July 1.

Huang Haojie and a group of researchers from the Institute of Seawater Desalination are discussing some issues of seawater desalination technology.

Theoretical calculations show that graphene can be applied to seawater desalination, and the single-layer nanoporous two-dimensional film has ultra-high selective separation efficiency compared with traditional seawater desalination film.

However, the grain boundaries existing in large-area graphene will reduce the mechanical properties of graphene, and the introduction of nanopores will further reduce the mechanical properties, resulting in local rupture of the separated films, which greatly reduces the separation efficiency and selectivity.

Of course, this problem is nothing to Galaxy Technology, and atomic manipulation technology can solve this problem perfectly.

Current graphene desalination membranes are divided into two categories.

The first is the nanoporous thin films with a single atomic layer thickness, represented by the team of MIT professor Rohit Karnik.

However, the mechanical strength of graphene with a single atomic layer thickness is weak, so the graphene used in experimental research is supported by polymer films.

In addition, sub-nanopores are introduced into graphene directly through high-energy electron beam bombardment or oxygen plasma etching, and the pore size distribution range is wide, which greatly reduces the separation efficiency, so it cannot be applied to practice.

The other category is the graphene oxide film researched by the team of Andre Geim, a professor at the University of Manchester and winner of the Physics of Explosives.

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 to expand the layer spacing, which reduces the desalination efficiency, so the existing research work mainly focuses on how to control the layer spacing between graphene oxide sheets.

In addition, there are also relevant research results in China.

That is a binary structure graphene film combining graphene nanosieves and carbon nanotubes, which has both the selective separation efficiency of the former and the strength advantage of the latter.

Nanoporous two-dimensional materials with a single atomic layer thickness have the smallest water transport resistance and the largest water permeation flow, which is an ideal material for constructing ultra-thin and efficient desalination membranes.

However, there are two major challenges in applying ultra-thin 2D materials to actual seawater desalination.

The first is how to prepare large-area crack-free nanoporous 2D films with excellent mechanical strength and flexibility.

Secondly, how to introduce sub-nanopores with high-density uniform pore size distribution inside the film to achieve efficient selective passage of water molecules and effective interception of salt ions/organic molecules.

For the first challenge, carbon nanotubes have excellent mechanical properties and are structurally similar to graphene, with π-π bonds and van der Waals forces interacting with each other.

The carbon nanotube film formed by the overlapping of carbon nanotubes is a porous network structure (average pore size 300 nm) that not only perfectly matches the structure of graphene, but also does not affect water permeability.

Therefore, domestic research institutions think of combining nanoporous graphene with carbon nanotubes to make up for the shortcomings of the former.

They first grew a single layer of graphene on top of the copper foil, and then covered some areas on top of the interconnected carbon nanotube network, and after dissolving the copper foil, they obtained a graphene film supported by carbon nanotubes.

In order to obtain high-density sub-nanopores with uniform pore size distribution, they grew a layer of mesoporous silicon oxide with uniform pore size distribution (average pore size 2 nm) on the surface of graphene as a mask, and used oxygen plasma etching to remove the graphene in the mesoporous silicon oxide pore size.

The longer the oxygen plasma etching time, the more graphene is etched away, and the larger the pore size of the graphene.

In this way, the pore size of the graphene nanosieve can be adjusted by adjusting the time of oxygen plasma etching. When the etching time is controlled at 10 seconds, the pore size is 0.63 nanometers, which can effectively allow water molecules with a diameter of 0.32 nanometers to pass through and block salt ions with a diameter of 0.7 nanometers.

The film can be suspended, bent, and stretched without polymer support without visible cracks.

The test and calculation results show that the new film can withstand 380.6 MPa stress and the Young's modulus reaches 9.7 GPa, which is three times that of carbon nanotube film, which is equivalent to 2.4 times the tensile stiffness and 10,000 times the bending stiffness of nanoporous graphene film.

So, they made a large and strong mesoporous graphene film.

So what about its filtration performance?

In less than 10 seconds, the permeability of the etched graphene nanosieve/carbon nanotube film can reach 20.6 liters per square meter per hour per atmosphere.

After 24 hours of osmosis, the salt ion rejection rate is greater than 97%.

Compared with the commercial triacetate cellulose desalination film, the water permeability of the new graphene nanosieve/carbon nanotube film is increased by 100 times, and the anti-pollution ability is stronger.

Moreover, because it is not constrained by the internal concentration polarization effect, the film can still maintain a high water permeability in a high concentration of 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 problems, that is, it is difficult to mass production, and if the mass production problem is solved, it can be applied on a large scale.

When Huang Haojie was eyeing graphene desalination technology, he dug up this domestic research team.

"Dr. Yuan, do you still have any problems with your desalination membrane?"

Hearing Huang Haojie's words, Yuan Quan replied with a smile:

"There are no big problems, the composite film of graphene and carbon nanotubes, with the help of atomic manipulation technology, has now achieved initial mass production, and our film quality is very strong."

She really admires Huang Haojie very much, atomic manipulation technology is definitely a revolutionary technology.

If her previous team developed a graphene carbon nanotube composite film, the desalination efficiency per square meter was 1; Then the graphene carbon nanofilm made by atomic manipulation technology has a desalination efficiency of 10 per square meter.

The reason for such a large gap is that the graphene carbon nanotube composite film made by atomic manipulation technology has no defects.

Although this new film is also a graphene carbon nanotube composite film, it is nearly ten times stronger, and the increase in strength can increase the air pressure to force the desalination of seawater.

At present, one square meter of film can produce 80 cubic meters of fresh water in one hour, and 700,000 cubic meters of fresh water in a year.

In other words, if you want to achieve an annual output of 40 billion cubic meters of fresh water, only 57,000 square meters of membrane are needed.

Of course, the service life of this film is about 4100 hours, which is equivalent to replacing it every six months.

The production cost of this desalination film is about 1,500 yuan per square meter.

In this way, a plant with an annual output of 40 billion cubic meters of fresh water needs to purchase 171 million yuan of desalination film every year.

Of course, Huang Haojie will not sell to a desalination plant at cost price, after all, this enterprise is the majority of the national team, and the ex-factory price of the film has increased by at least five times.

Combined with other electricity bills and the like, the annual operating cost of a factory with an annual output of 40 billion cubic meters of fresh water is about 1.2 billion yuan.

It is equivalent to 0.03 Chinese yuan per cubic meter of operating costs, plus the cost of equipment, infrastructure and transportation, the cost of fresh water per cubic meter can be compressed to about 0.17 Chinese yuan.

At present, the water charges of various industries are: 2.80 yuan/cubic meter for residents' domestic water; Administrative water: 3.90 yuan/cubic meter; Industrial and commercial water: 4.10 yuan/cubic meter;

Water used in hotels, restaurants, catering, etc.: 4.60 yuan/cubic meter; Water for bathing: 60 yuan/cubic meter; Car washing industry, pure water water: 40 yuan/cubic meter; Agricultural water: 0.60 yuan/cubic meter.

Even if it is agricultural water, it still has a profit margin of 400%.

Of course, due to the nature of this enterprise, except for agricultural water, the rest of the water is wholesaled to various cities for use, and the wholesale price is 0.5 Chinese yuan per cubic meter.