Section 3.5: Interface Geochemistry

Reactions at mineral surfaces greatly influence several key (bio)geochemical processes including biomineralization, nutrient and trace element cycling and contaminant dynamics. In this research theme, we focus on the mechanisms and kinetics of mineral nucleation and growth, and how these processes impact the speciation, sequestration or release/transport of various elements in Earth’s surface and (near-)subsurface environments. Currently, we are interested in the formation and/or transformation of calcium carbonates and sulfates, clay minerals, iron (oxyhydr)oxides and sulfides and phosphates (e.g., struvite) in natural and engineered environments.

In order to understand the key controlling factors affecting mineral formation and/or transformation, we make synthetic analogues of these mineral phases and perform experiments under simulated natural conditions. Such an approach allows us to perform in-depth investigations on the how minerals form and break, and how these impact critical (bio)geochemical cycles (e.g., iron, phosphorous, sulfur) as well as the mobility and toxicity of metals and metalloids (e.g., arsenic, chromium, nickel).

We integrate several analytical approaches from inorganic chemistry, materials chemistry and nanoscience to help us gain better understanding on their mineral chemistry. Specifically, we use a suite of laboratory-based solid state and aqueous phase characterization techniques (e.g., TEM, SEM, XRD, IR, ICP-OES/MS, IC) and combine them with synchrotron-based scattering (e.g., SAXS/WAXS, PDF), and spectroscopic (e.g., SXM, XAFS) techniques to probe these reactions at high spatial or temporal resolution.

Current research projects under this research theme include:

Our research is part of a large comparative sampling effort, in cooperation with other universities and institutions that aims to unravel the complex interplay between light absorbing impurities (LAI; e.g., microbes, mineral dust, black/brown carbon, etc.), and glacial ecosystems. In particular, we are interested in how LAI affect glacial melt rates (their impact on albedo), and their fate following melt events. We then hope to use these data to contribute to larger global melt models that attempt to predict climate change in the future.

The biodiversity of these glacial ecosystems will be explored using a variety of ‘omic’ techniques. These include targeted amplicon sequencing of ribosomal genes, metagenomic and metagenomic sequencing to infer microbial physiology, and mass spectrometry of metabolites in order to determine downstream metabolic processes. We are particularly interested in the role that snow and ice algae play in glacial ecosystem processes. Previous research has shown that algae can reduce albedo by up to 13%, which can dramatically increase melting. We’ve also documented that when melting starts, snow and later ice algae are the main primary photosynthetic organisms that bloom on glacier surfaces.

Additionally, we are interested in the geochemistry of these environments as it is very important in determining the types of organisms that are able to survive and thrive. Assessing the geochemistry can provide a picture of nutrients available on glacial surfaces which may be bioavailable for growth processes. Analysis of the geochemistry of the system is accomplished by evaluating cations and anions in snow, ice, and meltwaters.

Furthermore, algal blooms can produce large amounts of organic matter, however we understand little about the processes that control the production of organic compounds nor how and if they are further degraded by heterotrophic organisms or become exported through rivers to fuel ocean productivity. Assessing the carbon inputs to the system (anthropogenic or otherwise) can also help us to determine the carbon sources available to drive biological processes. Analyses are performed on glacial samples in order to determine dissolved and particulate organic matter composition. This data provides us with a comprehensive view of all organic and inorganic inputs through air and by snow to the surface of these glacial ecosystems.

Such combined microbial diversity and organic carbon data are crucial if we want to understand and quantify the fundamental processes leading to glacier melting and derive data sets that will be implemented in global numerical climate models.

Current research projects under this research theme include:

Electron microscopy allows characterization of geomaterials at the microscopic to nano-scale. We specialize in the characterization of a wide-range of minerals including, but not limited to, silicates, (oxyhydr)oxides, sulfides, sulfates, carbonates and phosphates. In addition, we are developing novel high-resolution electron imaging and spectroscopic tools, as well as complex sample environments, for the characterization of geomaterials.

The research group runs and manage the Potsdam Imaging and Spectral Analysis (PISA) Facility. The PISA facility currently houses 2 high-resolution Transmission Electron Microscopes (TEM), 1 high-resolution Scanning Electron Microscope (SEM) and 2 Focused Ion Beam combined with SEM (FIB-SEM).

Using our ZEISS Ultra Plus and FEI Quanta SEMs and associated Energy Dispersive X-ray (EDX) Spectroscopy, we can provide mineral surface structure and its chemical composition. Further mineral characterization of thin samples can be obtained using Transmission Electron Microscopy (TEM) and Scanning Transmission Electron Microscopy (STEM) on our FEI Tecnai and Thermo Fisher Scientific Titan Themis Z TEMs. On these TEMs, Energy Dispersive X-ray (EDX) Spectroscopy and Selected Area Electron Diffraction (SAED) can be used to provide chemical composition and crystal structure at the nano-scale. Focused Ion Beam (FIB) milling on FEI Helios electron microscope allows thin sample preparation for TEM observations. Finally, Liquid Phase Transmission Electron Microscopy (LPTEM) using Poseidon holder (Protochips) is a cutting-edge technique currently developed here to design experimental procedure that will allow us to investigate in situ mineral formation and mineral dissolution processes.

Current research projects from our group include:

External scientific users can request access to these microscopes. Please visit Potsdam Imaging and Spectral Analysis (PISA) Facility for more information.

Postdocs and PhD students of the Interface Geochemistry research group will provide semi-annual updates detailing their research to date. Research updates will be posted here as they become available.


Rebecca Volkmann, PhD Student

Besides our research on nucleation and growth kinetics of the mineral struvite, a new direction has opened up in the project: We found out that struvite crystals transform to the mineral newberyite after some time in air. This phenomenon is known in the literature, but no systematic data on temperatures and timescales are available. So, the project now also includes to monitor the transformation kinetics of struvite to other phosphate minerals.

The image shows two struvite crystals that have been exposed to air for six months under the petrographic microscope. On the left side colourless under linear polarised light, the twinned struvite crystal shows colourful 2nd order interference colours under crossed polars in the right-hand sided image. Highly refractive, dark phases can be seen at the phase boundaries, that are probably newly formed newberyite.

Alice Paskin, PhD Student

In my research work I mainly focus on the geological relevance and phosphate recovery potential of Fe(II) based phosphate mineral vivianite, which is found widely in anoxic water bodies and soils. The broader purpose of this project is to optimize vivianite formation and the effect of various physical (pH, temperature) and chemical (organic and inorganic additives) parameters towards its formation. The analytical methods I frequently employ for my research work are pH-based kinetics, IR, powder XRD, SEM, TEM and UV-vis spectrophotometry.

Zhengzheng Chen, PhD Student

During the first six months, I completed the first draft of the literature review and started co-precipitation experiments with different concentrations of organophosphorus and ferrihydrite, completing the adsorption of ferrihydrite on organophosphorus. The structural properties of the samples were characterized using XRD and IR. The surface area of the samples was determined by BET and so far, it appears that the surface area decreases with increasing organic concentrations.

Ruth Esther Delina, PhD Student

Tropical weathering of ultramafic rocks forms ferruginous Ni laterite deposits. These deposits are typically enriched in, and sought for Ni, Co, Sc, REEs, and PGEs. Nickel laterites also contain elevated amounts of Cr which may occur in its hexavalent form, a known toxic pollutant and carcinogen. To assess the speciation, mobility, and therefore, the environmental impact of Cr associated with Ni laterites, a sequential extraction procedure (SEP) tailored to Cr in ferruginous tropical soils will be optimized. The improved scheme should address problems with established SEPs (e.g. readsorption of Cr, partial dissolution of metal-substituted Fe-oxyhydroxides) identified through an extensive literature review. The optimization involves testing selected extractants on synthesized Fe-bearing minerals commonly found in Ni laterites (e.g. goethite, hematite, magnetite, etc.). In my first six months, I focused on the syntheses of pure Fe-bearing phases (Fig. A) and metal-substituted (e.g. Cr) varieties (Fig. C) through different pathways such as transformation of ferrihydrite (Fig. B), forced hydrolysis of Fe(III) solutions, and partial oxidation of Fe(II) solutions (Fig. D-E). I characterized them using x-ray diffraction and infrared spectroscopy and prepared them for elemental analysis. During this period, I learned the effects of different physicochemical parameters such as temperature, pH, redox environment, substituting cation, etc. to the formation of Fe-oxyhydroxides. Several attempts of synthesizing Cr-hematite, for example, exhibited how Cr substitution inhibits its formation.

Rebecca Volkmann, PhD Student

To evaluate nucleation and growth processes of the mineral struvite, we conducted synthesis experiments of pure solutions of different initial concentrations at standard conditions. The crystal formation has been followed by UV-Vis spectrometry to monitor changes in turbidity that occur during crystallization. Precipitates have been imaged with the (cryo-) SEM. The next steps will include syntheses under a temperature range from approximately 5 to 50 °C as well as sample analysis and imaging to see changes in formation depending on temperature.

The image shows a turbidity [%] vs. time [s] plot of struvite formation at different solution concentrations. The first rise in turbidity refers to the induction time of crystal formation, which presents the time window until the first mineral nucleates are created. The induction time increases from 20 to 350 s with decreasing concentrations of the struvite solutions.

Elisa Katharina Peter, PhD Student

Within the Deep Purple project, we investigate pigmented microorganisms which inhabit the Greenland Ice Sheet and accelerate the ice melt due to surface darkening. We aim to improve our understanding of the processes governing pigment formation by investigating the metabolome of purple-brown ice algae (1) as well as red (2) and green snow algae. The picture above shows polar (1A, 2A) and non-polar (1B, 2B) intracellular metabolite extracts of ice and snow samples from our summer 2020 fieldwork. We are excited to see indications of a high abundance of water-soluble purple pigments such as purpurogallin in the ice algae and the prevalence of less polar orange and red pigments such as carotenoids in the red snow algae and look forward to the LC- and GC-MS results of our untargeted metabolome study.

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