Closed-loop remediation of coal tar dense non-aqueous phase liquids (DNAPLs): Electrical resistance heating (ERH)-enhanced ionic liquid flushing with integrated activated carbon regeneration

Coal tar dense non-aqueous phase liquids (DNAPLs), widespread legacy contaminants at former industrial sites, pose significant environmental challenges due to high viscosity and persistent polycyclic aromatic hydrocarbons (PAHs) content. Conventional cosolvent flushing is hampered by solvent volatility, flammability, and recycling difficulties. We therefore developed a closed-loop system integrating ionic liquid (IL) flushing, electrical resistance heating (ERH), and activated carbon (AC) regeneration. Batch tests showed that 60% v/v 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF 4 ]) mobilized coal tar and solubilized > 60% of total PAH (ΣPAH) within 12 h at ambient temperature. Density functional theory calculations confirmed strong π–π stacking between [BMIM]⁺ and PAHs (e.g., interaction energy (ΔE) = –45.1 kJ/mol for phenanthrene). AC rapidly adsorbed PAHs from [BMIM][BF 4 ] solution with even greater affinity (ΔE = –59.1 kJ/mol for phenanthrene), enabling efficient IL regeneration. Column studies demonstrated enhanced ΣPAH removal at elevated temperature (93.6% at 60 °C vs. 76.8% at 20 °C within 2 h); this is particularly critical for the removal of high-molecular-weight PAHs (HMW PAH), attributed to a reduction in coal tar viscosity that promotes mobilization and intra-tar diffusion. A 2D sand tank experiment demonstrated 93.2% ΣPAH removal at 80 °C within 5 h under continuous AC regeneration and ERH-enhanced circular IL flushing. This innovative IL-ERH-AC system offers a safe, sustainable, and efficient solution for recalcitrant coal tar DNAPL remediation.