Research treats chromium in wastewater with photocatalysts

April 4, 2023
Scientists used hourglass-shaped photocatalysts to convert the toxic pollutant hexavalent chromium into chromium.

The Tsinghua University Press announced that scientists have published research showing how photocatalytic technology may be a solution for treating hexavalent chromium in wastewater.

The paper was published in Polyoxometalates on March 23.

Hexavalent chromium is a toxic heavy metal pollutant and a byproduct of industrial processes. Researchers have been trying to find more effective ways to remove the contaminant from wastewater.

“Rapid industrialization causes an increased release of wastewater containing heavy metal ions,” said Yuan-Yuan Ma, a researcher at Hebei Normal University in Shijiazhuang, China. “Hexavalent chromium, which has high carcinogenicity and teratogenicity, is widely found in wastewater and can easily enter food chains.”

Photocatalysis technology is an appealing solution for removing heavy metals from wastewater because it is sustainable, cost-effective, and environmentally friendly.

The researchers used hourglass-shaped photocatalysts from phosphomolybdate-based crystalline structures. Researchers chose this particular type of photocatalyst because of its molecular properties and well-defined hourglass-type structure, which give it a wide light absorption ability and the band structure necessary to reduce levels of hexavalent chromium.

“The semiconductor photocatalysts in photocatalytic processes can absorb photons matched with their band gap energy, leading to the conversion of toxic hexavalent chromium to less toxic chromium,” said Ma.

Researchers tested four “hybrid” photocatalysts and compared them to six other photocatalysts. The hybrids had slightly different compositions, but all had the same hourglass-type structure that could be maintained up to 200 degrees Celsius. They had wide visible-light absorption and similar energy band structures.

Researchers labeled these as Hybrid 1, 2, 3, and 4. After 80 minutes of exposure to a 10W LED light, hybrids 1 and 3 had around a 90% reduction in hexavalent chromium, while 2 and 4 had around a 74% and 71% reduction in hexavalent chromium, respectively.

The hybrids generally performed better than any of the tested photocatalysts. Hybrids 1 and 3, which performed best, were both Mn{P4MO6}2-based hybrids. Hybrids 2 and 4 were Co{P4MO6}2-based. Researchers suspect that the better performance was due to structural differences that gave hybrids 1 and 3 a narrower band gap.

Looking ahead, researchers will focus on improving the design of the photocatalysts, while also planning for how to best bring this technology to a real-world wastewater setting. 

Image courtesy Institute of Chemical Research of Catalonia (ICIQ).
All images courtesy of Ecosorb.