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

Arsenic removal consecutive to microbiological iron oxidation and precipitation is an effective process for treating As-rich acid mine drainage (AMD). Here, we studied the effect of hydraulic retention time (HRT) - a key operating parameter that drives water treatment process performances - on water physico-chemistry, precipitate mineralogy, and bacterial community that develops in a bench-scale bioreactor treating AMD from the Reigous Creek (Carnoulès mine, France). System efficiency regarding Fe(II) and As(III) oxidation and As removal was monitored during 19 days. At the end, the mineralogy (As and Fe bearing phases) and bacterial communities in the biogenic precipitate were analyzed by X-ray absorption spectroscopy and high-throughput 16S rRNA gene sequencing, respectively. 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 precipitates differed widely from that of the original feed water. Fe-oxidizing bacteria were dominant whatever the HRT but the proportion of Gallionella and Ferrovum genera shifted notably, from respectively 65 and 12 % at low HRT, to 23 and 51 % at high HRT. This shift was associated with physicochemical changes in pH, dissolved oxygen and Fe(III) concentrations in the treated water. A genetic potential for As(III) oxidation was evidenced through the detection of aioA genes at all HRT and As-oxidizing Thiomonas genera was always present as a minority member of the community but HRT did not appear to control As(III) oxidation in the present experiment. To our knowledge, this is the first evidence of the role of HRT as a driver of bacterial community structure in bioreactors exploiting the microbial Fe(II) oxidation process for AMD treatment.