Hexavalent Chromium Reduction and Accumulation by <Emphasis Type="Italic">Acinetobacter</Emphasis> AB1 Isolated from Fez Tanneries in Morocco

Hexavalent chromium reduction and accumulation by Acinetobacter AB1 isolated from Fez tanneries effluents were tested. The effects of some environmental factors such as pH, temperature, and exposure time on Cr(VI) reduction and resistance were investigated. We found that this strain was able to resist to concentrations as high as 400 mg/l of Cr(VI). Moreover, pH 10 and the temperature 30°C constitute favourable conditions to the growth and reduction of Acinetobacter AB1. Complete reduction of Cr(VI) was observed at low initial Cr(VI) concentrations of 50 mg/l after 72 h of incubation. Furthermore, Transmission electron microscope (TEM) analysis showed morphological changes in AB1 strain due 48H exposure to 100 mg/l chromate concentration and revealed circular electron dense (dark black point) inclusion within the cell cytoplasm suggesting chromium deposition within the cells.     

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.     

<Emphasis Type="Italic">Delftia</Emphasis> sp. JD2: a potential Cr(VI)-reducing agent with plant growth-promoting activity

A chromium (Cr)-resistant bacterium isolated from soil containing 6,000 mg/kg of Cr was identified based on 16S rRNA gene sequence analysis as Delftia, and designated as JD2. Growth of JD2 was accompanied with reduction of Cr(VI) to Cr(III) in liquid medium initially containing 100 mg/L Cr(VI), the maximum concentration allowing growth. JD2 showed NADH/NADPH-dependent reductase activity associated with the soluble fraction of cells. The results suggest that JD2 might be a good candidate for the treatment of highly Cr(VI)-contaminated water and/or industrial effluents. The isolate produced indole-3-acetic acid in the presence and absence of Cr(VI) and showed free-living nitrogen-fixing activity possibly attributable to a V-nitrogenase. JD2 did not counteract the harmful effect of Cr(VI) during leguminous plant growth and nodulation by rhizobial strains but functioned as a “helper” bacterium to enhance the performance of rhizobial inoculant strains during inoculation of alfalfa and clover (used as model plants to study plant growth-promoting activity) in the absence of Cr(VI).     

The Versatility of Delftia sp. Isolates as Tools for Bioremediation and Biofertilization Technologies

Two Pb(II)-resistant bacteria isolated from a soil containing 2,500 mg/kg of Pb were identified by 16S rRNA sequencing analysis as Delftia sp. and designated as 3C and 6C. Both isolates grew at a Pb(II) concentration of 62 mg/L and at the stationary phase showed a Pb(II)-sorption capability of 10 ± 1.5 (3C) and 5 ± 0.8 (6C) mg/g of biomass. Biochemical properties related to heavy metal resistance and plant growth promotion were analyzed and compared with the Cr(VI)-resistant plant growth-promoting Delftia sp. JD2, previously reported by our group. Both isolates and JD2 were resistant to Cr(VI), Pb(II) and many antibiotics, produced siderophores and the phytohormone indole-3-acetic, and showed clover growth-promoting activity in greenhouse conditions. Interestingly, the occurrence of integron class 1 was shown in all isolates. Our results add to previous reports and suggest that bacteria of the genus Delftia could be consider as good candidates for the design of technologies for cleaning up contaminated environments and/or the production of biofertilizers.     

Chromium(VI)-reducing <Emphasis Type="Italic">Chlorella</Emphasis> spp. isolated from disposal sites of paper-pulp and electroplating industry

We isolated four cultures of chromate resistant, unicellular, non-motile green algae from disposal sites of the paper-pulp and electroplating industries. These algae were maintained in Tris-acetate-glycerophosphate medium containing 30 μM K₂Cr₂O₇. The morphological features as well as analysis of the 500-bp fragment of 18S rDNA (NS 12 region) showed that these isolates belong to Chlorella spp. These isolates showed EC₅₀ values for chromate ranging from 60 to 125 μM. Uptake studies with radioactive ⁵¹Cr(VI) showed that 10–19% of total radioactivity was intracellular, and 1–2% was bound to the cell wall. The rest of the activity remained in the medium, suggesting that resistance was not related to accumulation of Cr(VI) in the cells. Interestingly, when these isolates were grown in the presence of 30 μM of K₂Cr₂O₇, a decrease in the Cr(VI) concentration in the medium was observed. Only live cells could deplete Cr(VI) from the supernatant, suggesting the presence of chromium reduction activity in these Chlorella isolates. Cr(VI) reduction activity of the cells of Chlorella was stimulated by light as well as by acetate and glycerophosphate. Treatment of Chlorella cells with 3-(3,4 dichlorophenyl),1,1dimethyl urea (DCMU) did not affect the Cr(VI) reduction. However, if the cells were treated with sodium azide, Cr(VI) reduction was severely affected. Though chromate resistance has been well documented in algae, the information on chromate reduction by algae is scant. This paper discusses the Cr(VI) reduction by Cr(VI) resistant Chlorella, which may find a use in the effective bioremediation of Cr(VI).     

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.     

Evaluation of chromium(VI) removal behaviour by two isolates of Synechocystis sp. in terms of exopolysaccharide (EPS) production and monomer composition

Chromium(VI) removal and its association with exopolysaccharide (EPS) production in cyanobacteria were investigated. Synechocystis sp. BASO670 produced higher EPS (548 mg L⁻¹) than Synechocystis sp. BASO672 (356 mg L⁻¹). While the EC₅₀ of the Cr(VI) for Synechocystis sp. BASO670 and Synechocystis sp. BASO672 were determined as 11.5 mg L⁻¹, and 2.0 mg L⁻¹, respectively, there was no relation between Cr(VI) removal and EPS production. Synechocystis sp. BASO672, which has higher EPS value, removed (33%) more Cr(VI) than Synechocystis sp. BASO670. Monomer compositions of EPS of each of the isolates were determined differently. Synechocystis sp. BASO672 which removed higher Cr(VI), had higher values of uronic acid and glucuronic acid (192 μg/mg and 89%, respectively). Our results showed that EPS might play a role in Cr(VI) tolerance. Monomer composition, especially uronic acid and glucuronic acid content of EPS may have enhanced Cr(VI) removal.     

Oxidative stress response and fatty acid changes associated with bioaccumulation of chromium [Cr(VI)] by a fresh water cyanobacterium Chroococcus sp.

Cr(VI) at 2.5, 5, 7.5 and 10 mg/l was removed over 1–5 days by a freshwater cyanobacterium, Chroococcus sp. 2.5 mg Cr(VI)/l gave the optimum rate. With 5 mg Cr(VI)/l, activities of superoxide dismutase and catalase were increased. Amounts of palmitic (16:0), stearic (18:0) and oleic acid (18:1) in the cell also increased after exposure to Cr(VI).     

Detoxification and accumulation of chromium from tannery effluent and spent chrome effluent by Paecilomyces lilacinus fungi

The tannery industry process involves chromium (Cr) salts as a main constituent of the process. The Cr recovery is a part of the process where other salts are used to achieve separation and recovery for using Cr back in the process. The process steps may contain both forms of Cr [Cr(VI): hexavalent and Cr(III): trivalent]. The recovery of Cr from tannery industry effluent through biological systems is much needed. The diverse physicochemical characteristics of these effluents may limit the growth of microorganisms and hence the limitation towards possible practical application of microorganisms in real industrial effluent conditions. The present study attempted the ability of the Cr-resistant fungus Paecilomyces lilacinus [isolated through an enrichment culture technique at 25 000 mg l⁻¹ of Cr(III)] to grow and remove Cr [Cr(VI) and Cr(III)] from two physicochemically different undiluted tannery industry effluents (tannery effluent and spent chrome effluent) in the presence of cane sugar as a carbon source. Such attempts are made keeping in view the potential integration of biological processes in the overall Cr removal and recovery processes to improve its efficiency and environmental sustainability. The fungus has broad pH tolerance range and can reduce Cr(VI) both in acidic (pH 5.5) and alkaline (pH 8.0) conditions. The fungus showed the ability to remove Cr(VI) (1.24 mg l⁻¹) and total Cr (7.91 mg l⁻¹) from tannery effluent below the detection level within 18 h and 36 h of incubation, respectively, and ability to accumulate 189.13 mg Cr g⁻¹ of dry biomass within 600 h of incubation from spent chrome effluent [containing 3731.4 mg l⁻¹ of initial Cr(III) concentration].At 200 mg l⁻¹ of Cr(VI) in growth media, with 100% detoxification and with only 10.54% of total Cr accumulation in the biomass, P. lilacinus showed Cr(VI) reduction as a major mechanism of Cr(VI) detoxification. The time-course study revealed the log phase of the growth for the maximum specific reduction of Cr(VI) and stationary phase of the growth for its maximum specific accumulation of both the forms of Cr [Cr(III) and Cr(VI)] in its biomass. In growth media at 50 mg l⁻¹ and 200 mg l⁻¹ of Cr(VI), P. lilacinus showed 100% reduction within 36 h and 120 h of incubation, respectively. The high degree of positive correlation and statistically high degree of relationship (r² = 0.941) between the fungal growth and % Cr(VI) reduction by the fungus support the role of metabolically active cellular growth in Cr(VI) reduction by the fungus. Results indicate that expanded solid (sludge) retention times (SRTs) (stationary phase) can be recommended for the removal of Cr(III) through accumulation. In case of Cr(VI), reduction needs a priority; therefore, a non-expanded SRT is recommended for designing a continuous-flow completely stirred bioreactor so that a log phase of cellular growth can be maintained during the reduction process. This study reveals the strong potential of P. lilacinus fungi for the removal of Cr from tannery effluent and spent chrome effluent.     

Chromium uptake,retention and reduction in photosynthetic <Emphasis Type="Italic">Euglena gracilis</Emphasis>

Photosynthetic Euglena gracilis grown with different K₂CrO₄ concentrations was analyzed for its ability to take up, retain and reduce Cr(VI). For comparison, cells were also exposed to CrCl₃. Cellular Cr(VI) uptake at pH 7.2 showed a hyperbolic saturation pattern with K _m of 1.1 mM, V _m of 16 nmol (h × 10⁷ cells)⁻¹, and K _i sulfate of 0.4 mM. Kinetic parameters for sulfate uptake were similar, K _m = 0.83 mM, V _m = 15.9 nmol (h × 10⁷cells)⁻¹ and K _i chromate = 0.3 mM. The capacity to accumulate chromium depended on the ionic species, external concentration and pH of the incubation medium. Cr(VI) or Cr(III) accumulation was negligible in the acidic (pH 3.5) culture medium, in which Cr(VI) was abiotically reduced to Cr(III). At pH 7.2 Cr(VI) was fully stable and high accumulation (>170 nmol/1 × 10⁷ cells at 1 mM K₂CrO₄) was achieved; surprisingly, Cr(III) accumulation was also significant (>35 nmol/1 × 10⁷ cells at 1 mM CrCl₃). Cr(VI) was reduced by cells at pH 7.2, suggesting the presence of an external reductive activity. Cr(VI) induced an increased cysteine and glutathione content, but not in phytochelatins suggesting that chromium accumulation was mediated by monothiol compounds.     

Modulation of tolerance to Cr(VI) and Cr(VI) reduction by sulfate ion in a <Emphasis Type="Italic">Candida</Emphasis> yeast strain isolated from tannery wastewater

The main aim of this study was to investigate the influence of the sulfate ion on the tolerance to Cr(VI) and the Cr(VI) reduction in a yeast strain isolated from tannery wastewater and identified as Candida sp. FGSFEP by the D1/D2 domain sequence of the 26S rRNA gene. The Candida sp. FGSFEP strain was grown in culture media with sulfate concentrations ranging from 0 to 23.92 mM, in absence and presence of Cr(VI) [1.7 and 3.3 mM]. In absence of Cr(VI), the yeast specific growth rate was practically the same in every sulfate concentration tested, which suggests that sulfate had no stimulating or inhibiting effect on the yeast cell growth. In contrast, at the two initial Cr(VI) concentrations assayed, the specific growth rate of Candida sp. FGSFEP rose when sulfate concentration increased. Likewise, the greater efficiencies and volumetric rates of Cr(VI) reduction exhibited by Candida sp. FGSFEP were obtained at high sulfate concentrations. Yeast was capable of reducing 100% of 1.7 mM Cr(VI) and 84% of 3.3 mM Cr(VI), with rates of 0.98 and 0.44 mg Cr(VI)/L h, with 10 and 23.92 mM sulfate concentrations, respectively. These results indicate that sulfate plays an important role in the tolerance to Cr(VI) and Cr(VI) reduction in Candida sp. FGSFEP. These findings may have significant implications in the biological treatment of Cr(VI)-laden wastewaters.     

Encapsulation of <Emphasis Type="Italic">Pannonibacter phragmitetus</Emphasis> LSSE-09 in alginate–carboxymethyl cellulose capsules for reduction of hexavalent chromium under alkaline conditions

Cr(VI) was efficiently reduced to Cr(III) by Pannonibacter phragmitetus LSSE-09 encapsulated in liquid-core alginate–carboxymethyl cellulose capsules under alkaline conditions. Taking into account the physical properties of the capsules, the activity of encapsulated cells, and total Cr(III) concentration in the supernatant, optimal conditions (0.5% w/v sodium alginate; 2% w/v sodium carboxymethyl cellulose; 0.1 M CaCl₂; 30-min gelation time) for LSSE-09 encapsulation were determined. At optimal conditions, a relatively high reduction rate of 4.20 mg g_{(dry weight)}⁻¹  min⁻¹ was obtained. Total Cr(III) concentration in the supernatant was significantly decreased after reduction, because 63.7% of the formed soluble organo-Cr(III) compounds compared with those of free cells were captured by the relatively smaller porous structure of alginate capsules. The optimal pH value (9.0) for Cr(VI) reduction was not changed after encapsulation. In addition, encapsulated LSSE-09 showed no appreciable loss in activity after eight repeated cycles at 37°C, and 85.7% of its initial activity remained after 35-day storage at 4°C. The results suggest that encapsulated LSSE-09 in alginate–carboxymethyl cellulose capsules has potential biotechnological applications for the detoxification of Cr(VI)-contaminated wastewater.     

Individual and Joint Toxicity Effects of Cu,Cr(III), and Cr(VI) on Pakchoi: A Comparison Between Solution and Soil Cultures

The single and joint toxicity effects of Cu, Cr(III), and Cr(VI) on the root elongation of pakchoi in solution and soil were investigated. The median effective concentration (EC₅₀) was determined to examine the toxic thresholds of the test elements. The results showed that individual contamination by Cu, Cr(III), or Cr(VI) can inhibit the root elongation of pakchoi. The EC₅₀ values of the test elements were 2.02 mg/L and 195.8 mg/kg, 62.2 mg/L and 1,773 mg/kg, and 6.88 mg/L and 8.08 mg/kg in solution and soil, respectively. Toxic unit (TU) was introduced to determine the outcome in combined tests, and different behaviors were observed in both solution and soil. The coexistence of Cu and Cr(III) in solution exhibited an antagonistic effect (EC_50mix = 1.76 TU_mix), whereas a synergistic effect was observed in soil (EC_50mix = 0.76 TU_mix). In contrast, combined Cu–Cr(VI) showed a less than additive toxicity both in solution and soil, with EC_50mix values of 3.31 and 1.24 TU_mix. In conclusion, the coexistence of toxicity in Cu–Cr(III) and Cu–Cr(VI) differs from the toxicity exhibited individually by Cu, Cr(III), and Cr(VI). Heavy metal interaction also changes depending on the medium.     

Cathodic reduction of hexavalent chromium [Cr(VI)] coupled with electricity generation in microbial fuel cells

A novel approach to Cr(VI)-contaminated wastewater treatment was investigated using microbial fuel cell technologies in fed-batch mode. By using synthetic Cr(VI)-containing wastewater as catholyte and anaerobic microorganisms as anodic biocatalyst, Cr(VI) at 100 mg/l was completely removed during 150 h (initial pH 2). The maximum power density of 150 mW/m² (0.04 mA/cm²) and the maximum open circuit voltage of 0.91 V were generated with Cr(VI) at 200 mg/l as electron acceptor. This work verifies the possibility of simultaneous electricity production and cathodic Cr(VI) reduction.     

Pollution control in pulp and paper industrial effluents using integrated chemical–biological treatment sequences

The main objective of the present study was to improve the quality of pulp and paper industrial wastewater of two local mills RAKTA and El-Ahlia, Alexandria, Egypt, and to bring their pollutant contents to safe discharge levels. Quality improvement was carried out using integrated chemical and biological treatment approaches after their optimization. Chemical treatment (alum, lime, and ferric chloride) was followed by oxidation using hydrogen peroxide and finally biological treatment using activated sludge (90 min for RAKTA and 60 min for El-Ahlia effluents). Chemical coagulation produced low-quality effluents, while pH adjustment during coagulation treatment did not enhance the quality of the effluents. Maximum removal of the tested pollutants was achieved using the integrated treatment and the pollutants recorded residual concentrations (RCs) of 34.67, 17.33, 0.13, and 0.43 mg/l and 15.0, 11.0, 0.0, and 0.13 mg/l for chemical oxygen demand (COD), biochemical oxygen demand (BOD₅), tannin and lignin, and silica in RAKTA and El-Ahlia effluents, respectively, all of which were below their maximum permissible limits (MPLs) for the safe discharge into water courses. Specific oxygen uptake rate (SOUR) and sludge volume index (SVI) values reflect good conditions and healthy activated sludge. Based on the previous results, optimized conditions were applied as bench scale on the raw effluents of RAKTA and El-Ahlia via the batch chemical and the biological treatment sequences proposed. For RAKTA effluents, the sequence was as follows: (1) coagulation with 375 mg/l FeCl₃, (2) oxidation with 50 mg/l hydrogen peroxide, and (3) biological treatment using activated sludge with 2,000 mg/l initial concentration and 90 min hydraulic retention time (HRT), while for El-Ahlia raw effluents, the sequence was (1) coagulation with 250 mg/l FeCl₃, (2) oxidation with 45 mg/l hydrogen peroxide, and (3) biological treatment using activated sludge with 2,000 mg/l initial concentration and 60 min HRT. In conclusion, results confirmed that the application of the proposed sequential treatments removed almost all COD, BOD₅, high molecular weight compounds, and silica from RAKTA and El-Ahlia influents and produced high-quality effluents, thus achieving the main objective of this study.     

Bioremediation of Cr(VI) from Chromium-Contaminated Wastewater by Free and Immobilized Cells of Cellulosimicrobium cellulans KUCr3

In this report, possible utilization of a chromium-reducing bacterial strain Cellulosimicrobium cellulans KUCr3 for effective bioremediation of hexavalent chromium (Cr(VI))-containing wastewater fed with tannery effluents has been discussed. Cr(VI) reduction and bioremediation were found to be related to the growth supportive conditions in wastewater, which is indicative of cell mass dependency for Cr(VI) reduction. Cr(VI) reduction was determined by measuring the residual Cr(VI) in the cell-free supernatant using colorimetric reagent S-diphenylcarbazide. Nutrient availability and initial cell density showed a positive relation with Cr(VI) reduction, but it was inhibited with increasing concentration of Cr(VI) under laboratory condition. The optimum temperature and pH for effective Cr(VI) reduction in wastewater were found to be 35°C and 7.5, respectively. The viable cells of KUCr3 were successfully entrapped in an agarose bead that was used in continuous column and batch culture for assaying Cr(VI) reduction. In packed bed column (continuous flow) experiment, approximately 25% Cr(VI) reduction occurred after 144 h. Cr(VI) was almost 75% and 52% reduced at concentrations of 0.5 mM and 2 mM Cr(VI), respectively, after 96 h in batch culture experiment in peptone-yeast extract-glucose medium, whereas it could decrease the Cr(VI) content up to 40% from the water containing tannery waste. This study suggests that KUCr3 could be used as a candidate for possible environmental clean up operation with respect to Cr(VI) bioremediation.     

Isolation and Growth Characteristics of Chromium(VI) and Pentachlorophenol Tolerant Bacterial Isolate from Treated Tannery Effluent for its Possible Use in Simultaneous Bioremediation

The bacterial strains resistant to pentachlorophenol (PCP) and hexavalent chromium [Cr(VI)] were isolated from treated tannery effluent of a common effluent treatment plant. Most of the physico-chemical parameters analyzed were above permissible limits. Thirty-eight and four bacterial isolates, respectively were found resistant to >50 μg/ml concentration of [Cr(VI)] and the same level of PCP. Out of the above 42 isolates, only one was found simultaneously tolerant to higher levels of both PCP (500 μg/ml) and Cr(VI) (200 μg/ml), and hence was selected for further studies. To the best of our knowledge, this is the first report in which a native bacterial isolate simultaneously tolerant to such a high concentrations of Cr(VI) and PCP has been reported. The culture growth was best at 0.4% (w/v) glucose as an additional carbon source and 0.2% (w/v) ammonium chloride as a nitrogen source. The growth results with cow urine as a nitrogen source were comparable with the best nitrogen source ammonium chloride. The isolate exhibited resistance to multiple heavy metals (Pb, As, Hg, Zn, Co & Ni) and to antibiotics nalidixic acid and polymixin-B. The efficacy of bacterial isolate for growth, PCP degradation (56.5%) and Cr(VI) bioremediation (74.5%) was best at 48 h incubation. The isolate was identified as Bacillus sp. by morphological and biochemical tests. The 16S rDNA sequence analysis revealed 98% homology with Bacillus cereus. However, further molecular analysis is underway to ascertain its likelyhood of a novel species.     

Effect of Zn(II) on the reduction and accumulation of Cr(VI) by <Emphasis Type="Italic">Arthrobacter</Emphasis> species

Natural habitats are often characterized by the coexistence of Zn and Cr. This study assessed the potential of two Gram-positive, Cr(VI)-reducing, aerobic bacterial strains belonging to Arthrobacter genera, which were isolated from basalt samples taken from the most polluted region of the Republic of Georgia, to remediate Cr(VI) in environments in the presence of Zn(II). Our batch experiments revealed that the addition of Zn(II) to the tested bacterial cells significantly enhanced the accumulation of Cr. According to electron spin resonance (ESR) measurements, the presence of Zn(II) ions did not change the nature of Cr(V) and Cr(III) complexes generated during the microbial reduction of Cr(VI). The efficiency of Cr(VI) reduction also remained unchanged after the addition of 50 mg/l of Zn(II) to the bacterial cells. However, at high concentrations of Zn(II) (higher than 200 mg/l), the transformation of Cr(VI) to Cr(V) and Cr(III) complexes decreases significantly. In addition, it was shown that the accumulation pattern of Zn in the tested bacterial species in the presence of 100 mg/l of Cr(VI) fits the Langmuir–Freundlich model well. The two tested bacterial strains exhibited different characteristics of Zn accumulation.     

Isolation and characterization of chromium(VI)-reducing bacteria from tannery effluents and solid wastes

In the present investigation, five novel Cr(VI) reducing bacteria were isolated from tannery effluents and solid wastes and identified as Kosakonia cowanii MKPF2, Klebsiella pneumonia MKPF5, Acinetobacter gerneri MKPF7, Klebsiella variicola MKPF8 and Serratia marcescens MKPF12 by 16S rDNA gene sequence analysis. The maximum tolerance concentration of Cr(VI) as K₂Cr₂O₇ of the bacterial isolates was varying up to 2000 mg/L. Among the investigated bacterial isolates, A. gerneri MKPF7 was best in terms of reduction rate. The optimum temperatures for growth and Cr(VI) reduction by the bacterial isolates were 35 and 40 °C, respectively except A. gerneri MKPF7 which grew and reduced Cr(VI) optimally at 40 °C. The optimum pH for growth and Cr(VI) reduction by K. cowanii MKPF2, A. gerneri MKPF7 and S. marcescens MKPF12 was 7.0 whereas the optimum pH for growth and Cr(VI) reduction by K. pneumoniae MKPF5 and K. variicola MKPF8 were 7.0, 8.0 and 6.0, 7.0, respectively. All the bacterial isolates showed maximum tolerance against Ni²⁺ and Zn²⁺ whereas minimum tolerance was observed against Hg²⁺ and Cd²⁺. The bacteria isolated in the present study thus can be used as eco-friendly biological expedients for the remediation and detoxification of Cr(VI) from the contaminated environments.     

Performance evaluation of various bioreactors for the removal of Cr(VI) and organic matter from industrial effluent

Performances of various bioreactors under different operating conditions were evaluated with respect to hexavalent chromium (Cr(VI)) reduction and COD removal. Continuous reactor studies were carried out with (i) aerobic suspended growth system, (ii) aerobic attached growth system, and (iii) anoxic attached growth system, using both synthetic and actual industrial wastewater. Arthrobacter rhombi-RE (MTCC7048), a Cr(VI) reducing strain enriched and isolated from chromium contaminated soil, was used in all the bioreactors for Cr(VI) biotransformation and COD removal. Aerobic and anoxic batch experiments were conducted to evaluate the bio-kinetic parameters. The bio-kinetic parameters for aerobic system were: μ_max = 2.34/d, K_s = 190 mg/L (as COD), K_i = 3.8 mg/L of Cr(VI), and Y_T = 0.377. These parameters for anoxic conditions were: μ_max = 0.57/d, K_s = 710 mg/L (as COD), K_i = 8.77 mg/L of Cr(VI), and Y_T = 0.13. Aerobic attached growth system, operated at a hydraulic retention time (HRT) of 24 h and an organic loading rate (OLR) of 3 kg/m³/d, performed better than aerobic suspended and the anoxic attached growth systems operated under identical conditions, while treating synthetic wastewater as well as industrial effluent.