Measurement and Determination of the Importance of Total Sulfhydryl Binding Site Concentrations in a Wide Range of Environmental Samples: A Novel UHPLC-MS Approach

Heavy metal contamination of surface waters and groundwater systems affects water quality across the industrialized world, and the form in which heavy metals are present in these waters controls the environmental fate and bioavailability of these elements. The aqueous speciation of many metal contaminants is controlled by binding onto sulfur-based binding sites on bacteria and on dissolved organic matter.

Despite the importance of these sulfur-based binding sites, very little is known about the abundance of the sites in natural waters or the processes that affect their abundance primarily due to analytical difficulties in measuring the concentration of these site types. In this study, we will use an analytical approach that we have developed for measuring sulfur-based binding sites both in water and sediment samples from a wide range of environments.

The research will be transformative by providing baseline information that can be used to assess the importance of sulfur-based sites in controlling metal speciation and behavior in natural and human-impacted water systems. Furthermore, the ability to measure the sulfur-based binding site concentrations in aqueous samples and on environmental solid-phase surfaces at the sensitivity of our new approach enables the investigators to test several hypotheses about the role of these important sites in surface and near-surface water-rock systems.

The sponsored research project will involve outreach and coordination with local high school science research programs in South Bend, Indiana schools. A geomicrobiology/environmental chemistry module will be created and taught in these high school science research programs (a short-course of 3-4 lectures and demonstrations), and 2-3 high school students will be recruited each year to conduct some of the sample analyses.

Organic compounds with sulfhydryl metal-binding sites are ubiquitous in natural and engineered surface water, groundwater, and soil settings, yet the role that they play in controlling the environmental behavior and fate of metals is poorly understood due to the difficulty of measuring their concentrations in aqueous and solid samples. Although other binding site types are typically more abundant, sulfhydryl sites likely control the fate, transport, and bioavailability of chalcophile elements in the environment due to the much higher binding affinities that these elements have with sulfhydryl sites compared to with other binding site types.

A first step in determining the role of organo-sulfhydryl binding sites in controlling metal speciation and bioavailability in natural systems is to measure the total concentration of these site types in surface and groundwaters and on environmental surfaces. Total sulfhydryl site concentrations in natural waters are likely to be, typically, in the nM to μM range, but sulfhydryl sites at these concentrations are not directly detectable using current approaches.

In this study, the investigators will use a novel ultra-high performance liquid chromatography – mass spectrometry (UHPLC-MS) analytical approach that they have developed and tested for measuring total sulfhydryl site concentrations in aqueous and solid-phase samples, and conduct a survey of sulfhydryl site concentrations in a wide range of environments. The UHPLC-MS approach dramatically lowers the detection limit relative to previous analytical methods, measuring the total sulfhydryl site concentration in a sample indirectly by measuring the change in concentration, upon exposure to sulfhydryl sites, of a molecule that binds specifically and essentially irreversibly to sulfhydryl sites on aqueous molecules or solid phase surfaces.

Because the concentration of the sulfhydryl-binding molecule can be determined very precisely using UHPLC-MS, the analytical approach has the potential to lower the detection limit for total sulfhydryl site concentration measurements by 2-3 orders of magnitude over current state-of-the-art techniques and to obtain detection limits of approximately 10 nM. The research team will use this approach to test several hypotheses about the role and occurrence of sulfhydryl sites in lake systems and on soil, microbiological, and root systems.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.