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Poly-electrolyte layers determine the efficiency of desalination membranes

Increasing water scarcity and pollution with micropollutants are responsible for the increasing cost of drinking water. Desalination of sea water and better wastewater treatment are necessary to counter this trend. Membranes can desalinate and remove most wastewater pollution. However, a lot of energy is required. Therefore, modern membranes must be more efficient in order to achieve satisfactory results.

Nano-filtration membranes consisting of poly-electrolytic layers are a promising approach to treat water more efficiently. Accordingly, the composition of poly-electrolytic layers has stirred up much interest in the production of nano-filtration membranes. Such membranes are manufactured layer by layer, which enables a good tuning of membrane properties for different purposes.

Commercially available nano-filtration membranes are usually a trade-off between high water permeability and good salt retention (desalination). This trade-off impacts either the quality or the volume of the cleaned water. Nano-filtration membranes that are produced in layer by layer can have a positive impact on this trade-off balance due to the formation of nano scaled layers. It is important to know which component plays a crucial role in the layer forming process.

A research group of the Technical University Eindhoven in the Netherlands had therefore undertaken the task of examining these layer components more closely. They specifically investigated the poly-electrolyte concentration. It is known that a higher poly-electrolyte concentration produces thicker layers. However, their impact on the membrane performance has so far been unknown. They now published their work, in which the researchers used two well-known strong poly-electrolytes: PDADMAC and PSS (polydiallyldimethylammonium chloride and poly(sodium-4-styrene sulfonate)). The membrane output was examined with regard to water permeability, the molecular weight cutoff and salt retention.

In the first double layer, the membranes made with a 50 mM saline solution showed a lower water permeability and molecular weight cutoff, as well as better salt retention (magnesium sulfate) due to the higher poly-electrolyt concentration. After a certain number of double layers, the molecular weight cutoff and the salt retention efficiency for all poly-electrolyte concentration leveled off. The higher the poly-electrolyte concentration, the sooner the plateau value was reached.

The membranes prepared with a 1 M salt concentration had a lower or comparable salt retention efficiency with one exception. The scientists concluded that the poly-electrolyte concentration significantly changed the membrane properties. A plateau was reached with seven or more double layers. The thicker layers showed a lower water permeability than those that were coated with poly-electrolyte solutions using a 50 mM salt concentration. Due to the reduced swelling of these membranes, they all had better salt retention efficiency, with the exception of magnesium chloride.

The results showed that increasing the poly-electrolyte concentration also increased the amount of poly-electrolyte adsorption. Due to a higher coating thickness, this led to lower permeability with pure water. However, this did not lead to a lower molecular weight cutoff or salt retention. The additional poly-electrolyte adsorption resulted in fewer links between the individual layers. The higher diffusivity of PDADMAC compared to PSS resulted in highly positively charged membranes, which in turn led to a better salt retention of magnesium and sodium chloride.

Overall, increased poly-electrolyte concentration and the salt concentration influenced the membrane charge exclusion significantly due to a higher charged surface, which led to better salt retention. However, the membrane size exclusion has not changed, which led to the same plateau values. The study presented here will allow chemists to produce better tuned desalination membranes, which will reduce the energy requirement and raw material requirements during production.

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Nanostructured membranes improve the gas separation of carbon dioxide

To reduce greenhouse gas emissions, various technologies are in development requiring the separation of mixed gases, such as  CO2 and methane or CO2 and nitrogen gas (CO2/CH4 and CO2/N2). Compared to other separation technologies, polymer membranes are  good candidates for industrial use. This is due to their low operating costs, high energy efficiency and simple scalability.

The gas permeability and selectivity, as well as the cost of these polymer membranes are the crucial criteria for their industrial use. These criteria are influenced by molecular order processes during polymerization at nano- and micrometer levels. However, the processes regulating the molecular order of most common membranes do not occur on these levels. Hence, there is little control over them during manufacturing. Not much is known about materials with self organizing properties and their influence on molecular order and gas separation.

Chemists at the Technical University of Eindhoven in the Netherlands examined the effects of the layer distance within the membrane and its halogenation on the gastrunge and published their results in the MDPI Membranes journal. They focused on the separation of helium, CO2 and nitrogen. The researchers used liquid crystal membranes for their investigation. Liquid crystal molecules can align in various nanostructures. These structures vary depending on the manufacturing process and can therefore be controlled. As a result, liquid crystal membranes are ideal in order to investigate the influence of nanostructures on gas separation.

A frequently used manufacturing method is to commence the self organization of the reactive liquid crystal molecules in a cell with spacers. This helps to better control the membrane thickness and alignment and ultimately control the molecular orientation. The final network of the liquid crystal molecules and their fixation in nanostructures is required to achieve mechanical strength. For example, high ordered crystal membranes (i.e. not liquid crystals) have a lower gas permeability. Nonetheless, they also are characterized by a higher selectivity for helium and CO2 compared to nitrogen.

A lamellar morphology and the flow direction of the gas also have a great influence on selectivity and permeability of the membrane. It is also known that halogen atoms such as chlorine or fluorine improve CO2 permeability and selectivity by affecting both gas solubility and diffusion.

In the presented experiments, all liquid crystal membranes with similar chemical compositions, but different halogenated alkyl chains, were aligned. The CO2 sorption and the entire gas permeation were better if their layers were further apart. The gas solubility itself had no impact. This was confirmed by the increased gas diffusion coefficients, which were also determined in the experiments.

Bulky halogens had only limited influence on gas permeability and selectivity. The CO2 permeability of all halogenated liquid crystal membranes increased due to a slightly higher CO2 solubility and diffusion coefficients, which led to improved selectivity for CO2. The layer distance in particular was a crucial factor that directly influenced the diffusion coefficient. The researchers recommended that future investigations should focus on improving separation performance, for example by reducing the membrane thickness.

At Frontis Energy, we are looking forward to a good commercial product that can separate CO2 from gas mixtures, such as biogas, effectively and cheap.

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