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(MS Thesis) Subsurface bacterial community assembly following electrokinetic in situ leaching of citrate-treated copper mine tailings

Abstract: Rising global demand for copper (Cu), alongside the growing inventory of Cu-bearing mine wastes, has positioned them as a promising alternative resource. However, Cu in tailings is often present in low concentrations and locked within complex mineral matrices, making extraction difficult. Conventional methods, which require physical excavation for ex situ treatment, pose significant environmental and technical challenges due to their invasive nature. In contrast, electrokinetic in situ leaching (EK-ISL) has recently emerged as a promising, non-invasive approach for metal recovery from industrial waste. Its application to mine tailings, however, remains largely unexplored, with a key knowledge gap regarding the environmental fate of post-treated material. This study examines the impact of EK-ISL, using citric acid (0.5 M), on microbial health and pedogenesis. Using MiSeq-based 16S rRNA high-throughput metabarcoding, the study analyzed the subsurface bacterial composition and predicted functional shifts in response to citrate-driven chemical gradients. Results demonstrated that citrate significantly enhanced Cu leaching efficiency during EK-ISL but caused significant microbiome restructuring, reducing bacterial diversity and homogenizing the community structure. Chloroflexi, slow-growing K-strategists linked to carbon assimilation, were found to be characteristic of pre-treated (Cu-rich) tailings. In contrast, fast-growing r-strategists such as Proteobacteria and Firmicutes, mostly nitrogen-fixing bacteria, were characteristic of post-treated (Cu-depleted) tailings. Predicted metabolic functions of the citrate-modified microbiome revealed enrichment in genes associated with metabolism and environmental information processing pathways, suggesting an adaptive shift favoring rapid growth and colonization. Although citrate induced a disturbance-driven ecological succession, it did not result in irreversible soil sterilization, allowing for a new ecological baseline to emerge over time. A timeline model for this recovery in the post-citrate scenario is proposed. The findings demonstrated that citrate facilitates Cu leaching from low-grade sources without significantly altering pedogenesis potential, positioning it as a promising leaching agent for integrated mining strategies that balance efficient metal extraction with ecological recovery.

Keywords: microbiome, mine tailings, ecological succession, microbial ecology, metabarcoding