293 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
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
Performance Metrics for the Objective Assessment of Capacitive Deionization Systems
In the growing field of capacitive deionization (CDI), a number of
performance metrics have emerged to describe the desalination process.
Unfortunately, the separation conditions under which these metrics are measured
are often not specified, resulting in optimal performance at minimal removal.
Here we outline a system of performance metrics and reporting conditions that
resolves this issue. Our proposed system is based on volumetric energy
consumption (Wh/m) and throughput productivity (L/h/m) reported for a
specific average concentration reduction, water recovery, and feed salinity. To
facilitate and rationalize comparisons between devices, materials, and
operation modes, we propose a nominal standard testing condition of removing 5
mM from a 20 mM NaCl feed solution at 50% water recovery for CDI research.
Using this separation, we compare the desalination performance of a
flow-through electrode (fte-CDI) cell and a flow between membrane (fb-MCDI)
device, showing how significantly different systems can be compared in terms of
generally desirable desalination characteristics. In general, we find that
performance analysis must be considered carefully so to not allow for ambiguous
separation conditions or the maximization of one metric at the expense of
another. Additionally, for context we discuss a number of important underlying
performance indicators and cell characteristics that are not performance
measures in and of themselves but can be examined to better understand
differences in performance
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
Global NO and HONO emissions of biological soil crusts estimated by a process-based non-vascular vegetation model
The reactive trace gases nitric oxide (NO) and nitrous acid (HONO) are
crucial for chemical processes in the atmosphere, including the formation of
ozone and OH radicals, oxidation of pollutants, and atmospheric
self-cleaning. Recently, empirical studies have shown that biological soil
crusts are able to emit large amounts of NO and HONO, and they may therefore
play an important role in the global budget of these trace gases. However,
the upscaling of local estimates to the global scale is subject to large
uncertainties, due to unknown spatial distribution of crust types and their
dynamic metabolic activity. Here, we perform an alternative estimate of
global NO and HONO emissions by biological soil crusts, using a process-based
modelling approach to these organisms, combined with global data
sets of climate and land cover. We thereby consider that NO and HONO are
emitted in strongly different proportions, depending on the type of crust and
their dynamic activity, and we provide a first estimate of the global
distribution of four different crust types. Based on this, we estimate global
total values of 1.04 Tg yr−1 NO–N and 0.69 Tg yr−1 HONO–N
released by biological soil crusts. This corresponds to around 20 % of
global emissions of these trace gases from natural ecosystems. Due to the low
number of observations on NO and HONO emissions suitable to validate the
model, our estimates are still relatively uncertain. However, they are
consistent with the amount estimated by the empirical approach, which
confirms that biological soil crusts are likely to have a strong impact on
global atmospheric chemistry via emissions of NO and HONO.</p
High potential for weathering and climate effects of non-vascular vegetation in the Late Ordovician
It has been hypothesized that predecessors of today’s bryophytes significantly increased global chemical weathering in the Late Ordovician, thus reducing atmospheric CO2 concentration and contributing to climate cooling and an interval of glaciations. Studies that try to quantify the enhancement of weathering by non-vascular vegetation, however, are usually limited to small areas and low numbers of species, which hampers extrapolating to the global scale and to past climatic conditions. Here we present a spatially explicit modelling approach to simulate global weathering by non-vascular vegetation in the Late Ordovician. We estimate a potential global weathering flux of 2.8 (km3 rock) yr−1, defined here as volume of primary minerals affected by chemical transformation. This is around three times larger than today’s global chemical weathering flux. Moreover, we find that simulated weathering is highly sensitive to atmospheric CO2 concentration. This implies a strong negative feedback between weathering by non-vascular vegetation and Ordovician climate
Exceptional Water Desalination Performance with Anion-Selective Electrodes
Capacitive deionization (CDI) typically uses one porous carbon electrode that is cation adsorbing and one that is anion adsorbing. In 2016, Smith and Dmello proposed an innovative CDI cell design based on two cation-selective electrodes and a single anion-selective membrane, and thereafter this design was experimentally validated by various authors. In this design, anions pass through the membrane once, and desalinated water is continuously produced. In the present work, this idea is extended, and it is experimentally shown that also a choice for anion-selective electrodes, in combination with a cation-selective membrane, leads to a functional cell design that continuously desalinates water. Anion-selective electrodes are obtained by chemical modification of the carbon electrode with (3-aminopropyl)triethoxysilane. After chemical modification, the activated carbon electrode shows a substantial reduction of the total pore volume and Brunauer–Emmett–Teller (BET) surface area, but nevertheless maintains excellent CDI performance, which is for the first time that a low-porosity carbon electrode is demonstrated as a promising material for CDI.</p
- …