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

Some terms

Reactions to help understand AMD from coal mining

                                                      dissolved     ferrous                                                   
   pyrite             water         oxygen         iron             sulfate           "acid"

1) FeS2, s    +     H 2O     +    7/2O2    =     Fe 2+    +   2SO42-   +     2H+

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

        ferrous      dissolved     hydrogen       ferric                            
       iron           oxygen           ion             iron
          water         
2) Fe 2+    +    1/4O2     +      H+      =  Fe 3+   +   1/2H 2O

This rxn causes pH to go up

        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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