Environmental remediation using natural clay mineral based semiconductor photocatalysts

Semiconductor materials in combination with natural clay minerals have become viable hybrid systems in sustainable environmental remediation. The structure, properties, and environmental functions of major natural clays are thoroughly analyzed in this review, which describes them as having large surface area, layered structure, ion-exchange capacity and many surface hydroxyl groups. These properties render them a great choice of adsorbent and sturdy support medium of semiconductor photocatalysts. The review also covers a broad spectrum of clay-based semiconductor photocatalytic systems, including TiO 2 , ZnO, Fe 2 O 3 , WO 3 , and CdS composites, and specifically kaolinite and montmorillonite-based materials. Different methods of synthesis and fabrication are covered in detail: sol-gel, hydrothermal, impregnation, and surface immobilization. Combined adsorption and photocatalysis synergistic activity between clay minerals and semiconductor particles increases the level of pollutant removal via the dual mechanism of adsorption followed by degradation of contaminants by reactive oxygen species under light irradiation. The importance of clay minerals in the enhancement of the photocatalytic performance is critically considered that is, enhanced dispersion of the active phases, decreased electron and hole recombination, improved light utilization, and structural stability. In addition, the review also analyses the mechanism underlying the charge transfer and oxidative degradation. The main issues, including the low visibility of activity, stability of nanoparticles, scalability, and long-term stability are also addressed, as well as future opportunities that are based on new material design and sustainable synthesis technology. On the whole, the clay-semiconductor composites are the inexpensive and eco-friendly resources that have a high potential in the cleanup of water, soil, and air contaminants.