Normally I just post whatever interesting bit of chemistry catches my interest on a given week, but today I'm posting about something of special interest to me. It's not about my work specifically, but it is about pollution remediation—and that, in a broad sense, is what I do.
This fascinating bit of cleanup chemistry targets some of the most difficult to remove pollutants. The unsightly colour of lignin stained water coming out of a pulp mill, pharmaceuticals passing through sewage plants (page 2), pesticide and herbicide runoff from farms, parks, golf courses, and lawns, and many others. Chemical warfare agents are even on the list of targets.
However, one of the things we have to consider when cleaning up pollution is whether the cleaning agent is itself a pollutant, or if the cleaning agent is specific enough that if any is left after doing its cleanup job, it doesn't go and "clean" stuff that's meant to be there. Or, similar to the case of chlorine, a highly effective disinfectant, what the reaction byproducts of the cleaning are and how to control the system so that those are maintained at a safe level while still getting the cleaning done to a safe level.
This particular cleaner is actually an entire class of chemicals called TAML activators or TAML catalysts (tetra-amido macrocyclic ligand). The core of TAML catalysts are all the same, an iron atom surrounded by four nitrogen atoms. Surrounding that and bonded to the nitrogen is a variety of shapes formed by the ever-useful carbon.
As this is a catalyst, not a reactive disinfectant like chlorine, it takes far less TAML than you'd think to clean a given amount of pollution. Specifically, one molecule of TAML can break up far more than one molecule of pollutant, which is the neat thing about catalysts. Fortunately, catalysts also decay over time and eventually stop working, because a catalyst that's good at breaking apart large organic molecules isn't something you want out in the wild where life depends on large organic molecules. Turns out the variety of things around the iron/nitrogen core affects several things, including how long the catalyst lasts before falling apart, how active it is, how non-toxic it is, and so on.
As far as reaction byproducts go, TAML seems to chop the pollutants up into pretty small, innocuous pieces.
If you don't understand reaction pathways then just follow the arrows to see that the pollutant (top left) is broken into little bits which are not pollutants (bottom right). The TAML does a pretty thorough job on the pesticide in that example.
The above example reaction is one tiny bit of a very interesting publication on the mechanisms of how TAML catalysts actually work.
I have to admit I was particularly amused at the short description on the TAML website about how they designed the different carbon surrounds that fit around the iron core. Even if you don't know organic chemistry or have any understanding of how they picked different structures, it was basically a series of: test to destruction, figure out where it broke, fix that, test to destruction again. Very systematic and effective trial-and-error, and it gives them lots of options if they need a short-lived catalyst for some application, as well as the more durable ones that last longer.