Aquatic Thermal and Photochemical Reactivity of N-(1,3-Dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD), N-Isopropyl-N′-phenyl-p-phenylenediamine (IPPD), and 6PPD-quinone
Kathryn L. Platt, Oleksandr Yushchenko, Juliana R. Laszakovits, Yiwen Zhang, Nicholas C. Pflug, Kristopher McNeill · 2025 · 22 citationsRead the paper
High Resolution Image Download MS PowerPoint Slide A ubiquitously used tire rubber antidegradant, 6PPD ( N -(1,3-dimethylbutyl)- N ′-phenyl- p -phenylenediamine), and its toxic ozonation product, 6PPD-quinone ( N -(1,3-dimethylbutyl)- N ′-phenyl- p -phenylenediamine quinone), have become recognized as important environmental pollutants since 6PPD-quinone (6PPD-Q) was identified as the likely cause of decades of mass Coho salmon kills. The reactivity of 6PPD, 6PPD-Q, and similar phenylenediamines requires study to better understand their environmental fate. This study explores the aquatic reactivity of 6PPD, N -isopropyl- N ′-phenyl-1,4-phenylenediamine (IPPD), and 6PPD-Q through thermal and photochemical pathways using both steady-state photochemistry and time-resolved laser spectroscopy techniques. 6PPD was found to rapidly degrade in the dark, with its degradation rate being highly dependent on the pH, temperature, and oxygen concentrations. IPPD behaves similarly to 6PPD. In contrast, 6PPD-Q is much more stable in the dark. All three chemicals are degraded via direct photochemistry. Regarding indirect photochemistry, 3 CDOM* plays a role in the degradation of 6PPD and IPPD but not 6PPD-Q, while 1 O 2 does not play a significant role for any of the compounds. Reaction rate constants are reported as well as 6PPD-Q molar yields from 6PPD, which were minimal for all aqueous pathways examined. 6PPD-Q may have a longer environmental lifetime as there are fewer degradation pathways. This research will help us to better understand and control these chemicals in the environment.
What the full text shows
Shown in the lab- 6PPD and IPPD degrade rapidly in the dark, with 6PPD at pH 7 being the most reactive. — k = 4.5 x 10^-5 s^-1 (6PPD pH 7) > 2.9 x 10^-5 s^-1 (IPPD pH 7) > 0.42 x 10^-5 s^-1 (6PPD pH 5) (19 °C, 12.5 mM phosphate buffer)
- 6PPD-Q is highly stable in the dark compared to 6PPD and IPPD. — No depletion over 30 h (19 °C, 12.5 mM phosphate buffer)
- Direct photolysis is more efficient under UVB than UVA for 6PPD. — Quantum yield: 0.92% (UVB) vs 0.06% (UVA) (6PPD pH 7, 21% O2)
- 6PPD-Q degradation is primarily driven by direct photochemistry, specifically UVA. — '' (Aquatic solution)
- Indirect photochemistry via triplet-state organic matter (3CDOM*) enhances degradation of 6PPD and IPPD, but not 6PPD-Q. — '' (5 mg C/L MRNOM or SRNOM)
- 6PPD-Q is not produced in substantial amounts from 6PPD via any aqueous pathway examined. — Minimal molar yields (Dark, direct, and indirect aqueous photochemical pathways)
- Setup
- Samples of 6PPD, IPPD, and 6PPD-Q in 12.5 mM phosphate buffer (pH 5 and 7) or 50/50 ACN/buffer; DOM sources included MRNOM and SRNOM (5 mg C/L).
- How
- The study used steady-state photochemistry, time-resolved laser spectroscopy (phosphorescence and transient absorption), and HPLC-QQQ/UV analysis to measure degradation rates and product yields. Thermal reactions were monitored via UV-vis spectrophotometry using BQMI as a proxy for 6PPD decay.
- Limitations
- Low water solubility of 6PPD and 6PPD-Q required ACN mixtures for laser experiments. · 6PPD-Q stability was observed over a relatively short window (30h) compared to some cited literature (47 days).
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