Nanotechnology scientists have found that carbon nanotubes can provide a low energy way to desalinate water. As well as saving money, the discovery has the potential to solve one of mankind’s most pressing problems.

Population growth is putting intense pressure on mankind’s ability to access drinkable water, and the problem is getting worse as climate change impacts the planet’s natural fresh water reserves. As the Intergovernmental Panel on Climate Change (IPCC) calculated, “For each degree of global warming, about seven percent of the world's population -- half-a-billion people -- will have 20 percent less freshwater.”

Too much salt also affects agriculture and the challenge of watering crops and feeding livestock. Today, the US Geological Society estimates that, “30% of the world's irrigated areas suffer from salinity problems.”

As a result, “Since 2015, the World Economic Forum's annual Global Risk Report has consistently ranked ‘water crises’ as among the global threats -- above natural disasters, mass migration, and cyber-attacks,” reported the state news channel France24 in January 2019.

Current desalination techniques for making seawater drinkable have been in use for centuries. They typically evaporate the sea water and capture the salt-free distilled water; a process that is very energy intensive and therefore increases human impact on climate change.

This is a point highlighted by the journal Scientific American, which notes that, “the process of desalinization burns up many fossil fuels… As such, the very proliferation of desalinization plants around the world‚ some 13,000 [including 300 in the USA] already supplying fresh water in 120 nations, primarily in the Middle East, North Africa and Caribbean, is both a reaction to and one of many contributors to global warming.”

But nanotechnology may help solve these problems, as researchers from Lawrence Livermore (LLNL) together with a team from Northeastern University have developed carbon nanotube pores that can extract the salt from seawater passed through it. The team also found that carbon nanotubes (CNTs) with diameters smaller than a nanometre (0.8 nm) have better water permeability than wider carbon nanotubes.

Like many great discoveries, the experiments with carbon nanotubes were based on natural processes. As Alex Noy, LLNL principal investigator on the CNT project and a senior author on the paper explained, “We are basically mimicking nature, because in nature there are also biological membranes which have a matrix with pores. What we have been able to do is to make the same pore function but these are man-made.”

First-principles MD simulations showing different H-bonding patterns for water in the bulk state (A) and CNTPs of different diameters [(B) and (C)].

By experimenting with different sized pores the team was able to show that carbon nanotubes hold the key to cheaper desalination techniques. As the online scientific journal, Phys.org, reports, “Computer simulations and experimental studies of water transport through CNTs with diameters larger than 1 nm showed enhanced water flow, but did not match the transport efficiency of biological proteins and did not separate salt efficiently, especially at higher salinities. The key breakthrough achieved by the LLNL team was to use smaller-diameter nanotubes that delivered the required boost in performance.”

A representation of the membrane with carbon nanotube pore

“Carbon nanotubes are a unique platform for studying molecular transport and nanofluidics.” Adds Noy, “Their sub-nanometre size, atomically smooth surfaces and similarity to cellular water transport channels make them exceptionally suited for this purpose, and it is very exciting to make a synthetic water channel that performs better than nature's own.”

“These studies revealed the details of the water transport mechanism,” adds Meni Wanunu, a physics professor at Northeastern University and co-author on the study. “… and showed that rational manipulation of these parameters can enhance pore efficiency.”

In contrast to logical thinking, the team found that water passed through a thinner carbon nanotube than a wider one. As Ramya Tunuguntla, an LLNL postdoctoral researcher and co-author, explained, “We found that carbon nanotubes with diameters smaller than a nanometre bear a key structural feature that enables enhanced transport. The narrow hydrophobic channel forces water to translocate in a single-file arrangement, a phenomenon similar to that found in the most efficient biological water transporters.”

Pictorial image showing an LUV (Large unilamellar vesicle)with embedded CNTPs (not to scale). (Inset) Water molecules escape the interior of the LUV through the CNTP to relieve an applied osmotic gradient.

You can learn more about this research through a YouTube clip made by the Lawrence Livermore team here.

Meanwhile, in a separate study, a team from the University of Nebraska, found an alternative method to remove salt from seawater, based on the positive or negative charge placed at the opening of the nanotube. As the university press release explains, “The chemists found that simultaneously increasing the positive charge of carbon atoms and negative charge of hydrogen atoms on the rims helped remove sodium better than did nanotubes with smaller charges. The team also identified a diameter that best balances sodium removal and water passage among such nanotubes.”

Schematics of the CNT system after a 200 ps equilibration run in the NVT ensemble without applying external pressure. The red and white colours represent oxygen and hydrogen atoms, respectively, in the water molecule. The turquoise and yellow balls represent carbon atoms and hydrogen atoms at the CNT rim, respectively. Na+ and Cl ions are denoted by green and blue, respectively. After the equilibration run, external pressure is applied to the seawater as denoted by the blue arrow. 

The need to find an improved desalination technique is of the upmost importance. As population increases and climate change raises sea levels, more and more people will look to the oceans to provide their drinking water. Additionally, rising agricultural and industrial requirments will increase freshwater consumption as population grows and economies advance.

Already nanotechnology has made countless breakthroughs in raw ingredient manufacturing, composite material development, and pharmacuetical applications. Now it seems that carbon nanotubes may have found one of their most important roles; providing the world with drinkable water.


Photo credit: YouTube, Offgridity, University of Nebraska, Science, RSC, ClimateCentral, RNSML, Ensia & MachineDesign