Résumé
Arsenic is one of the most hazardous elements associated to acid mine drainage with concentrationsranging from ≤ 1 µg L-1 to hundreds of mg L-1 in these leachates. To date, there is no affordable methodto efficiently treat As-rich AMD over the long term. Natural attenuation processes based on oxidationof iron and arsenic by autochthonous bacteria present in AMD promote precipitation of these elements.The exploitation of these processes offer a promising method for the treatment of As-rich acid minedrainage (AMD).In the present study, the Fe(II) oxidation rates and Fe and As removal rates have been investigated in alaboratory pilot-scale system with a continuous circulation of AMD water under controlled conditions(temperature, light, water-height and residence time). The system was fed with AMD from the Carnoulèsmine with a permanent acid character (pH 2-4), an average iron concentration of 0.5 g L-1 in the reducedform (Fe(II)) and an average arsenic concentration of 40 mg L-1 predominantly under As(III) oxidationstate.The efficiency and rates of Fe(II) oxidation and of Fe and As removal were determined in the pilot as afunction of experiment duration and for a range of residence times, in presence or absence of a floatingfilm that develops naturally at the surface of the water. The Fe- and As-rich biofilms formed in the pilotwere characterized for their mineralogy, redox As speciation, and bacterial quantification.During the early stage of experiment, results evidenced that the gradual coverage of the pilot channelbottom with an Fe-As precipitate, that contained 6 ± 3 x 106 bacterial cell/g (dry wt.), increased theFe(II) oxidation efficiency and the Fe and As removal by two orders of magnitude. At the steady state, 90 % Fe(II) was oxidized and 30 % Fe and 80 % As were precipitated within a residence time of200 min. Rate values were slightly lower than those measured in the field in the natural AMD stream atCarnoulès. The development of a thin floating film at the surface of water in the pilot channel affectedoxygen diffusion and retarded these rates. All these results improve our understanding of naturalprocesses responsible for arsenic attenuation in AMD, and give some indications that might beconsidered when designing an Fe and As removal passive treatment in AMDs.