Removal of endocrine-disrupting compounds from wastewater using molecular imprinting

Sammanfattning: Endocrine-disrupting compounds (EDCs) such as hormones and certain pharmaceuticals (i.e. birth control pills, oestrogen replacement products and other steroids) can interfere with the regulatory systems of humans and wildlife. Many of these compounds are excreted by humans and released with sewage. Pesticides and various industrial chemicals such as dioxin and PCBs are also EDCs that often find their way into waterways. EDCs are typically found at trace concentrations (ng/l-µg/l) in contaminated streams, which is especially challenging when engineering their removal. There is thus an urgent need to develop novel specific, and highly effective, methods to enrich trace contaminants in water treatment processes prior to their removal and destruction. This thesis describes a novel method for the selective and efficient removal of EDCs using molecularly imprinted polymers (MIPs) synthesized by guided polymerization of functional monomers around a target molecule (a so-called template). MIPs were thus synthesized with oestradiol (E2), the most potent oestrogen known, and were tested as an adsorbent in solid phase extraction (defined as MISPE). Complete recovery of E2 was achieved after applying 2 litres of E2 solution (2 µg/l), while the control, non-imprinted polymers (NIPs) (without template) removed only 77% of the E2. Adsorption to MIPs is based on the same property that makes the pollutants so harmful, namely their capacity to bind to selective sites; natural receptors in the case of hormones, and artificial receptors in the case of synthetic polymers. This would also allow the removal of unknown contaminants having the capacity to bind to natural receptors. Thus, knowledge of the nature, concentration and toxicity of each contaminant is no longer required and, if a molecule is toxic because it can bind to a natural biological receptor, it should be removed by the synthetic receptor analogues. This was demonstrated in this work by the removal of unknown compounds with oestrogenic activity from a wastewater sample by adsorption through E2-MIPs, using an in vitro assay, e.g. a transformed yeast strain (Saccharomyces cerevisiae, strain BJ3505) containing the human oestrogen receptor. Significant oestrogenic activity was only observed in the extracts from the MISPE percolated with wastewater, and was equivalent to the effect of 22 ± 4 ng E2/l in wastewater. Despite the excellent efficiency and selectivity of MIPs to retain EDCs at trace concentrations, clogging and back-pressure problems occurred when handling sewage. To overcome these problems, a new method was developed to embed the imprinted polymers into high-flow macroporous gels. Macroporous gels, also known as cryogels, were prepared from a polyvinyl alcohol (PVA) gel precursor and cast at subzero temperatures, where ice crystals form, leaving interconnected pores after melting. Macroporous molecularly imprinted cryogels were either synthesized as monoliths (denoted MIP/PVA-MGMs) tested in solid phase extraction or cast in Kaldnes carriers (so-called macroprous gel particles and denoted MIP/PVA-MGPs). The MIP/PVA-MGMs were able to recover 110 ± 10% of the E2 from a spiked wastewater sample (2 µg/l) at a flow rate of 50 ml/min, while the non-imprinted adsorbent medium (NIP/PVA-MGMs) could only remove 74 ± 9%. MIP/PVA-MGPs were prepared using E2, atrazine and nonylphenol (NP) as template, and tested in a moving-bed MGPs reactors, which consisted of a glass column filled with MIP(NIP)/PVA-MGPs equipped with an air pump to ensure constant motion of the MGPs in the reactor. These compounds are all classified as EDCs and are commonly found in aquatic environments. Their removal from spiked aqueous solutions was performed separately at environmentally relevant concentrations (0.5 µg E2/l, 10 µg atrazine/l and 5 µg NP/l), at a very short hydraulic retention time (4 min). Both E2-MIP/PVA-MGPs and atrazine-MIP/PVA-MGPs showed highly selective and efficient removal, while NP-MIP/PVA-MGPs were less selective. Final destruction of the contaminants is necessary to avoid the generation of highly concentrated hazardous waste. Biological degradation was first investigated as it is an economic and environmentally friendly alternative. Within 14 days of inoculation, a pure bacterial suspension of E2-degrading Novosphingobium tardaugens was able to remove 85 ± 3% of the E2 present in aqueous solution, showing that this pure strain is suitable for the treatment of enriched levels of contaminant. However, the enriched contaminants are normally recovered in the organic solvent used for the elution of the template from the MIPs. Thus, MIP-extraction was combined with phototreatment (UV-visual). This study showed that simultaneous irradiation and elution was feasible, with 90% of the extracted E2 being degraded within 10 hours. Moreover, the acetone used for elution and the irradiated imprinted polymer could both be re-used in a new adsorption-elution cycle without any loss of efficiency.