Interaction between 6PPD/6PPD-Q and natural Fe-Mn nodules: Performance and mechanism of adsorption and oxidative transformation

The widely used rubber antioxidant N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and its ozonated product, 6PPD-quinone (6PPD-Q), are highly toxic to aquatic life, yet understanding on their environmental behaviors is limited. This study comprehensively investigated their adsorption and transformation processes on natural Fe-Mn nodules (NFMN), which commonly exist in sediments and soils through a combination of diverse experimental and computational methods. The maximum adsorption capacity of 6PPD-Q (719.2 μg·g^-1) is significantly higher than that of 6PPD (133.8 μg·g^-1) at 293 K, and it is more difficult to desorb. They follow different kinetic and isothermal adsorption models, and environmental conditions (including temperature, pH, and anions) exert distinct influences on the adsorption of the two substances. Adsorption mechanisms involving electrostatic attraction, charge transfer, hydrogen bonding, and Lewis acid-base complexation were unveiled. For 6PPD, electrostatic adsorption and Lewis acid-base complexation contribute significantly to its adsorption. Conversely, for 6PPD-Q, the contribution of Lewis acid-base complexation outweighs that of hydrogen bonding, while the effect of electrostatic adsorption is relatively negligible. The stronger electrostatic attraction, more efficient charge transfer, and a greater number of binding sites for hydrogen bonding and Lewis acid-base complexation with NFMN results in more robust adsorption of 6PPD-Q. Furthermore, 6PPD can transform into 6PPD-Q on NFMN, facilitated by dissolved Mn(III). This study advances understanding of the adsorption behavior and mechanism of 6PPD and 6PPD-Q, and highlights a new pathway for 6PPD-Q formation, which provides valuable reference for assessing the water body exposure risks and formulating environmental remediation strategies for such pollutants. ENVIRONMENTAL IMPLICATION: This study offers the first comprehensive insight into the interactions between 6PPD/6PPD-Q and NFMN, illuminating their environmental behavior in water and soil systems. It reveals the adsorption discrepancy between 6PPD and 6PPD-Q, and elucidates the mechanisms underlying the difference in adsorption. Additionally, it uncovers a novel pathway for 6PPD-Q formation, offering valuable implications for risk assessment and environmental remediation strategies.