Funktionalisierung von Ultrafiltrationsmembranen zur Integration von spezifischen Adsorbereigenschaften

Heavy metal pollution in aquatic systems is a serious problem rooted in the rising industrialization and urbanization observed in the world, since heavy metals like copper or lead possess the potential to disrupt ecosystems via bioaccumulation or cause diseases after chronic exposure. Membrane processes have gained a lot of attention in recent decades concerning wastewater treatment. However, in terms of heavy metal pollution membranes with high fluxes based solely on size exclusion like microfiltration or ultrafiltration are not suitable and only high pressure and therefore cost intensive processes like nanofiltration or reverse osmosis can be applied.

To achieve a type of membrane able to selectively remove heavy metals in flow-through conditions and retain high fluxes, the combination of ultrafiltration and adsorption was selected to create membrane adsorbers.

Polysulfone was chloromethylated and the newly introduced functional group was substituted with sodium azide to produce azidomethylated polysulfone which directly served as membrane polymer or was blended with unsubstituted polysulfone before usage. These membranes were cast via non-solvent induced phase separation and showed characteristics typical for ultrafiltration membranes. Different approaches for membrane casting were followed to eventually achieve finger and sponge-like pore structures.

Functional polymers able to bind heavy metals were equipped with alkyne groups which served as complimentary groups for the membrane’s azide groups to react in an Azide-alkyne click reaction. The two types of functional polymers used for the reaction were polyethyleneimine and poly acrylamide. Polyethyleneimine was functionalized with glycidyl propargyl ether to introduce the alkyne group.  The latter was synthesized from poly acrylic acid by forming an activated ester species and subsequently equipped with different amines to introduce alkyne groups and groups of 5‑Amino‑8‑hydroxyquinoline to enable the click reaction and chelation of metal ions respectively.

After showing the feasibility of immobilizing propargyl alcohol and alkyne-terminated poly ethylene glycol as model molecules for low and high molecular weight components in the membrane pore structure via cycloaddition, aqueous solutions of functional polymers based on polyethyleneimine and poly acrylamide were created, and membranes made from azidomethylated polysulfone were infiltrated from the back side of the membrane to reject them inside the membrane’s pore structure with subsequent reaction induced either catalysed by Cu+ or by application of heat at 60 °C. A significant loss of flux was observed; however, the parameters of the infiltration process were adjusted to minimize said loss and retain fluxes of around 400 L/m2hbar  for membranes loaded with polyethyleneimine finger pore structure and 200 L/m2hbar  for membranes loaded with poly acrylamide and sponge-like pore structure.

Membrane adsorbers equipped with polyethyleneminine were used to bind Cu2+ and membranes loaded with poly acrylamide for binding Pb2+ in static and dynamic experiments reaching an adsorption capacity of 7,8 mg/g  for copper and 3,4 mg/g  for lead under static conditions. The data suggests that the adsorption follows the Freundlich isotherm model in case of copper and membrane adsorbers based on polyethyleneimine. In addition, these membranes showed capacities of 40 mg/g  for adsorption of copper, while complete loading was not reached after reaching a specific filtration volume of 350 L/m2  .

Since it could be shown that the retained functional polymers have the potential to reach higher capacities, these membrane adsorbers represent a promising approach to small scale and potentially large-scale heavy metal removal applications.



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