Field-based assessment of in-stream contaminant fate

Water quality of rivers worldwide is affected by the increasing use of organic mi-cropollutants for human purposes. Most pollutants enter rivers via urban areas and wastewater treatment plants either freely dissolved, or attached to particles and may undergo transformation processes during their transport in rivers. There is still a lack of a holistic assessment of relevant processes, as well as comprehensive and representative field-based studies that describe and quantify the attenuation of these compounds in rivers. This thesis tackles this knowledge gap and aims to bet-ter understand processes that determine the in-stream contaminant fate. Processes potentially relevant for the fate of organic micropollutants under field-conditions are summarized in this thesis in an integrated way including on the one hand hydrological aspects such as transport processes, groundwater and tributary inflow, and hyporheic exchange, as well as reactive processes comprising sorption, biodegradation, volatilisation and photodegradation. State of the art measurement approaches to capture these processes are compiled. The abundance of relevant processes under field-conditions and the fact that they take place simultaneously clearly shows the need for an appropriate sampling strategy to disentangle attenua-tion processes of pollutants in rivers. For this purpose, a field study was conducted aiming at the identification of relevant water fluxes that contribute to the river discharge since they may lead either to an in-stream dilution or concentration increase of pollutants. Different potentially useful groundwater tracers were methodologically compared for this purpose. The results illustrate that the proper choice of tracer is crucial to quantify and localize ground-water inflows. In addition, these results clearly evidence the need for a holistic un-derstanding of water fluxes in catchments, with respect to the water quality of in-flowing water especially in terms of the in-stream chemical status and thus pollutant turnover processes in rivers. On this basis, the goal of a following field study was the quantification of repre-sentative transformation rate constants of selected organic micropollutants as they are transported downstream from the wastewater treatment plant (selected indicator substances). The same water parcels were compared at two sequential reaches to investigate relevant environmental factors that may differ between both reaches and thus influence the fate. The results demonstrate the important role of photolysis for the attenuation of dissolved organic pollutants despite the generally comparably low reactivity in the selected river system. The latter is, inter alia, attributed to the an-thropogenic character of the river that leads to a fast travel time. A holistic assessment of organic micropollutant transport in rivers compiles equally the dissolved and particle-bound pollutants which is the reason why selected hydro-phobic compounds associated to suspended particles were traced through the catchment during high discharge conditions. The hydrophobic character of these compounds allows the identification of particle origin in the river. Moreover, the in-teraction between particle transport and the sediment storage is identified as im-portant mechanism for particle associated pollutant transport in rivers. In summary, the holistic approach of this thesis gives insights into relevant process-es that determine the fate of organic micropollutants in fluvial systems. This work provides a sound basis for future field-based fate studies that aim to quantify atten-uation processes in rivers

La1-xSrxMnO3±δ as a nonstoichiometric model system for the catalysis of oxygen evolution reaction

The main issue of electrochemical water splitting is the search for suitable catalyst materials for the kinetically hindered oxygen evolution reaction (OER) at the anode. The state-of-art precious metal catalysts suffer from their high price and insufficient long term stability in the common electrolytes. Binary or multinary transition metal oxides in alkaline medium are an cost-efficient alternative, since they can achieve both lower overvoltages as well as better long-term stability than precious metal oxides. One of the best-studied materials for SOFC applications is the perovskite system La1-xSrxMnO3±δ (LSMO). Due to its general nonstoichiometry and the large variety of possible defect species it is a perfect model system for studying the influence of defect chemistry on the catalytic activity in OER. We have systematically investigated LSMO as a nonstoichiometric model catalyst system for OER in alkaline media. Nanocrystalline powders over the whole composition range have been prepared by a sol-gel based auto combustion method. XPS and XRD analysis verified the presence of pure phase materials with a continuous change of manganese oxidation state from Mn3+ to Mn4+. Measurements in an electrochemical RDE setup showed a clear trend in catalytic activity in OER with the highest values at medium La/Sr compositions. An equivalent trend could also be observed in the electrical conductivity of the powders, leading to the assumption of a higher polaron hopping probability at medium La/Sr compositions. Additional annealing of pristine powder samples in oxidizing and reducing atmospheres caused a further change in manganese oxidation state and ongoing electrochemical measurements should reveal whether the defined adjustment of the nonstoichiometry will lead to an improvement of catalytic activity compared to the untreated catalysts

Unravelling Charge Carrier Mobility in d₀ ‐Metal‐based Spinels

Enabling high Mg ion mobility, spinel-type materials are promising candidates for cathode or solid electrolyte applications. To elucidate the factors governing the observed high mobility of multivalent ions, periodic DFT calculations of various charge carriers (A=Li, Na, K, Mg, Ca, Zn and Al) in the ASc₂S₄ and ASc₂Se₄ spinel compounds were performed, resulting in the identification of a Brønsted-Evans-Polanyi-type scaling relation for the migration barriers of the various charge carriers. Combining this scaling relation with the derivation of a descriptor, solely based on easily accessible observables, constitutes a conceptual framework to investigate ion mobility in d₀-metal-based spinel chalcogenides with significantly reduced computational effort. This approach was exemplarily verified for various d₀-metal-based spinel chalcogenide compounds AB₂X₄ (B=Sc, Y, Ga, In, Er and Tm; X=O, S and Se) and led to the identification of d₀-metal-based CaB₂O₄ spinels as promising compounds possibly enabling high Ca ion mobility

Anthropogenic activities significantly increase annual greenhouse gas (GHG) fluxes from temperate headwater streams in Germany

Anthropogenic activities increase the contributions of inland waters to global greenhouse gas (GHG; CO$_2$, CH$_4$, and N$_2$O) budgets, yet the mechanisms driving these increases are still not well constrained. In this study, we quantified year-long GHG concentrations, fluxes, and water physico-chemical variables from 28 sites contrasted by land use across five headwater catchments in Germany. Based on linear mixed-effects models, we showed that land use was more significant than seasonality in controlling the intra-annual variability of the GHGs. Streams in agriculture-dominated catchments or with wastewater inflows had up to 10 times higher daily CO$_2$, CH$_4$, and N$_2$O emissions and were also more temporally variable (CV > 55 %) than forested streams. Our findings also suggested that nutrient, labile carbon, and dissolved GHG inputs from the agricultural and settlement areas may have supported these hotspots and hot-moments of fluvial GHG emissions. Overall, the annual emission from anthropogenic-influenced streams in CO$_2$ equivalents was up to 20 times higher (∼ 71 kg CO$_2$ m$^{−2}$ yr$^{−1}$) than from natural streams (∼ 3 kg CO$_2$ m$^{−2}$ yr$^{−1}$), with CO$_2$ accounting for up to 81 % of these annual emissions, while N$_2$O and CH$_4$ accounted for up to 18 % and 7 %, respectively. The positive influence of anthropogenic activities on fluvial GHG emissions also resulted in a breakdown of the expected declining trends of fluvial GHG emissions with stream size. Therefore, future studies should focus on anthropogenically perturbed streams, as their GHG emissions are much more variable in space and time and can potentially introduce the largest uncertainties to fluvial GHG estimates

Swabian MOSES 2021: An interdisciplinary field campaign for investigating convective storms and their event chains

The Neckar Valley and the Swabian Jura in southwest Germany comprise a hotspot for severe convective storms, causing tens of millions of euros in damage each year. Possible reasons for the high frequency of thunderstorms and the associated event chain across compartments were investigated in detail during the hydro-meteorological field campaign Swabian MOSES carried out between May and September 2021. Researchers from various disciplines established more than 25 temporary ground-based stations equipped with state-of-the-art in situ and remote sensing observation systems, such as lidars, dual-polarization X- and C-band Doppler weather radars, radiosondes including stratospheric balloons, an aerosol cloud chamber, masts to measure vertical fluxes, autosamplers for water probes in rivers, and networks of disdrometers, soil moisture, and hail sensors. These fixed-site observations were supplemented by mobile observation systems, such as a research aircraft with scanning Doppler lidar, a cosmic ray neutron sensing rover, and a storm chasing team launching swarmsondes in the vicinity of hailstorms. Seven Intensive Observation Periods (IOPs) were conducted on a total of 21 operating days. An exceptionally high number of convective events, including both unorganized and organized thunderstorms such as multicells or supercells, occurred during the study period. This paper gives an overview of the Swabian MOSES (Modular Observation Solutions for Earth Systems) field campaign, briefly describes the observation strategy, and presents observational highlights for two IOPs

Analyzing Particle-Associated Pollutant Transport to Identify In-Stream Sediment Processes during a High Flow Event

Urban areas are a leading source of polycyclic aromatic hydrocarbons (PAHs) that result from combustion processes and are emitted into rivers, especially during rain events and with particle wash-off from urban surfaces. In-stream transport of suspended particles and attached PAHs is linked strongly to sediment turnover processes. This study aimed to identify particle exchange processes that contribute to the transport of suspended particles during flood events. An urban high-flow signal was tracked in high temporal resolution at two sampling sites in the Ammer River (South-western Germany). Samples were analyzed for turbidity, total suspended solids concentrations (TSS), particle-size distribution, organic carbon, and PAH. Maximum discharge and the highest TSS occurred nearly simultaneously at the upstream sampling site, whereas a temporally shifted course was observed for downstream. The total load of particles was similar, yet a decrease of PAH mass (~28%) and an increase of the particulate organic carbon (POC) content (~3.5%-points) occurred. Coarser particles (≥26 µm) dominated at the beginning of the event at both sampling sites. The signal of remobilized riverbed sediment increases downstream and leads to well-established, robust linear correlations between TSS and PAHs. This study highlights that riverbed sediment acts as intermediate storage for contaminated particles from upstream sources that shape, together with the fresh urban input, the “particle signature” of suspensions moving through catchments during high discharge conditions