Acid-Modified and Surfactant-Functionalized Bentonite and Clinoptilolite for Naproxen Adsorption from Water

Pharmaceutical contamination of aquatic environments, particularly by nonsteroidal anti-inflammatory drugs such as naproxen, remains a persistent challenge for conventional water treatment. This study evaluated the naproxen adsorption performance of bentonite and clinoptilolite in their natural, acid-activated, and hexadecyltrimethylammonium (HDTMA)-modified forms. Materials were characterized by Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and nitrogen physisorption, confirming that acid activation increased surface area and porosity while HDTMA introduced an organophilic character without disrupting the crystalline framework. Adsorption experiments identified pH 6 as optimal for all HDTMA-modified materials. Sequentially modified clinoptilolite (C-AS) achieved the highest capacity, reaching 14.8, 16.5, and 19.9 mg g^-1 at 25, 30, and 35 °C, respectively, outperforming all bentonite-based materials, whose interlayer pore blockage limited surfactant benefits. Kinetic data were best described by the pseudo-second-order model for most materials, while the Elovich and Avrami fractional-order models provided superior fits overall, with sub-unitary Avrami exponents indicating a heterogeneous, multi-pathway adsorption mechanism; Weber-Morris analysis further confirmed that intraparticle diffusion was not the sole rate-limiting step. Equilibrium data showed mixed Langmuir-Freundlich behavior that was resolved using the Sips model, and thermodynamic parameters derived from a three-temperature (25-35 °C) study confirmed spontaneous, endothermic adsorption governed by physisorption. These results demonstrate that the pairing of acid activation with surfactant functionalization is markedly more effective for zeolitic frameworks than for layered clays, offering a low-cost strategy for pharmaceutical removal from water.