Freshwater scarcity is a global issue as the world population grows, increasing the need for resources, especially water. Scientists are working to develop an inexpensive method of seawater purification by utilizing graphene oxide sheets, which produce a higher volume of freshwater at a lower cost, significantly aiding in combating the global freshwater crisis..
Graphene oxide sheets are made by treating graphene sheets with functional groups and act as highly permeable membranes that stack to form layers of 2D nanochannels. Stacked graphene oxide sheets have salt rejection properties that are exhibited by means of size exclusion of ions, electrostatic interactions, and ion adsorption, while maintaining excellent water flux. Fabricating graphene oxide sheets is a relatively simple oxidation process using chemical exfoliation and water exposure, making it more economically feasible than to utilize over current desalination methods. This method of desalination could provide cheap, fresh water to areas of the world that are running out of water and contribute to solving the global freshwater crisis. This issue must be addressed, because it is an international concern, and could be dangerous to future generations. By providing solutions to the lack of clean drinking water around the world, people can live in safer and more secure environments and eventually further advance technology until having clean drinking water is no longer a concern.
Key Words — Desalination, Graphene, Graphene Oxide, Membrane, Freshwater Crisis
The global freshwater crisis
Is Desalination the Solution?
According to the BBC, water demand is expected to increase by 55% between 2000 and 2050 as populations grow and global temperatures rise [1]. The demand is driven by agriculture and irrigation, which accounts for 70% of global freshwater use, and food production will need to rise to feed the growing population. The scarcity of freshwater around the world due to sources being drained faster than they are being replenished, paired with this increase in water demand, will lead to a global freshwater crisis. It is proposed that the shortage due to water pollution and climate change could result in conflicts between nations in need. Solutions proposed at this critical point include “recycling water” and desalination, and these processes have worked for Israel. The question is whether the world can just desalinate its way out of this disaster with current methods, but graphene oxide membranes could improve desalination technologies, effectively resolving the freshwater crisis.
The crisis stems from depletion of already scarce natural resources. 70% of Earth’s surface is water, but 97.5% of it is seawater unsuitable for human consumption [1]. The water table, or the level below which the ground is saturated with water, is dropping across the world, and there’s not an infinite reservoir of water. A state such as California, which has suffered from major droughts, doesn’t have enough water to accomplish all that it desires, as aquifers below the ground aren’t instantly refilled when it rains. More than just having an inadequate water supply, these shortages will lead to wars. The Syrian civil war is an example of conflict as a result of the severe droughts, which have decimated communities. With a lack of natural resources and water, young people realize that there are no livelihood opportunities, and that leads to them being more easily radicalized [1]. Making water more obtainable could reduce the likelihood of struggle. In fact, Israel views water availability as a national security issue, and it is most important as a nation in the Middle East to provide freshwater to its citizens.
Israel has nearly perfected the principle of water conservation sustainability, effectively recapturing and recycling 86% of the water that goes down the drain. One widespread system that accounts for over 40% of Israel’s agricultural water needs is reprocessing effluent water [1]. Wastewater, including sewage, is collected to be reused as water for irrigation or methane for renewable energy instead of being dumped into the sea. The Dan Region Wastewater Treatment Plant (Shafdan), located near Tel Aviv, collects, treats and reclaims municipal wastewater in high density urban areas and industrial zones [2]. According to Igudan Environmental Infrastructure’s website, Shafdan’s main goals are, “to minimize environmental pollution and avoid health risks by constructing a sewage collector and disposal system, to prevent the discharge of raw sewage into rivers and the sea, and to contribute toward protecting and preserving the state’s dwindling water resources through appropriate treatment of sewage water for purposes of its reuse [1].” No other country has performed up to Israel’s standard in this field.
Israel is currently the global leader in desalination, the process of turning seawater into potable drinking water. IDE Technologies, specializing in the engineering and operation of industrial water treatment plants, is one of the most trusted suppliers of desalinated water and manages some of the largest seawater reverse osmosis plants in the world [3]. This Israeli company constructed the Sorek plant, which is capable of producing 624,000 meters cubed per day of potable water, sets conventions in desalination capacity and water cost and has been able to reduce the country’s water shortage problem [3]. In the BBC article, Anders Berntell, the executive director of 2030 Water Resources Group, states, “If Israel can do it, a country located in a desert, it proves that with the right technology, economic resources and political determination, you can make it happen [1].” If desalination operations have been successful at sourcing over half of Israel’s drinking water, it’s quite possible that desalination is the solution, but current technologies must be improved in order to lower the cost, decrease energy consumption, and make the new desalination method’s use more universal.
Desalination Processes
The two main categories of desalination used today are thermal and membrane-based desalination. Thermal desalination is an energy intensive method in which water is heated to evaporate the solvent from leftover salt in solution. There are different thermal methods for desalination units of varying scales, including multi-stage flash and vapor distillation. Membrane-based distillation uses semi-permeable properties of differing membranes, as well as reverse osmosis, to filter water. Common types of membrane distillation include nano-filtration, ultra-filtration, and micro-filtration. An energy and cost analysis can be done to determine the best desalination technique and what needs improvement in the creation of new technology.
Multi-stage flash (MSF) distillation is based on the principle of flash evaporation, where water is evaporated by reducing the pressure as opposed to raising the temperature. Low pressure steam is supplied by an external power plant to initially heat the seawater in the brine heater. As Figure 1 shows, the heated seawater flows into the evaporator flash chambers to go through many stages of reducing the pressure of the brine in order to reach the equilibrium vapor pressure required for boiling [4]. Finally, the flashed water vapor is cooled and condensed by colder seawater flowing in tubes to produce distillate. Through this continuous process, seawater is able to be evaporated repeatedly without adding more heat, and the heat of condensation released is used to gradually raise the temperature of more incoming seawater.
FIGURE 1 [6]
The thermal desalination process of multi-stage flash distillation
Specification changes for different parts of the operation of a multi-stage flash distillation plant can have a significant effect on efficiency and cost of production. The quantity of water vapor formation is dependent on the pressure maintained in each stage. The rate of production of distillate increases as the temperature of seawater decreases because flash range increases when the temperature of seawater decreases [4]. In addition, increasing the temperature at which the plant operates to greater than the current temperature limits of 120ºC can increase efficiency, but this also speeds up the corrosion of the metal surfaces that are in contact with the seawater. Rate is also dependent on the number of stages in the MSF plant, and according to the Advances in Seawater Desalination Technologies review article, “An increase in stages providing more heat transfer area improves the plant efficiency, but it also increases the plant capital cost [4].” While it is possible to increase the efficiency of MSF distillation plants, it is not without some cost, and other desalination procedures must be explored.
Vapor compression distillation (VCD) is the process in which the heat for evaporating seawater comes from the compression of vapor, based on the principle of reducing the boiling point temperature by reducing the pressure. The two methods used to condense water vapor are either through the utilization of a mechanical compressor or a steam jet. A mechanical compressor is electrically driven and compresses vapor in the evaporator and then condenses it inside of a tube bundle. The seawater is sprayed on the outside of the tube where it boils and produces more vapor [4]. The steam-jet unit, or a thermocompressor, extracts water vapor, compresses it and condenses the mixture on the tube walls to provide heat of condensation for the evaporation of seawater on the other side of the tube walls in the evaporator. Vapor distillation is a reliable and efficient process, carried out at a low temperature and requiring power. The high capacity compressor allows for operation at low temperatures below 70ºC, effectively reducing corrosion and scale formation. However, VCD is used for small-scale desalination units and will not account for the widespread demand for fresh water.
Membrane-based desalination can have more widespread use than thermal desalination in the future because it is more cost effective and requires less energy. Micro-, ultra- or nano-filtration with reverse osmosis is a non-thermal membrane separation process in which water is salvaged from a pressurized saline solution. The membrane is able to filter out the salt ions, allowing only the water to pass through. The main types of membranes have differentiated degrees of permeation abilities. For microfiltration, water and any dissolved salts will pass through the membrane, and it is possible for some microorganisms to permeate the membrane. Ultrafiltration allows mainly water and dissolved salt ions to pass through, but microorganisms are completely rejected [5]. At the nanofiltration level, further rejection of ions will occur, and reverse osmosis will take all of the dissolved salt out of solution to produce deionized water.
Seawater Reverse Osmosis (SWRO) is based on the concept of overcoming osmotic pressure by applying an external pressure higher than the osmotic pressure exhibited by the seawater. This causes water to flow in the opposite direction of the natural flow across a membrane, leaving the dissolved salts behind without requiring heat or phase separation changes [4]. The energy required for reverse osmosis processes is for pressurizing the seawater feed. The membrane qualities required for reverse osmosis include the strength to withstand the entire pressure drop across it, the quantity of flux, and permeate salinity. Problems from material corrosion are significantly less than that of thermal desalination because of the ambient temperature conditions [4].
Neither method of desalination is currently energy efficient enough to be a sustainable source of drinking water universally. While membrane-based desalination is less energy intensive than thermal distillation, there is still a significant amount of power required to carry out the process of seawater desalination. According to Figure 2, the operating cost in dollars per cubic meter of desalinated water produced per day is greater for MSF than SWRO technology. While the process of reverse osmosis doesn’t require thermal energy, it requires more electrical energy than multi-stage flash distillation [7]. However, the cost of the electrical energy required for SWRO is less than the total energy cost for MSF, and using a membrane is more cost competitive than using thermal methods of desalination. The cost of desalination is related to the location of the plant and the amount of energy used. The choice between the types of desalination processes comes down to how each process applies to specific situations.
FIGURE 2 [7]
Cost data for MSF and SWRO desalination
In the case of the critical point of freshwater resources, current water desalination technology is unable to provide for the scope of this crisis. For countries that lack access to natural resources, a new technology must be created to resolve cost and energy issues. The solution is to use graphene oxide as a membrane for desalination.
The next generation of desalination
Graphene, the “Wonder Material”
The term “graphene” was first used in 1986 and referred to a single layer of graphite, but despite having a name, it wasn’t properly studied for nearly two more decades. The one atom thick layer could not be isolated from the crystalline and metal surfaces where they were supported, until 2004 when Andre Geim and Kostya Novoselov found out how to separate the atoms. When they were finally able to be isolated, it allowed for inexpensive research of graphene and its excellent properties to flourish.
They are made up of a lattice of hexagonal carbon rings; a single layer of graphene involves the sp2 hybridization of the carbon atoms, which causes pi bond to delocalize their charge and leads to the development of a “cloud” of electrons. This cloud of electrons is what makes graphene an acceptable semiconductor. In a study conducted by Lee et. al in 2008, free-standing graphene layers were stressed via nanoindentation to measure their elastic response, in which graphene was established as the strongest material ever measured [8]. The electrochemical properties of graphene are due to the highly mobile electrons that delocalize from the pi orbitals. These properties combined with graphene’s high thermal conductivity not only make it an interesting substance in isolation, but also a viable candidate for many contexts of its use, including water filtration through membrane-based desalination.
According to K.A. Mahmoud et al, the ideal membrane for membrane-based desalination, such as reverse osmosis, should be resistant to fouling, have high selectivity or rejection rates, and allow for high water flux [9]. Membranes with these types of qualities would reduce the energy consumption for desalination processes by utilizing both the permeability and the selectivity of the membrane for filtration [10]. Also, the mechanical strength of the membrane must be considered in order for it to withstand the intense pressure and friction that can be experienced with membrane-based desalination, such as reverse osmosis.
Standard graphene is impermeable to all liquids and gases due to the electron density present around the carbon rings, thus modifications must be made in order for it to act as a semipermeable membrane. Nanopores can be introduced to the graphene layer by diblock copolymer templating, chemical etching, and most commonly, helium ion beam drilling [11]. In the process of helium ion beam drilling, a helium ion microscope focuses on a specific point on the graphene sheet until the the particles are ejected, which is repeated to etch a series of small and uniform holes [12]. This allows the previously impermeable graphene sheets to be semi-permeable and more selective for filtration. Nanoporous graphene has gained traction as a viable candidate in desalination and as a purification membrane due to its unique structural qualities, mechanical strength, and thinness. Monolayer nanoporous graphene is two-dimensional, being only one atom thick. The extremely small width combined with its permeability allows for high flux, meaning a higher volume of product could pass through, meaning there would be a larger output of desalinated water.
There are other carbon-based nanomaterials that have garnered support due to their similar qualities to nanoporous graphene, such as mechanical strength and thermal stability. In particular, carbon nanotubes have proven to effectively adsorb metallic ions and transport water. However, its high cost of production can make similar technologies with more industry-friendly costs, such as the aforementioned nanoporous graphene and graphene oxide sheets, more appealing.
Graphene Oxide
As ideal as nanoporous graphene seems, its “carbon backbone” can allow for further specializations to improve its performance [9]. One of these specializations, the addition of oxides, creates graphene oxide, which proves to be an even better competitor for industrial-scale desalination than graphene itself. Graphene oxide is similar in structure and properties to nanoporous graphene, with a few highlighted differences. Free-standing graphene oxide membranes are produced by chemical or thermal exfoliation of graphene. This treatment disperses functional groups, such as hydroxyl, carboxyl, carbonyl, and epoxide, to bond with the carbon atoms, producing an asymmetrical structure. The addition of these specific functional groups gives graphene oxide an additional mechanism of water desalination; while nanoporous graphene can only desalinate water through exclusion due to ion size and electrostatic interactions, graphene oxide is also able to desalinate through ion adsorption. The physical processes illustrated in Figure 3 below are nanoporous graphene and a stacked graphene oxide membrane use during desalination.
FIGURE 3 [11]
Desalination via (a)nanoporous graphene membrane vs. (b) stacked graphene oxide membrane
The process is as follows: water is adsorbed to the hydrophilic functional groups of stacked graphene oxide sheets, then promptly diffused along the hydrophobic carbon core, which creates 2D nanochannels through which the water can permeate [11][13].
As previously stated, nanoporous graphene and carbon nanotubes function similarly, but carbon nanotubes are more expensive to produce. However, graphene oxide sheets appear to have the advantage of all three. Firstly, they are relatively inexpensive to produce. An et. al state that “graphene oxide membranes are able to be mass-produced at low costs by means of vacuum filtration, chemical oxidation, or ultrasonic exfoliation [11].” They also have ionic adsorption properties that nanoporous graphene lacks. The functional groups on the surface of graphene oxide contain oxygen, which has a high electronegativity. The large negative charge on the surface of the sheet, positively charged impurities, such as sodium and other metal ions, can be adsorbed. They have the ability to form 2D channels when stacked to have the highest possible rate of flux [9]. The 2D channels through stacked graphene oxide sheets are highly selective. However, stacked graphene oxide sheets also have their challenges. Effective filtration of metal ions requires that the pores of the separate sheets be aligned, as well as there being very low interlayer spacing. When filtering water, stacked graphene oxide sheets can swell and increase in width. This results in the increase of interlayer spacing, allowing sodium ions to flow along with the water that is being filtered [11].
Production of Graphene Oxide and its Variations
There are three major applications of graphene oxide that are commonly used in membranes: as a freestanding membrane, as a surface modification to existing membrane, and as a composite mixed-matrix membrane. The BBC has called both freestanding graphene oxide membranes and mixed-matrix membranes the “most promising” for water flux [11]. Hummer’s method is one of the most popular techniques used to created free-standing graphene oxide sheets. In an experiment conducted by Zaaba et. al, the original method was modified and the new method consisted of mixing a 9:1 ratio of sulfuric acid and phosphoric acid, then adding graphite powder and potassium permanganate, then stirring the mixture for about six hours until it turned dark green. After the mixture is complete, hydrogen peroxide was slowly dropped in to remove excess potassium permanganate, resulting in an exothermic reaction, releasing heat. Once the solution cooled down, hydrochloric acid and deionized water were added and the entire solution was placed in a centrifuge. This last step was repeated three more times and then was placed in an oven at 90 degrees Celsius for twenty four hours to completely dry out the solution, leaving only the graphene oxide powder behind for further use [14]. The final chemical makeup of a single graphene oxide molecule from a graphene oxide sheet after going through Hummer’s method is shown below in Figure 4.
FIGURE 4 [15]
Chemical makeup of a graphene oxide molecule.
Free standing membranes are made by oxidizing graphite via Hummer’s method, then thermally or chemically exfoliating the sheet, then exposing it to hydroxyl, epoxide, and carboxyl functional groups. Graphene oxide can be deposited to the surface of polymer membranes by vacuum filtration, drop casting, or through layer by layer coating, which improves the mechanical strength and separation performance of the membranes [11]. Mixed matrix membranes are made by mixing a solution of graphene oxide to polymer solutions and casting them into hybrid membranes. In an investigation conducted by Ganesh et al. [11], the combination of the graphene oxide solution with polydopamine matrices significantly improved the compound’s salt rejection performance and water flux of the original polydopamine matrices.
Future applications
Graphene Oxide Sheets Addressing Current Desalination Issues
If graphene oxide sheets were to be the world’s primary desalination method, it would decrease costs significantly, allowing for desalination to be used more often in areas where it is needed. Since more seawater could be desalinated, a greater number of countries would have easier access to clean drinking water. A higher volume of water could be produced at a lower cost and it would diminish the concern of not having enough clean drinking water [16] in a lot of places. Since the 1980’s, when membrane desalination was first becoming popularly commercialized, engineers have been focusing on minimizing the cost of membrane desalination as well as minimizing the cost of constructing the plants where desalination takes place. Costs for membrane desalination over the years have been cut from approximately $1.50 per unit meter cubed to about $0.60 per unit meter cubed, while thermal process costs have remained around $2.50 per unit meter cubed since they first started being used [17]. In both membrane and thermal processes, the largest percentage of the cost is taken up by capital expenditure charges, as is shown in Figure 5 below. By using a chemical and physical process instead of a thermal process, ten to fifteen percent of energy will be saved, so the process will not be as difficult to sustain for long periods of time [17].
FIGURE 5 [17]
Price comparison between thermal and membrane desalination methods from 1972 to 2010.
Goldman Sachs specifically has taken an interest in graphene oxide sheets for their “applications from cancer treatment to water filtration,” [18] and has openly expressed interest in funding further research for their production and use.
However, even given all of the advantages of using graphene oxide sheets as the primary desalination method in the world, the process of desalination itself presents a major environmental issue: where does the salt go? No matter what method is used, salt and other impurities are removed from the seawater to produce pure water for drinking. When the salt and impurities are removed, they must go somewhere, and currently it is all being dumped onto the shores of the nearest bodies of water. The excess salt that is dumped onto nearby shores is washed back into the ocean and creates water with an even higher salt content, essentially creating Dead Seas wherever desalination is used, making it so that nothing can live in those bodies of water. According to Jeffrey Graham from the Scripps Institute of Oceanography’s Center for Marine Biotechnology and Biomedicine, “the salty sludge leftover after desalinization for every gallon of freshwater produced, another gallon of doubly concentrated salt water must be disposed of can wreak havoc on marine ecosystems if dumped willy-nilly offshore.”[19] This essentially means that if the salt is dumped out without a second thought, the additional salt could negatively affect the organisms that are currently living in that water, destroying the existing ecosystem. The more this process is used, the higher the existing salt content rises, and eventually the salt content will become so high that it will kill any organisms that once lived there. Once this issue with desalination itself is solved, graphene oxide sheets may truly become the most economically feasible option for producing clean drinking water from the available seawater. Of course, even once the graphene oxide sheet method of desalination is implemented, it only solves one aspect of the issue. This would solve the problem of purification, but not distribution, so not every place in the world that needs water would necessarily have access to it. It would make it easier for countries with the resources to obtain this technology to continuously have access to potable water, but for any areas of the world that could not obtain graphene oxide sheets or are not anywhere near seawater to purify, they would still have issues with having enough drinking water.
Graphene oxide sheets to solve the freshwater crisis
With graphene oxide sheets allowing more seawater to be purified, many countries like Israel will have easier access to enough drinking water for its residents, meaning that the freshwater crisis could be solved. Graphene oxide sheets are a better alternative to many of the other options in thermal and membrane desalination methods for multiple reasons, from reducing cost and minimizing electricity use to maximizing water volume output. The amount of pure water graphene oxide sheets can produce at the relatively low cost it takes to produce and use the technology would play a large role in combating the lack of drinking water throughout the world, as it would allow a larger amount of clean drinking water to be produced. Of course, this method is not perfect yet since there is no place to properly dispose of the salt and it does not solve the issue of even water distribution, but research has shown that, in terms of desalination methods, graphene oxide sheets are one of the best options in terms of cost and volume of output. This technology is especially crucial to continue researching because of its international importance and the role it could play in improving the quality of life around the world for years to come. It could have the ability to help solve a problem as big as the freshwater crisis by providing a relatively cheap and large amount of drinking water in areas of the world that are lacking. The first step is to implement graphene oxide technology where it is needed and once that task is completed, future generations will be one step closer to being able to live in safer and more secure environments where having clean drinking is no longer a concern.
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