Scientists develop ‘nanopores’ that inexpensively filter the salt out of seawater

Mohammad Heiranian/University of Illinois

Just think what this could mean.

There’s filtration and then there’s filtration. Engineers in the US have been working on the latter, coming up with a new markedly more energy-efficient way of taking the salt out of seawater, which could deliver huge advantages in terms of providing people with access to drinking water and help combat problems like drought.

The researchers have developed a material that allows high volumes of water to pass through extremely tiny holes called ‘nanopores’ while blocking salt and other contaminants. The material they’re using – a nanometre-thick sheet of molybdenum disulphide (MoS2) riddled with these nanopore holes – is the most efficient of a number of thin-film membranes that the engineers modelled, filtering up to 70 percent more water than graphene.

“Even though we have a lot of water on this planet, there is very little that is drinkable,” said Narayana Aluru, a professor of mechanical science and engineering at the University of Illinois and leader of the study. “If we could find a low-cost, efficient way to purify sea water, we would be making good strides in solving the water crisis."

Molybdenum disulphide coupled with nanopores could be that solution. While desalination isn’t a new concept, the efficiency gains with this kind of new material – both in terms of the energy required to make the filtration work, and also the cost of keeping a desalination system running – could make a world of difference when it comes to processing large amounts of seawater.

“Finding materials for efficient desalination has been a big issue, and I think this work lays the foundation for next-generation materials,” said Aluru. “These materials are efficient in terms of energy usage and fouling, which are issues that have plagued desalination technology for a long time.”

Conventional desalination relies on reverse osmosis to channel seawater through a thin plastic membrane, but the process suffers from a number of bottlenecks. While the membrane appears thin to the eye, from a microscopic perspective it’s more tube- or tunnel-like than a sheet that’s only a nanometre in thickness, which means it requires more pressure (and thus energy) to operate. They’re also susceptible to more clogging, which ramps up operational costs.

In comparison, the extreme thinness of the molybdenum disulphide membrane allows water to pass through with much less resistance, lessening or negating many of the above drawbacks. But the ingenuity behind the system isn’t just in its engineering.

“MoS2 has inherent advantages in that the molybdenum in the centre attracts water, then the sulphur on the other side pushes it away, so we have much higher rate of water going through the pore,” said Mohammad Heiranian, first author of the study. “It’s inherent in the chemistry of MoS2 and the geometry of the pore, so we don’t have to functionalise the pore, which is a very complex process with graphene.”

There you have it, folks – the world’s first thirsty water filter. We love it! The next steps for the researchers are partnering with manufacturers who can bring their modelled desalination technique to life. The first step will be testing, but they’re confident their findings – which are published in Nature Communications – could be applied on an industrial scale for everybody’s benefit.

“I’m in California now, and there’s a lot of talk about the drought and how to tackle it,“ said Amir Barati Farimani, a postdoctoral fellow at Stanford University who worked on the research at Illinois as a graduate student. ”I’m very hopeful that this work can help the designers of desalination plants."

ORIGINAL: Science Alert

PETER DOCKRILL

12 NOV 2015

Nanocabons Clean Water Without the use of Chemicals and Transform Waste Heat into Electricity

Scientists at INM are working on a desalination method which does not require the addition of chemicals and which is highly energy-efficient. This environmentally friendly process can even be used to generate electricity.

When purifying waste water, chemical reactions are used or it is subjected to elaborate filtering methods to remove salts and heavy metals. Now scientists in Saarbrücken are working on a desalination method which does not require the addition of chemicals and which is highly energy-efficient: in the process known as capacitive de-ionization (CDI), electrodes are used to extract the ions from the water and collect them on the electrodes. The result is clean water and ions which have been enriched on the electrodes. This environmentally friendly process can even be used to generate electricity: emissions such as carbon dioxide are also suited to generating electrical energy when they are dissolved in water as ions.

At an international conference in Saarbrücken, more than 110 experts from 20 countries will be exchanging their knowledge from 26th to 28th October in order to further understand and improve the materials and processes which form the basis of CDI. The conference is being organized on the Saarbrücken Campus by the INM – Leibniz Institute for New Materials.

Nanoporous carbon materials for electrochemical water treatment via capacitive deionization. Copyright: Uwe Bellhäuser /Volker Presser (INM)

The principle of CDI not only serves the purpose of removing unwanted ions from the water. Volker Presser, head of the Energy Materials Group at INM explains, “The generation of electricity can also be the main desired effect, to use emissions from power plants to produce electricity, for example”. For this purpose, the emissions merely has to be present in the water as ions. “Carbon dioxide, for example, is very well suited to this purpose” Professor Presser said and added: “It is particularly exciting for us that, thanks to the electrosorption process, we can even convert waste heat into electricity”. He said this worked because the electrodes are charged at low temperatures and discharged at higher temperatures. As a result of the temperature increase, the electrical voltage increases so that electrical energy can be recovered directly during discharging.

The basic principle of CDI functions without chemical reactions thanks to ion-electrosorption: the water to be purified flows between two electrodes made of porous carbon to which a voltage is applied. The positively charged electrode extracts the ions which have a negative charge from the water and the negatively charged electrode, located opposite, extracts the ions with a positive charge from the water. The ions are stored in the nanopores of the electrode material and, at the end of the process, clean water flows out.

“To achieve the highest possible degree of efficiency, it is not sufficient to simply use ‘any’ porous activated carbon as electrode material”, Presser, the young Saarbrücken-based researcher commented. One focus of his work and an important topic of discussion at the conference is therefore the synthesis and characterisation of new carbon-nanomaterials such as graphenes or so-called carbon nano-onions.

The CDI&E International Conference on Capacitive Deionization and Electrosorption will take place from 26th to 28th October in the auditorium of the campus of the Saarland University. The conference is chaired by the German-Israeli team consisting of Professor Volker Presser and Professor Matthew Suss from Technion – the Israel Institute of Technology.

A large number of participants come from Asian and Arab regions since countries where there is a marked scarcity of water are doing intensive research into the technology. CDI is also an attractive option for processing water from mine workings: not only do you get clean water; the ions or precious metals retained as a highly enriched fluid can be a valuable resource in industry.

In ten programme units with supplementary posters, scientists will discuss their results and findings regarding possible materials and mechanisms of electrodes in CDI, energy generation, extraction of ions and ionic processes. Professor emeritus Bertel Kastening, a well-known pioneer of electro-chemistry, has been invited as guest of honour.

ORIGINAL: Leibnitz Institute

15. October 2015