Transport of polycyclic aromatic hydrocarbons (PAHs) in DOM-influenced complex groundwater systems: Insights into aromatic ring number and π-electron characteristics

Polycyclic aromatic hydrocarbons (PAH) contamination in groundwater is a global environmental and public health concern, strongly influenced by dissolved organic matter (DOM) and geochemical conditions. In DOM-rich and redox-active groundwater systems, coupled physicochemical and microbial processes regulate PAHs transport, yet the relationships between molecular characteristics and transport behavior under such geochemical conditions remain poorly understood. Herein, we investigated the coupled processes of sorption, transport, and biodegradation of naphthalene (NAP, 2-ring) and phenanthrene (PHE, 3-ring) across molecular to pore scales, from the perspectives of aromatic ring number and π-electron characteristics. Specifically, PHE exhibited slower transport than NAP, with higher retardation factors ( R = 4.38 vs. 2.94), consistent with its stronger sorption affinity ( K F = 2.96 vs. 0.23 at 318 K). Molecular simulations suggested that enhanced π-electron delocalization of PHE increased electrostatic polarization and interactions with hydrated metal species ([Fe(H 2 O) 6 ] 3+ and [Mn(H 2 O) 6 ] 2+ ), resulting in stronger adsorption energies (−65.56 to −60.13 vs. −57.13 to −56.92 kJ mol −1 ), shorter interaction distances (2.08 Å vs. 2.22 Å), and lower diffusion coefficients (0.031 vs. 0.077 Å 2 ps −1 ) compared to NAP. Under DOM-influenced conditions, PAHs transport exhibited scale-dependent behavior: π-mediated interactions with DOM and hydrated metal species were associated with reduced molecular mobility at the molecular-scale, whereas DOM-associated processes may facilitate PAHs co-transport at the pore-scale. Anaerobic enrichment of Desulfosporosinus and Trichoderma was associated with enhanced microbial biodegradation of PAHs. Overall, this study provides new insights into contaminant transport behavior, contributing to improved prediction, risk assessment, and groundwater management in complex groundwater systems.