The reason it binds (and can unbind) is that it has an affinity to calcium carbonate surfaces, just like it does to phosphate binders like GFO or aluminum oxide. It gets onto the surface of calcium carbonate in place of carbonate, as calcium phosphate.
It binds an amount directly related to the concentration in the water. the more in the water, the more binds. The less in the water, the less binds.
I discuss it in the contact of binding to fresh CaCO3 surfaces made in the tank by precipitation here:
The phosphorus atom is one of living matter's basic building blocks. It is present in every living creature and in every reef aquarium's water. Unfortunately, it is often present in excess in reef aquaria, and that excess has the potential to cause at least two substantial problems for...
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One possible mechanism could be through calcium phosphate precipitation, as outlined above. A second mechanism for potential phosphate reduction when using high pH additives is the
binding of phosphate to calcium carbonate surfaces. The absorption of phosphate from seawater onto aragonite is pH dependent, with the
binding maximized at around pH 8.4 and with less binding occurring at lower and higher pH values. Habib Sekha (owner of Salifert) has pointed out that limewater additions may lead to substantial precipitation of calcium carbonate in reef aquaria. This idea makes perfect sense. After all, it is certainly not the case that large numbers of reef aquaria exactly balance calcification needs by replacing all evaporated water with saturated limewater. And yet, many aquarists find that calcium and alkalinity levels are stable over long time periods with just that scenario. One way this can be true is if the excess calcium and alkalinity, which such additions typically add to the aquarium, are subsequently removed by precipitation of calcium carbonate (such as on heaters, pumps, sand, live rock). It is this ongoing precipitation of calcium carbonate, then, that may reduce the phosphate levels; phosphate binds to these growing surfaces and becomes part of the solid precipitate.
If the calcium carbonate crystal is static (not growing), then this process is reversible, and the aragonite can act as a reservoir for phosphate. This reservoir can inhibit the complete removal of excess phosphate from a reef aquarium that has experienced very high phosphate levels, and may permit algae to continue to thrive despite all external phosphate sources having been cut off. In such extreme cases, removal of the substrate may even be required.
If the calcium carbonate deposits are growing, then phosphate may become buried in the growing crystal, which can act as a sink for phosphate, at least until that CaCO3 is somehow dissolved. Additionally, if these crystals are in the water column,
e.g., if they form at the local area where limewater hits the aquarium water, then they may become coated with organics and skimmed out of the aquarium.
If phosphate binds to calcium carbonate surfaces to a significant extent in reef aquaria, then this mechanism may be attained with other high pH additive systems (such as some of the two-part additives, including
Recipe #1 of my DIY system). However, this potential precipitation of phosphate on growing calcium carbonate surfaces will not be as readily attained with low pH systems, such as those using calcium carbonate/carbon dioxide reactors or those where the pH is low due to excessive atmospheric carbon dioxide, because the
low pH inhibits the precipitation of excess calcium and alkalinity as calcium carbonate, as well as inhibiting the binding of phosphate to calcium carbonate.
