GEOCHEMICAL AND MICROBIAL COMMUNITY DYNAMICS OF A REMEDIATION SYSTEM TREATING COAL-DERIVED ACID MINE DRAINAGE

Mobility of toxic elements and the overall treatment efficiency of coal-generated acid mine drainage (AMD) depends on the dynamic evolution of biogeochemical processes taking place in the system. Tab Simco is an abandoned coal mine near Carbondale, Illinois that produces AMD with pH ~2.4 and average concentration (ppm) of dissolves ions: 600 Fe, 150 Al, 40 Mn and 3500 SO₄. To abate this problem, a passive treatment system comprised of open limestone drains, a SO₄-reducing bioreactor, and an oxidation pond was built in 2007.

Over the past five years, within the bioreactor, a horizontally stratified Al-and Fe-rich layer with a thickness of ~0.7 m has precipitated above the compost layer. This layer consists of finely laminated, optically distinguishable microcrystalline sub-layers. Structural order within this layer suggests stratified zones of redox conditions control organization of precipitates. To model the fate of toxic elements in the system, we followed the chemical, mineralogical and biological evolution through a multi-analytical approach (XRD, SEM, ICP-MS, 16S rRNA gene analysis) applied to surface precipitates and associated waters. The precipitates evolve through a temporal transition from amorphous Fe and Al phases, to schwertmannite and halotrichite, and finally to goethite and clay minerals. Results indicate that Fe(III) oxide nanoparticles play a paramount role in remediation due to high sorption capacities for metal and anionic contaminants. Still, experiments mimicking a static system show that alkalinity produced from fresh limestone is inhibited by the formation of Fe-oxides that coat the surface.

Molecular analysis of the bacterial 16S rRNA gene sequences present in the effluent indicated that bacterial oxidation of Fe(II) is primarily mediated by Betaproteobacteria, with 49% of the identified sequences classified in this group. While most of these phylotypes shared the highest DNA similarity to uncharacterized environmental clones, the most closely related isolate in pure culture is Sideroxydans lithotrophicus. To further quantify the biogeochemical interplay, additional data must be collected at lab and field scale. Connecting the response of the system to changes that propagate through time is critical in the design of an effective remediation strategy for Tab Simco.