This objective of this work is to predict condensation of atmospheric water vapor in the vadose zone of organic-bearing soils. It will primarily develop knowledge on the formation of condensed forms of water that are crucial for biogeochemical functioning of unsaturated terrestrial environments. This is important because mineral-bound thin water films are the media through which nutrients, metabolites and contaminants are transported in unsaturated porous media, and because organic molecules are of widespread occurrence in soils.
The proposed experimental work will resolve the mechanisms through which (1) water vapor binds to minerals coated by environmentally relevant organic molecules, and (2) how packing modes of minerals affect water adsorption and condensation. To achieve these goals we will follow water adsorption and condensation reactions in representative soil minerals reacted with environmentally relevant organic molecules (e.g. oxalic acid, shikimic acic, humic acid). We will also test how packing motifs of particles affect water condensation, as driven by inter-particle capillaries.
Ongoing Fourier Transform Infrared Spectroscopy and Dynamic Vapor Adsorption work in our laboratory has already guided us with the mineral-organic assemblages needed to achieve our scientific goals. Access to the three following key EMSL capabilities will open new possibilities along this front. First, Sum Frequency Generation Spectroscopy (SFG) will probe the molecular-scale nature of water films
associated to organic-mineral assemblages. This technique will resolve detailed and unparalleled information on water binding and condensation at the top-most surface of the samples that cannot be achieved using conventional forms of vibration spectroscopy. Second, scattering-type infrared scanning near-field optical microscopy (s IR-SNOM) will show how organics are distributed in mineral-organic aggregates. This relatively novel technique will be crucial for assessing the impact of chemical and physical heterogeneity on water condensation. Finally, environmental scanning electron microscopy (ESEM) will give details on the dynamics of water condensation in organic-mineral aggregates. ESEM will provide a means to visualize water condensation and, as such, will be interdependent with IR-SNOM measurements. It will also open new possibilities for following phase transitions to ice, which are strongly relevant to soils of the Cryosphere. Collectively, these three EMSL capabilities will allow us to devise a cohesive vision of water condensation in unsaturated media, and thereby constrain ideas on Earth System models.
The proposed work directly addresses the needs outlined by the Terrestrial & Subsurface Ecosystems Science Theme (TSE) of the EMSL. It is also in line with the vision of the Terrestrial Ecosystems Science (TES) program of the DOE Office of Biological and Environmental Research (BER), focusing on carbon cycling in nature. This work will consequently directly benefit the Soil Biogeochemistry community while at the same time advance fundamental knowledge on water condensation mechanisms in minerals.