Standards vary from state to state for reasons which I find impossible to explain. For example, Pennsylvania has set an iron limit of 7 mg/liter (7 ppm) on the discharge from a treatment plant. West Virginia, on the other hand, has adopted 10 mg/liter (10 ppm) in the receiving stream as a satisfactory iron level, thus giving proper credit for the dilution effect of the stream. As you all know, public hearings are currently being held in all fifty states, attempting to develop stream standards which will be satisfactory to both the states and the Federal Water Pollution Control Administration. So, we still are aiming at a moving target.
Now, let’s consider the fate of the ferrous iron and the acid in the water as it moves—miles, in many cases—from its point of formation to its point of discharge. As all of you know, substantial amounts of limestone or calcium carbonate are associated with coal deposits.
The first reaction (Equation 1)—which occurs rapidly—is the reaction of the acid with, say, limestone. Bicarbonate ion is formed and acid in the mine water is neutralized.
After, or simultaneously with neutralization of the acid, another important reaction begins. You will remember that I stated that ferrous, or divalent iron, was formed as a result of the pyrite-water-air reaction which gave birth to our contaminants. One property of iron in this form is that it is quite soluble, even in neutral solutions. Another is its tendency to resist oxidation to the trivalent form (Fe+³) in acid solution. However, by reaction with limestone, we have removed a major portion of the acid, thus raising the pH. Now, further oxidation of the divalent iron can and does proceed much more rapidly at these higher pH levels, resulting in the formation of ferric or trivalent iron hydroxide which is extremely insoluble in neutral solution.
Now, let’s make the further assumption that the mine water in its passage toward its eventual point of discharge contacts alkaline rock in the presence of additional air. Ideally, a series of combined neutralization and oxidation reactions will occur and with long residence times, iron is oxidized from its ferrous form to the insoluble ferric form and is dropped out as ferric hydroxide or “Yellowboy”. If residence time is sufficiently long, almost all of the iron will be removed and the discharge collected in the final sump will be essentially iron- and acid-free, although it may still contain large amounts of sulfate.
Instead of having shallow flowing underground streams with high turbulence to effect aeration and reaeration many, many times, we must depend upon natural oxidation of the ferrous ion in relatively quiescent deep ponds on the surface. Alternately, we must force-aerate with compressors. And, of course, in the case of acid discharges, we must supply our own alkaline materials to raise the pH to acceptable levels so that ferrous oxidation can proceed at a more rapid rate.
Now, the rate of this reaction is not infinitely fast but is extremely susceptible to both pH level and water temperatures. Every case has its optimum solution. I realize that this is scant comfort for an operator who is looking for design parameters upon which to base a treatment plant. Frankly, he needs more than a bit of divine guidance, particularly since he cannot yet profit from the experience of others as would be the case if he were, say, designing a sewage disposal plant. However, build something he must, or face a shutdown of his operation.