The double edges of the “wonder material” Graphene in water and air filtration, food processing and drug delivery
Graphene has grabbed a lot of attention lately, being termed a “wonder material” because of its unique properties. Some of these are (graphene-info.com):
- The thinnest material known to man
- Approximately 200 times stronger than steel
- Excellent heat and energy conductor
- Light absorption properties
Graphene, which is a hexagonal arrangement of carbon atoms that is one atom thick, is a component of graphite, and was first isolated in 2004. Since then, it’s been a source for innovation in a myriad of industries such as batteries, touch screens, sports equipment, solar cells and today’s topic, water filters.
Drinking water shortages in various parts of the world have spurred research into more cost-effective ways of filtering water. “Reverse osmosis” or RO is the most common way of filtering seawater into potable (drinkable) water, but it does require a lot of energy and capital for equipment. Graphene, however, is many times more water-permeable than standard RO systems and requires less energy.
There are several ways to make graphene into a filter to allow only H2O molecules to pass through it and not salts (like NaCl, which are larger). In order to allow water to pass through at a 90 degree angle, the graphene or graphene oxide sheet would need to have holes punched in it, such as with ion bombardment. However, this is not a practical way to produce a filter sheet on an industrial scale. A variant of graphene, graphene oxide (GO), is a solution to this problem. GO is produced when graphene is oxidated and later exfoliated (youtube video), producing a three-dimensional surface out of a flat one (graphene). GO is hydrophilic, meaning that water flows through it quickly. Instead of allowing the water to flow through at a 90 degree angle to a graphene sheet, it’s better to allow it to flow parallel between graphene oxide sheets, controlling the sheet distance from one sheet to another to disallow salts and other impurities from passing through. GO membranes are also easier to manufacture than graphene membranes on a large scale, but the two can be used successfully together when graphene sheets (non-hydrophilic) are used between the GO sheets to control the pore size. The UK firm G2O is working on fulfilling its first commercial contract for water filtration membranes enhanced with GO (graphene-info.com).
A new water filter using graphene sheets shows a lot of promise. Pioneered by Australian scientists and named “Graph Air”, the graphene itself is made from soybean oil, which is safe, cheap and renewable. The process for making the graphene is easier, cost-effective, and more eco-friendly than the typical process, which can require costly volatile gases. The most important advantage of Graph Air is its resistance to fouling, which causes downtime, backflushing and eventual filter replacement in most RO systems. The test situation showed that the Graph Air system produced clean water from Sydney Harbor water, filtering out 100% salt and 99% of impurities without fouling.
A Russian study showed how GO can purify water from E.Coli. GO injected into a mixture of saline water and nutrient medium (to simulate the human body and provide a food source for the bacteria) contaminated with E. Coli, “captures” the bacteria, forming flakes that can be extracted from the water. Furthermore, the GO can be recycled using ultrasound, enabling its re-use.
Researchers at Rice University employed graphene in a new air filter designed to create an “air curtain” between people, much as plastic barriers do. The filter pulls in air through a graphene foam filter, which is energized with electricity (graphene is highly conductive), zapping bacteria and viruses, and pushes out the air to form a “wall” of clean air. It’s hence called the VirusWall, sort of like a bug-zapper or electric fence for pathogens.Here’s another sort of air filtration: graphene layered on the outside of face masks provided repellent, photothermal and self-cleaning properties to the masks. Here’s how the masks work: several layers of graphene on the outside caused incoming droplets (that can carry virus) to bounce off the mask. Then, when the masks are exposed to sunlight, the high conductivity of the graphene causes the mask to quickly heat to over 80 deg C (176 deg F!) to destroy any pathogens on it (self-sterilization), so that the mask can be reused. Finally, when the mask has become stained or unwearable, it can be recycled and the graphene used for desalinating saltwater into drinking water.
Other substances derived from graphene are termed graphene family nanoparticles (GFNs). In the food industry, these are being researched for their ability to absorb phycotoxins from shellfish (such as clams, oysters and mussels), alfa-toxins in peanuts, mycotoxins, and residual pesticides in foods (nanografi.com) In addition, the antibacterial properties of silver (Ag) are magnified when it is synthesized onto reduced graphene oxide (rGO) sheets, presumably because the extremely sharp edges of the rGO causes cellular damage in addition to the silver ions that are released.
GFNs are also the subject of many studies in drug delivery. Binding drugs to GFNs allow the unique properties of graphenes to target and deliver drugs in the body like never before. Here are just a few of GFNs’ superb abilities as drug carriers (review paper):
- High surface area of GFNs enables them to be an exceptional carrier of drugs
- pH-assisted delivery: Polyethylene-grafted GO (PEG-GO) will release more drugs in a shorter time in the acidic area around tumors, than without GO
- thermal-assisted delivery: drugs delivered by GO can be selectively released by near-infrared lasers
- Magnetic and light assisted delivery: drugs can be accumulated for release near tumors when magnets or near-infrared light (lasers) are applied in the area
- GFNs are also candidates for gene therapy delivery due to their ability to penetrate cells.
As wonderful as the prospective applications are, GFNs are toxic to humans. It has been discovered that nanoparticles with diameters <100 nm can enter cells, and those with diameters <40 nm can enter the nucleus (Nanotechnology as a double-edged sword…), and can cause toxicity to many different organs (table), depending on type of particle, dosing and delivery. This causes concern for those manufacturing products with graphene, as well as anyone accidental ingesting graphene due to unforeseen breakdown of the product (such as filters that filter water, or process food, or purify air).
Let’s hope that graphene and GFNs get their “due diligence” whenever they are used in close contact with water, air, foods, and drugs, so that this wonder material will not leave us wondering why we employed it for our safety!