A VERY BRIEF INTRODUCTION TO ACID AND ALKALINE MINE DRAINAGE AND THEIR TREATMENT BY PASSIVE METHODS

Some terms

• pH = -log [H_⁺] = negative log of hydrogen ion concentration; as hydrogen ion concentration goes up, pH goes down . (Actually it's hydrogen ion activity, not concentration.)
• mg/L ppm = parts per million
• Alkalinity = ability to neutralize acid; mainly from HCO_3^- or added limestone
• Acidity = ability to neutralize base; mainly from metals in solution (Fe, Al, Mn) with some contribution from H_⁺
• Net alkalinity = Alkalinity - Acidity (It's actually more complicated; download Applied Geochemistry papers from Kirby main page.)
  • If this number is positive, there is "net Alkalinity"; may be able to use wetland alone
  • If negative there is "net acidity"; must add alkalinity, wetland alone insufficient
  • Both can be expressed as mg/L HCO_3^- or mg/L CaCO₃ (can be converted to either units)
• Alkalinity added in two ways
  • Adding a base like lime or limestone
  • Achieving sulfate reduction

Reactions to help understand AMD from coal mining

• The process starts deep underground or in spoils piles with the oxidation of the mineral pyrite (fool's gold), which is associated with coal and some shales

Fe2+ does not precipitate from solution unless pH is quite high

• If there is sufficient oxygen, ferrous iron oxidizes to make ferric iron which stays in solution if pH is low

        ferrous      dissolved     hydrogen       ferric                            
       iron           oxygen           ion             iron          water         
2) Fe _²⁺    +    1/4O₂     +      2H_⁺      =  Fe _³⁺   +   1/2H ₂O

This rxn causes pH to go up

• If pH is > approximately 3.5, the ferric iron reacts with water to form a solid iron hydroxide precipitate (yellow boy); the higher the pH, the more yellow boy will form, until most of the iron leaves the solution

        ferric                                     solid ferric                          
           iron                water              hydroxide            "acid"        
3) Fe _³⁺    +      3H ₂O      =      Fe(OH)_{3, s}    +      3H_⁺

This rxn causes pH to go down; Al _³⁺  behaves like Fe _³⁺, i.e., it forms a white aluminum hydroxide, though pH must be approximately 5.5 for aluminum to precipitate

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