Luca Righetto

Surface Interactions of Actinides with Alumina Colloids

Results are given on the influence of pH, carbonates and humic substances on the adsorption of Am (III), Th(IV) and Np(V) onto alumina colloids at fixed ionic strength. Surface interactions are oxidation state dependent, with adsorption edges shifting toward high pH in the order Th(IV), Am (III), Np(V). Carbonate ions markedly reduce the adsorption of neptunium, americium and thorium at relatively high pH. The addition of humic substances increases the actinide uptake at low pH and lowers the adsorption by the colloidal particles in the neutral to alkaline pH range.

Degradation of some chloro-aliphatic water contaminants by photocatalytic membranes immobilizing titanium dioxide☆

Abstract The TiO 2 -mediated photodegradation of chloroethanoic acid, trichloroethene and tetrachloroethene, chosen as model molecules of aliphatic chloro-organics was studied at 303±2 K by employing polymeric membranes photografted on cellulose to immobilize the TiO 2 (3.0–5.6 mg cm −2 ). The radiant flux in the absorption range and the volume-surface ratio in the photoreactor cell were kept constant at 2.5±0.2 mW cm −2 and 1.1±0.1 mL cm −2 respectively. The initial rate of photodegradation was studied as a function of the initial concentration of reactants by the linearized form of the Langmuir—Hinshelwood equation, by which rate constants k and equilibrium adsorption constants K were evaluated. Values of k (5.7±0.5 μmol h −1 ) are independent of the chemical nature of the three reactants investigated, in agreement with other literature findings. Values of K are compared with those measured in aqueous suspensions of TiO 2 with the same chloro-organics and with the values predicted by a mechanism involving hydroxyl radicals as the primary oxidant adsorbed on the photocatalyst surface. Satisfactory performance and potential technological advantages of photocatalytic membranes are discussed.

Flow dynamical characterization of sorbents immobilized as composites in membranes prepared by photochemical grafting onto polymers

Abstract Permeability measurements of water and n-hexane were carried out at 20°C through membranes prepared by photochemically grafting an epoxy-diacrylate copolymer or its composites onto cellulose. Composite membranes immobilizing 30.0 wt.% of calcite, seven calcium oxide samples obtained from calcination of calcite in different experimental conditions, α-iron(III) oxide and cobalt peroxide were thus tested. Hydraulic resistances as a function of membrane thickness were studied. Experimental results in the Poiseuille s laminar flow regime are compatible with a normal Kozeny-Carman constant and a porosity of nominally 50%, indicating a regular packing of isometrically shaped particles in the microporous medium. Surface areas of membranes, both without additions and in the presence of filler materials, could thus be obtained by liquid permeametry measurements and successive treatment of data by the Kozeny-Carman equation. A general analysis of surface area characterization of immobilized sorbents is presented and compared with experimental values of B.E.T. method (liquid nitrogen adsorption). Results of flow dynamical investigation confirm the peculiar regularity of structure of the dense layer of photosynthetic membranes, as was found from morphology, from correlation between mean pore diameter and water fluxes per unit applied pressure drop, as well as from rejection characteristics in preceding studies.

Transport of oxygen facilitated by peroxo-bis [N, N′-ethylene bis-(salicylideneiminato)-dimethylformamide-cobalt (III)] embedded in liquid membranes immobilized by photografting onto cellulose☆

Abstract A novel method to prepare liquid membranes, suitable to obtain facilitated transport of oxygen, incorporating the title complex ( I ) or the same without dimethylformamide ( II ), is reported. These membranes were obtained by photochemically grafting an epoxy-diacrylate copolymer onto cellulose in the presence of ( I ) or ( II ) together with dimethylformamide. Sorption isotherms of oxygen into deoxygenated membranes were obtained by Langmuir plots at 343–373 K. By these isotherms the molar ratio (0.5) between sorbed oxygen and cobalt in the membrane corresponding to saturation was evaluated, and constants K (bar−1) of heterogeneous equilibrium (Equation (2) in the text) involving ( I ) and ( II ) were obtained as a function of temperature. From K values, a mean enthalpy change of − 84 ± 1 kJ mol−1 and entropy change of −220 ± 3 J mol−1 K−1 were calculated. This latter reflects the high order in binuclear dioxygen complex formation. Permeability coefficients of oxygen and nitrogen from air through the photosynthetic membranes at 343 K (4.60 × 10−5 and 9.23 × 10−7 cm2 s−1 bar−1, respectively) were measured: These values show, within a certain range of pressure drop of air, an efficient O2/N2 separation, with a very advantageous ratio (50) between their fluxes.

The Triple Layer Model Revised

Abstract The triple layer model (TLM) and the procedures for searching out its parameters, i.e. statistical optimization and the double extrapolation, have been critically reviewed and modified. The Gouy-Chapman equation and the assumption of constant values for the capacitances have been demonstrated to be inconsistent with the rest of the model. However, the constant capacitance model physically resembles the TLM near the point of zero charge (PZC). A new set of independent variables and functions has been employed. The dependence of these functions on the electrolyte activity and adsorption density at the PZC has been shown to have a very simple analytical form. A new statistical optimization procedure has been developed. It needs no approximations, two adjustable parameters, and an independent knowledge of two or three quantities among surface charge, surface potential, inner layer capacitance, total surface site density and inert electrolyte adsorption density. Only the data around the PZC may be employed to estimate the system constants.

Photosynthetic membranes. A model study of the influence of photoinitiated grafting and graft polymerization of 1,6-hexanediol diacrylate and 4-(N-ethyl,N-2-acryloxyethyl)amino-4′-nitro-azobenzene, as co-monomers, onto polypropylene ribbon, on permeation and diffusion coefficients of oxygen and nitrogen

Abstract Photoinitiated grafting and graft polymerization of 1,6-hexanediol diacrylate (I) and 4-(N-ethyl,N-2-acryloxyethyl)amino-4′-nitro-azobenzene (II) as co-monomers with a fixed molar ratio I:II equal to 10, in the presence of 1,2-diphenyl-2,2-dimethoxyethanone (III) as photoinitiator with a ratio R of its molar concentration of 0.005-0.38 with respect to the sum of molar concentrations of I and II, were studied using a polypropylene ribbon as a model system of a macroporous substrate. By polychromatic irradiation at 45°C with energy inputs of 90 mW cm−2, ultrathin layers of co-polymer, able to function as asymmetric membranes, could thus be obtained. Mean polychromatic quantum yields of grafting and graft polymerization, as a function of R, were also measured. Membrane specimens with constant values of grafted (200±20) and graft polymerized (500±80) molecules of monomer units in co-polymer per unit apparent surface (A2) have been subjected to measurements of permeabilities (P) and diffusivities (D) at 25°C for oxygen and nitrogen, as a function of R. Results show that only an optimum R minimizes P and D, and a narrow range of R-values allows a normal membrane behaviour.

Thermodynamic study of adsorption of linear aliphatic monohypric alcohols and monocarboxylic acids from n-Heptane and n-Decane solutions onto α-iron (III) oxide immobilized in photosynthetic membranes☆

Abstract Differential heats of adsorption at 298.2 K of a series of linear aliphatic monohydric alcohols CnH2n +1OH (n = 2–14) and linear aliphatic monocarboxylic acids CnH2n +1COOH (n = 1–13), from n -heptane and n -decane solutions (0.00005-0.03 molar fractions), onto α-Fe2O3 immobilized by photosynthetic membranes, prepared by photochemically grafting an epoxy-diacrylate copolymer onto cellulose, were measured. Langmuir-type treatment of data allowed us to calculate thermodynamic constants and adsorption heats for monolayer saturation, from which thermodynamic functions (free energy, enthalpy, and entropy) (saturated system as standard) were then computed. Mean areas occupied by adsorbed molecules in the monolayer coincided with those obtained for the unsupported sorbent, by allowing only about 20% of decrease of surface area, as a consequence of immobilization. Behaviour of immobilized sorbent is compared with that of ‘free’ α-Fe2O3. The sensitivity of adsorption measurements both to study configuration of adsorbed molecules, solvent-solute and solute-solute interactions

Gas separation membranes containing carbon immobilized by photochemical grafting onto polymers

Abstract Steady state permeabilities and diffusivities of carbon dioxide, oxygen, nitrogen, methane, ethane, ethylene and propylene have been measured as a function of temperature in the range 298-373 K through membranes immobilizing 30 wt.% of active carbon, which were prepared by photochemically grafting an epoxy-diacrylate prepolymer onto cellulose. From the experimental data, solubility coefficients for the above gases in the membranes have been calculated as a function of temperature. Results show that mass transfer across photosynthetic membranes immobilizing carbon may occur either by a strongly activated process, seemingly involving surface migration of an adsorbed unimolecular layer of gas, or by Knudsen type diffusion. The first type of mechanism prevails for molecules, such as the hydrocarbons examined, for which relatively strong adsorption interactions with active sites occur. As a consequence of the observed permeability and diffusivity pattern, efficient separation of gas mixtures may be achieved, with high selectivities at high permeabilities between molecules possessing different transport mechanism. The role of sorption enthalpies is discussed in terms of these mechanisms, as well as in terms of the structure and performance of photosynthetic membranes immobilizing sorbents.

Thermodynamic study of sorption of n-alkanes onto 5A zeolites immobilized in photosynthetic membranes

Abstract Equilibrium adsorption isotherms of n-heptane and n-dodecane onto composite photosynthetic membranes, prepared by photochemically grafting an epoxy-diacrylate copolymer containing 30 wt.% of zeolite 5A, were determined gravimetrically in the temperature range 298-373 K at partial pressures ranging from 0.01 to 500 Torr. Parallel adsorption runs were carried out onto unsupported, free zeolite. Experimental data were fitted to a virial equation describing the dependence of the amount of adsorbate on pressure by means of adsorbate-adsorbent and adsorbate-adsorbate interaction coefficients dependent on temperature. By these latter, thermodynamic standard values for monolayer coverage were calculated and compared with some recent literature values measured on free sorbent, although in a higher temperature range than that of the present study. Furthermore, from monolayer surface coverage at 298 K, areas occupied by single molecules of adsorbed species could be calculated, which indicate a relaxed conformation parallel to adsorption planes (coaxial configuration in the zeolite cavities). No difference has been found between adsorbed amounts per unit mass of sorbent in free and immobilized conditions, within the limits of experimental uncertainty; however, when adsorption was carried out from solution a small, but definite, decrease of surface area was noted. This fact points to an “ideal” behaviour of the immobilized sorbent in the membrane, particularly where gas adsorption and transport are concerned.

Influence of photocatalytic systems on the photo-cross-linking of diallyl oxydiethylene dicarbonate grafted onto cellulose☆

Abstract The photo-cross-linking of 18.4-20.0 μmol of diallyl oxydiethylene dicarbonate ( I ) grafted onto cellulose (per square contimetre of apparent cellulose surface) was examined kinetically at 30±2 °C, on exposure to an incident polychromatic radiation flux of (2.05±0.08) × 10 −7 einstein s −1 cm −2 , in the presence of catalytic amounts (20-200 ppm) of three vanadium- ( V ) organometallic compounds. As with the diacrylate systems studied previously, the kinetics of photo-cross-linking of grafted I followed a multi-relaxation spectrum in terms of two parameters (width of spectrum β and mean lifetime τ). The β values (0.42±0.04) were almost the same in all experiments and were similar to that (0.44±0.02) observed for the thermal decay of double bonds of pre-irradiated I in bulk. The β parameter therefore seems to reflect the cross-linked character of the linear polymer pattern, rather than the cross-linking process, either thermal or photocatalytic. In contrast, the τ values reflect, in a very sensitive manner, the effect of the concentration and type of photocatalytic system employed. An acceleration factor of about 190 was reached with 200 ppm vanadium (synergic mixture of vanadium (V) isopropoxide and isobutoxide). Some relevant features of the photo-cross-linking mechanism are discussed in the light of the proposed kinetic model.