A manganese-oxidizing bacterial consortium and its biogenic Mn oxides for dye decolorization and heavy metal adsorption

Biogenic Mn Oxide Generation and Mn(II) Removal by a Manganese Oxidizing Bacterium Bacillus sp. Strain M2

Mn(II)-oxidizing bacteria (MOB) are widely distributed in natural environments and can convert soluble Mn(II) into insoluble Mn(III) and Mn(IV). The biogenic manganese oxides (BioMnOx) produced by MOB have been considered for remediating heavy metal pollution and degrading organic pollutants in an eco-friendly manner. In this study, a manganese-oxidizing bacterium was isolated from Mn-polluted rivulet sediment and identified as Bacillus sp. strain M2 by PCR, phylogenetic tree construction, transmission electron microscopy (TEM), and physiological and biochemical indices. Strain M2 grew well under Mn(II) stress. BioMnOx with nanosized irregular geometric shapes and loose structures generated by strain M2 were found on the surface of the bacterial cells. The content of Mn in the bacteria was as high as 5.36%. Approximately 71.24% and 47.52% of Mn(II) was oxidized to Mn(III/IV) in the cell and in the deposits, respectively, within 3 d of cultivation with Mn(II). Extracellular enzymes contributed to the Mn removal and oxidation. In conclusion, Bacillus sp. strain M2 has a high potential for use in the remediation of Mn-contaminated sites.

Can xenobiotics support the growth of Mn(II)-oxidizing bacteria (MnOB)? A case of phenol-utilizing bacteria Pseudomonas sp. AN-1

Biogenic manganese oxides (BioMnOx) produced by Mn(II)-oxidizing bacteria (MnOB) have garnered considerable attention for their exceptional adsorption and oxidation capabilities. However, previous studies have predominantly focused on the role of BioMnOx, neglecting substantial investigation into MnOB themselves. Meanwhile, whether the xenobiotics could support the growth of MnOB as the sole carbon source remains uncertain. In this study, we isolated a strain termed Pseudomonas sp. AN-1, capable of utilizing phenol as the sole carbon source. The degradation of phenol took precedence over the accumulation of BioMnOx. In the presence of 100 mg L-1 phenol and 100 µM Mn(II), phenol was entirely degraded within 20 h, while Mn(II) was completely oxidized within 30 h. However, at the higher phenol concentration (500 mg L-1), phenol degradation reduced to 32% and Mn(II) oxidation did not appear to occur. TOC determination confirmed the ability of strain AN-1 to mineralize phenol. Based on the genomic and proteomics studies, the Mn(II) oxidation and phenol mineralization mechanism of strain AN-1 was further confirmed. Proteome analysis revealed down-regulation of proteins associated with Mn(II) oxidation, including MnxG and McoA, with increasing phenol concentration. Notably, this study observed for the first time that the expression of Mn(II) oxidation proteins is modulated by the concentration of carbon sources. This work provides new insight into the interaction between xenobiotics and MnOB, thus revealing the complexity of biogeochemical cycles of Mn and C.

Characteristics of biological manganese oxides produced by manganese-oxidizing bacteria H38 and its removal mechanism of oxytetracycline

Oxytetracycline (OTC) is widely used in clinical medicine and animal husbandry. Residual OTC can affect the normal life activities of microorganisms, animals, and plants and affect human health. Microbial remediation has become a research hotspot in the environmental field. Manganese oxidizing bacteria (MnOB) exist in nature, and the biological manganese oxides (BMO) produced by them have the characteristics of high efficiency, low cost, and environmental friendliness. However, the effect and mechanism of BMO in removing OTC are still unclear. In this study, Bacillus thuringiensis strain H38 of MnOB was obtained, and the conditions for its BMO production were optimized. The optimal conditions were determined as follows: optimal temperature = 35 °C, optimal pH = 7.5, optimal Mn(Ⅱ) initial concentration = 10 mmol/L. The results show that BMO are irregular or massive, mainly containing MnCO3, Mn2O3, and MnO2, with rich functional groups and chemical bonds. They have the characteristics of small particle size and large specific surface area. OTC (2.5 mg/L) was removed when the BMO dosage was 75 μmol/L and the solution pH was 5.0. The removal ratio was close to 100 % after 12 h of culture at 35 °C and 150 r/min. BMO can adsorb and catalyze the oxidation of OTC and can produce ·O2-, ·OH, 1O2, and Mn(Ⅲ) intermediate. Fifteen products and degradation pathways were identified, and the toxicity of most intermediates is reduced compared to OTC. The removal mechanism was preliminarily clarified. The results of this study are convenient for the practical application of BMO in OTC pollution in water and for solving the harm caused by antibiotic pollution.

Cd immobilization efficacy of biogenic Mn oxide formed by Cladosporium sp. XM01 and its biological response in sediment

Biogenic Mn oxides (BMOs), the main component of natural Mn oxides, closely relate to Cd in sediment. However, the immobilization behavior of Cd in sediments by BMOs is currently unclear. This study explores the role of BMO produced by the Mn-oxidizing fungus Cladosporium sp. XM01 in mediating the Cd immobilization and its biological response in sediment. A comparison is made with those of a chemical Mn oxide (CMO, triclinic birnessite). After 45 d of remediation, the results showed that the application of BMO reduced the extractable Cd by 32.20-64.40% based on the TCLP (toxicity characteristic leaching procedure) and by 26.16-51.43% based on the PBET (physiologically based extraction test). Additionally, BMO was more effective at immobilizing Cd than CMO in sediments. The BCR (Community Bureau of Reference) extraction results suggested that BMO converted some acid-soluble components (20.63-33.23%) of Cd into residual components (9.40-20.68%). Moreover, the urease and catalase activity gradually increased within the first 25 days and then stabilized after applying BMO. Microbial community analysis revealed that the addition of a high-dose BMO was more conducive to increasing microbial abundance and biodiversity. This study verifies that BMO is a low-cost, high-efficiency, and eco-friendly material for immobilizing Cd in sediment.

Treatment of wastewater containing thallium(I) by long-term operated manganese sand filter: Synergistic action of MnOx and MnOM

The long-term and stable removal of thallium (Tl) from industrial wastewater generated by mining and smelting operations remains challenging. While sand filters are commonly applied for the simultaneous removal of Mn(II) and other heavy metals, they have limited efficacy in treating Tl-contaminated wastewater. To address this gap, we operated a lab-scale Mn sand filter (MF) without added microorganisms to investigate the efficiency and mechanisms of Mn(II) and Tl(I) removal. Trends in effluent Mn(II) and Tl(I) concentrations indicated three operational stages: start-up, developing and maturation. Over time, the removal efficiency of Tl(I) gradually improved, plateauing at approximately 80 % eventually. Throughout operation, Tl(I) was sequestrated via surface complexation and ion exchange. Besides, enrichment of Sphingobium and other typical manganese oxidizing microorganisms (MnOM) during operation facilitated Mn(II) and Tl(I) oxidation and sequestration by generating biogenic manganese oxides (BioMnOx). Additionally, the accurate control of water quality and operating conditions during operation could also enhance removal efficiency. In summary, physicochemical actions of Mn oxides and biochemical actions of microorganisms synergistically contributed to the sequestration of Mn(II) and Tl(I). These findings provided a novel and sustainable method for the long-term and stable treatment of industrial wastewater containing thallium.

An overview of in situ remediation for groundwater co-contaminated with heavy metals and petroleum hydrocarbons

Groundwater is an important component of water resources. Mixed pollutants comprising heavy metals (HMs) and petroleum hydrocarbons (PHs) from industrial activities can contaminate groundwater through such processes as rainfall infiltration, runoff and discharge, which pose direct threats to human health through the food chain or drinking water. In situ remediation of contaminated groundwater is an important way to improve the quality of a water environment, develop water resources and ensure the safety of drinking water. Bioremediation and permeable reactive barriers (PRBs) were discussed in this paper as they were effective and affordable for in situ remediation of complex contaminated groundwater. In addition, media types, technology combinations and factors for the PRBs were highlighted. Finally, insights and outlooks were presented for in situ remediation technologies for complex groundwater contaminated with HMs and PHs. The selection of an in situ remediation technology should be site specific. The remediation of complex contaminated groundwater can be approached from various perspectives, including the development of economical materials, the production of slow-release and encapsulated materials, and a combination of multiple technologies. This review is expected to provide technical guidance and assistance for in situ remediation of complex contaminated groundwater.

Green synthesis of MnO2-embedded Rauvolfia tetraphylla leaves (MnO2@RTL) for crystal violet dye removal and as an antibacterial agent

The application of green synthesized nanocomposites for the prevention of environmental pollution is increasing nowadays. Here, a green composite has been synthesized by embedding MnO2 on Rauvolfia tetraphylla leaves using its leaf extract hereinafter termed as MnO2@RTL, and demonstrated for crystal violet (CV) dye removal from simulated and real wastewater. The surface properties of the material were determined by scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX), Fourier transform infrared spectra (FTIR), X-ray diffraction (XRD), and Brunauer-Emmet-Teller (BET) surface area, pHZPC, and zeta potential. The material exhibits a remarkable adsorption capacity of 61.162 mg/g at 328 K and pH 7. The adsorption was best fitted with Pseudo-second-order kinetic (R2 = 0.998) and a combination of Langmuir and Freundlich isotherm model (R2 = 0.994-0.999). The thermodynamic study revealed spontaneous (ΔG values =  - 2.988 to - 4.978 kJ/mol) and endothermic (ΔH values = 6.830 to 11.018 kJ/mol) adsorption. After adsorption, 80% regeneration occurred with 50% methanol, and recycled up to five times. Advantageously, the material was able to remove CV dye in the presence of coexistent ions and from industrial wastewater, confirming field applicability. The adsorption capacity of the material is superior to previously reported materials. The standard deviation and relative standard deviations have been evaluated to be 0.000422-0.000667 and 0.473-0.749%, which suggests the reliability of the experiments. The exhausted material, after recycling, was pyrolyzed to overcome the disposal problem. It was established as a secondary adsorbent with 73% efficiency which makes the material win-win. The material showed antibacterial properties with Staphylococcus aureus bacteria with a zone of inhibition 5 mm.

The evolution of nitrogen transformation microorganism consortium under continued manganese domestication conditions

Manganese redox-stimulated bioremediation of nitrogen wastewater is receiving increasing attention. However, the nitrogen metabolic capacity and community evolution during manganese-mediated nitrogen transformation process under continued manganese domestication conditions are ambiguous. In this study, nitrogen- metabolizing microbial consortiums were incubated with synthesized Mn-humic acid complex (Mn-HA) for one month (M1), three months (M2) and six months (M3), respectively. During the Mn-HA incubation period, Bio-MnOx accompanying with bacterial consortiums (MnOB consortiums) with high TIN removal capacities were obtained. The TIN removal rates in M1, M2 and M3 were 0.220, 1.246 and 4.237 mg·L-1·h-1, respectively, which were 15.961, 90.006 and 1550.006 times higher than CK (Control Check group, no Mn-HA added group) (0.014 mg·L-1·h-1), respectively. Functional genes (amoA, AMX and narG) were most abundant in M3, which was associated with the highest nitrogen removal rate in M3. MnOB1 (bacterial consortium in M1), including Geobactor, Geothrix, Anaeromyxobacter and Bacillus, may be responsible for the Mnammox-NDMO (MnOx reduction coupled to ammonium oxidation - nitrate/nitrite-dependent low-valent Mn oxidation) process. MnOB3 (bacterial consortium in M2) enriched nitrifying bacteria Ellin6067, and denitrifying bacteria Denitratisoma, which dominated nitrogen transformation. MnOB6 (bacterial consortium in M3) enriched denitrifiers Denitratisoma, nitrifiers Ellin6067 and potential anammox bacteria SM1A02, Candidatus_Brocadia. Combined with the reduced abundance of Nitrospirae, a short-cut partial nitrification and denitrification (PND) or partial nitrification, denitrification and anammox (PNDA) could occurred in M2 and M3. It is suggested that community may have evolved into an energetically efficient short-cut nitrification, denitrification and anammox consortium to replace the full-range nitrification and denitrification community in M1 and CK under the continued manganese domestication conditions. Enhanced metabolic pathways of hydroxylamine oxidation and the nitric oxide reduction may confirm that PND or PNDA occurred in M2 and M3.

Enhancing the Mn-Removal Efficiency of Acid-Mine Bacterial Consortium: Performance Optimization and Mechanism Study

In this study, an acclimated manganese-oxidizing bacteria (MnOB) consortium, QBS-1, was enriched in an acid mine area; then, it was used to eliminate Mn(Ⅱ) in different types of wastewater. QBS-1 presented excellent Mn removal performance between pH 4.0 and 8.0, and the best Mn-removal efficiency was up to 99.86% after response surface methodology optimization. Unlike other MnOB consortia, the core bacteria of QBS-1 were Stenotrophomonas and Achromobacter, which might play vital roles in Mn removal. Besides that, adsorption, co-precipitation and electrostatic binding by biological manganese oxides could further promote Mn elimination. Finally, the performance of the Mn biofilter demonstrated that QBS-1 was an excellent inoculant, which indicates good potential for removing Mn contamination steadily and efficiently.

Manganese-mediated ammonium removal by a bacterial consortium from wastewater: Experimental proof and biochemical mechanisms

Manganese-redox-mediated nitrogen transformation is promising for ammonium wastewater treatment. However, due to the limited contact between insoluble Mn and the microbe, extracellular electron transfer (EET) inefficiencies become a technical bottleneck in the technical practical application. To overcome this obstacle, humic acid (HA) was introduced to synthesize manganese-humic acid complex (Mn-HA) to increase Mn solubility. The TIN (Total Inorganic Nitrogen) removal rate constant k was 3.18, 1.08, 3.56, 1.13 and 1.05 times higher than CK (Control group) at 10, 15, 20, 40 and 60 mg/L influent nitrate in the MH group, respectively. Mn-HA was inferred to stimulated the nitrogen removal by providing more reaction active sites, bridging Mn-O bonds to transfer electrons and playing a redox role in the respiratory chain. A Mnammox-NDMO (manganese oxide reduction-coupled ammonium oxidation - nitrate/nitrite- dependent manganese oxidation) bacteria consortium was enriched in MH group, containing Mnammox bacteria Geothrix, Geobacter and NDMO bacteria Pseudomonas and Bacillus.

Detoxification and metabolism of glyphosate by a Pseudomonas sp. via biogenic manganese oxidation

Biogenic manganese oxides (BMO) are widely distributed in groundwater and provides promise for adsorbing and oxidizing a wide range of micropollutants, however, the continuous biodegradation and bioavailability of micropollutants via cycle biogenic Mn(II) oxidation remains to be elucidated. In this study, glyphosate was degraded and to serve as the nutrient source by a Pseudomonas sp. QJX-1. The addition of glyphosate will not affect the Mn(II) oxidation function of the strain but will affect its Mn(II) oxidation process and effect. The glyphosate degradation products could further be used as the C, N and P sources for bacterium growth. Analysis of the RNA-seq data suggested that Mn(II) oxidation driven by oxidoreductases for glyphosate degradation. The long-term column experiments using biological Mn(II) cycling to realize continuous detoxification and metabolism of glyphosate, and thus revealed the synergism effects of biological and chemical conversion on toxic micropollutants and continuous metabolism in an aquatic ecosystem.

A Review of Manganese-Oxidizing Bacteria (MnOB): Applications, Future Concerns, and Challenges

Groundwater serving as a drinking water resource usually contains manganese ions (Mn2+) that exceed drinking standards. Based on the Mn biogeochemical cycle at the hydrosphere scale, bioprocesses consisting of aeration, biofiltration, and disinfection are well known as a cost-effective and environmentally friendly ecotechnology for removing Mn2+. The design of aeration and biofiltration units, which are critical components, is significantly influenced by coexisting iron and ammonia in groundwater; however, there is no unified standard for optimizing bioprocess operation. In addition to the groundwater purification, it was also found that manganese-oxidizing bacteria (MnOB)-derived biogenic Mn oxides (bioMnOx), a by-product, have a low crystallinity and a relatively high specific surface area; the MnOB supplied with Mn2+ can be developed for contaminated water remediation. As a result, according to previous studies, this paper summarized and provided operational suggestions for the removal of Mn2+ from groundwater. This review also anticipated challenges and future concerns, as well as opportunities for bioMnOx applications. These could improve our understanding of the MnOB group and its practical applications.

Role of intracellular energy metabolism in Mn(Ⅱ) removal by the novel bacterium Stenotrophomonas sp. MNB17

Microorganism-mediated Mn(Ⅱ) removal has gained increasing attention as a valuble bioremediation approach. In this study, a novel strain Stenotrophomonas sp. MNB17 - obtained from marine sediments - was found to show Mn(Ⅱ) removal efficiencies of 98.51-99.38% within 7 days and 92.24% within 20 days at Mn(Ⅱ) concentrations of 10-40 mM and 50 mM, respectively. On day 7, 80.44% of 50 mM Mn(Ⅱ) was oxidized to Mn(Ⅲ/Ⅳ), whereas only 2.11-2.86% of 10-40 mM Mn(Ⅱ) was oxidized. This difference in the proportion of Mn-oxides suggested that the strain MNB17 could remove soluble Mn(Ⅱ) via distinct mechanisms under different Mn(Ⅱ) concentrations. At 10 mM Mn(Ⅱ), indirect mechanisms were employed by strain MNB17 to remove Mn(Ⅱ). The sufficient energy generated by increased cellular respiration led to enhanced ammonification, and MnCO₃ was the main component of the Mn-precipitates (97.27%). Meanwhile, intracellular fatty acids were degraded and served as an important carbon source for respiration. At 50 mM Mn(Ⅱ), most of the soluble Mn(Ⅱ) was oxidized, and Mn-oxides dominated the Mn-precipitates (80.44%). Mn(Ⅱ) oxidation likely contributed to electrons for energy production, as the down-regulation of respiratory pathways resulted in a deficit of electron supply, which warrants futher study. The exogenous addition of tricarboxylic acid cycle substrates (malate, α-ketoglutarate, oxaloacetate, succinate, and fumarate) was found to accelerate Mn(Ⅱ) removal as MnCO₃ at a concentration of 50 mM. Overall, this study reports a novel strain MNB17 with the biotechnological potential of Mn(Ⅱ) removal and elucidates the function of cellular energy metabolism during the Mn(Ⅱ) removal process. In addition, it demonstrates the potential of aerobic respiration-related substrates in accelerating the removal of high concentrations of Mn(Ⅱ) for the first time.

Biosorption of heavy metals by microorganisms: Evaluation of different underlying mechanisms

Globally, ecotoxicologists, environmental biologists, biochemists, pathologists, and other experts are concerned about environmental contamination. Numerous pollutants, such as harmful heavy metals and emerging hazardous chemicals, are pervasive sources of water pollution. Water pollution and sustainable development have several eradication strategies proposed and used. Biosorption is a low-cost, easy-to-use, profitable, and efficient method of removing pollutants from water resources. Microorganisms are effective biosorbents, and their biosorption efficacy varies based on several aspects, such as ambient factors, sorbing materials, and metals to be removed. Microbial culture survival is also important. Biofilm agglomerates play an important function in metal uptake by extracellular polymeric molecules from water resources. This study investigates the occurrence of heavy metals, their removal by biosorption techniques, and the influence of variables such as those indicated above on biosorption performance. Ion exchange, complexation, precipitation, and physical adsorption are all components of biosorption. Between 20 and 35 °C is the optimal temperature range for biosorption efficiency from water resources. Utilizing living microorganisms that interact with the active functional groups found in the water contaminants might increase biosorption efficiency. This article discusses the negative impacts of microorganisms on living things and provides an outline of how they affect the elimination of heavy metals.

A manganese-oxidizing bacterium-Enterobacter hormaechei strain DS02Eh01: Capabilities of Mn(II) immobilization, plant growth promotion and biofilm formation

While biogenic Mn oxides (BioMnO_x) generated by Mn(II)-oxidizing bacteria (MOB) have attracted increasing attention, a MOB strain isolated from Mn-polluted sediments was identified and assigned as Enterobacter hormaechei DS02Eh01. Its Mn(II) immobilization activity, plant growth-promoting traits, and biofilm formation capability were investigated. The results showed that strain DS02Eh01 was found to be able to tolerate Mn(II) up to 122 mM. The strain immobilized Mn(II) in aquatic media mainly through extracellular adsorption, bio-oxidation and pH-induced precipitation as well as manganese oxidation. DS02Eh01-derived BioMnO_x are negatively charged and have a larger specific surface area (86.70 m²/g) compared to the previously reported BioMnO_x. The strain can immobilize Mn(II) at extreme levels, for instance, when it was exposed to 20 mM Mn(II), about 59% of Mn(II) were found immobilized and 17% of Mn(II) were converted to MnO_x. The SEM and TEM observation revealed that the DS02Eh01-derived BioMnO_x were aggregates doped with granules and microbial pellets. The precipitated Mn(II) and the Mn(III)/Mn(IV) oxides co-existed in BioMnO_x, in which Mn(II) and Mn(IV) were found dominant with Mn(II) accounting for 49.6% and Mn(IV) accounting for 41.3%. DS02Eh01 possesses plant growth-promoting traits and biofilm formation capacity even under Mn(II) exposure. Mn(II) exposure at 5 mM was found to stimulate strain DS02Eh01 to form biofilms, from which, the extracted EPS was mainly composed of aromatic proteins. This study reveals that E. hormaechei strain DS02Eh01 possesses the potential in environmental ecoremediation via coupling processes of macrophytes extraction, biochemical immobilization and biosorption.

Fe(II) oxygenation inhibits bacterial Mn(II) oxidation by P. putida MnB1 in groundwater under O2-perturbed conditions

Bacterial Mn(II) oxidation plays a crucial role in Mn cycling and the associated biogeochemistry in natural waters and is of practical concern in the clean-up of excess Mn from drinking water. Fe(II) usually occurring together with Mn(II) in groundwater is oxidized prior to Mn(II) when perturbed by O₂, but the impact of Fe(II) oxygenation on the subsequent bacterial Mn(II) oxidation remains unknown. Here we demonstrated that Fe(II) oxygenation largely inhibited the Mn(II)-oxidizing ability of MnB1 belong to Pseudomonas putida which is ubiquitous in groundwater. The mechanisms of the inhibition varied under different Fe(II) concentrations. At high Fe(II) concentrations (≥ 1 mM), the inhibition of bacterial Mn(II) oxidation was mainly because of cell death caused by intracelluar reactive oxygen species. At low Fe(II) concentrations (≤ 0.05 mM), the inhibition of bacterial Mn(II) oxidation was attributed to Fe(III) oxyhydroxides generated from Fe(II) oxygenation. Fe(III) oxyhydroxides attached to cell surface and damaged the cell membrane, resulting in the influx of dissolved Fe into the cell. Transcriptomic analysis revealed that the intracellular Fe suppressed the transcription initiation process and the subsequent generation of multicopper oxidases which were responsible for Mn(II) oxidation. These findings implicate that the inhibition effect of Fe(II) oxygenation on bacterial Mn(II) oxidation should be considered in groundwater-surface water interaction zone and the biological treatment of Fe-Mn containing drinking water.

Insights into the synergistic removal mechanisms of thallium(I) by biogenic manganese oxides in a wide pH range

The behavior and mechanism of thallium (Tl) adsorption by biogenic manganese oxides (BMnOx) are poorly understood. In this study, BMnOx was applied for Tl(I) removal from aqueous solution, and the adsorption interactions were systematically revealed for the first time. BMnOx was successfully prepared with high productivity by effectively oxidizing Mn(II) with a manganese oxide bacterium in an optimal Mn(II) concentration range of 4.0-28 mg/L. Compared with other adsorbents, the prepared BMnOx achieved high Tl(I) adsorption capacity over a wide pH range from 3.0 to 9.0 and high humic acid (HA) concentration (40 mg/L) interference. The experimental results were well depicted by pseudo-second-order kinetics and the Langmuir isotherm model, indicating that chemisorption played the dominant role during the adsorption process. The adsorption mechanisms were verified as synergetic interactions of oxidation-precipitation, electrostatic attraction, ion exchange and surface complexation. X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) results suggested that 19.46% of the highly toxic Tl(I) was transformed into the much less toxic product Tl₂O₃ after adsorption onto BMnOx. This study provides theoretical guidance for high-concentration Tl(I) decontamination from groundwater by biogenic manganese oxides.

Microbial approaches for sustainable remediation of dye-contaminated wastewater: a review

The coloured effluents produced from different industries, such as textile, plastics, printing, cosmetics, leather and paper, are extremely toxic and a tremendous threat to the aquatic organisms and human beings. The removal of coloured dye pollutants from the aqueous environment is a great challenge and a pressing task. The growing demand for low-cost and efficient treatment approaches has given rise to alternative and eco-friendly methods, such as biodegradation and microbial remediation. This work summarizes the overview and current research on the remediation of dye pollutants from the aqueous environment by microbial bio-sorbents, such as bacteria, fungi, algae, and yeast. In addition, dye degradation capabilities of microbial enzymes have been highlighted and discussed. Further, the influence of various experimental parameters, such as temperature, pH, and concentrations of nutrients, and dye, has been summarized. The proposed mechanism for dye removal by microorganisms is also discussed. The object of this review is to provide a state-of-the-art of microbial remediation technologies in eliminating dye pollutants from water resources.

Microbial Consortia Are Needed to Degrade Soil Pollutants

Soil pollution is one of the most serious environmental problems globally due to the weak self-purification ability, long degradation time, and high cost of cleaning soil pollution. The pollutants in the soil can be transported into the human body through water or dust, causing adverse effects on human health. The latest research has shown that the clean-up of soil pollutants through microbial consortium is a very promising method. This review provides an in-depth discussion on the efficient removal, bio-adsorption, or carbonated precipitation of organic and inorganic pollutants by the microbial consortium, including PAHs, BPS, BPF, crude oil, pyrene, DBP, DOP, TPHP, PHs, butane, DON, TC, Mn, and Cd. In view of the good degradation ability of the consortium compared to single strains, six different synergistic mechanisms and corresponding microorganisms are summarized. The microbial consortium obtains such activities through enhancing synergistic degradation, reducing the accumulation of intermediate products, generating the crude enzyme, and self-regulating, etc. Furthermore, the degradation efficiency of pollutants can be greatly improved by adding chemical materials such as the surfactants Tween 20, Tween 80, and SDS. This review provides insightful information regarding the application of microbial consortia for soil pollutant removal.

Experimental study on treatment of heavy metal-contaminated soil by manganese-oxidizing bacteria

There are many studies on the treatment of heavy metals by manganese-oxidizing bacteria and the reaction is good; the problem of compound pollution of heavy metals in soil has been difficult to solve. In this study, the application of manganese-oxidizing bacteria in soil was studied. The tolerance of manganese-oxidizing strains (Pseudomonas taiwanensis) to environmental conditions and the treatment effect of heavy metals As, Pb, and Cd in aqueous solution were investigated, and the effect of iron-manganese ratio on the treatment effect was discussed. The results showed that the suitable pH conditions for the growth of P. taiwanensis were 5-9, and the salt tolerance was 6% (by sodium chloride). The tolerant concentrations for heavy metals As(V) and Mn(II) were 500 mg L^-1 and 120 mg L^-1, respectively. The strains were enriched by nutrient broth medium. After the logarithmic phase, the bacterial suspension was mixed with ATCC#279 medium at a ratio of 1:10, and a certain amount (10 mg L^-1) of Mn(II) was added. The results of As, Pb, and Cd removal in the composite polluted water phase were 22.09%, 30.75%, and 35.33%, respectively. The molar ratio of manganese and iron affected the removal efficiency of single arsenic, the highest efficiency is 68%, and the ratio of iron to manganese is 1:5. However, when the soil was treated by the same method, the results showed that not all metals were passivated, such as Cu. At the same time, for As, Pb, and Cd, the treatment effects in soil were worse than those in water, perhaps more consideration should be given to environmental conditions, such as soil moisture and temperature, when manganese-oxidizing bacteria are used to treat soil.

Structural and functional study on cysteine 495, coordinating ligand to T1Cu site in multicopper oxidase CopA

Excessive intake of manganese seriously affects human health. Manganese oxidizing bacteria can efficiently remove manganese, among which manganese oxidase plays a decisive role. Multicopper oxidase, one of the manganese oxidases, has 4 copper binding sites, among them, T1Cu coordinates with two histidine, one cysteine and one axial residue, mainly transferring electrons from the substrate to T2Cu and T3Cu. Here, we conducted site-directed mutagenesis on T1Cu coordinating 495 amino acid site from cysteine to aspartic acid, histidine and methionine in multicopper oxidase CopA from Brevibacillus panacihumi MK-8, through the enzyme kinetics and structure models, finding that the enzyme catalytic efficiency (k_cat/K_m) of the mutated C495H with Mn²⁺ and ABTS reached 9.03 min^-1 mM^-1 and 8863 s^-1 mM^-1, 1.47 times and 1.67 times that of CopA. And it was found strain Rosetta-pET-copA^C495H could remove 91.67% manganese after 7-day culture, which was 11.65% higher than the original strain. To sum up, these results provide a vision for the future application of protein engineering in biological manganese removal.

Combination of the endophytic manganese-oxidizing bacterium Pantoea eucrina SS01 and biogenic Mn oxides: An efficient and sustainable complex in degradation and detoxification of malachite green

Recently, Mn oxides (MnOxs) have been attracting considerable interest in the oxidation of organic pollutants. However, the reduction of MnOx in these reactions leads to the deactivation of the catalyst, which must be frequently regenerated. We evaluated the application of a manganese-oxidizing bacterium (MOB) and MnOx in removing toxic dyes. We studied the co-function of a plant-endophytic MOB, Pantoea eucrina SS01, with its bio-generated MnOx and evaluated the detoxification activity and chemical transformation mechanisms of the complex in malachite green (MG) degradation. We found a synergistic effect between MnOx and the strain. Particularly, strain SS01 could adsorb MG but could not degrade it, whereas the addition of Mn(II) promoted MG degradation by the formation of a complex containing the bacterium and MnOx aggregates (SS01-bio-MnOx), with distinct morphology characteristics. The complex showed a marked sustainability in the degradation of MG into less toxic or non-toxic metabolites. In this process, strain SS01 might have enhanced the regeneration of MnOx, accelerating MG degradation. Our data not only contribute to understanding the mechanism of MG removal by the SS01-bio-MnOx complex, but also provide a scientific basis for the future application of MOB and MnOx.

Mn-incorporated ferrihydrite for Cr(VI) immobilization: Adsorption behavior and the fate of Cr(VI) during aging

Chromium(VI) (Cr(VI)) is an environmental priority pollutant, and its mobility in natural environment is strongly controlled by ferrihydrite. Ferrihydrite always contains various ions, which may change the properties of ferrihydrite, thereby affecting the behavior of pollutants. This study aims to investigate the adsorption of Cr(VI) by Mn-incorporated ferrihydrite and the mobility behavior of Cr(VI) during aging. Results showed that the incorporation of Mn enhanced the adsorption of Cr(VI) on ferrihydrite, and the adsorption performance increased with the increase of Mn content. The maximum adsorption capacity for Cr(VI) reached to 48.5 mg/g with molar ratio of Mn/Fe 5%, while it was 36.1 mg/g for pure ferrihydrite. After aging for 7 days, ferrihydrite transformed into goethite and hematite. The adsorbed Cr(VI) on the surface of ferrihydrite was released into the solution during aging. The incorporation of Mn retarded the transformation of ferrihydrite, which inhibited the migration of adsorbed Cr(VI). Nevertheless, the incorporation of Mn resulted in the transformation of adsorbed Cr(VI) to non-desorbed Cr(VI), thereby enhancing the retention of Cr(VI). Our results suggest that the incorporation of Mn into ferrihydrite has an important role on the mobility of Cr(VI), which enhances our understanding of the behavior of Cr(VI) in the environment.

Distribution, characteristics of extracellular polymeric substances of Phanerochaete chrysosporium under lead ion stress and the influence on Pb removal

The distribution, characteristics of extracellular polymeric substances (EPS) of Phanerochaete chrysosporium under Pb²⁺ stress and the influence on Pb removal were investigated. Polysaccharides was found to be the main composition in both soluble EPS (SEPS) and bounded EPS (BEPS). More polysaccharides and protein in BEPS were detected with the increased Pb²⁺ concentration. The ratio of Pb amount distributed in BEPS to the total Pb removed by the fungal biomass gradually decreased from 91.66 to 61.27% in group with 50 mg/L of initial Pb²⁺, but kept at about 35% or 25% in groups with higher Pb²⁺. It implies that BEPS played a certain role in the lead removal process, and the role of BEPS was relatively more important in the removal of lower concentration of Pb²⁺ and in the initial period of Pb removal. With FTIR analysis and Pb²⁺ adsorption experiment, more effective functional groups and better Pb²⁺ adsorption capacity was demonstrated in BEPS than in SEPS. SEM-EDS analysis demonstrated that part of Pb immobilized in BEPS was in the form of Pb precipitation. The increased molecular weight in SEPS and more polysaccharides in BEPS were probably beneficial for the adhesion of Pb precipitation.

Polymer/BiOBr-Modified Gauze as a Dual-Functional Membrane for Heavy Metal Removal and Photocatalytic Dye Decolorization

It is crucial to remove heavy metals and dyes before discharging industrial effluents. Gauze substrate was surface-modified by coating with a polymeric adsorbent and a spray coating of BiOBr photocatalyst to develop a novel dual-functional membrane, polymer/BiOBr-modified gauze, for water remediation. The polymeric adsorbent was crosslinked to prevent the dissolving of the adsorbent during operation in contaminated water. The morphology and surface chemistry of the modified gauze were characterized before and after the adsorption of Ni²⁺. The surface wettability, isotherms, and kinetics of Ni²⁺ adsorption were studied. We also studied the effect of pH, initial Ni²⁺ concentration, monomer molar ratio, and monomer chemical structure on the Ni²⁺ adsorption capacity. To achieve a high Ni²⁺ adsorption capacity and good photocatalytic decolorization activity, the amount of decorated BiOBr was tuned by changing the spray-coating time to optimize the exposed BiOBr and polymer on the surface. The optimized dual-functional membrane PB20 possesses excellent adsorption capacity (650 mg g^-1) for Ni²⁺ ions and photocatalytic decolorization activity (100% degradation of RhB within 7 min). Decorating the optimized amount of BiOBr on the surface can introduce photocatalytic decolorization activity without sacrificing the adsorption capacity for Ni²⁺.

Carboxymethyl cellulose improved adsorption capacity of polypyrrole/CMC composite nanoparticles for removal of reactive dyes: Experimental optimization and DFT calculation

In this study, polypyrrole/carboxymethyl cellulose nanocomposite particles (PPy/CMC NPs) were synthesized and applied for removal of reactive red 56 (RR56)and reactive blue 160 (RB160) as highly toxic dyes. The amount of CMC was found significantly effective on the surface adsorption efficiency. Different optimization methods including the genetic programming, response surface methodology, and artificial neural network (ANN) were used to optimize the effect of different parameters including pH, adsorption time, initial dye concentration and adsorbent dose. The maximum adsorption of RR56 and RB160 were found under the following optimum conditions: pH of 4 and 5, adsorption time of 55 min and 52 min for RR56 and RB160, respectively, initial dye concentration of 100 mg/L and adsorbent dose of 0.09 g for both dyes. were obtained for RR56 and RB160, respectively. Also, the results indicated that ANN method could predict the experimental adsorption data with higher accuracy than other methods. The analysis of ANN results indicated that the adsorbent dose is the main factor in RR56 removal, followed by time, pH and initial concentration, respectively. However, initial concentration mostly determines the RB160 removal process. The isotherm data for both dyes followed the Langmuir isotherm model with a maximum adsorption capacity of 104.9 mg/g and 120.7 mg/g for RR56 and RB160, respectively. In addition, thermodynamic studies indicated the endothermic adsorption process for both studied dyes. Moreover, DFT calculations were carried out to obtain more insight into the interactions between the dyes and adsorbent. The results showed that the hydrogen bondings and Van der Waals interactions are dominant forces between the two studied dyes and PPy/CMC composite. Furthermore, the interaction energies calculated by DFT confirmed the experimental adsorption data, where PPy/CMC resulted in higher removal of both dyes compared to PPy. The developed nanocomposite showed considerable reusability up to 3 cylces of the batch adsorption process.