Alberto E. Regazzoni

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.

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.

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 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.

Boric acid adsorption on magnetite and zirconium dioxide

Abstract The adsorption of boric acid on magnetite and zirconium dioxide in aqueous suspensions has been characterized through electrokinetic measurements and potentiometric titrations. Both the pzc and the iep shift to lower pH values upon adsorption, remaining equal at all boric acid concentrations studied. From the shifts in pzc, values for the adsorption equilibrium constants are derived using the site-binding model and assuming that boron is placed in the zero plane rather than in the β-plane. The predictions of the model for the pH-dependence of adsorption are compared with available literature values.

SYNTHESIS OF METAL OXIDE PARTICLES FROM AQUEOUS MEDIA: THE HOMOGENEOUS ALKALIN1ZAT1ON METHOD

Abstract An analysis of the urea method as a way to synthesize uniform colloidal particles is presented. Various selected cases of precipitation and coprecipitation are reviewed to illustrate the inherent complexities of the method. The importance of the kinetics of urea hydrolysis is recognized. The concept of trajectory is used to rationalized the evolution of the systems upon aging. The influence of thermodynamic and kinetic factors in the definition of the nature of the precipitating solid phases is analyzed; the overriding role of the latter is stressed. The factors that influence particle morphology are also addressed.

Precipitation of copper(II) hydrous oxides and copper(II) basic salts

The precipitation of copper(II) hydrous oxides and basic salts from solutions that became homogeneously alkaline by forced hydrolysis of urea has been studied as a function of solution composition, ageing time and temperature. The time required for the onset of precipitation increased with the initial copper(II) concentration, [CuII]0. The precipitate was found to be amorphous copper hydroxide containing variable amounts of carbonate; at the highest [CuII]0, basic copper nitrate also precipitated. Upon further ageing of the systems, tenorite or malachite particles were formed at the expense of amorphous copper hydroxide; the nature of the final solid phase depended on [CuII]0. In the presence of added sulfate anions, brochantite particles were precipitated; in these cases precipitation took place earlier. All processes occurred at rates that were strongly dependent on temperature, although the sequence of events remained essentially the same.Thermodynamic and kinetic factors influencing the formation of all possible solid phases are discussed. It is shown that spherical monodispersed copper(II) hydrous oxide particles can be synthesized when these factors are taken into account adequately.

Amylase-Functionalized Mesoporous Silica Thin Films as Robust Biocatalyst Platforms

A robust biocatalyst was produced by immobilization of alpha-amylase into mesoporous silica thin films with local order pore structure and 11 nm pore diameter, supported on glass stripes. The activity of this novel catalyst was evaluated for direct starch degradation. The catalyst films show excellent activity, and enhanced stability with respect to free enzyme at extreme conditions of pH and temperature. In addition, they can be easily separated from the reaction media and reused several times.