Silvia García

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.

Redox and complexation chemistry of the CrVI/CrV–D-galacturonic acid system

The oxidation of d-galacturonic acid by Cr(VI) yields the aldaric acid and Cr(III) as final products when a 30-times or higher excess of the uronic acid over Cr(VI) is used. The redox reaction involves the formation of intermediate Cr(IV) and Cr(V) species, with Cr(VI) and the two intermediate species reacting with galacturonic acid at comparable rates. The rate of disappearance of Cr(VI), Cr(IV) and Cr(V) depends on pH and [substrate], and the slow reaction step of the Cr(VI) to Cr(III) conversion depends on the reaction conditions. The EPR spectra show that five-coordinate oxo-Cr(V) bischelates are formed at pH < or = 5 with the uronic acid bound to Cr(V) through the carboxylate and the alpha-OH group of the furanose form or the ring oxygen of the pyranose form. Six-coordinated oxo-Cr(V) monochelates are observed as minor species in addition to the major five-coordinated oxo-Cr(V) bischelates only for galacturonic acid : Cr(VI) < or =10 : 1, in 0.25-0.50 M HClO(4). At pH 7.5 the EPR spectra show the formation of a Cr(V) complex where the vic-diol groups of Galur participate in the bonding to Cr(V). At pH 3-5 the Galur-Cr(V) species grow and decay over short periods in a similar way to that observed for [Cr(O)(alpha-hydroxy acid)(2)](-). The lack of chelation at any vic-diolate group of Galur when pH < or = 5 differentiates its ability to stabilise Cr(V) from that of neutral saccharides that form very stable oxo-Cr(V)(diolato)(2) species at pH > 1.

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.

Detection and structural characterization of oxo-chromium(V)-sugar complexes by electron paramagnetic resonance.

This article describes the detection and characterization of oxo-Cr(V)-saccharide coordination compounds, produced during chromic oxidation of carbohydrates by Cr(VI) and Cr(V), using electron paramagnetic resonance (EPR) spectroscopy. After an introduction into the main importance of chromium (bio)chemistry, and more specifically the oxo-chromium(V)-sugar complexes, a general overview is given of the current state-of-the-art EPR techniques. The next step reviews which types of EPR spectroscopy are currently applied to oxo-Cr(V) complexes, and what information about these systems can be gained from such experiments. The advantages and pitfalls of the different approaches are discussed, and it is shown that the potential of high-field and pulsed EPR techniques is as yet still largely unexploited in the field of oxo-Cr(V) complexes. Subsequently, the discussion focuses on the analysis of oxo-Cr(V) complexes of different types of sugars and the implications of the results in terms of understanding chromium (bio)chemistry.

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.

Application of chitosan in removal of molybdate ions from contaminated water and groundwater

Water pollution by heavy metals represents a serious problem around the world. Among various treatment techniques for water remediation, adsorption is an effective and versatile method due to the low cost, effectiveness and simplicity. Chitosan is a cationic polysaccharide with an excellent adsorption capacity of heavy metal ions. Chitosan has a high molybdate adsorption capacity (265±1mgg-1) at 20°C and pH 2.7. Participation of hydroxyl groups in the adsorption of molybdate anions was confirmed by FT-IR analysis. SEM images showed that morphological surface changes happen after MoVI adsorption. Continuous adsorption data were best fitted by Modified Dose- Response model. Scale-up of continuous processes was achieved applying bed depth service time (BDST) model. Application of chitosan in molybdate removal from real groundwater samples suggest that this polysaccharide is a good option to be used for household purposes.

The EPR pattern of [CrO(cis-1,2-cyclopentanediolato)2]- and [CrO(trans-1,2-cyclopentanediolato)2]-

The addition of a large excess of 1,2-cyclopentanediol to a 1:1 mixture of glutathione and CrVI at pH 7.5 stabilises the intermediate CrV species formed by the one-electron reduction of CrVI by glutathione. The isotropic EPR parameters (giso and Aiso) of the CrV species formed with both cis- and trans-1,2-cyclopentanediol correspond to those calculated for five-coordinate oxo-CrV complexes with four alcoholato donors [Cr(O)(1,2-cyclopentanediolato)2]−. The five-coordinate oxo-CrV species formed with both 1,2-cyclopentanediol isomers show very similar EPR superhyperfine patterns, but differ in their stability and the conditions required for their formation due to the different chelation ability of the cis- vs. trans-1,2-diolato moiety.

Redox and ligand-exchange chemistry of chromium(VI/V)-methyl glycoside systems

In order to establish a general pattern for the redox and coordination chemistry of glycosides with Cr(VI) and Cr(V), reactions of a series of methyl glycosides with Cr(VI) and Cr(V) have been studied at different acidities. Oxidations of methyl α- and β-D-glucopyranoside (Glc1Me), methyl α- and β-D-mannopyranoside (Man1Me), methyl α- and β-D-galactopyranoside (Gal1Me) and methyl α- and β-D-ribofuranoside (Rib1Me) by Cr(VI) proceed rapidly at pH ≤ 1, and yield Cr(III) and methyl glycofuranuronolactone as final products when an excess of methyl glycoside over Cr(VI) is used. At constant [H+], the reaction follows the rate law −d[Cr(VI)]/dt = kH [Gly1Me] [Cr(VI)]. Relative reactivities of methyl glycosides toward Cr(VI) reduction are: β-Rib1Me > α-Gal1Me > α-Rib1Me ≈ β-Gal1Me > β-Man1Me > α-Man1Me > α-Glc1Me > β-Glc1Me. This sequence is interpreted in terms of the degree of unfavorable steric interactions induced by the nonbonded 1,3-diaxial interactions in the respective Gly1Me-Cr(VI) monochelate, which is formed in rapid equilibrium that precedes the rate determining step. For all the glycosides, the oxidation rate decreases with an increase in pH value and becomes negligible at pH > 5. At pH 5.5 and 7.5, addition of an excess of α-Man1Me or α(β)-Gal1Me to an equimolar cysteine-Cr(VI) mixture, afforded two EPR triplets at giso1 1.9802 and giso2 1.9800/1 with Aiso 16.5(3) × 10−4 cm−1 in a 50 ∶ 50 giso1 ∶ giso2 ratio. The EPR spectral parameters and the superhyperfine pattern of the signal are consistent with the presence of two geometric isomers of the [CrVO(cis-O3,O4-Gal1Me)2]− and [CrVO(cis-O2,O3-Man1Me)2]− complexes. The same final spectral pattern is observed at pH 7.5 for the ligand-exchange reaction of Man1Me and Gal1Me with [CrVO(ehba)2]− (ehba = 2-ethyl-2-hydroxybutanoato(2−)). No EPR signal is observed when an excess of Xil1Me or Glc1Me is added to an equimolar cysteine-Cr(VI) mixture. In the ligand-exchange reactions of [CrVO(ehba)2]− at pH 7.5 with Xil1Me or Glc1Me, a very low intensity EPR singlet is observed at giso 1.9799. These results show that only glycosides with one cis-diolato group (such as Man1Me and Gal1Me) are effective for stabilizing Cr(V) at pH 5.5 and 7.5. The high redox reactivity of methyl glycosides with Cr(V) at high [H+] is attributed to the formation of [CrVO(O,O-glycoside)(OH2)3]+ species (giso 1.9716), which are not observed at pH 5.5–7.5, where only the five-coordinate bis-chelate oxochromate(V) species are observed.

Diabetes primary prevention program: New insights from data analysis of recruitment period

Primary Prevention of Diabetes Program in Buenos Aires Province evaluates the effectiveness of adopting healthy lifestyle to prevent type 2 diabetes (T2D) in people at high risk of developing it. We aimed to present preliminary data analysis of FINDRISC and laboratory measurements taken during recruitment of people for the Primary Prevention of Diabetes Program in Buenos Aires Province in the cities of La Plata, Berisso, and Ensenada, Argentina.

Relación entre las tasas de desempleo y los indicadores de absentismo por incapacidad temporal durante periodos de crisis económica

espanolIntroduccion: Estudios realizados en varios paises presentan una relacion inversa entre las tasas de desempleo (TD) y los indicadores de absentismo por incapacidad temporal (IT) durante periodos de crisis economica. Objetivo: Evaluar la relacion entre indicadores de absentismo y el desempleo en Espana (2009-2015). Metodos: Se obtuvieron tasas de incidencia (TI) y absentismo (TA) por IT de origen comun (ITcc) a partir de datos de la Seguridad Social, asi como las TD segun datos del Instituto Nacional de Estadistica. Se describio graficamente la evolucion temporal de los indicadores de ITcc y el desempleo en el periodo 2009-2015. Para explorar dicha relacion tambien se realizaron graficos de dispersion de los indicadores frente a las TD. Posteriormente, se ajustaron modelos de regresion lineal simple. Resultados: Las TI y TA descienden en 2009-2013 (TI 2009=28,07%, TI 2013=20,41%, TA 2009=2,53%, TA 2013=1,86%) momento en que cambia su tendencia, iniciando una fase ascendente hasta 2015 (TI 2015=22,52%, TA 2015=2,12%). La TD asciende en 2009-2013 (TD 2009=17,86%, TD 2013=26,10%), pasando a descender hasta 2015 (TD 2015=22,06%). Las TD y los indicadores de absentismo presentan una relacion lineal inversa. Conclusiones: Se observo una relacion inversa entre el desempleo y los indicadores de absentismo en Espana. Son necesarios estudios especificos para poder evaluar hipotesis explicativas como las asociadas a los efectos de disciplina, seleccion y/o cambios en la fuerza de trabajo. EnglishIntroduction: Different countries have shown an inverse relationship between unemployment rates (UR) and indicators of sickness absence during periods of economic recession. Objective: To evaluate the relationship between indicators of sickness absence and unemployment in Spain during 2009-2015. Methods: We obtained incidence and absenteeism rates of non-work related sickness absence (NWSA) from Social Security data, and UR from the National Institute of Statistics. The relationship between indicators of NWSA and UR in the period 2009 to 2015 was graphically described using time trend plots. Scatter plots of NWSA indicators were also made against UR. Finally, we fitted linear regression models. Results: Incidence (IR) and absenteeism rates (AR) of NWSA showed downward trends in 2009-2013 (IR 2009=28.07%, IR 2013=20.41%; AR 2009=2.53% and AR 2013=1.86%), changing to an upward trend up to 2015 (IR 2015=22.52%, AR 2015=2.12%). Unemployment rates trended upward in 2009-2013 (UR 2009=17.86%, UR 2013=26.10%), and then changed to a downward trend through 2015 (UR 2015=22.06%). There was an inverse relationship between UR and NWSA indicators. Conclusions: As in previous international studies, we found an inverse relationship between unemployment rates and indicators of sickness absence in Spain. More detailed studies are needed to evaluate explanatory hypotheses, such as those associated with the effects of discipline, selection and/or changes in the workforce.