Systematic insights into 6PPD-quinone biotransformation by acclimated aerobic sludge: Kinetics, pathways, toxicity, and enzymatic mechanisms.

Abstract

This study acclimated aerobic (AeS), nitrifying (NS), and denitrifying (DS) sludge systems to N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPD-Q) at 1-20 mg·L⁻¹ . Within an environmental to high concentration range (1 μg·L⁻¹ to 5 mg·L⁻¹), acclimated AeS removed 88.9%-95.2% of 6PPD‑Q within 14 days, significantly outperforming the NS and DS systems. The first-order degradation rate constant of the aerobic sludge ranged from 0.175 to 1.006 d⁻¹ . Based on the Haldane kinetic model, its substrate inhibition constant was 29.3 mg·L⁻¹ , demonstrating the broadest substrate tolerance range. This study proposed 13 biotransformation products (7 of which were reported for the first time), and constructed a complete degradation pathway accordingly. Ecological Structure-Activity Relationship modeling and in vitro bioassays jointly confirmed that AeS treatment significantly reduced the acute toxicity of 6PPD-Q. Metagenomic binning identified Sphingopyxis, Achromobacter, and Pseudomonas as the dominant genera, while molecular docking further implicated cytochrome P450 and peroxidase may serve as key enzymes in the catabolism of the parent compound. A 40‑day experiment showed that combining acclimated aerobic sludge with activated sludge significantly reduced 6PPD‑Q concentrations in real wastewater effluents. This study systematically elucidated the degradation pathway, toxicity evolution, and enzymatic mechanisms of 6PPD-Q in activated sludge, providing new insights for its bioremediation.