Bacterial chromate reductase, a potential enzyme for bioremediation of hexavalent chromium: a review.

Geochimica et Cosmochimica Acta

The anthropogenic inputs of hexavalent chromium [Cr(VI)] have increased enormously during the past few decades and has become a challenge for life on earth and hence removal of this carcinogen has become the need of the hour. Cr(VI) removal through common physicochemical techniques is highly expensive and inappropriate at low concentration. Microbial reduction of Cr(VI) to trivalent form is considered a favorable technique for Cr(VI) removal from wastewater, as it reduces the highly toxic form of Cr to less toxic form and therefore the article conveys essential fundamental information on removal of Cr(VI) by bacteria. For efficient bioremoval of Cr(VI),the main machinery of the process, the microbes, and their conditions, which decide the fate of this heavy metal, should be appropriate. Hence, the authors cover vast information about the isolation of chromium-resistant bacteria from various environment and their Cr(VI) resistance capability. An extensive report is given on information pertaining to the fa...

Chromium-Resistant Bacteria and Their Environmental Condition for Hexavalent Chromium Removal: A Review

BACKGROUND: The reduction of highly mobile and toxic hexavalent chromium by bacterial strains is considered to be a viable alternative to reduce Cr(VI) contamination, in soils and water bodies, emanating from the overburden dumps of chromite ores and mine drainage. The present study reports the isolation of Cr(VI) resistant bacterial strains from an Indian chromite mine soil and their potential use in reduction of hexavalent chromium.

RESULTS: Among the isolates, a bacterial strain (CSB-4) was identified as Bacillus sp. based on standard biochemical tests and partial 16SrRNA gene sequencing, which was tolerant to as high as 2000 mg L−1 Cr(VI) concentration. The strain was capable of reducing Cr(VI) to Cr(III) in different growth media. Under the optimized conditions pH ∼7.0, 100 mg L−1 Cr(VI), 35 °C temperature and stirring speed 100 rpm, CSB-4 reduced more than 90% of Cr(VI) in 144 h. The time course reduction data fitted well an exponential rate equation yielding rate constants in the range 3.22 × 10−2 to 6.5 × 10−3 h−1 for Cr(VI) concentration of 10–500 mg L−1. The activation energy derived from temperature dependence rate constants between 25 and 35 °C was found to be 99 kJ mol−1. The characterization of reduced product associated with bacterial cells by SEM-EDS, FT-IR and XRD was also reported.

Reduction of hexavalent chromium by Bacillus sp. isolated from chromite mine soils and characterization of reduced product.

Cr(VI) biotreatment has attracted a substantial amount of interest due to its cost effectiveness and environmental friendliness. However, the slow Cr(VI) bioreduction rate and the formed organo-Cr(III) in solution are bottlenecks for biotechnology application. In this study, a novel strain, Acinetobacter sp. HK-1, capable of reducing Cr(VI) and immobilizing Cr(III) was isolated. Under optimal conditions, the Cr(VI) reduction rate could reach 3.82 mg h(-1) g cell(-1). To improve the Cr(VI) reduction rate, two quinone/graphene oxide composites (Q-GOs) were first prepared via a one-step covalent chemical reaction. The results showed that 2-amino-3-chloro-1,4-naphthoquinone-GO (NQ-GO) exhibited a better catalytic performance in Cr(VI) reduction compared to 2-aminoanthraquinone-GO. Specifically, in the presence of 50 mg L(-1) NQ-GO, a Cr(VI) removal rate of 190 mg h(-1) g cell(-1), which was the highest rate obtained, was achieved. The increased Cr(VI) reduction rate is mainly the result of NQ-GO significantly increasing the Cr(VI) reduction activity of cell membrane proteins containing dominant Cr(VI) reductases. X-ray photoelectron spectroscopy analysis found that Cr(VI) was reduced to insoluble Cr(III), which was immobilized by glycolipids secreted by strain HK-1. These findings indicate that the application of strain HK-1 and NQ-GO is a promising strategy for enhancing the treatment of Cr(VI)-containing wastewater.

Cr(VI) reduction and Cr(III) immobilization by Acinetobacter sp. HK-1 with the assistance of a novel quinone/graphene oxide composite.

The present work highlighted the studies on Cr(VI) reduction by cells of Acinetobacter haemolyticus (A. haemolyticus). The strain tolerated 90 mg Cr(VI) l−1 in LB broth compared to only 30 mg Cr(VI) l−1 in LB agar. From the FTIR analysis, the Cr(III) species formed was also most likely to form complexes with carboxyl, hydroxyl, and amide groups from the bacteria. A TEM study showed the absence of precipitates on the cell wall region of the bacteria. Instead, microprecipitates were observed in the cytoplasmic region of the cells, suggesting the transportation of Cr(VI) into the cells. Intracellular reduction of Cr(VI) was supported by a reductase test using soluble crude cell-free extracts. The specific reductase activity obtained was 0.52 µg Cr(VI) reduced per mg of protein an hour at pH 7.2 and 37°C. Our results indicated that A. haemolyticus can be used as a promising microorganism for Cr(VI) reduction from industrial wastewaters.

Chromium(VI) resistance and removal by Acinetobacter haemolyticus

Several types of microorganisms have been reported to reduce Cr(VI) to the less toxic Cr(III) via enzymatic reactions. The main purpose of this study is to determine the reduction of Cr(VI) by a locally isolated bacterium, Acinetobacter haemolyticus (A. haemolyticus) using the x-ray absorption fine structure (XAFS) spectroscopy. XAFS analysis consisted of both the x-ray absorption near-edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) spectra. XANES spectra of the rinsed Cr(VI)-laden A. haemolyticus biomass indicated that chromium was only present in the form of Cr (III). The nearest atoms coordinated to Cr(III) were oxygens in an octahedral geometry. The longer Cr-O bond lengths (~1.97 A) obtained from the EXAFS spectra corroborated the presence of chromium as Cr(III) bonded to oxygen. These findings demonstrate the ability of A. haemolyticus to reduce Cr(VI) to Cr(III) that was bound to oxygen atoms of the ligands in A. haemolyticus.

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Quek Hsiao Pei

Papers

Chromium(VI) resistance and removal by Acinetobacter haemolyticus

Determination of Chromium(VI) Reduction by Acinetobacter haemolyticus using X-ray Absorption Fine Structure Spectroscopy