Acid mine drainage and acid rock drainage contain sulfuric acid together with heavy metals. Biological treatment often relies on sulfate reducing bacteria which use organic electron donating substrates to enable bacteria to reduce sulfate to sulfide, subsequently sulfides precipitate heavy metals. However, excess sulfides are released from the treatment system, so the process is not very effective in removing sulfur. Excess sulfides have oxygen demand, are corrosive and malodorous. A process developed at the University of Arizona uses zero valent iron (ZVI) either alone or mixed with organic substrates. The main advantage of using ZVI is that ferrous iron (Fe2+) released from its corrosion will precipitate sulfides formed by sulfate reduction, thereby avoiding the discharge of excess sulfides from the barrier system. Additionally ZVI has other advantages. ZVI is a slow release electron that can supply electrons equivalents for sulfate reduction over prolonged periods of time. ZVI itself can directly reduce heavy metals such as copper to metallic forms and thus provides an additional mechanism of removing heavy metals. Lastly the corrosion of ZVI creates substantial alkalinity which is useful for neutralizing severly acid rock drainage.
Two laboratory-scale packed bed column experiments were conducted to study the impact of ZVI on the treatment of acid rock drainage by sulfate reduction, imitating a biologically active permeable reactive barrier (PRB). A control column was packed with a compost and limestone mixture. A complete treatment reactor was composed of a compost, limestone and ZVI mixture (ZVI 10% by volume). Both reactors were inoculated with a mixed culture containing sulfate reducing bacteria. The reactors were fed with a synthetic acid rock drainage (SARD) containing 250 mg/l of sulfate and copper (10 to 25 ppm). The SARD was fed at a hydraulic retention time of 24 h. Initially the pH of the synthetic acid rock drainage was set at 7; however. the pH of this influent was progressively decreased to 3 so as to imitate the severely acidic conditions of real acid rock drainage.
The complete treatment with ZVI provided: two-fold greater levels of sulfate reduction while discharging 3-fold less sulfide compared to the control reactor. Sulfide formed in the ZVI-containing reactor was thus effectively precipitated as FeS. The ZVI containing column had effluent pH values that were on the average 3 units higher compared to the effluent of the control reactor lacking ZVI, emphasizing the large impact of ZVI on generating additional alkalinity. During the operation of both columns, copper was effectively removed. The copper removal efficiency was 96.8% (±1.1) and 93.4% (±2.2) in the treatment and control columns, respectively.
The results taken as a whole clearly indicate that inclusion of a small percentage of ZVI in the PRB greatly increased the increased sulfate reduction, decreased release of sulfide, and produced more alkalinity compared to the control column. This was achieved while maintaining nearly complete removal of copper.