Michael Mader

Advances in Stable Isotope Monitoring of CO2 Under Elevated Pressures, Temperatures and Salinities: Selected Results from the Project CO2ISO-LABEL

The BMBF project CO2ISO-LABEL (Carbon and Oxygen ISOtopes under extreme conditions LABoratory EvaLuations for CO2-storage monitoring) investigated stable isotope methods in laboratory studies for transferral to carbon capture and storage (CCS) field sites including enhanced gas and oil recovery (EGR and EOR). The isotope composition of injected CO2 and water are useful tracers for migration and water-rock-gas interactions during such operations. However, quantification of carbon and oxygen equilibrium isotope effects at elevated pressures and temperatures are so far scarce. They thus need more investigations under p/T conditions that are characteristic for reservoirs and overlying aquifers. With this, the main objective of the project was to improve stable carbon and oxygen isotope methods for monitoring CO2 storage sites and their impact of injected CO2 on reservoir geochemistry under controlled laboratory settings. An important finding was that isotope fractionations of carbon between CO2 and dissolved inorganic carbon (DIC) were not significantly different from each other in experiments with pure CO2 and pressures between 59 and 190 bar. Furthermore, influences of rock types (limestone, dolomite and sandstone) and fluid salinities were found to be negligible for carbon isotope fractionation between CO2 and DIC. Another finding was that water oxygen isotope ratios changed systematically in response to different CO2/H2O molar ratios in closed system equilibration experiments. This helps to reconstruct the amounts of CO2 that equilibrated with formation waters. Results of the project will enable better assessment of geochemical conditions in underground carbon storage sites or other subsurface systems where large amounts of CO2 interact with water and rocks.

Stable isotope mass balances versus concentration differences of dissolved inorganic carbon – implications for tracing carbon turnover in reservoirs

ABSTRACT The aim of this study was to identify sources of carbon turnover using stable isotope mass balances. For this purpose, two pre-reservoirs in the Harz Mountains (Germany) were investigated for their dissolved and particulate carbon contents (dissolved inorganic carbon (DIC), dissolved organic carbon, particulate organic carbon) together with their stable carbon isotope ratios. DIC concentration depth profiles from March 2012 had an average of 0.33 mmol L–1. Increases in DIC concentrations later on in the year often corresponded with decreases in its carbon isotope composition (δ13CDIC) with the most negative value of –18.4 ‰ in September. This led to a carbon isotope mass balance with carbon isotope inputs of −28.5 ‰ from DOC and −23.4, −31.8 and −30.7 ‰ from algae, terrestrial and sedimentary matter, respectively. Best matches between calculated and measured DIC gains were achieved when using the isotope composition of algae. This shows that this type of organic material is most likely responsible for carbon additions to the DIC pool when its concentrations and δ13CDIC values correlate negatively. The presented isotope mass balance is transferable to other surface water and groundwater systems for quantification of organic matter turnover.

Insights into agricultural influences and weathering processes from major ion patterns

Department of Geography and Geosciences, Friedrich‐Alexander‐University Erlangen‐ Nuremberg (FAU), GeoZentrum Nordbayern, Schlossgarten 5, 91054 Erlangen, Germany Gehrenhagweg 6, CH‐5420 Ehrendingen, Switzerland Correspondence Robert van Geldern, Department of Geography and Geosciences, Friedrich‐ Alexander‐University Erlangen‐Nuremberg (FAU), GeoZentrum Nordbayern, Schlossgarten 5, 91054 Erlangen, Germany. Email: robert.van.geldern@fau.de

River recharge versus O 2 supply from the unsaturated zone in shallow riparian groundwater: A case study from the Selke River (Germany)

Besides gas-water-exchange in surface waters, respiratory consumption of dissolved oxygen (DO) in adjacent riparian groundwater may trigger the addition of so far hardly explored sources from the unsaturated zone. These processes also systematically influence stable isotope ratios of DO and were investigated together with Cl- as a conservative tracer for water mixing in a near-river riparian groundwater system. The study focused on a losing stream section of the Selke River at the foot of the Harz Mountains (Germany). The study area exposed steep DO gradients between the stream water and riparian groundwater between April 2016 and May 2017. Our results indicated dominant influences of microbial community respiration with observed DO concentration gradients. These observations can be explained by DO from the river that is subject to fractionation by microbial respiration with a typical fractionation factor (αr) of 0.982. However, with such respiration dominance, we expected a simultaneous enrichment of δ18ODO towards values that are more positive than the well-known atmospheric O2 signal of +23.9‰ versus the Vienna Standard Mean Ocean Water standard (VSMOW). Surprisingly, our measurements revealed much lower δ18ODO values between +22‰ and +18‰ in the near-river groundwater. Mass balance calculations revealed that the DO pool in the shallow and unconfined aquifer receives contributions of up to about 80% by diffusion of oxygen from the vadose zone with a distinctly lower isotope value than the one of the atmosphere. This finding about additional oxygen sources from the unsaturated zone has numerous ramifications for oxygen related processes in near-river environments including the oxidation of pollutants, nutrients and ecosystem health.

Direct oxygen isotope effect identifies the rate-determining step of electrocatalytic OER at an oxidic surface

Understanding the mechanism of water oxidation to dioxygen represents the bottleneck towards the design of efficient energy storage schemes based on water splitting. The investigation of kinetic isotope effects has long been established for mechanistic studies of various such reactions. However, so far natural isotope abundance determination of O2 produced at solid electrode surfaces has not been applied. Here, we demonstrate that such measurements are possible. Moreover, they are experimentally simple and sufficiently accurate to observe significant effects. Our measured kinetic isotope effects depend strongly on the electrode material and on the applied electrode potential. They suggest that in the case of iron oxide as the electrode material, the oxygen evolution reaction occurs via a rate-determining O−O bond formation via nucleophilic water attack on a ferryl unit.Understanding reaction mechanisms is crucial for catalyst design. Here, natural-abundance isotope quantifications of O2 yield mechanistically significant reaction kinetic isotope effects for water oxidation over metal oxide electrodes, the bottleneck step of water electrolysis.