Evaluation of photocatalytic ZnO-geopolymer composites: degradation of methylene blue and α-pinene

Alkali-activated ZnO–geopolymer composites were synthesised using metakaolin to investigate their photocatalytic degradation efficiency toward methylene blue and α-pinene under 253.7 nm irradiation. Composites prepared with bulk Si/Al molar ratios (1 and 2), ZnO/Al loadings (0.10, 0.15, and 0.20), were characterised using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), N2-sorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and UV-vis spectroscopy. A custom-built flow cell was used for gas-phase α-pinene degradation, while aqueous methylene blue degradation was evaluated under batch conditions. A key finding of the study was that increasing ZnO content led to matrix densification, reduced surface area, and blue shifting of the adsorption band. Structural studies revealed that ZnO was integrated into the geopolymer framework through bonding with aluminium within the composite. The highest 160 mg/L methylene blue removal (91.05%) was achieved by a geopolymer with Si/Al ratio of 2 and attributed to naturally occurring TiO2 phases in the metakaolin coupled with a high surface area. These samples exhibited superior continuous α-pinene degradation, with a maximum removal of 26.93%, which was attributed to reduced Al–OH surface groups and lower hydrophilicity, favouring VOC adsorption. Significantly, this research identifies the critical parameters of incorporating ZnO into a geopolymer framework, suggesting that both the concentration of ZnO added, and the microstructural modification induced by ZnO play important roles in determining photocatalytic activity. This study revealed how ZnO influences the physicochemical characteristics of geopolymers, and the remediation mechanism of pollutants from aqueous and gaseous environments.