Insolubilization of hexavalent chromium in highly alkaline cement sludge by anaerobic bioremediation

Chemical treatment using reducing agents such as ferrous sulfate is the most popular method for remediation of hexavalent chromium–contaminated sludge. However, the use of such chemical agents poses the risk of secondary pollution through the elution of other heavy metals. Therefore, a bioremediation method was developed to remediate high-alkali cement sludge containing hexavalent chromium. When a biomediator mainly composed of lignocellulose and lignoarabinoxylan was added to hexavalent chromium–contaminated sludge under anaerobic conditions, the amount of hexavalent chromium eluted from the treated sludge decreased significantly to below the level of the environmental standard value in Japan and its pH was reduced to 8. Moreover, the oxidation-reduction potential of the treated sludge decreased and its microbiota changed. These results indicate that anaerobic microbes can facilitate the change of hexavalent chromium to an immobilized form of trivalent chromium. Nucleotide sequence analysis suggested that anaerobic microbes activated in the sludge were Exiguobacterium aurantiacum, which are known to tolerate high pH environments and produce organic acids, even in the cement sludge. Finally, treated sludge did not elute hexavalent chromium during shaking in acid or alkaline solution.     

细菌六价铬还原及吸附机制研究进展

六价铬[Hexavalent chromium,Cr(Ⅵ)]是一种致癌物,其毒性远大于三价铬,因此会对人体健康和生态环境造成危害。Cr(Ⅵ)污染场地中的细菌主要通过生物还原和生物吸附降低Cr(Ⅵ)的毒性和迁移能力。Cr(Ⅵ)还原细菌的抗性机制与还原过程已被多次讨论,但现有综述还缺乏细菌类别、铬酸盐还原酶活性与吸附机制的总结。因此,本文通过系统发育树展示常见Cr(Ⅵ)还原细菌的类别,归纳细菌的Cr(Ⅵ)还原机制,总结现阶段铬酸盐还原酶的酶活性参数与反应条件,并讨论环境影响因子对细菌Cr(Ⅵ)还原的影响。其次,本文综述了细菌对Cr(Ⅵ)的吸附现象与机理。最后,本文对未来细菌修复Cr(Ⅵ)污染的机理研究进行了展望,以期加深对细菌Cr(Ⅵ)还原和吸附过程的了解。     

Isolation and characterization of a highly effective bacterium Bacillus cereus b-525k for hexavalent chromium detoxification

The chromate resistant Gram-positive Bacillus cereus strain b-525k was isolated from tannery effluents, demonstrating optimal propagation at 37 °C and pH 8. The minimum inhibitory concentration (MIC) test showed that B. cereus b-525k can tolerate up to 32 mM Cr⁶⁺, and also exhibit the ability to resist other toxic metal ions including Pb²⁺ (23 mM), As³⁺ (21 mM), Zn²⁺ (17 mM), Cd²⁺ (5 mM), Cu²⁺ (2 mM), and Ni²⁺ (3 mM) with the resistance order as Cr ⁶⁺ > Pb²⁺ > As³⁺ >Zn²⁺ >Cd²⁺ >Ni²⁺ >Cu²⁺. B. cereus b-525k showed maximum biosorption efficiency (q) of 51 mM Cr⁶⁺/g after 6 days. Chromate stress elicited pronounced production of antioxidant enzymes such as catalase (CAT) 191%, glutathione transferase (GST) 192%, superoxide dismutase (SOD) 161%, peroxidase (POX) 199%, and ascorbate peroxidase (APOX) (154%). Within B. cereus b-525k, the influence of Cr⁶⁺ stress (2 mM) did stimulate rise in levels of GSH (907%) and non-protein thiols (541%) was measured as compared to the control (without any Cr⁶⁺ stress) which markedly nullifies Cr⁶⁺ generated oxidative stress. The pilot scale experiments utilizing original tannery effluent showed that B. cereus b-525k could remove 99% Cr⁶⁺ in 6 days, thus, it could be a potential candidate to reclaim the chromate contaminated sites.     

Lignin degradation by microorganisms: A review

Lignin is an abundant plant-based biopolymer that has found applications in a variety of industries from construction to bioethanol production. This recalcitrant branched polymer is naturally degraded by many different species of microorganisms, including fungi and bacteria. These microbial lignin degradation mechanisms provide a host of possibilities to overcome the challenges of using harmful chemicals to degrade lignin biowaste in many industries. The classes and mechanisms of different microbial lignin degradation options available in nature form the primary focus of the present review. This review first discusses the chemical building blocks of lignin and the industrial sources and applications of this multifaceted polymer. The review further places emphasis on the degradation of lignin by natural means, discussing in detail the lignin degradation activities of various fungal and bacterial species. The lignin-degrading enzymes produced by various microbial species, specifically white-rot fungi, brown-rot fungi, and bacteria, are described. In the end, possible directions for future lignin biodegradation applications and research investigations have been provided.     

生物处理石油污染的研究概况

自20世纪80年代以来,人们对微生物降解石油进行了深入的研究,并逐步将这一技术应用于实际环境的处理中。处理环境中菌群的数量及组成、生物降解的有效性、实际环境中出现的影响因子对降解的影响、石油的毒性和极限环境下的生物降解石油等,许多学者都进行了广泛的研究。此外,一些研究者也尝试着通过构建生物降解模型,可以使微生物降解石油的研究能从经验上升到理论。     

Synthesis of zero-valent copper-chitosan nanocomposites and their application for treatment of hexavalent chromium

This study used ionotropic crosslinking to synthesize chitosan-tripolyphosphate chelating resin beads, which are used to fabricate zero-valent copper-chitosan nanocomposites. The copper nanoparticles were dispersed on chitosan-tripolyphosphate beads, and were thus able to maintain appropriate dispersion and stability, which greatly improves their applicability. The fabrication process contains two steps: using chitosan-tripolyphosphate beads to adsorb Cu(II) ions, followed by chemical reduction to reduce Cu(II) ions to zero-valent copper. This study explored the adsorption of synthesized chitosan-tripolyphosphate beads to Cu(II) ions, and used SEM/EDS, XPS, and TEM to examine the properties of zero-valent copper-chitosan nanocomposites. The results showed that, the adsorption behavior of hexavalent chromium from aqueous solution onto fabricated nanocomposites has better adsorption capacity than that of the chitosan-tripolyphosphate beads.     

The uptake and detoxification of antimony by plants: A review

Many crops cultivated in mining areas have been found to accumulate high levels of antimony (Sb) in their edible parts, thereby causing potential risks to human health. Understanding the behaviours of Sb in plants is important, particularly the mechanisms involved in its uptake, toxicity, detoxification and accumulation in crops. Many factors affect the uptake of Sb in plants, including water management, Sb speciation and some coexisting ions in soils. At present, the mechanisms of Sb uptake by plants have not been fully elucidated so far. The uptake of Sb has been proposed to occur mainly through the passive pathway; however, it is possible that an active pathway exists as well. Antimony can damage plants, including growth retardation, inhibition of photosynthesis, decreases in the uptake of certain essential elements and decreases in the synthesis of certain metabolites. Plants often have defence mechanisms to alleviate Sb toxicity; e.g., a highly efficient antioxidative system and the ability to immobilise Sb in the cell wall or compartmentalise Sb in the cytosol. Such mechanisms have been widely reported in Sb-tolerant and Sb-accumulating plants. In view of the above knowledge, several questions remain: (1) What is the actual uptake pathway of Sb in plants? (2) Does Sb participate in redox reactions within plants? (3) What is the role of metabolic reactions of Sb in Sb toxicity to plants? (4) Can Sb be methylated, and if so, how? (5) How does Sb induce bursts of reactive oxidative species (ROS)?     

真菌还原Cr（VI）的研究

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

Removal of hexavalent chromium in tannery wastewater by Bacillus cereus

Bacillus cereus was used to remove chromium (Cr(VI)) from medium containing tannery wastewater under different conditions. The maximum rate of Cr(VI) removal was attained at a temperature of 37?°C, pH of 7.0-9.0, and biomass of 20 g/L when the initial Cr(VI) concentration was less than 50?mg/L. Under the optimum conditions, the Cr(VI) in tannery wastewater was treated with each cellular component of B. cereus to detect its ability to reduce Cr(VI). The results showed that the removal rate of Cr(VI) for the cell-free extracts could reach 92.70%, which was close to that of the whole cells (96.85%), indicating that the Cr(VI) reductase generated by B.?cereus is primarily intracellular. Additionally, during continuous culture of the B. cereus, the strain showed good consecutive growth and removal ability. After treatment of 20?mg/L Cr(VI) for 48?h, the B. cereus was observed by SEM and TEM-EDX. SEM images showed that the B.?cereus used to treat Cr(VI) grew well and had a uniform cellular size. TEM-EDX analysis revealed large quantities of chromium in the B. cereus cells used to treat Cr(VI). Overall, the results presented herein demonstrate that B. cereus can be used as a new biomaterial to remove Cr(VI) from tannery wastewater.     

Soil chromium bioremediation: Synergic activity of actinobacteria and plants

The aim of this work was to evaluate a strategy to reduce the bioavailable chromium fraction in soil, using a Cr(VI) resistant microorganism, Streptomyces sp. MC1, under non sterile conditions, with maize plants as bioindicator and/or bioremediator.Soil samples were contaminated with 100, 200 and 400 mg kg⁻¹ of Cr(VI) or Cr(III). Bioavailable chromium (35%) was only detected in samples with Cr(VI). Soil samples with Cr(VI) 200 mg kg⁻¹ were inoculated with Streptomyces sp. MC1, and bioavailable chromium decreased up to 73%.Zea mays seedlings were planted in soil samples contaminated with chromium. Plantlets accumulated chromium mainly as Cr(III), and biomass decreased up to 88%. Streptomyces sp. MC1 was inoculated in soil samples contaminated with 200 mg kg⁻¹ of Cr(VI) and Z.mays seedlings were planted.Streptomyces sp. MC1 caused Z.mays biomass increase (57%), chromium accumulation and bioavailable chromium decreased up to 46% and 96%, respectively.This work constitutes the first contribution of cooperative action between actinobacteria and Z.mays in the bioremediation of Cr(VI) contaminated soil. The large removal capacity of bioavailable chromium by Streptomyces sp. MC1 and Z.mays infers that they could be successfully applied together in bioremediation of soils contaminated with Cr(VI).     

Occupational exposure to hexavalent chromium and cancers of the gastrointestinal tract: A meta-analysis

Introduction: We conducted a systematic literature review and meta-analysis of oral cavity, esophageal, stomach, small intestine, colon, and rectal cancers among workers occupationally exposed to Cr(VI). Methods: Using PubMed, studies published from 1950 to 2009 evaluating the relationship between Cr(VI) exposure and GI cancers were identified. Measures of effect and variability were extracted from 32 studies meeting specific inclusion criteria, and meta-analysis summary relative risk measures were calculated using random effects models and inverse variance weighting methods. Results: Meta-standardized mortality ratios (SMRs) were, for cancer of the: oral cavity [1.02 (95% CI = 0.77–1.34)]; esophagus [1.17 (95% CI = 0.90–1.51)]; stomach [1.09 (95% CI = 0.93–1.28)]; colon [0.89 (95% CI = 0.70–1.12)]; and rectum [1.17 (95% CI = 0.98–1.39)]. Analyses of more highly exposed subgroups included in the studies or subgroups based on geographic region or by industry with recognized Cr(VI) exposures (welding, chrome plating, chromate production, and pigment production) did not result in elevated meta-SMRs except for esophageal cancer among US cohorts [meta-SMR = 1.49 (95% CI = 1.06–2.09)]. However, that finding was based on a subgroup of only four studies, one of which was a PMR study. Potential confounding by socioeconomic status (SES), diet and/or smoking, or limitations due to the healthy-worker effect (HWE) were evaluated, and while smoking, diet and SES may be important factors that may have upwardly biased the meta-SMRs, HWE is not likely to have significantly affected the summary results. None of three studies reporting small intestine cancers observed a statistically significant increased risk. Discussion: These meta-analyses and literature review indicate that Cr(VI)-exposed workers are not at a greater risk of GI cancers than the general population.     

Understanding the diversity in catabolic potential of microorganisms for the development of bioremediation strategies

Molecular ecological approaches have detected diverse microorganisms that occur in response to pollution and bioremediation; however, most of these organisms have not been isolated, and their physiological traits are poorly understood. One important objective in current bioremediation studies would therefore be an assessment of the physiology and functions of the diverse microbial population at a polluted site. Among the parameters relating to the diversity of the microbial catabolic potential, e.g., substrate specificity, inducer specificity, number of catabolic routes and kinetics of catabolic enzymes, our studies have focused on the kinetic diversity of phenol-degrading bacteria. In one example, a kinetic analysis allowed functionally important phenol-degrading bacteria to be identified in activated sludge; this information could be used to improve the performance of phenol-degrading activated sludge. In an analysis of phenol-degrading bacteria present in trichloroethylene (TCE)-contaminated aquifer soil, the kinetic data could be linked to group-specific monitoring of their phenol-hydroxylase genes. The results have suggested that one group of phenol-degrading bacteria can effectively contribute to TCE bioremediation, while other groups work poorly. Based on this information, we have succeeded in developing a high-performance TCE-degrading bioreactor. We suggest that a careful analysis of the diversity of microbial catabolic potential, particularly of the kinetic traits, may facilitate the development of new bioremediation strategies. This revised version was published online in August 2006 with corrections to the Cover Date.     

Suicidal genetically engineered microorganisms for bioremediation: need and perspectives

In the past few decades, increased awareness of environmental pollution has led to the exploitation of microbial metabolic potential in the construction of several genetically engineered microorganisms (GEMs) for bioremediation purposes. At the same time, environmental concerns and regulatory constraints have limited the in situ application of GEMs, the ultimate objective behind their development. In order to address the anticipated risks due to the uncontrolled survival/dispersal of GEMs or recombinant plasmids into the environment, some attempts have been made to construct systems that would contain the released organisms. This article discusses the designing of safer genetically engineered organisms for environmental release with specific emphasis on the use of bacterial plasmid addiction systems to limit their survival thus minimizing the anticipated risk. We also conceptualize a novel strategy to construct "Suicidal Genetically Engineered Microorganisms (SGEMs)" by exploring/combining the knowledge of different plasmid addiction systems (such as antisense RNA-regulated plasmid addiction, proteic plasmid addiction etc.) and inducible degradative operons of bacteria.     

A comparison of the in vitro genotoxicity of tri- and hexavalent chromium

Chromium can be found in the environment in two main valence states: hexavalent (Cr(VI)) and trivalent (Cr(III)). Cr(VI) salts are well known human carcinogens, but the results from in vitro studies are often conflicting. Cr(VI) primarily enters the cells and undergoes metabolic reduction; however, the ultimate product of this reduction, Cr(III) predominates within the cell. In the present work, we compared the effects of tri- and hexavalent chromium on the DNA damage and repair in human lymphocytes using the alkaline single cell gel electrophoresis (comet assay). Potassium dichromate induced DNA damage in the lymphocytes, measured as the increase in comet tail moment. The effect was dose-dependent. Treated cells were able to recover within a 120-min incubation. Cr(III) caused greater DNA migration than Cr(VI). The lymphocytes did not show measurable DNA repair. Vitamin C at 50 microM reduced the extent of DNA migration. This was either due to a decrease in DNA strand breaks and/or alkali labile sites induced by Cr(VI) or to the formation of DNA crosslinks by Cr(VI) in the presence of vitamin C. Vitamin C, however, did not modify the effects of Cr(III). Catalase, an enzyme inactivating hydrogen peroxide, decreased the extent of DNA damage induced by Cr(VI) but not the one induced by Cr(III). Lymphocytes exposed to Cr(VI) and treated with endonuclease III, which recognizes oxidized pyrimidines, displayed greater extent of DNA damage than those not treated with the enzyme. Such an effect was not observed when Cr(III) was tested. The results obtained suggest that reactive oxygen species and hydrogen peroxide may be involved in the formation of DNA lesions by hexavalent chromium. The comet assay did not indicate the involvement of oxidative mechanisms in the DNA-damaging activity of trivalent chromium and we speculate that its binding to cellular ligands may play a role in its genotoxicity.     

Insights into environmental bioremediation by microorganisms through functional genomics and proteomics

Environmental pollutants in the soil are a major concern worldwide. Bioremediation mediated by microorganisms is a highly promising technology that is environmentally friendly, safe, and effective. However, incomplete biological information regarding the cellular responses in many microbial communities restricts progress in the site-specific mineralization process. The application of proteomics in environmental bioremediation research provides a global view of the protein compositions of the microbial cells and offers a promising approach to address the molecular mechanisms of bioremediation. With the combination of proteomics, functional genomics provide an insight into global metabolic and regulatory networks that can enhance the understanding of gene functions. This article deals with the applications of functional genomics and proteomics to dissect the cellular responses to environmental stimuli, such as stress response, induction and expressions of regulatory proteins/enzymes in response to aromatic hydrocarbons and heavy metals. An understanding of the growth conditions governing the expression of the proteome (for example, enzymes and regulatory proteins of aromatic hydrocarbon degradation, energy generation pathways, transport and stress-related proteins) in a specific environment is essential for developing rational strategies for successful bioremediation.     

Intensification of composting processes by aerobic microorganisms: A review

Microbiological and biotechnological characteristics of intensification of aerobic processing of organic waste have been reviewed, with a view for revealing two types of correlations: (1) between the quality of the composts obtained and the microorganisms involved in composting and (2) between physicochemical parameters and consumer properties of the composts.     

A novel technology for biosorption and recovery hexavalent chromium in wastewater by bio-functional magnetic beads

The goal of this study was to develop an applied technique for the removal and recovery of heavy metal in wastewater. It is novel that the Cr(VI) could be adsorbed and recovered by bio-functional magnetic beads. Furthermore, the magnetic separation technology would make their separation more convenient. The beads were constituted by the powder of Rhizopus cohnii and Fe(3)O(4) particles coated with alginate and polyvinyl alcohol (PVA). The parameters effecting Cr(VI) removal were obtained: the optimum pH 1.0 and optimum temperature 28 degrees C. The biosorption took place mainly in form of Cr(VI) and R. cohnii biomass played a key role in Cr(VI) adsorption. The model of Langmuir isotherm and Lagergren could be better used to fit the sorption process and kinetics, respectively. The beads still maintained predominant characteristics of adsorption, recovery and magnetism after five cycles for adsorption-desorption. The mechanism of adsorption was gained by Fourier transform infrared spectroscopy (FTIR), raman spectroscopy (RS) and scanning electron microscopy (SEM). The groups of -NH(3)(+), -NH(2)(+)-, and NH- played an important role in the Cr(VI) adsorption. Consequently, the beads exhibited the superior performances in Cr(VI) cleanup, separation and recovery and the perspective potential in application.