It is well accepted that hydroxyl radicals, unlike other radical species, react non-selectively with different organic micro-pollutants. But, it has also been demonstrated in the past few years that hydroxyl radicals can’t degrade (i.e. inability to react with) certain organic micro-pollutants like perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS). But, authors of scientific journal articles still refer hydroxyl radicals as non-selective oxidizing species. Should the notion of non-selective reaction of hydroxyl radicals towards organic micro-pollutants now be changed? Can you share your comments/views?
@Trevor. This is not about atmospheric chemistry and VOCs. Hydroxyradicals are important for the turnover of many chemicals in surface water and their re-activity are utilized in a family of engineered processes known as advanced oxidation processes (AOP).
It has been known for decades that hydroxyradicals reacts slower with highly halogenated chemicals like chloroform than with most other organic molecules.
The labeling of hydroxyradicals as non-selective is in my opinion mostly based on that their reaction has very similar rate constants for reactions with most organic molecules in water solution which is quite unusual.
Since we have known about the exceptions from the generally similar re-activity of hydroxyradicals a long time I don't think the discovery of a few more chemicals that do not follow the general patterns should prevent us from using a term like "mostly unselective" for the reactivity of hydrocyradicals.
Indeed, I share the same opinion. Generally, rate constants only vary over 3 orders of magnitude (10^8-10^10), with most of them in the order of 10^9 M-1s-1. It is known that halogenated compounds have lower reactivity due to the electron withdrawing properties of the halogens. But even a significant number of halogenated organics still excert high reactivity towards HO. As 99% of the organics follow the general rule, the term "unselective" is appropriate.
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