Silica is great for removing humidity from air. But what I want is to remove oxygen from air, so my food stays fresh longer. Any clean and easy to use substance/method for that purpose?
Oxygen absorbers. Little paper packets of iron powder.
Yeah don't. Like many metals, finely dispersed iron can self-combust on air. I'd suggest an air-tight packaging filled with CO2, or, if your food is susceptible to acid, N2, if at all feasible. Industry uses additives similar to hydroquinone, mixed directly into, e.g., plastics. Plenty of them are food-safe, but I wouldn't know where to buy them if you aren't a business.
You leave the iron powder in the packets. No need to disperse it, and certainly not in the air.
I think a lot of us might even confuse the two. The packets look fairly similar too.
Radiolab did an amazing episode on food freshness. They talk about plastic produce packaging, which is a bit of a modern marvel.
https://radiolab.org/podcast/forever-fresh
Tldr: those plastic bags that salads come in are WAY more interesting than you think. They're selectively permeable membranes that only allow certain gasses in/out.