Molecular Sorting – Membrane adsorbers for the separation of recyclable materials and micropollutants

Nowadays various types of membranes for water filtration are already available through commercial channels. A common feature of these membranes is that substantially different separation cut-offs are used for size exclusion. On the other hand, the underlying porous structure, which provides a highly specific surface, remains unused. Membranes for nanofiltration (NF) and reverse osmosis (RO) can in fact partially retain molecular and ionic substances. However, high pressures are necessary for this, which pushes up both the investment and the operating costs.

In principle adsorbers can be used to remove molecular contaminants. Typical adsorber materials are microporous, so as to provide a large specific surface for adsorption. A disadvantage of these materials is the limited mass transport, since the micropollutants have to diffuse into the inner porous structure of the adsorbents.

There is therefore a need for new integrated separation systems. For this purpose we are developing mixed-matrix membranes that, in addition to their filtration function, can adsorptively bind substances dissolved in water.

To achieve this, functional submicroparticles are manufactured by means of miniemulsion polymerization. These are between 50 nm and 500 nm in size and can be synthesized from a variety of different, commercially available monomers. The particles provide the best compromise between a high specific surface, safety and functionality and are compatible with the phase inversion process for the manufacture of porous membranes.

The variation of the particle surface and the combination of different particles enables us to manufacture membrane adsorbers with separation characteristics that can be adapted flexibly for applications in the areas of drinking water, process water and waste water. A large number of particles with different functional surface groups are now available. The spectrum of functional groups ranges from the fairly hydrophobic pyridine, by way of cationic ammonium compounds to anionic phosphonates and also thiourea.

In a first step the particles were embedded in polyethersulfone flat membranes by means of a phase inversion process (Fig. x/1). This showed that, quantitatively, up to 40 percent by weight of the particles can be integrated in the membranes. Most of the particles are easily accessible inside the pores. It was also shown that different particles can be combined in one membrane. In this way various micropollutants, for example, can be removed with just one membrane adsorber [1].

If one compares the adsorption behavior of silver on various membrane adsorbers, it can be seen that the reference membrane without particles exhibits practically no unspecific adsorption. On the other hand, the membrane with thiourea groups selectively binds over 0.8 g silver per m².

However, if one compares the adsorption behavior of various metal ions on a phosphonate membrane adsorber, it can be seen that practically no silver is bound, whereas copper and especially lead are adsorbed very well by it (e.g. over 5 g lead per m²).

The regenerability of the systems is important for the cost-effectiveness of the membrane adsorbers. So far we have been able to find suitable solutions for a quantitative desorption in all the adsorptions investigated. Thus copper, for instance, can be completely removed from the membrane adsorber using small amounts of diluted nitric acid. A pre-enrichment of copper by a factor of 100 is therefore possible. But membrane adsorbers for micropollutants such as bisphenol A can also be completely regenerated by means of a pH shift [1].

In further studies we intend to transfer the principle of the membrane adsorbers presented here to hollow fiber membranes. This makes possible both a higher specific separation area and a higher specific adsorption volume. The resulting systems are then to be used to remove toxic substances such as micropollutants or heavy metals from drinking water direct at its point of use. The membrane adsorber systems can also be used to recover valuable metals such as rare earths from process streams.

[1] Niedergall, K.; Bach, M.; et al. (2014) Removal of micropollutants from water by nanocomposite membrane adsorbers, Separation and Purification Technology 131: 60-68

[2] Niedergall, K.; Kopp, D.; et al. (2015) Mixed-matrix membrane adsorbers for the selective binding of metal ions from diluted solutions, Chemie Ingenieur Technik (submitted)

We would like to thank the Fraunhofer-Gesellschaft for funding the project "Molecular Sorting for Resource Efficiency" within the scope of the “Markets Beyond Tomorrow” research program.