Autotrophic Uranium (VI) bioreduction in groundwater by siderite: Comparison with sulfur and zero-valent iron

Uranium (U), recognized as a significant health risk in groundwater, has become a key focus in environmental remediation efforts. While numerous electron donors have been investigated for the removal of U(VI) through microbial processes, the potential of abundant and economical Fe(II)-containing minerals remains unexplored. Here, a new inorganic electron donor, siderite (FeCO₃) was proposed. Although siderite demonstrates a lesser electron-donating capacity than Fe(0) and S(0), the Siderite-B bioreactor successfully enriched microbes belonging to the Azotobacter genus, which are known for their nitrogen-fixing ability. Within this system, Azotobacter facilitated the oxidation of Fe(II) coupled with the reduction of U(VI). Initially, Fe(II) donated electrons to the NAD⁺/NADH couple. Subsequently, NADH transferred these electrons to the Rnf/Fix complex, which in turn donated them to ferredoxin, catalyzing the reduction of U(VI) to U(IV). The Siderite-B autotrophic bioreactor achieved a U(VI) removal efficiency of 93.40 ± 0.47 % over 144 h, which was slightly lower than the S(0)-B bioreactor (97.12 ± 0.50 %) and the Fe(0)-B bioreactor (95.58 ± 0.95 %). In contrast, S(0)-B and Fe(0)-B bioreactors were enriched with microbes belonging to the Thiobacillus genus, which reduced U(VI) mainly through Fe-S oxidoreductase and Cytochrome C mediated electron transfer. Over a 90-day continuous-flow experiment, the Siderite-B bioreactor exhibited high U(VI) removal efficiencies of 96.83 ± 1.12 %, 95.92 ± 1.84 %, and 85.28 ± 1.41 % at influent U(VI) concentrations of 10, 20, and 30 mg/L, respectively. The findings highlight the potential of siderite as an effective and autotrophic electron donor for U(VI) reduction, offering a cost-effective and environmentally friendly alternative for groundwater uranium remediation.