Inclusion of Graphene Oxide in Desalination Film for Saline Water Purification

Chemistry Research Investigation- Empirical Essay Draft

Claim

Development of organic and inorganic nanomaterials is important to meet a range of contemporary needs, including consumer products, healthcare, transportation, energy and agriculture.

Rationale:

With the ever-decreasing supply of fresh water, there is a great demand for treatments in maintaining water supply. With salt water covering 97% of Earth’s water supply, it has great potential in providing fresh water for consumption. However, methods of desalination such as Reverse Osmosis only provides 1% of the freshwater supply and are cost inefficient. Therefore, nanomaterial and nanotechnology have been researched to meet the needs of fresh water supply.

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The nano-material, graphene, is one of the potential solutions. It is non-reactive, strong and impermeable to gas or liquid, viable to desalination. Graphene membranes obtained by graphene oxide layers covered by graphene oxide solution. The graphene embedded carbon nanotubes can function as nano-filters, which can used for contaminate rejection in polluted water, salt ions removal and acts as antifouling agent. Therefore, derive the question of how reasonable is it to include graphene oxide in desalination film for saline water purification, in terms of effectiveness(antifouling and water flux rate) and cost efficiency.

Research question:

How will the inclusion of graphene oxide in desalination film for saline water purification, in terms of effectiveness (antifouling and water flux rate) and cost efficiency of the nanomaterial?

Background Theory: define water flux and anti fouling and dechlorination

Graphene membranes are 2D array of graphene oxide molecules that are able to effectively perform molecule separation in gas or liquid state. The graphene membrane has demonstrated to have high water flux range, which is the flow rate of filtrate of water passing through the membrane, measured in add units. Graphene membranes has have strong bond between graphene sheets and proteins that acts as an effective anti-fouling agents. Both flux rate and antifouling ability relates and will be used to analyse effectiveness of the nanomaterial in the research question.

The water flux rate are greatly influenced by the membrane thickness, the greater the thickness the lower the flux rate, therefore they are inversely proportional. Since, graphene membranes has nanothickness, it is promising in increasing of water permeability. Benefits of graphene membrane compared to the reverse osmosis membrane include nano thickness and greater mechanical strength, as it allows the water to be move with lower pressure and in different environments, hence more efficient. Desalination plants usually utilise reverse osmosis, which is a process where seawater move across polymer membrane under pressure. Polymer molecules intertwine and leaves gaps for water molecules but enable sodium and chloride ions, which is salt, to pass as they are half a nanometers (half a billionth of a metre) across.

Except for the fact that the outlet in which the water pass through would required to be punctured, graphene works the same as reverse osmosis. Therefore, bringing 3 potential benefits. First, are able engineered to be of the optimum size (1.2 nm) across, which allows water flow more smoothly than a polymer membrane, while able to hold back chloride ions, sodium ions. Second, the holes would all be of the consistent, hence enabling sodium and chloride ions to pass. Third, since polymer are snake structured graphene’s straight structure would maximise the speed in which the water molecules pass. (The economist, 2013). Furthermore, experimental studies have conducted to explore graphene membrane for performance. Using molecular dynamics simulations, it was found that water flux rate ranges from 10−100 L/ cm2/day/MPa and maintains salt rejection at 2 to 3 orders of magnitude higher than reverse osmosis membranes. (Cohen-Tanugi & Grossman, 2012). Therefore, simulations demonstrates that graphene membrane can efficiently increase water permeability at magnitude higher than reverse osmosis while actively rejecting salt ions.

 

Analysis & Interpretation:

In terms of cost efficiency, it have been identified a by G2O Water, a graphene membrane can be used to reduce energy costs of any filters by 80-90% and decrease water costs by 40%, which is equivalent in reducing the cost of production by $30 million per year for desalination plants, that produces 189270589 litres a day. (The Graphene Council, 2019) According to Stetson, seawater in graphene membrane would require pressure of approximately 28 bar (400lb per square inch), which equals half to a third the amount pressure required for other plants. Hence, estimated to decrease the cost of production by 15-20%, while having greater outputs. (The Economist, 2013).

Additionally, polyamide membranes which has low production price of approximately of $1 per square foot, degrades quickly when exposed to chlorine. Hence, increasing the production cost as two additional processes must be undertaken to which is the removal of chlorine before the process then addition of chlorine into drinking water to act as disinfectant. Furthermore, without chlorine, the membranes are prone to growing biological matter that decrease the efficiency of membrane by clotting, therefore the graphene oxide membranes is a more economically viable option. With production cost at $4-$5 per square, graphene oxide membranes, considering its effectiveness and no need of additional process would be more cost efficient than polyamide as is not significantly more expensive. The nanothickness of the graphene oxide membranes allows molecule to be transported through membrane more efficiently than the polyamide. Therefore, reduced the energy requires to pump water through them as desalination of ground water and seawater would require 46 % and 15% less energy, respectively. (Sumner & Thomas, 2016). The lower energy requirements and exclusion of chlorine processes, the graphene membranes are proven to be more cost efficient than traditional methods .

Below is comparison diagrams on water vapour flux and salt rejection of the graphene and PDFE membranes under different conditions, assessing the destination performance.

 

Diagram 1: Comparison of PTFE membrane and Graphene membrane

The Diagram 1, is the comparison of membranes under conditions of 70 g/L of sodium chloride solution for 72 hours, where (a) is PTFE membrane and (b) permeable graphene membrane. It can be identified that the for both PTFE membrane and Graphene membrane fulfil 100% salt rejection, while the flux differ for the two. For PTFE membrane it consistently decrease overtime from 40 L m2/ hr to 30L m2/ hr, whereas graphene membrane maintained at consistent rate of 50 L m2/ hr. Therefore, despite no difference in salt rejection, graphene is more efficient in water flux.

Diagram 2: Comparison of PTFE membrane and Graphene membrane

But, when compared in second diagram, in the saline solutions containing surfactants graphene membrane are more effective. In the second diagram is comparisons of membrane under conditions70 g/ L sodium solution and 1 mM sodium dodecyl sulphate. For the PTFE membrane, over 32 h there was a obvious decrease in water flux performance from 40 L m2/hr to 14.2Lm2/ hr and salt rejection also decreased, from 100 to 97.1%. While, the graphene membrane demonstrated consistently higher water flux 50 L m2/ hr and substantial salt rejection of 100% in the duration of 72 hours (Chemical Engineering, 2019). Therefore, results demonstrate that the permeable graphene membranes has greater water flux and salt rejection performance than PDFE membranes in direct contact membrane distillation processes conditions in laboratories (Seo et.al, 2018)

Furthermore, based on molecular dynamics simulations, it was found that are graphene membranes are able to transport water at 66 L per cm2, with greater than 9o% salt rejection. This when compared the reverse osmosis membrane, that transports water at 0.01–0.05 L per cm2, with similar salt rejection, graphene membranes is more viable. (Wang & Karnik, 2012)

However, challenges still exist to make graphene membranes viable at a large scale. Firstly, the graphene membranes are unable reject salt at high pressure as salt ions is more affected by pressure increases than water molecules, due to salty ion’s greater molecular volume . Hence, manufacturing of large graphene membranes with low cost and maintain structure with a narrow size distribution are be a challenge. Furthermore, they need to be modified to remove salt ions at high flow rates (i.e.10) (Wang & Karnik, 2012) Additionally , there are some challenges in fabricating and applying graphene oxide based nano-filters for water desalination. The challenges include mechanical challenges if nano-filters are in the form of nano-sheets, cost plans, surface limitations, and production.Therefore, there are more research done for the betterment of the society

Conclusion & Evaluation:

In summary, graphene nanomaterial application in desalination membrane surpasses the traditional methods of desalination, in terms of cost efficiency and effectiveness in laboratory conditions. As nano-filter’s mechanical strength and properties of the membrane allows efficient desalination. However, scientists identifies that a range of mechanical challenges that must be overcame to make graphene membrane more viable. The potent improvements that can be carry out include: extended research how to maintain graphene efficiency under high pressures and the actual manufacturing of graphene at a large scale while maintaining its effectiveness and the actual application of graphene in real life as most results are derived from simulations. Hence, if these 3 improvements are applied the claim then better evaluation of the the claim can be done.

Four credible educational results are utilised to verify the viability to use graphene membrane for desalination of saline water. Three sources supports that the graphene based membrane would greatly reduce the cost of desalination due to their lower energy requirements, hence have high cost efficiency. And two other sources supporting the effectiveness of graphene. Hence, the response to the claim “How reasonable is it to include graphene oxide in desalination film for saline water purification, in terms of effectiveness and cost efficiency of the nanomaterial?”, is that the graphene membrane indeed is a effective and cost efficient method to desalinate water and meet the contemporary need for fresh water in laboratory and controlled environment. However, there is development that needed to be done to make application of graphene membrane at a large scale in real life.

Therefore, in response to the claim “Development of organic and inorganic nanomaterials is important to meet a range of contemporary needs, including consumer products, healthcare, transportation, energy and agriculture” is validity of the claim can only be answered when extended research in the field are conducted as the simulations, where variables are controlled, may not apply to real life. This demands further research and developments to be undertaken.

Reference List

 

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