Partition-Buffered Leaching of 6PPD-Quinone from Tire Wear Particles

Abstract 6PPD-quinone (6PPDQ; N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine-quinone) forms on tire-derived solid surfaces, including those of intact tires and environmentally dispersed tire wear particles (TWPs), yet its leaching kinetics remain insufficiently characterized for confident use in watershed and coastal fate-and-transport models. Here, we conducted time-resolved saturated column leaching experiments to quantify 6PPDQ leaching from lab-generated TWPs and to derive a physically based parametrization. Across hour-to-month observation windows, TWPs exhibited sustained, nearly linear 6PPDQ release (α ≈ 1) and weak particle-size scaling of the characteristic time scale, inconsistent with whole-particle Fickian diffusion. We tested a mechanistic hypothesis that strong TWP-water partitioning buffers aqueous concentrations and limits 6PPDQ mass export. Using a well-mixed column reactor (WMCR) representation, we estimated the TWP-water partition coefficient Ksw for the columns. The regression yielded log10 Ksw = 4.37 ± 0.22. Building on this mechanistic interpretation, we derived a Newtonian first-order leaching term suitable for hydrologic and coastal modeling. A two-compartment formulation further indicated that the TWP concentration, cTWP, strongly controls the apparent fraction of 6PPDQ released to the dissolved phase. These findings may help explain variability across prior leaching studies while bridging inconsistencies between experimental and field measurements.