Hydroxyapatite-Based Metal Oxide Photocatalysts for Water Remediation: Review

Water pollution by persistent organic residues represents a major environmental challenge. Photocatalysis is an advanced oxidation process based on the activation of suitable semiconductor catalysts by light, which possess specific structural and textural characteristics, enabling the generation of reactive species capable of degrading and mineralizing pollutants. It plays a substantial role in water treatment and environmental protection due to the use of clean energy and its operation under mild conditions. This review summarizes recent progress on hydroxyapatite-based metal oxide nanocomposites, with an emphasis on the mechanisms and synergistic effects that enhance the photocatalytic activity of pharmaceutical residue degradation. Structural characteristics, porosity, and charge dynamics of these photocatalysts are identified as key factors influencing photocatalytic reactions. Their catalytic efficiency was analyzed by highlighting the underlying mechanisms, the main kinetic factors, and their main advantages, including an increased active surface area, reduced electron–hole recombination, and easy recovery of the photocatalyst. Key challenges, including material stability, limited visible-light absorption, and scaling difficulties, are highlighted with the aim of improving photocatalytic efficiency and developing sustainable hybrid systems for water treatment.