Mechanistic insights into the transformation and toxicity evolution of IPPD-Q during chlorination and chloramination in drinking water

As the tire-derived quinone, IPPD-Q emerges as one of the most acutely toxic emerging contaminants in urban waters, its fate during final disinfection remains unclear. This study demonstrates that IPPD-Q reacts rapidly with both chlorine and chloramine, exhibiting second-order rate constants of 110.4 ± 10.9 M^-1 s^-1 and 46.2 ± 3.8 M^-1 s^-1 at pH 7.0, respectively. HOCl dominates over OCl⁻ in chlorination, whereas NH₂Cl is the sole reactive species in chloramination during pH 6.0 ∼ 8.0. Eighteen chlorinated and twelve chloraminated transformation products (TPs) were structurally elucidated, revealing that HOCl drives sequential Cl substitutions at C2, C9 and C12 or OH substitution at C9 and C12 sites followed by benzoquinone cleavage. Whereas, NH₂Cl yields both Cl-substituted TPs via Cl substitution at C2 and C12 sites and unique NH₂-substituted TPs via nitrogen incorporation at C9 site. Density functional theory (DFT) calculations show that electrophilic attack is governed by the lowest activation free energy at C2 (ΔG^‡ = 11.95 kcal/mol for HOCl and 10.82 kcal/mol for NH₂Cl), aligning with the observed preferential formation of highly toxic chlorinated TPs. Zebrafish embryo assays combined with ECOSAR predictions reveal that some TPs are more toxic than IPPD-Q itself, with chlorination TPs exhibiting the highest lethality. Dynamic electron-distribution analysis unravels synchronous σ_C-H/π_CC bond rupture and σ_C-Cl/C-N/C-O bond formation directed by frontier-orbital interactions. This work provides the first molecular-orbital-based roadmap for predicting and mitigating the risks associated with tire-derived Quinones during drinking water disinfection.