A low-cost water hyacinth-based adsorbent for free fatty acids removal from waste cooking oil: kinetic, isotherm, and thermodynamic studies

The pre-treatment of waste cooking oil (WCO) by removing free fatty acids (FFAs) is pivotal for biodiesel production. This study develops a low-cost, efficient adsorbent derived from water hyacinth (Eichhornia crassipes), activated with 5 M NaOH, termed WHA-H, for this purpose. Characterization through FT-IR and SEM analyses revealed that NaOH activation significantly enhanced the surface roughness and functional group availability on WHA-H, leading to improved adsorption capabilities. Nitrogen adsorption–desorption isotherms of WHA-H confirmed a complex pore structure with Type II and Type IV isotherms combination, indicating the presence of both meso- and macroporosity. Kinetic studies conformed to the pseudo-second-order model, suggesting chemisorption as the primary FFA adsorption mechanism, while isotherm data were best described by the Langmuir model, indicating monolayer coverage on a homogeneous surface. WHA-H exhibited a maximum FFA adsorption capacity (q_m) of 1666.67 mg g⁻¹. Thermodynamic parameters indicated that the adsorption process is spontaneous and exothermic, with desorption studies establishing diethyl ether as an effective solvent for WHA-H regeneration. Our study not only demonstrates WHA-H’s potential as a sustainable adsorbent for improving WCO quality but also offers an eco-friendly approach to managing the invasive water hyacinth.