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Science

Nanofiber Membrane Filters 99.9% of Salt From Seawater Within Minutes (interestingengineering.com) 117

Posted by BeauHD from the cheap-and-quick dept.
A team of Korean scientists may have found a way to desalinate seawater in minutes. An anonymous reader quotes a report from Interesting Engineering: When using membranes to filter seawater, the membrane must remain dry for long periods of time. If the membrane becomes wet, the filtration process becomes ineffective and allows large amounts of salt to pass through the membrane. For long term operations, progressive membrane wetting has been observed regularly, which be resolved by changing the membrane. Researcher Yunchul Woo and his team at the Korea Institute of Civil Engineering and Building Technology (KICT) have now developed a membrane that is less susceptible to wetting and is stable in the long term.

The membrane is made of nanofibers that have been fabricated into a three-dimensional hierarchical structure, This was achieved by using a type of nanotechnology called electrospinning. Using this technology, the researchers were able to fabricate a membrane that is highly hydrophobic -- i.e. water repellent. The hydrophobic nature of the membrane is helpful because it is designed to not allow water molecules to pass. Instead, a temperature difference is applied on the two sides of the membrane that causes water from one end to evaporate into water vapor. The membrane allows water vapor to pass, which then condenses onto the cooler side. Called, membrane distillation, this is a commonly used method of desalination using membranes. Since the salt particles are not converted to the gaseous state, they are left out on one side of the membrane, giving highly purified water on the other side. The Korean researchers also used silica aerogel in their membrane fabrication process which further enhanced the flow of water vapor through the membrane, providing quicker access to desalinated water. The team tested their technology for continuous operation for 30 days and found that the membrane continued to filter out 99.9 percent salt without any wetting issues. The study has been published in the Journal of Membrane Science.
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Nanofiber Membrane Filters 99.9% of Salt From Seawater Within Minutes More

Nanofiber Membrane Filters 99.9% of Salt From Seawater Within Minutes

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  • can the membrane be made long? (Score:4, Interesting)

    by FudRucker ( 866063 ) on Wednesday July 07, 2021 @09:05AM (#61558587)
    long enough that it can be made to function like a conveyor belt, keep the filter moving so it can be dried and reused/rotated to filter seawater over & over again, if a dummy like me can think it up i bet an engineer could build a system that worked like that

    • ... by Stella Andrassy (and implemented by Charlie Parker). Not exactly the same, but it was a still with a rotating drum which had a wicking material. Heat could be supplied by solar energy or an electric heater or other source. The idea was that the wicking material accelerated the evaporation. Not sure how much her patents reflect the final variation.

      Related to that:
      "US4235678A - Solar powered water desalination system with a regenerative fixture"
      https://patents.google.com/pat... [google.com]

      This is another related e

  • Do you know what this means? (Score:2, Insightful)

    by Anonymous Coward
    We'll have enough salt to last forever!

    • Re: (Score:3, Informative)

      by Anonymous Coward

      "We'll have enough salt to last forever!"

      Alas not. Since all sea-water nowadays contains plastic particles, ocean salt has been avoided by more and more people, because it is full of it, especially the 'Fleur de sel' type.

      People go back to salt from deep inside mountains, from a time when there were neither plastic nor people on earth.

    • Nice quote from the movie Top Secret [wikipedia.org].

  • As long as it is cheap enough and doesn't degrade under real world conditions, this can make a huge difference to billions of people. Unfortunately, the article only speaks of salt water, not dirty well or actual sea water. Time will tell.

    • Re: (Score:2, Offtopic)

      by polyp2000 ( 444682 )

      It is refreshing to hear replies like this here on Slashdot and a good sign that attitudes are changing.

    • Re: (Score:2)

      by Moloth ( 2793915 )

      If its converted to steam then I would say the water would be very pure.

      • Water vapor, not steam. Ie, clouds are water vapor but not steam. But the original water is touching the membrane, and even though the membrane is hydrophobic you don't want lots of seawater gunk gumming it up. So although the summary did not say I was presume that obviously that there is filtering of the water first.

        • Steam and water vapor are the same thing.

          • Steam is water vapor. Water vapor is not always steam. I know this is not what they told you in kindergarten. The morning fog is not steam but most definitions of that word. No one says "boy, it sure looks steamy this morning" when they see the fog. Even the clear air around you has water vapor (unless you're in some very dry desert), that humidity is not called "steam" normally.

            Definition of steam (Entry 1 of 2)
            1 : a vapor arising from a heated substance
            2a : the invisible vapor into which water is converted when heated to the boiling point
            b : the mist formed by the condensation on cooling of water vapor
            3a : water vapor kept under pressure so as to supply energy for heating, cooking, or mechanical work also : the power so generated
            b : active force : power, momentum got there under his own steam sales began to pick up steam also : normal force at full steam
            c : pent-up emotional tension needed to let off a little steam

    • Unfortunately, the article only speaks of salt water, not dirty well or actual sea water.

      ... or Vodka

      That's the most important question. Can it turn cheap vodka into good vodka?

    • The way I'm reading this, the quantities of water purified will be low and the energy consumption possibly rather high (?). That will not be a lot of help for - say - Los Angeles, it will also not be a lot of help for inland communities. Still, Great Oaks, tiny acorns and all that.

      • Re: (Score:2)

        by bws111 ( 1216812 )

        The advantage of this is supposed to be lower energy cost than traditional distillation methods or reverse osmosis.

        • The advantage of this is supposed to be lower energy cost

          Neither the TFS nor TFA says that this process uses less energy.

          Why would it?

          The process described uses a vapor phase while RO does not, so it appears to use more energy.

          The headline implies that the difference is that it is faster than RO, although the process described doesn't sound faster, and speed is not an important metric anyway.

          • Re: (Score:3)

            by bws111 ( 1216812 )

            The advantage of membrane distillation in general is lower energy cost. From https://emis.vito.be/en/bat/to... [emis.vito.be]

            "The potential advantages of MD, in comparison with conventional separation processes, are found primarily in the lower working temperature and pressure, and thus the lower energy costs and less stringent mechanical properties. In contrast to distillation and RO, supply solutions can be separated at a temperature well below the boiling point and under atmospheric pressure. Typical supply temperatur

          • Imagine a plexi glass pipe, the sun shines in, the upper half contains enough water vapour, that "membrane" extracts it: win!

      • Steam generation is pretty easy. The result of generating power could be piped into this. Cutting down on the amount that currently evaporates into the atmosphere via a cooling tower, or pond.

      • No, it's low energy. Don't let "water vapor" make you think "steam", which involves high energy. This is potentially a huge breakthrough.

        • The enthalpy of vaporization is the same for evaporation and boiling, because boiling is a type of evaporation. They both require the same amount of energy input.

          • Facepalm.

            Nitpicker with no clue?

            Boiling: you have to add heat.

            Evapouration: a slight stream of air can cause it. Same energy on paper in your napkin calculation: big difference for my power bill.

    • Re: (Score:2)

      by ceoyoyo ( 59147 )

      One of the attractions of membrane distillation is apparently that it works with less pre-treatment than other methods. The paper says it's currently used for sea and waste water purification.

      https://www.sciencedirect.com/... [sciencedirect.com]

  • Instead, a temperature difference is applied on the two sides of the membrane that causes water from one end to evaporate into water vapor. The membrane allows water vapor to pass, which then condenses onto the cooler side. Called, membrane distillation, this is a commonly used method of desalination using membranes. Since the salt particles are not converted to the gaseous state, they are left out on one side of the membrane, giving highly purified water on the other side.

    So basically it is the same as desalination, only .... what? Still have to impart enough energy to cause the evaporation, etc.

    I can see where this could be a neat material to do cool things with... but I really don't think desalination is one of them.

    • Bingo, that was my observation, this sounds just like distillation the old fashion way, adding energy to effect a phase transition from liquid to vapor. I'm a bit at a loss what the membrane is doing if what is being passed is water vapor.

      Thoughts?

      • Re:ok... so what is special? (Score:5, Informative)

        by bws111 ( 1216812 ) on Wednesday July 07, 2021 @09:42AM (#61558709)

        According to https://emis.vito.be/en/bat/to... [emis.vito.be]

        "The potential advantages of MD, in comparison with conventional separation processes, are found primarily in the lower working temperature and pressure, and thus the lower energy costs and less stringent mechanical properties. In contrast to distillation and RO, supply solutions can be separated at a temperature well below the boiling point and under atmospheric pressure. Typical supply temperatures around 30-60C permit re-use of residual heat flows, and the use of alternative energy sources such as sun, wind and geothermics."

        • Re: (Score:2)

          by TWX ( 665546 )

          Okay, so they're saying that they can achieve phase-transition using this particular membrane and technique for a lower energy budget than current phase-transition methods use?

          • Re: (Score:3)

            by bws111 ( 1216812 )

            I think that is the case. Some quick reading indicates that membrane distillation has been worked on since the 1960s because it shows promise as a cheap way to distill. The problem has been that once the membrane gets too wet the process breaks down. This advance is a way to keep the membrane from getting wet.

    • Evaporation is not boiling. It is a lot more complicated than you think. Even I know that air pressure changes the temperature you need to boil water.

      Human sweat cools you via evaporation, and I have sweated at 79 degrees. So it clearly works at temperatures under 80 degrees.

      Normal desalination works via boiling at over 200 F degrees. That takes a lot of energy.

      If this material allows mere evaporation at even 100 degrees, that is a HUGE energy savings. More than enough to make it viable and profitabl

    • The article exists to be read. It's membrane distillation with one of the biggest kinks worked out. Evaporation over a short distance does not take a whole lot of energy. You don't need to boil the water to make steam, you just need a temperature differential and some pressure. This technique has been used before, the difference is that this membrane can remain effective for much longer than with the current methods. And this has potential to be much more efficient that the common reverse osmosis metho

    • Still have to impart enough energy to cause the evaporation, etc.
      In a desert that should not be a problem, or?

  • "When using membranes to filter seawater, the membrane must remain dry for long periods of time."

    Huh? How does it remain dry when you're filtering water , or am I missing something here?

    • water vapor is dry, and that's what it filters. of course once condensation occurs, everything gets wet. unless you have very hydrophobic membranes.

      • Yup, and with existing membranes used for this, getting wet means it allows more salt to get through. Think of trying to filter your coffee using tissue paper, you get a few drops through then it's wet and it all falls apart. So the existing membranes work, but they can only be used for short periods of time.

      • Re: (Score:2)

        by Viol8 ( 599362 )

        Water vapour can have salt dissolved in it can it? Wow, you've discovered a whole new chemistry there!

    • Re:Remain dry? (Score:5, Informative)

      by Rhipf ( 525263 ) on Wednesday July 07, 2021 @01:31PM (#61559833)

      I had to do some searching before the summary made sense. The material is actually touching the liquid on one side but the membrane is highly hydrophobic so the material doesn't actually absorb the liquid (salt water in this case). The pores in the membrane are such that the salt water also will not pass through the membrane due to the surface tension of the salt water. The H2O molecules will evaporate off the surface in these membrane pore areas though and can then pass through as water vapour. Once on the other side of the membrane the vapour cools and condenses back into liquid H2O without the salt (since the salt molecules don't evaporate from the salt water solution).
      If the hydrophobic coating on the membrane isn't hydrophobic enough eventually the membrane will absorb the liquid and at that point the membrane becomes ineffective for distillation.

  • Do you even need a membrane if you are evaporating the water?

    • Re:am I missing something? (Score:5, Informative)

      by Junta ( 36770 ) on Wednesday July 07, 2021 @09:50AM (#61558729)

      So I looked a little bit, and evidently:
      "In contrast to distillation and [reverse osmosis], supply solutions can be separated at a temperature well below the boiling point and under atmospheric pressure. Typical supply temperatures around 30-60C permit re-use of residual heat flows, and the use of alternative energy sources such as sun, wind and geothermics."

      So yes you are doing phase change, but managing it without having to produce low pressure environment or having to hit 100C.

      I didn't understand myself and frankly it took a couple of tries to hit a page that actually stated it simply, but all I have is 5 minutes of internet search to speak to accuracy.

    • Re: (Score:2)

      by ceoyoyo ( 59147 )

      The description in the summary and the article isn't very good. Membrane distillation seems to be similar to regular old filtering except that instead of excluding large particles and passing small ones, you pass vapour phase water and exclude liquid phase. Basically, you exclude the water molecules that are stuck to other water molecules.

      You still have a reservoir of water in contact with the membrane just like you would in reverse osmosis, dialysis or whatever. That's why wetting is a problem. If the liq

    • Yes. This is a technique that is decades old called Membrane Distillation (on wikipedia but removed to get past the "looks like ascii art" filter). The advance here is NOT the evaporation but in having a membrane that lasts longer. Water on one side, hotter than on the other side of the membrane, and some pressure. The water is vapor only very briefly and for a very short distance (the width of the membrane).

      What is not happening here is evaporating the water separately and then pushing the vapor throug

  • Bell Curve; Temperature differential (Score:4, Informative)

    by NEDHead ( 1651195 ) on Wednesday July 07, 2021 @09:39AM (#61558693)

    No one said boil the water, just temp difference between the sides. Diffusion results of the water portion that resides to the right side of the curve.

  • If we can pump oil through pipes for thousands of miles, we can certainly pump salt water into the Sahara and let nature do the desalination process.

    • we want to keep the water and discard the salt, not the other way around. hence the name: desalination.

    • If we can pump oil through pipes for thousands of miles, we can certainly pump salt water into the Sahara and let nature do the desalination process.

      No need for pumps. Just dig a trench from the Mediterranean Sea to the Qattara Depression [wikipedia.org].

      Qattara can hold about 1000 cubic kilometers. That is about one year of global sea-level rise.

      The 130-meter drop from sea level can be used to generate hydropower.

      As the water evaporates, the salinity gradient with the Mediterranean can be used to generate electricity with osmotic power [wikipedia.org].

      Basically, the QD will be a 20,000 sq km solar energy collector.

      • Sooner or later we will have to put up a dike between Atlantic and Mediterranean anyway, and same for Suez Channel. So that proposal could work. Same for the dead sea.

        • I am a big fan of geoengineering, but I don't see how damming the Med. will work. As the Med. drops, the rest of the world's oceans will rise. That is going to be unacceptable to billions of people.

          If you want to sequester water, another idea is to flood the Tarmin Basin [wikipedia.org].

          It may hold 100,000 cubic km. That will be enough to hold back global sea-level rise for a century.

          The only problem is, where do you get the water? Unlike Qattara, there is no ocean nearby.

          • As the Med. drops, the rest of the world's oceans will rise. That is going to be unacceptable to billions of people.
            If it is not dammed, the Med will rise just as the rest of the ocean.
            As plenty of cities/towns are only 10cm (1/3rd of a foot) above sea level: they will drown (at least partially). So, assuming we have a rise of about 1m (~1 yard), damming the street of Gibraltar, would have an neglectable effect for the rest of the world, but would keep the Med safe. And: despite the low difference in hight,

  • That method of desalination has been used for ages. Turning water into vapor for later condensing, even without a fancy membrane.

  • If this can reduce the cost of desalinization using traditional means, even by a modest amount, it is a real game changer, because fresh water supplies are becoming increasingly scarce and unreliable.

  • Incremental Science (Score:3)

    by kamakazi ( 74641 ) on Wednesday July 07, 2021 @10:43AM (#61558915)

    This isn't "new" technology, it is an incremental step in increasing the efficiency of desalination. I seem to recall there are also people working on using graphene as a membrane, or graphene oxide, which is water permeable, but not so much to salt and other impurities. That is a liquid water, pressure driven filtration rather than a distillation process. Water supply is a significant problem in many parts of the world, and it appears that it will get more significant in some quite densely populated areas as the climate changes. This is not trivial work being done by academics to publish papers, it is Serious Science with life and death repercussions. It is also the kind of science that because of global necessities can foster international cooperation, so potential side effects include helping to temper isolationism and hard line nationalism.

  • How is membrane distillation any more energy efficient than normal distillation?

  • "If the membrane becomes wet, the filtration process becomes ineffective and allows large amounts of salt to pass through the membrane."

    *If* the membrane becomes wet...when filtering WATER.... ? What do they mean "if"?

    Can someone clarify how the membrane could possibly NOT get "wet" when dealing with the "essence of wetness" (Zoolander)?

    not trying to be snarky, honestly confused.

    • Re: (Score:2)

      by bws111 ( 1216812 )

      Pour some water into oil. The oil does not become 'wet', the water just separates out. In this case, the membrane is made such that it repels water and does not become wet (there may be some water on the surface, but the membrane does not become 'soggy').

    • ....ahhh. Not until further down do they explain that it's filtering water VAPOR. Could have included that closer to the top to keep us plebs from getting stuck at the beginning.

  • I keep reading desalination articles, and then looking for products, but so far I haven't seen any. At-sea surimi production takes a huge throughput of fresh water, and it'd be very nice to have something that works better than big evaps or reverse osmosis. So far, all the articles about better new ways to desalinate have turned into about as many results as medicines resulting from mouse studies. Sell us a pre-filter that does just 70% of the work of the RO's, so they could all be second pass, and it wou

  • I don't know how much salt renders water undrinkable and I didn't see that question addressed in the article.

    Either way, this is good. Gradual progress is still progress.

    LK

  • There have been a few of these announced over the years but for real-world application there are a number of practicalities that have to be overcome as well:
    - cost
    - durability
    - energy input.

    From TFA, the water needs to be vaporized to pass through the membrane. I'm not sure I understand how this is a significant improvement on simple distillation which doesn't need the membrane and equally produces salt-free clean water much more simply?

  • Cost (Score:2)

    by u19925 ( 613350 )

    The most important criteria for successful salination technologies are capital cost and energy use. While scientists may not be able to estimate capital cost, I am sure they can analyze the energy usage. The paper does not mention that either. Without this, the paper is a curious scientific study rather than a "solution".

  • How much water? If a membrane with a square meter of surface area filters 99.9% of salt out of a microliter of water "in minutes," so what?

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