Anaerobic Nitrate Reduction with Oxidation of Fe(II) by Citrobacter Freundii Strain PXL1: A Potential Candidate for Simultaneous Removal of As and Nitrate from Groundwater

Arsenic and nitrate are the most common pollutants in groundwater, especially in some rural endemic arsenic poisoning areas. Arsenic is highly toxic to human being and can cause many serious diseases, and nitrate is another common toxic pollutant that can be transformed to carcinogenic N-nitroso compounds in human digestive system. Arsenic pollution is usually caused by geological reasons and mining activities while nitrate pollution is mainly due to intense application of nitrogen fertilizers.

Although nitrate and arsenic have been frequently detected together in groundwater, there are biotechnologies that can only remove arsenic or nitrate separately. There are no technologies that can simultaneously remove nitrate and arsenic by a single species of microorganism. It is urgently necessary to seek cost-effective biological methods that can clean up groundwater contaminated with nitrate and arsenic.

Therefore, researchers investigated the potential of simultaneous removal of nitrate and arsenite from groundwater by a nitrate-reducing Fe(II) oxidizing anaerobe, Citrobacter freundii strain PXL1. The biogenic iron oxides and their adsorption of arsenite were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) spectroscopy, and Fourier transform infrared (FTIR) spectroscopy.

The strain PXL1 could grow well in arsenite concentration of 2.85–13.65 μM and efficiently oxidize Fe(II), reduce nitrate and remove As(III) from water. Scanning electron microscopy, X-ray diffraction spectroscopy, and Fourier transform infrared spectroscopy analysis showed that the strain PXL1 produced poorly crystalline Fe(III) oxides. As(III) was removed mainly by adsorption and co-precipitation of the biogenic Fe(III) oxides. The anaerobic nitrate-dependent Fe(II) oxidizing bacteria including C. freundii PXL1 are promising microbes for in situ remediation of nitrate and arsenite contaminated groundwater. The result showed that C. freundii strain PXL1 is an excellent bacterial species for bioremediation of nitrate and arsenite polluted groundwater.

The result was published in Ecological Engineering in April 2015.