User:Nwbeeson/sandbox/CPPD-quinone

6PPD-quinone is an organic chemical widely distributed in the environment which is toxic to salmon. It is created by the use of 6PPD as a stabilising additive in rubbers, such as natural rubber, and the man-made rubbers styrene-butadiene and Butyl rubber; all of which are common in vehicle tires. During the life of the tire, the 6PPD is converted to 6PPD-quinone by ozone in the air. This prevents the ozone from destroying the rubber the tire is constructed from.

Although it is not directly used in tire manufacture

Application
6PPD is a common rubber antiozonant, with major application in vehicle tires. It is mobile within the rubber and slowly migrates to the surface via blooming. Here it forms a "scavenger-protective film", reacting with the ozone more quickly than the ozone can react with the rubber. This process forms aminoxyl radicals and was first thought to degrade only to the quinone diimine, but has since been understood to continue to oxidize to quinones, amongst other products. Despite 6PPD being used in tires since the mid 1960s, its transformation to quinones was first recognized in 2020. The oxidized products are not effective antiozonants, meaning that 6PPD is a sacrificial agent.


 * 6PPD ozone-oxidation overview.svg

The tendency of 6PPD to bloom towards the surface is protective because the surface film of antiozonant is replenished from reserves held within the rubber. However, this same property facilitates the transfer of 6PPD and its oxidation products into the environment as tire-wear debris. The 6PPD-quinone (6PPD-Q, CAS:2754428-18-5) is of particular and increasing concern, due to its toxicity to fish.

Environmental impact
6PPD and 6PPD-quinone enter the environment through tire-wear and are sufficiently water-soluble to enter river systems via urban runoff. From here they become widely distributed (at decreasing levels) from urban rivers through to estuaries, coasts and finally deep-sea areas.

6PPD-quinone is of environmental concern because it is toxic to coho salmon, killing them before they spawn in freshwater streams.

A 2022 study also identified the toxic impact on species like brook trout and rainbow trout. The published lethal concentrations are:
 * coho salmon: LC50 = 0.095 μg/L
 * brook trout: LC50 = 0.590 μg/L
 * rainbow trout: LC50 = 1.00 μg/L

It is not known why the ozone-oxidised 6PPD is toxic to coho salmon. The Nisqually and nonprofit Long Live the Kings are trying out a mobile stormwater filter at a bridge in the Ohop Valley. The Washington Department of Ecology, Washington State University and the US Tire Manufacturer's Association are working on regulation and education.

6PPD itself is deadly to rotifers, especially in combination with sodium chloride, though not at the level generally found in the runoff from road salt. A small-scale biomonitoring study in South China has shown shown both 6PPD and 6PPDQ to be present in human urine; concentrations were low but the health implications are unknown. A synthetic route to the 6PPD-quinone has been posted on ChemRxiv.