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

Water Quality Degradation Due to Heavy Metal Contamination: Health Impacts and Eco-Friendly Approaches for Heavy Metal Remediation

Water quality depends on its physicochemical and biological parameters. Changes in parameters such as pH, temperature, and essential and non-essential trace metals in water can render it unfit for human use. Moreover, the characteristics of the local environment, geological processes, geochemistry, and hydrological properties of water sources also affect water quality. Generally, groundwater is utilized for drinking purposes all over the globe. The surface is also utilized for human use and industrial purposes. There are several natural and anthropogenic activities responsible for the heavy metal contamination of water. Industrial sources, including coal washery, steel industry, food processing industry, plastic processing, metallic work, leather tanning, etc., are responsible for heavy metal contamination in water. Domestic and agricultural waste is also responsible for hazardous metallic contamination in water. Contaminated water with heavy metal ions like Cr (VI), Cd (II), Pb (II), As (V and III), Hg (II), Ni (II), and Cu (II) is responsible for several health issues in humans, like liver failure, kidney damage, gastric and skin cancer, mental disorders and harmful effects on the reproductive system. Hence, the evaluation of heavy metal contamination in water and its removal is needed. There are several physicochemical methods that are available for the removal of heavy metals from water, but these methods are expensive and generate large amounts of secondary pollutants. Biological methods are considered cost-effective and eco-friendly methods for the remediation of metallic contaminants from water. In this review, we focused on water contamination with toxic heavy metals and their toxicity and eco-friendly bioremediation approaches.

Nitrate Bioreduction under Cr(VI) Stress: Crossroads of Denitrification and Dissimilatory Nitrate Reduction to Ammonium

This study explored the response of NO₃^--N bioreduction to Cr(VI) stress, including reduction efficiency and the pathways involved (denitrification and dissimilatory nitrate reduction to ammonium (DNRA)). Different response patterns of NO₃^--N conversion were proposed under Cr(VI) suppress (0, 0.5, 5, 15, 30, 50, and 80 mg/L) by evaluating Cr(VI) dose dependence, toxicity accumulation, bioelectron behavior, and microbial community structure. Cr(VI) concentrations of >30 mg/L rapidly inhibited NO₃^--N removal and immediately induced DNRA. However, denitrification completely dominated the NO₃^--N reduction pathway at Cr(VI) concentrations of <15 mg/L. Therefore, 30 and 80 mg/L Cr(VI) (R₄ and R₆) were selected to explore the selection of the different NO₃^--N removal pathways. The pathway of NO₃^--N reduction at 30 mg/L Cr(VI) exhibited continuous adaptation, wherein the coexistence of denitrification (51.7%) and DNRA (13.6%) was achieved by regulating the distribution of denitrifiers (37.6%) and DNRA bacteria (32.8%). Comparatively, DNRA gradually replaced denitrification at 80 mg/L Cr(VI). The intracellular Cr(III) accumulation in R₆ was 6.60-fold greater than in R₄, causing more severe oxidant injury and cell death. The activated NO₃^--N reduction pathway depended on the value of nitrite reductase activity/nitrate reductase activity, with 0.84-1.08 associated with DNRA activation and 1.48-1.57 with DNRA predominance. Although Cr(VI) increased microbial community richness and improved community structure stability, the inhibition or death of nitrogen-reducing microorganisms caused by Cr(VI) decreased NO₃^--N reduction efficiency.

Heavy Metal Contamination in the Aquatic Ecosystem: Toxicity and Its Remediation Using Eco-Friendly Approaches

Urbanization and industrialization are responsible for environmental contamination in the air, water, and soil. These activities also generate large amounts of heavy metal ions in the environment, and these contaminants cause various types of health issues in humans and other animals. Hexavalent chromium, lead, and cadmium are toxic heavy metal ions that come into the environment through several industrial processes, such as tanning, electroplating, coal mining, agricultural activities, the steel industry, and chrome plating. Several physical and chemical methods are generally used for the heavy metal decontamination of wastewater. These methods have some disadvantages, including the generation of secondary toxic sludge and high operational costs. Hence, there is a need to develop a cost-effective and eco-friendly method for the removal of heavy metal ions from polluted areas. Biological methods are generally considered eco-friendly and cost-effective. This review focuses on heavy metal contamination, its toxicity, and eco-friendly approaches for the removal of heavy metals from contaminated sites.

Simultaneous removal of ternary heavy metal ions by a newly isolated Microbacterium paraoxydans strain VSVM IIT(BHU) from coal washery effluent

In the present work, the removal of Cr (VI), Cd (II) and Pb (II) at 50 mg/L of each metal ion concentration was investigated by Microbacterium paraoxydans strain VSVM IIT(BHU). The heavy metal binding on the bacterial cell surface was confirmed through X-ray photoelectron spectroscopy and energy dispersive X-ray. X-ray photoelectron spectroscopy analysis also confirmed the reduction of Cr (VI) to Cr (III). Heavy metal removal dynamics was investigated by evaluating dimensionless, and the value of N_k (9.49 × 10^-3, 9.92 × 10^-3 and 1.23 × 10^-2 for Cr (VI), Cd (II) and Pb (II) ions) indicated that the removal of heavy metals by bacterial isolate was mixed diffusion and transfer controlled. It was found that both the experimental and predicted values for isolated bacterial strain coincided with each other with a good R² value in the L-M Algorithm range of 0.94-0.98 for the ternary metal ion system. The bacterial isolate presented a maximum heavy metal ion removal efficiency of 91.62% Cr (VI), 89.29% Pb (II), and 83.29% Cd (II) at 50 mg/L.

Bio-capture of Cr(VI) in a denitrification system: Electron competition, long-term performance, and microbial community evolution

Chromium is widely applied in industries as an important metal resource, but the discharge of Cr(VI) containing wastewater leads to the loss of chromium resources. This study proposed a bio-capture process of chromium in a denitrification system. The bio-capture potentiality was explored by investigating the electron competition between Cr(VI) and nitrogen compounds reduction, the long-term bio-capture performance, and the microbial community evolution. In the competition utilization of electron donors, both NO₃^--N and NO₂^--N took precedence over Cr(VI), and NO₂^--N reduction was proved to be the rate-limiting step. Under the optimum conditions of 20 mg/L NO₃^--N and 6 h HRT, 99.95% of 30 mg/L Cr(VI) could be reduced, and 220980 μg Cr/g MLSS was captured by the biofilm, which was fixed in intercellular as Cr(III). Microbiological analysis confirmed that the bio-reduction of Cr(VI) and NO₃^--N was mediated by synergistic interactions of a series of dominant bacteria, including genera Acidovorax, Thermomonas, and Microbacterium, which contained both the denitrification genes (narG, narZ, nxrA, and nirK) and chromate reduction genes (chrA and chrR). This study proved the feasibility of chromium bio-capture in denitrification systems and provided a new perspective for the Cr(VI) pollution treatment.

Nanocatalysts for Oxidative Desulfurization of Liquid Fuel: Modern Solutions and the Perspectives of Application in Hybrid Chemical-Biocatalytic Processes

In this paper, the current advantages and disadvantages of using metal-containing nanocatalysts (NCs) for deep chemical oxidative desulfurization (ODS) of liquid fuels are reviewed. A similar analysis is performed for the oxidative biodesulfurization of oil along the 4S-pathway, catalyzed by various aerobic bacterial cells of microorganisms. The preferences of using NCs for the oxidation of organic sulfur-containing compounds in various oil fractions seem obvious. The text discusses the development of new chemical and biocatalytic approaches to ODS, including the use of both heterogeneous NCs and anaerobic microbial biocatalysts that catalyze the reduction of chemically oxidized sulfur-containing compounds in the framework of methanogenesis. The addition of anaerobic biocatalytic stages to the ODS of liquid fuel based on NCs leads to the emergence of hybrid technologies that improve both the environmental characteristics and the economic efficiency of the overall process. The bioconversion of sulfur-containing extracts from fuels with accompanying hydrocarbon residues into biogas containing valuable components for the implementation of C-1 green chemistry processes, such as CH4, CO2, or H2, looks attractive for the implementation of such a hybrid process.

New Plant Growth-Promoting, Chromium-Detoxifying Microbacterium Species Isolated From a Tannery Wastewater: Performance and Genomic Insights

Hexavalent chromium [Cr(VI)], widely generated by tannery activities, is considered among the most toxic substances and causes a serious damage for the environment and for human health. Interestingly, some microorganisms have a potential of bioremediation of chromium-contaminated wastewaters and soils through the reduction of Cr(VI) (soluble and harmful form) into Cr(III) (stable and non-toxic form). Here, we present the full genome sequence of a novel heavy-metal-resistant, plant growth-promoting bacterium (PGPB), Microbacterium metallidurans TL13, which was isolated from a Tunisian leather industry. The strain TL13 was resistant to many heavy metals, such as chromium, copper, nickel, cobalt, and arsenic. The 50% TL13 growth inhibitory concentration (IC₅₀) values of HgCl₂, CoCl₂, K₂Cr₂O₇, CuSO₄, NiCl₂, FeSO₄, and Na₂HAsO₄ are 368, 445, 676, 1,590, 1,680, 4,403, and 7,007 mg/L, respectively, with the following toxicity order: HgCl₂ > CoCl₂ > K₂Cr₂O₇ > CuSO₄ > NiCl₂ > FeSO₄ > Na₂HAsO₄. This new strain was also able to promote the growth of the hybrid tomato (Elika F1) under chromium metal stress. Its whole genome sequence length was estimated to be 3,587,460 bp (3,393 coding sequences) with a G + C content of 70.7%. Functional annotation of the genome of TL13 revealed the presence of open reading frames (ORFs) involved in adaptation to metal stress, such as the chromate transport protein, cobalt–zinc–cadmium resistance protein, copper resistance protein, copper responsive transcriptional regulator, multidrug resistance transporters, arsenical resistance operon repressor, arsenate reductase, arsenic resistance protein, mercuric resistance operon regulatory protein, mercuric ion reductase, and organomercurial lyase. Moreover, genes for the production of glutathione peroxidase, catalase, superoxide dismutase, and thioredoxin reductase, which confer a higher tolerance to oxidative/metal stresses, were identified in TL13 genome. In addition, genes for heat shock tolerance, cold shock tolerance, glycine-betaine production, mineral phosphate solubilization, ammonia assimilation, siderophores, exopolysaccharides, polyketides, and lytic enzymes (cellulase, chitinase, and proteases) production that enable bacteria to survive biotic/abiotic stress and to promote plant growth and health were also revealed. Based on genome analysis and experimental approaches, strain TL13 appears to have evolved from various metabolic strategies and could play a role in ensuring sustainable environmental and agricultural systems.

Progress Towards Bioelectrochemical Remediation of Hexavalent Chromium

Chromium is one of the most frequently used metal contaminants. Its hexavalent form Cr(VI), which is exploited in many industrial activities, is highly toxic, is water-soluble in the full pH range, and is a major threat to groundwater resources. Alongside traditional approaches to Cr(VI) treatment based on physical-chemical methods, technologies exploiting the ability of several microorganisms to reduce toxic and mobile Cr(VI) to the less toxic and stable Cr(III) form have been developed to improve the cost-effectiveness and sustainability of remediating hexavalent chromium-contaminated groundwater. Bioelectrochemical systems (BESs), principally investigated for wastewater treatment, may represent an innovative option for groundwater remediation. By using electrodes as virtually inexhaustible electron donors and acceptors to promote microbial oxidation-reduction reactions, in in situ remediation, BESs may offer the advantage of limited energy and chemicals requirements in comparison to other bioremediation technologies, which rely on external supplies of limiting inorganic nutrients and electron acceptors or donors to ensure proper conditions for microbial activity. Electron transfer is continuously promoted/controlled in terms of current or voltage application between the electrodes, close to which electrochemically active microorganisms are located. Therefore, this enhances the options of process real-time monitoring and control, which are often limited in in situ treatment schemes. This paper reviews research with BESs for treating chromium-contaminated wastewater, by focusing on the perspectives for Cr(VI) bioelectrochemical remediation and open research issues.

Interlining Cr(VI) remediation mechanism by a novel bacterium Pseudomonas brenneri isolated from coalmine wastewater

A bioremedial approach was investigated on the removal of Cr(VI) from aqueous solution using a novel chromium reducing bacteria isolated from coalmine wastewater. Cr(VI) removal efficacy of the bacterium was determined in a series of batch studies under the influence of various parameters viz., pH (1-7), temperature (20-40 °C), initial metal concentration (1-150 mg/L), agitation speed (80-150 rpm) and substrate concentration (1-5 mg/L). Oxygen involvement in the removal process was determined by different incubation conditions. Substrate consumption and its resultant biomass generation were considered for determining the viability of the microbe under varied metal concentration. The microbial isolate survived in Cr(VI) tainted solution with initial concentration of 1-140 mg/L, among which maximum remediation was found in 60 mg/L Cr(VI) loaded solution. The bacterial species also survived in other metal solution viz., Fe(II), As(V), Cu(II), Pb(II), Zn(II), Mg(II), Mn(II) apart from Cr(VI). Multiple approaches were tested to facilitate understanding of the bacterial Cr(VI) removal mechanism. The bacteria accumulated metal ions in the exponential growth phase both on and within the cell. Underlying latent factors which governed the bacterial growth and its removal activity was determined with the classical Monod equation. The isolated bacterium also survived in the bimetallic solutions with significant removal of Cr(VI). The microbial species isolated from mining area was identified as Pseudomonas brenneri by 16s rRNA molecular characterization. Hence, the isolated novel bacterium illustrated promising involvement towards bio-treatment of Cr(VI) laden wastewater.

Comparative genomics of 16 Microbacterium spp. that tolerate multiple heavy metals and antibiotics

A total of 16 different strains of Microbacterium spp. were isolated from contaminated soil and enriched on the carcinogen, hexavalent chromium [Cr(VI)]. The majority of the isolates (11 of the 16) were able to tolerate concentrations (0.1 mM) of cobalt, cadmium, and nickel, in addition to Cr(VI) (0.5–20 mM). Interestingly, these bacteria were also able to tolerate three different antibiotics (ranges: ampicillin 0–16 μg ml⁻¹, chloramphenicol 0–24 μg ml⁻¹, and vancomycin 0–24 μg ml⁻¹). To gain genetic insight into these tolerance pathways, the genomes of these isolates were assembled and annotated. The genomes of these isolates not only have some shared genes (core genome) but also have a large amount of variability. The genomes also contained an annotated Cr(VI) reductase (chrR) that could be related to Cr(VI) reduction. Further, various heavy metal tolerance (e.g., Co/Zn/Cd efflux system) and antibiotic resistance genes were identified, which provide insight into the isolates’ ability to tolerate metals and antibiotics. Overall, these isolates showed a wide range of tolerances to heavy metals and antibiotics and genetic diversity, which was likely required of this population to thrive in a contaminated environment.

Insight into Cr6+ reduction efficiency of Rhodococcus erythropolis isolated from coalmine waste water

A microbial treatment of Cr⁶⁺ contaminated wastewater with a chromium reducing bacteria isolated from coal mine area was investigated. In a series of batch study metal removal was executed under different parametric conditions which include pH (2-7), temperature (20-50 °C), initial Cr⁶⁺ concentration (1-100 mg/L), substrate utilization and its overall effect on biomass generation. Impact of oxygen availability was checked at different agitation speed and its role on the remedial process. Liquid phase reduction of Cr⁶⁺ was measured in terms of substrate reduction and total biomass yield. The bacterium species isolated was able to tolerate Cr⁶⁺ over a wide range from 1 to 100 mg/L before it reached minimum inhibition concentration. Apart from Cr⁶⁺, the bacterial isolate showed tolerance towards Fe, As, Cu, Ag, Zn, Mn, Mg and Pb. Removal mechanism adopted by the bacterium recommended that it employed accumulation of Cr⁶⁺ as Cr³⁺ both within and outside the cell. Classical Monod equation was used to determine the biokinetics of the bacterial isolate along with the interference of metal ion concentration and substrate utilization. Cr⁶⁺ removal was found prominent even in bimetallic solutions. The bacterial isolate was confirmed to be Rhodococcus erythopolis by 16s rRNA molecular characterization. Thus the bacterial isolate obtained from the coal mine area proved to be a potential agent for microbial remediation of Cr⁶⁺ laden waste water.

Actinobacteria: Current research and perspectives for bioremediation of pesticides and heavy metals

Actinobacteria exhibit cosmopolitan distribution since their members are widely distributed in aquatic and terrestrial ecosystems. In the environment they play relevant ecological roles including recycling of substances, degradation of complex polymers, and production of bioactive molecules. Biotechnological potential of actinobacteria in the environment was demonstrated by their ability to remove organic and inorganic pollutants. This ability is the reason why actinobacteria have received special attention as candidates for bioremediation, which has gained importance because of the widespread release of contaminants into the environment. Among organic contaminants, pesticides are widely used for pest control, although the negative impact of these chemicals in the environmental balance is increasingly becoming apparent. Similarly, the extensive application of heavy metals in industrial processes lead to highly contaminated areas worldwide. Several studies focused in the use of actinobacteria for cleaning up the environment were performed in the last 15 years. Strategies such as bioaugmentation, biostimulation, cell immobilization, production of biosurfactants, design of defined mixed cultures and the use of plant-microbe systems were developed to enhance the capabilities of actinobacteria in bioremediation. In this review, we compiled and discussed works focused in the study of different bioremediation strategies using actinobacteria and how they contributed to the improvement of the already existing strategies. In addition, we discuss the importance of omic studies to elucidate mechanisms and regulations that bacteria use to cope with pollutant toxicity, since they are still little known in actinobacteria. A brief account of sources and harmful effects of pesticides and heavy metals is also given.

Use of combined microscopic and spectroscopic techniques to reveal interactions between uranium and Microbacterium sp. A9, a strain isolated from the Chernobyl exclusion zone

Although uranium (U) is naturally found in the environment, soil remediation programs will become increasingly important in light of certain human activities. This work aimed to identify U(VI) detoxification mechanisms employed by a bacteria strain isolated from a Chernobyl soil sample, and to distinguish its active from passive mechanisms of interaction. The ability of the Microbacterium sp. A9 strain to remove U(VI) from aqueous solutions at 4 °C and 25 °C was evaluated, as well as its survival capacity upon U(VI) exposure. The subcellular localisation of U was determined by TEM/EDX microscopy, while functional groups involved in the interaction with U were further evaluated by FTIR; finally, the speciation of U was analysed by TRLFS. We have revealed, for the first time, an active mechanism promoting metal efflux from the cells, during the early steps following U(VI) exposure at 25 °C. The Microbacterium sp. A9 strain also stores U intracellularly, as needle-like structures that have been identified as an autunite group mineral. Taken together, our results demonstrate that this strain exhibits a high U(VI) tolerance based on multiple detoxification mechanisms. These findings support the potential role of the genus Microbacterium in the remediation of aqueous environments contaminated with U(VI) under aerobic conditions.

Removal of ammonium in surface water at low temperature by a newly isolated Microbacterium sp. strain SFA13

The strain SFA13, isolated from Songhua River, demonstrates ability to convert ammonium to nitrogen under aerobic conditions at low temperature. On the basis of 16S rRNA gene sequence, the strain SFA13 was a species in genera Microbacterium. The isolate showed unusual ability of autotrophic nitrification with the ratio of 0.11 mg NH4(+)-N/L/h at 5°C. Ammonium was consumed by the strain SFA13 with the biodegradation of organic carbon and without nitrite or nitrate accumulation. NO3(-)-N or NO2(-)-N was reduced by the strain SFA13. The denitrification ratio was 0.24mgNO3(-)-N/L/h. Hydroxylamine oxidase, nitrite reductase and nitrate reductase were detectable. The putative nitrogen removal process by the strain SFA13 was as follows: NH4(+)→NH2OH→NO2(-)→NO3(-), then NO3(-)→NO2(-)→N2. Biological activated carbon attached with the strain SFA13 could effectively remove ammonium in surface water with the rate of 2.68±0.27-3.16±0.25 mg NH4(+)-N/L/h at C/N 2-10, temperature 10°C, and DO>5.2 mg/L.

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).

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.

Characterization of the ars gene cluster from extremely arsenic-resistant Microbacterium sp. strain A33

The arsenic resistance gene cluster of Microbacterium sp. A33 contains a novel pair of genes (arsTX) encoding a thioredoxin system that are cotranscribed with an unusual arsRC2 fusion gene, ACR3, and arsC1 in an operon divergent from arsC3. The whole ars gene cluster is required to complement an Escherichia coli ars mutant. ArsRC2 negatively regulates the expression of the pentacistronic operon. ArsC1 and ArsC3 are related to thioredoxin-dependent arsenate reductases; however, ArsC3 lacks the two distal catalytic cysteine residues of this class of enzymes.

Biodegradation of wastewater pollutants by activated sludge encapsulated inside calcium-alginate beads in a tubular packed bed reactor

The wastewater treatment plants produce large quantities of biomass (sludge) that require about one-third of the total inversion and operation plant costs for their treatment. By the microorganisms immobilization it is possible to handle high cell concentration in the reactor, increasing its efficiency, reducing the loss of biomass and the wash out is avoided. Moreover, there is no cell growth then the sludge production is reduced. In this study, the COD removal and VSS variation were modeled in a tubular reactor with activated sludge immobilized in Ca-alginate. Moreover, two aspects that are commonly not considered in the performance of the actual reactors of this kind were introduced; the performance in non-steady state and the dispersion effect. The model was calibrated with an actual wastewater taken out from a Mexican wastewater treatment plant. The results of the performance of the tubular bioreactor at different scenarios (i.e., different residence time and VSS in the reactor) are presented. With longer residence times and higher VSS concentration in the Ca-alginate beads in the tubular bioreactor it is possible to increase the time operation of the bioreactor and to treat higher volumes of wastewater. During the process, the sludge generation was drastically reduced and it is possible to remove nitrogen form the wastewater making this process more attractive.

Isolation and diversity analysis of arsenite-resistant bacteria in communities enriched from deep-sea sediments of the Southwest Indian Ocean Ridge

Microorganisms play an important role in the geobiocycling of arsenic element. However, little is known about the bacteria involved in this process in oceanic environments. In this report, arsenite-resistant bacteria were detected in deep-sea sediments on the Southwest Indian Ridge. From arsenite enriched cultures, 54 isolates were obtained, which showed varied tolerance to arsenite of 2-80 mM. Phylogenetic analysis based on 16S rRNA showed that they mainly belonged to Proteobacteria and Actinobacteria. Denaturing gradient gel electrophoresis revealed that Microbacterium esteraromaticum was the dominant member in the arsenite enriched communities, and this was reconfirmed by 16S rRNA gene library analyses. Thus, M. esteraromaticum showed highest resistant to arsenite among the detected bacteria. These results indicate that there are quite diverse bacteria of arsenite resistance inhabiting the deep sea sediment, which may play a role in the geobiocycling of arsenic element in marine environments.

Chromate reduction by immobilized palladized sulfate-reducing bacteria

Resting cells of Desulfovibrio vulgaris NCIMB 8303 and Desulfovibrio desulfuricans NCIMB 8307 were used for the hydrogenase-mediated reduction of Pd(II) to Pd(0). The resulting hybrid palladium bionanocatalyst (Bio-Pd(0)) was used in the reduction of Cr(VI) to the less environmentally problematic Cr(III) species. The reduction of Cr(VI) by free and agar-immobilized Bio-Pd(0) was evaluated. Investigations using catalyst suspensions showed that Cr(VI) reduction was similar ( approximately 170 nmol Cr(VI)/h/mg Bio-Pd(0)) when Bio-Pd(0) was produced using D. vulgaris or D. desulfuricans. Continuous-flow studies using D. vulgaris Bio-Pd(0) with agar as the immobilization matrix investigated the effect of Bio-Pd(0) loading, inlet Cr(VI) concentration, and flow rate on the efficiency of Cr(VI) reduction. Reduction of Cr(VI) was highest at a D. vulgaris Bio-Pd(0) loading of 7.5 mg Bio-Pd(0)/mL agar (3:1 dry cell wt: Pd(0)), an input [Cr(VI)] of 100 microM, and a flow rate of 1.75 mL/h (approx. 3.5 column volumes/h). A mathematical interpretation predicted the activity of the immobilized Bio-Pd(0) for a given set of conditions within 5% of the value found by experiment. Considering the system as an 'artificial enzyme' analog and application of applied enzyme kinetics gave an apparent K(m) value (K(m app)) of 430 microM Cr(VI) and a determined value of flow-through reactor activity which differed by 11% from that predicted mathematically.