Time-Gradient Nitric Acid Modification of CF Biofilm-Carrier and Surface Nature Effects on Microorganism Immobilization Behavior in Wastewater

Nitric acid oxidation and urea modification of carbon fibres as biofilm carriers

Carbon fibres (CF) are commonly used as carriers in biofilm-based wastewater treatment. The surface properties of the CF are herein modified using a combination of nitric acid oxidation and urea to optimise the carrier to immobilise bacterial cells. The capacity of the CF carriers to immobilise bacterial cells and activated sludge is evaluated using bacterial cell adhesion and sludge immobilisation tests. The total interaction energy profiles between the CF supports and bacterial cells were calculated according to the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory to explain the mechanism by which these modifications enhance this immobilisation capacity. CF-U has a high capacity for immobilising bacterial cells and activated sludge (3.7 g-sludge/g-CF supports) owing to its low total interaction energy. Nitric acid oxidation reduced the diiodomethane contact angle of CF from 55.1° to 38.5°, which reduced the Lifshitz-van der Waals interaction energy, while urea modification further increased the zeta potential of CF from 12.8 mV to -0.7 mV, thereby reducing the electrostatic interaction energy. Experiments and DLVO theory both determined that a combination of nitric acid oxidation and urea modification significantly enhanced the ability of CF to immobilise microorganisms.

Order-of-magnitude enhancement in photocurrent generation of Synechocystis sp. PCC 6803 by outer membrane deprivation

Biophotovoltaics (BPV) generates electricity from reducing equivalent(s) produced by photosynthetic organisms by exploiting a phenomenon called extracellular electron transfer (EET), where reducing equivalent(s) is transferred to external electron acceptors. Although cyanobacteria have been extensively studied for BPV because of their high photosynthetic activity and ease of handling, their low EET activity poses a limitation. Here, we show an order-of-magnitude enhancement in photocurrent generation of the cyanobacterium Synechocystis sp. PCC 6803 by deprivation of the outer membrane, where electrons are suggested to stem from pathway(s) downstream of photosystem I. A marked enhancement of EET activity itself is verified by rapid reduction of exogenous electron acceptor, ferricyanide. The extracellular organic substances, including reducing equivalent(s), produced by this cyanobacterium serve as respiratory substrates for other heterotrophic bacteria. These findings demonstrate that the outer membrane is a barrier that limits EET. Therefore, depriving this membrane is an effective approach to exploit the cyanobacterial reducing equivalent(s).

The low extracellular electron transfer activity hampers the application of cyanobacteria in biophotovoltaics. Here, the authors report an order-of-magnitude enhancement in photocurrent generation of the cyanobacterium by deprivation of the outer cell membrane.

Electrospun nanofibers enhance trehalose synthesis by regulating gene expression for Micrococcus luteus fermentation

In this study, mesoporous polyacrylonitrile (PAN)/thermoplastic polyurethane (TPU) blended nanofibers were prepared to immobilize Micrococcus luteus for enhancing the conversion of trehalose. The images of SEM showed the cells were adsorbed on the surface and pores due to the unique pore structure. The results of contact angle, Zeta potential and water holding ratio exhibited the good hydrophilicity and stability of PAN/TPU-P2. Besides, it was indicated that the biomass and immobilization efficiency were increased to 0.633 g/L and 0.153 g/g, respectively. It was the most noteworthy that the trehalose yield could reach 23.46 g/L, which was 71.62 % higher than that of the control in the multi-batch fermentation. Moreover, the reactive oxygen species (ROS) level was decreased to 12.8 % while the enzyme concentration was increased to 11.176 mg/mL. Meanwhile, it was also found that PAN/TPU-P2 immobilization substantially increased the expression of target gene MtreY by 3.500 times. In other words, the mechanism by which immobilized cells increased trehalose yield was that PAN/TPU-P regulated gene expression of MtreY. Therefore, this research provided theoretical foundation for the metabolic regulation of sufficient trehalose production by immobilized cells.

Methane Elimination Using Biofiltration Packed With Fly Ash Ceramsite as Support Material

Methane is a greenhouse gas and significantly contributes to global warming. Methane biofiltration with immobilized methane-oxidizing bacteria (MOB) is an efficient and eco-friendly approach for methane elimination. To achieve high methane elimination capacity (EC), it is necessary to use an exceptional support material to immobilize MOB. The MOB consortium was inoculated in biofilters to continuusly eliminate 1% (v/v) of methane. Results showed that the immobilized MOB cells outperformed than the suspended MOB cells. The biofilter packed with fly ash ceramsite (FAC) held the highest average methane EC of 4.628 g h^–1 m^–3, which was 33.4% higher than that of the biofilter with the suspended MOB cells. The qPCR revealed that FAC surface presented the highest pmoA gene abundance, which inferred that FAC surface immobilized the most MOB biomass. The XPS and contact angle measurement indicated that the desirable surface elemental composition and stronger surface hydrophilicity of FAC might favor MOB immobilization and accordingly improve methane elimination.

Optimizing carbon fibre supports for bioreactors by nitric acid oxidation and calcium ion coverage according to extended DLVO theory

Optimizing supports for microorganisms is required for bioreactors. Carbon fibres (CF) were employed as supports for microorganisms. To optimize CF supports for immobilizing bacterial cells, we used methods of nitric acid oxidation and calcium ion coverage. We evaluated the capacity of these CF supports (untreated CF, nitric acid oxidation CF and Ca²⁺-covered CF) via bacterial cell adhesion tests, based on extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory. The results implied that because of the high hamaker constants, oxidized CF supports had higher capacity in this regard than untreated CF supports. However, the growing oxygen groups increased the negative zeta potential of CF supports, thus likely to reduce their capacity, in accordance with XDLVO theory. Since the Ca²⁺ coverage could decrease the negative zeta potentials of CF without reducing the hamaker constants, it could enhance the capacity of oxidized CF supports. We concluded that a combination of nitric acid oxidation and Ca²⁺ coverage could increase the capacity of CF supports to immobilize bacterial cells.

Effect of different nitric acid concentrations on manganese/activated carbon-modified catalysts for the catalytic ozonation of toluene

In this work, the effect of nitric acid modification on activated carbon (AC) and on properties of Mn/AC ozone catalysts was studied.

Improved methane elimination by methane-oxidizing bacteria immobilized on modified oil shale semicoke

Methane is a greenhouse gas with significant global warming potential. The methane-oxidizing bacteria (MOB) immobilized on biocarrier could perform effectively and environmentally in methane elimination. To further improve the efficiencies of MOB immobilization and methane elimination, the surface biocompatibility of biocarrier needs to be improved. In this work, the oil shale semicoke (SC) was chemically modified by sodium p-styrenesulfonate hydrate (SS) and 2-(methacryloyloxy)ethyltrimethylammonium chloride (DMC) to promote surface hydrophilicity and positive charge, respectively. Results revealed that, under methane concentrations of ~10% (v/v) and ~0.5% (v/v), the MOB immobilized on semicoke modified with 1.0 mol L^-1 of SS permitted improved methane elimination capacities (ECs), which were 15.02% and 11.11% higher than that on SC, respectively. Additionally, under methane concentrations of ~10% (v/v) and ~0.5% (v/v), the MOB immobilized on semicoke modified with 0.4 mol L^-1 of DMC held superior ECs, which were 17.88% and 11.29% higher than that on SC, respectively. The qPCR analysis indicated that the MOB abundance on modified semicoke were higher than that on SC. In consequence, the surface biocompatibility of semicoke could be promoted by SS and DMC modifications, which potentially provided methods for other biocarriers to improve surface biocompatibility.

Carbon nanotubes/carbon fiber hybrid material: a super support material for sludge biofilms

Carbon fiber (CF) is widely used as a sludge biofilm support material for wastewater treatment. Carbon nanotubes/carbon fiber (CNTs/CF) hybrid material was prepared by ultrasonically assisted electrophoretic deposition (EPD). CF supports (CF without handling, CF oxidized by nitric acid, CNTs/CF hybrid material) were evaluated by sludge immobilization tests, bacterial cell adsorption tests and Derjaguin -Landau -Verwey -Overbeek (DLVO) theory. We found that the CNTs/CF hybrid material has a high capacity for adsorbing activated sludge, nitrifying bacterial sludge and pure strains (Escherichia coli and Staphylococcus aureus). CNTs deposited on CF surface easily wound around the curved surface of bacterial cell which resulted in capturing more bacterial cells. DLVO theory indicated the lowest total interaction energy of CNTs/CF hybrid material, which resulted in the highest bacteria cell adsorption velocity. Experiments and DLVO theory results proved that CNTs/CF hybrid material is a super support material for sludge biofilms.

Experimental study on treating agate dyeing wastewater with sulfate-reducing bacteria strengthening peanut shells and scrap iron

To solve the problems of high concentrations of Cr⁶⁺, SO₄^2- and H⁺ in agate dyeing industrial wastewater and heavy pollution and high treating cost, single-factor and orthogonal experiments were conducted to determine the optimum particle size, the ratio of adsorbents dosing and hydraulic retention time based on peanut shells and scrap iron. Experiments, using five dynamic columns filled with the peanut shells, scrap iron and sulfate-reducing bacteria (SRB), were also conducted to determine the effect and mechanism of treating the wastewater. The results show that the best treatment effect was obtained when the diameter of peanut shells was 3 mesh, scrap iron being 60 mesh size, scrap iron and peanut shells with a ratio of 1:2, and hydraulic retention time being 24 h. By the comprehensive comparison of five groups of columns, the treating effect of column 4 was best, in which the removal rate of SO₄^2- and Cr⁶⁺ was 30.17% and 88.36% respectively before adding the microorganisms, and 25.34% and 99.31% respectively after adding the microorganisms. The average of chemical oxygen demand (COD) release quantity was 62.11 and 513.75 mg·L^-1, and the average effluent pH was 7.09 and 7.93 before and after addition of microorganisms respectively. In conclusion, peanut shells, scrap iron and SRB had a certain synergistic effect on treating agate dyeing wastewater.

Nitric Acid-Treated Carbon Fibers with Enhanced Hydrophilicity for Candida tropicalis Immobilization in Xylitol Fermentation

Nitric acid (HNO₃)-treated carbon fiber (CF) rich in hydrophilic groups was applied as a cell-immobilized carrier for xylitol fermentation. Using scanning electron microscopy, we characterized the morphology of the HNO₃-treated CF. Additionally, we evaluated the immobilized efficiency (IE) of Candida tropicalis and xylitol fermentation yield by investigating the surface properties of nitric acid treated CF, specifically, the acidic group content, zero charge point, degree of moisture and contact angle. We found that adhesion is the major mechanism for cell immobilization and that it is greatly affected by the hydrophilic–hydrophilic surface properties. In our experiments, we found 3 hto be the optimal time for treating CF with nitric acid, resulting in an improved IE of Candida tropicalis of 0.98 g∙g⁻¹ and the highest xylitol yield and volumetric productivity (70.13% and 1.22 g∙L⁻¹∙h⁻¹, respectively). The HNO₃-treated CF represents a promising method for preparing biocompatible biocarriers for multi-batch fermentation.