Bioremediation of Heavy Metals from Soil and Aquatic Environment: An Overview of Principles and Criteria of Fundamental Processes

Effect of rhizobacteria on arsenic uptake by macrophyte Eichhornia crassipes (Mart.) Solms

Wastewater flowing in streams and nallahs across India carries several trace metals, including metalloid arsenic (As), which are considered serious environmental contaminants due to their toxicity, and recalcitrant nature. In this study, we determined the phytoremediation of As by Eichhornia crassipes (Mart.) Solms either alone or in association with plant growth-promoting rhizobacteria. Pseudomonas and Azotobacter inoculation to E. crassipes resulted in enhanced As removal compared to uninoculated control. Co-inoculation with a consortium of Pseudomonas, Azotobacter, Azospirillum, Actinomyces, and Bacillus resulted in a higher As (p < 0.05) phytoaccumulation efficiency. P. aeruginosa strain jogii was found particularly effective in augmenting As removal by E. crassipes. Our findings indicate that the synergistic association of E. crassipes and various rhizobacteria is an effective strategy to enhance removal of As and thus may be utilized as an efficient biological alternative for the removal of this metalloid from wastewaters.

Rapid classification of heavy metal-exposed freshwater bacteria by infrared spectroscopy coupled with chemometrics using supervised method

Rapid, cost-effective, sensitive and accurate methodologies to classify bacteria are still in the process of development. The major drawbacks of standard microbiological, molecular and immunological techniques call for the possible usage of infrared (IR) spectroscopy based supervised chemometric techniques. Previous applications of IR based chemometric methods have demonstrated outstanding findings in the classification of bacteria. Therefore, we have exploited an IR spectroscopy based chemometrics using supervised method namely Soft Independent Modeling of Class Analogy (SIMCA) technique for the first time to classify heavy metal-exposed bacteria to be used in the selection of suitable bacteria to evaluate their potential for environmental cleanup applications. Herein, we present the powerful differentiation and classification of laboratory strains (Escherichia coli and Staphylococcus aureus) and environmental isolates (Gordonia sp. and Microbacterium oxydans) of bacteria exposed to growth inhibitory concentrations of silver (Ag), cadmium (Cd) and lead (Pb). Our results demonstrated that SIMCA was able to differentiate all heavy metal-exposed and control groups from each other with 95% confidence level. Correct identification of randomly chosen test samples in their corresponding groups and high model distances between the classes were also achieved. We report, for the first time, the success of IR spectroscopy coupled with supervised chemometric technique SIMCA in classification of different bacteria under a given treatment.

Nickel phytoextraction through bacterial inoculation in Raphanus sativus

A pot experiment was conducted to evaluate the potential of two plant growth promoting rhizobacteria (PGPR) viz. Bacillus sp. CIK-516 and Stenotrophomonas sp. CIK-517Y for improving the growth and Ni uptake of radish (Raphanus sativus) in the presence of four different levels of Ni contamination (0, 50, 100, 150 mg Ni kg^-1 soil). Plant growth, dry biomass, chlorophyll and nitrogen contents were significantly reduced by the exogenous application of Ni, however, bacterial inoculation diluted the negative impacts of Ni stress on radish by improving these parameters. PGPR strain CIK-516 increased root length (9-27%), shoot length (8-27%), root dry biomass (2-32%), shoot dry biomass (9-51%), root girth (6-48%), total chlorophyll (4-38%) and shoot nitrogen contents (11-15%) in Ni contaminated and non-contaminated soils. Positive regulation of chlorophyll and nitrogen contents by the inoculated plants shows plant tolerance mechanism of Ni stress. Bacterial strain (CIK-516) exhibited indole acetic acid and 1-amino-cyclopropane-1-carboxylate deaminase potentials which would have helped radish plant to stabilize in Ni contaminated soil and thereby increased Ni uptake (24-257 in shoot and 58-609 in root mg kg^-1 dry biomass) and facilitated accumulation in radish (bioaccumulation factor = 0.6-1.7) depending upon soil Ni contamination. Based on the findings of this study, it might be suggested that inoculation with bacterial strain CIK-516 could be an efficient tool for enhanced Ni phytoextraction in radish.

Microbial remediation of fluoride-contaminated water via a novel bacterium Providencia vermicola (KX926492)

The present study emphasizes on the isolation, identification and characterization of a fluoride-resistant bacteria from contaminated groundwater of a severely affected rural area. The isolate was investigated for its possible role towards bioremediation of fluoride. Bacterial growth was determined by various carbon and nitrogen sources. Influence of parameters like initial fluoride concentration (5-25 mg L^-1), pH (3-9) and temperature (15-42 °C) on fluoride removal by Providencia sp. KX926492 were also examined. SEM, EDX and FTIR were performed to analyse the surface texture, elemental composition and functional groups of the bacterium involved in the uptake of fluoride ions. 16S rRNA sequencing was performed to identify the isolate. Plackett-Burman design was employed to optimize the various parametric conditions of fluoride removal. Maximum removal of 82% was achieved when the initial fluoride concentration was 20 mgL^-1 at pH 7 and 37 °C temperature with dextrose and nitrogen concentrations of 5 and 4 g per 50 mL respectively. Results suggested that Providencia vermicola (KX926492) could be a potential bacterium in removal of fluoride from contaminated water.

Microbial and Plant-Assisted Bioremediation of Heavy Metal Polluted Environments: A Review

Environmental pollution from hazardous waste materials, organic pollutants and heavy metals, has adversely affected the natural ecosystem to the detriment of man. These pollutants arise from anthropogenic sources as well as natural disasters such as hurricanes and volcanic eruptions. Toxic metals could accumulate in agricultural soils and get into the food chain, thereby becoming a major threat to food security. Conventional and physical methods are expensive and not effective in areas with low metal toxicity. Bioremediation is therefore an eco-friendly and efficient method of reclaiming environments contaminated with heavy metals by making use of the inherent biological mechanisms of microorganisms and plants to eradicate hazardous contaminants. This review discusses the toxic effects of heavy metal pollution and the mechanisms used by microbes and plants for environmental remediation. It also emphasized the importance of modern biotechnological techniques and approaches in improving the ability of microbial enzymes to effectively degrade heavy metals at a faster rate, highlighting recent advances in microbial bioremediation and phytoremediation for the removal of heavy metals from the environment as well as future prospects and limitations. However, strict adherence to biosafety regulations must be followed in the use of biotechnological methods to ensure safety of the environment.

Optimal reduction of chemical oxygen demand and NH3-N from landfill leachate using a strongly resistant novel Bacillus salmalaya strain

Background

When the unavoidable waste generation is considered as damaging to our environment, it becomes crucial to develop a sustainable technology to remediate the pollutant source towards an environmental protection and safety. The development of a bioengineering technology for highly efficient pollutant removal is this regard. Given the high ammonia nitrogen content and chemical oxygen demand of landfill leachate, Bacillus salmalaya strain 139SI, a novel resident strain microbe that can survive in high ammonia nitrogen concentrations, was investigated for the bioremoval of ammonia nitrogen from landfill leachate. The treatability of landfill leachate was evaluated under different treatment parameters, such as temperature, inoculum dosage, and pH.

Results

Results demonstrated that bioaugmentation with the novel strain can potentially improve the biodegradability of landfill leachate. B. salmalaya strain 139SI showed high potential to enhance biological treatment given its maximum NH₃–N and COD removal efficiencies. The response surface plot pattern indicated that within 11 days and under optimum conditions (10% v/v inoculant, pH 6, and 35 °C), B. salmalaya strain139SI removed 78% of ammonia nitrogen. At the end of the study, biological and chemical oxygen demands remarkably decreased by 88% and 91.4%, respectively. Scanning electron microscopy images revealed that ammonia ions covered the cell surface of B. salmalaya strain139SI.

Conclusions

Therefore, novel resistant Bacillus salmalaya strain139SI significantly reduces the chemical oxygen demand and NH₃–N content of landfill leachate.

Graphical abstract

Leachate treatment by B. salmalaya strain 139SI within 11 days.

Streptomyces sp. is a powerful biotechnological tool for the biodegradation of HCH isomers: biochemical and molecular basis

Actinobacteria are well-known degraders of toxic materials that have the ability to tolerate and remove organochloride pesticides; thus, they are used for bioremediation. The biodegradation of organochlorines by actinobacteria has been demonstrated in pure and mixed cultures with the concomitant production of metabolic intermediates including γ-pentachlorocyclohexene (γ-PCCH); 1,3,4,6-tetrachloro-1,4-cyclohexadiene (1,4-TCDN); 1,2-dichlorobenzene (1,2-DCB), 1,3-dichlorobenzene (1,3-DCB), or 1,4-dichlorobenzene (1,4-DCB); 1,2,3-trichlorobenzene (1,2,3-TCB), 1,2,4-trichlorobenzene (1,2,4-TCB), or 1,3,5-trichlorobenzene (1,3,5-TCB); 1,3-DCB; and 1,2-DCB. Chromatography coupled to mass spectrometric detection, especially GC-MS, is typically used to determine HCH-isomer metabolites. The important enzymes involved in HCH isomer degradation metabolic pathways include hexachlorocyclohexane dehydrochlorinase (LinA), haloalkane dehalogenase (LinB), and alcohol dehydrogenase (LinC). The metabolic versatility of these enzymes is known. Advances have been made in the identification of actinobacterial haloalkane dehydrogenase, which is encoded by linB. This knowledge will permit future improvements in biodegradation processes using Actinobacteria. The enzymatic and genetic characterizations of the molecular mechanisms involved in these processes have not been fully elucidated, necessitating further studies. New advances in this area suggest promising results. The scope of this paper encompasses the following: (i) the aerobic degradation pathways of hexachlorocyclohexane (HCH) isomers; (ii) the important genes and enzymes involved in the metabolic pathways of HCH isomer degradation; and (iii) the identification and quantification of intermediate metabolites through gas chromatography coupled to mass spectrometry (GC-MS).

Microbes from mined sites: Harnessing their potential for reclamation of derelict mine sites

Derelict mines pose potential risks to environmental health. Several factors such as soil structure, organic matter, and nutrient content are the greatly affected qualities in mined soils. Soil microbial communities are an important element for successful reclamation because of their major role in nutrient cycling, plant establishment, geochemical transformations, and soil formation. Yet, microorganisms generally remain an undervalued asset in mined sites. The microbial diversity in derelict mine sites consists of diverse species belonging to four key phyla: Proteobacteria, Acidobacteria, Firmicutes, and Bacteroidetes. The activity of plant symbiotic microorganisms including root-colonizing rhizobacteria and ectomycorrhizal fungi of existing vegetation in the mined sites is very high since most of these microbes are extremophiles. This review outlines the importance of microorganisms to soil health and the rehabilitation of derelict mines and how microbial activity and diversity can be exploited to better plan the soil rehabilitation. Besides highlighting the major breakthroughs in the application of microorganisms for mined site reclamation, we provide a critical view on plant-microbiome interactions to improve revegetation at the mined sites. Also, the need has been emphasized for deciphering the molecular mechanisms of adaptation and resistance of rhizosphere and non-rhizosphere microbes in abandoned mine sites, understanding their role in remediation, and subsequent harnessing of their potential to pave the way in future rehabilitation strategies for mined sites.

Alleviation of Heavy Metal Stress in Plants and Remediation of Soil by Rhizosphere Microorganisms

Increasing concentration of heavy metals (HM) due to various anthropogenic activities is a serious problem. Plants are very much affected by HM pollution particularly in contaminated soils. Survival of plants becomes tough and its overall health under HM stress is impaired. Remediation of HM in contaminated soil is done by physical and chemical processes which are costly, time-consuming, and non-sustainable. Metal–microbe interaction is an emerging but under-utilized technology that can be exploited to reduce HM stress in plants. Several rhizosphere microorganisms are known to play essential role in the management of HM stresses in plants. They can accumulate, transform, or detoxify HM. In general, the benefit from these microbes can have a vast impact on plant’s health. Plant–microbe associations targeting HM stress may provide another dimension to existing phytoremediation and rhizoremediation uses. In this review, applied aspects and mechanisms of action of heavy metal tolerant-plant growth promoting (HMT-PGP) microbes in ensuring plant survival and growth in contaminated soils are discussed. The use of HMT-PGP microbes and their interaction with plants in remediation of contaminated soil can be the approach for the future. This low input and sustainable biotechnology can be of immense use/importance in reclaiming the HM contaminated soils, thus increasing the quality and yield of such soils.

Decontamination of multiple heavy metals-containing effluents through microbial biotechnology

To decontaminate heavy metal-containing waste water, a microbial biotechnology was developed by using the synergy between Sulfate reducing bacteria (SRB), Bacillus cereus (B. cereus) and Camellia oleifera cake (COC). In this process the COC degradation assisted by B.cereus, created an anoxic environment and provided energy and nutrition for SRB. Both of B. cereus and SRB played significant roles through biosorption, bioaccumulation and biosurfactant production. Meanwhile, a flotation technology commonly used in many effluent treatments has been led into this system for increasing the efficiency as well. After desorption and regeneration with acid and deionized water, the biosorbents could be reused to adsorb metal ions. 97% of heavy metals removal was achieved by the proposed technology. For multiple heavy metals-containing solutions, the capacities are in the order of Cd²⁺>Zn²⁺>Cu²⁺.

Advances in microbe-assisted reclamation of heavy metal contaminated soils over the last decade: A review

Contamination of agricultural soils with trace metals present lethal consequences in terms of diverse ecological and environmental problems that entail entry of metal in food chain, soil deterioration, plant growth suppression, yield reduction and alteration in microbial community. Metal polluted soils have become a major concern for scientists around the globe. Phytoremediation involves the hyperaccumulation of metals in different plant parts. Phytoremediation of metals from polluted soils could be enhanced through inoculation with metal resistant plant growth promoting (PGP) bacteria. These PGP bacteria not only promote plant growth but also enhance metal uptake by plants. There are a number of reports in the literature where PGP bacterial inoculation improves metal accumulation in different plant parts without influencing plant growth. Therefore, there is a need to select PGP bacterial strains which possess the potential to improve plant growth as well as expedite the phytoremediation of metals. In this review, we have discussed the mechanisms possessed by PGP bacteria to promote plant growth and phytoremediation of metals. The central part of this review deals with the recent advances in microbial assisted-phytoremediation of metals.

Alleviation of heavy metal toxicity and phytostimulation of Brassica campestris L. by endophytic Mucor sp. MHR-7

Heavy metal (HM) pollution is of great concern in countries like Pakistan where a huge proportion of human population is exposed to it. These toxic metals are making their way from water bodies to soil where it not only interferes with plant growth and development but also initiates serious health issues in human consuming the produce of such soils. Bioremediation is one of the most viable and efficient solution for the problem. Purpose of the current study was to isolate endophytic fungi from plants grown on HM contaminated soil and screen them for their ability to tolerate multiple HM including chromium (Cr⁶⁺), manganese (Mn²⁺), cobalt (Co²⁺), copper (Cu²⁺) and zinc (Zn²⁺). Out of 27 isolated endophytes, only one strain (MHR-7) was selected for multiple heavy metals tolerance. The strain was identified as Mucor sp. by 18S and 28S ribosomal RNA internal transcribed spacer (ITS) 1 and 4 sequence homology. The strain effectively tolerated up to 900µgmL^-1 of these heavy metals showing no remarkable effect on its growth. The adverse effect of the heavy metals, measured as reduction of the fungal growth increased with increasing concentration of the metals. The strain was able to remove 60-87% of heavy metals from broth culture when supplied with 300µgmL^-1 of these metals. A trend of decline in bioremediation potential of the strain was observed with increasing amount of metals. The strain removed metals by biotransformation and/or accumulation of heavy metal in its hyphae. Application of Mucor sp. MHR-7 locked down HM in tis mycelium thereby making them less available to plant root reducing HM uptake and toxicity in mustard. Besides its bioremediation potential, the strain was also able to produce IAA, ACC deaminase and solubilize phosphate making it excellent phytostimulant fungus. It is concluded that MHR-7 is an excellent candidate for use as biofertilizer in fields affected with heavy metals.

Integral use of sugarcane vinasse for biomass production of actinobacteria: Potential application in soil remediation

The use of living actinobacteria biomass to clean up contaminated soils is an attractive biotechnology approach. However, biomass generation from cheap feedstock is the first step to ensure process sustainability. The present work reports the ability of four actinobacteria, Streptomyces sp. M7, MC1, A5, and Amycolatopsis tucumanensis, to generate biomass from sugarcane vinasse. Optimal vinasse concentration to obtain the required biomass (more than 0.4 g L^-1) was 20% for all strains, either grown individually or as mixed cultures. However, the biomass fraction recovered from first vinasse was discarded as it retained trace metals present in the effluent. Fractions recovered from three consecutive cycles of vinasse re-use obtained by mixing equal amounts of biomass from single cultures or produced as a mixed culture were evaluated to clean up contaminated soil with lindane and chromium. In all cases, the decrease in pesticide was about 50% after 14 d of incubation. However, chromium removal was statistically different depending on the preparation methodology of the inoculum. While the combined actinobacteria biomass recovered from their respective single cultures removed about 85% of the chromium, the mixed culture biomass removed more than 95%. At the end of the reused vinasse cycle, the mixed culture removed more than 70% of the biological oxygen demand suggesting a proportional reduction in the effluent toxicity. These results represent the first integral approach to address a problematic of multiple contaminations, concerning pesticides, heavy metals and a regionally important effluent like vinasse.

Plant responses to environmental stresses-from gene to biotechnology

Increasing global population, urbanization and industrialization are increasing the rate of conversion of arable land into wasteland. Supplying food to an ever-increasing population is one of the biggest challenges that agriculturalists and plant scientists are currently confronting. Environmental stresses make this situation even graver. Despite the induction of several tolerance mechanisms, sensitive plants often fail to survive under environmental extremes. New technological approaches are imperative. Conventional breeding methods have a limited potential to improve plant genomes against environmental stress. Recently, genetic engineering has contributed enormously to the development of genetically modified varieties of different crops such as cotton, maize, rice, canola and soybean. The identification of stress-responsive genes and their subsequent introgression or overexpression within sensitive crop species are now being widely carried out by plant scientists. Engineering of important tolerance pathways, like antioxidant enzymes, osmolyte accumulation, membrane-localized transporters for efficient compartmentation of deleterious ions and accumulation of essential elements and resistance against pests or pathogens is also an area that has been intensively researched. In this review, the role of biotechnology and its successes, prospects and challenges in developing stress-tolerant crop cultivars are discussed.

Nanotechnology in Sustainable Agriculture: Recent Developments, Challenges, and Perspectives

Nanotechnology monitors a leading agricultural controlling process, especially by its miniature dimension. Additionally, many potential benefits such as enhancement of food quality and safety, reduction of agricultural inputs, enrichment of absorbing nanoscale nutrients from the soil, etc. allow the application of nanotechnology to be resonant encumbrance. Agriculture, food, and natural resources are a part of those challenges like sustainability, susceptibility, human health, and healthy life. The ambition of nanomaterials in agriculture is to reduce the amount of spread chemicals, minimize nutrient losses in fertilization and increased yield through pest and nutrient management. Nanotechnology has the prospective to improve the agriculture and food industry with novel nanotools for the controlling of rapid disease diagnostic, enhancing the capacity of plants to absorb nutrients among others. The significant interests of using nanotechnology in agriculture includes specific applications like nanofertilizers and nanopesticides to trail products and nutrients levels to increase the productivity without decontamination of soils, waters, and protection against several insect pest and microbial diseases. Nanotechnology may act as sensors for monitoring soil quality of agricultural field and thus it maintain the health of agricultural plants. This review covers the current challenges of sustainability, food security and climate change that are exploring by the researchers in the area of nanotechnology in the improvement of agriculture.

The role of forest in mitigating the impact of atmospheric dust pollution in a mixed landscape

Atmospheric dust pollution, especially particulate matter below 2.5 μm, causes 3.3 million premature deaths per year worldwide. Although pollution sources are increasingly well known, the role of ecosystems in mitigating their impact is still poorly known. Our objective was to investigate the role of forests located in the surrounding of industrial and urban areas in reducing atmospheric dust pollution. This was tested using lichen transplants as biomonitors in a Mediterranean regional area with high levels of dry deposition. After a multivariate analysis, we have modeled the maximum pollution load expected for each site taking into consideration nearby pollutant sources. The difference between maximum expected pollution load and the observed values was explained by the deposition in nearby forests. Both the dust pollution and the ameliorating effect of forested areas were then mapped. The results showed that forest located nearby pollution sources plays an important role in reducing atmospheric dust pollution, highlighting their importance in the provision of the ecosystem service of air purification.

Cadmium in rice: Transport mechanisms, influencing factors, and minimizing measures

Cadmium (Cd) accumulation in rice and its subsequent transfer to food chain is a major environmental issue worldwide. Understanding of Cd transport processes and its management aiming to reduce Cd uptake and accumulation in rice may help to improve rice growth and grain quality. Moreover, a thorough understanding of the factors influencing Cd accumulation will be helpful to derive efficient strategies to minimize Cd in rice. In this article, we reviewed Cd transport mechanisms in rice, the factors affecting Cd uptake (including physicochemical characters of soil and ecophysiological features of rice) and discussed efficient measures to immobilize Cd in soil and reduce Cd uptake by rice (including agronomic practices, bioremediation and molecular biology techniques). These findings will contribute to ensuring food safety, and reducing Cd risk on human beings.

Biological diversity of Salix taxa in Cu, Pb and Zn phytoextraction from soil

The aim of the study was to estimate the efficiency of copper (Cu), lead (Pb) and zinc (Zn) phytoextraction by 145 Salix taxa cultivated in an area affected by industrial activity. Survivability and biomass of plants were also analyzed. The highest Cu, Pb and Zn content in shoots was 33.38 ± 2.91 (S. purpurea × viminalis 8), 24.64 ± 1.97 (S. fragilis 1) and 58.99 ± 4.30 (S. eriocephala 7) mg kg^-1 dry weight, respectively. In the case of unwashed leaves, the highest content of these metals was 135.06 ± 8.14 (S. purpurea 26), 67.98 ± 5.27 (S. purpurea 45) and 142.56 ± 12.69 (S. alba × triandra 2) mg kg^-1 dw, while in washed leaves it was 106.02 ± 11.12 (S. purpurea 45), 55.06 ± 5.75 (S. purpurea 45) and 122.87 ± 12.33 (S. alba × triandra 2) mg kg^-1 dw, respectively. The differences between the highest and lowest values for Cu, Pb and Zn were 545%, 20500% and 535% in shoots; 2692%, 2560% and 7500% in unwashed leaves; and 3286%, 2221% and 6950% in washed leaves, respectively. S. acutifolia was able to effectively accumulate all three metals jointly, producing shoots that were well developed in both length and diameter when compared with the other tested willows-an ability that would suggest its high suitability for practical application.

Biofertilizers: a potential approach for sustainable agriculture development

The worldwide increase in human population raises a big threat to the food security of each people as the land for agriculture is limited and even getting reduced with time. Therefore, it is essential that agricultural productivity should be enhanced significantly within the next few decades to meet the large demand of food by emerging population. Not to mention, too much dependence on chemical fertilizers for more crop productions inevitably damages both environmental ecology and human health with great severity. Exploitation of microbes as biofertilizers is considered to some extent an alternative to chemical fertilizers in agricultural sector due to their extensive potentiality in enhancing crop production and food safety. It has been observed that some microorganisms including plant growth promoting bacteria, fungi, Cyanobacteria, etc. have showed biofertilizer-like activities in the agricultural sector. Extensive works on biofertilizers have revealed their capability of providing required nutrients to the crop in sufficient amounts that resulted in the enhancement of crop yield. The present review elucidates various mechanisms that have been exerted by biofertilizers in order to promote plant growth and also provides protection against different plant pathogens. The aim of this review is to discuss the important roles and applications of biofertilizers in different sectors including agriculture, bioremediation, and ecology.

A New Strategy for Heavy Metal Polluted Environments: A Review of Microbial Biosorbents

Persistent heavy metal pollution poses a major threat to all life forms in the environment due to its toxic effects. These metals are very reactive at low concentrations and can accumulate in the food web, causing severe public health concerns. Remediation using conventional physical and chemical methods is uneconomical and generates large volumes of chemical waste. Bioremediation of hazardous metals has received considerable and growing interest over the years. The use of microbial biosorbents is eco-friendly and cost effective; hence, it is an efficient alternative for the remediation of heavy metal contaminated environments. Microbes have various mechanisms of metal sequestration that hold greater metal biosorption capacities. The goal of microbial biosorption is to remove and/or recover metals and metalloids from solutions, using living or dead biomass and their components. This review discusses the sources of toxic heavy metals and describes the groups of microorganisms with biosorbent potential for heavy metal removal.

Bioremediation of copper-contaminated soils by bacteria

Although copper (Cu) is an essential micronutrient for all living organisms, it can be toxic at low concentrations. Its beneficial effects are therefore only observed for a narrow range of concentrations. Anthropogenic activities such as fungicide spraying and mining have resulted in the Cu contamination of environmental compartments (soil, water and sediment) at levels sometimes exceeding the toxicity threshold. This review focuses on the bioremediation of copper-contaminated soils. The mechanisms by which microorganisms, and in particular bacteria, can mobilize or immobilize Cu in soils are described and the corresponding bioremediation strategies-of varying levels of maturity-are addressed: (i) bioleaching as a process for the ex situ recovery of Cu from Cu-bearing solids, (ii) bioimmobilization to limit the in situ leaching of Cu into groundwater and (iii) bioaugmentation-assisted phytoextraction as an innovative process for in situ enhancement of Cu removal from soil. For each application, the specific conditions required to achieve the desired effect and the practical methods for control of the microbial processes were specified.

Metal-resistant rhizobacteria isolates improve Mucuna deeringiana phytoextraction capacity in multi-metal contaminated soils from a gold mining area

Phytoremediation consists of biological techniques for heavy metal remediation, which include exploring the genetic package of vegetable species to remove heavy metals from the environment. The goals of this study were to investigate heavy metal and bioaugmentation effects on growth and nutrient uptake by Mucuna deeringiana; to determine the metal translocation factor and bioconcentration factor and provide insight for using native bacteria to enhance heavy metal accumulation. The experiment was conducted under greenhouse conditions using a 2 × 4 factorial scheme with highly and slightly contaminated soil samples and inoculating M. deeringiana with three highly lead (Pb⁺²)-resistant bacteria Kluyvera intermedia (Ki), Klebsiella oxytoca (Ko), and Citrobacter murliniae (Cm) isolated from the rhizosphere of native plants identified as Senecio brasiliensis (Spreng.) Less., Senecio leptolobus DC., and Baccharis trimera (Less) DC., respectively. The increased heavy metal concentrations in soil samples do not decrease the root dry mass of M. deeringiana, concerning the number and dry weight of nodules. The shoot dry mass is reduced by the increasing concentration of heavy metals in soil associated with Kluyvera intermedia and Klebsiella oxytoca bacteria. The number of nodules is affected by heavy metals associated with Citrobacter murliniae bacteria. The bacteria K. intermedia, C. murliniae, and K. oxytoca increase the lead and cadmium available in the soil and enhanced metal uptake by Mucuna deeringiana. The M. deeringiana specie has characteristics that make it hyperaccumulate copper and zinc. The translocation and bioconcentration factors for M. deeringiana characterize it as a promising candidate to phytostabilize multi-metal contaminated soils.

Whole resting cells vs. cell free extracts of Candida parapsilosis ATCC 7330 for the synthesis of gold nanoparticles

The cell free extracts of Candida parapsilosis ATCC 7330 are more efficient than the whole resting cells of the yeast in the synthesis of directly usable gold nanoparticles as revealed by this systematic study. Cell free extracts yielded gold nanoparticles of hydrodynamic diameter (50-200 nm). In this study, the total protein concentration influences the nanofabrication and not only the reductase enzymes as originally thought. Powder X-ray diffraction studies confirm the crystalline nature of the gold nanoparticles. Fourier Transform Infra Red spectroscopy and thermal gravimetric analysis suggests that the biosynthesized gold nanoparticles are capped by peptides/proteins. Dispersion experiments indicate a stable dispersion of gold nanoparticles in pH 12 solutions which is also confirmed by electron microscopic analysis and validated using a surface plasmon resonance assay. The effectiveness of the dispersed nanoparticles for the reduction of 4-nitrophenol using sodium borohydride as a reductant further confirms the formation of functional gold nanoparticles. It is also reported that gold nanoparticles with mean particle diameter of 27 nm are biosynthesized inside the whole cell by transmission electron microscopy analysis. With optimized reaction conditions, maximum gold bioaccumulation with the 24 h culture age of the yeast with cellular uptake of ~1010 gold atoms at the single cell level is achieved but it is not easy to extract the gold nanoparticles from the whole resting cells.

Value added phytoremediation of metal stressed soils using phosphate solubilizing microbial consortium

The presence of heavy metals in the soil is a matter of growing concern due to their toxic and non-biodegradable nature. Lack of effectiveness of various conventional methods due to economic and technical constraints resulted in the search for an eco-friendly and cost-effective biological techniques for heavy metal removal from the environment. Until now, phytoremediation has emerged as an innovative technique to address the problem. However, the efficiency of phytoremediation process is hindered under the high metal concentration conditions. Hence, phosphate solubilizing microbes (PSM) assisted phytoremediation technique is gaining more insight as it can reduce the contamination load even under elevated metal stressed conditions. These microbes convert heavy metals into soluble and bioavailable forms, which consequently facilitate phytoremediation. Several studies have reported that the use of microbial consortium for remediation is considered more effective as compared to single strain pure culture. Therefore, this review paper focuses on the current trends in research related to PSM mediated uptake of heavy metal by plants. The efficiency of PSM consortia in enhancing the phytoremediation process has also been reviewed. Moreover, the role of phosphatase enzymes in the mineralization of organic forms of phosphate in soil is further discussed. Biosurfactant mediated bioremediation of metal polluted soils is a matter of extensive research nowadays. Hence, the recent advancement of using biosurfactants in enhanced phytoremediation of metal stressed soils is also described.

Remediation of metalliferous mines, revegetation challenges and emerging prospects in semi-arid and arid conditions

Understanding plant behaviour in polluted soils is critical for the sustainable remediation of metal-polluted sites including abandoned mines. Post-operational and abandoned metal mines particularly in semi-arid and arid zones are one of the major sources of pollution by soil erosion or plant hyperaccumulation bringing ecological impacts. We have selected from the literature 157 species belonging to 50 families to present a global overview of 'plants under action' against heavy metal pollution. Generally, all species of plants that are drought, salt and metal tolerant are candidates of interest to deal with harsh environmental conditions, particularly at semi-arid and arid mine sites. Pioneer metallophytes namely Atriplex nummularia, Atriplex semibaccata, Salsola kali, Phragmites australis and Medicago sativa, representing the taxonomic orders Caryophyllales, Poales and Fabales are evaluated in terms of phytoremediation in this review. Phytoremediation processes, microbial and algal bioremediation, the use and implication of tissue culture and biotechnology are critically examined. Overall, an integration of available remediation plant-based technologies, referred to here as 'integrated remediation technology,' is proposed to be one of the possible ways ahead to effectively address problems of toxic heavy metal pollution. Graphical abstract Integrated remediation technology (IRT) in metal-contaminated semi-arid and arid conditions. The hexagonal red line represents an IRT concept based on remediation decisions by combination of plants and microbial processes.

Maize (Zea mays L.) performance in organically amended mine site soils

Organic amendments play an important role in the eco-friendly remediation of degraded mine site soils. This study investigated the quality (essential nutrients and heavy metal content) of maize grown on organically amended soils from three active mines in Nigeria. Soil samples were collected randomly at 0-15 cm depth, air-dried and sieved. Five kg of soil were amended with poultry manure and sawdust (poultry manure only, sawdust only, poultry manure-sawdust mixtures in 3:1, 2:1 and 1:1 ratios) at 10 g kg(-1). Maize (Zea mays L.) seeds were planted and watered for two consecutive periods of 8 weeks, with the control and treatment experiments set up in the screenhouse in quadruples. Harvested tissues were weighed, dried, ground and digested. Chemical properties were determined using standard methods while atomic absorption spectrophotometry was used to determine total metal concentrations (Ca, Mg, Fe, Zn, Pb, Cd and Cu). ANOVA was used to test for significant differences among treatment groups in the various parameters. Application of poultry manure-sawdust mixtures significantly (p < 0.05) enhanced tissue dry matter yield, as well as N, P, K, and Na contents while Zn, Cd, Cu and Pb were immobilized to approximately 50-100%. Treatment with sawdust alone reduced tissue nutrient content resulting in depressed plant yield while poultry manure only though enhanced crop yield, contained higher heavy metal contents. Soil amendments comprised of poultry manure-sawdust mixtures can be effective remediation strategy for mine site soils, as these organic materials help replenish soil nutrients, immobilize heavy metals, and enhance food productivity.

Phytoremediation of iron from red soil of tropical region by using Centella asiatica

The accumulation and removal efficiency of Fe by Centella asiatica was carried out at various Fe concentrations in soil treatments (0, 50, 100, 150 and 200 mg Fe/kg soil). Iron accumulation in different parts of C. asiatica (leaf, stem and root) was analyzed by atomic absorption spectrophotometer (AAS). Factorial experiment with a completely randomized design and Duncan's test were used for data analyses. The results revealed that C. asiatica have the ability to uptake and accumulate Fe significantly (p < 0.05; r  =  0.977) in the aerial parts. The different soil treatments had significant effect on the total Fe accumulations in C. asiatica (p < 0.05). The potential of C. asiatica as a metal hyperaccumulator plant, harvested for analysis, shows efficient accumulation of Fe at high concentration (p < 0.05; r  =  0.977). The root showed the highest accumulation of Fe followed by the leaves (p < 0.05) and the stem (p < 0.05). The Pearson correlation coefficient between leaves and root have showed highly significant correlation (p < 0.01; r  =  0.785) as compared to the leaves and stem (p < 0.01; r  =  0.780). The efficiency of Fe removal by C. asiatica from the contaminated soil has been evaluated by bioconcentration factor and translocation factor, found to be >1 and <1, respectively, further supporting its metal hyperaccumulator properties.

Heavy metals in aquatic organisms of different trophic levels and their potential human health risk in Bohai Bay, China

Fourteen aquatic organism samples were collected from Bohai Bay, and concentrations of five heavy metals were measured to evaluate the pollution levels in aquatic organisms and the potential risk to human health. The concentrations of Zn and Cu were much higher than those of Cd, Cr, and Pb in all the organisms. In general, the heavy metal concentration levels were in the order phytoplankton < zooplankton < fish < shrimp < shellfish. Heavy metal concentrations in higher trophic-level aquatic organisms in Bohai Bay were compared to those in the organisms from other worldwide coastal waters. The concentration levels of most heavy metals were higher than the 75th percentile, except that Pb concentration was between the 25th and 50th percentiles. The calculated bioconcentration factors (BCF) of Cr, Cu, and Pb for phytoplankton were less than 100, indicating no accumulation in primary producers. The bioaccumulation factor (BAF) of Pb for zooplankton was the highest, indicating significant Pb accumulation in zooplankton. For higher trophic-level aquatic organisms, the order of BAF values was fish < shrimp < shellfish for most metals except for Pb. The human health risk assessment suggests that strict abatement measures of heavy metals must be taken to decrease the health risk caused by consuming aquatic products.

Bioremediation of lead contaminated soil with Rhodobacter sphaeroides

Bioremediation with microorganisms is a promising technique for heavy metal contaminated soil. Rhodobacter sphaeroides was previously isolated from oil field injection water and used for bioremediation of lead (Pb) contaminated soil in the present study. Based on the investigation of the optimum culturing conditions and the tolerance to Pb, we employed the microorganism for the remediation of Pb contaminated soil simulated at different contamination levels. It was found that the optimum temperature, pH, and inoculum size for R. sphaeroides is 30-35 °C, 7, and 2 × 10(8) mL(-1), respectively. Rhodobacter sphaeroides did not remove the Pb from soil but did change its speciation. During the bioremediation process, more available fractions were transformed to less accessible and inert fractions; in particular, the exchangeable phase was dramatically decreased while the residual phase was substantially increased. A wheat seedling growing experiment showed that Pb phytoavailability was reduced in amended soils. Results inferred that the main mechanism by which R. sphaeroides treats Pb contaminated soil is the precipitation formation of inert compounds, including lead sulfate and lead sulfide. Although the Pb bioremediation efficiency on wheat was not very high (14.78% root and 24.01% in leaf), R. sphaeroides remains a promising alternative for Pb remediation in contaminated soil.

Treatment of petroleum drill cuttings using bioaugmentation and biostimulation supplemented with phytoremediation

This study sought to compare the effectiveness of bioaugmentation and biostimulation, as well as the combination of both techniques, supplemented with phytoremediation, in the decontamination of petroleum drill cuttings. Drill cuttings with relatively low concentration of total petroleum hydrocarbons (TPH) and metals were mixed with soil in the ratio 5:1 and treated with three different combinations of the bioremediation options. Option A entailed bioaugmentation supplemented with phytoremediation. Option B had the combination of biostimulation and bioaugmentation supplemented with phytoremediation. While biostimulation supplemented with phytoremediation was deployed in option C. Option O containing the drill cuttings-soil mixture without treatment served as untreated control. Fertilizer application, tillage and watering were used for biostimulation treatment, while spent mushroom substrate (Pleurotus ostreatus) and elephant grass (Pennisetum purpureum) were employed for bioaugmentation and phytoremediation treatment, respectively. The drill cuttings-soil mixtures were monitored for TPH, organic carbon, total nitrogen, pH, metal concentrations, and fungal counts, over time. After 56 days of treatment, there was a decline in the initial TPH concentration of 4,114 mg kg(-1) by 5.5%, 68.3%, 75.6% and 48% in options O, A, B and C, respectively. Generally, higher TPH loss resulted from the phytoremediation treatment stage. The treated options also showed slight reductions in metal concentrations ranging from 0% to 16% of the initial low concentrations. The results highlight the effectiveness of bioaugmentation supplemented with phytoremediation. The combination of bioaugmentation and biostimulation supplemented with phytoremediation, however, may prove better in decontaminating petroleum drill cuttings to environmentally benign levels.