Mercury bioremediation by mercury accumulating Enterobacter sp. cells and its alginate immobilized application

Clay-polymer hybrid hydrogels in the vanguard of technological innovations for bioremediation, metal biorecovery, and diverse applications

Polymeric hydrogels are among the most studied materials due to their exceptional properties for many applications. In addition to organic and inorganic-based hydrogels, "hybrid hydrogels" have been gaining significant relevance in recent years due to their enhanced mechanical properties and a broader range of functionalities while maintaining good biocompatibility. In this sense, the addition of micro- and nanoscale clay particles seems promising for improving the physical, chemical, and biological properties of hydrogels. Nanoclays can contribute to the physical cross-linking of polymers, enhancing their mechanical strength and their swelling and biocompatibility properties. Nowadays, they are being investigated for their potential use in a wide range of applications, including medicine, industry, and environmental decontamination. The use of microorganisms for the decontamination of environments impacted by toxic compounds, known as bioremediation, represents one of the most promising approaches to address global pollution. The immobilization of microorganisms in polymeric hydrogel matrices is an attractive procedure that can offer several advantages, such as improving the preservation of cellular integrity, and facilitating cell separation, recovery, and transport. Cell immobilization also facilitates the biorecovery of critical materials from wastes within the framework of the circular economy. The present work aims to present an up-to-date overview on the different "hybrid hydrogels" used to date for bioremediation of toxic metals and recovery of critical materials, among other applications, highlighting possible drawbacks and gaps in research. This will provide the latest trends and advancements in the field and contribute to search for effective bioremediation strategies and critical materials recovery technologies.

Exploring the origins and cleanup of mercury contamination: a comprehensive review

Mercury is a global pollutant that poses significant risks to human health and the environment. Natural sources of mercury include volcanic eruptions, while anthropogenic sources include industrial processes, artisanal and small-scale gold mining, and fossil fuel combustion. Contamination can arise through various pathways, such as atmospheric deposition, water and soil contamination, bioaccumulation, and biomagnification in food chains. Various remediation strategies, including phytoremediation, bioremediation, chemical oxidation/reduction, and adsorption, have been developed to address mercury pollution, including physical, chemical, and biological approaches. The effectiveness of remediation techniques depends on the nature and extent of contamination and site-specific conditions. This review discusses the challenges associated with mercury pollution and remediation, including the need for effective monitoring and management strategies. Overall, this review offers a comprehensive understanding of mercury contamination and the range of remediation techniques available to mitigate its adverse impacts.

Biofilm formation by selected microbial strains isolated from wastewater and their consortia: mercury resistance and removal potential

Wastewater often contains an increased amount of mercury and, at the same time, resistant microorganisms. During wastewater treatment, a biofilm of indigenous microorganisms is often unavoidable. Therefore, the objective of this research is to isolate and identify microorganisms from wastewater and investigate their ability to form biofilms for possible application in mercury removal processes. The resistance of planktonic cells and their biofilms to the effects of mercury was investigated using Minimum Biofilm Eradication Concentration-High Throughput Plates. The formation of biofilms and the degree of resistance to mercury were confirmed in polystyrene microtiter plates with 96 wells. Biofilm on AMB Media carriers (Assisting Moving Bad Media) was quantified using the Bradford protein assay. The removal of mercury ions by biofilms formed on AMB Media carriers of selected isolates and their consortia was determined by a removal test in Erlenmeyer flasks simulating MBBR. All isolates in planktonic form showed some degree of resistance to mercury. The most resistant microorganisms (Enterobacter cloacae, Klebsiella oxytoca, Serratia odorifera, and Saccharomyces cerevisiae) were tested for their ability to form biofilms in the presence and absence of mercury, both in polystyrene plates and on ABM carriers. The results showed that among planktonic forms, K. oxytoca was the most resistant. A biofilm of the same microorganisms was more than 10-fold resistant. Most consortia biofilms had MBEC values > 100,000 μg/mL. Among individual biofilms, E. cloacae showed the highest mercury removal efficiency (97.81% for 10 days). Biofilm consortia composed of three species showed the best ability to remove mercury (96.64%-99.03% for 10 days). This study points to the importance of consortia of different types of wastewater microorganisms in the form of biofilms and suggests that they can be used to remove mercury in wastewater treatment bioreactors.

Bioremediation: An effective approach of mercury removal from the aqueous solutions

Mercury (Hg(II)) is the 16th rarest element present in the earth's crust. Due to rapid industrialization and urban expansions, the mercury concentration has been elevated in the environment. Hg(II) contamination in the aqueous environment has become a great challenge for human beings. The main source of Hg(II) in the aqueous phase is untreated effluent industries (such as the paper industry). Hg(II) is non-biodegradable in nature and even its trace amount in an aqueous environment can pose chronic threats among the humans (damage to the central nervous system, respiratory system, and cardiovascular system, mutation of DNA), animals, and aquatic creatures. Therefore, the removal of mercury from aqueous solutions is an urgent need of the modern era. The conventional techniques such as ion exchange, precipitation, membrane filtrations are costly and also generate byproducts in the environment. Bioremediation is a sustainable, environmentally sound, and cost-effective technique to remove Hg(II) from the aqueous solutions. In this process, naturally occurring microorganisms are utilized to remove the Hg(II) from the aqueous solutions. Lentinus edodes, U. lactuca, and Typha domingensis are found to have great potential to remove mercury from water ranged from ~100 mg g^-1 to 337 mg g^-1.

Unraveling the Underlying Heavy Metal Detoxification Mechanisms of Bacillus Species

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

Mercury alters the rhizobacterial community in Brazilian wetlands and it can be bioremediated by the plant-bacteria association

This study examined how soil mercury contamination affected the structure and functionality of rhizobacteria communities from Aeschynomene fluminensis and Polygonum acuminatum and how rhizobacteria mediate metal bioremediation. The strains were isolated using culture-dependent methods, identified through 16S rDNA gene sequencing, and characterized with respect to their functional traits related to plant growth promotion and resistance to metals and antibiotics. The bioremediation capacity of the rhizobacteria was determined in greenhouse using corn plants. The isolated bacteria belonged to the phyla Actinobacteria, Deinococcus-Thermus, Firmicutes, and Proteobacteria, with great abundance of the species Microbacterium trichothecenolyticum. The rhizobacteria abundance, richness, and diversity were greater in mercury-contaminated soils. Bacteria isolated from contaminated environments had higher minimum inhibitory concentration values, presented plasmids and the merA gene, and were multi-resistant to metals and antibiotics. Enterobacter sp._C35 and M. trichothecenolyticum_C34 significantly improved (Dunnett's test, p < 0.05) corn plant growth in mercury-contaminated soil. These bacteria helped to reduce up to 87% of the mercury content in the soil, and increased the mercury bioaccumulation factor by up to 94%. Mercury bioremediation mitigated toxicity of the contaminated substrate. Enterobacter sp._C35, Bacillus megaterium_C28, and Bacillus mycoides_C1 stimulated corn plant growth and could be added to biofertilizers produced in research and related industries.

A selective and easily recyclable dimer based on a calix[4]pyrrole derivative for the removal of mercury(ii) from water

A recyclable mercury(ii) selective dimer based on a calix[4]pyrrole derivative has been synthesised and characterised by mass and FT-IR spectrometry, Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray (EDX). Information regarding the ability of the dimer to interact with metal cations was obtained from FTIR and SEM-EDX analyses. A striking feature of micrographs of the loaded dimer is the change of morphology with the cation. Based on these results, optimal conditions for removing cations from water were assessed under different experimental conditions. Results obtained demonstrate that the removal process is fast. Capacity values and selectivity factors show that the dimer is selective for Hg(ii) in single and multiple component metal solutions relative to other cations. Single-ion transfer Gibbs energies from water to a solvent containing common functionalities to those of the dimer were used to assess the counter-ion effect on the removal process. Agreement is found between these data and energy calculations derived from molecular simulation studies. Studies on polluted water in the presence of normal water components in addition to toxic metal cations are reported. Further experimental work on wastewater from the mining industry is in progress.

Effects of Pseudomonas alkylphenolica KL28 on immobilization of Hg in soil and accumulation of Hg in cultivated plant

OBJECTIVE

The available content of mercury (Hg) in farmland soil is directly related to the safety of agricultural products. Meanwhile, humans may accumulate high concentrations of Hg through the food chain, resulting in health damage. Regarding the remediation technologies of Hg-contaminated soil, research and development is mainly concentrated on the immobilisation of Hg in soil and efficient extraction by accumulators. Therefore, in this work, the highly Hg-tolerant strain Pseudomonas alkylphenolica KL28 was used to study the removal effect of Hg in a solution, immobilization effect of Hg in soil, and its effect on growth, Hg accumulation and photosynthetic characteristics of Brassica campestris L.

RESULTS

KL28 could effectively remove Hg²⁺ in the solution, with the removal ratio of 96.0% at 24 h. This strain could reduce decreases in shoots' and roots' dry weights by 31% and 16%, respectively, at a Hg concentration of 20 mg/L. The available Hg in the soil decreased to 4.7-9.4% in 8 days treated with KL28 bacterial solution at a dosage of 100 L/hm². Meanwhile, with increases in Hg concentrations, Fv/Fm, Y(II), Y(I) and Y(NPQ) in the leaves of B. campestris showed a downward trend while Y(ND) and Y(NO) displayed an upward trend. Under the stress of 20 mg/L Hg²⁺, KL28 could reduce the Fv/Fm from 11.2 to 6.1%.

CONCLUSIONS

KL28 could effectively remove Hg in the solution, immobilize Hg in soil, promote growth, decrease Hg accumulation and affect photosynthetic characteristics of B. campestris, indicating its potential use in Hg contaminated soils.

Multiple-pathway remediation of mercury contamination by a versatile selenite-reducing bacterium

Mercury contamination is a global concern because of its high toxicity, persistence, bioaccumulative nature, long distance transport and wide distribution in the environment. In this study, the efficiency and multiple-pathway remediation mechanisms of Hg²⁺ by a selenite reducing Escherichia coli was assessed. E. coli can reduce Hg²⁺ to Hg⁺ and Hg⁰ and selenite to selenide at the same time. This makes a multiple-pathway mechanisms for removal of Hg²⁺ from water in addition to biosorption. It was found that when the original Hg²⁺ concentration was 40μgL^-1, 93.2±2.8% of Hg²⁺ was removed from solution by E. coli. Of the total Hg removed, it was found that 3.3±0.1% was adsorbed to the bacterium, 2.0±0.5% was bioaccumulated, and 7.3±0.6% was volatilized into the ambient environment, and most (80.6±5.7%) Hg was removed as HgSe and HgCl precipitates and Hg⁰. On one hand, selenite is reduced to selenide and the latter further reacts with Hg²⁺ to form HgSe precipitates. On the other hand Hg²⁺ is successively reduced to Hg⁺, which forms solid HgCl, and Hg⁰. This is the report on bacterially transformation of Hg²⁺ to HgSe, HgCl and Hg⁰ via multiple pathways. It is suggested that E. coli or other selenite reducing microorganisms are promising candidates for mercury bioremediation of contaminated wastewaters, as well as simultaneous removal of Hg²⁺ and selenite.

Media optimization for economic succinic acid production by Enterobacter sp. LU1

Enterobacter sp. LU1 could efficiently convert glycerol to succinic acid under anaerobic conditions after the addition of lactose. In this study, media constituents affecting both Enterobacter sp. LU1 biomass and succinic acid production were investigated employing response surface methodology (RSM) with central composite design. Statistical methods led to the development of an efficient and inexpensive microbiological media based on crude glycerol, whey permeate as carbon sources and urea as a nitrogen source. The optimized production of bacterial biomass in aerobic conditions was predicted and the interactive effects between crude glycerol, urea and magnesium sulfate were investigated. As a result, a model for predicting the concentration of bacterial biocatalyst biomass was developed with crude glycerol as a sole carbon source. In addition, it was observed that the interactive effect between crude glycerol and urea was statistically significant. Response surface methodology was also employed to develop the model for predicting the concentration of succinic acid produced. Validity of the model was confirmed during verification experiments wherein actual results differed from predicted values by 0.77%. The applied statistical methods proved the feasibility for anaerobic succinic acid production on crude glycerol without expensive yeast extract addition. In conclusion, the RSM method can provide valuable information for succinic acid scale-up fermentation using Enterobacter sp. LU1.

Optimization of microbial detoxification for an aquatic mercury-contaminated environment

Mercury (Hg) reduction performed by microorganisms is well recognized as a biological means for remediation of contaminated environment. Recently, studies demonstrated that Hg-resistant microorganisms of Tagus Estuary are involved in metal reduction processes. In the present study, aerobic microbial community isolated from a highly Hg-contaminated area of Tagus Estuary was used to determine the optimization of the reduction process in conditions such as the contaminated ecosystem. Factorial design methodology was employed to examine the influence of glucose, sulfate, iron, and chloride on Hg reduction. In the presence of several concentrations of these elements, microbial community reduced Hg in a range of 37-61% of the initial 0.1 mg/ml Hg²⁺ levels. The response prediction through central composite design showed that the increase of sulfate concentration led to an optimal response in Hg reduction by microbial community, while the rise in chloride levels markedly decreased metal reduction. Iron may exert antagonistic effects depending upon the media composition. These results are useful in understanding the persistence of Hg contamination in Tagus Estuary after inactivation of critical industrial units, as well as data might also be beneficial for development of new bioremediation strategies either in Tagus Estuary and/or in other Hg-contaminated aquatic environments.

Screening of mercury-resistant and indole-3-acetic acid producing bacterial-consortium for growth promotion of Cicer arietinum L

Mercury resistant (Hg^R ) bacteria were screened from industrial effluents and effluents-polluted rhizosphere soils near to districts Kasur and Sheikhupura, Pakistan. Out of 60 isolates, three bacterial strains, Bacillus sp. AZ-1, Bacillus cereus AZ-2, and Enterobacter cloacae AZ-3 showed Hg-resistance as 20 μg ml^-1 of HgCl₂ and indole-3-acetic acid (IAA) production as 8-38 μg ml^-1 . Biochemical and molecular characterization of selected bacteria was confirmed by 16S ribotyping. Mercury resistant genes merA, merB, and merE of mer operon in Bacillus spp. were checked by PCR amplification. The merE gene involved in the transportation of elemental mercury (Hg⁰ ) via cell membrane was first time cloned into pHLV vector and transformed in C43(DE3) Escherichia coli cells. The recombinant plasmid (pHLMerE) was expressed and purified by nickel (Ni⁺² ) affinity chromatography. Chromatographic techniques viz. thin layer chromatography (TLC), high performance liquid chromatography (HPLC), and Gas chromatography-mass spectrometry (GC-MS) confirmed the presence of Indole-3-acetic acid (IAA) in supernatant of selected bacteria. The strain E. cloacae AZ-3 detoxified 88% of mercury (Hg⁺² ) from industrial effluent (p < 0.05) after immobilization in Na-alginate beads. Finally, Hg-resistant and IAA producing bacterial consortium of two strains, Bacillus sp. AZ-1 and E. cloacae AZ-3, inoculated in mercury amended soil with 20 μg ml^-1 HgCl₂ resulted 80, 22, 64, 116, 50, 75, 30, and 100% increase as compared to control plants in seed germination, shoot and root length, shoot and root fresh weight, number of pods per plant, number of seeds and weight of seeds, respectively, of chickpea (Cicer arietinum L.) in pot experiments (p < 0.05).

Detoxification of mercury pollutant leached from spent fluorescent lamps using bacterial strains

The spent fluorescent lamps (SFLs) are being classified as a hazardous waste due to having mercury as one of its main components. Mercury is considered the second most toxic heavy metal (arsenic is the first) with harmful effects on animal nervous system as it causes different neurological disorders. In this research, the mercury from phosphor powder was leached, then bioremediated using bacterial strains isolated from Qatari environment. Leaching of mercury was carried out with nitric and hydrochloric acid solutions using two approaches: leaching at ambient conditions and microwave-assisted leaching. The results obtained from this research showed that microwave-assisted leaching method was significantly better in leaching mercury than the acid leaching where the mercury leaching efficiency reached 76.4%. For mercury bio-uptake, twenty bacterial strains (previously isolated and purified from petroleum oil contaminated soils) were sub-cultured on Luria Bertani (LB) plates with mercury chloride to check the bacterial tolerance to mercury. Seven of these twenty strains showed a degree of tolerance to mercury. The bio-uptake capacities of the promising strains were investigated using the mercury leached from the fluorescent lamps. Three of the strains (Enterobacter helveticus, Citrobacter amalonaticus, and Cronobacter muytjensii) showed bio-uptake efficiency ranged from 28.8% to 63.6%.

Engineering tailored nanoparticles with microbes: quo vadis?

In the quest for less toxic and cleaner methods of nanomaterials production, recent developments in the biosynthesis of nanoparticles have underscored the important role of microorganisms. Their intrinsic ability to withstand variable extremes of temperature, pressure, and pH coupled with the minimal downstream processing requirements provide an attractive route for diverse applications. Yet, controlling the dispersity and facile tuning of the morphology of the nanoparticles of desired chemical compositions remains an ongoing challenge. In this Focus Review, we critically review the advances in nanoparticle synthesis using microbes, ranging from bacteria and fungi to viruses, and discuss new insights into the cellular mechanisms of such formation that may, in the near future, allow complete control over particle morphology and functionalization. In addition to serving as paradigms for cost-effective, biocompatible, and eco-friendly synthesis, microbes hold the promise for a unique template for synthesis of tailored nanoparticles targeted at therapeutic and diagnostic platform technologies.

Sources and remediation techniques for mercury contaminated soil

Mercury (Hg) in soils has increased by a factor of 3 to 10 in recent times mainly due to combustion of fossil fuels combined with long-range atmospheric transport processes. Other sources as chlor-alkali plants, gold mining and cement production can also be significant, at least locally. This paper summarizes the natural and anthropogenic sources that have contributed to the increase of Hg concentration in soil and reviews major remediation techniques and their applications to control soil Hg contamination. The focus is on soil washing, stabilisation/solidification, thermal treatment and biological techniques; but also the factors that influence Hg mobilisation in soil and therefore are crucial for evaluating and optimizing remediation techniques are discussed. Further research on bioremediation is encouraged and future study should focus on the implementation of different remediation techniques under field conditions.