Entity

Time filter

Source Type


Villalobos M.,National Autonomous University of Mexico | Escobar-Quiroz I.N.,National Autonomous University of Mexico | Salazar-Camacho C.,National Autonomous University of Mexico | Salazar-Camacho C.,Technological University of Choco
Geochimica et Cosmochimica Acta | Year: 2014

Sorption and oxidation reactions in the environment may affect substantially the mobility of redox-sensitive toxic trace elements and compounds. Investigating the environmental factors that influence these reactions is crucial in understanding and predicting the geochemical fate of these environmental species, as well as to design appropriate engineered remediation schemes. Arsenic is a widespread contaminant of concern, especially in its oxidized forms, and Mn oxide minerals are some of the major contributors to its oxidation. The goal of this work was to investigate the influence of particle size and structural differences of environmentally-relevant Mn(IV) birnessites on the adsorption of As(V) and on the oxidation of As(III). An acid birnessite of 39m2/g and a δ-MnO2 of 114m2/g were used. Both birnessites sorbed a maximum Pb(II) of 0.3 Pb/Mn, indicating a significantly larger layer cationic vacancy content for acid birnessite, and a density of reactive edge sites for both of 12sites/nm2. As(V) forms a bidentate bridging complex on singly-coordinated surface sites at the birnessite particle edges regardless of loading, pH, birnessite type, and presence of pre-sorbed metals(II). Maximum As(V) adsorption, under repulsive electrostatic pH conditions did not yield adsorption congruency behavior between both birnessites at constant pH, presumably because the increase in internal vacancy content causes negative electrostatic repulsion towards external As(V) oxyanion binding.At pH 4.5 As(III) oxidation on birnessites was fast and quantitative at As/Mn ratios of 0.3-0.33, the reaction being largely driven by the proton concentration. At pH 6 δ-MnO2 oxidized As(III) faster and to a higher extent than acid birnessite, at equal masses; but the reverse at equal total surface areas. The oxidation driving force (independently from protons) was higher at pH 6 than at pH 4.5 because of Mn(II) product removal by sorption to interlayer vacancies, which overcomes reactive surface site blockage by this species, provided sufficient vacancies are present. Metals(II) pre-sorbed on birnessites always decreased the initial stages of As(III) oxidation rates as compared to the metal(II)-free systems presumably through site blockage. But after 24h the Pb(II)-equilibrated birnessites at pH 6 reached equal and sometimes higher oxidation extents through removal of As(V) from solution, by stabilizing its (binary) adsorption to edge sites through a decrease in electrostatic repulsion by the sorbed Pb(II).This work provides useful insights on the influence of particle size and structure (vacancy content) of birnessite minerals analogous to biogenic Mn oxides relevant to the environment, especially as it pertains to reactivity towards sorption of Pb(II), Zn(II), and As(V), and to oxidation of As(III), all of which are significant processes that dictate their transport and fate in aqueous geochemical environments. © 2013 Elsevier Ltd. Source


Bechara M.,Technological University of Choco | Velez I.,University of Antioquia
Revista Mexicana de Biodiversidad | Year: 2010

Eight of 40 Rhinella marina Linnaeus, 1758 (24 males and 16 females) from Aburra Valley, Antioquia, Colombia, were infected with 2 digenea species: Pseudosonsinotrema chabaudi (Caballero y Caballero, 1969) Sullivan, 1974 (Pleurogenidae), which represents new host and country records, and Mesocoelium monas (Rudolphi, 1819) Freitas, 1958 (Brachycoeliidae), which was previously reported for the country. Pseudosonsinotrema chabaudi in San Antonio de Prado reached the highest prevalence levels (40%) and mean abundance (14.7), whereas the highest mean intensity was recorded for M. monas in cane toads from Barbosa (46.7). Source


De Cordoba P.F.,Polytechnic University of Valencia | Isidro J.M.,Polytechnic University of Valencia | Perea M.H.,Technological University of Choco
International Journal of Geometric Methods in Modern Physics | Year: 2014

It has been argued that gravity acts dissipatively on quantum-mechanical systems, inducing thermal fluctuations that become indistinguishable from quantum fluctuations. This has led some authors to demand that some form of time irreversibility be incorporated into the formalism of quantum mechanics. As a tool toward this goal, we propose a thermodynamical approach to quantum mechanics, based on Onsager's classical theory of irreversible processes and Prigogine's nonunitary transformation theory. An entropy operator replaces the Hamiltonian as the generator of evolution. The canonically conjugate variable corresponding to the entropy is a dimensionless evolution parameter. Contrary to the Hamiltonian, the entropy operator is not a conserved Noether charge. Our construction succeeds in implementing gravitationally-induced irreversibility in the quantum theory. © 2014 World Scientific Publishing Company. Source


Mosquera L.H.,Polytechnic University of Valencia | Mosquera L.H.,Technological University of Choco | Moraga G.,Polytechnic University of Valencia | Martinez-Navarrete N.,Polytechnic University of Valencia
Food Research International | Year: 2012

The adsorption isotherms (20 °C) and the relationship between water content and glass transition temperature were modeled in freeze-dried strawberry powder studying the effect of the addition of maltodextrin and arabic gum. Both compositional and physicochemical analyses of strawberry pulp were performed. If the midpoint of the glass transition is considered, the critical water activity that ensures the glassy state of the product during storage at 20 °C increased from 0.094 to 0.237-0.341 when maltodextrin and arabic gum were added, respectively. The increase in the critical water content was not so marked and it was noticeable only in arabic gum added sample (from 7.5 to 8.9. g water/100. g product), being this sample more stable. © 2011 Elsevier Ltd. Source


Igual M.,Polytechnic University of Valencia | Ramires S.,Catholic University of Portugal | Mosquera L.H.,Technological University of Choco | Martinez-Navarrete N.,Polytechnic University of Valencia
Powder Technology | Year: 2014

The spray drying of lulo was optimized by using the central composite design of the response surface methodology, to study the effect of inlet air temperature (120-180. °C), arabic gum concentration (0-10% w/w), and maltodextrin DE16.5-19.5 concentration (0-10% w/w) on some product and process aspects. Arabic gum and maltodextrin, more than inlet air temperature, improved the product yield, reduced the hygroscopicity and the water content of the obtained powder, and contributed to the retention of its nutritive and functional properties through an increase in ascorbic acid, vitamin C, total phenol and total flavonoid content and antioxidant capacity. Significant (p. <. 0.05) response surface models were obtained in every case, with the linear terms of solute concentration being the factors that affected the response variables most significantly. The overall optimum spray drying conditions for obtaining lulo powder were 125. °C inlet air temperature, 3% (w/w) arabic gum, and 13.4% (w/w) maltodextrin DE16.5-19.5. © 2014 Elsevier B.V. Source

Discover hidden collaborations