Mechanisms and possible applications of the Al–H₂O reaction under extreme pH and low hydrothermal temperatures

The reaction of Al and H₂O is a promising method for the renewable production of H₂ (an environmentally friendly fuel whose combustion produces only water), because it does not directly include fossil fuels conversion. This reaction was studied at extreme pH values as low as 1 and as high as 13.5 with HCl, H₂SO₄, and NaOH, and at low hydrothermal temperatures of 40-100 °C. Factors such as pH, temperature, and solution medium influenced H₂ production, which was considerably greater at higher temperatures and more extreme pH (acidic or alkaline). Alkaline conditions consistently favored more rapid H₂ production than acidic conditions. Under the most extreme conditions, the activation energy was ∼60 kJ mol⁻¹ for both acidic and alkaline reactions. A model predicting H₂ production in acidic reactions was derived from the reaction mechanism and kinetics. The model yielded a good fit to on-site measurements at the Tamagawa and Zao hot springs in northeast Japan. This study would aid the development of industrial H₂ production systems using natural acidic hot springs or alkaline industrial wastewater.