A simple biogeochemical process removing arsenic from a mine drainage water

Arsenic is a toxic element commonly found in mining environments. The present study aimed to determine the influence of the indigenous bacterial population on the biogeochemical evolution of arsenic concentration in a mine drainage water. Biological As(III)-oxidizing activity was detected in diverse micro-environments along the water stream, from the source to the discharge point. Laboratory experiments showed that the bacterial population promoted As(III) and Fe(II) oxidation in conditions close to those of the site, i.e., temperature, water composition and oxygen availability. Immobilization of bacteria on pozzolana in a column bioreactor increased oxidation rates compared to on-site natural conditions: As concentration in the bioreactor outlet was less than 50 mu g L-1, whereas on-site total As concentration at the discharge point is close to 100 mu g L-1. The highest As(III) removal rate in the inoculated column reached 1900 mu g L-1 h(-1) and As(III) removal rate in the non-inoculated blank column was 42 mu g L-1 h(-1). Two strains of As(III)-oxidizing bacteria, related to Variovorax paradoxus and Leptothrix cholodnii, were respectively isolated from a laboratory reactor and directly from site sludge. A diverse population of bacteria affiliated to nine phylogenetic groups belonging to the Proteobacteria and to the Bacteroidetes was identified in a laboratory reactor by molecular analyses of 16S rRNA genes. Amongst these, microorganisms already known to be potentially responsible for As(III) or Fe(II) oxidation, as well as for As(V) reduction co-existed: they included members of the genera Gallionella, Pseudomonas, Ralstonia, Sphingomonas, Methylobacterium and Methylophilus. On-site experiments confirmed the laboratory results, i.e. the removal of As from the contaminated effluent, and a passive on site treatment is currently developed for improving the quality of the discharged water.