Magnesium oxide (MgO) plays a crucial role in fluoride removal processes, involving both ionic reactions (Mg²⁺ reacting with F⁻ to form MgF₂ precipitate) and adsorption behaviors. Both of these processes are highly dependent on the particle surface area and surface activity of the magnesium oxide. As the size of MgO particles decreases, especially when entering the nanoscale, their specific surface area increases rapidly, and surface active sites also significantly increase, thereby markedly affecting the efficiency of fluoride removal.
1. Comparison between Micron-Sized and Nano-Sized Magnesium Oxide
Traditional magnesium oxide is mostly in the form of micron-sized powder. These materials exhibit a relatively slow hydrolysis reaction in water, resulting in a lower release rate of Mg²⁺ ions and a limited reaction surface. In short-term treatment or flow-through reaction systems, micron-sized MgO may not provide sufficient reaction sites or rate, leading to inadequate fluoride removal efficiency.
In contrast, nano-sized magnesium oxide, due to its extremely small particle size, possesses a much higher specific surface area and reactivity. Under the same mass loading, it can provide more reaction contact interfaces. Studies have shown that nano-MgO can react with fluoride ions in a very short time, significantly reducing the treatment duration and improving fluoride removal rates.
2. Enhanced Adsorption Mechanism
Smaller particles lead to a greater number of surface charges and structural defects on the magnesium oxide, properties that are beneficial for enhancing the adsorption capacity of fluoride ions. Particularly in nanoscale MgO, a large number of oxygen vacancies and edge active sites exist, which can strongly adsorb F⁻, forming more stable surface complexes or composite precipitates. Therefore, smaller particles not only promote the reaction rate but also increase the fluoride removal capacity.
3. Experimental and Research Support
Numerous experimental comparison results demonstrate that nano-sized magnesium oxide exhibits a significantly higher fluoride removal rate than ordinary magnesium oxide under the same reaction conditions. For example, one experiment showed that under a fluoride concentration of 10 ppm, nano-magnesium oxide could reduce the fluoride concentration to below 1 ppm within 10 minutes, while micron-sized magnesium oxide required several hours and might not even achieve the same effect.
Furthermore, some studies have explored the fitting results of adsorption isotherms for magnesium oxide with different particle sizes. They found that nanoparticles fit the Langmuir model better, indicating that their surface adsorption tends to form a monolayer and achieve saturated adsorption, suggesting that their adsorption behavior is more regular and efficient.
4. Balance Issues in Practical Applications
Despite the performance advantages of nano-sized magnesium oxide, some issues exist in practical applications. The first is the agglomeration phenomenon: nanoparticles tend to agglomerate into larger particles in water, reducing the effective specific surface area and affecting the reaction efficiency. The second is the difficulty in separation: after the reaction, nano-magnesium oxide and its products are difficult to separate by ordinary sedimentation or filtration methods, requiring more complex recovery techniques such as ultrafiltration or magnetic modification.
Therefore, in practical use, researchers often load nano-MgO onto porous materials or supporting matrices, such as activated carbon, zeolite, glass fiber, or prepare it into granular or composite filter media. This approach aims to balance the need for high reaction efficiency with convenient operation.