Valorization of Industrial and Agro-Industrial Wastes in the Green Synthesis of Mordenite Zeolite for Efficient Heavy Metal Adsorption

This study presents a sustainable synthesis route for mordenite zeolite (MOR) using rice husk ash (RHA) as the silica source and waste porcelain (WP) as the aluminum source, with tetraethylammonium hydroxide (TEAOH) employed as the organic structure-directing agent (OSDA). Zeolites were synthesized with controlled Si/Al molar ratios (10, 20, and 30) to assess how framework composition influences their structural, textural, and preliminar adsorption properties. The materials were characterized using X-ray diffraction (XRD), X-ray fluorescence (XRF), Fourier-transform infrared spectroscopy (FTIR), nitrogen physisorption (BET), thermogravimetric analysis (TGA), temperature-programmed desorption of ammonia (NH3-TPD) and cation exchange capacity (CEC). Increasing the Si/Al ratio (SAR) resulted in higher surface area, enhanced pore development, and improved thermal stability. The synthesized zeolites were then exploratorily evaluated for the removal of Pb2+, Cu2+, and Cd2+ ions under controlled pH conditions (4 and 8) at initial metal concentrations of 1–5 mg L−1, simulating textile effluents. The highest adsorption efficiencies were observed at pH 8 and for the sample with SAR = 10, particularly for Pb2+, followed by Cu2+ and Cd2+. This behavior is attributed to the greater density of framework negative charges and increased effective accessibility of acid sites in the more aluminum-rich material. Conversely, the SAR = 30 sample exhibited higher surface area and microporosity but lower affinity for metal cations, highlighting the role of framework composition in governing adsorption selectivity. Overall, the findings indicate the technical feasibility and environmental relevance of converting industrial and agro-industrial residues into zeolitic materials with promising performance for heavy-metal removal from aqueous systems. This approach promotes waste valorization while supporting the development of cleaner and potentially cost-effective water-treatment technologies.