Techniques à membranes appliquées à l'élimination des matières en suspension dans un circuit semi-fermé d'aquaculture

Les piscicultures en circuits semi-fermés sont confrontées au problème de l'élimination permanente des matières en suspension (M.E.S.) et des substances azotées. Les procédés conventionnels utilisés pour retenir les M.E.S. (décantation, hydrocyclones, filtres mécaniques à tambour rotatif, filtration gravitaire) ne donnent par entière satisfaction. Par contre, la filtration sur membranes permet d'arrêter en totalité les particules en suspension et les bactéries.On montre d'abord que les teneurs en M.E.S. et leurs répartitions granulométriques mesurées sur des échantillons prélevés en bassins d'aquaculture varient avec la taille des poissons et l'heure du prélèvement et on met en évidence la présence en nombre important de particules submicroniques.Différents essais de filtration sur membranes sont ensuite présentés :- d'une part, avec des membranes d'ultrafiltration capillaires à peau interne utilisées en potabilisation des eaux : on examine l'influence des paramètres hydrodynamiques (pression transmembranaire, vitesse de recirculation) afin de rechercher les conditions optimales de fonctionnement. Le flux de perméat ne dépasse pas dans le meilleur des cas 100 l.h-1.m-2.- d'autre part, avec des membranes de microfiltration organiques planes en fluorure de polyvinylidène (PVDF) et tubulaires en céramique. Les flux obtenus avec les membranes organiques sont de l'ordre de 250 l.h-1.m-2Dans tous les cas, la rétention des M.E.S. est totale.Cependant l'estimation de l'investissement et des coûts de fonctionnement pour une pisciculture en circuit fermé de taille industrielle conduit à des prix trop élevés pour que l'utilisation des membranes dans ce domaine soit à ce jour économiquement envisageable.A problem confronting semi-closed circuit aquaculture is the need for continuous elimination of suspended matter (SM) and nitrogenous substances. Conventional processes used to retain SM (settling tanks, hydrocyclones, rotating-drum mechanical filters, gravity filtration) are not entirely satisfactory. However, membrane filtration has recently been shown to allow removal of suspended particles and bacteria. The present study evaluates the performance of different ultrafiltration and microfiltration membranes for water processing in a semi-closed aquaculture system. A brief economic analysis of treatment costs is proposed based on the results.The marine aquafarm studied produces about 5 tons of turbot per year with a plant volume of about 100 m3. The water processing line is fitted with a rotating-drum mechanical filter that stops the largest particles and ejects 1 m3 h-1 of loaded water into the surrounding environment. Another 2 m3 h-1 are cleared out by overflowing the pumping pit. These volumes are renewed at a rate of 3% per hour by pumping saltwater from an underground source. Crossflow filtration was performed on rejections from both the mechanical filter and pumping pit overflow. SM contents and granulometric distributions determined by laser diffractometry were found to vary with sample source and withdrawal time, and size of fish in the pens. A comparison of granulometric distributions in volume percent and numerical percent underscores the presence of a great number (> 98 %) of submicron particles.To limit the risk of mechanical-pore fouling due to blockage by particles, organic membranes in the form of internal-skin capillaries (pore diameters of about 10 to 20 nm) were initially employed. These membranes, used in drinking water production, are relatively inexpensive. The experimental device was fitted with an interchangeable volumetric pump (with gears or monoscrew). Adjustable parameters were transmembrane pressure and circulation velocity within the module. Analysis of the influence of these hydrodynamic parameters revealed that pressures higher than 1 bar were unnecessary, as beyond this point permeate flux no longer increased. Optimal flux did not exceed 100 L h-1 m-2 with the gear pump. Replacing the latter with a monoscrew pump improved permeate flux up to 70 %.Tests were also performed with flat microfiltration organic membranes of polyvinylidene fluoride (PVDF) with pore diameters ranging from 0.1 to 8 µm. The flux obtained with these membranes was roughly 250 L h-1 m-2 and presented little variation with varying pore diameter. Comparative tests carried out on tubular membranes showed lower fluxes than those obtained with organic membranes which, considering their much higher cost, makes them less attractive in this context. The use of membranes in aquafarming is without precedent. An economic analysis of the practice was carried out based on financial assessments of processing of surface waters into drinking water, for which outputs to be treated and SM contents were of the same order of magnitude. With operating costs from 0.35 to 0.95 FF per cubic meter of filtered water, expected investment for a fishfarm producing 100 tons of fish a year is currently 3 to 4 times too great to consider economically profitable the use of membranes for water treatment in closed-circuit aquafarming

On the Thermal Stability of Graphone

Molecular dynamics simulation is used to study thermally activated migration of hydrogen atoms in graphone, a magnetic semiconductor formed of a graphene monolayer with one side covered with hydrogen so that hydrogen atoms are adsorbed on each other carbon atom only. The temperature dependence of the characteristic time of disordering of graphone via hopping of hydrogen atoms to neighboring carbon atoms is established directly. The activation energy of this process is found to be Ea=(0.05+-0.01) eV. The small value of Ea points to extremely low thermal stability of graphone, this being a serious handicap for practical use of the material in nanoelectronics.Comment: 3 figure

Shape resonance for the anisotropic superconducting gaps near a Lifshitz transition: the effect of electron hopping between layers

The multigap superconductivity modulated by quantum confinement effects in a superlattice of quantum wells is presented. Our theoretical BCS approach captures the low-energy physics of a shape resonance in the superconducting gaps when the chemical potential is tuned near a Lifshitz transition. We focus on the case of weak Cooper-pairing coupling channels and strong pair exchange interaction driven by repulsive Coulomb interaction that allows to use the BCS theory in the weak-coupling regime neglecting retardation effects like in quantum condensates of ultracold gases. The calculated matrix element effects in the pairing interaction are shown to yield a complex physics near the particular quantum critical points due to Lifshitz transitions in multigap superconductivity. Strong deviations of the ratio $2\Delta/T_c$ from the standard BCS value as a function of the position of the chemical potential relative to the Lifshitz transition point measured by the Lifshitz parameter are found. The response of the condensate phase to the tuning of the Lifshitz parameter is compared with the response of ultracold gases in the BCS-BEC crossover tuned by an external magnetic field. The results provide the description of the condensates in this regime where matrix element effects play a key role.Comment: 12 pages, 6 figure

Observation of Multi-Gap Superconductivity in GdO(F)FeAs by Andreev Spectroscopy

We have studied current-voltage characteristics of Andreev contacts in polycrystalline GdO$_{0.88}$F$_{0.12}$FeAs samples with bulk critical temperature ${T_c}$ = (52.5 \pm 1)K using break-junction technique. The data obtained cannot be described within the single-gap approach and suggests the existence of a multi-gap superconductivity in this compound. The large and small superconducting gap values estimated at T = 4.2K are {\Delta}L = 10.5 \pm 2 meV and {\Delta}S = 2.3 \pm 0.4 meV, respectively.Comment: 5 pages, 4 figures, submitted to JETP Letter

Mechanical and Electronic Properties of MoS2_2 Nanoribbons and Their Defects

We present our study on atomic, electronic, magnetic and phonon properties of one dimensional honeycomb structure of molybdenum disulfide (MoS$_2$) using first-principles plane wave method. Calculated phonon frequencies of bare armchair nanoribbon reveal the fourth acoustic branch and indicate the stability. Force constant and in-plane stiffness calculated in the harmonic elastic deformation range signify that the MoS$_2$ nanoribbons are stiff quasi one dimensional structures, but not as strong as graphene and BN nanoribbons. Bare MoS$_2$ armchair nanoribbons are nonmagnetic, direct band gap semiconductors. Bare zigzag MoS$_2$ nanoribbons become half-metallic as a result of the (2x1) reconstruction of edge atoms and are semiconductor for minority spins, but metallic for the majority spins. Their magnetic moments and spin-polarizations at the Fermi level are reduced as a result of the passivation of edge atoms by hydrogen. The functionalization of MoS$_2$ nanoribbons by adatom adsorption and vacancy defect creation are also studied. The nonmagnetic armchair nanoribbons attain net magnetic moment depending on where the foreign atoms are adsorbed and what kind of vacancy defect is created. The magnetization of zigzag nanoribbons due to the edge states is suppressed in the presence of vacancy defects.Comment: 11 pages, 5 figures, first submitted at November 23th, 200

Graphite and Hexagonal Boron-Nitride Possess the Same Interlayer Distance. Why?

Graphite and hexagonal boron nitride (h-BN) are two prominent members of the family of layered materials possessing a hexagonal lattice. While graphite has non-polar homo-nuclear C-C intra-layer bonds, h-BN presents highly polar B-N bonds resulting in different optimal stacking modes of the two materials in bulk form. Furthermore, the static polarizabilities of the constituent atoms considerably differ from each other suggesting large differences in the dispersive component of the interlayer bonding. Despite these major differences both materials present practically identical interlayer distances. To understand this finding, a comparative study of the nature of the interlayer bonding in both materials is presented. A full lattice sum of the interactions between the partially charged atomic centers in h-BN results in vanishingly small monopolar electrostatic contributions to the interlayer binding energy. Higher order electrostatic multipoles, exchange, and short-range correlation contributions are found to be very similar in both materials and to almost completely cancel out by the Pauli repulsions at physically relevant interlayer distances resulting in a marginal effective contribution to the interlayer binding. Further analysis of the dispersive energy term reveals that despite the large differences in the individual atomic polarizabilities the hetero-atomic B-N C6 coefficient is very similar to the homo-atomic C-C coefficient in the hexagonal bulk form resulting in very similar dispersive contribution to the interlayer binding. The overall binding energy curves of both materials are thus very similar predicting practically the same interlayer distance and very similar binding energies.Comment: 18 pages, 5 figures, 2 table

Phonon-assisted carrier transport through a lattice-mismatched interface

We showed the distinctive unconventional junction effect of MoS2 junctions: a lattice mismatched MoS2. It is unique to observe the difference originated from the atomic interrelation at the interface. The results revealed the dominant scattering source at the conventional naturally stepwise junction, while the misorientationally stacked layer exhibited effectively decoupled behavior and a significantly smaller junction resistance via phonon assist carrier. Therefore, our finding in this paper clearly shows the different mechanisms in carrier transport at both junction interface of MoS2

Enhancement of chemical activity in corrugated graphene

Simulation of chemical activity of corrugated graphene within density functional theory predicts an enhancement of its chemical activity if the ratio of height of the corrugation (ripple) to its radius is larger than 0.07. Further growth of the curvature of the ripples results in appearance of midgap states which leads to an additional strong increase of chemisororption energy. These results open a way for tunable functionalization of graphene, namely, depending of curvature of the ripples one can provide both homogeneous (for small curvatures) and spot-like (for large curvatures) functionalization.Comment: 7 pages 3 figures. One figure added, description of the shape of ripples expanded. Final version, to be published in J. Phys. Chem.

Size Quantization in Planar Graphene-Based Heterostructures: Pseudospin Splitting, Interface States, and Excitons

A planar quantum-well device made of a gapless graphene nanoribbon with edges in contact with gapped graphene sheets is examined. The size-quantization spectrum of charge carriers in an asymmetric quantum well is shown to exhibit a pseudospin splitting. Interface states of a new type arise from the crossing of dispersion curves of gapless and gapped graphene materials. The exciton spectrum is calculated for a planar graphene quantum well. The effect of an external electric field on the exciton spectrum is analyzed.Comment: 15 pages, 14 figure