SELECTIVE RECOVERY OF RARE EARTH ELEMENTS FROM ACID MINE DRAINAGE USING INDIGENOUS BACTERIA IN BIOELECTROCHEMICAL SYSTEMS

Author

PARTHA PROTIM BHOWMIK, Southern Illinois University CarbondaleFollow

Date of Award

8-1-2025

Degree Name

Master of Science

Department

Civil Engineering

First Advisor

Liu, Jia

Abstract

Rare earth elements (REEs) are essential components in many modern technologies. The heterogeneous global REE reserves, increasing demand, and environmental concerns associated with traditional methods from primary resources have driven researchers to strive for sustainable REE extraction techniques from secondary and low-grade non-conventional sources. This study aims to sustainably extract REEs from acid mine drainage (AMD)—a low-grade secondary source—through the attached growth of indigenous species in a two-chamber bioelectrochemical system (BES) under aerobic and anaerobic conditions. In the first study, AMD was used as a seed to cultivate an indigenous bacterial consortium under aerobic conditions. A two-chamber BES was employed to treat the AMD, where PAN-based carbon fiber brushes (CFBs) served as biocathodes, graphite rods as anodes, synthetic wastewater as the anolyte, and raw AMD as the catholyte. A 300 Ω external resistor and an anion exchange membrane (AEM) were used to facilitate electrical connection and ionic separation between the two chambers, respectively. Nitric acid treatment followed by coating with lab-synthesized Fe3O4 nanoparticles increased surface roughness and enhanced the biofilm-holding capacity of the CFBs. Lysinibacillus fusiformis and Bacillus spp. were the dominant bacterial species on the biocathodes before and after BES operation. A 5-day BES treatment significantly increased the pH of AMD in the cathode chamber, resulting in the selective bioprecipitation of REEs over other critical metals. The use of surface-modified CFBs, a greater number of biocathodes, an initial pH increase of the raw AMD, and multiple stripping cycles were all positively correlated with REE extraction efficiency. A three-electrode setup with three stripping cycles achieved REE extraction efficiencies of 68.06 ± 2.44% for total REEs, 69.31 ± 2.30% for light REEs, and 66.16 ± 2.64% for heavy REEs. Active attached growth and electrical connection between the two chambers were essential for REE recovery under aerobic conditions, as control tests showed minimal extraction when either factor was missing. Approximately 15% of the other major metals were extracted by the system, while ~ 60% remained in the catholyte after the BES operation, encompassing considerable selectivity towards REEs. Increasing the solution pH led to a higher precipitation percentage but reduced the overall purity of the final REE product, as indicated in precipitation studies using 10% (w/v) oxalic acid. A solution pH of 4.0 precipitated ~ 55% of REEs from the REE-loaded liquor but also co-precipitated large amounts of Fe and Cu, suggesting the need for further purification steps. In contrast, a lower pH range (1.0-2.0), combined with a higher initial REE concentration, may lead to improved recovery and product purity. In the second study, indigenous anaerobic and sulfate-reducing bacterial species were cultivated in liquid culture media from AMD. Both a sequential extraction technique—employing initial pH enhancement and REE fractionation in an aerobic BES, followed by REE recovery in anaerobic bioreactors—and a completely anaerobic BES were used for REE extraction. Improved attached growth formation was observed in SEM analysis following nitric acid treatment of the hydrophobic, non-polar CFB surface. The presence of Desulfosporosinus auripigmenti and Clostridium subterminale in the attached growth before and after treatment confirmed the establishment of anaerobic conditions and active sulfate-reducing metabolism. A 5-day treatment in the aerobic BES resulted in the extraction of ~ 33% of TREEs from AMD with excellent selectivity. Subsequent treatment of the catholyte with anaerobic biofilm in the bioreactors resulted in an additional ~ 47% of TREEs extraction, leading to a combined efficiency of 80.67 ± 5.88% for TREEs, 79.95 ± 5.98% for LREEs, and 81.79 ± 5.72% for HREEs. Initial pH adjustment to 3.0 helped reduce interference from other metals, and the completely anaerobic BES achieved nearly 100% removal of REEs from raw AMD with excellent selectivity. A single stripping step recovered only ~ 40% of TREEs from the attached biomass, suggesting the need for multiple stripping cycles and extended contact time to improve recovery. Subsequent precipitation using 10% (w/v) oxalic acid at an optimized pH range of 1.0-2.0 can further enhance the separation of REEs from the REE-enriched liquor. Based on the findings from both aerobic and anaerobic systems, this study demonstrates the technical feasibility and selectivity of using indigenous microbial consortia and surface-modified biocathodes for sustainable REE recovery from AMD. The integration of sequential extraction and optimized pH control significantly enhances REE fractionation while minimizing the co-extraction of competing metals. These results underscore the potential for scaling up bioelectrochemical approaches using flow-through reactor designs and advanced electrode modifications to support environmentally responsible REE recovery technologies.