LFER-Ansatz zur Bestimmung der Gibbs freien Energien der Adsorption von Oxoanionen an den isotypen Fe/Al-Hydroxiden Goethit und Diaspor
Zusammenfassung der Projektergebnisse
The interest in environmental geochemistry on thermodynamics of aqueous systems at elevated temperatures have increased over the last decade. Adsorption and desorption reactions on hydrous oxide surfaces are particularly important mobility controlling reactions, because they are widespread in the hydrogeologic environment as a coating on other solids, and because their surfaces have a relatively high point-of-zero-charge (pH PZC) which facilitates oxyanion adsorption at ambient groundwater pH values. The effect of various physicochemical conditions including temperature on oxyanion adsorption was investigated on the macroscopic scale with adsorption isotherms. In the first part of the project, after detailed learning of the CD-MUSIC surface complexation modelling concept as affected by the code Visual MINTEQ, we have first re-calculated previous data modelled by the outdated and no more available code ECOSAT. This recalculation was also necessary because in earlier modelling the one-term Van’t Hoff extrapolation was used with the invalid assumption of nearly zero entropy. A two-term extrapolation was thus implemented for reliable entropy estimation. This provided the PhD student with the benefit to learn the concept from a more fundamental point of view. At the same time, goethite adsorbent material was synthesized and characterized in laboratory. In the second year, laborious (>1000) batch equilibrium adsorption experiments were performed for chromate, molybdate and vanadate onto the goethite which has a high Cr(VI), Mo(VI) (and V(V)?) adsorption capacity under acidic pH conditions. Experimental results indicate that adsorption of chromate was not very strong and exothermic, since it decreased as the ionic strength increased and as the temperature increased. A CD-MUSIC model framework was developed to allow chromate adsorption under all pH/C/I/T conditions to be quantified. The model data suggest that the inner-sphere complex (≡FeO)2CrO2 dominates at acidic pH, and that the outer-sphere complex ≡FeOH2CrO4 is important only at circumneutral pH values. The molybdenum experiment was performed in reverse order. On basis of the chromate data, first a simulation have been set up to derive at best experimental conditions. From the LFER concept using the chromate, silicate and selenate data, an entropy value for the adsorption reaction could be predicted of around 320 J K−1 mol−1 for the inner-sphere surface complex (≡FeO)2MoO2. With the 2nd outer-sphere surface complex ≡FeOH2MoO4, molybdate adsorption becomes strongly pH and ionic strength dependent as later shown by the experimental data, with a maximum adsorption in the acidic pH range. With increasing solution pH, adsorption decreased gradually to as low as 5% at pH 10. The thus predicted data were to 90% fitted by the experimental data. In parallel, experiments were conducted to study the behavior of vanadate adsorption on granular ferric hydroxide. This was not main task of the project, but a request by the waterworks association as one of main stakeholders interested in the project outcome. A combined equilibrium and kinetic model approach thus developed allows any combinations of environmental conditions to be used in scenario models to predict lifetimes of the adsorbent columns. On the other hand, high-T vanadate adsorption experiments planned for the last part of the project could not yet be finalized due to Corona crisis and laboratory lockdown. In general, our model developments and experimental data enable to improve the design and quality of adsorbents for specific industrial applications. These models allow to simulate and predict accurately adsorbent efficiency under harsh conditions on basis of conditional data like pH values, adsorbate/adsorbent concentration, ionic strength, and last but not least also temperature.
Projektbezogene Publikationen (Auswahl)
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2019. LFER and the effect of temperature on oxyanion adsorption by goethite. E3S Web of Conferences 98, 10001
Dabizha, A., Vlasova, N., Kersten, M.
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2020. Exothermic adsorption of chromate by goethite. Applied Geochemistry 123, 104785
Dabizha, A., Kersten M.
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2020. Predicting breakthrough of vanadium in fixed-bed absorbent columns with complex groundwater chemistries: A multi-component granular ferric hydroxide−vanadate−arsenate−phosphate−silicic acid system. Water Research X9, 100061
Dabizha, A., Bahr, C., Kersten, M.
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2021. Comment on “Enthalpy of Uranium Adsorption onto Hematite”. Environ. Sci. Technol.
Kersten, M.
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2021. Predicting Breakthrough of Ternary Ca–Uranyl–Carbonate Species in Mineral Water treated by a Fixed-bed Granular Ferric Hydroxide Adsorbent. ES&T Water 1, 366-375
Bahr, C., Jekel, M.R., Kersten, M.
