"Breakthrough" osmosis and unusually high power densities in Pressure-Retarded Osmosis in non-ideally semi-permeable supported membranes

Osmosis is the movement of solvent across a membrane induced by a solute-concentration gradient. It is very important for cell biology. Recently, it has started finding technological applications in the emerging processes of Forward Osmosis and Pressure-Retarded Osmosis. They use ultrathin and dense membranes supported mechanically by much thicker porous layers. Until now, these processes have been modelled by assuming the membrane to be ideally-semipermeable. We show theoretically that allowing for even minor deviations from ideal semipermeability to solvent can give rise to a previously overlooked mode of “breakthrough” osmosis. Here the rate of osmosis is very large (compared to the conventional mode) and practically unaffected by the so-called Internal Concentration Polarization. In Pressure-Retarded Osmosis, the power densities can easily exceed the conventional mode by one order of magnitude. Much more robust support layers can be used, which is an important technical advantage (reduced membrane damage) in Pressure-Retarded Osmosis.Peer ReviewedPostprint (published version

Evaporation-driven electrokinetic energy conversion: critical review, parametric analysis and perspectives

Energy harvesting from evaporation has become a hot topic in the last couple of years. Researchers have speculated on several possible mechanisms. Electrokinetic energy conversion is the least hypothetical one. The basics of pressure-driven electrokinetic phenomena of streaming current and streaming potential have long been established. The regularities of evaporation from porous media are also well known. However, coupling of these two classes of phenomena has not, yet, been seriously explored. In this critical review, we will recapitalize and combine the available knowledge from these two fields to produce a coherent picture of electrokinetic electricity generation during evaporation from (nano)porous materials. For illustration, we will consider several configurations, namely, single nanopores, arrays of nanopores, systems with reduced area of electrokinetic-conversion elements and devices with side evaporation from thin nanoporous films. For the latter (practically the only one studied experimentally), we will formulate a simple model describing correlations of system performance with such principal parameters as the nanoporous-layer length, width and thickness as well as the pore size, pore-surface hydrophilicity, effective zeta-potential and electric conductivity in nanopores. These correlations will be qualitatively compared with experimental data available in the literature. We will see that experimental data not always are in agreement with the model predictions, which may be due to simplifying model assumptions but also because the mechanisms are different from the classical electrokinetic energy conversion. In particular, this concerns the mechanisms of conversion of evaporation-driven ion streaming currents into electron currents in external circuits. We will also formulate directions of future experimental and theoretical studies that could help clarify these issues.Comment: 41 pages, 7 figure

.“Breakthrough” osmosis in “leaky” supported membranes: A breakthrough in PRO?

It is shown theoretically that allowing for a certain (even quite small) extent of membrane “leakiness” (deviations from ideal perm-selectivity) can give rise to a previously overlooked mode of “breakthrough” osmosis in thin polymeric membranes supported mechanically by porous layers. This result is obtained by using the well-established Spiegler-Kedem model for the description of solute and volume transfer across the membrane barrier layer and the classical convection-diffusion equation for modelling the solute transfer within the porous support layer. The use of these simple modelling tools enables one to obtain transparent analytical solutions and simple criteria for the occurrence of the “breakthrough” mode. In particular, we show that it occurs only in the PRO configuration (barrier layer facing the concentrated solution), only when the solute concentration in the dilute solution is very low and only when the draw-solution concentration is sufficiently high (exceeds a threshold value). We demonstrate that in this mode the rate of osmosis is very large (compared to the conventional mode) and practically independent of the diffusion permeance of the support layer. This opens interesting opportunities in the utilization of this phenomenon in Pressure-Retarded Osmosis (PRO) because the support layers can be made much more robust mechanically without compromising the PRO performance by the Internal Concentration Polarization. Besides, the estimated power densities achievable in the “breakthrough” mode can easily exceed by one order of magnitude those predicted in the conventional mode. This can help resolve two principal problems encountered in PRO: insufficient power density and mechanical collapse of thin and loose membranes into the openings of dilute-side spacers. Our analysis shows that for the “breakthrough” mode to occur the membrane must combine a certain extent of “leakiness” (solute reflection coefficients of ca. 0.95 – 0.995) with sufficiently low diffusion permeability. Reliable experimental detection of minor deviations from ideal perm-selectivity can be difficult (primarily in view of concentration-polarization phenomena). Therefore, it is not clear if the corresponding membranes currently exist. Nonetheless, this analysis provides membrane material scientists with clear guidance on the desired properties of membranes that could become a potential game-changer in renewable energy

Implications of inhomogeneous distribution of concentration polarization for interpretation of pressure-driven

A number of CFD studies have demonstrated that there is a considerable inhomogeneity of extent of Concentration Polarization (CP) over the membrane surface especially in spacer-filled feed channels. However, the consequences of this inhomogeneity for the interpretation of measurements of solute rejection in pressure-driven membrane processes have received little attention. This study uses a simple model of locally-1D CP combined with a postulated probability distribution of unstirred-layer thickness over the membrane thickness. In this way, we obtain transparent analytical results and can consider qualitative consequences of inhomogeneous distribution of CP over membrane surface. Our analysis shows that disregarding the CP distribution under-estimates the CP of strongly positively-rejected solutes and over-estimates the CP for the negatively-rejected ones. This observation is especially important for the interpretation of ion rejection from multi-ion solutions in nanofiltration where strong positive and pronounced negative rejections can occur simultaneously for solutes of different charges. We conclude that for reliable interpretation of pressure-driven membrane measurements it is desirable to reduce the inhomogeneity of CP distribution to a minimum in membrane-testing devicesPeer ReviewedPostprint (author's final draft

Photoinduced 3D orientational order in side chain liquid crystalline azopolymers

We apply experimental technique based on the combination of methods dealing with principal refractive indices and absorption coefficients to study the photoinduced 3D orientational order in the films of liquid crystalline (LC) azopolymers. The technique is used to identify 3D orientational configurations of trans azobenzene chromophores and to characterize the degree of ordering in terms of order parameters. We study two types of LC azopolymers which form structures with preferred in-plane and out-of-plane alignment of azochromophores, correspondingly. Using irradiation with the polarized light of two different wavelengths we find that the kinetics of photoinduced anisotropy can be dominated by either photo-reorientation or photoselection mechanisms depending on the wavelength. We formulate the phenomenological model describing the kinetics of photoinduced anisotropy in terms of the isomer concentrations and the order parameter tensor. We present the numerical results for absorption coefficients that are found to be in good agreement with the experimental data. The model is also used to interpret the effect of changing the mechanism with the wavelength of the pumping light.Comment: uses revtex4 28 pages, 10 figure

Influence of tree-crown density on dominant plant species of the herb-shrub stratum in the zone of mixed forests

Forest ecosystems are among the most complex and dynamic biological systems of our planet. They play an important role in sustaining biodiversity, regulating the climate, and preserving water resources. Furthermore, they serve as natural filters, improving the quality of soil and air, and also preventing erosive processes. Forests create unique conditions for life of various species of plants and animals, which contributes to maintenance of the natural biodiversity and supports the stability of the ecosystem. Likewise, forests are important for the carbon cycle. They absorb a large amount of carbon, thus hindering global warming. Therefore, forest ecosystems are of paramount ecological value and their preservation is crucial for a balanced functioning of the planet. Our studies were carried out in the forest ecosystems of the Desna-Starohutskyi National Park, which is in the Ukrainian Polissia. The materials and methods of the study included systematic collection of the data on density of tree crowns, and also records of diversity of plants of the herb-shrub stratum in the chosen forest areas. Those data were analyzed using statistical methods. The study results revealed that the crown density has a significant effect on diversity of herb-shrub plants in the lower forest strata. Increase in crown density correlated with decrease in the light availability in the herb-shrub stratum. Change in the crown density towards increase significantly altered the conditions for competition between herbaceous and shrub species. Decrease in light availability led to shift in the competition ratio between the species, promoting dominance of more shade-loving species. Increase in crown density, which often reached 100%, made the competition more severe, especially for key resources (light, water, and nutrients). Because of this, species diversity in the herb-shrub stratum of the forest ecosystems was observed to decrease, and less adapted species were extruded. In general, change in tree-crown density in the forest ecosystem had a significant effect on the dynamics of herbaceous and shrub species, changing competitive relations and the structure of those plant communities. The results we obtained expand the knowledge about interactions between crown density and the structure of herb-shrub stratum, which gives perspectives for more efficient management of forest resources, and can also improve scientific identification and implementation of measures for protection of forest ecosystems

Current-induced ion concentration polarization at a perfect ion-exchange patch in an infinite insulating wall

This research examines, theoretically, the ion concentration polarization, ion fluxes, and electrostatic fields near an ion- exchange patch in the wall of an electrified fluidic channel. These phenomena are important in related microfluidic ion- preconcentration systems. Under an electric field, counter ions enter the ion-exchange patch at one side and leave at the other, with salt depletion occurring near the entrance and accumulation near the exit. The high patch conductivity and the concentration profiles lead to local electric field perturba- tions that may facilitate preconcentration. This study includes analytical expressions of ion concentrations and electrochemical potentials at small to moderate electric fields, as well as numerical simulations. Additionally, a simple matrix of poly- nomial coefficients (obtained via fitting of numerical data) enables analytical calculation of the two-dimensional concen- tration profiles at all electric fields within the range investigated in the numerical simulations. This is possible because a single dimensionless parameter controls this problem.Postprint (updated version