Plant ash prevents acid mine drainage from sulfur-bearing tailings through multiple actions — A low-cost alkaline material

Plant ash, an inexpensive alkaline material containing organic nutrients, can provide electron donors for microbial reactions and neutralize water acidity, making it an excellent candidate for use in preventing acid mine drainage production from tailings. However, the efficacy of using plant ash alone to treat tailings and the role of microorganisms remain unclear. Therefore, the effectiveness of plant ash in acid neutralization, inhibiting oxidation of sulfur-bearing minerals, and reducing acid mine drainage production is compared to that of three other alkaline materials—volcanic ash, limestone, and fly ash—in this study. Their potential microbial mechanisms are also explored. The 63-day experimental results showed that all four alkaline materials effectively neutralized the acidity of the sulfur-bearing tailings and dramatically decreased heavy metal leaching (Fe, Mn, Zn, Pb, As, and Cd). Plant ash had a higher removal efficiency of Fe (99.9%), Mn (94.0%), and Zn (99.7%) than the other three materials, which was related to its ability to provide better precipitation and adsorption conditions. XRD observed gypsum in each treatment column, the formation of a new phase of calcium carbonate was observed in the plant ash column, and SEM showed that precipitation wrapped around the surface of tailings inhibited their oxidation process. 16 S rRNA sequencing confirmed that plant ash could inhibit AMD production through microbial action. The plant ash column had higher biodiversity than the control column, and the eutrophic flora in the plant ash column could use organic matter to consume dissolved oxygen, inhibiting tailing oxidation; however, the growth of some reducing flora resulted in As reduction, causing a slight increase in As in the leachate.