Mechanistic insights into Levonorgestrel adsorption onto sawdust-derived activated carbon: kinetics, isotherms, and density functional theory investigation

Abstract

The increasing detection of pharmaceutical contaminants in aquatic environments poses significant ecological and human health risks. This study investigated the removal of levonorgestrel (LNG), a synthetic progestin commonly found in contraceptive formulations, from aqueous solution using chemically modified and carbonised sawdust adsorbents. Sawdust precursor was chemically activated by steeping in saturated ammonium chloride solution for 24 h to obtain activated sawdust (ASD). A portion was subsequently carbonised at 623 K for 30 min, yielding carbonised sawdust (CSD). Both adsorbents were characterised using FTIR, SEM, and physicochemical parameters, including surface area, pHpzc, bulk density, and attrition. Batch adsorption experiments evaluated the effects of initial pH (2–11), adsorbent dosage (0.5–2.5 g), initial LNG concentration (60–300 mg/L), contact time (0–120 min), and temperature (298–328 K). Optimal removal efficiencies of 97.60% and 84.98% were achieved at pH 6 for CSD and ASD, respectively, with 0.5 g adsorbent dosage. Equilibrium data fitted well to both the Freundlich and Temkin isotherms, while kinetic data followed the pseudo-second-order model (R 2  > 0.98). Intra-particle diffusion studies indicated that pore diffusion governed the adsorption process. Density Functional Theory (DFT) calculations in both gas and aqueous phases revealed that LNG's HOMO (− 6.305 eV) and LUMO (− 1.347 eV) energies facilitate electron transfer interactions with the adsorbent surface. CSD demonstrated superior adsorption capacity compared to ASD, attributed to its higher surface area and porosity. These findings establish sawdust-derived adsorbents as cost-effective, sustainable materials for removing pharmaceutical micropollutants from wastewater.