Purification by Sunlight

An Australian-based research team has made a breakthrough in synthesising a novel photocatalytic titanium dioxide (TiO2) material capable of rapid and effective disinfection of water using natural sunlight.

By William Steel

Effective purification of water using sunlight is a tantalising prize. Indeed the new material has demonstrated superior performance over existing titanium dioxide (TiO2) disinfection technologies (which require artificial ultraviolet (UV) light), and even holds potential to rival chemical-free UV disinfection treatments.

Removing the need for artificial lighting, the breakthrough paves the way for novel TiO2 disinfection solutions that require lower energy demands and carry reduced capital and operations and maintenance (O&M) costs.

So attractive is the proposition that the researchers are already in collaboration with industry players to take the recently published research to the next level.

Professor Yun Liu, head of the functional materials research group at the Australian National University, is the lead researcher. Speaking to WWi magazine, Lui says: “This is an important breakthrough for science and the industry. Researchers have been working in this field for well over ten years; attempting to shift catalytic effects into the visible light range. Although some improvements were found, it’s only now a breakthrough has been made.”

Liu’s own team had been working on the research for four years.

“Our objective was not to develop a photocatalyst with the specific intention of being utilised in water treatment,” she says . “Visible light catalysts, or VLCs, which respond to sunlight can be used across a wide variety of applications in many industries. But water treatment is certainly a very exciting possibility for the material we’ve developed.”

She continues: “A photocatalyst that can purify water with natural sunlight instead of UV may significantly reduce system costs for operators and use solar energy to the maximum extent.”

Potential economic advantages of the new TiO2 material exist in comparison to existing, standard TiO2 water disinfection products as well, as Liu explains: “Considering manufacturing costs in isolation, our TiO2 [costing] around $150 per 100g would be far cheaper than existing products. And relevant here is that the methods we used to synthesise the material are quite conventional, so there shouldn’t be an issue there even when scaling up.”

Particular niche applications for sunlight driven water disinfection are apparent too. Removing the requirement for electricity means that the technology may be well applied in off-grid or natural disaster circumstances.

Design and testing

Commercially available, standard TiO2 is well established as a photocatalyst and has found application across a range of contexts including water disinfection. When exposed to light, TiO2 dispersed within the water destroys organic matter pollutants by breaking down organic bonds through oxidisation.

Important to highlight is that while standard TiO2 does respond to visible light, it relies upon the presence of non-natural lighting, typically UV, to enhance photocatalytic activity to a level at which water disinfection is effective. This limitation has constrained adoption of standard TiO2 for the purposes of large-scale water disinfection.

The Australian team’s research breakthrough rests in a new approach to designing and synthesising a modified form of TiO2 material based around so-called defect chemistry design.

More precisely, what the researchers demonstrated was how the addition of paired nitrogen and niobium ions into TiO2 improves its performance as a photocatalyst to the extent that it requires only sunlight to produce the necessary interactions that lead to effective decomposition of organic matter pollutants.

The validity of the new modified TiO2 material has so far only been demonstrated under experimental conditions using 13W daylight-simulation LEDs. “We’ve shown that the new TiO2 photocatalyst decomposes organic pollutants in wastewater in twenty minutes, compared with the leading commercialised TiO2 product, which takes one hour to decompose only 26 percent of the same pollutants,” explains Liu.

Although the laboratory tests saw the researchers simulate organic pollutants using several dyes, including Rhodamine B (RhB), Liu believes that viruses and pathogens would be equally susceptible to the interactions. Key targets for treatment include Cryptosporidium and Giardia.

Liu adds: “Using dye as a model pollutant is standard for experimental proofs of this type as they exhibit similar chemical properties as seen in organic matter you’d want to remove during treatment. Of course, in the future we intend to demonstrate the effects under real-world conditions.”

Expanding on this, Liu said: “We’re dealing with companies who are very keen to obtain modified-TiO2 samples to test on wastewater. We hope to have more data on the modified-TiO2 capabilities within the year.”

It ought to be noted that, as with UV-based disinfection, VLC treatment remains reliant on water being clear such that microorganisms are effectively exposed to light; so pre-filtering remains a necessity even with the modified TiO2 breakthrough.

Industry interest

Outside of Liu’s lab, scientists have been working on TiO2 catalytic water treatment for a number of years. Effective solutions, however, have proven hard to deliver. The central challenge - now cracked by Liu’s team - was shifting photocatalytic performance into the visible light spectrum.

A few years back, in 2014, Panasonic appeared to have come close as the company unveiled a VLC solution of its own. WWi reached out to Panasonic to enquire about the status of the research. A company spokesperson explains that the research had been discontinued and although little information could be disclosed, adds that decision was taken as Panasonic “could not find marketability on [the] technology.”

Nevertheless there is optimism that modified TiO2 out of ANU may find greater success. Indeed, as Liu explains, the material has already caught the attention of national and international water industry players.

“We have a patent application to protect the IP. But we’re already engaged with industry partners for different tests to carry the research forward, and progress from fundamental research to commercialisation,” says Liu.

In the most immediate timeframe, collaboration will further investigations seeking answers to outstanding questions. Of questions that remain, Liu highlights: “Water temperature is unlikely to be a problem, but we need to investigate the impact of pH variability. We have funding to investigate this matter further. We hope that within the year we’ll have some new results to publish, and inform what direction to take next.”

Collaboration with industry will be particularly critical in facilitating the scale up towards a viable, commercial solution.

“We’re very keen to collaborate with a variety of groups as the requirements for applying the technology vary over different contexts,” explains Liu.

“Different levels of water treatment will need particular considerations for instance. In this regard, the water industry and its engineers have expertise that will be required for developing a system that makes optimal use of our TiO2 material.”

Liu acknowledges there are practical issues to consider in integrating TiO2 into a market-ready water treatment system. With a particle size less than ten nanometers, one matter to tackle is a process for cost-effective removal of the TiO2 material following treatment.

“We can remove the particles via centrifuge quite easily in the laboratory environment, but that’s not ideal for industrial settings. Membranes may be an option,” she adds.

There are sound reasons for recovery of the material. “Although it is not toxic, as a nanoparticle there are health issues to consider. Second to this, in principle we can recycle the TiO2 many times over. How many times, we don’t know for certain yet but we haven’t encountered any problems in the lab. Recovery makes economic sense.”

Hinting at the road ahead, Liu adds: “We’re looking to undertake a larger scale demonstration in partnership with potential end-users - we may identify some problems that require optimising, and we’ll need to engineer facilities to accommodate the solution. After this, we’ll look towards large-scale manufacture of the material for commercialisation.”

Stiff competition

While the new TiO2 photocatalyst holds potential as a faster, cheaper alternative to standard TiO2 UV-reliant water purification, the path to a competitive, market-ready product is beset with obstacles.

Aside from the possibility of any technical challenges yet to reveal themselves and product development, chemical-free UV disinfection represents a well-established solution with widespread adoption throughout the industry.

Widely regarded as a cost-effective technology for treatment of pathogens (notably Cryptosporidium and Giardia) in water, wastewater and water reuse settings, UV disinfection systems represent strong competition for modified TiO2 to go up against as an alternate treatment solution.

“One of the reasons drinking water systems have implemented UV to the extent we see is that it’s cost-effective and typically low cost compared to other unit treatment processes,” Paul Swaim, principal technologist at CH2M tells WWi.

“With upwards of three hundred water treatment facilities using UV for drinking water disinfection in North America, it’s also used significantly in Europe, Australia and many other regions.”

He adds: “In wastewater disinfection, UV is also well adopted; the market share between UV and chlorination is about half and half.”

Key benefits of UV disinfection across all three domains, Swaim explains, include its fast disinfection times, reduction of byproducts compared to chemical disinfection, and not requiring additional chemicals, such as catalytic substances, to enhance the process.

Additionally, Swaim adds that the UV disinfection process is “largely independent of issues like water temperature and pH. So there’s no conditioning required to ensure effectiveness.”

Precisely how well modified TiO2, implemented within a full-scale system, compares to the performance and economic characteristics of UV treatment remains to be seen.

Still, the market will surely respond to new technologies proven to provide significant benefits. If modified-TiO2 can deliver on its promise of cost-effective sunlight based disinfection it may very well emerge as a disruptive technology within the industry.

Based in Denmark, William Steel is a freelance contributor for WWi magazine. The TiO2 research described above was completed in collaboration with the Chinese Academy of Sciences, the University of New South Wales, Western Sydney University and the Australian Nuclear Science and Technology Organisation.