Does PO4 bind to rust?

[121]

If you put a piece of iron or an iron alloy into a tank, would PO4 bind to the rust that will form?

The rust could then be removed with an oxalic acid solution.

[Randy Holmes-Farley]

Yes, that is what GFO is, a very high surface area form of ferric oxide/hydroxide. :)

[disc1]

I would stay with pure iron and stay away from any alloys unless you know exactly what other metals are in them. Some of those metals are pretty toxic.

[allsps40]

I would not add any iron items to the tank and salt water is very corrosive. Iron is a good fuel for algae growth. My old sand that came with my tank had a lot and I mean a lot of iron in it, set a magnet on the sand it got covered in black iron sand. This was not good, I was having algae issues on the sand bed. Iron is in salt water but only in very small traces, too much can kill corals. I have seen a few people drop razor blades in the tank while cleaning and not know. Then months later they start having issues with the tank and corals, not knowing why. Then end up finding a totally rusted razor in the sand bed. Just my experience.

[bertoni]

I don't think there would be enough surface area on most iron objects to trap much phosphate, just as a practical concern.

[121]

I did a little reading on steel. Stainless steel contains around 18% chromium which is very toxic to marine life. Regular steel contains around 2% carbon and would be suitable. The rust will eat away at the iron and create fresh layers for the PO4 to bind to, so I don't think surface area would be a problem. Chromium free steel ball bearings would be ideal. You would need macro algae to absorb the excess iron. That would boost the macro algae growth which would also be beneficial. I thought about aluminum kitchen foil but then you get the excess aluminum in solution, creating the same problems as Phosguard. Would macro algae absorb aluminum?

I'll continue using commercial, iron based PO4 binders but I'd like to experiment with chromium free steel. What could be used as a cheap source? All steel seems to be stainless these days. I guess I'd have to put a cast iron skillet in my sump :lmao:

[Randy Holmes-Farley]

Yes, surface area is the problem. That is why GFO was invented by companies trying to treat potable water to remove arsenic (which looks chemically very similar to phosphate).

Corroding iron will flake apart and release tiny chunks of GFO like material which you will not be able to contain, nor can you harvest the bound phosphate from the tank that way (like you can with GFO). You might as well add soluble iron compounds if that is what you want to accomplish.

When reef aquarists have experimented in the past, they use iron wool, a very high surface area material which is primarily iron with an oxide surface. Not a good choice like GFO, but it can work. :)

[disc1]

Yes surface area is the thing. Those little granules of GFO aren't solid. They are very porous. Each one has a huge surface area. Maybe Randy knows how much exactly. But it's huge.

A rusting piece of solid iron has only the surface area that is exposed. That will be thousands of times less than the same amount of GFO.