“This is an exciting finding because it helps to address a problem that has been a real challenge for the water-treatment process. We anticipate that integrating our titanium-oxide photocatalyst into the current processes could improve its effectiveness in removing these chemicals, as well as reducing the amount of energy required to do so.”Michel Barsoum, Drexel University
Discharged in large quantities by textile, cosmetic, ink, paper and other manufacturers, dyes carry high-toxicity and can bring potential carcinogens to wastewater. It’s a major concern for wastewater treatment – but researchers in Drexel University’s College of Engineering may now have found a solution, using a tiny nanofilament.
A study lead Michel Barsoum, a professor in the College of Engineering, and his team found that a one-dimensional, lepidocrocite-structured titanium oxide photocatalyst material has the ability to break down two common dye pollutants — rhodamine 6G and crystal violet — when illuminated with visible light. Using an equal ratio of dye to catalyst, it was able to reduce concentrations of the two dyes in water by 90% and 64%, respectively, in just 30 minutes.
“This is an exciting finding because it helps to address a problem that has been a real challenge for the water-treatment process,” Barsoum said. “We anticipate that integrating our titanium-oxide photocatalyst into the current processes could improve its effectiveness in removing these chemicals, as well as reducing the amount of energy required to do so.”
On encountering a nanofilament, the dye adheres to its surface and then undergoes photocatalysis when illuminated, with the dye sensitizing the nanofilaments to visible light. This process accelerates degradation, causing the dye to break apart into harmless byproducts such as carbon dioxide and water.
The study, reported in a paper in Matter, found that the key to the dye degradation and self-sensitization process was the ability of the material to generate electron holes and reactive oxygen species.
The two dyes are common effluents in wastewater. Effluent, which literally means something that flows out, is different to the sewage found in wastewater. While solid waste can be filtered out before the water is purified, effluent is suspended in the water, making it hard to separate and remove.
Rhodamine 6G is a xanthene-derived dye primarily used in wood processing, paper dyeing, pen ink and cosmetics. Crystal violet, a triphenylmethane dye, is used to dye ink and textiles. These dyes are water soluble and any excess is discharged as effluent.
Wastewater is a major environmental concern worldwide and its existence has long-term impacts on the health of humans, aquatic plants and animals. Households and industry generate nearly 380 billion cubic tons of wastewater globally each year. Only 24% of this is treated sufficiently due to various challenges, including high energy consumption, the existence of residual chemicals, and the insufficient processing of complex and persistent contaminants, including dyes.
According to the researchers, the most common wastewater treatment methods, such as sedimentation, biological oxidation and chemical-physical treatment, are ineffective at removing dyes, due to the dyes’ complex molecular structure and water-soluble nature.
Adsorption of dyes by clay materials, activated carbon, iron oxide and natural materials such as coffee grounds has been tried before and exhibited high uptakes. However, these materials simply separate the dye from the water — the dye still exists and is simply attached to the adsorbent materials within the wastewater.
Photocatalysts, long thought to be the key to removing dyes from water, have thus far not produced a sustainable solution. According to Barsoum, many photocatalysts typically require UV light treatment, which uses extensive energy. The impact of the new nanofilament resides in its self-sensitization behavior, which makes the nanofilament more sensitive to visible light.
“The use of visible light – light the human eye can see – like the sun or other simulated light sources, could significantly reduce the financial and energy consumption costs associated with treatment, while still being highly effective at removing dyes from wastewater, eliminating the toxic effluents,” said Adam Walter, a doctoral student in Barsoum’s research group, and first author of the paper. “This also presents an exciting opportunity for expansion into other fields like solar cells or optical devices.”
The team used X-ray diffraction to characterize the arrangement of atoms in the nanofilaments. They further characterized them using scanning and transmission electron microscopy, which fires beams of electrons at the material to form an image. To assess dye decolorization, the team monitored the sample using ultraviolent-visible spectroscopy and quantified mineralization by chemical oxygen demand.
One of the most important findings of the study was strong evidence that the nanofilament is sensitized by the dye, representing a symbiotic relationship between the additive and effluent that resulted in cleaner, less toxic water. One way of thinking about this, Walter said, is that the dye catalyzes its own destruction.
This sensitization opens the door to using the nanofilament for applications in solar cells and optical devices. Earlier this year, the same nanofilament was studied by the team and found to harness sunlight for hydrogen separation, which could unlock its potential in green-fuel generation.
“We are just beginning to uncover the possibilities of this material,” Barsoum said. “As we better understand the processes enabling its behavior, we anticipate exploring new applications where it could improve the performance of technology that the world needs to move toward a more sustainable future.”
This story is adapted from material from Drexel University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.