Kinetics of adsorption of oxalic acid on different titanium dioxide samples.

FTIR-ATR kinetic studies on the adsorption of oxalic acid on different TiO(2) films were performed. The particulate films were obtained through the evaporation of TiO(2) suspensions. The evolution of the IR bands followed a pseudo-first-order behavior, as previously observed. Systematic studies as a function of the oxalic acid concentration afforded the specific rate constant for adsorption (k(a)) and desorption (k(d)). The influence of physical parameters of the samples, i.e., specific BET area, crystalline domain size, TiO(2) load, film area, and pore size, on the kinetic parameters k(a) and k(d) was analyzed. A mechanism in which the adsorption and desorption processes are controlled by the diffusion through the pores of the films is proposed (intraparticle diffusion). It is concluded that all the samples behave in the same way. Thicker films or those with smaller particle size (higher specific surface area, smaller pores) show the slowest rates of adsorption and desorption. These results are relevant for the design of efficient heterogeneous catalysts and sensors, and for the interpretation of pollutant adsorption.

Kinetics of adsorption of carboxylic acids onto titanium dioxide.

A FTIR-ATR kinetic study on the adsorption of carboxylic acids (oxalic, citric, malonic, succinic, gallic, EDTA and TTHA acids, where EDTA = ethylenediaminetetraacetate and TTHA = triethylenetetramine-N,N,N',N'',N''',N'''-hexaacetate, in the concentration range 6 x 10(-7) M-2 x 10(-5) M on TiO(2) (Degussa P25) by ATR-FT-IR is reported. The influence of carboxylic acid concentration, pH, ionic strength, TiO(2) load in the film is presented. The adsorption processes follows pseudo-first-order kinetics at constant ligand concentration, even though several adsorption modes have been reported in the literature. Plots of the pseudo first order constant k(obs)vs. carboxylic acid concentration are linear for all the studied ligands. The slopes of these plots (a) are not very sensitive to the nature of the ligands; a decrease in a is observed as the size increases. The intercept (b) is inversely related to the stability of the surface complexes. We propose that both the rate of adsorption, and the desorption rate are controlled by the diffusion through the pores of the film, although in the case of (b), the desorption rate is modulated by the stability of the surface complex. These results are relevant for oxide dissolution, remediation of water, pollutants removal, sensors design and heterogeneous photocatalysis.

Aerosol-assisted production of mesoporous titania microspheres with enhanced photocatalytic activity: the basis of an improved process.

An aerosol-based process was used to prepare mesoporous TiO(2) microspheres (MTM) with an average diameter in the range of 0.5-1 microm. The structural characteristics and photocatalytic properties of the synthesized materials were determined. As-prepared MTM materials and those heated in air from 400 to 600 degrees C exhibited mesoporous texture with a narrow size distribution and an inorganic framework that consisted of 4-13 nm anatase crystallites. Pore volumes for the MTM materials were in the range of 0.17-0.34 cm(3) g(-1). Microspheres heated to 400 degrees C presented a locally ordered mesopore structure and possessed X-ray diffraction d spacings between 9.8 and 17.3 nm. Heating above 400 degrees C resulted in a loss of the mesoscopic order, a decrease of the surface area, retention of the porosity, and an increase of the anatase nanoparticle size to 13 nm. The accessibility of the pore volume was measured by monitoring the uptake of gallic acid (GA) using Fourier transform IR. The MTM materials made excellent catalysts for the photodegradation of GA, with the performance being higher than that of an equivalent sample of Degussa P25. The present MTM materials are advantageous in terms of their ease of separation from the aqueous phase, and hence a novel photocatalytic process is proposed based on separate adsorption and photocatalytic decomposition steps with an improved and more rational use of both catalyst and sunlight.

Adsorption of oxalate on anatase (100) and rutile (110) surfaces in aqueous systems: experimental results vs. theoretical predictions.

A combined experimental and theoretical study of the adsorption of oxalic acid from the aqueous phase at the surface of anatase nanoparticles has been performed. The interfaces were investigated by ATR-FTIR measurements and quantum-chemical calculations using the semiempirical method MSINDO. The vibration spectra of the most stable surface complexes have been calculated and used for the interpretation of experimental results. The theoretical studies have been done using the anatase (100) surface to model the adsorption of oxalic acid and water. The effect of interaction of water and oxalic acid on the adsorption mechanism and the vibration spectra was taken into account in the theoretical models. Inclusion of solvation effects was found crucial to determine the type of denticity and structure of adsorbed complexes. By comparison of experimental data and theoretical calculations the most likely surface species and the effects of hydration in their relative stabilities were determined. The present results are compared to previous studies preformed also by combination of experimental and theoretical calculations of analogous systems using nanoparticulate rutile [C. B. Mendive et al., Phys. Chem. Chem. Phys., 2008, 10, 1960, ref. 1]. Differences between surface complexes on anatase and rutile lie mainly on the denticity type. Whilst in the case of rutile the most stable species consist of two bidentate surface complexes followed in third place by a monodentated form, anatase allows the formation of four species in which the stability order is reversed with respect to the denticity type. In the case of anatase, the main contributors to the surface speciation are two monodentate species differing in the position of the H atom within the molecule (being more stable when it is placed in the O-C-O moiety not bound to the surface); and two bidentate species, one deprotonated and one monoprotonated, in which the C-C bond was parallel or perpendicular to the TiO2 surface, respectively.

The adsorption and photodegradation of oxalic acid at the TiO2 surface.

ATR-FTIR measurements in combination with quantum chemical calculations were performed to study chemical reactions taking place at the surface of a thin TiO2 layer immersed in an aqueous oxalic acid solution under UV(A) illumination. It was found that the adsorption of oxalic acid on TiO2 in the dark can be explained in terms of two surface complexes for the anatase phase. Under UV(A) illumination, one of the adsorbed species on the anatase phase preferably undergoes photo-degradation and at the same time more molecules of oxalic acid are adsorbed at the TiO2 surface which is thus enriched in the second complexation mode. The spectral changes observed under UV(A) illumination are explained in the light of different theories: photo-desorption of water molecules as a thermal mechanism induced by the absorption of photons, surface reconstruction, and newly exposed surface area provided by the de-aggregation of the TiO2 particles.

Homogeneous precipitation of layered Ni(II)-Cr(III) double hydroxides.

An adequate account of the hydrolytic properties of Cr3+ and Ni2+ allows setting the conditions for homogeneous nucleation of layered Ni(II)-Cr(III) double hydroxides; water exchange and hydrolysis rate constants indicate that, at very high rates of base dosing, formation of heteronuclear Cr(III)-Ni(II) hydroxo species should prevail over precipitation of active Cr(OH)3. This is realized by the urea method under microwave-assisted hydrothermal conditions. This approach yields crystalline Ni1-xCrx(OH)2(CO3)x/2nH2O (x approximately 0.32-0.36) in less than 5 min at 453 K; higher degrees of crystallinity are obtained at higher temperatures and/or longer aging times. Formation of Ni(II)-Cr(III) LDHs upon microwave-assisted hydrothermal aging of freshly coprecipitated Ni(OH)2+Cr(OH)3 mixtures takes longer, due to a different operating mechanism. The implications of the advanced rationale for the design of synthesis procedures are stressed. It is proposed that homogeneous nucleation of Ni(II)-Cr(III) LDHs involves the edge-on condensation of planar heteronuclear Cr(III)-Ni(II) hydroxo trimers. Ordered aggregation of primary particles leads to the final platelet crystals, a process that involves the exchange of CO2-(3) ions dangling at the crystallites' edges by bridging OH-.

ATR-FTIR measurements and quantum chemical calculations concerning the adsorption and photoreaction of oxalic acid on TiO2.

The adsorption and photoreaction of oxalic acid on the surface of anatase and rutile TiO2 nanoparticles have been studied using a combined experimental and theoretical approach. In the dark, the experimental adsorption reaches an equilibrium state that can be described as a mixture of adsorbed water and oxalic acid molecules, with the latter forming two different surface complexes on anatase and one on rutile particles. When the system is subsequently illuminated with UV(A) light, the surface becomes enriched with absorbed oxalic acid, which replaces photo-desorbed water molecules, and one of the adsorbed oxalic acid structures on anatase is favoured over the other.

Interaction of catechol and gallic acid with titanium dioxide in aqueous suspensions. 1. Equilibrium studies.

The adsorption isotherms of catechol (1,2-dihydroxybenzene) and gallic acid (3,4,5-trihydroxybenzoic acid) onto titanium dioxide (Degussa P-25) were measured at various pH values and room temperature using attenuated total reflection Fourier transform infrared (FTIR-ATR) data, processed by singular value decomposition. The affinity is largely pH independent, although the deprotonatation of the carboxylic group in gallic acid might produce a slight increase in the affinity. Catechol was shown to form two complexes, with Langmuir stability constants log K of 4.66 (strong mode) and 3.65 (weak mode). Both complexes have the same spectral signature, and mononuclear and binuclear chelate structures are proposed for them. Gallic acid chemisorbs by complexation through two -OH groups and forms one complex only, log K = 4.70. The third -OH and the pendant carboxylate do not influence much the stability of the surface complex. Comparison with literature data demonstrates that the affinity of 4-chlorocatechol is also similar, whereas 2,3-dihidroxynaphthalene and 4-nitrocatechol form more stable complexes, probably because of the solvation contribution to the overall Gibbs adsorption energy. All quoted constants refer to the surface complexation equilibria written as follows: ([triple bond]Ti-OH)2 + H2L = ([triple bond]Ti)2-L + 2H2O, i.e., as electroneutral processes. The FTIR-ATR spectra of the surface complexes are also discussed.

Remoción de As mediante el método RAOS en Los Pereyra, provincia de Tucumán, Argentina

Describe el comportamiento del método RAOS (Remoción de Arsénico por Oxidación Solar) para la remoción del arsénico de aguas de consumo humano en regiones rurales de la prov. de Tucumán

Dissolution of Nickel Ferrite in Aqueous Solutions Containing Oxalic Acid and Ferrous Salts.

The dissolution of nickel ferrite in oxalic acid and in ferrous oxalate-oxalic acid aqueous solution was studied. Nickel ferrite was synthesized by thermal decomposition of a mixed tartrate; the particles were shown to be coated with a thin ferric oxide layer. Dissolution takes place in two stages, the first one corresponding to the dissolution of the ferric oxide outer layer and the second one being the dissolution of Ni(1.06)Fe(1.96)O(4). The kinetics of dissolution during this first stage is typical of ferric oxides: in oxalic acid, both a ligand-assisted and a redox mechanism operates, whereas in the presence of ferrous ions, redox catalysis leads to a faster dissolution. The rate dependence on both oxalic acid and on ferrous ion is described by the Langmuir-Hinshelwood equation; the best fitting corresponds to K(1)(ads)=25.6 mol(-1) dm(-3) and k(1)(max)=9.17x10(-7) mol m(-2) s(-1) and K(2)(ads)=37.1x10(3) mol(-1) dm(-3) and k(2)(max)=62.3x10(-7) mol m(-2) s(-1), respectively. In the second stage, Langmuir-Hinshelwood kinetics also describes the dissolution of iron and nickel from nickel ferrite, with K(1)(ads)=20.8 mol(-1) dm(3) and K(2)(ads)=1.16x10(5) mol(-1) dm(3). For iron, k(1)(max)=1.02x10(-7) mol of Fe m(-2) s(-1) and k(2)(max)=2.38x10(-7) mol of Fe m(-2) s(-1); for nickel, the rate constants k(1)(max) and k(2)(max) are 2.4 and 1.79 times smaller, respectively. The factor 1.79 agrees nicely with the stoichiometric ratio, whereas the factor 2.4 implies the accumulation of some nickel in the residual particles. The rate of nickel dissolution in oxalic acid is higher than that in bunsenite by a factor of 8, whereas hematite is more reactive by a factor of 9 (in the absence of Fe(II)) and 27 (in the presence of Fe (II)). It may be concluded that oxalic acid operates to dissolve iron, and the ensuing disruption of the solid framework accelerates the release of nickel. Copyright 2000 Academic Press.

Tratamiento de residuos nucleares líquidos por fotocatálisis heterogénea

Se construyó un equipo a escala de banco para el tratamiento de EDTA por fotocatálisis heterogénea bajo radiación ultravioleta con dióxido de titanio, y se procesaron soluciones de EDTA a concentraciones y PH análogos a los resultantes de la descontaminación de centrales nucleares. Bajo irradiación a 366 nm se observó la degradación total del EDTA en 10 horas y la reducción del 95

del carbono orgánico total al cabo de 39 horas. Se considera al sistema apto para el cambio de escala

Tratamiento de residuos nucleares líquidos por fotocatálisis heterogénea

Se construyó un equipo a escala de banco para el tratamiento de EDTA por fotocatálisis heterogénea bajo radiación ultravioleta con dióxido de titanio, y se procesaron soluciones de EDTA a concentraciones y PH análogos a los resultantes de la descontaminación de centrales nucleares. Bajo irradiación a 366 nm se observó la degradación total del EDTA en 10 horas y la reducción del 95 del carbono orgánico total al cabo de 39 horas. Se considera al sistema apto para el cambio de escala