Influence of goethite on adsorption and bioreduction of arsenate under anoxic conditions

Arsenic is a metalloid contaminant that due to its risk to plants, animals and human health is of impressive concern in the environment. The most environmental problems are the result of mobilization of arsenic under anaerobic conditions. In these conditions, reducing bacteria have ability for bioreduction of arsenate to arsenic and increasing its mobilization. The objective of this study was to determine the bioreduction of arsenate in aqueous and adsorbed on goethite in the presence of shewanella sp in two different sections.
In the first section, bioreduction of arsenate in aqueous phase and in the second section bioreduction of adsorbed arsenate on goethite was studied. The change in the number of viable cells over the course of the experiments was monitored by viable cell counts.
The results of the first section revealed that Shewanella sp bacterium is capable of bioreduction of arsenate in aqueous phase at a concentration of 0.05 mM. The results also showed that during 6 days, goethite is capable to adsorb arsenate at a concentration of 0.05 mM totally and Shewanella sp reduced adsorbed arsenate on goethite. The rate of bioreduction of arsenate in aqueous phase was more than the adsorbed arsenate on goethite. The results also showed that Shewanella sp was able to bioreduce of ferric iron in the structure of goethite to ferrous iron. So, Shewanella sp was able to reduce both adsorbed arsenate on goethite and ferric iron in anoxic condition and Shewanella sp as an iron reducing bacterium (IRB) was able to reduce arsenate. But bioreduction of adsorbed arsenate on goethite was done with more priority in comparison with ferric iron in the structure of goethite in the presence of Shewanella sp. The concentration of arsenite in aqueous phase after 30 days of bioreduction process was 0.04 mM that revealed 80% of adsorbed arsenate reduced to arsenite and entered to aqueous phase. The amount of aqueous ferrous iron increased up to 10 days and after that significantly decreased. The amount of total ferrous iron increased up to 20 days and after that became almost constant. The results of the change in the number of viable cells over the course of the experiments also showed continuation of bacterium cell growth of Shewanella sp in the bioreduction experiments.
Bioreduction process in anaerobic ecosystems could create contamination in water sources and should be considered especially by the environmental specialists.