Abstract
A laboratory-scale study was carried out to evaluate the groundwater bioremediation potential of hexavalent chromium (Cr(VI)), taking into account the chromate pollution of an industrial site located in Southern Italy (Apulia Region). The reduction of Cr(VI) was studied on laboratory microcosms, set up in different experimental conditions, namely: ABIO (soil and water sterilized), BIO (soil and water not sterilized), LATT (with the addition of lactate), and YE (with the addition of yeast extract). Control test lines, set up by using sterilized matrices and amendments, were employed to assess the occurrence of the pollutant reduction via chemical processes. By combining molecular (microbial abundance, specific chromate reductase genes (ChR) and the Shewanella oinedensis bacterial strain) with chemical analyses of chromium (VI and III) in the matrices (water and soil) of each microcosm, it was possible to investigate the response of microbial populations to different experimental conditions, and therefore, to assess their bioremediation capability in promoting Cr(VI) reduction. The overall results achieved within this work evidenced the key role of amendments (lactate and yeast extract) in enhancing the biological reduction of hexavalent chromium in the contaminated aqueous phase of laboratory microcosms. The highest value of Cr(VI) removal (99.47%) was obtained in the YE amended microcosms at seven days.
Highlights
The main natural source of chromium in the environment is the erosive process of rocks containing chromite, generally ferric chromite, a mineral in which chromium is present in the oxidative state of+3, hereinafter Cr(III) or trivalent chromium
The overall result showed that the enrichment of microcosms by lactate or yeast extract led to a rapid decrease in the Cr(VI) concentration, enhancing the biological reduction of this pollutant
The control tests setup with autoclaved matrices and amendments, revealed a scarce degree of Cr(VI) reduction, in particular, in the LATT microcosm (4.79%), significantly lower than that registered in the not sterilized microcosms; in these ones, the highest Cr(VI) removal percentages were reached in the amended reactors, suggesting that the hexavalent chromium removal was mainly “biological”; in this sense, the role of both of the amendments was substantially to stimulate the ability of the autochthonous microbial populations involved in the reductive Cr(VI) process, providing them a carbon source useful for their growth
Summary
The main natural source of chromium in the environment is the erosive process of rocks containing chromite, generally ferric chromite, a mineral in which chromium is present in the oxidative state of+3, hereinafter Cr(III) or trivalent chromium. The main natural source of chromium in the environment is the erosive process of rocks containing chromite, generally ferric chromite, a mineral in which chromium is present in the oxidative state of. The principal Cr(III) reaction in water is the formation of chromium hydroxides and mixed iron chromium hydroxide (Cr(OH) and CrFe(OH)6 ), which have a low solubility and tend to precipitate under neutral to basic aqueous solutions [1]. The other form of chromium that is stable in a natural environment, the hexavalent chromium, or Cr(VI), is water soluble at any pH and is 100-fold more toxic than Cr(III) [2]. While there are natural sources of chromium in the environment, the majority of Cr(VI) inputs are derived from industrial processes, including metal plating, leather tanning, synthesis of paint Cell membrane damages caused by oxidative stress induced by Cr(VI) have been extensively reported, both in eukaryotic and prokaryotic cells [2,3,4].
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