Comparative effectiveness and acid stress regulation of bioaugmentation versus biostimulation in remediating uranium mine groundwater

Biostimulation and bioaugmentation are promising for remediating acid in-situ leaching (AISL) U mine groundwater, yet their efficiencies, mechanisms, and acid stress regulation remain unclear. This study collected groundwater from a decommissioned AISL sandstone-type U mine, and isolated an indigenous sulfate-reducing bacterial strain called Desulfovibrio multispirans H1SLC23D. The sulfate removal and U immobilization performance of biostimulation and bioaugmentation were first evaluated and compared at a baseline pH of 5.0. Bioaugmentation significantly accelerated the establishment of a reducing microenvironment (leading by about 7 d), and had higher remediation efficiencies of sulfate and total dissolved U (73.9% and 99%), compared with biostimulation (39.6% and 94.8%). Multiple solid-phase evidences revealed that bioaugmentation promoted the generation of crystalline phases compared to biostimulation, including uranyl phosphate ((UO)3(PO4)2) and metal sulfides (e.g., FeS and MoS2). To further understand the operational boundaries of the bioaugmented system, its response to acid stress was investigated by varying the influent pH. In bioaugmentation, decreasing the influent pH from 5.5 to 4.5 decreased the sulfate removal efficiency from 64.5% to 27.3%, indicating that the functional niche and remediation efficiency of the inoculated strain are highly pH-dependent. Acid stress suppressed microbial metabolic activity, reduced the expression of hindered the formation of stable U minerals during the bioaugmentation, thereby decreasing remediation efficiency. In addition, we found that the metabolic relay by Desulfovibrio and Sulfurospirillum was a critical process in biostimulation and bioaugmentation, facilitating a cascade reaction (S6 + → S0 → S2-). This study deepened our understanding of distinctions between biostimulation and bioaugmentation at a specific pH, and provides insights into the physiological constraints and acid stress response of bioaugmentation from a niche perspective.