Acids are neutralised and poured down the sink, heavy metals head into their own container, some things need to be handed to your laboratory heroes—the service room experts—to be handled. Widely-used organic solvents are particularly difficult to dispose of.
In a stroke of irony, one of the most wasteful chemistries is also one of the most important for sustainability: catalyst design. Dancing in the face of that irony, a group of chemists in Brazil have recently turned a heavy metal-based environmental pollutant into a catalyst that breaks down those tricky toxic solvents.
Catalysis is a tricky beast. It has tendrils in biochemistry and inorganic chemistry, physical chemistry and, of course, organic chemistry. A good catalyst is an aesthetic enigma: they are absurdly complex, nearly grotesque molecules, with lithe mechanisms that twirl and dip and move bits of substrates just so, coaxing energetically exhausting reactions into reality. Catalyst design is a sort of frustrating art. There are lots of failed attempts before the masterpiece.
This Brazilian group have truly created a work of art in their new green catalyst. They start with a gel made of chitosan and add iron ions to make beads. Chitosan is a biopolymer that comes from treating crustacean shells with a base, so it’s a common waste product from the fishing industry. Not only that, they then take chromium ions, which are a useless waste product from leather tanning processes, and add them directly onto the chitosan-iron (CT-Fe) beads. Chromium ions are also a heavy metal water contaminant, which means that they are highly toxic to humans, so removing them from water by direct adsorption onto the CT-Fe beads actually protects us.
Those so-called useless, toxic chromium ions are the key ingredient in the catalyst, in that they actually become a promoter in the catalytic cycle in the next step of the process. This process is called the Fenton Process, and involves making highly reactive free radicals (yes, like the ones you don’t want in your body or in the ozone layer!) that tear apart difficult organic compounds. Ripping these organic compounds, like solvents, apart, leaves compounds that are often easier to dispose of, or even better, useful as starting points for synthesising new, useful things.
This new catalyst is a trifecta of sustainability, taking two unwanted, even dangerous waste products and constructing something to break down other tricky products. It’s a beautiful example of the creativity and resourcefulness that make up the core of catalyst design. It reminds us that even though chemistry can be hugely wasteful, it’s not a huge waste.
Chagas, P. M. B.; Caetano, A. A.; Tireli, A. A.; Cesar, P. H. S.; Corrêa, A. D.; Guimarães, I. do R. Use of an Environmental Pollutant From Hexavalent Chromium Removal as a Green Catalyst in The Fenton Process. Sci. Rep. 2019, 9 (1), 12819. doi.org/10.1038/s41598-019-49196-9.