what are the natural chemical processes by which dissolved trace metals are removed from acid mine drainage?

Question

Trace metals in geochemistry is defined as 'all elements except the eight abundant rock-forming elements: oxygen, silicon, iron, calcium, potassium, and magnesium'

Answer 1

Carbonate neutralization:
Generally, limestone or other calcareous strata that could neutralize acid.
Ion exchange:
ion exchangers would remove potentially toxic heavy metals from mine runoff.
Wetlands:
A spectrum of bacteria and archaea, in consortium with wetland plants, may be used to filter out heavy metals and raise pH. Anaerobic bacteria in particular are known to be capable of reverting sulfate ions into sulfide ions. These sulfide ions can then bind with heavy metal ions, precipitating heavy metals out of solution and effectively reversing the entire process.
Precipitation of metal sulfides

See reference:
http://en.wikipedia.org/wiki/Acid_mine_drainage