Abstract
BackgroundChromium is a transition metal most commonly found in the environment in its trivalent [Cr(III)] and hexavalent [Cr(VI)] forms. The EPA maximum total chromium contaminant level for drinking water is 0.1 mg/l (0.1 ppm). Many water sources, especially underground sources, are at low temperatures (less than or equal to 15 Centigrade) year round. It is important to evaluate the possibility of microbial remediation of Cr(VI) contamination using microorganisms adapted to these low temperatures (psychrophiles).ResultsCore samples obtained from a Cr(VI) contaminated aquifer at the Hanford facility in Washington were enriched in Vogel Bonner medium at 10 Centigrade with 0, 25, 50, 100, 200, 400 and 1000 mg/l Cr(VI). The extent of Cr(VI) reduction was evaluated using the diphenyl carbazide assay. Resistance to Cr(VI) up to and including 1000 mg/l Cr(VI) was observed in the consortium experiments. Reduction was slow or not observed at and above 100 mg/l Cr(VI) using the enrichment consortium. Average time to complete reduction of Cr(VI) in the 30 and 60 mg/l Cr(VI) cultures of the consortium was 8 and 17 days, respectively at 10 Centigrade. Lyophilized consortium cells did not demonstrate adsorption of Cr(VI) over a 24 hour period. Successful isolation of a Cr(VI) reducing organism (designated P4) from the consortium was confirmed by 16S rDNA amplification and sequencing. Average time to complete reduction of Cr(VI) at 10 Centigrade in the 25 and 50 mg/l Cr(VI) cultures of the isolate P4 was 3 and 5 days, respectively. The 16S rDNA sequence from isolate P4 identified this organism as a strain of Arthrobacter aurescens, a species that has not previously been shown to be capable of low temperature Cr(VI) reduction.ConclusionA. aurescens, indigenous to the subsurface, has the potential to be a predominant metal reducer in enhanced, in situ subsurface bioremediation efforts involving Cr(VI) and possibly other heavy metals and radionuclides.
Highlights
Chromium is a transition metal most commonly found in the environment in its trivalent (Cr3+) and hexavalent (Cr6+) forms [1]
Many water sources are at low temperatures year round (≤15°C) and it is important to evaluate the possibility of remediating Cr6+ contamination using microorganisms adapted to these low temperatures
This study demonstrates that indigenous microbial populations present in Cr6+-contaminated aquifers are able to aerobically catalyze the removal of toxic and soluble Cr6+ from the media, most likely reducing it to the relatively nontoxic and insoluble Cr3+
Summary
Chromium is a transition metal most commonly found in the environment in its trivalent [Cr(III)] and hexavalent [Cr(VI)] forms. The EPA maximum total chromium contaminant level for drinking water is 0.1 mg/l (0.1 ppm). Chromium is a transition metal most commonly found in the environment in its trivalent (Cr3+) and hexavalent (Cr6+) forms [1]. Hexavalent chromium is water soluble, toxic, and carcinogenic, and is considered a pollutant by the United States Environmental Protection Agency (EPA) [4]. Chromium is the second most common inorganic contaminant of groundwater at hazardous waste sites [5]. The solubility and negative charge of its more common forms, chromate and dichromate (CrO42-, and HCrO4-), lead to limited adsorption in aquifer minerals, and results in high mobility of Cr6+ in aquifers [6]. The historical and present day contamination of groundwater and soils by Cr6+ is a result of its industrial uses, including metal plating (for corrosion resistance), pigment production, and lumber and wood products (for preservation) [7]
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