Huang Haojie and a group of researchers from the Seawater Desalination Research Institute are discussing some issues in seawater desalination technology.
Theoretical calculations prove that graphene can be used in seawater desalination, and the single-layer nanopore two-dimensional membrane produced has higher selective separation efficiency than traditional seawater desalination membranes.
However, the grain boundaries existing inside large-area graphene will reduce the mechanical properties of graphene, and the process of introducing nanopores will further reduce the mechanical properties, causing the separation film to easily undergo local rupture, greatly reducing the separation efficiency and separation selectivity.
Of course, this problem is nothing to Galaxy Technology, and atomic manipulation technology can perfectly solve this problem.
Current graphene desalination membranes are divided into two categories.
One type is the single-atom-thick nanoporous film studied by MIT professor Rohit Karnik’s team.
However, the mechanical strength of single-atom-thick graphene is weak, so the graphene used in experimental studies is supported by polymer films.
And sub-nanometer pores are directly introduced into graphene through high-energy electron beam bombardment or oxygen plasma etching. The pore size distribution range is wide, which greatly reduces the separation efficiency, so it cannot be applied in practice.
The other category is the graphene oxide film studied by the team of Professor Andre Geim, the winner of the Explosives Physics Prize at the University of Manchester.
Graphene oxide is easy to mass-produce, but after the graphene oxide film is immersed in the solution, the graphene oxide sheets will absorb water and expand the interlayer spacing, which reduces the seawater desalination efficiency. Therefore, existing research work mainly focuses on how to control graphene oxide.
The interlayer spacing between the ene sheets.
In addition, there are also relevant research results in China.
That is a binary structure graphene film that combines graphene nanosieves and carbon nanotubes. This film combines the selective separation efficiency of the former with the strength advantages of the latter.
Single-atom-thick nanoporous two-dimensional materials have the smallest water transmission resistance and the largest water penetration flux, making them ideal materials for constructing thin and efficient seawater desalination membranes.
However, applying thin two-dimensional materials to actual seawater desalination faces two major problems.
The first is how to prepare a large-area, crack-free two-dimensional nanopore film with excellent mechanical strength and flexibility.
The second is how to introduce sub-nanometer pores with high density and uniform pore size distribution inside the film to achieve efficient and selective passage of water molecules and effective interception of salt ions/organic molecules.
Regarding the first problem, carbon nanotubes have excellent mechanical properties and are similar in structure to graphene, and the two can interact through -bonds and van der Waals forces.
The carbon nanotube film formed by overlapping carbon nanotubes is a porous network structure (average pore diameter 3oo nanometers) film, which not only perfectly matches the structure of graphene, but also does not affect water permeability.
Therefore, domestic research institutions thought of combining nanopore graphene with carbon nanotubes to make up for the shortcomings of the former.
They first grew a single layer of graphene on copper foil, then covered some areas on it with a network of interconnected carbon nanotubes. After etching away the copper foil, they obtained a graphene film supported by carbon nanotubes.
In order to obtain high-density sub-nanometer pores with uniform pore size distribution, they grew a layer of mesoporous silicon oxide with uniform pore size distribution (average pore size 2 nanometers) on the surface of graphene as a mask, and used oxygen plasma etching to remove the mesoporous silicon oxide.
Graphene within pores.
The longer the oxygen plasma etching time is, the more graphene is etched away and the larger the pore size of the graphene is.
In this way, the pore size of the graphene nanosieve can be controlled by adjusting the oxygen plasma etching time. When the etching time is controlled at 1o seconds, the pore size is o.63 nanometers, which can effectively allow water molecules with a diameter of o.32 nanometers to pass through
And blocks salt ions with a diameter of o.7 nanometers.
This kind of film can be suspended, bent, and stretched without polymer support without obvious cracks.
Test and calculation results show that the new film can withstand 38o.6mpa stress and has a Young's modulus of 9.7gpa, which is three times that of a carbon nanotube film and equivalent to 2.4 times the tensile stiffness and 1,000 times the tensile stiffness of a nanohole graphene film.
Bending stiffness.
As a result, they made a large and strong graphene mesoporous film.
So what about its filtering performance?
Within 1o seconds, the permeability of the etched graphene nanosieve/carbon nanotube film can reach 2o.6 liters per square meter per hour per atmosphere.
After 24 hours of penetration, the salt ion rejection rate is greater than 97%.
Compared with the commercial cellulose triacetate desalination membrane, the water permeability of the new graphene nanosieve/carbon nanotube membrane is increased by 1oo times, and the anti-pollution ability is stronger.
And because it is not restricted by the internal concentration polarization effect, the membrane can still maintain a high water permeability in a high-concentration salt environment.
The new graphene nanosieve/carbon nanotube film made by domestic research institutions is strong and durable without polymer support, and has a variety of permeation efficiency advantages.
Of course, this seawater desalination technology is not without its problems, that is, it is difficult to mass produce. If the mass production problem is solved, it can be applied on a large scale.
When Huang Haojie set his sights on graphene seawater desalination technology, he recruited this domestic research team.
"Dr. Yuan, are there any other problems with your seawater desalination membrane?"
Hearing Huang Haojie's words, Yuan Quan smiled and replied:
"There is no big problem. With the help of atomic manipulation technology, the composite film of graphene and carbon nanotubes has already achieved initial mass production, and the quality of our film is very strong."
She really admired Huang Junjie. Atomic manipulation technology was definitely a revolutionary technology.
If the graphene carbon nanotube composite film developed by her previous team had a seawater desalination efficiency of 1 per square meter, then the graphene carbon nanotube film produced using atomic manipulation technology would have a seawater desalination efficiency of 1o per square meter.
The reason why there is such a big gap is that there are no defects in graphene-carbon nanotube composite films made with atomic manipulation technology.
Although this new type of film is also a graphene-carbon nanotube composite film, its strength is nearly ten times stronger. The increase in strength can increase atmospheric pressure to force seawater to desalinate faster.
At present, one square meter of membrane can produce 80 cubic meters of fresh water in one hour, and 700,000 cubic meters of fresh water in one year.
In other words, if we want to achieve an annual output of 400 billion cubic meters of fresh water, we only need 57,000 square meters of membrane.
Of course, the service life of this film is about 4100 hours, which means it needs to be replaced every six months.
The production cost of this seawater desalination film is about 1500 yuan per square meter.
Calculating this, a factory with an annual output of 4,000 million cubic meters of fresh water needs to purchase 171 million yuan in seawater desalination membranes every year.
Of course, Huang Haojie will not sell it to a seawater desalination plant at cost price. After all, this enterprise is dominated by the national team, and the factory price of the film will be at least five times higher.
Including other electricity bills and the like, the annual operating cost of a factory with an annual output of 4,000 million cubic meters of fresh water is about 1.2 billion Chinese yuan.
The equivalent operating cost per cubic meter is o.o3 Chinese yuan. Adding in equipment, infrastructure, and transportation costs, the cost of fresh water per cubic meter can be reduced to about o.o.17 Chinese yuan.
The current water charges for various industries are: residential water: 2.8o yuan/cubic meter; administrative water use: 3.9o yuan/cubic meter; industrial and commercial water: 4.1o yuan/cubic meter;
Water used in hotels, restaurants, catering industries, etc.: 4.6o yuan/cubic meter; water used in bathing industry: 6o yuan/cubic meter; water used in car washing industry, pure water: 4o yuan/cubic meter; agricultural water used: 0.6o yuan/cubic meter.
Even for agricultural water, there is still a 4oo% profit margin.
Of course, due to the nature of this enterprise, in addition to agricultural water, the remaining water is allocated to various cities for use, and the wholesale price is 0.5 Chinese yuan per cubic meter.