Evaluation of in vitro Cr(VI) reduction potential in cytosolic extracts of three indigenous Bacillus sp. isolated from Cr(VI) polluted industrial landfill

Three efficient Cr(VI) reducing bacterial strains were isolated from Cr(VI) polluted landfill and characterized for in vitro Cr(VI) reduction. Phylogenetic analysis using 16S rRNA gene sequencing revealed that the newly isolated strains G1DM20, G1DM22 and G1DM64 were closely related to Bacillus cereus, Bacillus fusiformis and Bacillus sphaericus, respectively. The suspended cultures of all Bacillus sp. exhibited more than 85% reduction of 1000 microM Cr(VI) within 30 h. The suspended culture of Bacillus sp. G1DM22 exhibited an ability for continuous reduction of 100 microM Cr(VI) up to seven consecutive inputs. Assays with the permeabilized cells and cell-free extracts from each of Bacillus sp. demonstrated that the hexavalent chromate reductase activity was mainly associated with the soluble fraction of cells and expressed constitutively. The Cr(VI) reduction by the cell-free extracts of Bacillus sp. G1DM20 and G1DM22 was maximum at 30 degrees C and pH 7 whereas, Bacillus sp. G1DM64 exhibited maximum Cr(VI) reduction at pH 6. Addition of 1mM NADH enhanced the Cr(VI) reductase activity in the cell-free extracts of all three isolates. Amongst all three isolates tested, crude cell-free extracts of Bacillus sp. G1DM22 exhibited the fastest Cr(VI) reduction rate with complete reduction of 100 microM Cr(VI) within 100 min. The apparent K(m) and V(max) of the chromate reductase activity in Bacillus sp. G1DM22 were determined to be 200 microM Cr(VI) and 5.5 micromol/min/mg protein, respectively. The Cr(VI) reductase activity in cell-free extracts of all the isolates was stable in presence of different metal ions tested except Hg(2+) and Ag(+).     

Reduction of toxic hexavalent chromium by Ochrobactrum intermedium strain SDCr-5 stimulated by heavy metals

A Cr(VI) resistant bacterial strain SDCr-5, identified as Ochrobactrum intermedium on the basis of 16S rRNA gene sequencing, was tolerant to high concentrations of Cr(VI) up to 15 mg ml(-1) in acetate minimal medium. O. intermedium SDCr-5 reduced Cr(VI) under a wide range of concentrations from 100 to 1500 microg ml(-1) and reduction was optimum at 37 degrees C and pH 7. It reduced 200 and 721 microg ml(-1) Cr(VI) within 72 and 96 h, respectively. The rate of Cr(VI) reduction increased with concentration from 100 to 1500 microg ml(-1). The presence of heavy metal cations such as Cu(2+), Co(2+), Mn(2+) and Ni(2+) stimulated Cr(VI) reduction. Strain SDCr-5 might be useful for Cr(VI) detoxification under a wide range of environmental conditions.     

Modulation of chromium(VI) toxicity by organic and inorganic sulfur species in yeasts from industrial wastes

Two chromium(VI) resistant yeast strains (Candida sp. and Rhodosporidium sp.) were isolated from industrial wastes. Four different yeasts, three from the Industrial Yeast Collection and one of pharmaceutical origin, were also studied in relation to chromate toxicity and its alleviation by sulfur species. The growth of yeasts from industrial wastes was inhibited by 50% by high concentrations of Cr(VI): Candida sp. by 4 mM Cr(VI) and Rhodosporidium sp. by 10 mM Cr(VI) in Sabouraud Broth medium. The other Cr(VI)-sensitive yeasts were inhibited by 0.1 mM Cr(VI). The general mechanism of chromium resistance in Candida sp. and Rhodosporidium sp. was due to reduced uptake of chromium, but not to biological reduction from Cr(VI) to Cr(III). In Cr(VI)-sensitive yeasts, chromium was accumulated as much as 10-fold, as in Saccharomyces cerevisiae. Cr(VI) toxicity in Candida sp. was modulated from Cr(VI)-resistance to Cr(VI)-hypersensitivity depending on the addition of methionine, cysteine, sulfate and djenkolic acid. If Candida sp. was grown in the presence of S-amino acids, especially methionine, it was more resistant than if the sulfur source was sulfate. When sulfate transport was enhanced by addition of djenkolic acid, Candida sp. became hypersensitive. Rhosporidium sp. was always resistant to Cr(VI) because sulfate transport was inefficient and it assimilated sulfur as S-amino acids. Cr(VI)-sensitive yeasts required larger amounts of S-amino acids, especially methionine, to tolerate Cr(VI) toxicity. Cysteine was toxic for C.famata 6016 above 50 microM.     

真菌还原Cr（VI）的研究

从不同来源的样品中分离筛选出几株抗Ｃｒ（ＶＩ）的真菌,他们能在含３００￣５００ｍｇ／ＬＫ２Ｃｒ２Ｏ７的蔗糖合成培养基中生长,其中ＢＳ－１菌株抗Ｋ２Ｃｒ２Ｏ７达９００ｍｇ／Ｌ．ＢＳ－１等４株真菌在含２００ｍｇ／Ｌ Ｋ２Ｃｒ２Ｏ７的培养基中生长４￣６ｄ后,培养液中的Ｃｒ（ＶＩ）已全部消失。这些真菌经鉴定为青霉菌ＢＳ－１和ＢＳ－３,黑曲霉ＢＲ－４和黄曲霉ＢＸ－１。经紫外可见光扫描及化学分析证实,高毒的Ｃ     

Heavy metal resistance patterns of Frankia strains

The sensitivity of 12 Frankia strains to heavy metals was determined by a growth inhibition assay. In general, all of the strains were sensitive to low concentrations (<0.5 mM) of Ag(1+), AsO(2)(1-), Cd(2+), SbO(2)(1-), and Ni(2+), but most of the strains were less sensitive to Pb(2+) (6 to 8 mM), CrO(4)(2-) (1.0 to 1.75 mM), AsO(4)(3-) (>50 mM), and SeO(2)(2-) (1.5 to 3.5 mM). While most strains were sensitive to 0.1 mM Cu(2+), four strains were resistant to elevated levels of Cu(2+) (2 to 5 mM and concentrations as high as 20 mM). The mechanism of SeO(2)(2-) resistance seems to involve reduction of the selenite oxyanion to insoluble elemental selenium, whereas Pb(2+) resistance and Cu(2+) resistance may involve sequestration or binding mechanisms. Indications of the resistance mechanisms for the other heavy metals were not as clear.     

A novel chromate reductase from Thermus scotoductus SA-01 related to old yellow enzyme

Bacteria can reduce toxic and carcinogenic Cr(VI) to insoluble and less toxic Cr(III). Thermus scotoductus SA-01, a South African gold mine isolate, has been shown to be able to reduce a variety of metals, including Cr(VI). Here we report the purification to homogeneity and characterization of a novel chromate reductase. The oxidoreductase is a homodimeric protein, with a monomer molecular mass of approximately 36 kDa, containing a noncovalently bound flavin mononucleotide cofactor. The chromate reductase is optimally active at a pH of 6.3 and at 65 degrees C and requires Ca(2+) or Mg(2+) for activity. Enzyme activity was also dependent on NADH or NADPH, with a preference for NADPH, coupling the oxidation of approximately 2 and 1.5 mol NAD(P)H to the reduction of 1 mol Cr(VI) under aerobic and anaerobic conditions, respectively. The K(m) values for Cr(VI) reduction were 3.5 and 8.4 microM for utilizing NADH and NADPH as electron donors, respectively, with corresponding V(max) values of 6.2 and 16.0 micromol min(-1) mg(-1). The catalytic efficiency (k(cat)/K(m)) of chromate reduction was 1.14 x 10(6) M(-1) s(-1), which was >50-fold more efficient than that of the quinone reductases and >180-fold more efficient than that of the nitroreductases able to reduce Cr(VI). The chromate reductase was identified to be encoded by an open reading frame of 1,050 bp, encoding a single protein of 38 kDa under the regulation of an Escherichia coli sigma(70)-like promoter. Sequence analysis shows the chromate reductase to be related to the old yellow enzyme family, in particular the xenobiotic reductases involved in the oxidative stress response.     

Ferrous iron-dependent volatilization of mercury by the plasma membrane of Thiobacillus ferrooxidans

Of 100 strains of iron-oxidizing bacteria isolated, Thiobacillus ferrooxidans SUG 2-2 was the most resistant to mercury toxicity and could grow in an Fe(2+) medium (pH 2.5) supplemented with 6 microM Hg(2+). In contrast, T. ferrooxidans AP19-3, a mercury-sensitive T. ferrooxidans strain, could not grow with 0.7 microM Hg(2+). When incubated for 3 h in a salt solution (pH 2.5) with 0.7 microM Hg(2+), resting cells of resistant and sensitive strains volatilized approximately 20 and 1.7%, respectively, of the total mercury added. The amount of mercury volatilized by resistant cells, but not by sensitive cells, increased to 62% when Fe(2+) was added. The optimum pH and temperature for mercury volatilization activity were 2.3 and 30 degrees C, respectively. Sodium cyanide, sodium molybdate, sodium tungstate, and silver nitrate strongly inhibited the Fe(2+)-dependent mercury volatilization activity of T. ferrooxidans. When incubated in a salt solution (pH 3.8) with 0.7 microM Hg(2+) and 1 mM Fe(2+), plasma membranes prepared from resistant cells volatilized 48% of the total mercury added after 5 days of incubation. However, the membrane did not have mercury reductase activity with NADPH as an electron donor. Fe(2+)-dependent mercury volatilization activity was not observed with plasma membranes pretreated with 2 mM sodium cyanide. Rusticyanin from resistant cells activated iron oxidation activity of the plasma membrane and activated the Fe(2+)-dependent mercury volatilization activity of the plasma membrane.     

Selection and properties of mutant yeast Pichia guilliermondii strains resistant to chromium (VI)

Yeast Pichia guilliermondii strains L3 and L2, exposed to UV mutagenesis, produced over 80 mutants capable of growing on media containing 1.5 mM bichromate (Cr(VI)). The mutations making the strains resistant to Cr(VI) were dominant or semidominant. The mutants varied in Cr(VI) resistance, the degree of chromium accumulation in the cells (from 0.1 to 11.6 mg/g dry cells), and the degree of Cr(VI) reduction (from 50% to complete disappearance of bichromate from the culture liquid). Chromium accumulation in mutant cells depended on medium composition, Cr(VI) concentration, and the time of exposure to Cr(VI). The resistance to bichromate can be caused by various reasons: decrease in chromium absorption, altered ability to reduce Nr(VI), or damage of sulfate transport mechanisms.     

Isolation and characterization of Cr(VI) reducing Cellulomonas spp. from subsurface soils: implications for long-term chromate reduction

Microbial enrichments from Cr(VI) contaminated and uncontaminated US Department of Energy Hanford Site sediments produced Cr(VI) reducing consortia when grown in the presence of Cr(VI) with acetate, D-xylose or glycerol as a carbon and energy source. Eight of the nine isolates from the consortia were Gram positive and four of these were identified by 16S rRNA sequence homology and membrane fatty acid composition as belonging to the genus Cellulomonas. Two strains, ES6 and WS01, were further examined for their ability to reduce Cr(VI) under growth and non-growth conditions. During fermentative growth on D-xylose, ES6 and WS01 decreased aqueous Cr(VI) concentrations from 0.04 mM Cr(VI) to below the detection limit (0.002 mM Cr(VI)) in less than three days and retained their ability to reduce Cr(VI) even after four months of incubation. Washed ES6 and WS01 cells also reduced Cr(VI) under non-growth conditions for over four months, both with and without the presence of an exogenous electron donor. K-edge XANES spectroscopy confirmed the reduction of Cr(VI) to Cr(III). The ability to reduce Cr(VI) after growth had stopped and in the absence of an external electron donor, suggests that stimulation of these types of organisms may lead to effective long-term, in situ passive reactive barriers for Cr(VI) removal. Our results indicate that Cr(VI) reduction by indigenous Cellulomonas spp. may be a potential method of in situ bioremediation of Cr(VI) contaminated sediment and groundwater.     

Reduction of high concentrations of chromate by Leucobacter sp. CRB1 isolated from Changsha, China

In recent years, more and more attentions are put on the remediation of Cr(VI) contamination with chromate resistant bacteria. Leucobacter sp. CRB1 was a novel chromate reducing bacteria isolated from the soil of chromite ore processing residue (COPR) disposal site in Changsha, China. The objectives of this study were to evaluate the Cr(VI) tolerance of Leucobacter sp. CRB1 as well as its tolerant mechanism, and Cr(VI) reduction ability. The results showed that Leucobacter sp. CRB1 was able to tolerate 4,000 mg/l of hexavalent chromium with 34.5% reduction efficiency. At the optimum pH 9.0, the maximum concentration of chromate be reduced completely was 1,818 mg/l in growing cells and 2,100 mg/l in resting cells. Scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDAX) showed that extracellular Cr(VI) reduction of Leucobacter sp. CRB1 contributed to its high tolerance and high reduction ability. With repeating spiking, 2,490 mg/l hexavalent chromium was reduced totally within 17 h. The results suggest Leucobacter sp. CRB1 has potential application for remediation of high concentration of Cr(VI) contamination.     

Selection and properties of mutant yeast <Emphasis Type="Italic">Pichia guilliermondii</Emphasis> strains resistant to chromium (VI)

Yeast Pichia guilliermondii strains L3 and L2, exposed to UV mutagenesis, produced over 80 mutants capable of growing on media containing 1.5 mM bichromate (Cr(VI)). The mutations making the strains resistant to Cr(VI) were dominant or semidominant. The mutants varied in Cr(VI) resistance, the degree of chromium accumulation in the cells (from 0.1 to 11.6 mg/g dry cells), and the degree of Cr(VI) reduction (from 50% to complete disappearance of bichromate from the culture liquid). Chromium accumulation in mutant cells depended on medium composition, Cr(VI) concentration, and the time of exposure to Cr(VI). The resistance to bichromate can be caused by various reasons: decrease in chromium absorption, altered ability to reduce Cr(VI), or damage of sulfate transport mechanisms.Translated from Prikladnaya Biokhimiya i Mikrobiologiya, Vol. 41, No. 2, 2005, pp. 204–209.Original Russian Text Copyright © 2005 by Babyak, Ksheminskaya, Gonchar, Yanovich, Fedorovich.     

Chromate removal by yeasts isolated from sediments of a tanning factory and a mine site in Argentina

Twenty-one yeast-like microorganisms were isolated from tannery effluents and from a nickel–copper mine in Argentina. They were tested for their Cu(II), Ni(II), Cd(II) and Cr(VI) tolerance in qualitative assays on solid medium. Three isolates were selected for their multiple tolerance to the different heavy metals and highest tolerance to Cr(VI). According to morphological and physiological analysis and 26S rDNA D1/D2 domain sequences the isolates were characterized as: Lecythophora sp. NGV-1, Candida sp. NGV-9 and Aureobasidium pullulans VR-8. Resistance of the three strains to high Cr(VI) concentrations and their ability to remove Cr(VI) were assessed using YNB-glucose medium supplemented with 0.5 and 1 mM Cr(VI). Chromate removal activity was estimated by measuring remaining Cr(VI) concentration in the supernatant using the colorimetric 1,5-diphenylcarbazide method and total chromium was determined by flame atomic absorption spectroscopy. The results indicate that the initial Cr(VI) concentration negatively influenced growth and the specific growth rate but stimulated the metabolic activity of the three strains; resistance to Cr(VI) by these strains was mainly due to reduction of Cr(VI) rather than chromium bioaccumulation. This study showed the potential ability of these strains as tools for bioremediation of Cr(VI) from contaminated sites.     

Reutilization of immobilized fungus Rhizopus sp. LG04 to reduce toxic chromate

Aims: Most of the researches investigating immobilized fungi in chromate [Cr(VI)] bioremediation have used dead cells to adsorb Cr(VI). Therefore, the aim was to identify a Cr(VI)‐reducing fungus with the ability of reducing the toxic Cr(VI) into the much less toxic Cr(III) and to apply the immobilized living fungus in continual reduction of Cr(VI). Methods and Results: Cr(VI) reduction occurred using both free fungi and immobilized living Rhizopus sp. LG04. The Cr(VI) bioreduction by the free fungi was achieved mainly by bioreduction coupled with a small amount of biosorption on the cell surfaces. LG04 spores immobilized with 3% polyvinyl alcohol and 3% sodium alginate produced the most stable and efficient biobeads. When the LG04 biobeads were washed and transferred into fresh medium containing 42 mg l^?1 of Cr(VI), the biobeads could be reused to reduce Cr(VI) for more than 30 cycles during an 82‐day operation period. Interestingly, as the cycles increased, the time required for complete reduction stabilized at approximately 2·5 days, which was faster than that obtained using the free fungi (4·5 days). The pH value of the solution decreased from 6·60 ± 0·10 to 3·85 ± 0·15 after each reduction cycle, which may be because the metabolic products of the fungus changed the environmental pH or because there was an accumulation of the organo‐Cr(III) complex. Conclusions: The results indicate that using the immobilized living fungus for the removal of Cr(VI) has the advantages in being stable, long‐term treatment, easy to re‐use and less biomass leakage. Significance and Impact of the Study: To our knowledge, this study reports the first successful use of immobilized living Rhizopus for the repeated reduction of Cr(VI).     

Removal of toxic chromate using free and immobilized Cr(VI)-reducing bacterial cells of Intrasporangium sp. Q5-1

Chromate-reducing microorganisms with the ability of reducing toxic chromate [Cr(VI)] into insoluble trivalent chromium [Cr(III)] are very useful in treatment of Cr(VI)-contaminated water. In this study, a novel chromate-reducing bacterium was isolated from Mn/Cr-contaminated soil. Based on morphological, physiological/biochemical characteristics and 16S rRNA gene sequence analyses, this strain was identified as Intrasporangium sp. strain Q5-1. This bacterium has high Cr(VI) resistance with a MIC of 17 mmol l⁻¹ and is able to reduce Cr(VI) aerobically. The best condition of Cr(VI) reduction for Q5-1 is pH 8.0 at 37°C. Strain Q5-1 is also able to reduce Cr(VI) in resting (non-growth) conditions using a variety of carbon sources as well as in the absence of a carbon source. Acetate (1 mmol l⁻¹) is the most efficient carbon source for stimulating Cr(VI) reduction. In order to apply strain Q5-1 to remove Cr(VI) from wastewater, the bacterial cells were immobilized with different matrices. Q5-1 cells embedded with compounding beads containing 4% PVA, 3% sodium alginate, 1.5% active carbon and 3% diatomite showed a similar Cr(VI) reduction rates to that of free cells. In addition, the immobilized Q5-1 cells have the advantages over free cells in being more stable, easier to re-use and minimal clogging in continuous systems. This study provides potential applications of a novel immobilized chromate-reducing bacterium for Cr(VI) bioremediation.     

Relationship of hydrogen bioavailability to chromate reduction in aquifer sediments

Biological Cr(VI) reduction was studied in anaerobic sediments from an aquifer in Norman, Okla. Microcosms containing sediment and mineral medium were amended with various electron donors to determine those most important for biological Cr(VI) reduction. Cr(VI) (about 340 microM) was reduced with endogenous substrates (no donor), or acetate was added. The addition of formate, hydrogen, and glucose stimulated Cr(VI) reduction compared with reduction in unamended controls. From these sediments, an anaerobic Cr(VI)-utilizing enrichment was obtained that was dependent upon hydrogen for both growth and Cr(VI) reduction. No methane was produced by the enrichment, which reduced about 750 microM Cr(VI) in less than six days. The dissolved hydrogen concentration was used as an indicator of the terminal electron accepting process occurring in the sediments. Microcosms with sediments, groundwater, and chromate metabolized hydrogen to a concentration below the detection limits of the mercury vapor gas chromatograph. In microcosms without chromate, the hydrogen concentration was about 8 nM, a concentration comparable to that under methanogenic conditions. When these microcosms were amended with 500 microM Cr(VI), the dissolved hydrogen concentration quickly fell below the detection limits. These results showed that the hydrogen concentration under chromate-reducing conditions became very low, as low as that reported under nitrate- and manganese-reducing conditions, a result consistent with the free energy changes for these reactions. The utilization of formate, lactate, hydrogen, and glucose as electron donors for Cr(VI) reduction indicates that increasing the availability of hydrogen results in a greater capacity for Cr(VI) reduction. This conclusion is supported by the existence of an enrichment dependent upon hydrogen for growth and Cr(VI) reduction.     

Development of a Pseudomonas sp. for aerobic chromate reduction

Summary Shake flask cultures using a chromate resistant isolate identified as Pseudomonas sp. in defined medium containing 60 mg Cr(VI)/1 gave maximum chromate reduction of 87% at 40°C and pH 9 in 72 h with urea as nitrogen source. Continuous culture at 30 C, pH 7.2–7.6 and dilution rate of 0.014/h in a 2-1 chemostat with the same isolate showed 81% chromate reduction with feed containing 124 mg Cr(VI)/1.     

In vitro reduction of hexavalent chromium by a cell-free extract of Bacillus sp. ES 29 stimulated by Cu2+

Chromium-resistant bacteria (CRB) isolated from soils can be used to reduce toxic Cr(VI) from contaminated environments. This study assessed in vitro reduction of hexavalent Cr using a cell-free extract (CFE) of CRB isolated from soil contaminated with dichromate. One isolate, ES 29, that substantially reduced Cr(VI) was identified as a Bacillus species by 16S rRNA gene-sequence homology. The isolate reduced Cr(VI) under aerobic conditions, using NADH as an electron donor and produced a soluble Cr(VI)-reducing enzyme stimulated by copper (Cu2+). The CFE of the bacterial isolate reduced 50% of Cr(VI) in 6 h. The Cr(VI)-reduction activity of the CFE had a Km of 7.09 microM and a Vmax of 0.171 micromol min(-1) mg(-1) protein. Mercury inhibited the enzyme, but not competitively, with a Vmax of 0.143 micromol min(-1) mg(-1) protein, a Km of 7.07 microM and a Ki of 1.58 microM. This study characterizes the enzymatic reduction of Cr(VI) by Bacillus sp. ES 29 which can be used for the bioremediation of chromate.     

Chromate reduction and 16S rRNA identification of bacteria isolated from a Cr(VI)-contaminated site

A gram-positive, hexavalent chromium [chromate: Cr(VI)]-tolerant bacterium, isolated from tannery waste from Pakistan, was identified as a Microbacterium sp. by 16S rRNA gene sequence homology. The strain (designated as MP30) reduced toxic Cr(VI) only under anaerobic conditions at the expense of acetate as the electron donor. The bacterium was able to grow aerobically in L-broth supplemented with 15 mM CrO4(2-) but then did not reduce Cr(VI). At a concentration of 2.4x10(9) cells/ml, 100 microM sodium chromate was reduced within 30 h; however, the maximum specific reduction rate was obtained at lower initial cell concentrations.     

Reduction of Cr(VI) by immobilized cells of Desulfovibrio vulgaris NCIMB 8303 and Microbacterium sp. NCIMB 13776

Hexavalent chromium, a carcinogen and mutagen, can be reduced to Cr(III) by Desulfovibrio vulgaris NCIMB 8303 and Microbacterium sp. NCIMB 13776. This study examined Cr(VI) reduction by immobilized cells of the two strains in a common solution matrix using various entrapment matrices. Chitosan and PVA-borate beads did not retain integrity and supported low or no reduction of Cr(VI) by the cells. A commercial preparation (Lentikats) was stable but also did not support Cr(VI) reduction. K-carrageenan beads were stable in batch suspensions but gel integrity was lost after only 5 h in a flow-through system in the presence of 100 microM Cr(VI). The best immobilization matrices were agar and agarose, where the initial rates of reduction of Cr(VI) (from 500 microM solution) for D. vulgaris NCIMB 8303 and Microbacterium sp. NCIMB 13776 were 127 (agar) and 130 (agarose), and 15 (agar) and 12 (agarose) nmol h(-1) mg dry cell wt(-1), respectively. The higher removal of Cr(VI) by D. vulgaris was also seen in 14-mL packed-bed flow-through columns, where, at a flow rate of 2.4 mL h(-1), the percentage removal of Cr(VI) was approximately 95% and 60% for D. vulgaris and Microbacterium sp., respectively (agar-immobilized cells). The Cr(VI) reducing activities of D. vulgaris and Microbacterium sp. were lost after 159 and 140 h, respectively. Examination of the beads for structural integrity within the columns in situ using magnetic resonance imaging after 24 and 100 h of continuous operation against Cr(VI) (with negligible Cr retained within the columns) showed that agar beads were more stable with time. The most appropriate system for development of a continuous bioprocess is thus the use of D. vulgaris NCIMB 8303 immobilized in an agar gel matrix.     

Studies on microbial chromate reduction by Pseudomonas Sp. In aerobic continuous suspended growth cultures

Reduction of hexavalent chromium was studied in three bench-scale continuous stirred tank reactors. The inoculum was a culture of Pseudomonas sp., capable of giving 83% to 87% chromate reduction in 72-h batch assays with 60 mg Cr(VI) L(-1) in synthetic medium. The continuous culture studies were conducted for about 100 days using synthetic feed containing different levels of chromate (5 to 124 mg L(-1)) at 28 degrees to 30 degrees C and pH 6.8. The feed rate was varied over the range 0.5 to 1 L d(-1) to obtain hydraulic retention time of 36 to 72 h. Chromate reduction efficiency was 81% to 91% and 100% for influent Cr(VI) concentrations of 15 to 124 and 5 mg L(-1), respectively, with a hydraulic retention time of 72 h. (c) 1994 John Wiley & Sons, Inc.