C. Gessa

Analytical and spectroscopic characterization of humic acids extracted from sewage sludge, manure, and worm compost.

Humic acids extracted from sewage sludges, manure, and worm compost have been characterized by chemical and spec-troscopic methods. Meaningful differences in the composition were revealed by FT IR, 1H, 13C NMR, and visible spectroscopies. These differences allow a differentiation amomg the products depending on the source from which they were obtained. Humic acid extracted from sewage sludges contains the highest percentage of aliphatic carbon, associated with polysaccharides and proteinaceous structures, and has characteristics close to those of aquatic humic acids. On the other hand, humic acids from manure and worm compost are similar to the humic acids originating from soil.

Coordination of transition-metal ions by polygalacturonic acid: A spectroscopic study

The interaction between polygalacturonic acid with Cu(II), VO(IV), Mn(II), Ni(II) and Co(II) has been studied with the aid of ESR, electronic and IR spectra. The results show that in fully hydrated gels Cu(II) and VO(IV) ions form inner-sphere carboxylate complexes, whereas [Mn(H2O)6]2+ retains its inner hydration sphere. On dehydration, all the ions are bound to carboxyl groups, whose binding mode is dependent on the nature of the ion. The changes observed in spectral parameters upon dehydration suggest the substitution of water molecules by sugar oxygen atoms in order to preserve the stereogeometry of the ions.

Zinc(II) adsorption on aluminium hydroxide

Abstract The adsorption of zinc(II) on aluminium hydroxide from ZnSO4 or ZnCl2 solutions in the presence of anions such as chloride, perchlorate or sulphate has been investigated. The results substantiate that the adsorption process depends on several interactive processes occurring in solution (e.g., zinc speciation) and at the oxide—solution interface (specific anion binding). It is suggested that chloride promotes the sorption of chloro-complexes, sulphate enhances the cationic adsorption by making the surface potential more negative, and perchlorate behaves as an indifferent anion. Specific adsorption of cationic species promotes the binding of adsorbing anions.

Interaction of transition-metal ions with polygalacturonic acid: A potentiometric study

Abstract The complexes formed by polygalacturonic acid (PGA) with Mn2+, Co2+ and Ni2+ were investigated by potentiometric titration of the polyelectrolyte in presence of varying amounts of metal ions. Formation functions, evaluated from the titration data and analyzed according to the Bjerrum plot method, were interpreted as evidence of: 1) the involvement of two carboxylate groups in the complex formation; 2) very little difference in preference for the binding of all the three ions by PGA. By taking into account Potentiometric and spectroscopic results (the latter reported elsewhere), an outer-sphere electrostatic structure is suggested for all the complexes. The relevance of this type of interaction in soil biochemistry is discussed.

Structure and internal motion of solvated beta-cyclodextrine: a molecular dynamics study

Abstract A 5xa0ns molecular dynamics simulation was performed on a system constituting 512 water molecules and one β -cyclodextrin molecule in a cubic cell with a 26xa0A length. The internal motion of the solute molecule was investigated and compared with reports from Molecular Mechanics, Neutron Scattering and Molecular Dynamics. The β -cyclodextrin structure is markedly distorted with the majority of the primary hydroxyl groups pointing towards the inner cavity. The structure and dynamics of the water molecules in the first solvation shells were also investigated. An average number of 25 water molecules compose the inner shell forming a network of hydrogen bonds with the cyclodextrin hydroxyl groups.

Anion-induced metal binding in amorphous aluminium hydroxide

Abstract The adsorption of VO 1+ and Cu 2+ ions on freshly-prepared aluminium hydroxide gels was studied using analytical and spectroscopic techniques. The cation adsorption is shown to involve hydrolytic complexes of metal ions. In addition, a correlation is found between the adsorption capacity of the gels and the nature of the anions present in the exchange solution, sulphate enhancing the copper uptake with respect to chloride. These findings are explained in terms of negative charges arising on the surfaces upon selective binding of sulphate, which promote the metal adsorption. Desorption experiments reveal that the adsorbed ions are quantitatively removed only by strong chelating agents. The role of such interactions in determining the soil nutrient status is discussed.

Analytical study of the reduction of chromium(VI) by D- galacturonic acid

Abstract The reduction of chromium (VI) to chromium(III) by d -galacturonic acid takes place to a significant extent only in the presence of metal ions such as copper(II) or chromium(III) itself. These metals can form stable complexes in which the opening of the sugar ring leads to a reducing free aldehydic group. The stoichiometry of the reaction, the dependence of the yield on the nature of the metal ion and the pH of the solution and the products formed, were studied. Stoichiometries and stability constants of the complexes present in the system chromium(III)- d -galacturonic acid were evaluated.

Complexing and redox properties of the system D-galacturonic acid-iron(III)

Abstract The stoichiometry of the reduction reaction and D -galacturonic acid with iron(III) has been determined, leading to a 4:1 ratio between the quantities of reduced iron and oxidized D -galacturonic acid, respectively. This last molecule is oxidized to formic acid, and the residue is thought to present a further terminal carboxylic group. The reducing predominant species appears to be the FeGal 3 complex.

Analytical study of the interactions of D-galacturonic acid with iron(III) and iron(II) in solution and with iron(III)-bentonite

Abstract The interaction of d -galacturonic acid (HG) with iron(III) and iron(II) ions was studied in aqueous 1 M sodium perchlorate solutions. At pH 3 where the ligand molecules coordinate essentially by the carboxylic group. At higher pH values, complexes with further deprotonated ligand molecules are the main species in the system. There is some evidence that the chelates formed involve the open form of the sugar molecule, the iron(III) ions binding the oxygen corresponding to the ring oxygen of the closed form. In the heterogeneous d -galacturonic acid/Fe(III)-clay system, the sugar molecule appears to interact strongly with the bentonite, suggesting the formation of complex species in which all three compounds of the ternary system are involved.

A molecular dynamics investigation on the occurrence of helices in polygalacturonic acid

Abstract Partially esterified polygalacturonic acid is the main component of pectin in higher plants. It constitutes the mucilaginous soil–root interface and acts as an accumulation phase for nutrients, being an important media for the diffusion of ions towards the root absorbing cells. The carboxylic groups and their methyl esters markedly affect the ability of the pectin molecules to bind oppositely charged ions and to form gels. Molecular Dynamics was employed to investigate the conformational equilibrium and the intermolecular interactions of a system constituted by two polygalacturonic acid chains, each formed by 24 units of galacturonic acid. The results suggest that, as evidenced for other polysaccharides, a helix based structure could be proposed for the polygalacturonic acid chains.