Hybrid Adsorbent from Construction and Mollusk Shell Wastes for Model Cationic Dye Removal: Kinetic Performance, Thermodynamic Insights, and Fenton Regenerability

Abstract

Dye-containing effluents remain a concern in wastewater treatment because many synthetic dyes are persistent, chemically stable, and potentially toxic. In parallel, the construction industry, although closely linked to economic and social development, generates large amounts of construction and demolition waste (CDW), which often lacks adequate disposal. Mollusk shells are another locally abundant residue with limited reuse. Converting these abundant residues into low-cost adsorbents offers a promising strategy for wastewater treatment and waste valorization. This study evaluates the efficiency of an innovative hybrid adsorbent composed of CDW and Mytella charruana (MC) shells for removing methylene blue (MB) from aqueous solution. The composite CDW-MC (75:25, m m–1) produced a synergistic effect, increasing the adsorption capacity to approximately 8.0 mg g–1 under initial screening conditions, a gain of about 14% and 37% relative to CDW alone (7.00 mg g–1) and MC alone (5.85 mg g–1), respectively. Under equilibrium conditions, the Toth model estimated a maximum adsorption capacity of 20.83 mg g–1 at 60 °C for the CDW-MC composite. Characterization by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), X-ray fluorescence (XRF), Brunauer–Emmett–Teller (BET) analysis, and point of zero charge (pHPZC) indicated that the composite had a heterogeneous surface containing carbonate-, silicate-, and metal oxide-related sites. The adsorption process was found to be spontaneous, exothermic, and predominantly physical in nature, with a positive entropic contribution. Regeneration of the saturated adsorbent via the Fenton reaction, optimized through a factorial experimental design, demonstrated that Fe2+ concentration was the decisive factor governing process efficiency, maintaining regeneration performance above 70% over five successive cycles. Overall, these results show that the CDW-MC composite offers a technically feasible and environmentally sustainable route for treating dye-contaminated wastewater while promoting waste valorization, adsorbent reuse, and circular-economy principles.