Adsorptive removal of phenanthrene and naphthalene from wastewater using coal derived carbon nanoparticles with kinetic and thermodynamic evaluation

This study presents the synthesis, comprehensive characterization, and application of coal-derived carbon nanoparticles (CNPs) for the highly efficient removal of polycyclic aromatic hydrocarbons (PAHs), specifically phenanthrene and naphthalene, from aqueous solutions. Subbituminous coals sourced from the Gombe and Kogi regions of Nigeria were transformed into CNPs via a single-step CO₂-assisted solid-state activation process conducted at two distinct temperatures: 550 °C and 650 °C. The synthesized materials underwent rigorous characterization using proximate/ultimate analysis, Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDX), Fourier Transform Infrared Spectroscopy (FTIR), Thermogravimetric Analysis Differential Thermal Analysis (TGA-DTA), and Brunauer-Emmett-Teller (BET) surface area analysis. Characterization results revealed predominantly spherical nanoparticles (10-100 nm) with high carbon purity (> 90%) and tunable oxygen functionalities. FTIR confirmed the presence of conjugated C = C domains, while TGA-DTA demonstrated superior thermal stability for 650 °C-activated samples, exhibiting less than 10% mass loss up to 800 °C, indicative of extensive carbonization. Batch adsorption experiments were systematically performed to optimize parameters such as contact time, initial concentration, adsorbent dosage, pH, and temperature for phenanthrene and naphthalene removal. The 650 °C-activated Gombe sample (GomCNp650) exhibited the highest monolayer adsorption capacities (Qmax) of 1.26 mg g⁻¹ for phenanthrene and 1.45 mg g⁻¹ for naphthalene, achieving equilibrium within 150 min and 120 min, respectively. Kinetic data fitted pseudo-second-order models (R² > 0.99), and equilibrium data were best described by the Langmuir isotherm (R² > 0.97), indicating monolayer chemisorption on uniform sites. Thermodynamic analysis revealed that phenanthrene adsorption onto 650 °C samples was spontaneous and exothermic (ΔG < 0; ΔH ≈ -8 to -10 kJ mol⁻¹), whereas naphthalene uptake required higher temperatures to become favorable due to entropy contributions. The study highlights that activation temperature critically tunes the balance between hydrophobic π-π interactions and surface functionality, influencing adsorption mechanisms. While the observed adsorption capacities are relatively low and reusability showed a significant decline over three cycles, these findings offer valuable insights into the fundamental interactions governing PAH adsorption on coal-derived carbon nanoparticles, informing future material design for enhanced performance.