[bertoni]

If you are trying to remove phosphate, I would use this procedure:

1) Place the rock in a tub outside.
2) Put on chemical-resistant gloves and goggles.
3) Add 10 parts water to cover the rock.
4) Add 1 part muriatic acid. Be careful. The fumes are toxic.
5) Let the mixture sit for 20-30 minutes.
6) Neutralize any remaining acid with baking soda. The bubbling will stop when the acid is gone.
7) Dispose of the water, but check with your sewer district to see what they have to say.
8) Rinse the rock well. In most places, tap water is fine.