A new water-treatment technique that combines two expensive methods could prove a cheaper and more efficient way to remove hard-to-clean contaminants. The technology combines photocatalysis, which uses light to break down pollutants, and electrochemical oxidation, which uses an electrical current to do the same.
Aicheng Chen, an associate professor and Canada Research Chair of material and environmental chemistry at Lakehead University, in Ontario, has filed for a patent on the process and says that it could be commercialized within two years. Chen combined the two water-treatment methods by creating a dual-purpose electrode. On one side, the electrode is coated with a photocatalyst, and on the other with an electrocatalyst. Chen tested the electrode’s ability to remove two different nitrophenols–chemicals that are frequently used to manufacture drugs, pesticides, fungicides, and dyes and are commonly found in industrial wastewater. The dual-function electrode removed between 85 and 90 percent of the notoriously hard-to-remove pollutants over three hours, compared with only 30 and 60 percent for either technique alone. Chen’s results were published last month in the journal Environmental Science and Technology.
Photocatalysis and electrochemical oxidation have both been studied extensively for their use in water treatment but are not widely employed because neither is efficient enough to justify the cost. In photocatalysis, ultraviolet radiation strikes a catalyst–often titanium dioxide–boosting electrons in the material to a higher energy state. This, in turn, leaves free positively charged holes to oxidize pollutants. But photocatalysis is not very efficient because often the electrons simply rebind to the holes.
Electrochemical oxidation works by passing a current through a catalyst in water to oxidize pollutants. When combined with photocatalysis, this process boosts the efficiency in part because the current prevents the electrons and holes generated through photocatalysis from recombining.
The most economical and commonly used water treatment employs bacteria to break down pollutants. But biological treatment is not always the most effective, particularly for effluents with high concentrations of organic or toxic compounds, so water has to be treated repeatedly, often with chemicals like chlorine, which adds to the cost.
“Biological treatment is not useful for all wastewater,” says Chen. “In water with high concentrations of pollutants, very high pH, or very low pH, it is difficult for the bacteria to survive.”
According to a recent report by Lux Research, water use is projected to grow globally to 40 percent by 2030, and water-related revenues are projected to grow from around $500 billion in 2007 to nearly $1 trillion by 2030. Such forecasts have led to a surge of interest in new, potentially more efficient water-treatment technologies in recent years. As demand for clean water continues to grow, researchers are looking for new ways to treat contaminated water; according to another Lux report, an array of options is needed because the number of hard-to-remove contaminants found in wastewater is also growing.
Lux Research senior analyst Heather Landis says that Chen’s technology is unique and has potential. Other companies have used titanium dioxide in photocatalysis, but so far, no one has combined photocatalysis with electrochemistry, she says. But according to Landis, Chen will need to demonstrate the technique on wastewater samples that contain multiple contaminants, as opposed to just the pollutant nitrophenol.
Alexander Orlov, an assistant professor of materials science and engineering at Stony Brook University, in New York, says that Chen’s approach could find niche applications, particularly for treating wastewater with high concentrations of nitrophenols. However, Orlov says that one potential problem could be with the titanium dioxide catalyst, which tends to lose its reactivity over time. Further testing will have to be done to demonstrate its long-term viability, he says. While Chen acknowledges that this could be an issue, he says that overall, titanium dioxide is a good catalyst because it is chemically inert as well as nontoxic. However, Chen is also experimenting with nanostructures of titanium dioxide, which should be more resilient in the long run.
How the technique will fare compared with biological treatment is still unknown. Because biological treatment uses bacteria and requires little in the way of upkeep, it is relatively low cost. Chen says that biological treatment will be cheaper at least at first. But because his method is superior at removing nitrophenols, he believes that it could be used in conjunction with biological treatment, particularly for treating heavily contaminated industrial or agricultural wastewater. Chen says that his approach could also have a leg up on types of water treatment that use chemical treatments such as chlorine, which are less environmentally friendly.
The next step is to test the method on other pollutants, perform a cost analysis, and scale the process up. Chen says that his group is now working on building a prototype treatment plant, which should be completed by the end of the year.