Nabila M. Guindy

Vacuum-ultraviolet (VUV) photolysis of water: oxidative degradation of 4-chlorophenol

Abstract A new Xe-excimer light source has been used for the oxidative degradation of 4-chlorophenol (4-CP) in aqueous solution. Owing to the very high absorption cross-section of water at the wavelength of excitation (172 nm), degradation of the organic substrate is initiated by hydroxyl radicals generated by water photolysis. Neither oxygen concentration, nor the presence of carbonates or nitrates affects the rate of 4-CP degradation; however, maximum oxygen concentration is required for higher rates of mineralization of organic carbon. Kinetic investigations are based on 4-CP, TOC and Cl − analysis. VUV photolysis as a means of oxidative degradation of organic pollutants in aqueous systems is showing the potential to compete with other photochemical oxidative processes.

Thermal analyses of mono- and divalent montmorillonite cationic derivatives

Abstract DTA and TG analyses were carried out for mono- and divalent cationic derivatives of Jelsovy-Potok montmorillonite. A certain relation between the charge density and the highest dehydration DTA peak was established for both series, since the two variables are a function of the magnitude of binding the hydrated water molecules. According to the charge density levels the cationic derivatives can be classified into three categories: (i) the single peak group falls in the range of charge density lower than 1 C m −2 ; (ii) the double peak group falls between 1 and about 2 C m −2 ; and (iii) for the higher charge densities three peaks are observed. The Ba derivative presents the only exception in this series. As the charge density increases, not only the number of dehydration peaks increases, but also the isolation between them becomes better, i.e., the overlapping of these peaks decreases in the order Ca > Li > Mg. The slopes of the TG curves in the temperature range 200–500°C increase with increase in charge density on the cationic derivative. This indicates that the rate of dehydration differs by changing the exchangeable cation of the surface, whereas the rate of dehydroxylation for all derivatives is the same since they all have the same origin.

Thermal dehydration of mono- and di-valent montmorillonite cationic derivatives

Abstract The kinetics of the isothermal dehydration of seven derivatives of Jelsovy-Potok montmorillonite comprising two series of mono- and di-valent cations were studied together with TG and DTA of these compounds. A certain correlation was established between the charge density and (1) the highest dehydration DTA peak, (2) the first order rate constant for the dehydration reaction. A diffusion controlled mechanism is suggested for the kinetics of theisothermal dehydration. This is supported by (1) the dependence of the first order rate constant on the cationic radius, (2) the relatively large values of the first order rate constants for the monovalent cations compared with those for the di-valent cations, and (3) the values of the energy of activation (≈ 23 kJ mol −1 ) for all the derivatives studied. Values of about 220 J K −1 were obtained for the entropy of activation supporting the hypothesis of a two dimensional diffusion reaction.

Thermal decomposition of oxalates. Part 17. Thermal decomposition of manganese(II) oxalate in a nitrogen atmosphere

Abstract Comparative studies of the kinetics of the isothermal and nonisothermal dehydration and decomposition of manganese(II) oxalate in an atmosphere of nitrogen are reported. Agreement between the values of the energy of activation for the isothermal and the nonisothermal dehydration at high heating rates was obtained. At low heating rate, the value obtained for the energy of activation is comparable with the enthalpy of dehydration. Values of 143 and 242 kJ mole −1 were obtained for the energy of activation of the isothermal and nonisothermal decomposition, respectively. The difference is attributed to the condition of the anhydrous salt used in both cases. The theory of absolute reaction rate is applied and the parameters obtained are discussed.

Photochemistry of metalloporphyrins in aqueous solutions

Abstract The fluorescence spectra of Ga(III), In(III), Ge(IV) and Sn(IV) tetramethyl pyridine porphyrin showed a small stokes shift ca 400 ± 100 cm −1 attributed to the similarity between their ground and excited states. The one and two electron reduction products of the metalloporphyrins were studied by steady-state method. The nature of the product was dependent upon the pH of the reaction medium. Chlorin and phlorin were obtained at low and high pHs, respectively. Sn TMPyP was the only metalloporphyrin which yielded the Π-radical anion whose stability is due to the high electronegativity of Sn(IV).

Nitrogen and water vapour adsorption on monovalent and divalent montmorillonite derivatives and their heats of immersion in polar liquid

Abstract Nitrogen and water vapour adsorption measurements were carried out at -195 °C and 35 °C respectively on some alkali metal and alkaline earth derivatives of Jelsovy Potok montmorillonite; the derivatives contained lithium, sodium, caesium, magnesium, calcium, strontium or barium. The nitrogen area was unaffected by the exchangeable ion and can be considered as a measure of the external surface area of montmorillonite. The water area was found to increase with decrease in the ionic radius of the exchangeable ions owing to their capacity for hydration. The crystallographic area provided information on the number of layers per particle and on the change in cross-sectional area of the water molecule whose packing factor was varied by changing the exchangeable ion.

Vapor adsorption on expanding and nonexpanding clay minerals

Adsorption of water and cyclohexane vapors at 35°C was measured on montmorillonite (an expanding-type clay mineral) and on illite (a nonexpanding clay mineral), both heated in vacuo at various temperatures for 5 hr. Specific surface areas were estimated from the BET method (SBET), and from the cumulative values of pore structure analysis (Scum). Water, but not cyclohexane, was noticed to penetrate between the layers of montmorillonite, and in such a case Scum > SBET. For the nonexpanding illite lattice SBET for water was found to be larger than Scum due to the presence of micropores in the nonexpanding mineral. Cyclohexane, as a bulkier molecule, was inaccessible to parts of the pore structure in both minerals. Heats of immersion in both water and cyclohexane were measured, and it is suggested that these values should be referred to the area actually accessible to the adsorbate molecule. For water/montmorillonite this value should be Scum and not SBET.

Kinetics of the acid and alkaline degradation of oxytetracycline hydrochloride

Abstract The kinetics of the acid degradation of oxytetracycline hydrochloride in absence and in presence of Fe(III) and/or Cu was followed spectrophotometrically at 245 nm. The reaction was found to be first order in oxytetracycline, in hydrogen ions and is inhibited by the presence of cations. Values of 96 and 54 kJ mol−1 were obtained for the overall energy of activation, and the corresponding values for the entropy of activation are −4.2 and −117 JK−1. The products of the reaction were identified by TLC as α- and β-apoterramycin. The alkaline degradation was traced at 380 nm. The reaction followed first order kinetics in both reactants and was unaffected by the presence of cations. Values of 65 kJ−1 mol−1 and −96 JK−1 were obtained for the activation parameters.

Surface properties of montmorillonite, an expanding-type clay mineral

Abstract The adsorption of nitrogen at -196 °C, and of water and cyclohexane vapours at 35 °C, was measured on montmorillonite samples prepared by heating the clay mineral at various temperatures between 30 and 1000 °C for 5 h in vacuo. Specific surface areas were estimated from both the BET method (SBET) and the cumulative values obtained from pore structure analysis, known as Scum. The results obtained indicate in general that water and nitrogen are able to penetrate between the layers due to the expansion properties of the mineral, and in such cases Scum >SBET. Cyclohexane, with a much bulkier molecule having a non-polar character, is unable to produce such an effect. Heats of immersion were also measured in water and in cyclohexane, and it is suggested that, for adsorbates which actually penetrate between the layers (e.g. H2O), the heat values should be referred to Scum and not to SBET.

Effect of structure on the mechanism of dehydration of clay minerals

Abstract The kinetics of the isothermal dehydration of two clay minerals, attapulgite and powdered vermiculite were studied both in vacuo and in the presence of constant water vapour pressure. The mechanism of dehydration was found to be interfacial where diffusion plays a dominant role and it is called “activated diffusion”. A comparison is made with some layered structure clay minerals.