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One possible way to make Fe3O4 nanoparticles leave an electron using light is through a photoreduction process. This can be achieved by irradiating the nanomaterials with a suitable light source, such as visible light or UV light, causing electrons to be excited and dislodged from the electrons’ valence band, thus reducing the particle's charge. This could then serve as an electron donor for methanogens in bio gas. Alternatively, some nanomaterials may also be able to donate an electron under certain conditions by allowing certain compounds to adhere to their surface and accept an electron.

From my point of view it is not possible to make Fe3O4 nanoparticles release an electron using light alone. Electron transfer from Fe3O4 nanoparticles to an electron acceptor typically occurs through a chemical or electrochemical reaction, not through light exposure. To introduce a new electron donor for methanogens in bio gas, you may need to consider modifying the composition or surface properties of the Fe3O4 nanoparticles, or finding alternative materials that are more suitable as electron donors in a biogas system.