Rust stains in your bathtub?
If you see reddish stains in your sink, bathtub, or heaven forbid, on your laundry, it’s a good bet you’ve got iron in your water. Don’t look for nuts and bolts dripping from your tap, this kind of iron is dissolved in the water. Where does it come from? Water is a great solvent and as it percolates through iron-rich rocks and soil and flows through pipes on its journey to your tap, it dissolves iron. We refer to this kind of iron as “ferrous” but it is not the only form of iron that can be found in water. “Ferric” iron is insoluble and occurs in water in the form of tiny particles.
If your tap water looks rusty, you’ve got ferric ions suspended in it. If the water is clear, but a rusty sediment settles to the bottom of a glass after a few minutes, ferrous iron is present. Ferric iron is actually the oxidized form of ferrous iron. Basically this means that ferrous iron has a plus two positive charge, and a loss of an electron converts it to ferric iron with a charge of plus three. This oxidation process happens when the water is exposed to air and oxygen steals and electron from ferrous iron.
Iron is a non-hazardous water contaminant, although it can become a problem because of its staining properties and in the worst conditions can even plug up pipes. Besides altering the appearance of the water, iron can also affect taste. Coffee or tea made with water that has a high iron content will taste different because iron forms complexes with tannins, compounds responsible for some of the flavor of these beverages.
The iron content of water can be dealt with by different techniques. At low levels of dissolved iron, phosphate treatment and water softeners are options. Phosphates readily form a complex with ferrous iron and keep it in solution. The trapped iron cannot react with oxygen in the air and the water stays clear. Phosphate treatment does have its limitations. Since the iron is not removed, the water retains its metallic taste. If the water is heated, the phosphate-iron complex breaks down and the iron is released. Also, phosphates are excellent fertilizers and can harm the environment by fostering the growth of plants in the water which use up the water’s oxygen content when they die and degrade.
Water softeners can remove both types of iron, ferric and ferrous. These filters exchange iron for sodium as the water flows through them. However, the added sodium can be a health problem for people on sodium-restricted diet. The best way to deal with iron in water on a large scale is via oxidation by air or with chemicals. When air is bubbled through water, ferrous iron is oxidized to ferric which is insoluble and can be filtered out. Aeration can, however, stimulate the growth of certain bacteria which clog the filter. Chemical oxidation proceeds in a similar fashion. Instead of oxygen, chlorine, potassium permanganate or hydrogen peroxide are used as oxidizing agents. This treatment has the additional advantage of getting rid of other organic matter as well. As with aeration, the precipitated iron is removed by filtration. Laboratories are available to determine the iron content of a water supply and can recommend appropriate treatments. It is easier to prevent those unsightly bathtub stains than eliminate them after they have formed.