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Separation and Characterization of Colloidal Mineral-Associated Organic Matter

Mineral-associated organic matter (MAOM) colloidal particles are a critical component in terrestrial ecosystems. They represent a substantial pool of soil organic matter (SOM) that regulates the biogeochemical cycling of organic carbon in our Earth system. Therefore, it is critical to understand the correlation between the nature (formation, type, and reactivity) of MAOM colloids in porewaters, bulk dissolved organic matter (OM) composition, geochemical conditions, and soil type to better predict the movement of OM from soil to porewater to atmosphere. 

The Environmental Molecular Sciences Laboratory (EMSL) developed a workflow to rapidly separate and characterize MAOM colloids that are <0.45 µm in size and prevalent in soil-water systems. This workflow is available to the research community through EMSL’s open calls for proposals. 

In this workflow, colloids will be aerosolized using a nebulizer. Then, the size-selected airborne colloids will be initially collected on transmission electron microscopy (TEM) grids using a cascade impactor. This process circumvents the labor-intense centrifugation and freeze-drying processes. The isolated particles on TEM grids will be sequentially characterized by a suite of nondestructive spectroscopic methods: scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX), X-ray photoelectron spectroscopy (XPS), TEM-EDX selected-area electron diffraction (SAED) for morphology, OM speciation, and MOAM associations. 

Research applications 

Supporting the Biogeochemical Transformations and Terrestrial-Atmospheric Processes Integrated Research Platforms, this workflow incorporates reactive transport models to better understand the impacts of colloidal MAOM particles on rates and extents of greenhouse gas releases from soil-water systems under varying biogeochemical conditions. 

List of related instrumentation and resources 

  • A medical nebulizer (8900-7-50, Salter Labs, Inc.) for generating colloidal particles. 

  • A nano Micro-Orifice Uniform Deposit Impactor (nano-MOUDI, model 110R, MSP, Inc.) for collecting particles. 

  • A computer-controlled SEM with EDX to probe their physicochemical properties. 

  • A scanning transmission electron microscope (STEM, Thermo Fisher, model Titan 80-300) coupled with EDX (Oxford Instruments) to probe detailed structures and elemental mapping. 

  • A Physical Electronics Quantera scanning X-ray microprobe for analyzing the elemental composition at the surfaces of colloidal particles. 

Tips for success 

  • Well-preserved “fresh” porewaters or water-extractable solutions, particularly for anoxic samples. 

  • Ambient temperature solutions to circumvent any inadvertent aggregation and precipitation of colloids from the “soluble” components.  

  • Approximately 100 mL of porewater samples with known OM and metal contents. 

  • If porewaters are not available, approximately 10 g of well-preserved soil will be used for water-extractable OM. 

Contributing teams and resources 

EMSL develops and deploys capabilities for the user program by conducting original research independently or in partnership with others and by adapting/advancing science and technologies developed outside EMSL. In some instances, EMSL directly deploys mature capabilities developed by others where there is value for the EMSL user community. The following grants/activities, principal investigators (PIs), and teams contributed to the development of this capability: 

  • Ravi Kukkadapu, Zezhen Cheng, Qian Zhao, Environmental Molecular Sciences Laboratory, S&T Developer: 60097. 

Contacts