Firearms are traditionally blue.

A. According to the change of valence, 3mol of Fe ~ Fe3O4 loses 8mol of electrons, so 5.6gFe(0. 1mol) participates in the reaction to transfer 0.38mol of electrons, so A is wrong;

B. NH3 is produced in this process, and the wastewater is alkaline, causing pollution, so B is wrong;

C. Reaction ③ The valence of iron has not changed, which belongs to non-redox reaction, so C is wrong;

D. With the increase of temperature, the reaction speed is accelerated, the amount of Fe3O4 generated is more, and the blue layer is thickened, so D is correct.

So choose D.