This paper aims to clarify the opportunities and benefits of a carbon dioxide based mild hydrothermal process for the selective production of hydrogen gas H-2 from water via the oxidation of pure metallic iron powder Fe-0 by focusing on (i) the effects of different process parameters such as temperature, initial carbon dioxide pressure and grain size of the metallic iron powder and (ii) the occurring reaction mechanisms. With the aid of process parameter optimisation, highly pure hydrogen gas (>99 mol%) with a percentage yield of approximately 80% could be experimentally obtained by treating metallic iron powder with a grain size of 5 mu m under mild hydrothermal conditions. The hydrogen formation reaction took place in a stirred 100 mL stainless steel autoclave, filled for 40 mL with a 1 M aqueous potassium hydroxide solution, at 160 degrees C for 16 h and made use of an absolute initial carbon dioxide pressure of 6 bar (6 bar CO2, 25 degrees C, taking place in a volume of 60 mL: 15 mmol CO2). Hence, this hydrothermal process operates at milder reaction conditions compared to previously reported similar processes, which operate at 300 degrees C. This paper shows that specifically under mild hydrothermal conditions, by complete dissolution of the introduced carbon dioxide gas in the 1 M potassium hydroxide solution, in situ formed carbonate ions CO32- play a key role in assisting the hydrogen gas formation by acting as catalysts. More specifically, the metallic iron oxidation into magnetite Fe3O4 is suggested to take place via the formation of iron(II) carbonate FeCO3 as an intermediate. (C) 2015 Elsevier Ltd. All rights reserved.