Iron Isotopes in Acid Mine Drainage: Extreme and Divergent Fractionation between Solid (Schwertmannite, Jarosite, and Ferric Arsenate) and Aqueous Species.

Examination of stable Fe isotopes is a powerful tool to explore Fe cycling in a range of environments. However, the isotopic fractionation of Fe in acid mine drainage (AMD) has received little attention and is poorly understood. Here, we analyze Fe isotopes in waters and Fe(III)-rich solids along an AMD flow-path. Aqueous Fe spanned a concentration and δ 56 Fe range of ∼420 mg L -1 and + 0.04‰ at the AMD source to ∼100 mg L -1 and -0.81‰ at ∼450 m downstream. Aqueous As (up to ∼33 mg L -1 ) and SO 4 2- (up to ∼2000 mg L -1 ), like aqueous Fe, decreased in concentration down the flow-path. X-ray absorption spectroscopy indicated that downstream attenuation in aqueous Fe, As, and SO 4 2- was due to the precipitation of amorphous ferric arsenate (AFA), schwertmannite, and jarosite. The Fe(III) in these solids displayed extreme variability in δ 56 Fe, spanning +3.95‰ in AFA near the AMD source to -1.34‰ in schwertmannite at ∼450 m downstream. Similarly, the isotopic contrast between solid Fe(III) precipitates and aqueous Fe (Δ 56 Fe ppt-aq ) dropped along the flow-path from about +4.1 to -1.1‰. The shift from positive to negative Δ 56 Fe ppt-aq reflects divergence between competing equilibrium versus kinetic fractionation processes.