Abstract
Heterogeneous photocatalysis using titanium dioxide (TiO2) and zinc oxide (ZnO) has been widely studied in various applications, including organic pollutant remediation in aqueous systems. The popularity of these materials is based on their high photocatalytic activity, strong photosensitivity, and relatively low cost. However, their commercial application has been limited by their wide bandgaps, inability to absorb visible light, fast electron/hole recombination, and limited recyclability since the nanomaterial is difficult to recover. Researchers have developed several strategies to overcome these limitations. Chief amongst these is the coupling of different semi-conductor materials to produce heterojunction nanocomposite materials, which are both visible-light-active and easily recoverable. This review focuses on the advances made in the development of magnetic ferrite-based titanium oxide and zinc oxide nanocomposites. The physical and magnetic properties of the most widely used ferrite compounds are discussed. The spinel structured material had superior catalytic and magnetic performance when coupled to TiO2 and ZnO. An assessment of the range of synthesis methods is also presented. A comprehensive review of the photocatalytic degradation of various priority organic pollutants using the ferrite-based nanocomposites revealed that degradation efficiency and magnetic recovery potential are dependent on factors such as the chemical composition of the heterojunction material, synthesis method, irradiation source, and structure of pollutant. It should be noted that very few studies have gone beyond the degradation efficiency studies. Very little information is available on the extent of mineralization and the subsequent formation of intermediate compounds when these composite catalysts are used. Additionally, potential degradation mechanisms have not been adequately reported.
Highlights
The findings demonstrated that even after three cycles, the photocatalytic activity of the magnetic nanocomposite ZnFe2 O4 /zinc oxide (ZnO) (65%)
The development and application of magnetic ferrite-based titanium oxide and zinc oxide nanocomposite as catalysts are extremely promising for the removal of organic pollutants from water and wastewater, as shown by various studies presented in this review
Studies demonstrated that these catalysts can be prepared by different methods such as sol-gel, co-precipitation, hydrothermal, and combustion
Summary
Titanium dioxide (TiO2 ) and zinc oxide (ZnO) are amongst the most widely studied photocatalysts due to their high photocatalytic activity, strong oxidation potential, superhydrophilicity, biological and chemical stability, prolonged durability, non-toxicity, and low cost [4,5,6,7,8,9] While these catalysts demonstrate excellent photocatalytic activity, their particle sizes, often in the nano range, negatively impact their recovery. Photocatalysis using these catalysts has limited the large-scale application in treatment of polluted water and wastewater due to the costs associated with loss of the material [4,10,11] This has necessitated the development of new, inexpensive materials with good photocatalytic efficiency, recoverability, and reusability properties. Conclusions are drawn, and challenges encountered in the use of these catalysts are cited
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