Structure-dependent selective adsorption and removal of 6PPD and 6PPD-quinone in chloride-enabled aluminum...

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and its oxidation product 6PPD-quinone (6PPD-Q) are tire-derived contaminants increasingly detected in urban water matrices. Here, we systematically investigate aluminum-based electrocoagulation (EC) for their removal and elucidate the governing electrochemical and molecular mechanisms. Removal performance was strongly electrolyte-dependent: in 0.1 mol L^-1 NaCl at 1.5 V (vs. Ag/AgCl), 6PPD and 6PPD-Q removals reached 80.7 ± 2.2% (60 min) and 90.5 ± 1.5% (5 min), respectively, whereas sulfate media yielded < 15% removal. Electrochemical impedance and polarization analyses revealed that chloride promotes anodic depassivation, lowers charge-transfer resistance, and sustains active aluminum dissolution. Comparative anode experiments (Al, Pt, and boron-doped diamond) demonstrated that removal in the Al system was not governed by reactive chlorine species. Despite minimal free chlorine accumulation, Al-based EC outperformed Pt-based oxidation, indicating that in situ coagulation dominated over indirect oxidation. Kinetic modeling favored a pseudo-second-order adsorption framework, indicating adsorption-controlled sequestration. Density functional theory (DFT) calculations and molecular dynamics (MD) simulations provided a molecular-level basis for this structure-dependent difference. Reactive-site analysis identified the carbonyl oxygens of 6PPD-Q as its most nucleophilic sites, and cluster-based calculations showed that 6PPD-Q binds more strongly than 6PPD to aluminum hydroxide surfaces across both coordination and hydrogen-bonding modes. MD simulations on a solvated surface corroborated this stronger interfacial affinity, together providing a molecular-level explanation for the faster removal of 6PPD-Q relative to 6PPD. Ultimately, these findings establish that molecular structure governs the adsorption behavior and removal kinetics of tire-derived contaminants on in-situ aluminum flocs, with chloride-promoted dissolution as the enabling condition, providing a mechanistic basis for treating TWP-contaminated waters.