The application of tender X-rays to environmental interfaces has grown tremendously over the past decade. Complimentary use of tender x-ray spectroscopy with other spectroscopic or microscopic tools and macroscopic data can enhance mechanistic understanding of environmental remediation processes. This presentation will describe two illustrative applications of tender X-rays associated with fouling of water purification membranes and calcite dissolution during geological carbon sequestration. Calcium bridging of carboxylate groups present in organic foulants and membrane surfaces contributes significant fouling and subsequent flux decline during water treatment of natural and waste waters containing natural organic matter (NOM), polysaccharides, and proteins. The combination of <i>ex-situ</i> Ca K-edge NEXAFS and Ca 2p XPS measurements of NOM fouled nanofiltration membranes provide evidence for depth-dependent NOM-Ca-NOM and NOM-Ca-membrane interactions. Resonant scattering profiles near the Ca K-edge reveal length scales consistent with calcium-organic complexes, showcasing the potential to link macroscopic performance metrics (e.g., flux decline) with specific fouling mechanisms. In studies examining C15-C18 olefin sulfonate (IOS) surfactant adsorption to calcite, the key role of electrostatic attraction was identified.  Sulfur K-edge NEXAFS in conjunction with ToF-SIMS and DFT simulations demonstrated that surfactant adsorption was significantly greater on energetically more favorable etch pits on the calcite surface compared to the cleaved 1014 surface.  Surfactant adsorption onto the etch pits decreased the rate of subsequent calcite dissolution. This finding suggests that tender X-ray spectroscopy has enough sensitivity to probe more detailed mechanisms of site-specific mono or bilayer adsorption, and is a promising technique for probing the role of surfactants in protecting carbonate minerals from dissolution during geological carbon sequestration.

Applications of tender X-ray spectroscopy to understand reactions at environmental interfaces

ACS Fall 2021

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