Lipids and Proteins Differentiation in Membrane Fouling Using Heavy Metal Staining and Electron Microscopy at Cryogenic Temperatures.

The detailed characterization of fouling in membranes is essential to understand any observed improvement or reduction on filtration performance. Electron microscopy allows detailed structural characterization, and its combination with labeling techniques, using electron-dense probes, typically allows for the differentiation of biomolecules. Developing specific protocols that allow for differentiation of biomolecules in membrane fouling by electron microscopy is a major challenge due to both as follows: the necessity to preserve the native state of fouled membranes upon real filtration conditions as well as the inability of the electron-dense probes to penetrate the membranes once they have been fouled. In this study, we present the development of a heavy metal staining technique for identification and differentiation of biomolecules in membrane fouling, which is compatible with cryofixation methods. A general contrast enhancement of biomolecules and fouling is achieved. Our observations indicate a strong interaction between biomolecules: A tendency of proteins, both in solution as well as in the fouling, to surround the lipids is observed. Using transmission electron microscopy and scanning electron microscopy at cryogenic conditions, cryo-SEM, in combination with energy-dispersive X-ray spectroscopy, the spatial distribution of proteins and lipids within fouling is shown and the role of proteins in fouling discussed.

Parameterization of a mesoscopic model for the self-assembly of linear sodium alkyl sulfates.

A systematic approach to develop mesoscopic models for a series of linear anionic surfactants (CH3(CH2)n - 1OSO3Na, n = 6, 9, 12, 15) by dissipative particle dynamics (DPD) simulations is presented in this work. The four surfactants are represented by coarse-grained models composed of the same head group and different numbers of identical tail beads. The transferability of the DPD model over different surfactant systems is carefully checked by adjusting the repulsive interaction parameters and the rigidity of surfactant molecules, in order to reproduce key equilibrium properties of the aqueous micellar solutions observed experimentally, including critical micelle concentration (CMC) and average micelle aggregation number (Nag). We find that the chain length is a good index to optimize the parameters and evaluate the transferability of the DPD model. Our models qualitatively reproduce the essential properties of these surfactant analogues with a set of best-fit parameters. It is observed that the logarithm of the CMC value decreases linearly with the surfactant chain length, in agreement with Klevens' rule. With the best-fit and transferable set of parameters, we have been able to calculate the free energy contribution to micelle formation per methylene unit of -1.7 kJ/mol, very close to the experimentally reported value.

New insights into the structure of membrane fouling by biomolecules using comparison with isotherms and ATR-FTIR local quantification.

The objective of this paper was to propose a deepened analyze of a microfiltration membrane fouling by two biomolecules: a protein (Bovine Serum Albumin) and a peptide (Glutathione). In addition to an analysis of flux decline, the mass of biomolecules accumulated on the membrane during filtration was quantified and compared to adsorption experiments, using Fourier Transform Infra Red spectroscopy in Attenuated Total Reflection mode (ATR-FTIR). It was demonstrated that the same quantity of accumulated biomolecules on the apparent membrane area can generate totally different flux declines because of different fouling mechanisms. On the one hand, Glutathione can adsorb in the whole porous media of the membrane, penetrating through the pores, modifying the hydrophilicity at low concentrations and generating pore constriction at high concentrations. On the other hand, BSA organize a dense irreversible fouling in the first minutes of filtration containing a quantity equivalent to more than 45 monolayers, leading to pore blocking and pore constriction. This structure is resistant to rinsing and NaOH cleaning. Then a reversible fouling, containing a quantity equivalent to more than 90 monolayers is settled. The above structure can be removed with an intensive water rinsing and corresponds to a rather porous cake leading to a low resistance to water permeation, whereas the intermediate structure can only be removed with chemical cleaning and has a higher impact on water permeation. The original approach detailed in this paper allowed to go deeper in the understanding of the membrane fouling by soft matter, not detailed in previous papers.

Shear-enhanced membrane filtration of model and real microalgae extracts for lipids recovery in biorefinery context.

In this work, the hydrodynamic effects of rotating disk filtration (with maximum shear rates of 16,000 s-1 and 66,000 s-1) were evaluated and compared with the crossflow filtration (16,000 s-1) in the recovery of lipids from a model solution that simulates the characteristics of Parachlorella kessleri aqueous extracts. Four polymeric membranes were tested. The PAN 500 kDa membrane along with the rotating disk filtration presented the best performances for lipid concentration and coalescence. The rotating disk filtration was tested with real microalgae extracts, confirming the total lipid retention and the limited membrane fouling.