Biomedical subjects

Josep Galceran

Publications and source records attributed to Josep Galceran.

8 recordsLinked to original sources

PubMed2006

Two grid refinement methods in the lattice Boltzmann framework for reaction-diffusion processes in complex systems.

This paper studies the optimisation of a numerical model and a computer code to solve numerically reaction-diffusion processes in environmental or biological systems with complicated geometries and mixtures of reactions including time and spatial scales extending over several order of magnitude. In particular, we consider different grid refinement techniques in the framework of a lattice Boltzmann solver for reaction-diffusion systems. Two new grid refinement methods are proposed, which are both quantitatively good. The first method is based on the matching of the concentration profiles and fluxes across two adjacent sub-domains, while the second method is based on nested subgrids. The focus of our study is the trade off between accuracy and CPU time. We show how the different parameters of the method, such as the refinement factors, the location of the boundary between different grids or coupling methods at the interface affect the quality of the numerical solution and the efficiency of the method.

Journal Article↗

PubMed2006

Lability of a mixture of metal complexes under steady-state planar diffusion in a finite domain.

The rigorous analytical solution for the fluxes from a mixture of 1:1 metal complexes toward an active surface under steady-state planar diffusion in a finite domain and excess ligand conditions allows for the computation of the global degree of lability of the system as well as particular degrees of lability of each complex in the mixture. This kind of system is found in a variety of fields ranging from electrochemical techniques (such as stripping chronopotentiometry at scanned deposition potential, SSCP) to analytical devices (such as diffusion gradients in thin-film gels, DGT). Among the specific effects arising from the presence of a mixture of ligands competing for the metal we highlight the following: (i) The degree of lability of a complex in the mixture differs from its degree of lability in an unmixed system with the same ligand concentration, and (ii) the degree of lability of one complex depends on (i.e., can be modified with) the concentrations of the ligands in the mixture. The impact of these characteristics on the metal flux crossing the active surface reaches the highest value when both complexes are partially labile. The complex contribution to the metal flux goes through a maximum when the thickness of the diffusion domain is varied. Thus, the thickness of the diffusion domain can be chosen to enhance the contribution of one particular complex. Lability criteria for each complex of the mixture within the reaction layer approximation are also reported. In particular, the reaction layer formulation for a complex is discussed in detail for two limiting cases: the rest of complexes are all nonlabile or the rest of complexes are all labile.

Journal Article↗

PubMed2006

Comparison of analytical techniques for dynamic trace metal speciation in natural freshwaters.

Several techniques for speciation analysis of Cu, Zn, Cd, Pb, and Ni are used in freshwater systems and compared with respect to their performance and to the metal species detected. The analytical techniques comprise the following: (i) diffusion gradients in thin-film gels (DGT); (ii) gel integrated microelectrodes combined to voltammetric in situ profiling system (GIME-VIP); (iii) stripping chronopotentiometry (SCP); (iv) flow-through and hollow fiber permeation liquid membranes (FTPLM and HFPLM); (v) Donnan membrane technique (DMT); (vi) competitive ligand-exchange/stripping voltammetry (CLE-SV). All methods could be used both under hardwater and under softwater conditions, although in some cases problems with detection limits were encountered at the low total concentrations. The detected Cu, Cd, and Pb concentrations decreased in the order DGT > or = GIME-VIP > or = FTPLM > or = HFPLM approximately = DMT (>CLE-SV for Cd), detected Zn decreased as DGT > or = GIME-VIP and Ni as DGT > DMT, in agreement with the known dynamic features of these techniques. Techniques involving in situ measurements (GIME-VIP) or in situ exposure (DGT, DMT, and HFPLM) appear to be appropriate in avoiding artifacts which may occur during sampling and sample handling.

Artifacts↗

PubMed2006

Model predictions of metal speciation in freshwaters compared to measurements by in situ techniques.

Measurements of trace metal species in situ in a softwater river, a hardwater lake, and a hardwater stream were compared to the equilibrium distribution of species calculated using two models, WHAM 6, incorporating humic ion binding model VI and visual MINTEQ incorporating NICA-Donnan. Diffusive gradients in thin films (DGT) and voltammetry at a gel integrated microelectrode (GIME) were used to estimate dynamic species that are both labile and mobile. The Donnan membrane technique (DMT) and hollow fiber permeation liquid membrane (HFPLM) were used to measure free ion activities. Predictions of dominant metal species using the two models agreed reasonably well, even when colloidal oxide components were considered. Concentrations derived using GIME were generally lower than those from DGT, consistent with calculations of the lability criteria that take into account the smaller time window available forthe fluxto GIME. Model predictions of free ion activities generally did not agree with measurements, highlighting the need for further work and difficulties in obtaining appropriate input data.

Benzopyrans↗

PubMed2006

Lability criteria for successive metal complexes in steady-state planar diffusion.

The lability of sequential metal complexes, ML, ML2, ML3, ... , up to a general 1:n metal/ligand stoichiometric ratio is considered for the case of metal ions (M) being accumulated at a surface (analytical sensor or organism). The analytical solution for the steady-state diffusion of M within a sequential complexation scheme allows quantification of the contribution from the dissociation of all of the complex species to the metal flux through the so-called lability degree, xi. A lability degree for each sequential complexation step is also defined which, due to the sequential character of the complexation scheme, depends not only on the proper kinetic constants of the given complexation step but also on the kinetics of the previous ones. When all contributions from the complexes are diffusion limited, the system is fully labile and xi=1. To provide simple lability criteria, the reaction layer approximation is extended to specifically deal with this sequential complexation scheme, so that a reaction layer thickness is defined when the existence of one particular rate-limiting step is assumed. Expressions for the classical lability parameter, L, are formulated using the reaction layer approximation. The change of the lability of the system as the diffusion layer thickness is modified is analyzed in detail. The contribution of the complex flux reflects the evolution of the system from labile to inert as the thickness of the sensor is appropriately decreased.

Journal Article↗

PubMed2005

LBGK method coupled to time splitting technique for solving reaction-diffusion processes in complex systems.

A new approach to numerically solve a reaction-diffusion system is given, specifically developed for complex systems including many reacting/diffusing species with broad ranges of rate constants and diffusion coefficients, as well as complicated geometry of reacting interfaces. The approach combines a Lattice Boltzmann (LB) method with a splitting time technique. In the present work, the proposed approach is tested by focusing on the typical reaction process between a metal ion M and a ligand L, to form a complex ML with M being consumed at an electrode. The aim of the paper is to systematically study the convergence conditions of the associated numerical scheme. We find that the combination of LB with the time splitting method allows us to solve the problem for any value of association and dissociation rate constant of the reaction process. Also, the method can be extended to a mixture of ligands. We stress two main points: (1) the LB approach is particularly convenient for the flux computation of M and (2) the splitting time procedure is very well suited for reaction processes involving association-dissociation rate constants varying on many orders of magnitude.

Chemical Industry↗

PubMed2004

Metal speciation dynamics and bioavailability: bulk depletion effects.

Under conditions of bulk depletion, the speciation and bioavailability of trace metals must be considered at two different time scales: (i) the time scale of the biouptake flux, as determined by diffusion of the bioactive free metal, dissociation of the bioinactive complex species, and the internalization rate; and (ii) the time scale of depletion of the bulk medium. The implications of these two time scales for the speciation dynamics are discussed in terms of experimental conditions. The geometry of the system is taken into accountvia a spherical cellular model. It considers a spherical organism depleting a spherical volume in a nonstirred medium and assumes linear adsorption of the metal atthe biointerface and first-order internalization kinetics. In cases where the rate of biouptake is fully controlled by the internalization step, concentration gradients in the medium are insignificant. Then the biouptake becomes independent of the geometry of the system, and the model has a much simpler solution. Examples of trace metal uptake by microorganisms are analyzed: (i) cobalt uptake by Prochlorococcus in the presence of NTA, under conditions where bulk depletion is the controlling process due to the large number of organisms and high internalization rates, (ii) silver uptake by Chlamydomonas reinhardtii with significant effects of bulk depletion, due to the high internalization rate; (iii) lead uptake by Chlorella vulgaris with pratically negligible bulk depletion due to the low internalization rate of the metal; and (iv) lead uptake by intestinal Caco-2 cells, illustrating the simplification of the bulk depletion model for a system with different geometry where internalization is the rate-controlling step.

Animals↗

PubMed2003

Theoretical and analytical characterization of a flow-through permeation liquid membrane with controlled flux for metal speciation measurements.

Speciation measurements with the permeation liquid membrane (PLM) technology require necessarily a good control of the flux of the analyte. In this perspective, a PLM-based multichannel flow-through cell has been designed. The first objective of this study has been to adapt the classical Levich model commonly used for electrochemical flow devices to the characteristic geometry of the PLM cell. In the latter case and contrary to the Levich model, the effects of the channel lateral walls on the flux of active species through the membrane have to be taken into account. The problem was solved by considering the existence of two parabolic Poiseuille profiles perpendicular to each other and developing along the fluid motion. The theoretical results obtained match satisfactorily with experimental data. The analytical study of this PLM system has been performed with copper(II) ions as test species and has shown that the preconcentration factor is (1) linear at least for preconcentration times of < or = 120 min, (2) reproducible on the same membrane as well as on different membranes, and (3) independent of the initial test metal ion concentration in the sample solution. The capabilities of this cell to determine metal speciation by considering lability of complexes and the flux of metal at variable flow rates of the test solution is also discussed by means of Cu(II)/sulfosalicylic complexes.

Journal Article↗