LFER Approach to predict the Gibbs free energy of oxoanion Adsorption by the isotype Fe/Al hydroxides goethite and diaspore

• Applicant: Professor Dr.-Ing. Michael Kersten
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: from 2016 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 314328699

The importance and demand in environmental geochemistry on thermodynamic data for aqueous systems at elevated temperatures has increased over the last decade. This is due to an intense debate on potential groundwater contamination with regard to geothermal power generation, carbon capture and sequestration, hydrofracking, and nuclear waste repositories. Deep aquifer brines recharged within the framework of these geo-engineering activities often contain elevated concentrations of toxic oxyanions such as arsenate and vanadate. However, the factors controlling the mobility of these oxyanions in geothermal waters at depth and with the pipe scales is not fully known. There is concern that they are more mobile at these than at ambient surface conditions, and may thus contaminate the shallower groundwater aquifer systems upon contact. In principle, much is known about water-rock interaction (adsorption, precipitation, etc.) processes governing retardation of the toxic oxyanions at room temperature, but much less about the processes controlling their solid-water partitioning along the temperature gradient. The hypotheses for this proposal are that (i) the adsorption behavior at elevated temperatures for all oxyanions can be predicted by a linear free energy relationship (LFER) between their Gibbs free energy of adsorption and of their aqueous protonation reaction, and that (ii) the entropy term can be predicted on basis of molecular level information on the surface complex structure. The data yet gained in the PIs lab for selenate, arsenite, and silicic acid do not suffice to validate these hypotheses. The purpose of the current work proposal is (1.) to perform experiments on the three binary adsorbate-adsorbent systems bromate-goethite (FeOOH), chromate-goethite and arsenate-goethite at higher temperatures to verify these hypotheses with the most critical data points identified by the LFER, and (2.) to extent by additional experiments the approach to the binary oxyanion-diaspore (AlOOH) system. The latter is even of greater importance with respect to water-rock interaction related to the geo-engineering activities. The aim is to set the LFER and entropy hypotheses on a reliable basis for prediction of standard molar Gibbs free energies of adsorption of all other oxyanions, for which no experimental data are available or difficult to obtain. This aim can be reached only with extensive experimental laboratory work to vary adsorbate/adsorbent ratio, pH value, ionic strength, and even temperature in a 4-D parameter space matrix suitable for the equilibrium thermodynamic modelling of the surface complexation reactions.

• DFG Programme: Research Grants
• International Connection: Ukraine
• Cooperation Partner: Professorin Dr. Nataliya Vlasova