The term bind/ bound as we are using it is a ploblem since we are using it to describe different states.
The PO4 sunk in the argonite crystal stucture won't release unless it dissolved by acidic conditions ,perhaps some localized low pH areas caused by bacterial activity. The PO4 species hanging around on rock surface will equilbrate with low PO4 water ,leaching back so to speak. The later is a common problem often solved by recurring the rock in low PO4 water ( via lanthanum chloride ) for a week or two .
A well maintained sand bed may provide enough surface area and bacterial colonization to keep up with PO4 in the water column assimilation of PO4 from the water column for some tanks., depending on the import level of PO4 mostly from foods. It won't be enough in heavily fed tanks ,ime.
+1. The surface of your aragonite rock/sand will adsorb phosphate quite efficiently when it's perfectly clean and phosphate free (albeit with a relatively low affinity by most standards). But, we have things that live in our tank, and we feed those things food that contains phosphate. So, you get an equilibrium between the phosphate in the water and the phosphate on the surface of the rock. When your phosphate level rises it will bind more, when it drops it will leach back.
That said, you really shouldn't count on it to keep phosphate levels in the tank low. As an example, if you are already at equilibrium and your phosphate level is 0.02 ppm because it's been there for a long time, and you all of a sudden start adding more phosphate and the level of your tank water goes to 0.1 ppm, the rock may absorb some of the extra phosphate so it won't go as high (until the new equilibrium is established), but it's not like a GFO product where it will bind phosphate so tightly that the level coming out will be near 0 and you can just toss it when it's exhausted. Along the same lines, if you took that rock that was at 0.1 ppm for a long time and put it in a tank with a level of 0.00 ppm, it will leech the phosphate back, actually raising the level in the water.
So, from a qualitative perspective, the aragonite can perhaps buffer some short-term small changes in phosphate level, but it cannot act as a long-term phosphate reduction method. At the end of the day you add phosphate when you feed the tank, and unless you yourself physically remove it through skimming, carbon dosing (which is probably just more skimming), macroalgae, GFO, etc., it will stay there.
