Structure–Property Relationships in Metakaolin Geopolymers Modified with Shell-Derived Calcium Particles for Multifunctional Wastewater Treatment

The sustainable valorization of marine shell waste as functional additives for geopolymer materials represents a promising strategy for developing multifunctional materials for environmental remediation. In this study, metakaolin-based geopolymers were modified with calcium-rich particles obtained by calcination of five marine shell species (Chamelea gallina, Mya arenaria, Mytilus edulis, Pecten maximus, and Rapana venosa) under identical synthesis conditions to evaluate the influence of shell mineralogy on the structural, textural, adsorption, and antibacterial properties of the resulting composites. The materials were comprehensively characterized by Fourier transform infrared spectroscopy in attenuated total reflectance (ATR-FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and nitrogen adsorption–desorption (BET/BJH) analyses. Functional performance was assessed through methylene blue (MB) adsorption experiments, adsorption kinetic modeling, and antibacterial tests against Escherichia coli (E. coli). ATR-FTIR and XRD analyses confirmed the formation of a stable amorphous geopolymer network containing residual crystalline phases together with shell-derived calcium carbonate, predominantly as calcite or aragonite depending on shell origin. The incorporation of shell-derived particles modified the pore architecture of the geopolymers. GP-SJ exhibited the highest BET specific surface area (94.30 m2 g−1) and the most developed mesoporous structure. Among the investigated formulations, GP-RP showed the most favorable overall combination of functional properties under the investigated conditions, exhibiting the highest methylene blue removal efficiency (63.97%) and experimental adsorption capacity at 160 min (9.60 mg g−1), together with a comparatively high reduction in recoverable E. coli colonies during preliminary antibacterial screening. The combined structural and functional analyses demonstrate that the environmental performance of shell-modified geopolymers cannot be predicted from a single parameter such as BET surface area or calcium content alone, but results from the synergistic interaction between mineralogical composition, particle dispersion, pore accessibility, and matrix compactness. Under the investigated conditions, these findings provide evidence for proposed structure–property correlations under the investigated conditions and suggests that shell-derived calcium particles act as microstructural regulators of geopolymer matrices, providing a basis for the further development of sustainable multifunctional materials for simultaneous dye removal and bacterial reduction in wastewater treatment.