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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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Bio-electric systems help PFAS removal

Per- and polyfluoroalkyl substances (PFAS) have been manufactured for various applications for many decades. These include medical applications, such as implants and catheters, or consumer products for firefighting, plastics, cookware, and cosmetics. Likewise, PFAS are required in countless industrial applications, such as in the automotive industry, the chemical industry and the energy sector, including hydrogen electrolysis and fuel cells (e.g. Nafion™). They help apparatuses to function properly, reduce wear and the risk of accidents. The widespread use of PFAS has led to traces of these substances entering the environment worldwide. Typical sites with higher environmental PFAS concentrations include airports, chemical plants, fire brigades, military facilities etc.

The long-term health effects of these substances are currently a matter of controversy, particularly with regard to their chemical stability (a desired property).

In addition to completely avoiding their entry into the environment, PFAS can also be eliminated from it. For example, activated charcoal is often used to adsorb PFAs onto it. However, this method is not efficient in soils. Ideally, the activated carbon itself would have to be further processed in order to reuse PFAS. This process is very energy intensive.

As for many treatment processes, microbes can be used also for PFAS. Such biological methods are called bioremediation. However, the carbon-fluorine (C-F) bonds in PFAS are among the strongest covalent bonds in organic chemistry. In addition, there are very few naturally occurring C-F bonds in nature. They only occur in small concentrations. A prominent example is fluoroacetic acid, a highly toxic compound produced by the South African poisonous gifblaar. Few microorganisms with the ability to break the C-F bond have been identified. Thus, bioremediation of PFAS is possible but a slow process.

As already described in our previous articles, bio-electrical systems can accelerate microbial conversion processes. With bio-electrical systems it is possible to offer microbes a greater electrochemical potential gradient. Since this leads to larger energy gain in microbial metabolism, such metabolic rates can be accelerated. This process is successfully employed to clean industrial waste water.

In bio-electrical systems, microorganisms along with contaminants are placed in an electrochemical apparatus. The electrodes of such a system serve as electron donors or acceptors. The biodegradation is can be measured via the electric current.

Indeed, bio-electric systems have been used to degrade fluorinated alkanes. For example, the anti-inflammatory drug dexamethasone was successfully eliminated using such an apparatus. As proposed for bio-electrical liquid fuel, designer microbiomes could also be studied for PFAS. Other drug residues, such as Prozac™, should also be examined to ensure absence from the environment.

At Frontis Energy we are looking forward to new developments for PFAS removal in bio-electrical systems.

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Decentralized waste energy systems produce biogas where it is needed

Among others, the current European energy crisis was caused by a surge in demand after the pandemic, the embargo on Russia, the reluctance of investors to finance fossil energy projects and the throttling of production by the OPEC countries. In this complex situation, European countries are forced to develop alternatives and renewable energy sources. At the same time, however, natural gas is difficult to replace in many industries. One exception is the food and beverage industry, which sits enormous untapped resources of biogas in their wastewater.

Wastewater is a resource of which 380 billion m³ are produced worldwide. It contains valuable nutrients and energy. Global production is projected to increase by 51% by 2050. Wastewater treatment consumes about 3-4% of the energy generated globally. The full reeovery of the energy that is contained in this wastewater would completely offset the energy consumption of its treatment and in many cases even produce a surplus. In addition, the entire global water treatment is estimated to account for up to 5% of man-made CO2 production. Unfortunately, many businesses and municipalities do not invest in complex and expensive wastewater treatment technologies and continue to waste this valuable resource. The European Biogas Association estimates that by 2050, a maximum of 65% of gas requirements (~167 billion m³) could be covered by biogas.

Europe is the largest cheese maker in the world. More than 9 million tons of cheese are produced annually. With every ton of cheese, 9 m³ of cheese whey remain. Despite its high nutritional value, whey is often treated like wastewater for various reasons. Yet, the very high organic load in the whey makes it difficult to treat. Wasted whey can also be used for biogas production. In addition to whey, regular wastewater is also produced by cheese makers. For example, a medium-sized cheese factory pays 1.5 million euros a year for its waste water. Reducing these costs by producing biogas would turn dairy industry wastewater into a valuable resource.

This situation is similar in many other food and beverage sectors such as breweries, distilleries, winemakers, bakeries etc. All of these sectors have high energy requirements. Renewable electrical energy cannot meet this need. The market for wastewater treatment in Europe and the US is around 12 billion euros.

Traditional wastewater treatment is a cascaded process including aeration and anaerobic sludge digestion followed by incineration. These methods often consume more than 70% of the energy in a wastewater treatment plant. If contaminants such as high-energy total organic carbon or ammonia were converted into biogas before the process, at least 80% of the energy needed for wastewater treatment could be saved. It is absurd that this energy is removed from the wastewater using even more energy.

An ever-increasing number of sewage treatment plants already recover the resources contained in their wastewater, apart from the water itself. The oldest recivered products are biogas and fertilizers made from sewage sludge. Due to its heavy metal content such as copper and mercury, sewage sludge is no longer used as fertilizer but incinerated.

Biogas is particularly popular in Europe as the produced volumes and prices are high enough to compete with natural gas. Biogas is also a green alternative to natural gas as no additional CO2 is emitted. (Hence, it is often called Renewable Natural Gas in North America.) A disadvantage of classic biogas is the CO2 and sulfide content. Another disadvantage is that anaerobic digestion is the terminal treatment step, wasting valuable wastewater resources in the preceding treatment. Finally, the size and complexity of current digestion requires significant commitment from users when it comes to capital expenditures. Most food manufacturers prefer to focus on making food rather than cleaning their wastewater.

Novel high-performance biogas reactors solve these problems through miniaturization. A 20-fold size reduction is achieved compared to conventional systems. The new technology used was developed in Japan in the early 1990s and is called microbial electrolysis. The electrolysis of wastewater is catalyzed by electroactive microorganisms on the anode (the positive electrode). The reaction products are CO2 (from organic matter) and nitrogen gas (N2 from ammonia).

Principle of a microbial electrolysis reactor. On the left anode, the organic material is oxidized to CO2. The free electrons are absorbed by the anode and transported to the cathode. Hydrogen gas (H2) is released there. CO2 and hydrogen form methane, the final microbial reaction product.

At the same time, hydrogen gas (H2) is generated at the cathode (the negative electrode). This hydrogen reacts with CO2 to form methane. The final methanation step completes the biocatalytic treatment of the wastewater. Gas grid injection is one possible use. But for cheese makers, the gas would be used on site to generate electricity and/or heat.

The reaction is accelerated using an applied voltage and is based on the laws of thermodynamics. As a result, the reactor volume can be reduced. The size reduction has several advantages. First, it makes biogas accessible in markets where it was previously not possible due to the high investment costs. Second, it enables higher throughput at a lower cost. Smaller units are mobile and can be shared, moved or rented. After all, food manufacturers want to do what they do best, which is to make food.

 

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Novel membranes from plant waste filter heavy metals from water

Unfortunately, water pollution is still an issue in many places. Heavy metals are a group of water pollutants that can accumulate in the human body and causing cancer and other diseases. Existing technologies for heavy metal removal, however, are very energy intensive.

Scientists from the Nanyang Technological University in Singapore and the Swiss Federal Institute of Technology Zurich (ETHZ) have created a new membrane out of byproducts from the vegetable oil industry. The membrane removes heavy metals from contaminated water. The team discovered that proteins, which originated from peanut or sunflower oil production bind heavy metal ions very effectively. In their tests, they showed that this adsorption process can purify contaminated water so much that it fulfills drinking water quality standards.

The researchers see their membranes as an inexpensive, simple, sustainable and scalable solution for heavy metal removal from water. Their results were published in the Chemical Engineering Journal.

The new protein based membranes were generated by an environmentally friendly process and needed little energy for their use. This makes them a promising water purification solution for industrialized nations as well as less developed countries.

The production of commercial vegetable oils generates protein rich waste products. These remnants remain from the raw plant after the oil extraction. For their membranes, the research team used sunflower and peanut oils. After the proteins had been extracted, they were transformed into nano-amyloid fibrils. These are rope-like structures built from tightly intertwined proteins. The protein fibrils strongly attract heavy metals and act like a molecular sieve. In the published experiments, the membranes removed up to 99.89 percent of heavy metals.

Among the three metals tested, lead and platinum were filtered most effectively, followed by chrome. Since platinum is often used as a catalyst in fuel cells or electrolyzers, the new membrane would be an elegant and cheap method to recover this metal.

The researchers combined the extracted amyloid fibrils with activated carbon. Due to the high surface volume ratio of the amyloid fibrils, they are particularly suitable for adsorption large amounts of heavy metals. The filter can be used for all types of heavy metals. In addition, organic pollutants such as perfluoralkyl and polyfluoralkyl compounds are filtered as well. These chemicals are used for a variety of consumer and industrial products, as well as in nafion membranes of fuel cells.

The concentration of heavy metals in contaminated water determines how much volume the membrane can filter. A hybrid membrane made of sunflower amyloids requires only 16 kg of protein to filter a swimming pool contaminated with 400 parts lead per billion. One kilogram of sunflower extract yields about 160 g of protein. The protein-rich sunflower and peanut oils are inexpensive raw materials. Since this is the first time that amyloid fibrils were obtained from sunflower and peanut proteins, the process must still be scaled and industrialized.

However, due to its simplicity and minimal use of chemical reagents, the process seems easy to scale. This makes it possible to recycle the waste product for further applications and to fully exploit such industrial food waste. The filtered metals can also be extracted and further recycled. After filtration, the membrane with the captured metals can simply be burned and leaving behind only the metals.

While toxic metals such as lead or mercury need safe disposal, other metals such as platinum can be re-used in the production of electronics and other high value devices, such as fuel cells. The recovery of the precious platinum, which costs 30,000 euros per kg, only requires 32 kg of protein, while the recovery of gold, which corresponds to almost 55,000 euros per kg, only requires 16 kg of protein. In view of the costs of less than 1 euro per kg of protein, the advantages are enormous.

The co-author of the article, Raffaele Mezzenga, had already discovered in 2016 that whey proteins from cow milk had similar properties. Back then, the researchers noticed that proteins from plant oil seeds could also have similar properties.

Another great advantage is that, unlike other methods such as reverse osmosis, this filtration does not require electricity. Gravity is completely sufficient for the entire filtration process. The method is also suitable for water purification in poorly developed areas.

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Bio-electrical system removes nitrogen from the wastewater

Hazardous compound removal from sewage such as organic matter and nitrogen makes wastewater treatment an energy intensive process. For example, treating activated sludge requires blowing oxygen or air into raw, unsettled sewage. This aeration significantly increases the cost of the wastewater treatment. About 5 kWh per kilogram nitrogen are required for aeration depending on the plant. The cost associated with energy consumption makes uof approximately EUR 500,000 per year in an average European wastewater treatment plant. This is up to one-third of the total operational costs of WWTP. It is therefore obvious that nitrogen removal from wastewater must become more economical.

Alternative approach: Microbial electrochemical technology

The conventional way of removing nitrogen is a cascade of nitrification and denitrification reactions. Nitrification that is, aerobic ammonium oxidation to nitrite and nitrate is carried out by ammonia-oxidizing bacteria. Subsequent denitrification is the reduction of nitrate to nitrogen gas (N2). In addition to the costly aeration process, the remaining intermediate products as nitrite and nitrate require further effluent treatment.

Instead of expensive pumping of oxygen into the wastewater, bioelectrical systems could accomplish the same result at a much lower cost. In such systems, an electron accepting anode is used as electron acceptor for microbial ammonium oxidation instead of oxygen, making aeration obsolete.

Complete conversion of ammonium to nitrogen gas

We previously reported the use of such an bio-electrical system to remove ammonia from wastewater in fed-batch reactors. Now, researchers of the University of Girona reported proof-of-concept on a novel technology. Their bioelectrical system is a complete anoxic reactor that oxidizes ammonium to nitrogen gas in continuous mode. The dual-chamber reactor nitrifies and denitrifies and ultimately removes nitrogen from the system.

The electricity-driven ammonium removal was demonstrated in continuously operated one-liter reactor at a rate of ~5 g / m3 / day. A complex microbial community was identified with nitrifying bacteria like Nitrosomonas as key organism involved anoxic ammonium oxidation.

From an application perspective, comparison between bioelectrical systems and aeration in terms of performance and costs is necessary. The researchers reported that the same removal range and treatment of the similar amounts of nitrogen was achieved but that their bioelectrical system converted almost all ammonium to dinitrogen gas (>97%) without accumulation of intermediates. Their system required about 0.13 kWh per kilogram nitrogen energy at a flow rate of 0.5 L / day. Using a bioelectrical system consumes 35 times less energy compared with classic aeration (~5 kWh per kilogram). At the same time, no hazardous intermediates like nitrite or NOx gases are formed.

Unveiling microbial-electricity driven ammonium removal

The new article also indicated potential clues for microbial degradation pathway that may lead to better understanding of the underlying processes of anoxic ammonium removal in bioelectrical systems.

The proposed nitrogen removal pathway was the bioelectrical oxidation of ammonia to nitrogen monoxide, possibly carried out by a microbe named Achromobacter. That was supposedly followed by the reduction of the nitrogen monoxide to nitrogen gas, a reaction that could have been performed by Denitrasisoma. Alternatively, three other secondary routes were considered: bioelectrical oxidation followed by anammox, or without nitrogen monoxide directly to N2. Some sort of electro-anammox may also be possible.

At Frontis Energy, we believe that the direct conversion of ammonium to nitrogen gas through the reversal of nitrogen fixation is a possibility as nitrogen fixation genes are ubiquitous in the microbial world and it would generate the universal bio-currency ATP rather than consuming it.

It was shown that Achromobacter sp. was the most abundant microbe (up to 60%, according to sequence reads) in the mixed community. However, anammox species (Candidatus Kuenenia and Candidatus Anammoximicrobium) and denitrifying bacteria (Denitratisoma sp.) have been also detected in the reactor.

Two possible electroactive reactions were identified: hydroxylamine and nitrite oxidation, reinforcing the role of the anode as the electron acceptor for ammonium oxidation. Data obtained from nitrite and nitrate tests suggested that both, denitrification and anammox based reactions could take place in the system to close the conversion.

As a result, ammonium was fully oxidized to nitrogen gas without accumulated intermediates. Taking it all together, it has been shown that ammonium can be removed in bioelectrical system operated in continuous flow. However, further reactor and process engineering combined with better understanding of the underlying microbial and electrochemical mechanisms will be needed for process scale up.

Experimental system set-up

  • The inoculum consisted of a 1:1 mix of biomass obtained from nitritation reactor and an aerobic nitrification reactor of an urban treatment plant
  • The reactor design was constructed of two 1 L rectangular chambers comprising an anode and cathode compartment
  • The separator, an anion exchange membrane,  was used to minimize the diffusion of ammonium to the cathode compartment
  • The anode and cathode chambers were filled with granular graphite as electrode support
  • Ag/AgCl reference electrode was used in the anode compartment
  • Two graphite rods were placed as current collectors in each chamber
  • The system was operated in batch and semi-continuous mode

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Water desalination and fluoride ions removal from water using electrodialysis

Clean freshwater is of the utmost importance for our health. Despite its central role for our lives, progressing global industrialization threatens freshwater resources around the world. Albeit a vital trace element, fluoride is a serious public health threat. Absorbed in larger quantities for a long time, fluoride causes fluorosis, a form permanent poising responsible for irreparable bone damage.

Fluoride bearing rocks are particularly common in India. Fluoride is leached into adjacent aquifers and contaminates the soil. Sometimes, the concentration of fluoride ions in Indian aquifers exceeds 30 mg/L. Toxic concentrations of 20-80 mg / day over a period of 10 to 20 years cause irreparable damage to the human body.

Fluoride ions in groundwater are removed for water treatment using membranes. However, such membranes foul easily, for example by bacteria present in wastewater or other deposits.  Fouling can become a serious threat to public health. Therefore, a particular focus in membrane research is on the development of fluoride removing membranes that prevent fouling. It can be accomplished when bacterial growth is slowed down or inhibited entirely. For water treatment, antimicrobial surface modifications are used in high-quality membranes for ultrafiltration, nanofiltration, reverse osmosis and electrodialysis.

Electrodialysis is often used to remove water contamination, because only little energy is needed for the process. For electrodialysis membranes, salt deposits are an economic risk that is to be avoided. Precipitates can occur when the concentration of bivalent ions in the water is too high. Added to precipitates comes the risk of biofouling caused by microbial growth. Both affect the performance of electrodialysis membranes, causing economic losses as the membranes must be cleaned or replaced. For efficient water treatment, it is therefore important to improve the thermal and mechanical properties of the membranes.

A group of scientists have synthesized a composite anion exchange membrane for water-salt altitude and fluoride ion removal by electrodialysis that has improved antimicrobial properties. She published her results in the journal ACS ES&T Water. The consortium consisted of researchers of the Academy of Scientific and Innovative Research in Ghaziabad, India and the University of Tokyo.

Their anion exchange membranes are based on cross-linked terpolymers with built-in silver nanoparticles to slow microbial growth. The membranes are suitable for water desalination and fluoride ion removal by electrodialysis. The preparation of the terpolymers and polyacrylonitrile copolymers was carried out by N-alkylation using various alkyl halides. N-alkylation of the terpolymer through various alkyl groups affected the water absorption, hydrophobicity, ion transport and ionic conductivity of the membrane. Long alkyl groups increased the effectiveness of fluoride removal as well as the oxidative and physical stability of the membranes. The suitability of the composite membranes was verified by testing removal efficiency of fluoride ions (5.5 and 11 mg/L) from a sodium chloride solution (2 g/L) by electrodialysis at an applied voltage of 2 V.

The incorporation of 0.03% silver nanoparticles in the quaternized polymer caused the desired antimicrobial effect. The uniform distribution of silver nanoparticles in the liquid and solid phases was detected by transmission electron microscopy and atomic force microscopy. The attachment of bacteria was quantified counting colony forming units and 100x lower when silver nanoparticles were present in the membrane. The reduced microbial attachment to the membrane surface is therefore due to the antimicrobial effect of the silver nanoparticles. The small amount of 0.03% silver nanoparticles was sufficient to achieve desired antimicrobial effect in the membrane.

After 15 days and at a water temperature of 50°C, no detectable silver leaching occurred. The novel membranes are thus an improved anion exchange solution with antimicrobial properties for efficient removal of fluorine and desalination by electrodialysis.

Methodology

The entire synthesis was carried out in four steps:

  • Step 1: Silver nitrate was diluted with deionized water to produce a 30 mm solution
  • Step 2: Terpolymer and quaternized terpolymers were prepared by free radical polymerization
  • Step 3: Composite additives were prepared by the reduction of silver nitrate with sodium borohydrite in the presence of dimethylformamide
  • Step 4: The membrane was networked with the silver nanoparticles

Characterization of the anion exchange membrane

The membrane was characterized using several analytical methods:

  • UV-VIS and IR spectroscopy
  • Incorporation of silver nanoparticles by scanning electron microscopy, atomic force microscopy and transmission electron microscopy
  • Thermal stability, tensile properties, solubility and further physicochemical and electrochemical properties of the silver nanoparticle composite
  • Desalination and fluoride removal
  • The effectiveness of silver nanoparticles on microbial attachment
  • Energy consumption and efficiency during water desalination and fluoride removal by the composite membrane
  • Membrane stability with respect to pH, temperature and Fenton’s Reagent was evaluated

Reference:

Pal et al. 2021 “Composite Anion Exchange Membranes with Antibacterial Properties for Desalination and Fluoride Ion Removal” ACS ES&T Water 1 (10), 2206-2216, https://doi.org/10.1021/acsestwater.1c00147

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Lead removal from water using shock electrodialysis

Lead was widely used in water pipes during the industrial revolution that triggered urbanization and exponential growth of the population in metropolitan centers. The reason for its popularity was the plasticity of the material used in service lines near the end user. The negative health effects have been known since the 1920s, but infrastructure modernization in industrialized countries remains an enormous economic challenge. Lead service lines therefore continue to circulate water in our supply systems. The city of Flint in the northwest of Detroit, for example, received much press attention due to its long struggle with lead poisoning (e.g. Flint Water Crisis). Dissolved lead is highly toxic in a very small concentration and accumulates in body tissues.

The biggest challenge when removing lead from the water cycle is that it is usually dissolved in very low concentrations. Other compounds “mask” the dissolved lead, which makes its removal difficult. Sodium, for instance, is concentrated ten thousand times higher than lead. While nowadays lead can be removed from water by reverse osmosis or distillation, these processes are not selective and thus ineffective. They consume a lot of energy, which in turn is an environmental issue in itself. High energy consumption makes water treatment also very expensive. At the same time, other minerals dissolved ion water are beneficial and therefore desired ingredients that should not be removed.

MIT engineers have developed a much more energy-efficient method to selectively remove lead from water and published their results in the journal ACS EST Water. The new system can remove lead from water in private households or industrial plants and hence from the water cycle. Through its efficiency, it is economically attractive and offers its users the clear advantage of not being poisoned.

The method is the most recent of a number of development steps. The researchers started with desalination systems and later developed it into radioactive decontamination method. With lead the engineers have found an attractive market. It is the first system that is also suitable for private households. The new approach uses a process that was named shock electrodialysis by the MIT engineers. It is essentially very similar to electrodialysis as we know it, as charged ions migrate into an electric field through the electrolyte. As a result, ions are enriched on one side while being depleted on the other.

The difference of the new method is that the electric field moves as a sort of shock wave through the electrolyte and drags dissolved ions along. The shock wave traverses from one side to the other is the voltage increases. The process leads to a lead reduction of 95%. Today, similar methods are also used to clean up aquifers or soil contaminated by solvents. In principle, the shock wave makes the process much cheaper than existing processes because the electrical energy is targeted to remove specifically lead while leaving other minerals in the water. Hence, a lot less energy is consumed.

As usual for bench top prototypes, shock electrodialysis is still too ineffective to be economically viable. Its up-scaling will take time. But the strong interest of potential users will certainly accelerate its industrialization. For a household whose water supply is contaminated by lead, the system could be placed in the basement and slowly clean the water carried by the supply pipes because high rates occur only during peak hours. For this purpose, a water reservoir is necessary, keeping a stock of purified water. This can be a fast and cheap solution for communities such as Flint.

The process could also be adapted for some industrial purposes. The mining and oil industries produce much heavily contaminated wastewater. One imagine to reclaim dissolved metals and sell them to the market. This would create economic an incentives for wastewater treatment. However, a direct comparison with currently existing methods is difficult because the longevity of the developed system is yet to be demonstrated.

At Frontis Energy we are thrilled by the idea of ​​creating economic incentives to help implementing environmentally friendly processes and are already looking forward to a commercial product.

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How infrared radiation influences the behavior of interfacial water

Despite a common belief, very little is known about the structure of water and interfacial interactions. Interfacial water that is adsorbed on the surface of the hydrophilic materials is formed by both water-surface and water-water interactions. It has been discovered that the interfacial water differs from the water in bulk and can exclude solutes and microspheres, and hence it is termed an exclusion zone (EZ). EZ water is known to have a higher refractive index, viscosity, and light adsorption at 270 nm. Charge separation is also caused by water-surface interactions. For example, the water EZ near Nafion™ membranes has an electrical potential of −200 mV.

Studies showed that electromagnetic energy can affect interfacial water. Infrared (IR) energy can cause expansion of the size of the EZ leading to charge separation. This study was conducted by researchers of the University of Washington with IR light of varying intensities and wavelengths to see if they can accelerate the process and bring protons into bulk water. The scientists attempted to shed light on the complex nature of aqueous  interfaces.

Experimental analysis

Materials used:

Deionized (DI) water with the resistivity of 18.2 MΩ × cm was purified with a Barnstead D3750 Nanopure Diamond water system. Other materials were a Nafion™ N117 membrane, a potassium phosphate buffer, a pH dye and carboxylate microspheres (1 µm diameter in a 2.5% suspension)

Sample preparation:

Carboxylate microsphere suspensions with a microsphere-to-water volume ratio of 1:300 and pH-sensitive dye with the dye-to-water volume ratio of 1:20 for better visualization were added.

Due to carbon dioxide absorption the water had a slightly acidic pH of 6.35 and was neutralized. To stabilize the pH, a 1 molar potassium phosphate buffer of pH 7.0 made from equal volumes of 1 molar K2HPO4 and KH2PO4 solutions and added at a final concentration of 1 mM.

A Nafion™ membrane of 3 × 20 mm size was pre-soaked in 1 liter of DI water for 24 hours before use.

Control and irradiation experiments:

A thick plastic block chamber was injected with the 1 mL water the containing buffer solution, pH dye, and microspheres. The chamber consisted of a glass slide and a groove in the central vertical plane of the chamber was used to hold the Nafion™ membrane. This setup was placed on the stage of an inverted microscope for observation over 10 min.

For irradiation experiments, mid-infrared (MIR) LED wavelengths at 3.0 μm, and three near-infrared (NIR) LEDs of different wavelengths were used. It was placed 2 mm above the water level in the chamber. The light was kept as continuous as possible with constant emission power. It shone for 5 mins onto the water surface. The temperature of the water samples was obtained using infrared cameras.

Results

Water zones differ from bulk water

Interfacial water excluded dye and microspheres by forming EZ water next to Nafion™. A red zone with of pH 4 was formed beyond the EZ water called proton zone (PZ). The researchers concluded that the protons accumulated there due to growing interfacial water. With the time of contact between Nafion™ and water progressing, the EZ size was doubled as did the PZ. The microspheres drifted away from Nafion™ with time.

Stability of EZ size and PZ size

It was evident from the observation that EZ water was not caused by the substance flowing out of Nafion™. It is believed that the ice-like structure of interfacial water cause EZ and PZ water. This network of hexagonal structure, several hundred microns. Electrostatic attractions exist between the EZ water layers.

Effect of IR radiation on EZ water and PZ water

The proton concentration in PZ water increased with IR intensity along with the size of EZ and PZ. Higher IR intensities weaken the OH bonds aiding those molecules to participate in EZ expansion. IR radiation also caused thermodiffusion with carboxylate microspheres moving away from the IR light spot with increasing intensity.

Effect of NIR on EZ and PZ waters

The study of the effect of NIR on interfacial water can help to better understand light therapy. Red wavelengths and NIR wavelengths are considered suitable due to their ability to deeply penetrate tissue. Light therapy aids in the synthesis of adenosine tri-phosphate (ATP), the universal biological energy currency. This could have medical benefits. Interfacial water could act as a photoreceptor in light therapy, as cells contain macromolecules and organelles. The use of NIR to establish a proton gradient requires further investigation.

Conclusions

The research showed that the  EZ and PZ zones in interfacial water stabilize after five minutes and that infrared radiation can considerably increase the size of these zones with intensity. This is possibly due to the special nature of water present on hydrophilic material surfaces.

It is also evident that IR radiation can help in building up microsphere-free zones − a phenomenon that in turn creates proton-rich zones. This is also  responsible for charge separation in interfacial water. In summary, some of the mysteries regarding the complexity of interfacial water, EZ, and PZ water zones have been clarified but much remains to be studied.

Outlook

As always, further research to understand the nature of EZ and PZ of water is required. For example the viability and the possibility of the use of NIR for light therapy using interfacial water as a photoreceptor should to be studied. This applications has the potential to make a positive impact on medical applications.

References: https://doi.org/10.1016/j.colcom.2021.100397 : Effect of infrared radiation on interfacial water at hydrophilic surfaces, Colloid and Interface Science Communications, Volume 42 , May 2021, 100397

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Self-cleaning membranes for biofouling control and prevention in water treatment

Membrane-based water treatment is critical for obtaining potable water, for example through wastewater treatment and seawater desalination. However, membrane fouling remains a common undesirable phenomenon affecting all membrane-based separation processes. Various efforts have been made to either directly control biofouling or to prevent it.

Ceramic membranes have better thermal and chemical stability along with higher fouling resistance and longer lifetimes when compared to polymeric membranes. These properties render ceramic membranes superior to polymers.

During the filtration process, the amount of water that can pass through a membrane is known as membrane flux. Due to membrane fouling, this flux is reduced and the affected membrane needs to be refurbished. Different membrane cleaning strategies have been researched including self-cleaning conductive polymeric membrane and electrically-assisted filtration but neither of them has shown a satisfactory flux recovery behavior.

Previous researches have suggested the use of ‘nano zeolite’ and carbon nanostructures for water treatment and desalination applications.

  • Zeolites are crystalline aluminosilicates possessing a well-defined inorganic structure, whose microporous 3-D channels and pores act as filters.
  • Carbon nanostructures consist of highly entangled carbon nanotubes which are made through a standardized chemical vapor deposition method.

To investigate the use of ceramic membranes made from nano zeolite and carbon nanostructures, a group of researchers at the New York University Abu Dhabi, United Arab Emirates, developed a new electro-ceramic membrane and evaluated its antifouling performance. Their research findings were published in the Chemical Engineering Journal.

Research Approach:

Zeolite / CNS membrane preparation:

Nano zeolite-Y (nano-Y) membranes were prepared by dispersing the desired amounts of nano-Y, carbon nanostructures, and polyvinylidene fluoride (PVDF) binder in a water-alcohol solution.

The suspension was vacuum filtered through a microfiltration membrane filter and the membrane was peeled off from it before drying it at room temperature.

Three different ratios of zeolite and carbon nanostructures were prepared initially, with 60, 70, and 80 wt% zeolite. The carbon nanostructures and the binder were prepared at a ratio of 1:1.

Membrane characterization:

The electrical conductivity and mechanical properties of the dried membranes were investigated.

The surface morphology of the zeolite carbon nanostructure membrane was studied through scanning electron microscopy and transmission electron microscopy.

Other tests including the membrane contact angle test were also performed on the different labeled membranes.

Membrane cleaning setup and antibacterial assessment:

Two foulants, yeast (200 mg / L) and sodium alginate (30 mg / L) were used as biofoulants.

A custom-made cell was designed and a fresh membrane was used for each electrochemical measurement performed using linear sweep voltammetry.

Antibacterial properties of the nano-Y carbon nanostructure membranes were determined by the disk diffusion method. Different bacteria were cultured overnight at 37°C in a shaking incubator at 100 rpm.

Results:

Membrane cross-sections showed a uniform distribution of nano-zeolite particles with the carbon nanostructure. Decreasing tensile strength was seen interpreted as successful nano zeolite incorporation. These values changed from 3.3 MPa to 2.1, 1.1 or 0.3 MPa, respectively for 60, 70 and 80 weight% nano-Y. In addition, a decrease in water contact angle from 84.7±2° to 18±4° was demonstrated within 4 min.

The composite membrane demonstrated enhanced electrocatalytic activity for hydrogen evolution in two foulants; yeast and sodium alginate.
These MF electro-ceramic self-cleaning, anti-bacterial membranes seem promising for various separation processes such as in wastewater treatment, dye separation and oil / water separation where fouling and bacterial growth are a major concern.

(Photo: WET GmbH, Attribution, via Wikimedia Commons)

Reference: https://doi.org/10.1016/j.cej.2020.128395 Electro-ceramic self-cleaning membranes for biofouling control and prevention in water treatment, Chemical Engineering Journal, Volume 415, 2021

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Pilot-scale microbial fuel cells produce electricity from wastewater

In wastewater treatment, aeration is an energy-intensive but necessary process to remove contaminants. Pumps blow air into the wastewater to supply the microbes in the treatment tank with oxygen. In return, these bacteria oxidize organic substances to CO2 and hence remove them from the wastewater. This process is the industrial standard and has proven itself for over a century. If the researchers at Washington State University and the University of Idaho have their way, that is changing now.

In their project, the researchers used a unique microbial fuel cell system they developed to replace aeration. Their novel wastewater treatment system cleans wastewater with the help of microorganisms that produce electricity. These microbes are called electrophiles.

The work should one day lead to less dependence on the energy-intensive treatment processes. Most of the energy in such processes is consumed in the activated sludge and its disposal. The energy consumption in water treatment produces around 4-5% of anthropogenic CO2 worldwide. to put that in perspective, according to the Air Transport Action Group in Geneva, international air transport produced 2.1% CO2 in 2019. The researchers published their work in the journal Bioelectrochemistry. In addition to cutting green house gas emissions, lowering the energy consumption of wastewater treatment would save billions in annual operation and maintenance costs.

Microbial fuel cells allow microbes to convert chemical energy into electricity, much like in a battery. In wastewater treatment, a microbial fuel cell can replace aeration while capturing electrons from wastewater organics. These electrons themselves are in turn a waste product of the microbial metabolism. All living organisms strive to discharge their excess electrons. This process is known as respiration or fermentation. The electricity generated the microbes can be used for useful applications in the wastewater treatment plant itself. The technology kills two birds with one stone. On the one hand, the treatment of the wastewater saves energy. On the other hand, it also generates electricity.

Up until now, microbial fuel cells have been used experimentally in wastewater treatment systems under ideal conditions, but under real and changing conditions they often fail. Microbial fuel cells lack regulation that controls the potential of anodes and cathodes and thus the cell potential. This can easily lead lead to a system failure. The entire cell must then be replaced.

To tackle this problem, the researchers added an additional reference electrode to the system that enables them to control their fuel cell. The system becomes more flexible. It can either work as a microbial fuel cell on its own and consume no energy, or it can be converted so that less energy is used for aeration while it purifies the wastewater more intensively. Frontis Energy uses a similar control system for its electrolysis reactors.

The system was operated for one year without major issues in the laboratory as well as a pilot in a wastewater treatment plant in Idaho. It removed contaminants at rates comparable to those in a classic aeration tanks. In addition, the microbial fuel cell could possibly be used completely independent of grid power. The researchers hope that one day it could be used in small wastewater treatment plants, such as cleaning livestock farms or in remote areas.

Despite the progress, there are still challenges to be overcome. They are complex systems that are difficult to build. At Frontis Energy we specialize in such systems and can help with piloting and commercialization.

(Photo: Wikipedia / National University of Singapore)