KAOLIN-BASED ADSORBENT FOR ENVIRONMENTAL APPLICATIONS

Abstract

Mercury contamination remains a critical global concern owing to its persistence, toxicity, and tendency to bioaccumulate within the ecosystem. Conventional methods for mercury remediation, including ion exchange and chemical precipitation, are often hindered by high cost, complexity, sludge generation, or limited regeneration potential. Adsorption has received great interest because of its simplicity, scalability, efficiency, and ability to minimize the generation of harmful sludge. However, most adsorbents used in adsorption processes are in their powdered form, thereby posing challenges in terms of handling and regeneration. In this thesis, a novel kaolin-based pellet adsorbent was developed and systematically investigated to address these shortcomings. Natural kaolin was initially calcined and subjected to sequential acid–base treatments to enhance its pore structure and expose additional active sites. The surface of the treated kaolin was then functionalized with 3-mercaptopropyltrimethoxysilane (3-MPTS) to introduce thiol groups, thereby strengthening mercury-binding capabilities. The functionalized kaolin was subsequently shaped into pellets via extrusion in the presence of a polyvinyl alcohol (PVA) binder. Comprehensive physicochemical characterizations using X-ray fluorescence, X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, energy-dispersive spectroscopy, Brunauer–Emmett–Teller surface analysis, and zeta potential measurements confirmed that acid–base modification and thiol grafting markedly improved the surface properties of kaolin and its affinity for mercury ions. Batch adsorption experiments revealed that mercury uptake was strongly dependent on contact time, solution pH, adsorbent dosage, and pellet size. Equilibrium was reached at 48 h, with a removal efficiency of 64.2%–79.1% across the studied temperature range (293–313 K). Adsorption kinetics fit the pseudo-second-order model (R2 > 0.98), signifying chemisorption as the dominant mechanism. Thermodynamic parameters (ΔH = 18.8 kJ/mol; ΔS = 55.7 J/molK) indicated an endothermic and spontaneous process, with negative ΔG values over all temperatures tested. Desorption experiments using ethylenediaminetetraacetic acid disodium salt dihydrate and potassium iodide confirmed that mercury was strongly bound to the thiol-functionalized surface, reflecting robust surface complexation or covalent bonding. These findings open promising avenues for industrial applications, where reliable performance, mechanical stability, and minimal generation of secondary waste streams are critical. Furthermore, the adsorption insight gained herein can guide the design of advanced adsorbents from natural materials for broader heavy metal remediation.