Novel chromium tolerant microorganisms: isolation, characterization and their biosorption capacity

Influence of polyethylene-type microplastics on long-term exposure to heavy metals in freshwater phytoplankton

The use of plastic materials has brought about significant social benefits but has also led to negative consequences, particularly their accumulation in aquatic environments. Studies have shown that small plastic particles, known as microplastics (MPs), can carry various harmful pollutants, such as heavy metals (HMs). Therefore, the aim of this research is to investigate the impact of polyethylene-type MPs on the long-term exposure of different HMs on freshwater microalgae Scenedesmus armatus and cyanobacteria Microcystis aeruginosa, in both isolated cultures and phytoplanktonic community conditions. Over a period of 28 days, the strains were subjected to concentrations of Ag+, Cu+2, and Cr+6 corresponding to their respective 72 h-EC10, with or without the presence of MPs. Throughout this period, the growth cell ratio, photosynthetic activity, and reactive oxygen species (ROS) were monitored. The findings indicated a substantial inhibitory impact on cell growth during the initial 7-14 days of exposure, followed by a reduction until reaching values like the controls after 28 days of exposure. There was a disturbance in photosynthetic activity during the first 72 h of exposure, which gradually returned to control levels, mainly significantly affected the respiration phase. Reactive oxygen species (ROS) activity was also affected during the initial 14 days of exposure. The presence or absence of MPs in the culture medium did not significantly alter the observed effects. However, interspecies competition created a more favorable environment for M. aeruginosa over the freshwater microalgae S. armatus. These findings suggest that the formation of MP-HMs complexes may have a limited impact on reducing the adverse effects of HMs in long-term exposures. However, because the impact depends on the specific HM involved, further studies are needed to gain a better understanding of the interaction between these pollutants.

Microbial Removal of Heavy Metals from Contaminated Environments Using Metal-Resistant Indigenous Strains

Contamination of soil with heavy metals has become a matter of global importance due to its impact on agriculture, environmental integrity, and therefore human health and safety. Several microbial strains isolated from soil contaminated by long-term chemical and petrochemical activities were found to manifest various levels of tolerance to Cr, Pb, and Zn, out of which Bacillus marisflavi and Trichoderma longibrachiatum exhibited above-moderate tolerance. The concentrations of target heavy metals before and after bioremediation were determined using electrochemical screen-printed electrodes (SPE) modified with different nanomaterials. The morpho-structural SEM/EDX analyses confirmed the presence of metal ions on the surface of the cell, with metal uptake being mediated by biosorption with hydroxyl, carboxyl, and amino groups as per FTIR observations. T. longibrachiatum was observed to pose a higher bioremediation potential compared to B. marisflavi, removing 87% of Cr and 67% of Zn, respectively. Conversely, B. marisflavi removed 86% of Pb from the solution, compared to 48% by T. longibrachiatum. Therefore, the fungal strain T. longibrachiatum could represent a viable option for Cr and Zn bioremediation strategies, whereas the bacterial strain B. marisflavi may be used in Pb bioremediation applications.

A comprehensive review on bio-stimulation and bio-enhancement towards remediation of heavy metals degeneration

Pollution of heavy metals is one of the risky contaminations that should be managed for all intents and purposes of general well-being concerns. The bioaccumulation of these heavy metals inside our bodies and pecking orders will influence our people in the future. Bioremediation is a bio-mechanism where residing organic entities use and reuse the squanders that are reused to one more form. This could be accomplished by taking advantage of the property of explicit biomolecules or biomass that is equipped for restricting by concentrating the necessary heavy metal particles. The microorganisms can't obliterate the metal yet can change it into a less harmful substance. In this unique circumstance, this review talks about the sources, poisonousness, impacts, and bioremediation strategies of five heavy metals: lead, mercury, arsenic, chromium, and manganese. The concentrations here are the ordinary strategies for bioremediation such as biosorption methods, the use of microbes, green growth, and organisms, etc. This review demonstrates the toxicity of heavy metal contamination degradation by biotransformation through bacterioremediation and biodegradation through mycoremediation.

The toxicity of hexavalent chromium to soil microbial processes concerning soil properties and aging time

Chromium (Cr) pollution has attracted much attention due to its biological toxicity. However, little is known regarding Cr toxicity to soil microorganisms. The present study assesses the toxicity of Cr(VI) on two microbial processes, potential nitrification rate (PNR) and substrate-induced respiration (SIR), in a wide range of agricultural soils and detected the abundance of soil bacteria, fungi, ammonia-oxidizing bacteria and archaea. The toxicity thresholds of 10% and 50% effective concentrations (EC₁₀ and EC₅₀) for PNR varied by 32.18- and 38.66-fold among different soils, while for SIR they varied by 391.21- and 16.31-fold, respectively. Regression model analysis indicated that for PNR, CEC as a single factor explained 27% of the variation in EC₁₀, with soil clay being the key factor explaining 47.3% of the variation in EC₅₀. For SIR, organic matter and pH were found to be the most vital predictors for EC₁₀ and EC₅₀, explaining 34% and 61.1% of variation, respectively. In addition, extended aging time was found to significantly attenuate the toxicity of Cr on PNR. SIR was mainly driven by total bacteria rather than fungi, while PNR was driven by both AOA and AOB. These results were helpful in deriving soil Cr toxicity threshold based on microbial processes, and provided a theoretical foundation for ecological risk assessments and establishing a soil environmental quality criteria for Cr.

Unraveling the Underlying Heavy Metal Detoxification Mechanisms of Bacillus Species

The rise of anthropogenic activities has resulted in the increasing release of various contaminants into the environment, jeopardizing fragile ecosystems in the process. Heavy metals are one of the major pollutants that contribute to the escalating problem of environmental pollution, being primarily introduced in sensitive ecological habitats through industrial effluents, wastewater, as well as sewage of various industries. Where heavy metals like zinc, copper, manganese, and nickel serve key roles in regulating different biological processes in living systems, many heavy metals can be toxic even at low concentrations, such as mercury, arsenic, cadmium, chromium, and lead, and can accumulate in intricate food chains resulting in health concerns. Over the years, many physical and chemical methods of heavy metal removal have essentially been investigated, but their disadvantages like the generation of chemical waste, complex downstream processing, and the uneconomical cost of both methods, have rendered them inefficient,. Since then, microbial bioremediation, particularly the use of bacteria, has gained attention due to the feasibility and efficiency of using them in removing heavy metals from contaminated environments. Bacteria have several methods of processing heavy metals through general resistance mechanisms, biosorption, adsorption, and efflux mechanisms. Bacillus spp. are model Gram-positive bacteria that have been studied extensively for their biosorption abilities and molecular mechanisms that enable their survival as well as their ability to remove and detoxify heavy metals. This review aims to highlight the molecular methods of Bacillus spp. in removing various heavy metals ions from contaminated environments.

Bioremediation of Chromium by Microorganisms and Its Mechanisms Related to Functional Groups

Heavy metals generated mainly through many anthropogenic processes, and some natural processes have been a great environmental challenge and continued to be the concern of many researchers and environmental scientists. This is mainly due to their highest toxicity even at a minimum concentration as they are nonbiodegradable and can persist in the aquatic and terrestrial environments for long periods. Chromium ions, especially hexavalent ions (Cr(VI)) generated through the different industrial process such as tanneries, metallurgical, petroleum, refractory, oil well drilling, electroplating, mining, textile, pulp and paper industries, are among toxic heavy metal ions, which pose toxic effects to human, plants, microorganisms, and aquatic lives. This review work is aimed at biosorption of hexavalent chromium (Cr(VI)) through microbial biomass, mainly bacteria, fungi, and microalgae, factors influencing the biosorption of chromium by microorganisms and the mechanism involved in the remediation process and the functional groups participated in the uptake of toxic Cr(VI) from contaminated environments by biosorbents. The biosorption process is relatively more advantageous over conventional remediation technique as it is rapid, economical, requires minimal preparatory steps, efficient, needs no toxic chemicals, and allows regeneration of biosorbent at the end of the process. Also, the presence of multiple functional groups in microbial cell surfaces and more active binding sites allow easy uptake and binding of a greater number of toxic heavy metal ions from polluted samples. This could be useful in creating new insights into the development and advancement of future technologies for future research on the bioremediation of toxic heavy metals at the industrial scale.

Response of microalgae Chlorella vulgaris to Cr stress and continuous Cr removal in a membrane photobioreactor

This study evaluated the toxicity of Cr(VI) to microalgae Chlorella vulgaris, and its removal by continuous microalgae cultivation in membrane photobioreactor (MPBR). Batch cultivation in photobioreactors showed that low concentration of Cr(VI) (0.5 and 1.0 mg L^-1) stimulated the growth of C. vulgaris, while 2.0 and 5.0 mg L^-1 Cr(VI) in the wastewater significantly inhibited the growth of C. vulgaris. Superoxide dismutase and catalase activities that represented cellular antioxidant capacity significantly increased at 0.5 and 1.0 mg L^-1 Cr(VI), and then gradually decreased with the continuous increase of Cr(VI) concentration. The content of malondialdehyde, which represents the degree of cellular oxidative damage, increased with the increase of Cr(VI) concentration and reached the peak value at 2.0 mg L-1 Cr(VI). C. vulgaris was then cultured in MPBR equipped with hollow-fiber ultrafiltration membrane module to achieve continuous removal of Cr from wastewater. With the in-situ solid-liquid separation function of the membrane module, solid retention time (SRT) and hydraulic retention time (HRT) of the reactor could be controlled separately. Experimental results showed that both SRT and HRT had significant effects on the algal biomass production and pollutants removal. During the continuous operation, MPBR achieved a maximum total Cr reduction of 50.0% at HRT of 3-day and SRT of 40-day, and a maximum volumetric removal rate of total Cr of 0.21 mg L^-1 d^-1 at HRT of 2-day and SRT of 40-day.

Cr(VI) removal by Penicillium oxalicum SL2: Reduction with acidic metabolites and form transformation in the mycelium

Bioremediation of Cr(VI) contamination using microorganisms is a promising method for reducing its environmental risks. The objective of this study was to clarify Cr(VI) removal by Penicillium oxalicum SL2 in terms of indirect Cr(VI) reduction by metabolites, interaction sites, and form transformation of chromium. Strain SL2 could sequentially remove Cr(VI) in the bioreactor. Oxalic acid produced by the fungus contributed to Cr(VI) reduction. Scanning transmissiony X-ray microscop (STXM) analysis suggested strain SL2 could partly reduce Cr(VI) to Cr(III) in the cell. Amine, carboxyl, and phosphate groups were related to Cr(VI) removal. Chromium K-edge X-ray absorption near edge structure (XANES) analysis implied Cr(III)-Cys potentially acted as an intermediate for the formation of chromium oxalate complexes during the process of treatment. This study would support the application of strain SL2 in Cr(VI) bioremediation and expand knowledge on the interaction of chromium with fungus.

A comprehensive study on the bacterial biosorption of heavy metals: materials, performances, mechanisms, and mathematical modellings

Discharges of waste containing heavy metals (HMs) have been a challenging problem for years because of their adverse effects in the environment. This article provides a comprehensive review of recent findings on bacterial biosorption and their performances for sequestration of HMs. It highlights the significance of HM removal and presents a brief overview on bacterial functionality and biosorption technology. It also discusses the achievements towards utilisation of bacterial biomass with biosorption of HMs from aqueous solutions. This article includes different types of kinetic, equilibrium, and thermodynamic models used for HM treatments using different bacterial species, as well as biosorption mechanisms along with desorption of metal ions and regeneration of bacterial biosorbents. Its fast kinetics of metal biosorption and desorption, low operational cost, and no production of toxic by-products provide attraction to many researchers. Bacteria can easily be produced using inexpensive growth media or obtained as a by-product from industries. A systematic comparison of the literature for a metal-binding capacity of bacterial biomass under different conditions is provided here. The properties of the cell wall constituents such as peptidoglycan and the role of functional groups for metal sorption are presented on the basis of their biosorption potential. Many bacterial biosorbents as reported in scientific literature have a high biosorption capacity, where some are better than commercial adsorbents. Based on the reported results, it seems that most bacteria have the potential for industrial applications for detoxification of HMs.

Confluence of montmorillonite and Rhizobium towards the adsorption of chromium(vi) from aqueous medium

Chromium in its hexavalent oxidation state is carcinogenic and wastewater from the electroplating industry is one of the principal sources of pollution. To reduce this toxicity and pave way towards environmental safety, a combination of environmental microbiology and chemistry is quite efficient for developing biosorbents to sequester chromium from waste water. Immobilization of Rhizobium in sodium montmorillonite provides a conducive environment to capture hexavalent chromium. Various characterization techniques such as FTIR, XPS and SEM-EDAX were performed and batch parameters such as pH variation, adsorbent dosage, concentration of metal ion and temperature were optimized. Pseudo second order kinetics coupled with a higher regression coefficient for Freundlich isotherm and a Langmuir adsorption capacity of 22.22 mg g⁻¹ was achieved for the adsorption process. The adsorption was enhanced by the charge interactions between the protonated clay-Rhizobium surface and Cr(vi) ions in acidic medium. The biosorbent was stable and easily regenerated using NaOH. Preliminary column studies were performed to test the efficiency of the developed biosorbent at higher volumes on a laboratory scale.

Chromium in its hexavalent oxidation state is carcinogenic and wastewater from the electroplating industry is one of the principal sources of pollution.

Screening of plant growth promoting traits in heavy metals resistant bacteria: Prospects in phytoremediation

Phytoremediation is considered as a novel environmental friendly technology, which uses plants to remove or immobilize heavy metals. The use of metal-resistant plant growth-promoting bacteria (PGPB) constitutes an important technology for enhancing biomass production as well as tolerance of the plants to heavy metals. In this study, we isolated twenty seven (NF1-NF27) chromium resistant bacteria. The bacteria were tested for heavy metals (Cr, Zn, Cu, Ni, Pb and Co) resistance, Cr(VI) reduction and PGPB characters (phosphate solubilization, production of IAA and siderophores). The results showed that the bacterial isolates resist to heavy metals and reduce Cr(VI), with varying capabilities. 37.14% of the isolates have the capacity of solubilizing phosphate, 28.57% are able to produce siderophores and all isolates have the ability to produce IAA. Isolate NF2 that showed high heavy metal resistance and plant growth promotion characteristics was identified by 16S rDNA sequence analysis as a strain of Cellulosimicrobium sp.. Pot culture experiments conducted under greenhouse conditions showed that this strain was able to promote plant growth of alfalfa in control and in heavy metals (Cr, Zn and Cu) spiked soils and increased metal uptake by the plants. Thus, the potential of Cellulosimicrobium sp. for both bioremediation and plant growth promotion has significance in the management of environmental pollution.

Biosorption of lead using Bacillus badius AK strain isolated from compost of green waste (water hyacinth)

The bacterial strain Bacillus badius AK isolated from water hyacinth compost was investigated for biosorption characteristics in Pb(II) removal. Batch mode experiments depicted the optimum conditions for biosorption as pH at 4, the temperature of 30°C, 150 rpm of the rotational speed at biomass concentration of 20 mL with 1.7 × 10¹⁶ colony forming unit per milliliter (CFU/mL) value, at 100-150 mg/L concentration of Pb(II). The bacterial biomass was used in its native and non-pretreated state, unlike the dried, freeze-dried or chemically treated biomass. The biosorption followed pseudo-second-order kinetics and isotherm fitted well to the Langmuir model. Maximum Pb(II) biosorption was observed at 1.7 × 10¹⁶ CFU/mL. Influence of Pb(II) on the growth of bacterial biomass was examined by fitting the monod's model. Specific growth rate and maximum specific growth rate of B. badius AK was observed as 0.05 and 2.54 h^-1, respectively; biomass yield coefficient was 11.81. The results indicated that bacterial biomass was efficient, robust and cheaper biosorbent for removal of Pb(II).

Draft Genome Sequence of Bacillus marisflavi TF-11T (JCM 11544), a Carotenoid-Producing Bacterium Isolated from Seawater from a Tidal Flat in the Yellow Sea

Bacillus marisflavi TF-11^T (JCM 11544) is a Gram-positive, spore-forming, and carotenoid-producing bacterium isolated from seawater from a tidal flat in the Yellow Sea. Here, we report the first draft genome sequence of B. marisflavi TF-11^T, which comprises 4.31 Mb in 11 scaffolds with a G+C content of 48.57%.

Isolating, screening and applying chromium reducing bacteria to promote growth and yield of okra (Hibiscus esculentus L.) in chromium contaminated soils

Hexavalent chromium [Cr (VI)], extensively used in different industries, is one of the most toxic heavy metals. The Cr (VI) reducing bacteria could be helpful in decreasing its toxic effects. The present study was conducted to evaluate the potential of Cr (VI) reducing bacteria to improve growth and yield of okra (Hibiscus esculentus L.) in Cr-contaminated soils. Most of the selected bacterial isolates significantly increased the growth and yield of okra. Maximum response was observed in the plants inoculated with the isolate K12 where plant height, root length, fruit weight and number of fruits per plant increased up to 77.5 percent, 72.6 percent, 1.4 fold and 2.9 fold, respectively. Moreover, inoculation with bacteria caused significant decrease in Cr (VI) concentration in soil and plant parts across all treatments. The maximum decrease of 69.6, 56.1 and 40.0 percent in Cr (VI) concentrations in soil, plant vegetative parts and plant reproductive parts, respectively, was observed in the treatment inoculated with the strain K12. Based on amplification, sequencing and analysis of 16S rDNA sequence, the strain K12 was found belonging to genus Brucella and was designated as Brucella sp. K12. These findings suggest that the strain K12 may serve as a potential bioresource to improve crop production in Cr-contaminated soils.

Biosensing and bioremediation of Cr(VI) by cell free extract of Enterobacter aerogenes T2

Hexavalent chromium or Cr(VI) enters the environment through several anthropogenic activities and it is highly toxic and carcinogenic. Hence it is required to be detected and remediated from the environment. In this study, low-cost and environment-friendly methods of biosensing and bioremediation of Cr(VI) have been proposed. Crude cell free extract (CFE) of previously isolated Enterobacter aerogenes T2 (GU265554; NII 1111) was prepared and exploited to develop a stable biosensor for direct estimation of Cr(VI) in waste water, by using three electrodes via cyclic voltammetry. For bioremediation studies, a homogeneous solution of commercially available sodium alginate and CFE was added dropwise in a continuously stirred calcium chloride solution. Biologically modified calcium alginate beads were produced and these were further utilized for bioremediation studies. The proposed sensor showed linear response in the range of 10-40 μg L(-1) Cr(VI) and the limit of detection was found to be 6.6 μg L(-1) Cr(VI). No interference was observed in presence of metal ions, e.g., lead, cadmium, arsenic, tin etc., except for insignificant interference with molybdenum and manganese. In bioremediation studies, modified calcium alginate beads showed encouraging removal rate 900 mg Cr(VI)/m(3) water per day with a removal efficiency of 90%, much above than reported in literature. The proposed sensing system could be a viable alternative to costly measurement procedures. Calcium alginate beads, modified with CFE of E. aerogenes, could be used in bioremediation of Cr(VI) since it could work in real conditions with extraordinarily high capacity.

Simultaneous Cr(VI) reduction and phenol degradation using Stenotrophomonas sp. isolated from tannery effluent contaminated soil

This study presents simultaneous hexavalent chromium (Cr(VI)) reduction and phenol degradation using Stenotrophomonas sp., isolated from tannery effluent contaminated soil. Phenol was used as the sole carbon and energy source for Cr(VI) reduction. The optimization of different operating parameters was done using Placket-Burman design (PBD) and Box-Behnken design (BBD). The significant operating variables identified by PBD were initial Cr(VI) and phenol concentration, pH, temperature, and reaction time. These variables were optimized by a three-level BBD and the optimum initial Cr(VI) concentration, initial phenol concentration, pH, temperature, and reaction time obtained were 16.59 mg/l, 200.05 mg/l, 7.38, 31.96 °C and 4.07 days, respectively. Under the optimum conditions, 81.27 % Cr(VI) reduction and 100 % phenol degradation were observed experimentally. The results concluded that the Stenotrophomonas sp. could be used to decontaminate the effluents containing Cr(VI) and phenol effectively.

Simultaneous analysis of photosystem responses of Microcystis aeruginoga under chromium stress

Chromium (Cr) is a toxic metal that poses a great threat to aquatic ecosystems. Information is limited on coinstantaneous responses of photosystems I (PSI) and II (PSII) to Cr(VI) stress due to lack of instruments that can simultaneously measure PSI and PSII activities. In the present study, responses of quantum yields of energy conversion and electron transport rates of PSI and PSII in Microcystis aeruginosa cells to Cr(VI) stress were simultaneously analyzed by a DUAL-PAM-100 system. Quantum yield of cyclic electron flow (CEF) under Cr(VI) stress and its physiological role in alleviating toxicity of Cr(VI) were also analyzed. At 5 mg L(-1) Cr(VI), quantum yield and electron transport rate of PSII decreased significantly, and light-induced non-photochemical fluorescence quenching lost. Cr(VI) also inhibited efficiency of PSII to use energy under high light more than of PSI. PSII showed lower maximal electron transport rate and light adaptability than PSI. Electron transport rate of PSI was higher and decreased less than that of PSII, implying less sensitivity of PSI to high light and Cr(VI). Energy dissipation through non-light-induced non-photochemical fluorescence quenching increased with increasing Cr(VI) concentration. CEF was stimulated under Cr(VI) treatment and made a significant contribution to quantum yield and electron transport of PSI, which was essential for protection of PSI from stresses of Cr(VI) and high light.

A Review of Biosorption of Chromium Ions by Microorganisms

Due to its widespread industrial use, chromium has become a serious pollutant in diverse environmental settings. The main source of chromium pollution including the Republic o Moldova is industry. It is a great need to develop new eco-friendly methods of chromium removal. Biosorption of heavy metals is a most promising technology involved in the removal of toxic metals from industrial waste streams and natural waters. This article is an extended abstract of a communication presented at the Conference Ecological Chemistry 2012.

Bioremediation of chromium by novel strains Enterobacter aerogenes T2 and Acinetobacter sp. PD 12 S2

PURPOSE

This study had an objective to identify the most potent chromium-resistant bacteria isolated from tannery effluent and apply them for bioremediation of chromium in tannery effluents.

METHODS

Two such strains (previously characterized and identified by us)--Enterobacter aerogenes (NCBI GenBank USA Accession no. GU265554) and Acinetobacter sp. PD 12 (NCBI GenBank USA Accession no. GU084179)--showed powerful chromium resistivity and bioremediation capabilities among many stains isolated from tannery waste. Parameters such as pH, concentration of hexavalent chromium or Cr (VI), and inoculum volume were varied to observe optimum bioconversion and bioaccumulation of Cr (VI) when the said strains were grown in M9 minimal salt media. E. aerogenes was used to remediate chromium from tannery effluents in a laboratory level experiment.

RESULTS

Observation by Scanning Electron Microscope and chromium peak in Energy Dispersive X-ray Spectroscopic microanalysis revealed that E. aerogenes helped remediate a moderate amount of Cr (VI) (8-16 mg L(-1)) over a wide range of pH values at 35-37°C (within 26.05 h). High inoculum percentage of Acinetobacter sp. PD 12 also enabled bioremediation of 8-16 mg L(-1) of Cr (VI) over a wide range of temperature (25-37°C), mainly at pH 7 (within 63.28 h). The experiment with real tannery effluent gave very encouraging results.

CONCLUSION

The strain E. aerogenes can be used in bioremediation of Cr (VI) since it could work in actual environmental conditions with extraordinarily high capacity.

Prediction of biosorption of total chromium by Bacillus sp. using artificial neural network

An artificial neural network (ANN) model was developed to predict the biosorption efficiency of Bacillus sp. for the removal of total chromium from aqueous solution based on 360 data sets obtained in a laboratory batch study. Experimental parameters affecting the biosorption process such as pH, contact time and initial concentration of chromium were studied. A contact time of 2 h was generally sufficient to achieve equilibrium. At optimal conditions, metal ion uptake increased with increasing initial metal ion concentration. The Freundlich model was applied to describe the biosorption isotherm. Chromium biosorption was most significantly influenced by pH, followed by the initial metal concentration of the solution. The findings indicated that the ANN model provided reasonable predictive performance (R(2) = 0.971) of chromium biosorption.

Effects of chromium on activated sludge and on the performance of wastewater treatment plants: A review

Chromium is a heavy metal of commercial importance, thus significant amounts are released in wastewaters. Chromium in wastewaters and in the aquatic environment is primarily encountered in oxidation stages +3 (Cr((III))) and +6 (Cr((VI))). Recent publications suggest that Cr((VI)) compounds are more toxic than Cr((III)) ones, while Cr((III)) has been identified as trace element, at least for complex organisms. With respect to chromium species mobility, Cr((VI)) can cross cellular membranes, which then may be oxidized to Cr((III)) and react with intracellular biomolecules. Clear conclusions cannot be derived about the critical chromium concentrations that affect activated sludge growth, as the latter is a function of a number of factors. Broadly, may be supported that activated sludge growth is stimulated at Cr((III)) concentrations up to 15 mg L(-1), above which is inhibited, with lethal doses lying above 160 mg Cr((III)) L(-1). On the other hand, literature data on Cr((VI)) effects on activated sludge are even more controversial. A number of reports support that Cr((VI)) is toxic to activated sludge at concentrations above 5 mg L(-1), while others report growth stimulation at concentrations up to 25 mg L(-1). However, all reports agree that Cr((VI)) is definitely an activated sludge growth inhibitor at higher concentrations, while 80 mg Cr((VI)) L(-1) have been identified as lethal dose. A number of factors have been identified to influence chromium toxicity on activated sludge, such as, pH, biomass concentration, presence of organic substances or other heavy metals, acclimation process, exposure time, etc. Naturally, the presence of chromium species in wastewaters may affect the performance of wastewater treatment plants often causing malfunctions, particularly for industrial wastewaters containing relatively high chromium concentrations. The present work reviews in a critical way the published literature on chromium effects on activated sludge, and on the operation of wastewater treatment plants.

Bioaccumulation characterization of zinc and cadmium by Streptomyces zinciresistens, a novel actinomycete

The bioaccumulation characteristics of Zn(2+) and Cd(2+) by a novel species, Streptomyces zinciresistens CCNWNQ0016(T), were investigated. S. zinciresistens accumulated Zn(2+) and Cd(2+) mainly on the cell wall followed by intracellular accumulation. The mycelium was deformed, aggregated and formed precipitate of zinc and cadmium on the cell surface. Electron dense granules were detected on the cell wall as well as within the cytoplasm. The amino, carboxyl, hydroxyl and carbonyl groups were responsible for the biosorption of Zn(2+) and Cd(2+). The Langmuir isotherm model fitted the experimental data of metals adsorption processes better than Freundlich isotherm model. Cu(2+) and Cr(3+) competed for adsorption sites on the cell surface with Zn(2+) and Cd(2+). 87.33% and 98.11% recovery of Zn(2+) and Cd(2+), respectively, could be obtained at pH≤2 from metal-loaded biomass of S. zinciresistens desorption.

Isolation and identification of chromium-resistant bacteria: Test application for prevention of chromium toxicity in plant

The purpose of this study was to isolate chromium resistant bacteria and characterize in depth before being applied for bioremediation or construction of chromium biosesnors. One of the major objectives was to identify the most useful strains with respect to chromium removal efficiency. Chromium resistant bacteria were isolated from tannery waste samples. These were characterized through biochemical tests and antibiotic assays. Cross metal reactivity, plasmid curing and cysteine quantifications were also performed. Gene sequencing was done for all the isolates and accession numbers received after submission to National Center for Biotechnology Information (NCBI). The removal capacity was also studied in broth cultures. The results were very encouraging since six new bacteria were recognised to withstand high concentrations of chromium. Out of these six bacteria, three i.e. Enterobacter aerogenes, Aeromonas sp., Acinetobacter sp. PD 12 performed very well by removing about 99% (average) from initial 19.8 ppm of Cr(VI) from a synthetic culture media. Enterobacter aerogenes, being the most efficient strain, was used to remediate hexavalent chromium from soil of potted plants- very encouraging results were obtained. These bacteria may be exploited for bioremediation of Cr (VI) and construction of microbial sensor in future.

Strategies for chromium bioremediation of tannery effluent

Bioremediation offers the possibility of using living organisms (bacteria, fungi, algae,or plants), but primarily microorganisms, to degrade or remove environmental contaminants, and transform them into nontoxic or less-toxic forms. The major advantages of bioremediation over conventional physicochemical and biological treatment methods include low cost, good efficiency, minimization of chemicals, reduced quantity of secondary sludge, regeneration of cell biomass, and the possibility of recover-ing pollutant metals. Leather industries, which extensively employ chromium compounds in the tanning process, discharge spent-chromium-laden effluent into nearby water bodies. Worldwide, chromium is known to be one of the most common inorganic contaminants of groundwater at pollutant hazardous sites. Hexavalent chromium poses a health risk to all forms of life. Bioremediation of chromium extant in tannery waste involves different strategies that include biosorption, bioaccumulation,bioreduction, and immobilization of biomaterial(s). Biosorption is a nondirected physiochemical interaction that occurs between metal species and the cellular components of biological species. It is metabolism-dependent when living biomass is employed, and metabolism-independent in dead cell biomass. Dead cell biomass is much more effective than living cell biomass at biosorping heavy metals, including chromium. Bioaccumulation is a metabolically active process in living organisms that works through adsorption, intracellular accumulation, and bioprecipitation mechanisms. In bioreduction processes, microorganisms alter the oxidation/reduction state of toxic metals through direct or indirect biological and chemical process(es).Bioreduction of Cr6+ to Cr3+ not only decreases the chromium toxicity to living organisms, but also helps precipitate chromium at a neutral pH for further physical removal,thus offering promise as a bioremediation strategy. However, biosorption, bioaccumulation, and bioreduction methods that rely on free cells for bioremediation suffer from Cr6 toxicity, and cell damage. Therefore, immobilization of microbial cell biomass enhances bioremediation and renders industrial bioremediation processes more economically viable from reduced free-cells toxicity, easier separation of biosorbents from the tannery effluent, ability to achieve multiple biosorption cycles, and desorption (elution) of metal(s) from matrices for reuse. Thus, microbial bioremediation can be a cost competitive strategy and beneficial bioresource for removing many hazardous contaminants from tannery and other industrial wastes.

Simultaneous Cr(VI) reduction and phenol degradation in pure cultures of Pseudomonas aeruginosa CCTCC AB91095

Simultaneous Cr(VI) reduction and phenol degradation were investigated in a reactor containing Pseudomonas aeruginosa CCTCC AB91095. Phenol was used as carbon source. P.aeruginosa utilized metabolites formed during phenol degradation as energy source for Cr(VI) reduction. Cr(VI) inhibited both Cr(VI) reduction and phenol degradation when Cr(VI) concentration exceeded the optimum value (20 mg/L), whereas phenol enhanced both Cr(VI) reduction and phenol degradation below the optimum initial concentration of 100 mg/L. Cr(III) was the predominant product of Cr(VI) reduction in cultures after incubation for 24 h. Both Cr(VI) reduction and phenol degradation were influenced by the amount of inocula. The concentration of Cr(VI) and phenol declined quickly from 20, 100 to 3.36, 29.51 mg/L in cultures containing of 5% (v/v) inoculum after incubation for 12 h, respectively. The whole study showed that P. aeruginosa is promising for the reduction of toxic Cr(VI) and degradation of organic pollutants simultaneously in the mineral liquid medium.