Journal of Molecular Pharmaceutics & Organic Process Research
Open Access

Photocatalysis; Advanced oxidation processes; Organic pollutant degradation; Titanium dioxide; Photocatalysts; Wastewater treatment; Air purification; Mineralization; Reaction mechanisms

Introduction

The increasing contamination of water bodies and air by organic pollutants has become a critical environmental concern. Traditional treatment methods such as chemical oxidation, biological treatment, and adsorption face limitations in effectively removing Persistent Organic Pollutants (POPs). In recent years, Advanced Oxidation Processes (AOPs) have emerged as promising technologies for the degradation of a wide variety of organic pollutants. Among these, photocatalytic AOPs have garnered considerable attention due to their efficiency, selectivity, and environmental sustainability. Photocatalysis, a process driven by light, facilitates the generation of highly reactive hydroxyl radicals (•OH) and other Reactive Oxygen Species (ROS), which can degrade organic contaminants into harmless by-products such as water and carbon dioxide. Titanium dioxide (TiO₂) is the most widely studied photocatalyst, but recent advancements have led to the development of novel photocatalytic materials that offer improved performance and broaden the range of degradable pollutants [1,2].

Description

Photocatalytic AOPs typically involve a photocatalyst, a light source, and the target organic pollutants. Under UV or visible light irradiation, the photocatalyst generates electron-hole pairs, which can then interact with water or oxygen molecules to produce ROS. These ROS, such as hydroxyl radicals (•OH), superoxide anions (O₂•−), and hydrogen peroxide (H₂O₂), are highly reactive and capable of decomposing organic pollutants.

The reaction process can be described as follows:

Excitation of photocatalyst

Upon illumination, the photocatalyst absorbs photons, leading to the excitation of electrons from the valence band to the conduction band, generating electron-hole pairs [3].

Generation of reactive oxygen species (ROS)

The electrons in the conduction band react with oxygen molecules (O₂) to form superoxide anions (O₂•−), while the holes in the valence band react with water or hydroxide ions (OH−) to produce hydroxyl radicals (•OH).

Degradation of pollutants

The ROS degrade organic pollutants by breaking chemical bonds and leading to mineralization (complete degradation) into simple by-products like CO₂, H₂O, and inorganic ions (e.g., NO₃⁻, SO₄²⁻) [4].

Recent developments in photocatalysts

Titanium dioxide (TiO₂) has long been the benchmark photocatalyst for AOPs due to its non-toxicity, stability, and high activity under UV light. However, TiO₂ faces limitations such as its wide bandgap (3.2 eV), which restricts its activation under visible light. To overcome these drawbacks, various strategies have been employed to modify TiO₂ and develop new photocatalysts:

Doping and composite formation: Doping TiO₂ with non-metals (e.g., nitrogen, sulfur, carbon) or metals (e.g., gold, silver, copper) has been shown to extend its absorption into the visible light region. Composite materials, such as TiO₂/graphene oxide or TiO₂/Carbon-based materials, enhance the photocatalytic performance by improving charge separation and increasing the surface area [5].

Noble metal and metal oxide photocatalysts: Nanostructured photocatalysts like ZnO, CuO, and g-C₃N₄ have been explored as alternatives to TiO₂. These materials offer better light absorption, charge carrier mobility, and catalytic activity under visible light.

Hybrid photocatalysts: Hybrid materials combining semiconductor photocatalysts with other materials like perovskites, semiconducting polymers, and carbon dots have also been studied to enhance photocatalytic efficiency by exploiting synergistic effects.

Mechanisms of organic pollutant degradation

The photocatalytic degradation of organic pollutants follows several reaction pathways, depending on the nature of the pollutant and the photocatalyst [6]. The general mechanisms include:

Direct electron transfer: Organic pollutants may directly interact with photogenerated electrons or holes, leading to the breaking of molecular bonds.

Indirect degradation via ROS: The most common degradation route involves ROS, especially hydroxyl radicals (•OH), which attack the pollutant molecules and degrade them into smaller, less toxic intermediates.

Mineralization: Complete mineralization involves the degradation of organic compounds to carbon dioxide, water, and mineral acids, which are environmentally benign [7].

Applications of photocatalytic AOPs

Photocatalytic AOPs are applied in various fields for the degradation of organic pollutants:

Wastewater treatment: Photocatalytic AOPs are effective for removing industrial effluents, pharmaceuticals, dyes, pesticides, and other toxic organic compounds from wastewater. Recent studies show that photocatalysis can be integrated with membrane filtration or electrochemical processes to improve efficiency and scalability.

Air purification: Photocatalysts are used to degrade volatile organic compounds (VOCs) and hazardous gases in indoor and outdoor air. Photocatalytic filters and coatings for building materials are being developed for sustainable air quality management [8].

Decontamination of food and water: Photocatalysis can also be applied to disinfect and degrade pollutants in food processing and drinking water purification, providing a safer alternative to traditional chemical treatments.

Challenges and future directions

Despite the promising capabilities of photocatalytic AOPs, several challenges remain:

Low efficiency under visible light: The majority of photocatalysts, including TiO₂, are still limited to UV light activation, which constitutes only a small fraction of sunlight. More research is needed to develop photocatalysts that are efficient under visible light or even solar light [9,10].

Reusability and stability: Photocatalysts often suffer from issues related to photodegradation and loss of activity over multiple cycles. Developing stable, recyclable photocatalysts is crucial for practical applications.

Scaling up: While laboratory-scale photocatalytic reactors have shown significant promise, scaling up these processes for industrial and commercial applications remains a challenge. Efforts are underway to design reactors that optimize light distribution, catalyst utilization, and pollutant adsorption.

Conclusion

Recent advancements in photocatalytic AOPs have led to the development of more efficient and sustainable technologies for organic pollutant degradation. Innovations in photocatalyst materials, reaction mechanisms, and reactor design have paved the way for practical applications in environmental remediation, such as wastewater treatment and air purification. However, challenges such as improving light absorption, enhancing catalyst stability, and scaling up the processes must be addressed for the widespread adoption of these technologies. Future research in photocatalytic AOPs will likely focus on optimizing catalyst properties, expanding pollutant degradation capacity, and integrating photocatalysis with other advanced treatment technologies to provide comprehensive solutions to environmental pollution.

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