21 research outputs found
Impact of tortuous flow on bacteria streamer development in microfluidic system during filtration
The way in which bacterial communities colonize flow in porous media is of importance but basic knowledge on the dynamic of these phenomena is still missing. The aim of this work is to develop microfluidic experiments in order to progress in the understanding of bacteria capture in filters and membranes. PDMS microfluidic devices mimicking filtration processes have been developed to allow a direct dynamic observation of bacteria across 10 or 20 micrometers width microchannels. When filtered in such devices, bacteria behave surprisingly: Escherichia coli, Pseudomonas aeruginosa or Staphylococcus aureus accumulate in the downstream zone of the filter and form large streamers which oscillate in the flow. In this study streamer formation is put in evidence for bacteria suspension in non nutritive conditions in less than one hour. This result is totally different from the one observed in same system with “inert” particles or dead bacteria which are captured in the bottleneck zone and are accumulated in the upstream zone. Observations within different flow geometries (straight channels, connected channels, staggered row pillars) show that the bacteria streamer development is influenced by the flow configuration and, particularly by the presence of tortuosity within the microchannels zone. These results are discussed at the light of 3D flow simulations. In confined systems and in laminar flow there is secondary flow (z-velocities) superimposed to the streamwise motion (in xy plane). The presence of the secondary flow in the microsystems has an effect on the bacterial adhesion. A scenario in three steps is established to describe the formation of the streamers and to explain the positive effect of tortuous flow on the development kinetics
Fate of colloids during estuarine mixing in the Arctic
The estuarine behavior of organic carbon (OC) and trace elements (TE) was
studied for the largest European sub-Arctic river, which is the Severnaya
Dvina; this river has a deltaic estuary covered in ice during several
hydrological seasons: summer (July 2010, 2012) and winter (March 2009)
baseflow, and the November–December 2011 ice-free period. Colloidal forms of
OC and TE were assessed for three pore size cutoffs (1, 10, and 50 kDa)
using an in situ dialysis procedure. Conventionally dissolved
(< 0.22 μm) fractions demonstrated clear conservative
behavior for Li, B, Na, Mg, K, Ca, Sr, Mo, Rb, Cs, and U during the mixing of
freshwater with the White Sea; a significant (up to a factor of 10)
concentration increase occurs with increases in salinity. Si and OC also
displayed conservative behavior but with a pronounced decrease in
concentration seawards. Rather conservative behavior, but with much smaller
changes in concentration (variation within ±30%) over a full range
of salinities, was observed for Ti, Ni, Cr, As, Co, Cu, Ga, Y, and heavy REE.
Strong non-conservative behavior with coagulation/removal at low salinities
(< 5‰) was exhibited by Fe, Al, Zr, Hf, and light REE.
Finally, certain divalent metals exhibited non-conservative behavior with a
concentration gain at low (~ 2–5‰, Ba, Mn) or intermediate
(~ 10–15‰, Ba, Zn, Pb, Cd) salinities, which is most likely
linked to TE desorption from suspended matter or sediment outflux.
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The most important result of this study is the elucidation of the behavior of
the "truly" dissolved low molecular weight LMW<sub>< 1 kDa</sub>
fraction containing Fe, OC, and a number of insoluble elements. The
concentration of the LMW fraction either remains constant or increases its
relative contribution to the overall dissolved
(< 0.22 μm) pool as the salinity increases. Similarly,
the relative proportion of colloidal (1 kDa–0.22 μm) pool for the
OC and insoluble TE bound to ferric colloids systematically decreased
seaward, with the largest decrease occurring at low
(< 5‰) salinities.
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Overall, the observed decrease in the colloidal fraction may be related to
the coagulation of organo-ferric colloids at the beginning of the mixing zone
and therefore the replacement of the
HMW<sub>1 kDa–0.22 μm</sub> portion by the
LMW<sub>< 1 kDa</sub> fraction. These patterns are highly reproducible
across different sampling seasons, suggesting significant enrichment of the
mixing zone by the most labile (and potentially bioavailable) fraction of the
OC, Fe and insoluble TE. The size fractionation of the colloidal material
during estuarine mixing reflects a number of inorganic and biological
processes, the relative contribution of which to element speciation varies
depending on the hydrological stage and time of year. In particular,
LMW<sub>< 1 kDa</sub> ligand production in the surface horizons of the
mixing zone may be linked to heterotrophic mineralization of allochthonous
DOM and/or photodestruction. Given the relatively low concentration of
particulate versus dissolved load of most trace elements, desorption from the
river suspended material was less pronounced than in other rivers in the
world. As a result, the majority of dissolved components exhibited either
conservative (OC and related elements such as divalent metals) or
non-conservative, coagulation-controlled (Fe, Al, and insoluble TE associated
with organo-ferric colloids) behavior. The climate warming at high latitudes
is likely to intensify the production of LMW<sub>< 1 kDa</sub> organic
ligands and the associated TE; therefore, the delivery of potentially
bioavailable trace metal micronutrients from the land to the ocean may
increase
Fibres creuses auto-nettoyantes en PVP-PVDF-TiO2 : évolution des propriétés sous irradiation UV
International audienc
Effects of natural organic matter and ionic species on membrane surface charge
The surface charges of clean and natural organic matter (NOM) adsorbed membrane surfaces of two different types of membranes (a UF and a NF membrane composed of the same material but having different pore sizes) were investigated Concentrated NOM and its fractionated constituents were used as adsorbate and interacting macromolecules near the membrane surface The potential and the acidity of membranes were measured using electrophoresis and potentiometric titration methods respectively from the perspective of charge characterization along with demonstration of ionic strength effects The membrane surface was also characterized with attenuated total refractive Fourier transform infrared spectra to determine intrinsic functional groups and those changes before and after NOM adsorption As a comparative study for the electrokinetic property of membrane the potentials for both examined polymeric membranes were determined by the electrophoresis and the streaming potential measurement methods as functions of ionic strength and the pH of measuring solution Selectivity tests were performed to decide the relative importance of charge valence of cation in terms of the surface charge of membrane It was demonstrated that divalent cations (Ca2+ Mg2+) increase zeta potentials relatively compared to monovalent cations (Na+ K+) because divalent cations have a greater potential in approaching membrane surfaces (i e inside the Stern layer) Thus divalent cations can provide a greater double layer compaction and when near the shear plane (available for both the potential measurement methods) exist to a lesser extent than monovalent cationsclose10311