In recent years, zero-valent aluminum (ZVAl) has been used to remove contaminants in water environment due to its strong reducibility, which is developing into a new water treatment technology. However, ZVAl can be readily oxidized to form a dense oxide layer when exposed to oxygen or water medium, which will decrease the reductive capacity of ZVAl, and become the biggest limiting factor for the further application of this technology. Therefore, it is crucial to understand the surface reaction mechanism of ZVAl in aqueous media comprehensively. Actually, ZVAl has been widely used in the field of hydrogen generation because the reaction between ZVAl and H₂O can generate hydrogen efficiently. In essence, the contaminants removal by ZVAl is the same as the hydrogen generation by Al/H₂O reaction. In both cases, ZVAl reduction reaction occurs, and releases electrons. The primary difference between the case of contaminant removal and the case of hydrogen generation is that their electron transfer targets are pollutants in water and the water itself, respectively. In order to overcome the limitation of surface oxide film, researchers from the two fields take measures to improve the reductive capacity of ZVAl, including film dissolution by acid or alkali, film destruction by mechanical ball milling or alloying, useful film generation by building electron channel, film phase transformation by high-heat treatment, and so on. In addition, the environment conditions of water medium, such as temperature, pressure, trace ions and organic acid in water, and (hydr)oxide added, have a significant effect on the surface change of ZVAl. Therefore, in this review, based on the oxide film's formation, dissolution, destruction, transformation and the impact of its external environment, etc., the latest research progress and the surface reaction mechanism of ZVAl in the fields of contaminant removal and hydrogen generation, are summarized and prospected. It is believed that ZVAl would be applied widely in two fields supposing that the limitations of oxide film are overcomed.
Contents
1 Introduction
2 Formation of surface oxide film
2.1 Formation of surface oxide film in air
2.2 Formation of surface oxide film in water
3 Change the surface oxide film to improve reducibility of ZVAl
3.1 Dissolution of film
3.2 Destruction of film
3.3 Phase transformation of film
3.4 Building electronic channels
4 Influence of water environment conditions on the mechanism of ZVAl surface
4.1 Temperature
4.2 Initial pressure
4.3 Trace ions and organic acid in water
4.4 (Hydr)oxide
5 Conclusion and outlook