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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Phosphate Export Using Binding Media: Granular Ferric Oxide/Hydroxide
In the past few years iron-based phosphate binding materials have become very popular among reef aquarists. These materials have been used commercially to treat drinking water (to remove arsenic, for example) and to treat wastewater (to remove a wide range of pollutants, including phosphate). They are sold to aquarists under a wide variety of different brand names, including PhosBan®, Phosphate Killer™, and ROWA®phos. These materials all range in color from reddish brown to nearly black. In a
previous article I detailed how they function as well as some of the concerns that aquarists have had when using this material.
Even though the commercial materials appear to be reasonably large particles (Salifert claims 0.2 – 2 mm on its product label), they actually have a high internal surface area, somewhat similar to activated carbon. Consequently, apparent particle size is an unreliable means by which to gauge available surface area (though it is reliable for nonporous solids such as table salt). I have seen no measures of accessible surface area for the commercial granular ferric oxide (GFO) sold to aquarists. There are various modifications to the standard material, such as forming it into pellets (to perhaps work better in some applications such as media bags) and enclosing the GFO in a polymeric matrix (reducing the potential for breakage of the particles).
Phosphate bound to GFO surfaces is still available to the water column by exchange, so the sequestering is temporary rather than permanent. This fact is known in the literature3, and can be easily demonstrated by adsorbing phosphate onto GFO, and adding enough so that a detectable concentration of phosphate (0.1 to 1 ppm) is in equilibrium with the solids. Then remove the solid GFO and add it to seawater with no detectable phosphate. The now-detectable phosphate in the new seawater shows that the phosphate can be released from the GFO media when the aquarium’s phosphate concentration drops low enough.
One concern when using GFO is that it may add soluble iron to the system. This iron will likely benefit growing macroalgae, and
I recommend adding soluble iron to systems that grow macroalgae. However, low bioavailability of iron may limit undesirable algae or cyanobacteria growth in some aquaria (and it can in parts of the ocean, as well), so adding iron might contribute to an algae or even cyanobacteria problem. In general, however, most aquarists find that the use of sufficient GFO causes a decline in algae, with the reduction in phosphate being more important to decreasing algae growth than the added iron is to promoting it.
A second concern with using GFO is that some aquarists find extensive precipitation of calcium carbonate near or on the GFO itself. It turns out that soluble iron can cause the precipitation of calcium carbonate. Such precipitation can turn bags of GFO into solid clumps, and may contribute to clogging pumps, but in general the effect, if noticed at all, is limited to objects very near the GFO. The extent of this effect may well depend on the degree to which calcium carbonate is supersaturated in the aquarium, as well as on the levels of magnesium and organics (both of which usually reduce the likelihood of calcium carbonate precipitation). Because of this potential for calcium carbonate precipitation, using this material in a reactor where it moves may be more important than for aluminum oxide media.
Finally, be sure to rinse these materials in fresh or saltwater before adding them to the aquarium, as fine particles may get loose in the aquarium, clouding and coloring the water, and possibly creating other problems. There is no efficiency drawback to this rinsing. Aquarists using the GFO in a fluidized bed reactor or canister filter can just run some fresh or salt water on it for a few minutes before putting it into the aquarium. A media bag of GFO can simply be rinsed with saltwater or RO/DI water a few times before adding it to the aquarium. Do not squeeze the GFO inside the bag when rinsing it, as that may break the particles into smaller bits that can escape the bag.
