How can acid mine drainage be prevented?

The removal of the source is the most obvious answer. Sealing off the pyrite so that it can not oxidize and flooding the abandoned mine are two solutions. Reclaiming land so that pyrite does not expose also helps.
However, it is very difficult to prevent pyrite from oxidizing once it has been disturbed and exposed to air.
You could coat the exposed pyrite with an impermeable coating of ferric phosphate to prevent exposure to air.

You could also curb acid mine drainage by suppressing the activity of Fe- and S-oxidizing bacteria. In the laboratory, this has been done with a low concentration of organic acid and the addition of antimicrobial compounds. However, this procedure appears to be impartial on a field scale.
And the alternative mechanism is to place the acid mine drainage in a lagoon with large quantities of organic waste "sawdust". Aerobic and anaerobic bacteria will lower the redox potential and produce degradation and intermediates that can be used by (SO4)2- reducers. Hydrogen sulfide generated by the (SO4)2- reducer will reduce the Fe3+ to Fe2+ chemically and then precipitated it as FeS. The pH should rise and both Fe3+ and (SO4)2- should decline. This seems to work on a small scale but has not been tried on a large scale.

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