Transitional copper stages during Acid Mine Drainage (AMD) sample exposure to biogenic H2S. (a) AMD sample predominating the soluble copper form before biogenic H2S exposure. (b) Initial soluble copper reduction to polysulfides by biogenic H2S. (c) Secondary polysulfides reduction to covellite by biogenic H2S. (d) Vacuum filtered and recovered dried covellite from AMD. Silva, et al. 2019. Metals. 9(2), 206-215.