Publications

Abstract The complexation of Cu(II) by two samples of fulvic acids (FA) extracted from the raw sewage sludge (RsFA) of a waste water processing plant and from the composted sludge (CsFA) obtained upon aerobical digestion was studied at pH 6.0. Synchronous molecular fluorescence spectroscopy was used to monitor the association of FA with Cu(II) and a self-modelling mixture analysis method (SIMPLISMA) was used to preprocess the spectral data to calculate the number of components with different quenching profiles as function of the Cu(II) concentration and their spectra. The stability constants and a rough estimation of the binding site concentrations were obtained by a Stern-Volmer analysis, by a non-linear least-square method and by a linear procedure. The analysis of the SyF spectra allowed the identification of two binding sites for both samples. The logarithm of the conditional stability constant corresponding to the 1:1 complex formed between the stronger binding site and Cu(II) is about 4 for both samples.

Abstract The association between alpha-cyclodextrin and glycerol has been studied using experimental and theoretical methods. The experimental technique used was microcalorimetric stepwise titration at 288.15, 298.15 and 313.15 K. The formation of a 1:1 complex between alpha-cyclodextrin and glycerol, in aqueous solutions, is indicated. There was no temperature dependence of the observed parameters. The thermodynamic parameters were K = 1.34 +/- 0.07, Delta H degrees = (-3.1 +/- 0.1) kJ mol(-1), Delta G degrees = (-0.7 +/- 0.1) kJ mol(-1) and Delta S degrees = (-8 +/- 4) J K-1 mol(-1) (Gibbs energy and entropy values calculated for 298.15 K). Molecular dynamics simulations have also been carried out. The average structures, obtained from 100 ps of dynamics, are compared with the solvated 'empty' alpha-cyclodextrin average structure. The dynamics calculations show that there is a structurally and energetically favourable association between the molecules, which is in good agreement with the experimental findings.

Abstract The enthalpies of solution of I-pentanol, 1-hexanol and 1-heptanol and water were determined in formamide and in ethylene glycol, at 25 degrees C, by macrosolution calorimetry. The observed enthalpies of solution far the n-alcohols are positive in the two solvents, and of similar magnitude. The enthalpy of solution of water is positive in formamide, and negative in ethylene glycol. From the enthalpies of solution, the enthalpies of salvation and the enthalpies of transfer for organic solvent-->water were calculated Using our values and literature data for alkanols, it was possible to see that both the enthalpies of solution and the enthalpies of salvation presented a constant CH2 increment for the entire series, in contrast with their behavior in water. The methylene increments for these properties in different solvents are compared with parameters considered to reflect the cohesive energy of the solvent.

Abstract Amino acids are a class of natural organic chelating agents which play an important role in the mobilisation of heavy metals in soils. In order to assess the ability of phenylalaline and tryptophan to extract heavy metal from a typical soil component, montmorillonite was purposely contaminated with Cu, Cd, Pb and Zn, The contaminated clay was suspended in solutions of amino acids at the pH range 7.6-3.3 and the release of metals was measured in presence of solid particles by DPP or DPASV, Leaching of heavy metals at neutral pH is more effective than at acidic pH and range from 100% to 7%. At a constant pH the leaching decreases following the order: Cu > Cd > Zn, Pb. The leaching by phenylalanine and tryptophan was also tested on three certified reference materials and the results obtained are compared to those obtained with EDTA, NTA and HNO3 as leaching agents.

Abstract The interaction of the aluminum ion [AI(III)I with the major constituent of soil organic matter, fulvic acids (FA), was investigated in the acid pH range. The complexation of a FA extracted from a humic cambisol soil (at a concentration of 83 mg/L) when titrated with Al(III) were monitored directly by synchronous fluorescence (SyF) spectroscopy at five pH values (3.0, 3.5, 4.0, 4.5, and 5.0), at 25 degrees C and with 0.1 N KNO3. Inverted second derivative SyF data were preprocessed by a self-modeling mixture analysis method to obtain fluorescence enhancement profiles as a function of the increasing AI(III) concentration. Assuming that the concentration of complexes formed between Al(VI) and FA is proportional to the fluorescence intensity, these profiles were adjusted to a 1:1 complexation model and the following conditional stability constants (log K-c) were calculated (standard deviation in parenthesis), pH = 3.0, 4.3 (1); pH = 3.5, 4.3 (1); pH = 4.0, 5.0 (1); pH = 4.5, 5.1 (3); end pH = 5.0, 5.3 (2). These constants show that the stability of the complexes decreases markedly at pH < 4. The environmental implication of this result is that an increase in the extent of the dissociation of the Al(III) + FA complexes is expected at pH < 4, increasing the free Al(III) concentration and probably its ecotoxicity.

Abstract The association of Al(III) with a soil fulvic acid (fua) was monitored in the pH range of 2.5-5.0 by synchronous fluorescence (SyF) spectroscopy. Sets of spectra of 100 mg/l fua solutions obtained at increasing Al(III) concentration (up to about 0.7 mM) at each pH (2.5, 3.0, 3.5, 4.0, 4.5 and 5.0) were preprocessed by the self-modeling mixture analysis method SIMPLISMA, with inverted second derivative spectra in the pure variable selection step, to determine the number and to calculate the spectra and concentration profiles of the components. This procedure detected two major components at all pH values: (i) one shows an increase of fluorescence which corresponds to a fluorescent complex formed between Al(III) and one of the binding sites of the fua; (ii) the other shows a decrease of fluorescence probably due to the decrease of the free concentration of a fluorescent binding site that complexes Al(III) forming a complex that does not show fluorescence. Considering the component that shows increase of fluorescence, conditional stability constants [log(K) with standard deviation in parenthesis] were estimated: pH=2.5, 4.5(3); pH=3.0, 4.6(3); pH=3.5, 4.8(2); pH=4.0, 5.2(1); pH=4.5, 6.1(2); and pH=5.0, 6.2(3). These results show that the stability of the complexes between fua and Al(III) decreases markedly for pH<4.5. This behavior may have an important influence on the changes of the environmental speciation of Al(III) upon acidification.

Abstract The following standard molar enthalpies of formation in the gaseous state at 298.15 K were determined from the enthalpies of combustion of the crystalline solids, and their enthalpies of sublimation and the mean (N-O) bond dissociation enthalpies were derived. Delta(f)H(m)(0)(g); [D(N-O)] (kJ mol(-1)): quinoxaline 1,4-dioxide, 227.1 +/- 2.4; 255.8 +/- 2.0; 2-methylquinoxaline 1,4-dioxide, 169.9 +/- 7.2; 268.3 +/- 4.9; 2-methyl-3-acetylquinoxaline 1,4-dioxide, 33.1 +/- 5.0; 251.6 +/- 4.2; 2-phenyl-3-benzoylquinoxaline 1,4-dioxide, 355.2 +/- 7.1; 227.3 +/- 5.4; 2-methyl-3-carbomethoxyquinoxaline 1,4-dioxide, -148.7 +/- 3.2; 242.3 +/- 3.9; pyrazine 1,4-dioxide, 186.5 +/- 1.9; 254.0 +/- 2.3; 2-methyl-5-pyrazinecarboxylic acid, -213.6 +/- 1.7. Unconstrained geometry optimizations by ab initio calculations showed the effect of steric hindrance on changes in extended delocalizations and were in accord with the trends in the mean bond dissociation enthalpies.

Abstract The standard (p(o) = 0.1 MPa) molar enthalpies of formation for crystalline 2-phenylquinoline and 2,2'-biquinoline were derived from the standard molar enthalpies of combustion, in oxygen, at the temperature T = 298.15 K, measured by static bomb combustion calorimetry; the Knudsen mass-loss effusion technique was used to measure the vapour pressures of the crystals as functions of temperature, and the standard molar enthalpies of sublimation, at T = 298.15 K, were derived using the Clausius-Clapeyron equation. [GRAPHICS] (C) 1997 Academic Press Limited.