Arsenic sequestration in biomats formed in Bangladesh groundwaters: a preliminary study.

Arsenic contamination is a dramatic problem in many Countries around the world. In Bangladesh, for example, 30 to 35 millions people are exposed to As concentrations in drinking water well above 50 μg L-1 (Smedley and Kinniburg, 2002). In waters rich in As and metals in solution the formation of Fe-As co-precipitation products is well documented and often enhanced by the presence of microorganisms (Bruneel et al., 2006). The presence of organic molecules in the co-precipitates, often from microbial origin, can stabilize the scarcely crystalline structure of the oxi-hydroxides in the long term, maintaining a high specific surface area and a high adsorption capacity. On the other side, these molecules can compete with As for adsorption or can enhance the weathering of the mineral phase or, perhaps, they could form complexes with the As anions by bridging with bi- or polyvalent cations or polycations, for examples of Fe3+ or Fe (hydr)oxides. The exact role of the organic phases included in the co-precipitates is still not fully understood, in spite of the ubiquitous presence of bio-organic-mineral system, which represents the key of metals and metalloids fixation/mobilization in most natural environments (Huang, 2008). Fe oxidizing bacteria are able to accelerate Fe(II) oxidation and As may be immobilized within the mixture of biogenic Fe oxides, organic material and microbial biomass. Hence, the role of the microorganisms in the formation of As-bearing co-precipitates, as well as in the solubilization of the co-precipitated As, deserves special attention for its environmental relevance. Co-precipitation products were sampled in a heavily arsenic affected area in South-West Bangladesh. Elemental analysis showed that these materials were formed mainly by Fe oxides, phosphates and carbonates; however, organic carbon made up to 6% of the total dry weight. Scanning electron microscopy imaging revealed the abundant presence of microorganisms, morphologically analogous to Fe-oxidizer bacterium Gallionella ferruginea, closely associated with the mineral component. The chemical and spectroscopic characterization of the organic material coupled with the microbial community analysis, could suggest that the co-precipitation products are organo-mineral materials formed by the incorporation of the mineral phase inside an organic matrix produced by microorganisms. The presence of the microbial organic phase affects the stability of the As inside the co-precipitates, This is partly due to the chemical interactions, and partly attributable to the composition of microbial consortia, where bacteria with potentially opposite effects on As fixation-mobilization have been detected, such as Fe-oxidizing bacteria, that could enhance the fixation of As on Fe oxides, and As-reducing bacteria, that increase the fraction of the trivalent form of As, more mobile in the environment.