Bio-precipitation of arsenic and antimony in a sulfate-reducing bioreactor treating real acid mine drainage water.

Arsenic (As) and antimony (Sb) from mining sites can seep into aquatic ecosystems by acid mine drainage (AMD). Here, the possibility of concomitantly removing As and Sb from acidic waters by precipitation of sulfides induced by sulfate-reducing bacteria (SRB) was investigated in a fixed-bed column bioreactor. The real AMD water used to feed the bioreactor contained nearly 1 mM As, while the Sb concentrations were increased (0.008 ± 0.006 to 1.01 ± 0.07 mM) to obtain an Sb/As molar ratio = 1. Results showed that the addition of Sb did not affect the efficiency of As bio-precipitation. Sb was removed efficiently (up to 97.9% removal) between the inlet and outlet of the bioreactor, together with As (up to 99.3% removal) in all conditions. Sb was generally removed as it entered the bioreactor. Appreciable sulfate reduction occurred in the bioreactor, which could have been linked to the stable presence of a major SRB operational taxonomic unit affiliated with the Desulfosporosinus genus. The bacterial community included polymer degraders, fermenters, and acetate degraders. Results suggested that sulfate reduction could be a suitable bioremediation process for the simultaneous removal of Sb and As from AMD.

Hydraulic retention time affects bacterial community structure in an As-rich acid mine drainage (AMD) biotreatment process.

Arsenic removal consecutive to biological iron oxidation and precipitation is an effective process for treating As-rich acid mine drainage (AMD). We studied the effect of hydraulic retention time (HRT)-from 74 to 456 min-in a bench-scale bioreactor exploiting such process. The treatment efficiency was monitored during 19 days, and the final mineralogy and bacterial communities of the biogenic precipitates were characterized by X-ray absorption spectroscopy and high-throughput 16S rRNA gene sequencing. The percentage of Fe(II) oxidation (10-47%) and As removal (19-37%) increased with increasing HRT. Arsenic was trapped in the biogenic precipitates as As(III)-bearing schwertmannite and amorphous ferric arsenate, with a decrease of As/Fe ratio with increasing HRT. The bacterial community in the biogenic precipitate was dominated by Fe-oxidizing bacteria whatever the HRT. The proportion of Gallionella and Ferrovum genera shifted from respectively 65 and 12% at low HRT to 23 and 51% at high HRT, in relation with physicochemical changes in the treated water. aioA genes and Thiomonas genus were detected at all HRT although As(III) oxidation was not evidenced. To our knowledge, this is the first evidence of the role of HRT as a driver of bacterial community structure in bioreactors exploiting microbial Fe(II) oxidation for AMD treatment.

Dynamics of Bacterial Communities Mediating the Treatment of an As-Rich Acid Mine Drainage in a Field Pilot.

Passive treatment based on iron biological oxidation is a promising strategy for Arsenic (As)-rich acid mine drainage (AMD) remediation. In the present study, we characterized by 16S rRNA metabarcoding the bacterial diversity in a field-pilot bioreactor treating extremely As-rich AMD in situ, over a 6 months monitoring period. Inside the bioreactor, the bacterial communities responsible for iron and arsenic removal formed a biofilm ("biogenic precipitate") whose composition varied in time and space. These communities evolved from a structure at first similar to the one of the feed water used as an inoculum to a structure quite similar to the natural biofilm developing in situ in the AMD. Over the monitoring period, iron-oxidizing bacteria always largely dominated the biogenic precipitate, with distinct populations (Gallionella, Ferrovum, Leptospirillum, Acidithiobacillus, Ferritrophicum), whose relative proportions extensively varied among time and space. A spatial structuring was observed inside the trays (arranged in series) composing the bioreactor. This spatial dynamic could be linked to the variation of the physico-chemistry of the AMD water between the raw water entering and the treated water exiting the pilot. According to redundancy analysis (RDA), the following parameters exerted a control on the bacterial communities potentially involved in the water treatment process: dissolved oxygen, temperature, pH, dissolved sulfates, arsenic and Fe(II) concentrations and redox potential. Appreciable arsenite oxidation occurring in the bioreactor could be linked to the stable presence of two distinct monophylogenetic groups of Thiomonas related bacteria. The ubiquity and the physiological diversity of the bacteria identified, as well as the presence of bacteria of biotechnological relevance, suggested that this treatment system could be applied to the treatment of other AMD.