Dead-end Filtration of Sponge-like Colloids: the Case of Casein Micelle
Résumé
In colloidal filtration, understanding the mechanisms responsible for deposit formation and determining the relations between filtration conditions and deposit characteristics are still major issues. Following the general - and now well admitted - description of colloidal filtration given by Bacchin et al. (2006, 2011), irreversible fouling occurs when a critical concentration in particles is reached at the membrane surface. At this concentration, the interactions between the particles get dominated by attractive forces and phase transition takes place from the liquid to the solid state (irreversible deposit – gel). In cross-flow filtration, this critical concentration is reached at a given flux (this is the well-known concept of critical flux) (Field et al., 1995); while it is reached at a critical filtered volume in dead-end filtrations conducted at constant flux (Bessière et al., 2005). Importantly, phase transition and deposit formation can also be interpreted in terms of a critical osmotic pressure crit, that can be viewed as the compressive pressure required to reach the critical concentration and go from the liquid to the solid state. The critical conditions for deposit formation, as well as the intrinsic characteristics of the deposit, are now well documented, especially for "hard" colloids like silica, latex, clays...; in opposition to "soft" colloids that can deform and even deswell when concentrated. As the "softness feature" might necessarily have an impact on the formation of a deposit and on its properties, it becomes essential to get new information about the behavior of soft colloids in filtration. It is all the more important that such soft objects (emulsions, microgels, protein aggregates...) are involved in an increasing number of industrial processes that use membrane operations.
In this work, we focus on the behavior of one particular soft and sponge-like natural colloid, the casein micelle that makes up to 80% of the protein content of cow milk. For that purpose, specific dead-end filtration experiments were designed and performed, involving successive filtrations interspersed with rinse periods at zero transmembrane pressure P = 0). This methodology allowed us to precisely monitor the build-up -or not- of a deposit at the membrane surface, as well as to estimate the critical conditions at which the deposit is formed. A detailed analysis of the properties of the deposit is also proposed, based on the variation in its specific resistance (α) with the conditions of filtration.
From the results obtained, we first demonstrate that deposit formation is intimately linked to the colloidal nature of the micelle, as no irreversible deposit is formed when caseins are not organized into micelles. The deposit, or gel, forms through intermicellar bonds that are created when the micelles are forced to come in contact with each other.
We then discuss the critical conditions for deposit formation and we estimate the corresponding critical osmotic pressure at which colloidal phase transition occurs. Interestingly, the obtained value is a bit higher than the one measured in "static" osmotic stress experiments; which could be a direct effect of liquid flow between the particles.
From our filtration experiments, we find that critical osmotic pressure crit Which corresponds to the compressive pressure that is required to reach the critical concentration and provokes phase transition, equals 31-35 kPa. Interestingly, this value is close to the one measured from "static" osmotic stress experiments. It is however sufficiently higher to postulate, among other assumptions, that the liquid flow between the particles and/or the kinetics of intermicellar bond formation have an effect on crit.
We finally demonstrate that our results are consistent with a deposit made of closely-packed and connected casein micelles that are compressed during filtration, and that fully relax to their "native" initial dimensions when pressure is released (Figure 1). The deposit is found to be both compressible and relaxable; two properties that are related to the soft and elastic nature of the casein micelle alone. In most cases, the resistance to flow through the deposit is entirely governed by the internal porosity of the casein micelle; this porosity being directly associated to the degree of compression of the micelle body.