Researchers from Hunan University and the Chinese Academy of Sciences have extended the use of covalent organic frameworks (COFs) to deal with problems facing removal of hexavalent chromium, or Cr(VI), in wastewater. According to Advanced Science News, the pores in their COF are Kagome-like in structure or similar in shape to the Star of David.
The framework is made up of this carbon- and nitrogen-based Kagome-lattice, with intermittent OH groups to create pores big enough to filter through and capture chromium ions.
Cr(VI) is a carcinogen found in tobacco smoke. However, Cr(VI) is also a common by-product in a number of industries; from electroplating to mining, leather tanning to printing, pigment manufacturing, and processes such as polishing and rust prevention.
According to Advanced Science News, Cr(VI) in effluent wastewater inevitably end up contaminating natural water sources and drinking water. In some cases, such contamination can easily be prevented.
For example, waste Cr(VI) makes it into some countries’ food supplies via contaminated animal feed or through safety breaches resulting in chemical spills in urban areas. According to Advanced Science News, these events pose a two-pronged risk; one to environmental protection and one to human health.
The United States’ Environmental Protection Agency sets a legal threshold of 100 µg of Cr(VI) per liter of drinking water, whereas the World Health Organisation sets a stricter limit of 50 µg, according to Advanced Science News. This has led to methods already in existence to remove Cr(VI) from wastewater, before it makes it into the environment and water supplies. However, these have undesirable drawbacks relating to cost, efficiency and drawn-out processes.
COFs offer a promising route away from these negative aspects of contemporary waste management methods, according to Advanced Science News. COFs are organic, crystalline, porous materials, which have already demonstrated successful removal of other heavy metals such as mercury, arsenic, neodymium and cadmium from water.
This COF is stable at room temperature, and appears to adsorb more Cr(VI) at lower pHs. According to Advanced Science News, this favors practical applications, since most Cr(VI)-containing industrial effluent is acidic. The COF has the ability to adsorb a lot of Cr(VI) very quickly and continues to adsorb more over a short period of time.