Abstract <p>The complexation of organic ligands, such as aspartic acid (Asp), with copper ions (Cu²⁺) in aquatic systems presents emerging challenges for water treatment processes. This study experimentally and theoretically investigates the impact of Cu²⁺-Asp complexes on halonitromethanes (HNMs) formation during chloramination and UV₂₅₄/chloramine treatment. Results demonstrated that the maximum concentrations of HNMs (e.g., CNM, DCNM, and TCNM) generated from Cu²⁺-Asp were 1.4-fold and 1.6-fold higher than those from Asp alone under chloramination and UV₂₅₄/chloramine treatment, respectively. Neutral pH and Cu²⁺ concentrations ranging from 1.0 to 3.0 mg L^-1 were identified as favorable conditions for HNMs formation. Mechanistically, the formation of a stable Cu²⁺-Asp complex facilitated a Cu²⁺/Cu¹⁺ redox cycle via Cu²⁺-Asp and Asp-Cu²⁺ charge-transfer interactions. Experimental analyses revealed that the enhanced HNMs formation was primarily attributed to the generation of Cl• radicals driven by Cu¹⁺ intermediates, rather than the catalytic effect of Cu²⁺ alone. Theoretically, machine learning models, particularly Gradient Boosting Regressor (R² = 0.968) and XGBoost (R² = 0.954), predicted HNMs formation with high accuracy, enabling optimization of disinfection parameters. Furthermore, plausible reaction pathways for HNMs formation from Asp were elucidated through experimental data and density functional theory (DFT) calculations. Finally, comparative assessments between simulated and actual water matrices confirmed the environmental relevance of the observed trends. This study offers novel insights into the catalytic and redox roles of Cu²⁺ complexes during disinfection, advancing the mechanistic understanding, predictive modeling, and risk assessment strategies for HNMs formation in water treatment systems.</p> Graphical Abstract <p></p>