Adsorption of the phytoestrogens, genistein and daidzein, onto the iro oxide-hydroxide goethite
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Genistein and daidzein are plant-derived compounds called phytoestrogens, and they are found in high concentrations in legumes, primarily soy. Phytoestrogens function as endocrine disrupting chemicals (EDCs) in animals when assimilated in high doses. Changed secondary sex characteristics and reduced fertility have been observed in fish due to these endocrine disrupting abilities of phytoestrogens. The role of phytoestrogens as environmental contaminants is highly relevant because of the growing soy-based biofuel industry. This thesis examines the extent to which genistein and daidzein adsorb onto goethite nanoparticles (-FeO(OH)). The goethite simulates sediment consisting of iron particles and suspended solids in water. The aim of this thesis is to investigate the fate of genistein and daidzein when they are discharged in water bodies with ferruginous sediments, and whether they get removed from the aqueous phase or stay dissolved. Standard batch adsorption experiments were carried out for genistein and daidzein onto goethite, and adsorption isotherms and sorption edges were made from the results. Furthermore the specific surface area (SSATEM) of the goethite nanoparticles was calculated using TEM-pictures, and the pKa values for goethite were found and calculated from standard potentiometric titrations. SSATEM of the goethite nanoparticles was found to be 248 m2/g ± 90 m2/g, while the specific surface area determined through BET-analysis, SSABET, was measured to 118.5 m2/g ± 2.6 m2/g. Furthermore the pKa values for goethite were calculated at pH 6.73 and 10.34, and the pH of point zero charge, pHpzc, was found at 8.3. The isotherm and sorption edge results showed that adsorption of genistein and daidzein onto goethite is pH-dependent and also independent of the adsorbate concentrations at these relatively low environmental relevant concentrations. For genistein, the adsorption decreased constantly with increasing pH. The highest adsorption capacity was at pH 4.7 with a KD at 0.00431 L/m2 and a percent mass adsorbed at 84 %. At pH higher than 7.4, the adsorption capacity started decreasing rapidly, to a minimum of 17 % mass adsorbed and a KD of 0.000168 L/m2 at pH 9.7. Daidzein was only removed to a limited degree and had an adsorption maxima at pH 7.4, with 27 % mass adsorbed and a KD at 0.000307 L/m2. KD and the percent mass adsorbed were slightly lower from pH 4.7 to 7.4. The minimum adsorption capacity for daidzein was found at pH 9.7, with a KD of 4.66691E-06 L/m2 and only 1 % mass adsorbed. Hydrophobic interactions or surface complexations were suggested as the adsorption mechanisms at low pH for both genistein and daidzein. Electrostatic repulsion was proposed to be the reason for the drop in adsorption capacity at higher pH, when the compounds get deprotonated. Applying the results to determine the fate of phytoestrogens in the environment, it is clear that genistein will be removed by adsorption to a greater extent than daidzein, when released to ferruginous natural waters. The removal of the phytoestrogens from the water phase through adsorption is significantly higher at low pH compared to high pH values, and thus the adsorption mechanism is more relevant for low pH waters.
Master's thesis in Environmental Technology