais preparados foi o de pseudo 2a ordem. A quantidade de íons fosfato adsorvida aumenta com a redução do pH da solução inicial. O modelo de isoterma de Langmuir simulou adequadamente os resultados de equilíbrio obtidos, sendo observada a seguinte ordem de capacidade de adsorção: ME2>PSHu>PC>PSHc. A adsorção de íons fosfato em Nb2O5.nH2O é espontânea e endotérmica. Os íons fosfatos podem ser dessorvidos da superfície do Nb2O5.nH2O através do ajuste do pH da solução. A adsorção de íons fosfato em óxido de nióbio hidratado envolve mecanismos de quimissorção, fisissorção e troca iônica.In the present study the parameters of Nb2O5.nH2O precipitation via reverse microemulsion were investigated with the objective to synthesize homogeneous nanoparticles according to morphology, structure and size. Basic synthesis parameters, such as addition order of microemulsions (O), ratio of water to surfactant (W), ratio of NbOF5-2 to water, were determined by design of experiments. The Nb2O5.nH2O was also prepared by co-precipitation and homogeneous solution method. The materials were characterized by X-ray diffraction (XRD), thermal analysis (TG/DTG), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR) and BET specific surface area measurements. The phosphate adsorption onto Nb2O5.nH2O was performed. The best way to prepared Nb2O5.nH2O via reverse microemulsion was used in experiment 2. MEV micrographs confirmed that materials prepared by microemulsion were smallest than materials prepared by co-precipitation and homogeneous solution. The kinetic data corresponded very well to the pseudo-second-order equation. The phosphate adsorption tended to increase with a decrease of pH. The data fitted well to the Langmuir model and the adsorption capacity for the Nb2O5.nH2O presented the following decreasing order: ME2>PSHu>PC>PSHc. The thermodynamic parameters evaluated reveal the spontaneous and endothermic nature of phosphate adsorption onto Nb2O5.nH2O. Phosphate can be desorbed from the surface of hydrous niobium oxide by adjusting the pH values of the solution. The phosphate adsorption occurs by the mechanisms of chemisorptions, physisorptions and ion-exchange

Synthesis of hydrous niobium oxide nanoparticles by reverse microemulsion

Com o objetivo de gerar um material mais homogêneo quanto à forma e tamanho das partículas formadas (nanopartículas), o presente trabalho visa o estudo das variáveis de preparação do Nb2O5.nH2O via microemulsão inversa (ME). Por meio de um planejamento fatorial completo, estudou-se a influência da concentração do agente precursor, da ordem de adição das microemulsões e da razão água/surfatante (W) no tamanho das partículas formadas. Para análise comparativa, Nb2O5.nH2O também foi preparado pelos métodos da precipitação convencional (PC) e da precipitação em solução homogênea (PSH). Os materiais preparados foram caracterizados por difratometria de raios X (DRX), termogravimetria (TG), microscopia eletrônica de varredura (MEV) e de transmissão (MET), espectrômetria de energia dispersiva (EDS), espectroscopia de absorção no infravermelho por transformada de Fourier (FTIR) e análise de área superficial específica pelo método B.E.T. A adsorção de íons fosfato em Nb2O5.nH2O foi estudada. A melhor forma de preparação do Nb2O5.nH2O via ME foi a utilizada no experimento 2. As micrografias MEV confirmam que os materiais preparados pelo método ME possuem partículas esféricas menores que as dos materiais preparados pelos métodos PC e PSH. Dos modelos cinéticos estudados, o que melhor se ajustou aos dados experimentais de adsorção de íons fosfato nos materiais preparados foi o de pseudo 2a ordem. A quantidade de íons fosfato adsorvida aumenta com a redução do pH da solução inicial. O modelo de isoterma de Langmuir simulou adequadamente os resultados de equilíbrio obtidos, sendo observada a seguinte ordem de capacidade de adsorção: ME2>PSHu>PC>PSHc. A adsorção de íons fosfato em Nb2O5.nH2O é espontânea e endotérmica. Os íons fosfatos podem ser dessorvidos da superfície do Nb2O5.nH2O através do ajuste do pH da solução. A adsorção de íons fosfato em óxido de nióbio hidratado envolve mecanismos de quimissorção, fisissorção e troca iônica.In the present study the parameters of Nb2O5.nH2O precipitation via reverse microemulsion were investigated with the objective to synthesize homogeneous nanoparticles according to morphology, structure and size. Basic synthesis parameters, such as addition order of microemulsions (O), ratio of water to surfactant (W), ratio of NbOF5-2 to water, were determined by design of experiments. The Nb2O5.nH2O was also prepared by co-precipitation and homogeneous solution method. The materials were characterized by X-ray diffraction (XRD), thermal analysis (TG/DTG), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR) and BET specific surface area measurements. The phosphate adsorption onto Nb2O5.nH2O was performed. The best way to prepared Nb2O5.nH2O via reverse microemulsion was used in experiment 2. MEV micrographs confirmed that materials prepared by microemulsion were smallest than materials prepared by co-precipitation and homogeneous solution. The kinetic data corresponded very well to the pseudo-second-order equation. The phosphate adsorption tended to increase with a decrease of pH. The data fitted well to the Langmuir model and the adsorption capacity for the Nb2O5.nH2O presented the following decreasing order: ME2>PSHu>PC>PSHc. The thermodynamic parameters evaluated reveal the spontaneous and endothermic nature of phosphate adsorption onto Nb2O5.nH2O. Phosphate can be desorbed from the surface of hydrous niobium oxide by adjusting the pH values of the solution. The phosphate adsorption occurs by the mechanisms of chemisorptions, physisorptions and ion-exchange

An investigation of phosphate adsorption from aqueous solution onto hydrous niobium oxide prepared by co-precipitation method

The synthetic hydrous niobium oxide has been used for phosphate removal from the aqueous solutions. The kinetic data correspond very well to the pseudo second-order equation The phosphate removal tended. to increase with a decrease of pH. The equilibrium data describe very well the Langmuir isotherm. The peak appearing at 1050 cm(-1) in IR spectra after adsorption was attributed to the bending vibration of adsorbed phosphate. The adsorption capacities are high, and increased with increasing temperature. The evaluated Delta G degrees and Delta H degrees indicate the spontaneous and endothermic nature of the reactions. The adsorptions occur with increase in entropy (Delta S positive) value suggest increase in randomness at the solid-liquid interface during the adsorption. A phosphate desorbability of approximately 60% was observed with water at pH 12, which indicated a relatively strong bonding between the adsorbed phosphate and the sorptive sites on the surface of the adsorbent. (C) 2008 Elsevier B.V. All rights reserved.FAPESP[2006/05421-0

Adsorção de íons fosfato em óxido de nióbio hidratado Adsorption of phosphate ions on hydrous niobium oxide

<abstract language="eng">The adsorption kinetics of phosphate on Nb2O5.nH2O was investigated at initial phosphate concentrations 0.25, 0.50 and 1.00 mg.L-1. The kinetic process was described by a pseudo-second-order rate model very well. The adsorption thermodynamics was carried out at 298, 308, 318, 328 and 338 K. The positive values of both &#916;H and &#916;S suggest an endothermic reaction and increase in randomness at the solid-liquid interface during the adsorption. &#916;G values obtained were negative indicating a spontaneous adsorption process. The Langmuir model described the data better than the Freundlich isotherm model. The effective desorption could be achieved using water at pH 12

Adsorption kinetic, thermodynamic and desorption studies of phosphate onto hydrous niobium oxide prepared by reverse microemulsion method

A type of Nb(2)O(5)center dot 3H(2)O was synthesized and its phosphate removal potential was investigated in this study. The kinetic study, adsorption isotherm, pH effect, thermodynamic study and desorption were examined in batch experiments. The kinetic process was described by a pseudo-second-order rate model very well. The phosphate adsorption tended to increase with a decrease of pH. The adsorption data fitted well to the Langmuir model with which the maximum P adsorption capacity was estimated to be 18.36 mg-Pg(-1). The peak appearing at 1050 cm(-1) in IR spectra after adsorption was attributed to the bending vibration of adsorbed phosphate. The positive values of both Delta H degrees and Delta S degrees suggest an endothermic reaction and increase in randomness at the solid-liquid interface during the adsorption. Delta G degrees values obtained were negative indicating a spontaneous adsorption process. A phosphate desorbability of approximately 68% was observed with water at pH 12, which indicated a relatively strong bonding between the adsorbed phosphate and the sorptive sites on the surface of the adsorbent. The immobilization of phosphate probably occurs by the mechanisms of ion exchange and physicochemical attraction. Due to its high adsorption capacity, this type of hydrous niobium oxide has the potential for application to control phosphorus pollution.Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP)[2006/05421-0

Synthesis of Nb(2)O(5)center dot nH(2)O nanoparticles by water-in-oil microemulsion

Hydrous niobium oxide (Nb(2)O(5)center dot nH(2)O) nanoparticles had been Successfully prepared by water-in-oil microemulsion. They were characterized by X-ray diffraction (XRD), thermal analysis (TG/DTG), Fourier transform infrared spectroscopy (FTIR), BET surface area measurement, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results showed that the nanoparticle was exactly Nb(2)O(5)center dot nH(2)O with spherical shape. Their BET surface area was 60 m(2) g(-1). XRD results showed that Nb(2)O(5)center dot nH(2)O nanoparticles with crystallite size in nanometer scale were formed. The crystallinity and crystallity size increased with increasing annealing temperature. TT-phase of Nb(2)O(5) was obtained when the sample is annealed at 550 degrees C. (C) 2009 Elsevier B.V. All rights reserved.FAPESP[2006/05421-0

The kinetic of mullite crystallization: Effect of water content

The kinetic of mullite crystallization from sol–gel method, with different water content, was investigated under non-isothermal conditions using DTA. The sols were obtained from Al(NO3)3.9H2O (ANN) and Si(OC2H5)4 (TEOS) mixtures by varying the water–alcohol content of the system. The crystalline phase changes were verified by X-ray diffraction (XRD). For a sample prepared using ethanol-based alkoxide solution (M0), only Al-poor mullite (p-mullite) crystallizes at 1000 °C; for the one synthesized with low water concentration (M6) Al-rich mullite (r-mullite) and spinel crystallize together; and for a sample prepared using a water-based alkoxide solution only spinel is formed. Thus, the variation of water contents during the synthesis caused great variations in the course of mullitization process. The average value of the apparent activation energy determined for p-mullite, r-mullite and spinel phase crystallization were found to be E = (899 ± 61) kJ mol−1, E = (1015 ± 272) kJ mol−1 and E = (980 ± 196) kJ mol−1, respectively. These results showed that sample M(0) was a monophasic gel, where aluminum and silicon atoms are mixed at a molecular level while sample M(100) was a diphasic gel, where silicon and aluminum atoms are distributed in a nanometric level. The fast reaction between TEOS and water molecules is responsible for this great difference in the sample's homogeneity. The kinetic model of the crystallization process was determined using Malek's procedure. It was established that the crystallization of p-mullite, r-mullite and spinel phase can be described by Šesták–Berggren autocatalytic model

Adsorption of phosphate from aqueous solution by hydrous zirconium oxide

Synthetic ZrO2 center dot nH(2)O was used for phosphate removal from aqueous solution. The optimum adsorbent dose obtained for phosphate adsorption on to hydrous zirconium oxide was 0.1 g. The kinetic process was described very well by a pseudo-second-order rate model. The phosphate adsorption tended to increase with the decrease in pH. The adsorption capacity increased from 61 to 66 mg g(-1) when the temperature was increased from 298 to 338 K. A phosphate desorption of approximately 74% was obtained using water at pH 12