The other tire contaminants: benzothiazoles, water treatment, and the hidden chemistry of road runoff
Two posts ago, the story was 6PPD-quinone: a tire antioxidant that converts into a compound lethal to coho salmon at parts-per-trillion concentrations, discovered because people happened to be watching a fish kill on a Seattle-area creek[1]. It deserved the headlines, but it also distorted the picture of what tire leachate actually is. Tire wear particles do not leach one interesting molecule, they leach a cocktail, and by mass the most abundant that cocktail are not antioxidant at all. They are the vulcanization accelerators, the sulfur chemistry that makes rubber a solid instead of a sticky mess, and their transformation products.
This post is about that other chemistry: benzothiazoles and their relatives, where they come from. The short version is that a treatment system designed for a different century's contaminants treats these compounds as someone else's problem, at every stage, for different reasons.
Where benzothiazoles come from
A tire is roughly half synthetic rubber by weight, and raw rubber is not useful. Polyisoprene chains are viscous, sticky, and slowly oxidizing. Vulcanization fixes that: sulfur bridges cross-link the chains into an elastic network, and accelerators make the cross-linking happen in minutes at manufacturing temperature instead of hours. The workhorse acccelerators are 2-mercaptobenzothiazoles (MBT) and the sulfenamides built from it. A typical tire contains percent-level quantities of this chemistry.
When tire wear particles land on a road, the accelerators do not stay put. UV light, moisture, and oxygen work on the sulfur-nitrogen heterocycle, and a fairly consistent cascade follows: MBT oxidizes toward benzothiazole (BTH); hydrolysis and oxidation produce 2-hydroxybenzothiazole (2-OH-BTH); methylthio variants appear along the way. The parents have log Kow values around 1.2 to 2.5, so they are polar enough to dissolve in road runoff, and 2-OH-BTH in particular is stable and water-soluble enough to keep going. Kloepfer and colleagues documented the result in 2005[2]: benzothiazoles passing through municipal wastewater plants largely untouched and showing up in the receiving water, with tire abrasion on impervious surface as a major pathway. Twenty years on, the monitoring literature has only widened the list of matrices where they show up: urban runoff, groundwater, road dust, and the influent of drinking water utilities downstream of cities.
The mass balance is worth sitting with. 6PPD-quinone is potent at sub-microgram-per-liter levels, which is exactly why it makes a good headline and a hard analytical target. Benzothiazoles occur at higher concentrations by one to two orders of magnitude in raw leachate. Toxicity per microgram is lower; exposure per microgram is higher. A risk assessment that keys only on the famous compound misses most of the tire signal.
Why the conventional train struggles
Run a stormwater or drinking water source through a standard treatment train and each stage fails against benzothiazoles, each for its own reason.
Coagulation and sedimentation remove particles. Tire debris that is still particulate gets swept out with the floc. The dissolved fraction does not care: at these log Kow values the compounds partition weakly to sludge surfaces and stay in the water column. After clarification, the benzothiazole load is essentially the same as before it.
Granular activated carbon looks like the answer, and for a while it is. MBT and BTH adsorb well, through a combination of hydrophobic partitioning and pi-pi stacking on the graphenic carbon surface. The failure mode is competition. Natural organic matter in any real surface water is present at milligrams per liter and adsorbs alongside everything else, occupying sites and blocking pores. The polar transformation products lose that competition first. 2-OH-BTH, the most water-soluble and least hydrophobic member of the family, breaks through earliest, and it breaks through while the carbon still looks nominally fresh by conventional bed-life metrics. If your monitoring tracks only the parent compounds, the carbon will appear to be working during exactly the window when the transformation products are coming through.
Then there is disinfection, which turns a mobility problem into a chemistry problem. Chlorine reacts with the electron-rich sulfur and nitrogen centers in MBT and BTH and produces chlorinated transformation products, sulfoxides, and in some conditions dimeric species. Post 4 argued that a large fraction of the disinfection byproduct signal in real treated water is unidentified. Tire-derived heterocycles are one of the feeds into that unidentified pool. A utility that meets every regulatory DBP number can still be forming compounds nobody has put on a list.
What ozone and carbon do differently
Ozone attacks benzothiazoles where chlorine does, but with different products and a different purpose. The sulfur atom and the heterocyclic nitrogen are electron-rich, and ozone reacts with them at rate constants that put these compounds among the faster-reacting micropollutants.[3] Parent MBT and BTH degrade quickly at conventional ozone doses.
The catch is that destroying the aromatic signal is not the same as destroying the compound. Ozonation opens the heterocycle and leaves a set of small, polar, oxygenated fragments: aliphatic carboxylic acids, sulfates, amines. These are chemically transformed, but they are also more mobile than what came in, and some of them are exactly the size and polarity range that a downstream adsorption step handles best.
That is the logic of the multi-barrier train. Ozone converts refractory heterocycles into biodegradable fragments, and biologically active carbon, GAC that has been colonized by a biofilm after ozone, consumes those fragments along with the residual dissolved organic carbon. Each step covers the other's weakness: carbon alone breaks through on polar compounds, ozone alone produces polar compounds, ozone followed by bioactive carbon handles both. It is worth being precise about what this buys you. Measured across chemical classes, ozone-reactive compounds (PPD antioxidants, the parents like MBT and BTH) come out first, partially oxidized intermediates are handled by the biologically active stage, and the most polar transformation products remain the hardest fraction, which is why class-based monitoring across the whole transformation cascade, rather than a parent-compound checklist, is the honest way to evaluate one of these plants.
What this means for monitoring
Three practical consequences follow.
First, monitoring only parents undercounts the problem. The benzothiazole story is a cascade: sulfenamide to MBT to BTH to 2-OH-BTH and beyond, with the mobile end-products persisting longest. A suspect list or LC-MS/MS panel built on parent accelerators will report a plant as clean while its most mobile products pass through. Non-target screening, or at minimum a multi-analyte panel covering the transformation products, is what catches this.
Second, the analytical tools for this are the ones this series keeps coming back to. These homologous series are exactly what high-resolution MS finds in a non-target run: related structures with common fragments, appearing as a family in the chromatogram. The question of whether an unknown feature belongs to the benzothiazole cascade is often answerable from the fragmentation pattern alone.
Third, the treatment design question is real. The long-term fix for 6PPD-quinone is replacing the antioxidant; the long-term fix for benzothiazoles is less obvious, because vulcanization accelerators are load-bearing chemistry. Safer alternatives exist and are an active research area, but in the meantime the exposure pathway runs through stormwater infrastructure that most cities have not budgeted to upgrade. Treatment plants downstream of dense road networks are, for now, the operational line of defense, and the multi-barrier design is the part of the toolkit that works.
The tire story that started with a dead salmon is really a story about a chemical system: additives, transformation products, and the infrastructure that either intercepts them or passes them downstream. 6PPD-quinone was one branch of that system. The benzothiazoles are another, and by most measures the larger one.
References
[1] Zhenyu Tian et al., A ubiquitous tire rubber–derived chemical induces acute mortality in coho salmon. Science 371, 185-189 (2021). DOI:10.1126/science.abd6951
[2] Achim Kloepfer, Martin Jekel, Thorsten Reemtsma; Occurrence, Sources, and Fate of Benzothiazoles in Municipal Wastewater Treatment Plants. Environ. Sci. Technol. 15 May 2005; 39 (10): 3792–3798. https://doi.org/10.1021/es048141e
[3] Lim, S.; Shi, J. L.; von Gunten, U.; McCurry, D. L. Ozonation of Organic Compounds in Water and Wastewater: A Critical Review. Water Research 2022, 213, 118053. https://doi.org/10.1016/j.watres.2022.118053