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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

Image source: Wikipedia

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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)

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Biochar from waste removes pharmaceuticals from wastewater

Biochar is a coal-like substance that is mainly made from agricultural waste products. It can remove contaminants such as pharmaceuticals from treated wastewater. This is the result of research carried out by scientists of the Pennsylvania State University and the Arid Lands Agricultural Research Center in Arizona. The biochar was made from two agricultural residues common in the US: cotton and guayule.

To test the ability of biochar to adsorb pharmaceuticals from treated wastewater, the scientists compared three common compounds. During adsorption, a material like a pharmaceutical adheres to the surface of solid biochar particles. In the case of absorption, in turn, one material is taken up into another, such as in a sponge.

The shrub guayule grows in the dry southwestern US and its waste was used for the biochar tested. Among bonatics, it is also called Parthenium argentatum. The shrub is cultivated as a source of rubber and latex. The plant is chopped to the ground and its branches crushed to extract the latex. The dry, mushy, fibrous residue that remains after the stalks are chopped up to extract the latex is called bagasse.

The results are important as they demonstrate the potential of biochar made from abundant agricultural waste. If it wasn’t re-used, this waste would have to be disposed at a cost. The production of biochar is an inexpensive additional processing step to reduce contamination in treated wastewater used for irrigation.

At the same time, most wastewater treatment plants are currently not equipped to remove emerging contaminants such as pharmaceuticals. If these toxic compounds were removed by biochar, the wastewater could be reprocessed in irrigation systems. This re-use is crucial in regions where water scarcity is a constraint for agricultural production.

The pharmaceutical compounds used in the study were: sulfapyridine, an antibacterial drug commonly used in veterinary medicine; docusate, a widely used laxative and stool softener, and erythromycin, an antibiotic used to treat infections and acne.

The results, published in the journal Biochar, suggest that biochar can effectively adsorb agricultural waste. The biochar obtained from cotton processing waste was a lot more efficient. It adsorbed 98% of the docusate, 74% of the erythromycin and 70% of the sulfapyridine from aqueous solutions. In comparison, the biochar obtained from guayule residues bagasse adsorbed 50% of the docusate, 50% of the erythromycin and only 5% of the sulfapyridine.

Research found that a temperature rise from about 340°C to about 700°C in the oxygen-free pyrolysis process used to convert agricultural waste materials to biochar resulted in a improved capacity for adsorption.

To date, there have been no studies on the use of guayule bagasse to make biochar and remove contaminants, nor are there any for cotton processing waste. Some research has been carried out into the possible removal of other contaminants. However, this is the first study to use cotton gin waste specifically to remove pharmaceuticals from water.

The research is more than theoretical. At Frontis Energy we hope that the technology will soon be available on industrial scale. With cotton gin waste being widespread even in the poorest regions, we believe this source of biochar holds great promise for decontaminating water. The next step would be to develop a mixture of biochar material to adsorb a wider variety of contaminants from water.

(Photo: Wikipedia)

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Highly efficient desalination using carbon nanotubes

Separating liquid compartments is not only important for generating energy in biological cells, respiration that is, but also for electrochemical cells and desalination through reverse osmosis and other processes. Therefore, scientists and engineers intensively research this field. We have already reported in several posts about promising attempts to make membranes cheaper and more effective. New nanomaterials have also been developed.

As a result of climatic changes caused by global warming, water scarcity is increasingly becoming a problem in many parts of the world. Settlements by the sea can secure their supply by desalinating water from seawater and brackish water sources. The process, however, is very energy intensive.

Now, researchers at California’s Lawrence Livermore National Laboratory (LLNL) have developed artificial pores made of carbon nanotubes that remove salt from water so efficiently that they are comparable to already available commercial desalination membranes. These tiny pores are only 0.8 nanometers in diameter. A human hair with a diameter of 60,000 nm. The researchers published the results in the journal Science Advances.

The predominant technology used to remove salt from water is reverse osmosis. A thin-film composite membrane (TCM) is used to separate water from ions. Hitherto the performance of these membranes has, however, been unsatisfactory. There is, for example, always a tradeoff between permeability and selectivity. In addition, exisiting membranes often show insufficient ion repulsion and are contaminated by traces of impurities. This requires additional cleaning stages, which again increase energy costs.

As is so often the case, the researchers got inspired by nature. Biological water channels, also known as aquaporins, are a great model for the structures that can improve performance. These aquaporins have extremely narrow internal pores that compress the water. This enables extremely high water permeability with transport rates of more than 1 billion water molecules per second per pore. Due to the low friction on the inner surfaces, carbon nanotubes represent one of the most promising approaches for artificial water channels.

The research group developed nanotube porins that insert themselves into artificial biomembranes. These engineered water channels simulate the functionality of aquaporin channels. The researchers measured the water and ion transport through their artificial porins. Computer simulations and experiments using the artificial porins in lipid membranes showed improved flux and strong ion repulsion in the channels of carbon nanotubes.

This measurement method can be used to determine the exact value of the water-salt permselectivity in such narrow carbon nanotubes. Atomic simulations provide a detailed molecular view of the novel channels. At Frontis Energy, we are excited about this promising approach and hope to see a commercial product soon.

(Image: Wikipedia)

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Reverse electrodialysis using Nafion™ membranes to produce renewable energy

In the order to address the global need for renewable and clean energy sources, salinity-gradient energy harvested by reverse electrodialysis (RED) is attracting significant interest in recent years. In addition, brine solution coming from seawater desalination is currently considered as a waste; however thanks to its high salinity it can be exploited as a valuable resource to feed RED. RED is an engineered adaptation of nature’s osmotic energy production where ions flow pass the cell membrane in order to produce the universal biological currency ATP. This energy is also harvested by the RED technology.

Now, more than ever there is need for sustainable and environmentally friendly technological solutions in order to keep up with ever growing demand for clean water and energy. The traditional linear way “produce and throw away” does no longer serve the society anymore and the new approach of circular economy has take a place, where any waste can be considered as a valuable resource for another process. In this respect, reverse electrodialysis is a promising electromembrane-based technology to generate power from concentrated solutions by harvesting the Gibbs free energy of mixing the solutions with different salinity. In particular, brine solutions produced in desalination plants, which is currently considered as a waste, can be used as concentrated streams in RED stack.

Avci et al. of the University of Calabria, Italy, have recently published their solution for brine disposal using RED-stack. They have realized that in order to maximize generated power, the high permselectivity and ion conductivity of membrane components in RED are essential. Although Nafion™ membranes are among the most prominent commercial cation exchange membrane solutions for electrochemical applications, no study has been done in its utilization toward RED processes. This was the first reported RED stack using Nafion™ membranes.

A typical RED unit is similar to an electrodialysis (ED) unit, which is a commercialized technology. ED uses a feed solution and the electrical energy, while producing concentrate and dilute, separately. On the other side, RED uses concentrated and dilute solutions that are mixed together in a controlled manner in order to produce spontaneously electrical energy. In a RED stack, repeating cells comprised of alternating cation and anion exchange membranes that are selective for anions and cations. The salinity gradient over each ion exchange membrane creates a voltage difference which is the driving force for the process. The ion exchange membranes are one of the most important components of a RED stack.

The performance of Nafion™ membranes (Nafion™ 117 and Nafion™ 115) have been evaluated under a high salinity gradient conditions for the possible application in RED. In order to simulate the natural environments of RED operation, NaCl solution as well as multicomponent NaCl + MgCl2 have been tested.

Gross power density under high salinity gradient and the effect of Mg2+ on the efficiency in energy conversion have been evaluated in single cell RED using Nafion™ 117, Nafion™ 115, CMX and Fuji-CEM-80050 as cation exchange membranes. Two commercial cation exchange membranes – CMX and Fuji-CEM 80050, frequently used for RED applications, have served as benchmark.

The results show that under the condition of 0.5 M / 4.0 M NaCl solutions, the highest Pd,max was achieved using Nafion™ membrane. This result is attributed to their outstanding permselectivity compared to other CEMs. In the presence of Mg2+ ions, Pd,max reduction of 17 and 20% for Nafion™ 115 and Nafion™ 117 were recorded, respectively. Both membranes maintained their low resistance; however a loss in permselectivity was measured under this condition. Even though, it was reported that Nafion™ membranes outperformed other commercial membranes such as CMX and Fuji-CEM-80050 for RED application.

(Photo: Wikipedia)

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Global wastewater resources estimated

In our last post on water quality in China, we pointed out a study that shows how improved wastewater treatment has a positive effect on the environment and ultimately on public health. However, wastewater treatment requires sophisticated and costly infrastructure. This is not available everywhere. However, extracting resources from wastewater can offset some of the costs incurred by plant construction and operation. The question is how much of a resource is wastewater.

A recent study published in the journal Natural Resources Forum tries to answer that question. It is the first to estimate how much wastewater all cities on Earth produce each year. The amount is enormous, as the authors say. There are currently 380 billion cubic meters of wastewater per year worldwide. The authors omitted only 5% of urban areas by population.

The most important resources in wastewater are energy, nutrients like nitrogen, potassium and phosphorus, and the water itself. In municipal wastewater treatment plants they come from human excretions. In industry and agriculture they are remnants of the production process. The team calculated how much of the nutrient resources in the municipal wastewater is likely to end up in the global wastewater stream. The researchers come to a total number of 26 million tons per year. That is almost eighty times the weight of the Empire State Building in New York.

If one would recover the entire nitrogen, phosphorus and potassium load, one could theoretically cover 13% of the global fertilizer requirement. The team assumed that the wastewater volume will likely continue to increase, because the world’s population, urbanization and living standards are also increasing. They further estimate that in 2050 there will be almost 50% more wastewater than in 2015. It will be necessary to treat as much as possible and to make greater use of the nutrients in that wastewater! As we pointed out in our previous post, wastewater is more and more causing environmental and public health problems.

There is also energy in wastewater. Wastewater treatment plants industrialized countries have been using them in the form of biogas for a long time. Most wastewater treatment plants ferment sewage sludge in large anaerobic digesters and use them to produce methane. As a result, some plants are now energy self-sufficient.

The authors calculated the energy potential that lies hidden in the wastewater of all cities worldwide. In principle, the energy is sufficient to supply 500 to 600 million average consumers with electricity. The only problems are: wastewater treatment and energy technology are expensive, and therefore hardly used in non-industrialized countries. According to the scientists, this will change. Occasionally, this is already happening.

Singapore is a prominent example. Wastewater is treated there so intensively that it is fed back into the normal water network. In Jordan, the wastewater from the cities of Amman and Zerqa goes to the municipal wastewater treatment plant by gravitation. There, small turbines are installed in the canals, which have been supplying energy ever since their construction. Such projects send out a signals that resource recovery is possible and make wastewater treatment more efficient and less costly.

The Frontis technology is based on microbial electrolysis which combines many of the steps in wastewater treatment plants in one single reactor, recovering nutrients as well as energy.

(Photo: Wikipedia)

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China has improved inland surface water quality

During the last decades, China has achieved rapid development in technology and economics, however at a huge environmental cost. The deterioration of inland surface water quality is considered one of the most serious environmental threats to ecosystem and ultimately public health.

Since 2001, China made major efforts to tighten the application of environmental rules in order to stop water pollution emitted by cities, farm and industry. According to the government’s “10th National Five-Year Plan”, large investments were made for pollution control and wastewater discharge regulation systems.

Small research studies showed that with this campaign, Chinese’s lakes and rivers got cleaner. Since then water quality has improved significantly − however, other parts of country still have problems with polluted water.

Now, a team of researchers of the at the Chinese Academy of Sciences in Beijing, has published one of the most comprehensive national investigation of China’s surface water quality in the renown journal Science. The researchers investigated all regions of the country to learn how surface water responds to multiple driving forces over time and space. Their report covers the assessment of water quality by means of three parameters: dissolved oxygen level (DO), chemical oxygen demand (COD) and ammonium nitrogen (N) in inland surface waters. They performed monthly site-level measurements at major Chinese rivers and lakes across the country between 2003 and 2017.

Due to regional variations in China’s inland water quality as well as the dynamics in multiple anthropogenic pollution sources, such studies are crucially important to identify the necessary regulation measures and water quality improvement policies adapted to ecosystem sustainability at all diverse country regions.

The results show that during the past 15 years, annual mean pollution concentration has declined across the country at significant linear rates or was maintained at acceptable levels. Consequently, the annual percentage of water quality have increased by 1.77% for COD, 1.83% for N and 1.45% for DO per year. While China has not yet implemented environmental water standards, the study shows that China’s water quality is improving nonetheless.

The best news is that the notoriously high pollution levels have declined as cities and industry have worked to clean up and reduce their discharges. According to the authors, the most visible alleviation was noticed in northern China, while in the western region of the country water quality remained at their low pollution level throughout the observation period. The reason is likely that pollution is caused by human activity, of which there is less in those parts of the country.

Despite large efforts toward decreased pollution discharges, urban areas are still considered as the major pollution centers. These areas face additional pressure due to the constant migration and fast urbanization of the rural regions. Especially in northern China, with high-density human activity and exploding urbanization, achieving and maintaining a clean environment is a permanent struggle.

To further reduce pollution and improve water quality, the authors recommend that future activities focus on water management systems and the water pollution control. For both, the central government issued guidelines to control and improve water use and pollution discharge at regional and national levels for 2020 and 2030.

At Frontis Energy, we certainly support activities in China that help improving the countries water quality and public health. The Frontis technology gives its user an incentive to to clean wastewater before discharge by extracting its energy. Our patent pending solutions are based on microbial electrolysis which helps to extract energy from wastewater and apply in particular to China.

Mima Varničić, 2020

(Photo: Gil Dekel / Pixabay)

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Ammonia energy storage #2

Recently, we reported on plans by Australian entrepreneurs and their government to use ammonia (NH3) to store excess wind energy. We proposed converting ammonia and CO2 from wastewater into methane gas (CH4), because it is more stable and easier to transport. The procedure follows the chemical equation:

8 NH3 + 3 CO2 → 4 N2 + 3 CH4 + 6 H2O

Now we have published a scientific article in the online magazine Frontiers in Energy Research where we show that the process is thermodynamically possible and does indeed occur. Methanogenic microbes in anaerobic digester sludge remove the hydrogen (H2) formed by electrolysis from the reaction equilibrium. As a result, the redox potentials of the oxidative (N2/NH3) and the reductive (CO2/CH4) half-reactions come so close that the process becomes spontaneous. It requires a catalyst in the form of wastewater microbes.

Pourbaix diagram of ammonium oxidation, hydrogen formation and CO2 reduction. At pH 7 and higher, the oxidation of ammonium coupled to methanogenesis becomes thermodynamically possible.

To prove our idea, we first searched for the right microbes that could carry out ammonia oxidation. For our experiments in microbial electrolysis cells we used microorganisms from sediments of the Atlantic Ocean off Namibia as starter cultures. Marine sediments are particularly suitable because they are relatively rich in ammonia, free from oxygen (O2) and contain less organic carbon than other ammonia-rich environments. Excluding oxygen is important because it used by ammonia-oxidizing microbes in a process called nitrification:

2 NH3+ + 3 O2 → 2 NO2 + 2 H+ + 2 H2O

Nitrification would have caused an electrochemical short circuit, as the electrons are transferred from the ammonia directly to the oxygen. This would have bypassed the anode (the positive electron accepting electrode) and stored the energy of the ammonia in the water − where it is useless. This is because, anodic water oxidation consumes much more energy than the oxidation of ammonia. In addition, precious metals are often necessary for water oxidation. Without producing oxygen at the anode, we were able to show that the oxidation of ammonium (the dissolved form of ammonia) is coupled to the production of hydrogen.

Oxidation of ammonium to nitrogen gas is coupled to hydrogen production in microbial electrolysis reactors. The applied potentials are +550 mV to +150 mV

It was important that the electrochemical potential at the anode was more negative than the +820 mV required for water oxidation. For this purpose, we used a potentiostat that kept the electrochemical potential constant between +550 mV and +150 mV. At all these potentials, N2 was produced at the anode and H2 at the cathode. Since the only source of electrons in the anode compartment was ammonium, the electrons for hydrogen production could come only from the ammonium oxidation. In addition, ammonium was also the only nitrogen source for the production of N2. As a result, the processes would be coupled.

In the next step, we wanted to show that this process also has a useful application. Nitrogen compounds are often found in wastewater. These compounds consist predominantly of ammonium. Among them are also drugs and their degradation products. At the same time, 1-2% of the energy produced worldwide is consumed in the Haber-Bosch process. In the Haber-Bosch process N2 is extracted from the air to produce nitrogen fertilizer. Another 3% of our energy is then used to remove the same nitrogen from our wastewater. This senseless waste of energy emits 5% of our greenhouse gases. In contrast, wastewater treatment plants could be net energy generators. In fact, a small part of the energy of wastewater has been recovered as biogas for more than a century. During biogas production, organic material from anaerobic digester sludge is decomposed by microbial communities and converted into methane:

H3C−COO + H+ + H2O → CH4 + HCO3 + H+; ∆G°’ = −31 kJ/mol (CH4)

The reaction produces CO2 and methane at a ratio of 1:1. Unfortunately, the CO2 in the biogas makes it almost worthless. As a result, biogas is often flared off, especially in places where natural gas is cheap. The removal of CO2 would greatly enhance the product and can be achieved using CO2 scrubbers. Even more reduced carbon sources can shift the ratio of CO2 to CH4. Nevertheless, CO2 would remain in biogas. Adding hydrogen to anaerobic digesters solves this problem technically. The process is called biogas upgrading. Hydrogen could be produced by electrolysis:

2 H2O → 2 H2 + O2; ∆G°’ = +237 kJ/mol (H2)

Electrolysis of water, however, is expensive and requires higher energy input. The reason is that the electrolysis of water takes place at a relatively high voltage of 1.23 V. One way to get around this is to replace the water by ammonium:

2 NH4+ → N2 + 2 H+ + 3 H2; ∆G°’ = +40 kJ/mol (H2)

With ammonium, the reaction takes place at only 136 mV, which saves the respective amount of energy. Thus, and with suitable catalysts, ammonium could serve as a reducing agent for hydrogen production. Microorganisms in the wastewater could be such catalysts. Moreover, without oxygen, methanogens become active in the wastewater and consume the produced hydrogen:

4 H2 + HCO3 + H+ → CH4 + 3 H2O; ∆G°’ = –34 kJ/mol (H2)

The methanogenic reaction keeps the hydrogen concentration so low (usually below 10 Pa) that the ammonium oxidation proceeds spontaneously, i.e. with energy gain:

8 NH4+ + 3 HCO3 → 4 N2 + 3 CH4 + 5 H+ + 9 H2O; ∆G°’ = −30 kJ/mol (CH4)

This is exactly the reaction described above. Bioelectrical methanogens grow at cathodes and belong to the genus Methanobacterium. Members of this genus thrive at low H2 concentrations.

The low energy gain is due to the small potential difference of ΔEh = +33 mV of CO2 reduction compared to the ammonium oxidation (see Pourbaix diagram above). The energy captured is barely sufficient for ADP phosphorylationG°’ = +31 kJ/mol). In addition, the nitrogen bond energy is innately high, which requires strong oxidants such as O2 (nitrification) or nitrite (anammox) to break them.

Instead of strong oxidizing agents, an anode may provide the activation energy for the ammonium oxidation, for example when poised at +500 mV. However, such positive redox potentials do not occur naturally in anaerobic environments. Therefore, we tested whether the ammonium oxidation can be coupled to the hydrogenotrophic methanogenesis by offering a positive electrode potential without O2. Indeed, we demonstrated this in our article and have filed a patent application. With our method one could, for example, profitably remove ammonia from industrial wastewater. It is also suitable for energy storage when e.g. Ammonia synthesized using excess wind energy.