FENNER CONFERENCE ON THE ENVIRONMENT
Nanotechnological solutions to water treatment and reuse
Professor Max Lu
Max Lu is a Federation Fellow and Professor of Nanotechnology in Chemical Engineering at the University of Queensland. He is Director, ARC Centre for Functional Nanomaterials. His research expertise is in nanoparticles and nanoporous materials for clean energy and water purification applications. With over 220 journal and 160 conference publications to his credit, he is also co-inventor of eight international patents. He is among the highest cited researchers in the world in chemical engineering and materials science. He has received numerous prestigious awards nationally and internationally, including the Orica Award, RK Murphy Medal, Le Fèvre Prize, IUMRS Young Scientist Award, Top 100 Most Influential Engineers in Australia. He is an elected Fellow of the Australian Academy of Technological Sciences and Engineering. He served on the ARC's College of Experts (2002–2004), and Expert Advisory Groups of the Prime Minister's Science, Engineering and Innovation Council (2004, 2005).
What I am going to do today, having heard the previous speakers talking about the various technological solutions and options for water management and we are really excited about how technology can contribute to the solutions is to go down to a much, much smaller scale in the materials world, to talk about some of the emerging opportunities in membranes and catalysts for advanced treatment.
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This is where I am from, at the University of Queensland. We have just had a new institute opened last October, the Australian Institute of Bioengineering and Nanotechnology. It is a brand new facility, a state-of-the-art institute funded jointly by Atlantic Philanthropies, the Queensland government and the University of Queensland. We have four programs, one of which is my area Nanotechnology for Energy and the Environment and the others are concerned with biological engineering themes. Currently we have about 280 scientists in the institute .
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My ARC centre has now become an ARC Centre of Excellence. It involves scientists from four universities physics scientists, chemists, and materials scientists. We do fundamental research around the nanoscale science, to develop novel materials platforms that will eventually be made into products useful for clean energy delivery, water purification (which is the topic I am going to touch on today) and also health care products.
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This slide shows you some images of the materials we deal with from nanotubes, which Tom Hatton talked about, and nanostructured carbon, from oxide nanorods to phosphate and silicate nanoporous materials. One of the common features of those materials is that they possess functionalities such as high specific surface area because of the reducing dimension, and they are very active as a catalyst. And because some of the porous materials we make have a very good molecular sieving effect they can separate small molecules from larger molecules that can be made into membranes. Also, some of the materials have ionic conducting properties that can be enhanced for use as desalination membranes and also membranes for conducting ions for fuel cells and other energy devices.
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In the area of water and energy we have projects in our centre addressing some of the fundamental issues of water purification this belongs to the more advanced stage treatment for recycling and reuse and also desalination. I will talk about these topics. But also some of the materials have been developed because they have the unique properties of being very sensitive to chemical molecules and can be used as sensors. Sensors are very important. Stuart Minchin talked about sensors in measuring water, but sensors can also be used for pre-warning in desalination systems and other water treatment processes.
We are also developing a lot of materials for fuel cell cars, which addresses global warming issues, and also some other energy storage issues.
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To talk about water treatment and recycling: this is a very busy slide produced by Dr Radcliffe as part of the Academy of Technological Sciences and Engineering's symposium in 2004. If you look at this flow chart, you see there is a primary process, a secondary process and a tertiary process. Current technologies are very efficient and very effective in removing the organic matters, the biological oxygen demands and the chemical oxygen demands, in water, and also suspended solids by mechanical filtration. But when you go to trace amount organochemical pollutants, such organic compounds cannot be degraded by microorganisms when in low concentrations, they are very refractory, very difficult to degrade, and you have to resort to some more advanced chemical processes. The diagram shows the chemical precipitation, chemical oxidation processes which we call advanced oxidation processes (AOP).
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This diagram shows the various options in biological treatment and physical and chemical treatment, as well as advanced oxidation processes. You can see on the three axes the various processes from coagulation to adsorption, which is a physico-chemical process. They involve a lot of materials and materials modifications to enhance their performance, such as an adsorption process and also membranes for micro- and ultrafiltration, and nanofiltration.
Some of you probably know that in Singapore there is a company called NuW ater. It is subsidised by the Singapore government for recycling water, using a combination of reverse osmosis and nanofiltration technologies to treat about 10 per cent of the recycled water with the normal water. That is for the drinking water supply. It is very well accepted in Singapore.
On the right-hand side of the diagram there are a number of emerging technologies termed as photocatalysis, Fenton processes and other UV-type chemical treatments. These are all the advanced oxidation processes. So advanced oxidation involves either a catalyst or a photocatalyst. You use either thermal energy or photons from solar energy to help oxidise and degrade refractory organic glutens in water. Photocatalysis, by the way, can also kill some of the pathogens. I will talk about photocatalysts.
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Titania is a typical oxide. It is a wide bandgap semiconductor. It is widely used in a larger particle size range as pigments in paints, but as we reduce titania particles to a few nanometres to tens of nanometres, the fundamental properties change so that they can absorb a lot of UV radiation from the solar radiation. So they can be very active in oxidising organic species; they can also kill bacteria and viruses.
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I borrowed this from Professor Fujishima, from the University of Tokyo, who is one of the pioneers in this area. This is a nanoparticle of titania. Upon irradiation with photons, the charge is separated, the electron and the hole are separated. The hole is very active in attacking the hydroxyl groups on the surface. Whether it is in water or in air, you inevitably have moisture and you have hydroxyl groups. And then they activate to degrade or mineralise any organic, including living, organisms. The skin of any bacteria and viruses can be harmed significantly by this process, and this is mineralised into water and CO2. This is the principle.
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How do we go about photocatalytic degradation? We have first to reduce the particle size of such materials, so nanotechnology is all about manipulating molecules and atoms as building blocks to assemble materials from the bottom up. In the way that nature does things, you assemble from the chemicals, molecules of titania. You assemble nanoparticles like this. They are highly crystalline, with a high surface area. They are very active in absorbing the pollutants in, and upon their irradiation with photons the oxidising power of the free radicals will perform the job of degrading the organic pollutants.
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However, there is a problem with titania-based nanoparticles. They tend to absorb UV radiation, but in order to utilise the full spectrum of the photons from the sun you need to do some bandgap engineering, or nanomodification. So what we did was to introduce nitrogen as an element to narrow the bandgap so that such nanoparticles not only absorb UV but also absorb the visible light.
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This is an X-ray diffraction pattern of these materials. One is P25, a commercial catalyst from Degussa, Germany, and the other is mesoporous titania that is nitrogen-doped. In terms of the crystallography information, they are not much different. Basically it consists of the active phase, which is called anatase crystals, in the particles. And if you look at the nitrogen adsorption isotherm, which is a way to characterise how high the surface area is, what the porosities are like in those materials, you see again that they are not much different.
But if you look at the optical absorption spectra the P25 mainly absorbs UV, the mesoporous titania, with a lot of pores, has been modified with nitrogen you can see the increase in the visible light range, which is great. This is what we want.
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Then we can test such nanocatalysts by putting them into a suspension reactor and shine some simulated sunlight. This is in about a 1-litre reactor. It is not large-scale; this is lab-scale data. We use rhodamine B, which is a coloured organic compound normally used as a model compound for organic degradation.
Under the UV, without catalysts you don't have any degradation. When we compare the mesoporous titania particles with the commercial catalyst, the commercial catalyst is much better than the mesoporous titania. But if you modify that with nitrogen doping, you change the bandgap to a narrower bandgap, and this is much more efficient. You can see the degradation rate much faster. In the visible light that is much more pronounced.
So that is what we hoped for and we designed for. Now this is a result that we have achieved.
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One of the other advantages of such catalysts is their recoverability and recyclability. If you suspend such particles in water, if you use P25, which is a commercial catalyst used by many companies around the world for groundwater treatment, this is what it looks like. So you have to resort to a pressure filter, or vacuum filter, to recover the catalyst after use, whereas the particles we make are large enough but more active than commercial catalysts and will settle within one minute. So that is an additional advantage this has been published and also patented in the way we make such mesoporous catalysts, doped with nitrogen.
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As Tom Hatton described, we are about to take part in a CSIRO cluster for new membrane materials for desalination.
In this regard we have a lot to learn from nature. We know that aquaporin is in all living cells. It is a unique protein in the lipid bi layer that can pump water . Of course, you need some energy, and the energy required in a living system is quite low, so it would be great to use or to understand such a protein structure so that you can mimic the membrane.
I took the figure you see here from Terry Turney, of CSIRO. Basically, if you have about 1 metre square, assuming 50 per cent packing density, you can have 260,000 litres per day. Using this sort of principle, if we can realise the engineering of nanostructures so that we can make new membranes, new ion channels, that preferentially pump water.
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Indeed, we have started some preliminary work on nanocomposites, mimicking how nature is doing things, by assembling some ionic conducting polymers with nanoparticles of silica or zeolite alumino-silicate, porous ionic-exchangers we can make these sorts of nanoparticles and then make them into films by self-assembly. We can coat, layer by layer; we can control the thickness precisely. This is the morphology of such a film.
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How can we use this in the desalination process? This is not a reverse osmosis. We are hoping to develop a new-generation membrane that will be used in this reverse electrodialysis process. So this will separate the salt, push the salt across the membrane, which is the lower concentration, the minor species, in sea water or brackish water. Therefore, you lower the energy cost. In this way you can expect a dramatic reduction in the cost. That is what we are working towards. We haven't really got a lot of results in this area yet.
This slide looks at the promises of nanotechnology. I think most of you have heard recently of the work of Jason Holt, from the Lawrence Berkeley National Laboratory. He has developed a membrane that incorporates carbon nanotubes and shows great water permeability. This can be incorporated into reverse osmosis membranes that will promise this sort of step-change in lowering the costs of desalinated water.
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I will just recap. Nanomaterials built from the bottom up have a lot of promising properties. They could become breakthrough technologies that would have a profound impact on water purification for recycling and also for desalination.
Unfortunately, I haven't got time to talk about our other projects in the area of water treatment. For example, we have a project on biogas utilisation, because biogas is a waste energy source in a lot of wastewater installations. If you use it wisely, you can produce the energy that is required for water treatment. We have patented technology to convert biogas into ethanol; it is currently being commercialised.
Discussion
Question: I am just wondering if there are any estimates about the energy, or even water, requirements to make these materials the nitrogen-doped titanium, or the nanotubes.
Max Lu: Of course, water can be recycled in our labs all the time. We haven't done a systematic analysis of water or energy usage in preparation, but I can tell you that the cost of such particles is slightly more expensive than the paints you buy, maybe double the price. So that gives you some sort of indicator. It is not grossly more expensive, not an order of magnitude higher. As to how that would translate into the costs of the whole water treatment system, no doubt it is going to be more expensive but whether it is going to raise the price to $2 or $3 I don't know. We haven't done that kind of analysis.
Question: I refer to your third last slide, in relation to membranes. Do we have the facility or the capability in Australia to fabricate the types of membranes that would be developed there and, I guess more importantly, to be able to test them on larger than laboratory scale?
Max Lu: At the moment, in the engineering scale, we do have the capability, mainly based in New South Wales. There is a UNESCO centre and there are also associated companies in Sydney MEMTEC and a few other companies.
For these kinds of new-generation membranes, when we are incorporating nanoparticles, in terms of the kind of equipment and processing facilities the processing won't be much different. It is just more the molecular engineering, the recipe, that will make a difference in the structure of the materials.
An objective of the CSIRO cluster which Tom Hatton talked about is to link the various universities that have facilities for testing the new membranes fast turnaround and within a month you get results whether it is in reverse osmosis or reverse electrodialysis processes.
Question: Thank you for your wonderful presentation. You talk about the new generation of membranes with nanoparticles in them moving into desalination, but there is a huge opportunity with this type of new generation of membranes to look at the recovery of water from the wastewater treatment plants, where you have got a low level of salt but a reasonable amount of complex organic compounds in it. So there is a potential in that. Most of the membranes are developed for drinking-water treatment systems. When you go for the recycling system, the fouling issues are there, and in industrial wastewater the reuse issues are huge as well. Have you had any thoughts on the application of your new-generation membranes to that sort of application rather than desalination of water?
Max Lu: I think there is a lot of potential for a hybrid system, where you have a membrane that can take some of the salt and also you have the photocatalyst. With these combined together you can kill the bacteria, all the pathogens, as well as the trace amount of organics. In fact, in the literature some people have already done a lot of work in this area.
Question: I am just wondering whether you could explain a little bit about the longevity issue. One of the things about membranes is that the more sophisticated they get the more sensitive, the more they need to be replaced, and that is a big cost factor. Is it possible to go down this pathway with this new generation and also address the issue of longevity of the individual membranes?
Max Lu: Yes, this is very possible, and it is the very aim of this CSIRO cluster. There are a couple of projects looking at the surface fouling , how to modify the surface chemistry so that they will last longer in terms of the surface fouling and also the stability of the polymers. More often than not, when you incorporate inorganic nanoparticles into a polymeric membrane, you would enhance the stability, among other property improvements.
