Adsorptive removal of polycyclic aromatic hydrocarbons from contaminated water by biomass from dead leaves of Halodule uninervis: kinetic and thermodynamic studies

Polycyclic aromatic hydrocarbons are carcinogenic organic pollutants that are produced primarily by daily industrial and personal activities. This work studies the use of an abundant seagrass powder (SG) as an adsorbent to remove the relatively highly soluble PAHs (i.e., Acenaphthylene (Ace), Phenanthrene (Phen), and Flouranthene (Flu)) from contaminated water. At an optimum dose of 2 g/L, Flu was found to have the highest removal efficiency of 90.54%, followed by Phen with a removal efficiency of 87.89% and Ace with an efficiency of 61.11%. Flu had a maximum adsorption capacity of 2.25 mg/g at an equilibrium time of 6 h, followed by Phen with q_e of 2.12 mg/g at an equilibrium time of 24 h, and finally Ace with q_e of 1.13 mg/g at an equilibrium time of 120 h. Linear and non-linear isotherm Langmuir, Freundlich, and Temkin models fit largely well with adsorption data for Flu, Phen, and Ace. This suggests that coexistence of heterogeneous adsorption mechanism. The kinetic data of all 3 PAHs were best described by the pseudo-second-order model. The adsorption of both Flu and Phen by SG was observed to be spontaneous with negative ΔG values. On the other hand, the adsorption of Ace was spontaneous only at a low temperature. The change of enthalpy (ΔH) for all the studied compounds was negative; therefore, their adsorption was found to be exothermic. Physisorption appeared to dominate the adsorption mechanism of Flu and Phen, with ΔH values of − 36.03 kJ/mol and − 36.65 kJ/mol respectively. However, Ace adsorption showed some presence of chemisorption, with ΔH values of − 114.43 kJ/mol.