Microbially-induced calcite precipitation in heterogeneous co-disposed coal waste systems

• Calcite generated via MICP improves the structural integrity of mine waste beds. • Calcite from MICP reduces voids and thus oxidant ingress in mine waste beds. • The packing of MICP-co-disposed beds affects sealing and calcite distribution. • Oxygen and nutrient access influence the success of the inoculation method. • The capillary barrier effect plays a pivotal role during cementation in mine waste. Acid rock drainage (ARD) is the acidic leachate obtained from weathered mine or mineral waste, caused by the oxidation of sulfide minerals by oxygen and water. Limiting the access of oxidants to the sulfide-containing mineral is paramount to prevent the oxidation reactions that promote acid-runoff. Co-disposal techniques include the co-mingling of complementary mine wastes with disparate ARD characteristics and particle sizes. This helps to reduce the permeation of oxidants to the acid-generating fractions. In this study, co-disposal was combined with a phenomenon known as microbially-induced calcite precipitation (MICP). In this process, ureolytic bacteria produce a calcite precipitate that causes long-range clogging in porous media. Calcite produced via MICP can be used to mitigate the exposure of sulfide-bearing coal waste rock to natural elements. This is achieved by creating a reactive barrier to oxidants that promote the onset of ARD. The calcite precipitate bifunctionally provides structural stability, supporting the reduction of seepage often associated with oxidative weathering. Twelve bioreactors were setup with different packing configurations (layered and blended) to determine the optimum physical conditions for MICP. Additionally, the bioreactors were inoculated via irrigation or agglomeration to explore the effect of the inoculation strategy on the calcite penetration depth. The packing protocol was found to play a significant role in the rate, penetration depth, and yield of MICP. Layered bioreactors yielded higher total calcite contents; however, blended bioreactors displayed a better calcite distribution than their layered counterparts. Further, the impact of the different inoculation methods was strongly dependent on the packing configuration.