Performance of a Semi-passive Sulfate-reducing Bioreactor for Acid Mine Drainage Treatment and Prediction of Environmental Behavior of Post-treatment Residues

The performance of a semi-passive sulfate-reducing bioreactor (SPSRB) was studied for the treatment of highly contaminated AMD (pH = 4.2, [Fe](i): 1600 to 6400 mg/L). The potential mobility of contaminants (metals and sulfates) from AMD post-treatment residue was also evaluated (e.g. metal speciation, weathering cells) to ensure their proper management. Sodium lactate (3500 mg/L) was continuously added to three SPSRBs composed of 60% birch chips, 20% calcite, and 20% poultry manure to enhance microbial activity. The SPSRBs were very effective during the total duration of the experiment (266 days) for the removal of Fe (average removal efficiency of 97.7%), Cd (> 99.4%), Cr (99.3%), Cu (99.9%), Ni (99.2%), Pb (99.8%), Al (92.4%), and Zn (90.7%). The study of the physicochemical stability of the post-treatment residues showed a net positive neutralization potential of 22.4 kg CaCO3/t and a NP/NA ratio of 1.80. Although the metal speciation estimates revealed that Fe, Mn, Ni, and Zn were mainly retained in the biofilter as oxides or hydroxides, a significant proportion of metals were also present in the form of sulfides, or bound to organic matter as exchangeable/soluble sulfate fractions. Finally, the kinetic tests (weathering cells) confirmed the high risk of metal release if the post-treatment residues are disposed of in an oxidizing environment.