Question

What are the main chemical reactions involved in AMD formation?

What is the “initiator reaction” and the “propagation cycle”? Why is this cyclical nature (regeneration of Fe2+) problematic?

How do microbes play a role in these reactions?

What is “yellow boy”?

Oxidation of Fe2+ is very slow at low pH values. Why does this reaction occur then in acidic environments?

Answer 1

The main chemical reaction is the oxidation of pyrites from the metal or coal mines:

--> (1)

Further oxidation of ferrous (Fe 2+) to ferric (Fe 3+) occurs in the presence of excess oxygen dissolved in water:

...(2)

...(3)

Reaction (1) is the initiator reaction and as we can see the process of productiong Fe2+ (ferrous ions) is propagated by reaction (2) and (3) as more and more ferrous ions are produced. This is highly problematic since each time oxidation of metal ore or Fe3+ ions produce more Fe2+ ions, there are H+ ions produced along with it during the process. This decreases the pH of the process making it highly acidic.

Presence of microbes which can survive in extreme acidic conditions usually favor the situation producing more H+ ions and further aggravating the process.  Thiobacillus ferrooxidans is one of main bacteria which can accelerate reactions (2) and (3). These bacteria will aid the production of ferrous ions even in the presence of low oxygen levels and hence accelerate the process of increasing the acidity.

This type of acidic environment is extremely unfavourable to aquatic life.

When neutralising chemicals are added to the acidic water from mines at a pH greater than 3, the ferric ions precipitate as iron hydroxide, a yellow solution, this is called as yellow boy. This will cause color in water affecting aquatic life, drinking water and so on. This can also favor further production of H+ ions in water.

Even though the oxidation of Fe2+ ion is slow at low pH values, they are accelerated by many orders of magnitude by microbes which can survive in these conditions. Hence, they proceed at significant level in these reaction proceed at these extreme conditions also.