phosphate remover (ferric oxide)

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Check out this article by R. Holmes-Farley
http://reefkeeping.com/issues/2004-11/rhf/index.php

What is iron oxide hydroxide?
A variety of different materials go by the general description of iron oxide hydroxide. One version is frequently referred to as Granular Ferric Oxide (GFO), and that name will be used throughout this article unless something else specific is intended. Ferric refers to iron in the +3 state, (called iron (III), or Fe+++), which is the most stable state of iron under aerobic conditions. The iron in GFO is ferric. Ferrous refers to iron in the +2 state (called iron (II) or Fe++). It is the more stable salt form of iron under anaerobic conditions. There is no ferrous iron in GFO (except perhaps trace impurities).
Iron hydroxide (Fe(OH)3) is composed of an Fe+++ ion surrounded by three hydroxide (OH-) ions. It is readily formed by combing any soluble form of Fe+++ with hydroxide ions. Adding Fe+++ (as in iron sulfate or chloride) directly to seawater will instantly form largely insoluble Fe(OH)3, which appears as a brown mud. This effect is the primary reason not to use unchelated ferric salts as iron supplements in marine aquaria, but that's a story for a different article.


At the other end of the extreme of iron (III) oxides and hydroxides is the dehydrated form ferric oxide, Fe2O3. It is composed of Fe+++ ions and O-- ions. Solid Fe(OH)3 spontaneously loses water to form a material that is in between these extremes, FeO(OH), which is what is often called iron oxide hydroxide, as shown in equations 1-3.

1. Fe+++ + 3OH- à Fe(OH)3

Ferric iron + hydroxide à iron hydroxide

2. Fe(OH)3 à FeO(OH) + H2O

Iron hydroxide à iron oxide hydroxide plus water

3. 2FeO(OH) à Fe2O3 + H2O

Iron oxide hydroxide à iron oxide plus water

Iron oxide hydroxide can be completely amorphous (having randomly arranged ions), completely crystalline (with an ordered arrangement of ions), or something in between. In nature it can take a variety of different crystalline forms, including goethite, lepidocrocite, and limonite. The detailed chemistry of these materials is beyond the scope of this article, but in short, all of the commercial GFO's sold to aquarists are comprised of a solid of Fe+++ and OH- and O-- ions. How crystalline the different commercial products are is unknown to me, although one manufacturer's representative confided in me the belief that the relative crystallinity is an important difference between some of them. Other differences are also important for phosphate binding, and these will be discussed in the following section.