Natural minerals such as hematite and birnessite have gained attention as low-cost photocatalysts for pollutant degradation due to their abundance, stability, and inherent catalytic properties. These minerals, often considered waste or low-value materials, can be activated and optimized for environmental applications without the need for complex synthesis processes. Their use in photocatalysis offers a sustainable alternative to engineered nanomaterials, particularly in resource-limited regions where cost and scalability are critical factors.

Hematite (α-Fe₂O₃) is an iron oxide mineral with a bandgap of approximately 2.2 eV, enabling visible light absorption. Its natural abundance makes it an attractive candidate for large-scale photocatalytic applications. However, pristine hematite suffers from rapid electron-hole recombination and low charge carrier mobility, limiting its efficiency. Impurities commonly found in natural hematite, such as silicon, aluminum, and titanium, can influence its photocatalytic behavior. While some impurities may introduce defect sites that enhance charge separation, others may act as recombination centers, reducing activity.

Birnessite, a layered manganese oxide (δ-MnO₂), exhibits redox-active properties and a tunable bandgap, making it suitable for degrading organic pollutants and heavy metals. Natural birnessite often contains interlayer cations (e.g., Na⁺, K⁺, Ca²⁺) and structural water, which affect its stability and reactivity. The presence of transition metal impurities, such as cobalt or nickel, can modify its electronic structure and improve catalytic performance. However, excessive impurities may disrupt the layered structure, reducing surface area and active sites.

Activation methods are essential to enhance the photocatalytic performance of natural minerals. Thermal treatment is a common approach to remove volatile impurities, improve crystallinity, and increase surface area. For hematite, heating between 300–600°C can reduce hydroxyl group coverage and enhance charge carrier mobility. Overheating, however, may lead to particle aggregation and reduced surface reactivity. Birnessite can be thermally treated to control interlayer water content and stabilize its structure, but excessive heating may collapse the layered framework, diminishing its catalytic properties.

Acid treatment is another effective activation method. Mild acid washing (e.g., with HCl or HNO₃) removes surface impurities and exposes active sites without significantly altering the mineral structure. For hematite, acid treatment can leach out silica and alumina impurities, improving surface reactivity. In birnessite, acid washing can exchange interlayer cations with protons, increasing acidity and enhancing adsorption capacity for cationic pollutants. However, strong acid treatment may dissolve the mineral or create excessive surface defects, negatively impacting performance.

Under solar irradiation, natural mineral photocatalysts demonstrate varying efficiencies depending on their composition and activation. Hematite exhibits moderate activity for degrading organic dyes and phenolic compounds due to its visible light absorption, but its performance is often limited by charge recombination. Coupling hematite with other natural minerals, such as titanium-bearing phases, can form heterojunctions that improve charge separation. Birnessite shows promise in oxidizing organic pollutants and reducing heavy metals (e.g., Cr(VI) to Cr(III)) due to its mixed oxidation states and redox-active sites. Solar-driven activation of birnessite is particularly effective in neutral to alkaline conditions, where its layered structure remains stable.

The lifecycle impacts of natural mineral photocatalysts are significantly lower than those of synthetic nanomaterials. The extraction and processing of hematite or birnessite require minimal energy compared to the high-temperature and chemical-intensive synthesis of engineered nanoparticles. Natural minerals also avoid the use of toxic precursors and solvents, reducing environmental and health risks. Scalability is a key advantage, as these materials can be sourced locally in many regions, minimizing transportation costs and supply chain dependencies.

In developing regions, the use of natural mineral photocatalysts aligns with low-cost and low-tech solutions for water and air purification. Simple activation methods, such as solar drying or mild acid washing, can be implemented with minimal infrastructure. Field studies have shown that crushed and activated hematite or birnessite can be incorporated into fixed-bed reactors or mixed directly with contaminated water for pollutant removal. The minerals can often be regenerated through mild thermal or chemical treatment, extending their usable lifespan.

Comparative studies indicate that while natural mineral photocatalysts may not match the efficiency of high-performance synthetic nanomaterials like TiO₂ or ZnO, their cost-effectiveness and sustainability make them viable for large-scale applications. For instance, hematite-based systems have demonstrated degradation efficiencies of 40–70% for common organic pollutants under natural sunlight, with performance improvements achievable through doping or composite formation with other natural minerals. Birnessite has shown comparable redox activity to some synthetic manganese oxides in heavy metal removal, particularly in systems where adsorption and photocatalysis act synergistically.

Challenges remain in optimizing natural mineral photocatalysts for widespread use. Variability in mineral composition due to geographic origin requires localized characterization and tuning of activation methods. Long-term stability under environmental conditions, especially in acidic or high-ionic-strength waters, needs further investigation. However, the potential for low-cost, scalable, and environmentally benign photocatalysts from natural minerals presents a compelling pathway for sustainable pollutant degradation in resource-constrained settings.

The integration of natural mineral photocatalysts into decentralized treatment systems could address pollution challenges in rural and peri-urban areas without relying on expensive or imported materials. Future research should focus on standardizing activation protocols, assessing real-world performance under diverse environmental conditions, and developing hybrid systems that combine natural minerals with other sustainable materials. By leveraging the inherent properties of hematite, birnessite, and similar minerals, photocatalytic pollutant degradation can become more accessible and environmentally sustainable globally.