Studies of cadmium(II), lead(II), nickel(II), cobalt(II) and chromium(VI) sorption on extracellular polymeric substances produced by Rhodococcus opacus and Rhodococcus rhodochrous

Exploration of an Extracellular Polymeric Substance from Earthworm Gut Bacterium (Bacillus licheniformis) for Bioflocculation and Heavy Metal Removal Potential

The present study shows the potential of an extracellular polymeric substance (EPS) produced by Bacillus licheniformis strain KX657843 isolated from earthworm (Metaphire posthuma) gut in the sorption of Cu(II) and Zn(II) and in flocculation. After harvesting bacterial cells from sucrose supplemented denitrifying culture medium, the EPS was extracted following ethanolic extraction method. The Fourier Transform Infrared Spectroscopy (FTIR) and 1H and 13C Nuclear Magnetic Resonance (NMR) of EPS revealed its functional groups, electronegative constituents, unsaturated carbon, and carbonyl groups. The negatively charged functional groups of carbohydrates and protein moiety of the EPS endowed it with heavy metal binding capacity through electrostatic interactions. The highest flocculation activity (83%) of EPS was observed at 4 mg L−1 and pH 11. The metal sorption by EPS increased with increasing pH. At pH 8, the EPS was able to remove 86 and 81% Cu(II) and Zn(II), respectively, from a 25 mg L−1 metal solution. 94.8% of both the metals at 25 mg L−1 metal solutions were removed by EPS at EPS concentration of 100 mg L−1. From Langmuir isotherm model, the maximum sorption capacities of EPS were calculated to be 58.82 mg g−1 for Cu(II) and 52.45 mg g−1 for Zn(II). The bacterial EPS showed encouraging flocculating and metal sorption properties. The potential to remove Cu(II) and Zn(II) implies that the EPS obtained from the earthworm gut bacteria can be used as an effective agent for environmental remediation of heavy metals and in bioflocculation.

Adaptation mechanisms of Rhodococcus sp. CNS16 under different temperature gradients: Physiological and transcriptome

Rhodococcus exhibits strong adaptability to environmental stressors and plays a crucial role in environmental bioremediation. However, seasonal changes in ambient temperature, especially rapid temperature drops exert an adverse effect on in situ bioremediation. In this paper, we studied the cell morphology and fatty acid composition of an aniline-degrading strain Rhodococcus sp. CNS16 at temperatures of 30 °C, 20 °C, and 10 °C. At suboptimal temperatures, cell morphology of CNS16 changed from short rod-shaped to long rod or irregular shaped, and the proportion of unsaturated fatty acids was upregulated. Transcriptomic technologies were then utilized to gain detailed insights into the adaptive mechanisms of CNS16 subjected to suboptimal temperatures. The results showed that the number of gene responses was significantly higher at 10 °C than that at 20 °C. The inhibition of peptidoglycan synthase expression and up-regulation of Filamentous Temperature Sensitive as well as unsaturated fatty acid synthesis genes at suboptimal temperatures might be closely related to corresponding changes in cell morphology and fatty acids composition. Strain CNS16 showed loss of catalase and superoxide dismutase activity, and utilized thioredoxin-dependent thiol peroxidase to resist oxidative stress. The up-regulation of carotenoid and Vitamin B2 synthesis at 10 °C might also be involved in the resistance to oxidative stress. Amino acid metabolism, coenzyme and vitamin metabolism, ABC transport, and energy metabolism are essential for peptidoglycan synthesis and regulation of cellular metabolism; therefore, synergistically resisting environmental stress. This study provides a mechanistic basis for the regulation of aniline degradation in Rhodococcus sp. CNS16 at low temperatures.

Mechanism and performance of trace metal removal by continuous-flow constructed wetlands coupled with a micro-electric field

Constructed wetlands coupled with a micro-electric field (CW-MEF) is a novel and efficient water treatment technology. The objective of this study was to investigate the mechanism and performance of trace metals (TMs) removal for CW-MEF systems during summer and winter. The mass distribution of TMs in plants and biofilms, physiological indices of wetland plants, and bacterial community structures on electrodes and in the rhizospheres were analyzed as well as to explore further the TM removal mechanism. Results show that the electric field intensities (EFI) of 100 and 200 mV cm^-1 had a significantly promoting effect on TM removal. Maximum removal efficiencies for Cu, Zn, Cd, Co, Ni and Pb were 95.6, 80.1, 74.0, 67.1, 69.8 and 99.6%, respectively, in summer with a 5d-hydraulic retention time (HRT). An EFI of 100 mV cm^-1 could alleviate the oxidative damage in plant cells by promoting the synthesis of reduced glutathione and an activity increase of catalase, thus increasing the phytoextraction for Cu, Zn and Cd. For biofilms, the MEF caused shifts in the bacterial community structures, and an EFI of 50 to 200 mV cm^-1 significantly promoted the enrichment of Cu, Zn, Cd and Co by biofilms. Moreover, microorganisms related to TM tolerance and enrichment exhibited a high abundance with an EFI of 100 and 200 mV cm^-1. It can be concluded that introducing MEF to CWs could intensify the TMs removal via the biological process and result in more efficient purification for TM-containing wastewater.

Characterization and chromium biosorption potential of extruded polymeric substances from Synechococcus mundulus induced by acute dose of gamma irradiation

This study characterized the extruded polymeric substances (EPS) secreted from Synechococcus mundulus cultures under the effect of 2-KGy gamma irradiation dose. The EPS demonstrated seven monosaccharides, two uronic acids and several chemical functional groups: O-H, N-H, =C-H, C=C, C=O, COO-, O-SO₃, C-O-C and a newly formed peak at 1593 cm^-1 (secondary imide). The roughness of EPS was 96.71 nm and only 28.4% total loss in weight was observed at 800 °C with a high degree of crystallinity quantified as CI_DSC (0.722) and CI_XRD (0.718). Preliminary comparative analyses of EPS exhibited high protein content in the radiologically modified (R-EPS) than control (C-EPS). Modified EPS were characterized with a high biosorption efficiency, which could be attributed to its high content of uronic acids, protein and sulphates as well as various saccharide monomers. Data revealed that 0.0213 mg L^-1 h^-1 is the maximum biosorption rate (SBR_max) of Cr(VI) for R-EPS, whereas 0.0204 mg L^-1 h^-1 SBR_max for the C-EPS respectively.

Effects of extracellular polymeric substances of Pseudomonas fluorescens, citrate, and oxalate on Pb sorption by an acidic Ultisol

The continuous production of low molecular weight (LMW) organic acids by plants and microorganisms coupled with the continuous presence of extracellular polymeric substances (EPS) in soils is a guarantee that the mobility of heavy metals in soils will be controlled. The effects of citrate, oxalate, and EPS on the adsorption of Pb by an acidic Ultisol were studied both as a function of pH and ionic strength. Electrokinetic potential measurements were also employed to observe to what extent each ligand affected the surface charge property of the Ultisol. All the ligands shifted the zeta potential of the Ultisol to the negative direction, implying that the surface charge of the soil became more negative. The effect on the zeta potential of the soil was observed in the order of oxalate ˃ citrate ˃ EPS. The quantity of Pb adsorbed at each pH (3.0-7.0) reflected the corresponding change in the zeta potential as induced by each ligand. The presence of the ligands shifted the isoelectric point of the Ultisol from 4.8 to 3.2 for the EPS system and below 3.0 for the citrate and oxalate systems. More Pb was adsorbed in the presence of oxalate than in the presence of citrate and EPS. The two most outstanding mechanisms that governed the adsorption of Pb by the Ultisol were (1) electrostatic attraction which was supported by the increase in negative zeta potential of the Ultisol and, (2) complexation which was supported by the lesser proportion of Pb adsorbed in the citrate system at higher pH and also by the spectroscopic data for EPS. The combination EPS + citrate + oxalate was more effective in enhancing the adsorption of Pb than the combination EPS + oxalate and EPS + citrate.

Mechanism of Cu(II) and Cd(II) immobilization by extracellular polymeric substances (Escherichia coli) on variable charge soils

Extracellular polymeric substances (EPS) found in soils can reduce the mobility of heavy metals through the use of both electrostatic and non-electrostatic mechanisms. Their effects vary from one soil type to another. The influence of EPS from Escherichia coli on the adsorption behaviors of Cu(II) and Cd(II) by two bulk variable charge soils, Oxisol and Ultisol, was studied at constant and varied pH, and the results were compared to a constant charge Alfisol. The maximum adsorption capacities of the soils were significantly (P < 0.05) enhanced in the presence of EPS, with Cu(II) adsorption being greater. Interaction of EPS with soils made the soil surface charge more negative by neutralizing positive charges and shifting the zeta potentials in a negative direction: from -18.6 to -26.4 mV for Alfisol, +5.1 to -22.2 mV for Oxisol, and +0.3 to -28.0 mV for Ultisol at pH 5.0. The adsorption data fitted both the Freundlich and Langmuir isotherms well. Preadsorbed Cd(II) was more easily desorbed by KNO₃ than preadsorbed Cu(II) from both the control and EPS treated soils. The adsorption of both metals was governed by electrostatic and non-electrostatic mechanisms, although more Cu(II) was adsorbed through the non-electrostatic mechanism. The information obtained in this study will improve our understanding of the mechanisms involved in reducing heavy metals mobility in variable charge soils and hence, their bioavailability.

Extraction and characterization of stratified extracellular polymeric substances in Geobacter biofilms

Extracellular polymeric substances (EPS) play crucial roles in promoting biofilm formation and contribute to electrochemical activities of biofilms in bioelectrochemical systems (BES). In this study, three stratified EPS fractions were extracted from Geobacter biofilms using EDTA-, ultrasound- and heating-based protocols and characterized with chemical, spectral and electrochemical analyses. Results suggested that, for Geobacter biofilms, ultrasound-based extraction protocol was more effective in EPS yield (62.1-66.5 mg C/g dry cell) than EDTA method, and had less cell lysis than heating method. The extraction methods greatly affected the proteins composition in the extracted EPS, indicated by the varied ratios of tryptophan/tyrosine protein-like substances. Electrochemical measurements demonstrated a good correlation between protein concentration and extracellular electron transfer function for both tightly-bound EPS and total EPS. This is the first study to extract and characterize stratified EPS fractions from Geobacter biofilms, and helpful for better understanding the function of EPS in BESs predominated by Geobacter.

Preparation of a bioflocculant by using acetonitrile as sole nitrogen source and its application in heavy metals removal

A novel bioflocculant, A-GS408, produced by Klebsiella oxytoca GS-4-08 cultured in acetonitrile (ACN) as sole nitrogen source was investigated in this study. A complete degradation of 1 g l^-1 of ACN was achieved in 350 h, and 4.6 g of crude A-GS408 can be obtained in one litter of synthetic medium. The as-prepared bioflocculant exhibits good flocculation efficiency (over 90%) toward Kaolin solution with the aid of Fe³⁺. Chemical analysis showed that the bioflocculant was mainly composed of polysaccharides (46.3%) and proteins (20.6%). FTIR and XPS results indicated the abundant carboxyl, amine and hydroxyl groups in A-GS408 s, which play an important role on Pd²⁺ and Cu²⁺ adsorption. The adsorption of heavy metals can be well stimulated by Freundlich isotherm equation, and the K_f was up to 439.2 mg^1-1/n l^1/n g^-1 and 112.2 mg^1-1/n l^1/n g^-1 for Pb²⁺ and Cu²⁺, respectively. The kinetic fitting results proved that the adsorption of heavy metals by A-GS408 was chemisorption. This study may provide a new method for preparation of bioflocculant, which can not only degrade toxic compound i.e., acetonitrile, but also can reuse considerable nitrogen source from nitrile-containing wastewater.

A novel efficient bioflocculant QZ-7 for the removal of heavy metals from industrial wastewater

In this study, a novel bioflocculant QZ-7 was produced from Bacillus salmalaya 139SI for industrial wastewater treatment. Biochemical analysis, FTIR, scanning electron microscopy-energy dispersive X-ray spectroscopy, and thermogravimetric analysis were performed. A synthetic wastewater sample was used to validate the performance of the prepared OZ-7 for the adsorption efficiency of As, Zn²⁺ Pb²⁺, Cu²⁺, and Cd²⁺ under optimal experimental conditions such as initial metal concentrations, pH, contact time (h) and QZ-7 adsorbent dosage (mg mL⁻¹). The maximum removal efficiency for Zn²⁺ (81.3%), As (78.6%), Pb²⁺ (77.9%), Cu²⁺ (76.1%), and Cd²⁺ (68.7%) was achieved using an optimal bioflocculant dosage of 60 mg L⁻¹ at 2 h shaking time, 100 rpm and pH 7. Furthermore, the obtained optimum experimental conditions were validated using real industrial wastewater and the removal efficiencies of 89.8%, 77.4% and 58.4% were obtained for As, Zn²⁺ and Cu²⁺, respectively. The results revealed that the prepared bioflocculant QZ-7 has the capability to be used for the removal of heavy metals from industrial wastewater.

In this study, a novel bioflocculant was produced using Bacillus salmalaya 139SI for industrial waste water treatment.

Advances in exopolysaccharides based bioremediation of heavy metals in soil and water: A critical review

Extracellular polysaccharides or Exopolysaccharides (EPS) are extensively studied bacterial byproducts with high molecular weight attributed to several applications. In spite of their application in the field of food, pharmaceutical, nutraceutical, herbicidal and cosmeceutical industries they were well known for their efficiency in the bioremediation of water and soil tainted with heavy metals. These heavy metals are comparatively high in density than water and are involved in several biological processes. But slight increase in levels can create toxicological bias. The techniques like electrodialysis, chemical precipitation, ion exchange and membrane separation have a lot of disadvantages akin to high energy consumption, high cost, partial exclusion, and creation of poisonous mire. In this context, EPS has a top role to play in the bioremediation of heavy metals. This review gives the critical assessment of the extensive work done to deal this issue by different groups in the last five years. It also explains how different natural circumstances have attributed to the advancement of EPS production, thereby increasing the capacity of bioremediation to deal the issue of heavy metal contamination in both soil and water. A detailed discussion of the EPS formation by bacteria and fungi with their applicability was reported.

Bio-recycling of metals: Recycling of technical products using biological applications

The increasing demand of different essential metals as a consequence of the development of new technologies, especially in the so called "low carbon technologies" require the development of innovative technologies that enable an economic and environmentally friendly metal recovery from primary and secondary resources. There is serious concern that the demand of some critical elements might exceed the present supply within a few years, thus necessitating the development of novel strategies and technologies to meet the requirements of industry and society. Besides an improvement of exploitation and processing of ores, the more urgent issue of recycling of strategic metals has to be enforced. However, current recycling rates are very low due to the increasing complexity of products and the low content of certain critical elements, thus hindering an economic metal recovery. On the other hand, increasing environmental consciousness as well as limitations of classical methods require innovative recycling methodologies in order to enable a circular economy. Modern biotechnologies can contribute to solve some of the problems related to metal recycling. These approaches use natural properties of organisms, bio-compounds, and biomolecules to interact with minerals, materials, metals, or metal ions such as surface attachment, mineral dissolution, transformation, and metal complexation. Further, modern genetic approaches, e.g. realized by synthetic biology, enable the smart design of new chemicals. The article presents some recent developments in the fields of bioleaching, biosorption, bioreduction, and bioflotation, and their use for metal recovery from different waste materials. Currently only few of these developments are commercialized. Major limitations are high costs in comparison to conventional methods and low element selectivity. The article discusses future trends to overcome these barriers. Especially interdisciplinary approaches, the combination of different technologies, the inclusion of modern genetic methods, as well as the consideration of existing, yet unexplored natural resources will push innovations in these fields.

Biosorption of Cadmium by Non-Toxic Extracellular Polymeric Substances (EPS) Synthesized by Bacteria from Marine Intertidal Biofilms

Cadmium is a major heavy metal found in polluted aquatic environments, mainly derived from industrial production processes. We evaluated the biosorption of solubilized Cd²⁺ using the extracellular polymeric substances (EPS) produced by Bacillus sp. MC3B-22 and Microbacterium sp. MC3B-10 (Microbactan); these bacteria were originally isolated from intertidal biofilms off the coast of Campeche, Mexico. EPS were incubated with different concentrations of cadmium in ultrapure water. Residual Cd²⁺ concentrations were determined by Inductive Coupled Plasma-Optic Emission Spectrometry and the maximum sorption capacity (Qmax) was calculated according to the Langmuir model. EPS were characterized by X-ray photoelectron spectroscopy (XPS) before and after sorption. The Qmax of Cd²⁺ was 97 mg g⁻¹ for Microbactan and 141 mg g⁻¹ for MC3B-22 EPS, these adsorption levels being significantly higher than previously reported for other microbial EPS. In addition, XPS analysis revealed changes in structure of EPS after biosorption and showed that amino functional groups contributed to the binding of Cd²⁺, unlike other studies that show the carbohydrate fraction is responsible for this activity. This work expands the current view of bacterial species capable of synthesizing EPS with biosorbent potential for cadmium and provides evidence that different chemical moieties, other than carbohydrates, participate in this process.

Radiation, radionuclides and bacteria: An in-perspective review

There has been a significant surge in consumption of radionuclides for various academic and commercial purposes. Correspondingly, there has been a considerable amount of generation of radioactive waste. Bacteria and archaea, being earliest inhabitants on earth serve as model microorganisms on earth. These microbes have consistently proven their mettle by surviving extreme environments, even extreme ionizing radiations. Their ability to accept and undergo stable genetic mutations have led to development of recombinant mutants that are been exploited for remediation of various pollutants such as; heavy metals, hydrocarbons and even radioactive waste (radwaste). Thus, microbes have repeatedly presented themselves to be prime candidates suitable for remediation of radwaste. It is interesting to study the behind-the-scenes interactions these microbes possess when observed in presence of radionuclides. The emphasis is on the indigenous bacteria isolated from radionuclide containing environments as well as the five fundamental interaction mechanisms that have been studied extensively, namely; bioaccumulation, biotransformation, biosorption, biosolubilisation and bioprecipitation. Application of microbes exhibiting such mechanisms in remediation of radioactive waste depends largely on the individual capability of the species. Challenges pertaining to its potential bioremediation activity is also been briefly discussed. This review provides an insight into the various mechanisms bacteria uses to tolerate, survive and carry out processes that could potentially lead the eco-friendly approach for removal of radionuclides.