A method for the removal of chromium from tanned leather wastes

The separation of Cr(III) from collagen in chromium-tanned leather (wet-blue) is achieved by a combination of the protein (collagen) stabilization (protective cross-linking) and a subsequent labilization of the Cr(III) species bound to the protein. During the separation collagen retains the tertiary triple helical and higher (fibril) structures. The model proposed for the protective cross-linking involves inter- rather than intramolecular bridging, that is, a supramolecular bridging. The process of chromium removal takes place in a restricted aqueous environment, and it can be characterized as semihomogeneous or semiheterogeneous. © 1998 Air and Waste Management Association

A Study on the Interaction of Eu2+(aq) with Pyridinecarboxylic Acids

Europous ion forms with isonicotinic, N-methylisonicotinic, nicotinic, and picolinic acids one to one complexes having several features, which are rather unusual for a lanthanide ion. They are formed in strongly acidic aqueous solutions and have absorption maxima around 420 nm. The formation constants are 0.15 1. mol-1 for nicotinic acid, 0.2 1. mol-1 for picolinic acid, 1.9 1. mol-1 for isonicotinic acid, and 0.4 1. mol-1 for N-methylisonicotinic acid, respectively. Evidence is presented that the complexes involve charge transfer from the metal ion to the ligand. The complexes of nicotinic and picolinic acids are stable toward further redox reaction. The complexes of isonicotinic acid and its N-methyl derivative, however, undergo further reduction leading in the first case to isonicotinaldehyde and in the second very likely to the dihydro derivative. In the presence of Eu3+(aq) the kinetics of the redox reaction of isonicotinic acid and its N-methyl derivative are second order in europous ion, first order in the organic acid, first order in hydrogen ion, and inverse first order in Eu3+(aq). A unified mechanism is proposed to explain the results for both of these acids, which is also consistent with the results obtained on complex formation and with the postulate of a charge transfer from europous ion to the ligand. © 1975, American Chemical Society. All rights reserved

Photocatalytic splitting of water: increase in conversion and energy storage efficiency

The yield of the photocatalytic splitting of water using tris-[1-(4-methoxyphenyl)-2-phenyl-1,2-ethylenodithiolenic-S,S′] tungsten as a photocatalyst-catalyst increases by more than threefold on going from 20 to 70°C, and there is no indication that the effect levels off at this temperature. The intensity of light (within the error limits of our experiments) does not have appreciable effect. The nature of the reversible electron acceptor also influences the energy storage efficiency, e.g. 1,1-dibenzyl-4,4′-bipyridiniumdichloride gives an energy storage efficiency approximately 10% higher than methylviologen. The energy storage efficiency also depends on the presence of electron donors; if Ph3N is added, the energy storage efficiency increases by 20%. With ethylenediaminetetraacetic acid (EDTA) the results are even more spectacular; there is a twofold increase, but only initially. At longer times the system is unstable. Overall light energy storage efficiencies can be as high as 7%, and the expectations for further improvement are very good. © 1994

Reduction of pyruvate by titanium(III)

Titanium(III) in aqueous solutions reacts with carbonate-like pyruvic acid and/or pyruvate to give a product of reductive coupling. The reaction was investigated kinetically over a range of hydrogen ion concentrations from 0.007 M to 2.5 M and over a wide range of concentrations of the other reactants. Under all conditions only one path was identified, corresponding to a second order rate law in TiIII, first order in Pyr, and inverse second order in H+. The data are interpreted by postulating the formation of an η2 precursor complex between TiIII and the carbonyl group. © 1984

A new series of organochromium complexes formed in aqueous solutions

Chromous ion reacts with 3-pyridineacrylic acid, 4-pyridineacrylic acid, maleic acid and fumaric acid to give remarkably inert organochromium(III) species. These species were isolated by ion exchange from acidic aqueous media and were characterized by a variety of techniques. An organochromium(III) compound was even isolated from the Cr(III)-3-pyridineacrylic acid aqueous mixture in solid form. In all the organometallic compounds reported in this paper chromium(III) is believed to be σ-bonded to carbonyl by the general schemeA figure is presented. © 1980

Photophysical study of the decomposition of water using visible light and tungsten tris(dithiolenes) as photosensitizers-catalysts

Excitation of four unsymmetrically substituted tungsten tris(dithiolenes), in various solvents with 355 nm laser pulses produce weak but easily measurable transient absorptions. The four complexes used are characterized as photocatalyst-catalysts (PC-C). for the photodecomposition of water and their names are tris-[1-(4-dimethylaminophenyl)-2-phenyl-1,2-ethylenodithiolenic-S,S′]tungsten,tris-[1-(4-methoxyphenyl)-2-phenyl-1,2-ethylenodithiolenic-S,S′]tungsten, tris-[1-(4-methoxy-phenyl)-1,2-ethylenodithiolenic-S,S′]tungsten and tris-[1-(phenyl)-1,2-ethylenodithiolenic-S,S′]tungsten. The lifetimes of the transient absorptions observed are long, persisting into the millisecond time domain. In acetone-water, and in the presence of MV2+ there are at least two transients, one corresponding closely to the known spectrum of MV 4 , assigned to the hydrogen precursor, the other living much longer and assigned to the O2 precursor. The results indicate that the electron is quickly removed from the excited state of the photocatalyst-catalyst and is temporarily stored in the electron acceptor, in a way that is unfavorable to recombination. The first stages of the process can be schematically represented as: hv MV2+-(W-L)-OH2→MV2+-(W--L+)-OH2→MV.+-(W-L)-+OH2→H2+(W-L)+O2. The dotted lines connecting the reagents with the PC-C (W-L) were put in this scheme in order to emphasize the supramolecular nature of the interactions. The opposite charges created by light absorption are quickly transformed into repulsive positive charges due to the presence of the electron acceptor MV2+ and two centers are created, two molecular diameters apart, one the precursor of the H2, the other the precursor of O2. The combination of a small space separation charges and of the electrostatic factor (transformation of these charges into repulsive ones) are perhaps crucial factors in the separation of the reduction from the oxidation. However, there is also a kinetic factor. The reduction is quick, irreversibly leading to H2 not to recombination. The splitting of water is time resolved. The reactions at the reduction end of the supramolecular complex (presumably because the involve fast proton and electron transfers) lead fast and irreversibly to H2 formation. © 1998 Elsevier Science S.A. All rights reserved

Homogeneous catalytic action of a nickel dithiolene complex, leading to dihydrogen formation from N,N'-dimethyl-4,4'-dipyridinium radical ion solutions

Bis(2-chlorodithiobenzyl)nickel(II) reacts with the methylviologen free-radical ion and is reduced to its mono-anion. The second-order rate constant for this reaction was found to be greater than 107M-1 s-1. The mono-anion of the complex acts as a homogeneous catalyst for the electron transfer reaction between the remaining excess of the methylviologen free-radical ion and water, leading to dihydrogen production. The kinetics and mechanism of the catalytic reaction were studied in acetone-water solutions. The reaction is second order with respect to the catalyst and depends on the water content of the solutions. The value of k2 is 8.2 ± 0.3 M-2 s-1 at 21 °C and at a 30:70 water: acetone ratio. The Arrhenius activation energy is 77 ± 3 kJ mol-1. A mechanism is proposed for the reaction. © 1985