Miguel A. Blesa

Interfacial properties of zirconium dioxide and magnetite in water

Abstract The interfacial properties of ZrO2 and Fe3O4 inmersed in water were studied through measurements of electrophoretic mobilities and surface charge densities obtained by potentiometric titrations. The pH(pzc) values were also determined by the addition method. The results are critically compared with previous data from the literature, and interpreted through the model of Davis, James, and Leckie [J. Colloid Interface Sci. 63, 480 (1978); 67, 90 (1978); 74, 32 (1980)]. Values of the constants p K a1 int , p K a2 int , p ∗ K anion int , and p ∗ K cation int are derived for both oxides in KNO3 solutions. The relative contribution of the two mechanisms (potential-determining ions exchange with solvent and complex pairs formation with swamping electrolyte) to charge development is very different in both cases. This behavior is discussed in terms of the DJL model.

Some observations on the composition and morphology of synthetic magnetites obtained by different routes

Abstract Magnetite has been synthetized by several different methods, and the products have been analysed in relation to the composition and morphology of the particles. It is shown that oxidation of ferrous salts in alkaline media gives much purer magnetite when hydrazine is added to the reaction mixture. A method is also described which permits one to synthetize spherical magnetite particles of uniform diameter. A discussion of the mechanisms of reaction is presented.

The influence of temperature on the interface magnetite-aqueous electrolyte solution

Values of the point of zero charge, pKa1int, pKa2int, p∗Kcationint, and the capacity of the innermost layer C1 for magnetite suspended in KNO3 solutions have been obtained in the temperature range 30–80°C from acid-base titration data. The dependency of the pzc on temperature indicates that ΔH∗, the enthalpy of transfer of protons from bulk to surface minus the enthalpy of transfer of hydroxide ions from bulk to surface, is negative, and pzc and (12)pKw become increasingly separated as temperature increases. These results are discussed in terms of the corresponding ΔH∗ and ΔS∗ values. The values of ΔS° and ΔH° of the ionization equilibria of the interface are also reported, and shown to be similar to equivalent magnitudes for aqueous Fe(II). C1 increases with temperature, and a tentative rationale for this is given. The site binding model is less adequate to fit data at higher temperatures, especially at low ionic strengths. The calculated profiles for the various potentials as a function of pH are given; ψ0/pH profiles deviate from the prediction of Nernst equation more markedly at higher temperatures. Ion pairs contribute less to the charging of the surface at higher temperatures, because of decreasing values of log Kanionint and log Kcationint.

Adsorption of EDTA and iron—EDTA complexes on magnetite and the mechanism of dissolution of magnetite by EDTA

Abstract The adsorption of EDTA on magnetite has been measured as a function of pH and EDTA concentration. The number of anchorage sites of EDTA on magnetite changes from 2 to 4 with increasing pH, while the affinity goes through a maximum in the pH interval 3–4. Adsorption of Fe(II) on EDTA has also been measured, the behavior being similar to that of other hydrolizable ions. The more complex composite systems EDTAFe(II), Fe(III)Fe3O4 are also analyzed and it is concluded that surface and dissolved iron ions compete for EDTA; thus the adsorptivity of the complex ions is less than the adsorptivity of either EDTA or aqueous iron ions. The rates of dissolution of magnetite in EDTA solutions with and without added Fe(II) have been measured and the results interpreted through a much faster rate of phase transfer for Fe(II) as compared to Fe(III). Heterogeneous electron transfer is involved in the dissolution of magnetite by ferrous ions, either exogenous or autogenerated. EDTA is necessary for the reductive pathway, as the larger stability constant of the Fe(III) complex over the Fe(II) complex provides the driving force for the heterogeneous electron transfer.

The reductive dissolution of iron oxides by ascorbate: The role of carboxylate anions in accelerating reductive dissolution

There are four general pathways of dissolution of reducible metal oxides in acidic aqueous solution: proton-assisted (acid), ligand-promoted acid, reductive, and ligand-promoted reductive dissolution. The presence and reactivity toward the surface of protons, chelating ligands, and reductants dictate the mechanism(s) controlling the dissolution. For the massive reductive dissolution of magnetic by ascorbic acid, the experimental rate law R = k[HA−]12[H+] suggests the involvement of surface ≡FeIII A− complexes. Adsorption isotherms of ascorbic acid onto hematite at pH 3 and 25°C yield a Langmuir-type surface complexation constant Ks = (9.57 × 108 M−1). Slow dissolution follows with an empirical rate law R = kobs(≡FeIIIA). It is concluded that the formation and kinetic reactivity of surface complexes determine the rate of dissolution. Dehydroascorbic acid also dissolves magnetite, but at slower rates. Oxalate accelerates the reductive dissolution of hematite by ascorbate even though it competes with ascorbate for surface sites; enhanced detachment of ≡FeII surface species by oxalate complexation may be involved. Autoacceleration of the reductive dissolution by dissolved FeII-carboxylate complexes is observed in EDTA/ascorbic acid mixtures; the rate reaches a maximum at intermediate [EDTA] values, where synergistic effects between EDTA and FeII-EDTA complexes are important. Autoacceleration may also operate in oxalate solutions.

Electrokinetic properties of the calcite/water interface in the presence of magnesium and organic matter

Abstract Electrophoretic mobilities of calcite particles immersed in saturated aqueous solutions (closed to atmospheric carbon dioxide) were measured as a function of solution composition at 25°C. Electrokinetic ζ potentials of calcite suspended in water are negative and slightly influenced by pH in the range 8.5 ⩽ pH ⩽ 10.5. At constant Ca 2+ equilibrium concentration, pH does not affect ζ potential values. The value of ζ is strongly dependent on p Ca 2+ or p CO 3 2− = p K so − p Ca 2+ ); below the isoelectric point of calcite, p Ca 2+ 2.7, ζ potentials are positive. It is demonstrated that Ca 2+ and CO 3 2− are the only potential determining ions (pdi). The effect of magnesium ions on calcite ζ potential values is heavily dependent on pH, aging time, and Mg 2+ concentration. At short equilibration times, the negative charge of calcite is reverted once the solution becomes supersaturated with respect to Mg(OH) 2 ; below these pH values, Mg 2+ behaves as an indifferent ion. At long equilibration times, positive surface charge develops in conditions of undersaturation with respect to Mg(OH) 2 but only at the highest studied Mg 2+ concentration. This finding is interpreted in terms of the formation of a surface layer of magnesium-bearing calcite, Mg 2+ , Ca 2+ , and CO 3 2− being the pdi. The adsorption of dodecyl sulfate anions produces a more negative surface charge. Adsorption of DS − is strongly dependent on p Ca 2+ ; this dependence is dominated by the electrostatic contribution to the overall adsorption Gibbs energy. The nonelectrostatic contribution derived from the data is indicative of a weak chemical interaction between calcium surface ions and the surfactant head group. The implications of these results for natural water systems are briefly discussed. The electrokinetic behavior of biogenic calcium carbonate secreted by the tube forming worm Ficopomatus enigmaticus is also reported.

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.

ATR–FTIR study of the stability trends of carboxylate complexes formed on the surface of titanium dioxide particles immersed in water

Abstract The use of titanium dioxide slurries or powders to enact the photocatalytic destruction of contaminants in water depends on its properties as a wide gap semiconductor. The charge transfer events at the water/semiconductor interface are strongly modified by the interaction of the titanium dioxide surface with solutes present in water. In this paper, we discuss the stability trend of the surface complexes formed by a series of organic anions (chemisorbed on TiO 2 ). The surface interaction was studied by ATR–FTIR, and the spectra obtained at different complexant concentrations and pH values were used to derive Langmuir-type stability constants. It is shown that surface complexation can be described as the dissociative chemisorption of the neutral acid, with the creation of a zwitterionic surface species. There is a linear Gibbs energy relationship (LGER) between the stability of the surface complexes and the first acidity constant of the ligand, with slope of 1.8.

Reductive dissolution of magnetite by solutions containing EDTA and FeII

Abstract The dissolution of magnetite particles in solutions containing EDTA and FeII was studied as a function of the total concentration of EDTA and FeII; the influence of pH was also studied. The rate shows a Langmuir-type dependence on [FeY2−] when [EDTA]0 ⩽ [FeII]0. At constant [EDTA]0, in the range where EDTA is in excess over FeII, the kinetic order on FeII is one; FeII in large excess has no influence on the rate. At constant [FeII]0, the rate of dissolution is maximum when the ratio of [EDTA]0 to [FeII]0 is close to 3. These results are interpreted in terms of fast solution and surface complexation processes followed by slow heterogeneous electron transfer from adsorbed FeY2− to surface >FeIII centers and fast phase transfer of >FeII. The inhibitory effect of excess EDTA results from competitive adsorption of FeY2− and EDTA. The rate increases with decreasing pH up to pH 3.1; at this value a maximum is achieved. The pH dependence of rate is the resultant of several factors, the most important being the influence of pH on the adsorption preequilibrium and the need for adsorbed protons adjacent to the reactive site. The stoichiometry of the dissolution reaction is not constant and the ratio of protons consumed to iron released is sensitive to experimental conditions. In the fatest reactions, this ratio is appreciably lower than the limiting value corresponding to the release of unhydrolyzed FeIII species. The implications of this result are discussed.

Growth and characterization of oxide layers on zirconium alloys

Abstract In the range 265–435°C Zr-2.5Nb corrosion takes place in two stages, as opposed to the cyclic behaviour of Zry-4. The Zry-4 corrosion stages are described by a single equation, in terms of the dense oxide layer thickness that decreases sharply at each transition. Tetragonal zirconia is present in the oxide layers of both alloys. In Zry-4, its volume fraction decreases as the oxide grows; it is barely discernible in Zr-2.5Nb in films below 1 μm, to later increase up to the transition. In both alloys, compressive stresses are developed associated with the oxide growth. Their relaxation at the transition correlates with the transformation of ZrO 2 (t) to ZrO 2 (m) and with the decrease of the dense oxide layer. In Zr-2.5Nb, oxide ridges form on the β-Zr phase filaments, at the very onset of film growth. The cyclic behaviour associated with the periodical breakdown of the dense oxide layer is therefore blurred, although optical microscopy shows that the scale retains the multilayered structure typical of Zry-4.