Al powder was stored in saturated water vapor, oxygen, nitrogen and drying air separately for a time period of up to ∼six months, the degradation behavior of Al activity was characterized by the reaction of Al with water. It was found that water vapor decreased the induction time for the beginning of Al–water reaction and reduced the total hydrogen generation per unit weight of Al, while oxygen increased the induction time and retarded the Al–water reaction. In contrast, the effect of nitrogen and drying air on Al activity was weak. The mechanism analyses indicated that water vapor promoted the hydration of the Al surface passive oxide film and sped up the reaction of Al with water, while oxygen thickened the passive oxide film of the Al surface and prolonged its hydration process. These results imply that water vapor rather than oxygen is responsible for the degradation of Al activity during storage in the atmospheric environment.

Different Al(OH)3 powders were used as adsorbents for fluoride removal from water. The results showed that the defluoridation performance of ultrasonically prepared Al(OH)3 (UAH) is much better than that of commercially available Al(OH)3 and is comparable to that of activated alumina, because the ultrasonic waves effectively break the agglomerates in the suspension so that the UAH particles are fine and have a beneficial phase constituent. Furthermore, the residual aluminum concentration in aqueous solution after defluoridation by Al(OH)3 was found to be one order of magnitude lower than that obtained with activated alumina which is below the World Health Organization (WHO) guideline for aluminum (0.2 mg L−1) in drinking water. The defluoridation dynamics and mechanism for UAH are discussed in detail.

•Heat-treatment of Al powder decreases the induction time of Al-water reaction.•Almost no induction time for oxide-modified Al even no heat-treatment is done.•The catalytic effect of γ-Al2O3 is the key factor to promote Al-water reaction.Previous works showed that oxide-modified Al powder could react with water and generate hydrogen under ambient condition. The activation mechanism was believed to be the weakening of passive oxide film on Al particle surface due to all its constituents transforming into loose γ-Al2O3 phase during the heat-treatment. In this work, a model experiment was performed, in which pure Al powder and the mixture of Al + γ-Al2O3 powders were heat-treated in vacuum from room temperature to 600 °C. It was found that the induction time for the reaction of heat-treated pure Al powder with water decreased obviously when the heat-treating temperature was above 200 °C. However, the reaction induction time of the mixture of Al + γ-Al2O3 powders was always considerably shorter than that of pure Al powder even if no heat-treatment was done. This implies that the catalytic effect of external oxides on the hydration of passive oxide film on Al particle surface plays a dominant role in promoting the Al-water reaction and hydrogen-generation. This clarification is also helpful in understanding the corrosion behavior of metal Al in different environment.

•Water type and quality have a significant impact on Al–water reaction.•Trace organic acid and F− ions in water retard the Al–water reaction.•Other cations and inorganic anions have a negligible impact on Al–water reaction.•The organic acid and F− ions form complexes with aluminol groups on Al surfaces.The Al–water reaction has attracted considerable attention in the past few years, because it is an economically viable way to supply hydrogen for portable and kW-grade fuel cells. The water type is found to have a significant impact on Al–water reaction. In this work, the effect of trace species in water on Al–water reaction is investigated systematically. It is found that the trace organic acids originated from the decay of botanies and animals in nature and trace F− ions, rather than other cations and anions, play a key role in the reaction dynamics of Al with water. The mechanism analyses reveal that the organic acids and F− ions form complexes with aluminol groups on Al particle surfaces to impede the hydration process of Al surface oxide film and retard the reaction of Al with water. The present results imply that a suitable water should be chosen for the Al–water reaction to generate hydrogen. At the same time, the organic acids and trace F− ions could be used as the agents to prevent undesirable Al–water reaction.

•Initial vacuum gas pressure has a significant impact on Al–water reaction.•The relation between the initial pressure and induction time is non-monotonic.•There is a minimum induction time with the variation of initial pressure.•Some vapor bubbles on Al surfaces due to boiling retard Al–water reaction.The effect of initial gas pressure on Al–water reaction was investigated systematically. It was found that there is a non-monotonic relationship between the initial pressure and the induction time for the beginning of Al–water reaction. The induction time decreases with decreasing the initial pressure at the first stage from atmospheric pressure, then reaches a minimum, and finally increases with further decreasing the initial pressure. The mechanism analyses revealed that the special vacuum pressure corresponding to the minimum induction time is close to the saturated vapor pressure, below which probably there are some vapor bubbles passing or adsorbing on Al particle surfaces due to boiling, retarding the hydration process of Al surface oxide film and increasing the reaction induction time. The present results imply that a suitable initial gas pressure should be chosen for Al–water reaction to generate hydrogen.

•Ultrasonically prepared Al(OH)3 suspension has a high activity and stability.•High-activity Al(OH)3 suspension considerably promotes Al-water reaction.•The activity of Al(OH)3 suspension is related to the reacted Al particle sizes.•Al(OH)3 in suspension promotes the hydration process of Al surface passive film.A high-activity Al(OH)3 suspension is prepared by the reaction of Al with water using an ultrasonic procedure. The above Al(OH)3 suspension could considerably promote the Al-water reaction and hydrogen-generation under ambient condition. This Al(OH)3 suspension has a good stability in air and its activity increases with decreasing the particle sizes of Al powder to prepare it. The mechanism analyses reveal that the Al(OH)3 particles in ultrasonically prepared Al(OH)3 suspensions are very fine, which could effectively dissociate water molecules and promote the hydration of the passive oxide film on Al particle surfaces, speeding up the Al-water reaction. As the present Al(OH)3 suspension is chemically neutral and there is no special treatment for Al powder, the present method provides a viable way to generate hydrogen for portable application.

Three different modification agents, γ-Al2O3, α-Al2O3, and TiO2 were used to modify Al particle surfaces. The effect of different modification agents on hydrogen-generation by the reaction of modified Al powder with water was investigated. It was found that different modification agents have different effect on the reaction dynamics of Al with water. The complete hydrogen-generation time of γ-Al2O3 modified Al powder is obviously shorter than that of α-Al2O3 and TiO2 modified Al powders, because of their different induction time for the beginning of the reaction. Possible mechanism analyses indicated that the phase transformation and resultant weakening of Al particle surface layer during processing strongly depends on the nucleation energy barrier on modification oxide grains. As γ-Al2O3 has a low nucleation energy barrier, the phase transformation of Al particle surface layer is relatively complete so that γ-Al2O3 modified Al powder has a short induction and complete reaction time with water. This implies that the Al surface modification probably originates from the phase change of its surface layer.