Removal of hexavalent chromium from wastewater using magnetotactic bacteria

Advanced materials for micro/nanorobotics

Autonomous micro/nanorobots capable of performing programmed missions are at the forefront of next-generation micromachinery. These small robotic systems are predominantly constructed using functional components sourced from micro- and nanoscale materials; therefore, combining them with various advanced materials represents a pivotal direction toward achieving a higher level of intelligence and multifunctionality. This review provides a comprehensive overview of advanced materials for innovative micro/nanorobotics, focusing on the five families of materials that have witnessed the most rapid advancements over the last decade: two-dimensional materials, metal-organic frameworks, semiconductors, polymers, and biological cells. Their unique physicochemical, mechanical, optical, and biological properties have been integrated into micro/nanorobots to achieve greater maneuverability, programmability, intelligence, and multifunctionality in collective behaviors. The design and fabrication methods for hybrid robotic systems are discussed based on the material categories. In addition, their promising potential for powering motion and/or (multi-)functionality is described and the fundamental principles underlying them are explained. Finally, their extensive use in a variety of applications, including environmental remediation, (bio)sensing, therapeutics, etc., and remaining challenges and perspectives for future research are discussed.

Bioremediation of hazardous heavy metals by marine microorganisms: a recent review

Heavy metals (HMs) like Zn, Cu, Pb, Ni, Cd, and Hg, among others, play a role in several environmental problems. The marine environment is polluted by several contaminants, such as HMs. A variety of physico-chemical methods usually available for sanitation HMs remediation suffer from either limitation. Bioremediation is a promising way of dealing with HMs pollution. Microbes have the ability with various potencies to resist HMs tension. The current review discusses the main sources and influences of HMs, the role of marine microorganisms in HMs bioremediation, as well as the microbial mechanisms for HMs detoxification and transformation. This review paper aims to provide an overview of the bioremediation technologies that are currently available for the removal of HMs ions from industrial and urban effluent by aquatic organisms such as bacteria, fungi, and microalgae, particularly those that are isolated from marine areas. The primary goals are to outline various studies and offer helpful information about the most important aspects of the bioelimination techniques. The biotreatment practices have been primarily divided into three techniques based on this topic. They are biosorption, bioaccumulation, bioleaching, and biotransformation. This article gives the brief view on the research studies about bioremediation of HMs using marine microorganisms. The current review also deals with the critical issues and recent studies based on the HMs biodetoxification using aquatic microorganisms.

Symbiotic hemolymph bacteria reduce hexavalent chromium to protect the host from chromium toxicity in Procambarus clarkii

Hexavalent chromium (Cr(VI)) is a cytotoxic heavy metal pollutant that adversely affects all life forms. Interestingly, the crustacean Procambarus clarkii exhibits a relatively high tolerance to heavy metals. The underlying mechanisms remain unclear. In this study, we investigated the role of symbiotic bacteria in P. clarkii in alleviating Cr(VI)-induced damage and explored their potential mechanisms of action. Through transcriptomic analysis, we observed that Cr(VI) activated P. clarkii's antimicrobial immune responses and altered the bacterial composition in the hemolymph. After antibiotic treatment to reduce bacterial populations, Cr(VI)-induced intestinal and liver damage worsened, and crayfish exhibited lower levels of GSH/CAT/SOD activity. The Exiguobacterium, the symbiotic bacteria in the hemolymph of P. clarkii, were proved to be primary contributor to Cr(VI) tolerance. Further investigation suggested that it resists Cr(VI) through the activation of the ABC transporter system and the reduction of Cr(VI) via the reductase gene nfsA. To validate the role of Exiguobacterium in Cr(VI) tolerance, crayfish treated with antibiotics then supplemented with Exiguobacterium H6 and recombinant E. coli (with the nfsA gene), reduced Cr(VI)-induced ovarian damage. Overall, this study revealed that the symbiotic bacteria Exiguobacterium can absorb and reduce hexavalent chromium, mitigating Cr(VI)-induced damage in P. clarkii. These findings provide new insights into hexavalent chromium tolerance mechanisms in crustaceans.

Hexavalent chromate bioreduction by a magnetotactic bacterium Magnetospirillum gryphiswaldense MSR-1 and the effect of magnetosome synthesis

Magnetotactic bacteria (MTB) are receiving attention for heavy metal biotreatment due to their potential for biosorption with heavy metals and the capability of the magnetic recovery. In this study, we investigated the characteristics of Cr(VI) bioreduction and biosorption by an MTB isolate, Magnetospirillum gryphiswaldense MSR-1, which has a higher growth rate and wider reflexivity in culture conditions. Our results demonstrated that the MSR-1 strain could remove Cr(VI) up to the concentration of 40 mg L^-1 and with an optimal activity at neutral pH conditions. The magnetosome synthesis existed regulatory mechanisms between Cr(VI) reduction and cell division. The addition of 10 mg L^-1 Cr(VI) significantly inhibited cell growth, but the magnetosome-deficient strain, B17316, showed an average specific growth rate of 0.062 h^-1 at the same dosage. Cr(VI) reduction examined by the heat-inactivated and resting cells demonstrated that the main mechanism for MSR-1 strain to reduce Cr(VI) was chromate reductase and adsorption, and magnetosome synthesis would enhance the chromate reductase activity. Finally, our results elucidated that the chromate reductase distributes diversely in multiple subcellular components of the MSR-1 cells, including extracellular, membrane-associated, and intracellular cytoplasmic activity; and expression of the membrane-associated chromate reductase was increased after the cells were pre-exposed by Cr(VI).

Application of multi-gene genetic programming technique for modeling and optimization of phycoremediation of Cr(VI) from wastewater

Background

Removal of Cr(VI) from wastewater is essential as it is potentially toxic and carcinogenic in nature. Bioremediation of heavy metals using microalgae is a novel technique and has several advantages such as microalgae remove metals in an environmentally friendly and economic manner. The present study deals with modeling and optimization of the phycoremediation of Cr(VI) from synthetic wastewater. The initial concentration of Cr(VI), initial pH, and inoculum size were considered as input factors, and the percentage removal of Cr(VI) was chosen as a response.

Results

An accurate data-driven genetic programming model was developed with the experimental data of other scientists to find a relation between the percentage removal of Cr(VI) and all input parameters. To maximize the removal of Cr(VI), the grey wolf optimization technique was applied to determine the optimal values of input parameters.

Conclusion

These optimum input parameters are difficult to get through experimentation using the trial-and-error method. The established modelling and optimization technique is generic and can be applied to any other experimental study.

Graphical Abstract

Sustainable enhancement of Cr(VI) bioreduction by the isolated Cr(VI)-resistant bacteria

Bioreduction of mobile Cr(VI) to sparingly soluble Cr(III) is an effective strategy for in situ remediations of Cr contaminated sites. The key of this technology is to screen Cr(VI)-resistant bacteria and further explore the sustainable enhancement approaches towards their Cr(VI) reduction performance. In this study, a total of ten Cr(VI)-resistant bacteria were isolated from a Cr(VI) contaminated site. All of them could reduce Cr(VI), and the greatest extent of Cr(VI) reduction (98%) was obtained by the isolated CRB6 strain. The isolated CRB6 was able to reduce structural Fe(III) in Nontronite NAu-2 to structural Fe(II). Compared with the slow bioreduction process, the produced structural Fe(II) can rapidly enhance Cr(VI) reduction. The resist dissolution characteristics of NAu-2 in the redox cycling may provide sustainable enhancement of Cr(VI) reduction. However, no enhancement on Cr(VI) bioreduction by the isolated CRB6 was observed in the presence of NAu-2, which was attributed to the inhibition of Cr(VI) on the electron transfer between the isolated CRB6 and NAu-2. AQDS can accelerate the electron transfer between the isolated CRB6 and NAu-2 as an electron shuttle in the presence of Cr(VI). Therefore, the combination of NAu-2 and AQDS generated a synergistic enhancement on Cr(VI) bioreduction compared with the enhancement obtained by NAu-2 and AQDS individually. Our results highlight that structural Fe(III) and electron shuttle can provide a sustainable enhancement of Cr(VI) reduction by Cr(VI)-reducing bacteria, which has great potential for the effective Cr(VI) in-situ remediation.

Bioaccumulation of Fluoride in Plants and Its Microbially Assisted Remediation: A Review of Biological Processes and Technological Performance

Fluoride is widely found in soil–water systems due to anthropogenic and geogenic activities that affect millions worldwide. Fluoride ingestion results in chronic and acute toxicity, including skeletal and dental fluorosis, neurological damage, and bone softening in humans. Therefore, this review paper summarizes biological processes for fluoride remediation, i.e., bioaccumulation in plants and microbially assisted systems. Bioremediation approaches for fluoride removal have recently gained prominence in removing fluoride ions. Plants are vulnerable to fluoride accumulation in soil, and their growth and development can be negatively affected, even with low fluoride content in the soil. The microbial bioremediation processes involve bioaccumulation, biotransformation, and biosorption. Bacterial, fungal, and algal biomass are ecologically efficient bioremediators. Most bioremediation techniques are laboratory-scale based on contaminated solutions; however, treatment of fluoride-contaminated wastewater at an industrial scale is yet to be investigated. Therefore, this review recommends the practical applicability and sustainability of microbial bioremediation of fluoride in different environments.

Enriched Synthesis of Magnetosomes by Expanding the Magnetospirillum magneticum AMB-1 Culture at Optimal Iron Concentration

The Magnetospirillum magneticum AMB-1 species is one of the most widely used magnetotactic bacterial strains for producing magnetosomes under laboratory conditions. Nevertheless, there exist several challenges in expanding and purifying the AMB-1 culture due to the restricted culture conditions. In an attempt to enrich the production of magnetosomes, this study reports the utilization of fermenter culture, which substantially promotes the cell densities at different concentrations of iron content. The experimental results confirmed magnetosomes’ high yield (production rate of 21.1 mg L−1) at the iron content of 0.2 μmol L−1. Moreover, different characterization techniques systematically confirmed the coated lipid membrane, particle size, dispersity, stability, and elemental composition of magnetosomes. Notably, the fermenter culture-based process resulted in highly discrete, dispersed, and stable magnetosomes with an average particle diameter of 50 nm and presented the integrated lipid membrane around the surface. The chemical composition by EDS of magnetosomes represented the presence of various elements, i.e., C, O, Na, P, and Fe, at appropriate proportions. In conclusion, the culture method in our study effectively provides a promising approach towards the culture of the magnetotactic bacterium for the enriched production of magnetosomes.

Research trends of heavy metal removal from aqueous environments

Heavy metals are a threat against human health. During the last century, with increased industrial activities, many water resources have been contaminated by heavy metals. Meanwhile the number of scientific studies about removing these toxic substances from aqueous environments has increased exponentially. According to bibliometric analysis the number of articles from 2000 to 2019 experienced a 1700% growth rate. China, India and the United States have published the greatest number of top-cited articles on the topic, with China in first place by a large margin. Six clusters of papers (by topic) were identified. From among the processes such as adsorption, membrane filtration, and ion exchange, adsorption has the lion's share of the investigations. Technical and efficiency considerations, as well as environmental impact and cost-effectiveness, were chosen as criteria to compare different methods. According to life cycle assessment, adsorption has the least amount of negative environmental effects compared to other trending methods such as membrane filtration and ion exchange. From a financial viewpoint, utilizing biosorbents and biochars for adsorption are the best options. Unlike other methods which depend on pretreatment processes and have a high energy demand, these sorbents are cost-effective and exhibit acceptable performance.

Selenium(Ⅳ) alleviates chromium(Ⅵ)-induced toxicity in the green alga Chlamydomonas reinhardtii

The wide range of industrial applications of chromium (Cr) has led to an increasing risk of water contamination by Cr(Ⅵ). However, efficient methods to remove or decrease the toxicity of Cr(Ⅵ) in situ are lacking. The main aim of this study was to investigate the mechanisms by which selenite alleviates chromium(Ⅵ)-induced toxicity in Chlamydomonas reinhardtii. Our results showed that K₂Cr₂O₇ had toxic effects on both the structure and physiology of C. reinhardtii in a dose-dependent manner. Adding selenite significantly alleviated chromium accumulation and toxicity in cells. RNA-seq data showed that the expression level of selenoproteins such as SELENOH was significantly increased. Both SELENOH-amiRNA knockdown mutants and selenoh insertional mutant produced more reactive oxygen species (ROS) and grew slower than the wild type, suggesting that SELENOH can reduce chromium toxicity by decreasing the levels of ROS produced by Cr(Ⅵ). We also demonstrated that selenite can reduce the absorption of Cr(Ⅵ) by cells but does not affect the process of Cr(Ⅵ) adsorption and efflux. This information on the molecular mechanism by which selenite alleviates Cr(Ⅵ) toxicity can be used to increase the bioremediation capacity of algae and reduce the human health risks associated with Cr(Ⅵ) toxicity.

Microbial Assisted Hexavalent Chromium Removal in Bioelectrochemical Systems

Groundwater is the environmental matrix that is most frequently affected by anthropogenic hexavalent chromium contamination. Due to its carcinogenicity, Cr(VI) has to be removed, using environmental-friendly and economically sustainable remediation technologies. BioElectrochemical Systems (BESs), applied to bioremediation, thereby offering a promising alternative to traditional bioremediation techniques, without affecting the natural groundwater conditions. Some bacterial families are capable of oxidizing and/or reducing a solid electrode obtaining an energetic advantage for their own growth. In the present study, we assessed the possibility of stimulating bioelectrochemical reduction of Cr(VI) in a dual-chamber polarized system using an electrode as the sole energy source. To develop an electroactive microbial community three electrodes were, at first, inserted into the anodic compartment of a dual-chamber microbial fuel cell, and inoculated with sludge from an anaerobic digester. After a period of acclimation, one electrode was transferred into a polarized system and it was fixed at −0.3 V (versus standard hydrogen electrode, SHE), to promote the reduction of 1000 µg Cr(VI) L−1. A second electrode, served for the set-up of an open circuit control, operated in parallel. Cr(VI) dissolved concentration was analysed at the initial, during the experiment and final time by spectrophotometric method. Initial and final microbial characterization of the communities enriched in polarized system and open circuit control was performed by 16S rRNA gene sequencing. The bioelectrode set at −0.3 V showed high Cr(VI) removal efficiency (up to 93%) and about 150 µg L−1 day−1 removal rate. Similar efficiency was observed in the open circuit (OC) even at about half rate. Whereas, purely electrochemical reduction, limited to 35%, due to neutral operating conditions. These results suggest that bioelectrochemical Cr(VI) removal by polarized electrode offers a promising new and sustainable approach to the treatment of groundwater Cr(VI) plumes, deserving further research.

Removal of iron and manganese in groundwater through magnetotactic bacteria

It is presented an alternative biological method based on biomineralization mechanisms of Magnetotactic Bacteria (MTB) for the removal in groundwater, of soluble elements such as Fe⁺²and Mn⁺². In first place, it was compared the effectiveness of MTB retention methods for obtention of concentrated volumes in microorganisms, then, it was carried out an inoculation process in groundwater samples and evaluate the removal rate of Fe⁺² and Mn⁺² in constant conditions of pH and temperature. It was identified electromagnetic method is more efficient in MTB retention, and that the inoculation processes of an enriched solution with MTB in groundwater samples allow to get average removal rates of 47.86% for Fe⁺² and 15.26% for Mn⁺². In addition, it was evaluated the removal rate of other metals due to magnetic properties of biominerals inside of MTB magnetosome. The highest removal in all cases occurred between the interval of 3 and 5 min of interaction and tended to stabilize in time.

Facile actuation of aqueous droplets on a superhydrophobic surface using magnetotactic bacteria for digital microfluidic applications

Magnetic actuation provides a low-cost, simple method for droplet manipulation on a digital microfluidic platform. The impetus to move the droplets on a low friction surface can come from internal superparamagnetic particles or paramagnetic salts. Recently, the use of microbes for bio-actuation has been established, where the thrust produced by the microbes can be exploited to exert the force required for droplet movement. This study presents biologically-driven magnetic actuation of droplets on a superhydrophobic surface using magnetotactic bacteria (MTB). MTB-droplets were impelled along various trajectories such as rectangular and figure-of-eight-shaped paths. Droplets were reproducibly actuated with speeds up of to 30 mm s^-1. We demonstrated the ability to sequentially merge and mix multiple droplets by merging a 10 μL MTB droplet with two 4 μL colored droplets. The reorientation of MTB in the droplet enhanced mixing rate of the merged fluids by ∼40% compared with the control experiment where no actuation was used. Biologically-driven magnetic actuation was compared with actuation by superparamagnetic particles and paramagnetic salts, in terms of controllability and speed. MTB droplet was moved with the same average speed as other two methods and showed higher response time as the magnet acceleration increased. Lastly, MTB were used to perform a phosphatase assay using endogenous enzyme. The relative absorbance at 405 nm, indicating the production of the yellow product, increased over time and levels off after 75 min.

Magnetosomes: biogenic iron nanoparticles produced by environmental bacteria

The scientific community's interest in magnetotactic bacteria has increased substantially in recent decades. These prokaryotes have the particularity of synthesizing nanomagnets, called magnetosomes. The majority of research is based on several scientific questions. Where do magnetotactic bacteria live, what are their characteristics, and why are they magnetic? What are the molecular phenomena of magnetosome biomineralization and what are the physical characteristics of magnetosomes? In addition to scientific curiosity to better understand these stunning organisms, there are biotechnological opportunities to consider. Magnetotactic bacteria, as well as magnetosomes, are used in medical applications, for example cancer treatment, or in environmental ones, for example bioremediation. In this mini-review, we investigated all the aspects mentioned above and summarized the currently available knowledge.

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.

Enhanced adsorption of hexavalent chromium by a biochar derived from ramie biomass (Boehmeria nivea (L.) Gaud.) modified with β-cyclodextrin/poly(L-glutamic acid)

This paper explored biochar modification to enhance biochar's ability to adsorb hexavalent chromium from aqueous solution. The ramie stem biomass was pyrolyzed and then treated by β-cyclodextrin/poly(L-glutamic acid) which contained plentiful functional groups. The pristine and modified biochar were characterized by FTIR, X-ray photoelectron spectroscopy, specific surface area, and zeta potential measurement. Results indicated that the β-cyclodextrin/poly(L-glutamic acid) was successfully bound to the biochar surface. Batch experiments were conducted to investigate the kinetics, isotherm, thermodynamics, and adsorption/desorption of Cr(VI). Adsorption capacities of CGA-biochar were significantly higher than that of the untreated biochar, and its maximum adsorption capacity could reach up to 197.21 mg/g at pH 2.0. Results also illustrated that sorption performance depended on initial solution pH; in addition, acidic condition was beneficial to the Cr(VI) uptake. Furthermore, the Cr(VI) uptake was significantly affected by the ion strength and cation species. This study demonstrated that CGA-biochar could be a potential adsorbent for Cr(VI) pollution control.

Yield cultivation of magnetotactic bacteria and magnetosomes: A review

Magnetotactic bacteria (MTB) have started to be employed for the biosynthesis of magnetic nanoparticles, due to the rapidly increasing demand for nanoparticles in biomedical, biotechnology and environmental protection. MBT are the group of prokaryotes that have the ability to produce bio-magnetic minerals or bio-magnetic crystals of either magnetite (Fe₃ O₄ ) or greigite (Fe₃ S₄ ) in numerous shapes and size ranges, known as magnetosomes (MS). MS compel MTB to respond to the applied external magnetic field. However, it is extremely difficult to grow MTB and produce high yield of MS under artificial environmental conditions, thus creating a major hurdle to relocate MTB technology from laboratory scale to industrial or commercial level. Therefore, to best of our knowledge this review is the first attempt to highlight existing research developments about the laboratory scale and mass production of MS by MTB. Moreover, the optimum culture media and environmental conditions used for the cultivation of MTB were also considered. Finally, future research is encouraged for the improvement of MS yield which will result in the development of advanced nanotechnology/magnetotechnology.

Cobalt separation by Alphaproteobacterium MTB-KTN90: magnetotactic bacteria in bioremediation

Bioremediation of toxic metals by magnetotactic bacteria and magnetic separation of metal-loaded magnetotactic bacteria are of great interest. This bioprocess technique is rapid, efficient, economical, and environmentally friendly. In this study, cobalt removal potential of a novel isolated magnetotactic bacterium (Alphaproteobacterium MTB-KTN90) as a new biosorbent was investigated. The effects of various environmental parameters in the cobalt removal and the technique of magnetic separation of cobalt-loaded bacterial cells were studied. Cobalt removal by MTB-KTN90 was very sensitive to pH solution; higher biosorption capacity was observed around pH 6.5-7.0. When biomass concentration increased from 0.009 to 0.09 g/l, the biosorption efficiency increased from 13.87 % to 19.22 %. The sorption of cobalt by MTB-KTN90 was rapid during the first 15 min (859.17 mg/g dry weight). With the increasing of cobalt concentrations from 1 to 225 mg/l, the specific cobalt uptake increased. Maximum cobalt removal (1160.51 ± 15.42 mg/g dry weight) took place at optimum conditions; pH 7.0 with initial cobalt concentration of 115 mg/l at 60 min by 0.015 g/l of dry biomass. The results showed maximum values for constants of Langmuir and Freundlich models so far. The biosorption mechanisms were studied with FTIR, PIXE, and FESEM analysis. Cobalt-loaded MTB-KTN90 had ability to separate from solution by a simple magnetic separator. Magnetic response in MTB-KTN90 is due to the presence of unique intracellular magnetic nanoparticles (magnetosomes). The orientation magnetic separation results indicated that 88.55 % of cobalt was removed from solution. Consequently, Alphaproteobacterium MTB-KTN90 as a new biosorbent opens up good opportunities for the magnetic removal of cobalt from the polluted aquatic environments.