Nanostructured Composite Adsorber Membranes for the Reduction of Trace Substances in Water: The Example of Bisphenol A

Abstract

The increasing contamination of water with biopersistent substances is considered to become one of the key environmental problems in the next decades. These so-called micropollutants show considerable toxic or otherwise negative effects even at concentrations in the ppt level or ppb level. For the experimental proof-of-concept study we chose 2,2-bis(4-hydroxyphenyl)-propane known under the name bisphenol A (BPA). The extensive use of BPA-based polymers has led to widespread environmental distribution of BPA. BPA is an endocrine disrupting compound which has an impact on the hormonal system (endocrine system) of humans and animals even at low concentrations. To tackle the problem of the micropollutants in the aquatic environment, efforts are being made to develop new adsorber materials with specific properties for extracting these various substances in one step with one membrane. For this reason cross-linked polymer nanoparticles were synthesized by miniemulsion polymerization and their adsorption properties were characterized. The properties of the polymer particles were adjusted for BPA by variation of the monomer, cross-linker and the stoichiometric ratio. Comparative adsorption tests were carried out with silica gel, commercial adsorber material (RP-C18), activated carbon, and the adjusted adsorber material P(4-VP-co-EGDMA). The advantage of the nanoscale, nonporous polymer particles compared with commercial adsorber material is the relatively large and rapidly accessible external surface. The adjusted adsorber material P(4-VP-co-EGDMA) and the activated carbon absorbed almost the complete quantity of BPA supplied. The advantage of the polymer adsorber material compared with activated carbon is the greater ease of regenerability. The BPA can be extracted from the polymer by means of ethanol or can be desorbed by means of a pH value shift. The adsorptive polymer particles were incorporated into a porous polymer matrix by an immersion phase separation process to build an affinity membrane. The production process for the membrane is controlled in such a way as to create an inner sponge-like structure in which the polymer nanoparticles are firmly embedded and nevertheless are easily accessible to the adsorbate. The sponge-like-structured membrane has the higher flux compared with a commercial UF membrane. The polymer particles were deposited in the pores of the sponge-like membrane structure during the membrane formation process and so are freely accessible to BPA. Tests were carried out in which the retained quantity of BPA of a commercially available UF membrane was compared with the retained quantity of BPA of a particle loaded membrane. This allows the construction of membranes for specific requirements in order to extract micropollutants ideally in one step with a functional membrane.