Toxic effects of iron on Aquatic animals and fish

Iron exists in two forms, soluble ferrous iron and insoluble ferric particulate iron. In most aquaculture systems there will be a high oxygen concentration, and all iron present in the water will be in the form of insoluble ferric Fe3+.  Ferric iron as a chemical is non toxic, however this doesn't means to say that it does not exert a pathological response.

Our experience has proved that in many aquaculture systems, the presence of iron at concentrations above 0.1mg/l,  the iron will damage the gills of the fish. This concentration is very much lower than the levels normally reported in the literature.   We find that the toxicity of iron will depend on the species of the fish, and the size of the fish. The gills of the fish are in effect acting as a mechanical filter, and small particles of iron with dimensions of a few microns are becoming trapped in the gill lamella.  This is why the species and size of the fish are of importance.

The presence of the small iron particles cause irritation of the gill tissues leading to gill damage and secondary bacterial and fungal infections. However the physical presence of the iron particles does not explain the extent of  the damage that can be caused by the iron,  there are therefore other mechanism involved in the process.

Iron acts like a catalysts in water, and will promote the dissociation of oxygen molecules in water to form free radicals. The free radicals are extremely reactive and short lived. We actually use this reaction in catalytic filters for the oxidation and disinfection of water.  However on the surface of the gills,  the free radicals formed by the iron will cause oxidation of the surrounding tissue, and this will lead to massive destruction of gill tissue and anaemia. The table below shows that iron actually has a higher oxidation potential than ozone, and certainly if you exposed the gills of fish to ozone, they will experience oxidation damage. The fact that iron has a higher oxidation potential than ozone is pretty good evidence that the iron can cause damage.

  Potential (Volts)
Iron 2.85
Ozone 2.07
Hydrogen peroxide 1.76
Permanganate 1.68
Chlorine 1.36
Chlorine dioxide 0.95

We have no definitive proof of the above mechanism,  but we know that ferric iron will generate free radicals and cause tissue oxidation,  we also know that if the levels are above 0.1mg/l (especially in very clean water with low organic concentrations) then there will usually be gill damage.  We have never actually been able to view iron particles on the gills,  or measure the iron content of the gills, however in all cases when an iron removal system is installed to lower the iron content of the water, the fish recover.  We consider this to be pretty good evidence, and the fact that it works is the important point.

Leading on from the above we have developed iron sequester HMF that when dosed into the water at concentrations of approximately 2mg/l,  will chelate the iron or prevent it from generating free radicals,  it is a simple and low cost solution to the problem.  

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