Luis F. Sala

Kinetics and mechanism of the oxidation of D-galactono-1,4-lactone by CrVI and CrV

Abstract The oxidation of d -galactono-1,4-lactone by CrVI yields d -lyxonic acid, carbon dioxide and Cr3+ as final products when an excess of sugar acid over CrVI is used. The redox reaction occurs through CrVI → CrIII and CrVI → CrV → CrIII paths. The complete rate law for the CrVI oxidation reaction is expressed by −d [CrVI] \dt = (k0+kH [H+] ) [gal] [CrVI] , where k0 = (31±3) ×10−4 M−1 s−1 and kH = (99±5) ×10−4 M−2 s−1, at 40°C. CrV is formed in a rapid step by reaction of the CO·− 2 radical with CrVI. CrV reacts with the substrate faster than does CrVI. The CrV oxidation follows the rate law : −d [CrV] \dt = ( k ′ 0 +k ′ H [H+] ) [gal] , where k ′ 0 = (15±2) ×10−3 M−1 s−1 and k ′ H = (34±4) ×10−3 M−2 s−1, at 40°C. The EPR spectra show that several intermediate [Cr (O) (gala) 2] − linkage isomers are formed in rapid pre-equilibria before the redox steps.

Oxidation of l-rhamnose and d-mannose by chromium(VI) in aqueous acetic acid

Abstract The oxidation of l -rhamnose and d -mannose by Cr(VI) in aqueous acetic acid follows the rate law: −d[Cr(VI)]/d t = ( k 2 + k 3 [H + ] [ aldose ] [Cr(VI)]/{1 + [H + ]/ K a + K 1 [H + ][ aldose ]}, where k 2 = 3.5 +- 0.8 × 10 −3 s −1 and 8.6 +- 1.0 × 10 −4 s −1 , K 3 = 6.8 ± 0.5 × 10 −3 M −1 s −1 and 5.1 ± 0.5 × 10 −3 M −1 s −1 , K a = 1–4 M and K 1 = 13∓_ 2 and 17±5 M −2 for l -rhamnose and d -mannose, respectively. This rate law corresponds to the reaction leading to the formation of l -1,4-rhamnonalactone and d -1,4-mannonalactone. No cleavage to carbon dioxide takes place when a 30-fold or higher excess of aldose over Cr(IV) is employed. The free radicals formed in the slow electron-transfer steps react with Cr(VI) to yield two intermediate Cr(V) complexes with EPR signals at g 1 = 1.978 and g 2 = 1.973. The mechanism and associated reactions kinetics are presented and discussed.

Complexes of cobalt(II) and nickel(II) with d-aldonic and d-alduronic acids in aqueous solution

Abstract The equilibrium reactions between d -glucoheptonic, d -gluconic, d -galactonic, d -ribonic, d -glucoronic and d -galacturonic acids with cobalt(II) and nickel(II) ions have been studied by potentiometric methods in aqueous solution. All measurements were carried out at t = 20°C and μ = 0.10 M, and the corresponding stability constants constatns were calculated by applying the computer program BEST. The interactions between the proposed cations and these biological ligands are compared. As an indicator of probable structures of d -alduronic acid complexes, relaxation rate measurements on 13C spectra of the ligand in the absence and presence of metal ion were performed.

Degradative oxidation of d-ribono-1, 4-lactone by CrVI in perchloric acid

Abstract The oxidation of d -ribono-1, 4-lactone by CrVI yielded d -erythronic acid, erythrose, carbon dioxide and CrIII as final products when a 15-fold or higher excess of sugar over CrVI was used. The kinetics of the redox processes involving the CrVI/CrIII couple were determined and a mechanism is proposed. The complete rate law for the CrVI oxidation reaction is expressed by: −d[CrVI]/dt = [a[S][H+] + c[H+]3] [CrVI]T, where a = 3.09 × 10−3M−1s−1, b = 4.98 × 10−2M−2, s−1, c = 1.07 × 10−3M−3s−1 and S refers to the total reductant concentration, at 60°C. CrVI is formed in a fast step by reaction of the CO2− radical and CrVI, and CrV reacts with the organic substrate faster than CrVI does. The EPR spectra show that the intermediate CrV complex (g= 1.978) is formed and decays by a first-order process.

Application of green seaweed biomass for MoVI sorption from contaminated waters. Kinetic, thermodynamic and continuous sorption studies

Spongomorpha pacifica biomass was evaluated as a new sorbent for Mo(VI) removal from aqueous solution. The maximum sorption capacity was found to be 1.28×10(6)±1×10(4) mg kg(-1) at 20°C and pH 2.0. Sorption kinetics and equilibrium studies followed pseudo-first order and Langmuir adsorption isotherm models, respectively. FTIR analysis revealed that carboxyl and hydroxyl groups were mainly responsible for the sorption of Mo(VI). SEM images show that morphological changes occur at the biomass surface after Mo(VI) sorption. Activation parameters and mean free energies obtained with Dubinin-Radushkevich isotherm model demonstrate that the mechanism of sorption process was chemical sorption. Thermodynamic parameters demonstrate that the sorption process was spontaneous, endothermic and the driven force was entropic. The isosteric heat of sorption decreases with surface loading, indicating that S. pacifica has an energetically non-homogeneous surface. Experimental breakthrough curves were simulated by Thomas and modified dose-response models. The bed depth service time (BDST) model was employed to scale-up the continuous sorption experiments. The critical bed depth, Z0 was determined to be 1.7 cm. S.pacifica biomass showed to be a good sorbent for Mo(VI) and it can be used in continuous treatment of effluent polluted with molybdate ions.

Interaction of zinc(II) ion withd-aldonic acids in the crystalline solid and aqueous solution

Abstract The interaction of zinc(II) ion with d -glucoheptonic acid, d -gluconic acid, d -gulonic acid, d -galactonic acid and d -ribonic acid has been investigated and compounds of the type Zn( d -glucoheptonate)2·3H2O, Zn( d -gluconate)2·3H2O, Zn( d -gulonate)2·3H2O, Zn( d -galactonate)2·3H2O and Zn( d -ribonate)2·H2O, have been isolated. These metal-sugar salts were characterized by elemental analysis, FT IR spectroscopy, thermogravimetric analysis and13C-NMR. Spectroscopic measurements showed similar patterns between these complexes and the structurally identified Mn( d -gluconate)2·2H2O. The zinc(II) ion is binding to two ligand molecules through the car☐ylate and OH groups of each sugar, as well as to water molecules. The potentiometric measurements in aqueous solutions for the systems formed by the sugar acids investigated and the zinc(II) ion at different metal-ligand ratios showed the 1:1 complexes formation. On the basis of the13C NMR, the participation of C-1 and C-2 in this complex formation was verfiied. Due to the hydroxide precipitation, quantitative evaluation of the stability constants was not performed.

Iron(III) complexes of lactobionic acid : equilibrium and structural studies in aqueous solution

The equilibrium constants for the protonation of lactobionic acid (4-O-β-D-galactopyranosylgluconic acid) and its co-ordination with FeIII were studied by potentiometric methods in aqueous solution, at 20.0 °C and I= 0.100 mol dm–3(NaNO3). The equilibrium data were processed with the Fortran computer program BEST. Correlations of chelate hydrolysis constants with those of the D-gluconic acid–iron(III) system were carried out. Co-ordination bonding sites and stereochemistry of metal–ligand interactions were inferred from UV and NMR spectroscopy.

Kinetics and mechanism of the oxidation of (±)-2-hydroxy-3-methyl butanoic acid by chromium(VI) in perchloric acid medium☆

Abstract The oxidation of 2-hydroxy-3-methylbutanoic acid by chromium(VI) in perchloric acid follows the rate law: −d[CrVI]/dt = (k0+kHh0) [S] [HCrO4−]; where [S] represents 2-hydroxy-3-methylbutanoic acid, k0 = 0.0359 M−1 s−1 and kH = 0.0183 M−2 s−1. This rate law corresponds to the formation of 2-methylpropanoic acid and carbon dioxide by CC cleavage when a 10-fold or higher excess of acid over chromium(VI) is employed. The reaction proceeds through a 1:1 acid/chromium(VI) complex, which decomposes by a three-electron step to the products. Chromium(V), formed in subsequent steps, decays at least four times faster than chromium(VI), as observed by EPR and visible spectrophotometry. When an excess of chromium(VI) over the substrate is employed, further CC cleavage takes place, and the final degradation products are acetone and carbon dioxide.

Reduction of hypervalent chromium in acidic media by alginic acid

Selective oxidation of carboxylate groups present in alginic acid by Cr(VI) affords CO2, oxidized alginic acid, and Cr(III) as final products. The redox reaction afforded first-order kinetics in [alginic acid], [Cr(VI)], and [H(+)], at fixed ionic strength and temperature. Kinetic studies showed that the redox reaction proceeds through a mechanism which combines Cr(VI)→Cr(IV)→Cr(II) and Cr(VI)→Cr(IV)→Cr(III) pathways. The mechanism was supported by the observation of free radicals, CrO2(2+) and Cr(V) as reaction intermediates. The reduction of Cr(IV) and Cr(V) by alginic acid was independently studied and it was found to occur more than 10(3) times faster than alginic acid/Cr(VI) reaction, in acid media. At pH 1-3, oxo-chromate(V)-alginic acid species remain in solution during several hours at 15°C. The results showed that this abundant structural polysaccharide present on brown seaweeds is able to reduce Cr(VI/V/IV) or stabilize high-valent chromium depending on pH value.

Removal of Chromium(VI) and Chromium(III) from Aqueous Solution by Grainless Stalk of Corn

Abstract Grainless stalk of corn (GLSC) was tested for removal of Cr(VI) and Cr(III) from aqueous solution at different pH, contact time, temperature, and chromium/adsorbent ratio. The results show that the optimum pH for removal of Cr(VI) is 0.84, while the optimum pH for removal of Cr(III) is 4.6. The adsorption processes of both Cr(VI) and Cr(III) onto GLSC were found to follow first-order kinetics. Values of k ads of 0.037 and 0.018 min−1 were obtained for Cr(VI) and Cr(III), respectively. The adsorption capacity of GLSC was calculated from the Langmuir isotherm as 7.1 mg g−1 at pH 0.84 for Cr(VI), and as 7.3 mg g−1 at pH 4.6 for Cr(III), at 20°C. At the optimum pH for Cr(VI) removal, Cr(VI) reduces to Cr(III). EPR spectroscopy shows the presence of Cr(V) + Cr(III)-bound-GLSC at short contact times and adsorbed Cr(III) as the final oxidation state of Cr(VI)-treated GLSC. The results indicate that, at pH ≈ 1, GLSC can completely remove Cr(VI) from aqueous solution through an adsorption-coupled reduction mechanism to yield adsorbed Cr(III) and the less toxic aqueous Cr(III), which can be further removed at pH 4.6.