Ferrous - ferric iron
Mine water with iron contamination may take on more than one chemical form. As it turns out, this can be a significant property, especially when we devise treatment strategies to remove the iron.
The iron will be in one of two oxidation states: ferrous having a +2 charge, or ferric having a +3 charge. Ferrous iron is soluble in water at any pH. If you see water containing only ferrous iron, the iron will be totally dissolved and the water will appear as crystal clear, no mater what pH it has. The situation is different with ferric iron. At a pH less of than about 3.5 ferric iron is soluble. But if the pH is higher than 3.5 the ferric iron will become insoluble and precipitate (form a solid) as an orange/yellow compound called yellowboy. This causes the familiar orange coatings on stream bottoms that tends to smother aquatic life. So, to put it in a nutshell, ferric iron will precipitate; ferrous iron will not.
Now to continue with another part of the story. Mine water may also have high levels of acidity, a situation that degrades the water quality. The most common property we associate with this is a low pH, less than 5 or thereabouts. To treat such water, we want to neutralize the acidity by adding alkalinity. Adding alkalinity will raise the pH. For passive treatment systems, limestone is the widely preferred neutralizing agent. Having mine water come in contact with limestone dissolves it, tending to neutralize it. As it does, the pH becomes higher. Okay, here's where the problem comes in. If this water also has iron in it, particularly ferric iron, as the pH rises above 3.5, the ferric iron will precipitate as yellowboy. In doing so, the yellowboy can deposit on the limestone forming a layer of yellowboy that protects the limestone from further dissolution. In other words, the limestone is rendered ineffective in further neutralization action because of the coating, also known as armoring. Armoring, in fact, is a failure mode of some treatment systems.
Let's go to yet another part of the story: to when the iron pollution is initially formed by pyrite weathering. When pyrite initially reacts with oxygen and water, one product is ferrous iron. (Equation 1 below) For ferrous to become ferric, more oxygen is needed. (Equation 2 below) However, underground the amount of oxygen can be very limited, and that conversion may not happen to any significant extent in the oxygen limited environment. [Sometimes certain bacteria can accelerate the ferrous to ferric reaction, however.] Often when mine pollution breaks out at the surface, very little of the iron is in the ferric form because of a lack of oxygen underground. This, however, can change quickly once the mine water is exposed to the atmosphere where plenty of oxygen is available. One treatment strategy for mine water having high acidity and virtually all the iron in the ferrous state is to keep oxygen from getting to it while it is passed through a channel of limestone rock. An anoxic limestone drain protects the water from oxygen while alkalinity is being added. If, on the other hand, significant amounts of iron are in the ferric state or adequate oxygen is present, a different strategy can be employed: removing the oxygen before adding limestone alkalinity. This is the case with a SAPS (Successive Alkalinity Producing System).
The three chemical reactions salient to this discussion are
4FeS2(s) + 14O2(g) + 4H2O(l) ---> 4Fe2+(aq) + 8SO42-(aq) + 8H+(aq) (1)
4Fe2+(aq) + O2(g) + 4H+(aq) ---> 4Fe3+(aq) + 2H2O(l) (2)
4Fe3+(aq) + 12 H2O(l) ---> 4Fe(OH)3(s) + 12H+(aq) (3)
Equation 1 describes the initial reaction of pyrite with water and oxygen to form ferrous ions. Equation 2 describes the reaction in which ferrous iron is converted to ferric iron. Equation 3 describes the actual hydrolysis and precipitation of ferric hydroxide (yellowboy).
When and where these reactions happen often drive the design of many passive treatment systems.