Simultaneous nitriles degradation and bioflocculant production by immobilized K. oxytoca strain in a continuous flow reactor

Responses of Bacillus sp. under Cu(II) stress in relation to extracellular polymeric substances and functional gene expression level

The production and composition of extracellular polymeric substances (EPS), as well as the EPS-related functional resistance genes and metabolic levels of Bacillus sp. under Cu(II) stress, were investigated. EPS production increased by 2.73 ± 0.29 times compared to the control when the strain was treated with 30 mg L^-1 Cu(II). Specifically, the polysaccharide (PS) content in EPS increased by 2.26 ± 0.28 g CDW^-1 and the PN/PS (protein/polysaccharide) ratio value increased by 3.18 ± 0.33 times under 30 mg L^-1 Cu(II) compared to the control. The increased EPS secretion and higher PN/PS ratio in EPS strengthened the cells' ability to resist the toxic effect of Cu(II). Differential expression of functional genes under Cu(II) stress was revealed by Gene Ontology pathway enrichment analysis and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis. The enriched genes were most obviously upregulated in the UMP biosynthesis pathway, the pyrimidine metabolism pathway, and the TCS metabolism pathway. This indicates an enhancement of EPS regulation-related metabolic levels and their role as a defense mechanism for cells to adapt to Cu(II) stress. Additionally, seven copper resistance genes were upregulated while three were downregulated. The upregulated genes were related to the heavy metal resistance, while downregulated genes were related to cell differentiation, indicating that the strain had initiated an obvious resistance to Cu(II) despite its severe cell toxicity. These results provided a basis for promoting EPS-regulated associated functional genes and the application of gene-regulated bacteria in heavy metal-containing wastewater treatment.

Sustainable and efficient technologies for removal and recovery of toxic and valuable metals from wastewater: Recent progress, challenges, and future perspectives

Due to their numerous effects on human health and the natural environment, water contamination with heavy metals and metalloids, caused by their extensive use in various technologies and industrial applications, continues to be a huge ecological issue that needs to be urgently tackled. Additionally, within the circular economy management framework, the recovery and recycling of metals-based waste as high value-added products (VAPs) is of great interest, owing to their high cost and the continuous depletion of their reserves and natural sources. This paper reviews the state-of-the-art technologies developed for the removal and recovery of metal pollutants from wastewater by providing an in-depth understanding of their remediation mechanisms, while analyzing and critically discussing the recent key advances regarding these treatment methods, their practical implementation and integration, as well as evaluating their advantages and remaining limitations. Herein, various treatment techniques are covered, including adsorption, reduction/oxidation, ion exchange, membrane separation technologies, solvents extraction, chemical precipitation/co-precipitation, coagulation-flocculation, flotation, and bioremediation. A particular emphasis is placed on full recovery of the captured metal pollutants in various reusable forms as metal-based VAPs, mainly as solid precipitates, which is a powerful tool that offers substantial enhancement of the remediation processes' sustainability and cost-effectiveness. At the end, we have identified some prospective research directions for future work on this topic, while presenting some recommendations that can promote sustainability and economic feasibility of the existing treatment technologies.

The role of anthraquinone-2-sulfonate on biohydrogen production by Klebsiella strain and mixed culture

The role of anthraquinone-2-sulfonate (AQS) on biohydrogen production was explored in this study. The hydrogen molar yield (HMY) of the batch experiment with 0.2 mM AQS was not significantly improved comparing to that without AQS, but the acetate and ethanol yields were increased by 12.1% and 9.82%, respectively. According to the metabolites flux analysis, e- balance calculations and stoichiometry results, the biogenic anthrahydroquinone-2,6,-disulfonate (AH2QS) preferred to improve the H2 generation that catalyzed by hydrogenase, but not likely affect the H2 generation that through formate cleavage pathway (FCP). However, comparing to the mix culture without AQS, the hydrogen production rate and hydrogen yield were increased by 20.97% of 1.96%, respectively, in the presence of AQS. The results of this study provided better understanding of the role of AQS on biohydrogen production by the strain that follows FCP, and the synergistic biohydrogen production by the co-culture that follows FCP and butyrate/acetate pathway.

Adsorption characteristics of methylene blue by a dye-degrading and extracellular polymeric substance -producing strain

Biosorption of dye by microbes and the extracellular polymeric substances (EPS) were of great environmental significance, especially for the dye-degrading and EPS-producing strain. Previous studies were mainly focused on the adsorption capacities and regeneration properties of pure culture, few were on the biosorption of dyes by the dye-degraders and the contributions of EPS on adsorption. In this study, a dye-degrading and EPS-producing strain i.e., Klebsiella oxytoca was used to evaluate its removal capacity to methylene blue. The maximum adsorption capacity (q_e) by the strain was calculated as 145 mg g^-1, which is superior to many reported bio-adsorbents and some synthetic materials. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy results suggested that CO, -NH₂ and P-OH groups were involved in the adsorption. High pressure steam sterilization (HPSS) increased the hydrophilicity of cell wall but did not significantly change the cell structure. Compared with the dead resting cell (DRC), the relative higher q_e obtained by live resting cell (LRC) possibly due to the loss of some cell structure during the HPSS process. Adsorption experiments by EPS-free LRC, confocal laser microscope and three-dimensional excitation-emission matrix fluorescence spectroscopy results confirmed that the EPS played a role in the adsorption of MB dye. The adsorption characteristics of the dye-degrader and the contributions of EPS on adsorption were investigated in detail in this study. The results were benefit for better understanding of the interaction mechanisms between the dye molecules and cells that before the biodegradation process, which were of great significance for the practical usage of residual sludge on removal of dyes.

Recent advances and perspectives in efforts to reduce the production and application cost of microbial flocculants

Microbial flocculants are macromolecular substances produced by microorganisms. Due to its non-toxic, harmless, and biodegradable advantages, microbial flocculants have been widely used in various industrial fields, such as wastewater treatment, microalgae harvest, activated sludge dewatering, heavy metal ion adsorption, and nanoparticle synthesis, especially in the post-treatment process of fermentation with high safety requirement. However, compared with the traditional inorganic flocculants and organic polymeric flocculants, the high production cost is the main bottleneck that restricts the large-scale production and application of microbial flocculants. To reduce the production cost of microbial flocculant, a series of efforts have been carried out and some exciting research progresses have been achieved. This paper summarized the research advances in the last decade, including the screening of high-yield strains and the construction of genetically engineered strains, search of cheap alternative medium, the extraction and preservation methods, microbial flocculants production as an incidental product of other biological processes, combined use of traditional flocculant and microbial flocculant, and the production of microbial flocculant promoted by inducer. Moreover, this paper prospects the future research directions to further reduce the production cost of microbial flocculants, thereby promoting the industrial production and large-scale application of microbial flocculants.

Cadmium removal by MgCl2 modified biochar derived from crayfish shell waste: Batch adsorption, response surface analysis and fixed bed filtration

Cadmium (Cd) pollution is regarded as a disturbing environmental problem due to its serious risks to the water body and human health. The removal of cadmium from wastewater is thus crucial to avoid its harmful effects on the ecosystem. This study comprehensively investigated Cd(II) adsorption onto MgCl₂ modified biochar (MgC600) and results showed that the adsorption capacity of MgC600 was more than twice of that of pristine biochar due to its enhanced ion exchange ability. Response surface analysis revealed that reaction time played a crucial role in the Cd(II) adsorption, followed by initial concentration and solution pH. Moreover, the optimal adsorption conditions and capacity were precisely given by the quadratic regression model and thus proved that the model can be applied to predict the operation conditions of Cd(II) adsorption. Finally, a new model defined as BJP model [Formula: see text] was proposed and proved to be more suitable for the fixed bed filtration process. Overall, our findings provide a promising material in treatment of Cd(II)-rich wastewater and give a clear picture of its application. More importantly, the newly developed BJP model can accurately describe the fixed bed filtration process and further promote its application in wastewater treatment.

Energy-saving preparation of a bioflocculant under high-salt condition by using strain Bacillus sp. and the interaction mechanism towards heavy metals

A highly efficient bioflocculant, i.e., Na-Bsp was successfully prepared by using a tolerant strain-Bacillus sp. under high-salt condition without sterilization. Salt-containing medium was not infected by other strains throughout the whole incubation period in 168 h. The as-prepared Na-Bsp was found to be cation-dependent, exhibiting high flocculant efficiency (FE) i.e., 97.69 ± 0.61%, towards kaolin particles by aid of Fe³⁺. High FE values were well maintained under a wide pH range and/or boiled water treatment, likely because of the main constituent of polysaccharide. The presence of hydroxyl, carboxyl, and amine groups on the bioflocculant surface were possibly responsible for strong interactions with heavy metals. The adsorption capacities of Pb²⁺, Cu²⁺ and Cr⁶⁺ were 1000.0, 434.8 and 384.6 mg g^-1, respectively. The changing of structure and configuration of bioflocculant during the metal adsorption were explored by the scanning electron microscope with electron energy loss spectroscopy and three-dimensional excitation-emission fluorescence spectrometry. This study provided a novel production method, whereby the conventional sterilization could be avoided, which is of great environmental significance for steam-saving. Furthermore, the as-prepared Na-Bsp exhibited high adsorption capacities toward heavy metals, which sheds lights on its potential usage as an alternative adsorbent.

Coadsorption behaviors and mechanisms of Pb(ii) and methylene blue onto a biodegradable multi-functional adsorbent with temperature-tunable selectivity

An entirely bio-degradable adsorbent based on lignin was synthesized by a crosslinking method and the adsorption of methyl blue (MB) and Pb(ii) onto the adsorbent were comparatively investigated, with adsorption behavior and mechanism of the two pollutants on the adsorbent (SLS) being assessed in single and binary systems. According to the results, SLS was capable of effective adsorption using MB and Pb(ii). The adsorption behavior of MB and Pb(ii) followed Langmuir and pseudo-first order models and showed temperature-dependent preferences. At 298 K MB was more preferred while at 318 K Pb(ii) adsorption was more favorable, which means that the selectivity of SLS can be tuned by changing the temperature. From a mechanism aspect, the adsorption of MB and Pb(ii) were both achieved through more than one route. Pb(ii) mainly interacts with sulfonate and hydroxyl groups on SLS, while MB can be bound on both anionic and aromatic groups due to its aromatic nature. Recycling and reuse experiments showed that used SLS can be readily reactivated and stably reused. The findings will guide adsorbent applications in wastewater containing heavy metals and aromatic compounds.

Enantioselective Uptake Determines Degradation Selectivity of Chiral Profenofos in Cupriavidus nantongensis X1T

Organophosphorus insecticides account for approximately 28% of the global commercial insecticide market, while 40% of them are chiral enantiomers. Chiral enantiomers differ largely in their toxicities. Enantiomers that are less active or inactive do not offer the needed efficacy but pollute the environment and cause toxicities to non-target species. Cupriavidus nantongensis X1^T, a recently isolated bacterial strain, could degrade S-profenofos 2.3-fold faster than R-profenofos, while the latter is the active enantiomer potently against pest insects and has greater mammalian safety. The degradation enzyme encoded by opdB was expressed via Escherichia coli and purified. The degradation kinetics of R- and S-profenofos showed that both the purified OpdB and crude enzyme extracts had no enantiomer degradation selectivity, which strongly indicated that the degradation selectivity occurred in the uptake process. Metabolite analyses suggested a novel dealkylation pathway. This is the first report of bacterial selective uptake of organophosphates. Selective degradation of S-profenofos over R-profenofos by the strain X1^T suggests a concept of co-application of racemic pesticides and degradation-selective bacteria to minimize contamination and non-target toxicity problems.