This study investigated the effect of concentration on biomethanation in electron donor additives (zero valent iron (ZVI) 10-40 g/L, activated carbon (AC) 1.0- 4.0 g/L and Fe3O4 0.25-1.0 g/L) when carbon dioxide (CO2) is the only carbon source in anaerobic digestion., and the researchers estimated the pathway of CO2. ZVI had 43-49 % biomethanation of injected CO2 but was not observed in AC and Fe3O4. It has been confirmed that the rate of hydrogen (H2) production from ZVI depending on its size. Furthermore, when there is a shortage of CO2, inhibition occurs due to the hydrogen generated from ZVI. The reason for the inhibition of methane (CH4) production in AC and Fe3O4 was found to be due to volatile fatty acids (VFAs) accumulation. The use of additives induces the conversion of CO2 to VFAs and inorganic carbon, which are necessary for biomethanation. The addition of ZVI increased the growth of hydrogenotrophic methanogens, thereby enabling CH4 production without VFAs accumulation.