Dear Junaid,

you can have some idea from the literature. Generally, The IO particles serve in electron exchange process which consequently enhances the whole digestion process. Please read the article http://www.sciencedirect.com/science/article/pii/S0043135415001517 

"Zero valent iron simultaneously enhances methane production and sulfate reduction in anaerobic granular sludge reactors"

I agree about ZERO VALENT iron Fe° (ZVI) but this does not answer Muhammad's  question about TRI-VALENT ferric Fe3+ or BI-VALENT ferrous Fe2+ iron in iron oxides.

Iron oxides FeO or Fe2O3 are insoluble (solids) and nanoparticles may permeate through the cell walls of methanogens which are strictly anaerobic which implies that  oxygen is toxic for them.

In practice we add ferrous chloride FeCl2 (fully dissolved) both as nutrient and as to reduce free sulfides S- - which could inhibit microbial activity in anaerobic digestion.