402 research outputs found
Attractive forces in microporous carbon electrodes for capacitive deionization
The recently developed modified Donnan (mD) model provides a simple and
useful description of the electrical double layer in microporous carbon
electrodes, suitable for incorporation in porous electrode theory. By
postulating an attractive excess chemical potential for each ion in the
micropores that is inversely proportional to the total ion concentration, we
show that experimental data for capacitive deionization (CDI) can be accurately
predicted over a wide range of applied voltages and salt concentrations. Since
the ion spacing and Bjerrum length are each comparable to the micropore size
(few nm), we postulate that the attraction results from fluctuating bare
Coulomb interactions between individual ions and the metallic pore surfaces
(image forces) that are not captured by meanfield theories, such as the
Poisson-Boltzmann-Stern model or its mathematical limit for overlapping double
layers, the Donnan model. Using reasonable estimates of the micropore
permittivity and mean size (and no other fitting parameters), we propose a
simple theory that predicts the attractive chemical potential inferred from
experiments. As additional evidence for attractive forces, we present data for
salt adsorption in uncharged microporous carbons, also predicted by the theory.Comment: 19 page
Theory of membrane capacitive deionization including the effect of the electrode pore space
Membrane capacitive deionization (MCDI) is a technology for water desalination based on applying an electrical field between two oppositely placed porous electrodes. Ions are removed from the water flowing through a channel in between the electrodes and are stored inside the electrodes. Ion-exchange membranes are placed in front of the electrodes allowing for counterion transfer from the channel into the electrode, while retaining the coions inside the electrode structure. We set up an extended theory for MCDI which includes in the description for the porous electrodes not only the electrostatic double layers (EDLs) formed inside the porous (carbon) particles, but also incorporates the role of the transport pathways in the electrode, i.e., the interparticle pore space. Because in MCDI the coions are inhibited from leaving the electrode region, the interparticle porosity becomes available as a reservoir to store salt, thereby increasing the total salt storage capacity of the porous electrode. A second advantage of MCDI is that during ion desorption (ion release) the voltage can be reversed. In that case the interparticle porosity can be depleted of counterions, thereby increasing the salt uptake capacity and rate in the next cycle. In this work, we compare both experimentally and theoretically adsorption/desorption cycles of MCDI for desorption at zero voltage as well as for reversed voltage, and compare with results for CDI. To describe the EDL-structure a novel modified Donnan model is proposed valid for small pores relative to the Debye length
Entropy production of soil hydrological processes and its maximisation
Hydrological processes are irreversible and produce entropy. Hence, the framework of non-equilibrium thermodynamics is used here to describe them mathematically. This means flows of water are written as functions of gradients in the gravitational and chemical potential of water between two parts of the hydrological system. Such a framework facilitates a consistent thermodynamic representation of the hydrological processes in the model. Furthermore, it allows for the calculation of the entropy production associated with a flow of water, which is proportional to the product of gradient and flow. Thus, an entropy budget of the hydrological cycle at the land surface is quantified, illustrating the contribution of different processes to the overall entropy production. Moreover, the proposed Principle of Maximum Entropy Production (MEP) can be applied to the model. This means, unknown parameters can be determined by setting them to values which lead to a maximisation of the entropy production in the model. The model used in this study is parametrised according to MEP and evaluated by means of several observational datasets describing terrestrial fluxes of water and carbon. The model reproduces the data with good accuracy which is a promising result with regard to the application of MEP to hydrological processes at the land surfac
Evaluating Interaction of Cord Blood Hematopoietic Stem/Progenitor Cells with Functionally Integrated Three-Dimensional Microenvironments
Despite advances in ex vivo expansion of cord blood-derived hematopoietic stem/progenitor cells (CB-HSPC), challenges still remain regarding the ability to obtain, from a single unit, sufficient numbers of cells to treat an adolescent or adult patient. We and others have shown that CB-HSPC can be expanded ex vivo in two-dimensional (2D) cultures, but the absolute percentage of the more primitive stem cells decreases with time. During development, the fetal liver is the main site of HSPC expansion. Therefore, here we investigated, in vitro, the outcome of interactions of primitive HSPC with surrogate fetal liver environments. We compared bioengineered liver constructs made from a natural three-dimensional-liver-extracellular-matrix (3D-ECM) seeded with hepatoblasts, fetal liver-derived (LvSt), or bone marrow-derived stromal cells, to their respective 2D culture counterparts. We showed that the inclusion of cellular components within the 3D-ECM scaffolds was necessary for maintenance of HSPC viability in culture, and that irrespective of the microenvironment used, the 3D-ECM structures led to the maintenance of a more primitive subpopulation of HSPC, as determined by flow cytometry and colony forming assays. In addition, we showed that the timing and extent of expansion depends upon the biological component used, with LvSt providing the optimal balance between preservation of primitive CB HSPC and cellular differentiation. Stem Cells Translational Medicine 2018;7:271–282
On the potential vegetation feedbacks that enhance phosphorus availability – insights from a process-based model linking geological and ecological timescales
In old and heavily weathered soils, the availability of P might be
so small that the primary production of plants is limited. However,
plants have evolved several mechanisms to actively take up P from
the soil or mine it to overcome this limitation. These mechanisms
involve the active uptake of P mediated by mycorrhiza, biotic
de-occlusion through root clusters, and the biotic enhancement of
weathering through root exudation. The objective of this paper is to
investigate how and where these processes contribute to alleviate P
limitation on primary productivity. To do so, we propose
a process-based model accounting for the major processes of the
carbon, water, and P cycles including chemical weathering at the
global scale. Implementing P limitation on biomass synthesis allows
the assessment of the efficiencies of biomass production across
different ecosystems. We use simulation experiments to assess the relative
importance of the different uptake mechanisms to alleviate P
limitation on biomass production.
We find that active P uptake is an essential mechanism for
sustaining P availability on long timescales, whereas biotic
de-occlusion might serve as a buffer on timescales shorter than
10 000 yr. Although active P uptake is essential for
reducing P losses by leaching, humid lowland soils reach P
limitation after around 100 000 yr of soil evolution. Given
the generalized modelling framework, our model results compare
reasonably with observed or independently estimated patterns and
ranges of P concentrations in soils and vegetation. Furthermore, our
simulations suggest that P limitation might be an important driver
of biomass production efficiency (the fraction of the gross primary
productivity used for biomass growth), and that vegetation on old
soils has a smaller biomass production rate when P becomes limiting.
With this study, we provide a theoretical basis for investigating
the responses of terrestrial ecosystems to P availability linking
geological and ecological timescales under different environmental
settings
Review on the science and technology of water desalination by capacitive deionization
Porous carbon electrodes have significant potential for energy-efficient water desalination using a promising technology called Capacitive Deionization (CDI). In CDI, salt ions are removed from brackish water upon applying an electrical voltage difference between two porous electrodes, in which the ions will be temporarily immobilized. These electrodes are made of porous carbons optimized for salt storage capacity and ion and electron transport. We review the science and technology of CDI and describe the range of possible electrode materials and the various approaches to the testing of materials and devices. We summarize the range of options for CDI-designs and possible operational modes, and describe the various theoretical–conceptual approaches to understand the phenomenon of CDI
Effect of electrode thickness variation on operation of capacitive deionization
In capacitive deionization (CDI) water is desalinated by applying an electrical field between two porous electrodes placed on either side of a spacer channel that transports the aqueous solution. In this work we investigate the equilibrium salt adsorption and the dynamic development of the effluent salt concentration in time, both as function of spacer and electrode thicknesses. The electrode thickness will be varied in a symmetric manner (doubling both electrodes) and in an asymmetric manner, by doubling and tripling one electrode but not the other. To describe the structure of the electrostatic double layer (EDL) which determines the salt adsorption in the micropores of activated carbons, a modified Donnan-model is set up which successfully describes the data, also for situations of very significant electrode thickness ratios. We develop a generalized CDI transport model accounting for thickness variations, which compares favorably with experimental data for the change of the effluent salt concentration in time. These experiments are aimed at further testing our equilibrium and transport models, specifically the assumption therein that in first approximation, for electrodes made of chemically unmodified activated carbon particles, the EDL structure is independent of the sign of the electronic charge. To investigate the relevance of chemical surface charge we also varied pH of the salt solution flowing into the cell
- …