Common Photo-oxidative Decarboxylation Mechanism in Iron Hydroxy Carboxylate Complexes.

Photochemical oxidation of dissolved organic matter is a crucial component of carbon cycling in surface waters. Photo-oxidation of iron(III)-carboxylate complexes is of particular interest because complexation with iron(III) can sensitize this functional group to photodecarboxylation. The photo-oxidation mechanism of ferrioxalate has been extensively characterized, but it is unclear whether the mechanism or timing is similar for other more complex carboxylates. In this study, we use time-resolved infrared spectroscopy to demonstrate that Fe(III)-citrate, an aliphatic carboxylate, and Fe(III)-salicylate, an aromatic carboxylate, follow the same photo-oxidation kinetics as ferrioxalate. Hence the data suggest a common mechanism for decarboxylation of iron hydroxy carbonates. Differences in the CO2 yield within 50 ps are qualitatively similar to the long-time-scale quantum yield for Fe(II) production.

River thorium concentrations can record bedrock fracture processes including some triggered by distant seismic events.

Fractures are integral to the hydrology and geochemistry of watersheds, but our understanding of fracture dynamics is very limited because of the challenge of monitoring the subsurface. Here we provide evidence that long-term, high-frequency measurements of the river concentration of the ultra-trace element thorium (Th) can provide a signature of bedrock fracture processes spanning neighboring watersheds in Colorado. River Th concentrations show abrupt (subdaily) excursions and biexponential decay with approximately 1-day and 1-week time constants, concentration patterns that are distinct from all other solutes except beryllium and arsenic. The patterns are uncorrelated with daily precipitation records or seasonal trends in atmospheric deposition. Groundwater Th analyses are consistent with bedrock release and dilution upon mixing with river water. Most Th excursions have no seismic signatures that are detectable 50 km from the site, suggesting the Th concentrations can reveal aseismic fracture or fault events. We find, however, a weak statistical correlation between Th and seismic motion caused by distant earthquakes, possibly the first chemical signature of dynamic earthquake triggering, a phenomenon previously identified only through geophysical methods.

Cr(VI) reduction by Fe(II) sorbed to silica surfaces.

The reaction kinetics of groundwater contaminants are integral to evaluating the fate and transport of toxic metals in the environment. For redox sensitive contaminants, such as chromium, the kinetics of different reaction pathways can vary by orders of magnitude. Species-specific rate constants for iron-chromium oxidation-reduction reactions are unknown for many systems, especially in the presence of sorbing surfaces. We investigate the role of quartz and amorphous silica (SiO2(am)) surfaces in mediating abiotic reduction of Cr(VI)aq by aqueous and sorbed Fe(II) using batch sorption and redox experiments. Sorption edges indicate outer-sphere (Fe(II)ads,OS) and inner-sphere (Fe(II)ads,IS) complexes are present on both silica surfaces, and their abundance depends on pH, ionic strength, and surface disorder. The rate constants for Cr(VI)aq reduction by Fe(II) species increase in the following order: kaq ≪ kads,OS,quartz < kads,OS,SiO2(am) < kads,IS,quartz < kads,IS,SiO2(am), suggesting that increasing proximity of Fe(II) to the negatively charged silica surface enhances the rate of reduction of Cr(VI)aq. However, we observe that experiments with larger amounts of sorbed Fe(II) reduce less total Cr(VI)aq over time, which we attribute to a portion of the sorbed Fe(II) being sequestered into the Cr(III)-Fe(III)-oxyhydroxide precipitates forming on the silica surface. Therefore, the balance between increases in the rate and decreases in the total amount of Cr(VI)aq reduction by various sorbed Fe(II) species must be considered when devising remediation strategies.

Kinetics and Products of Chromium(VI) Reduction by Iron(II/III)-Bearing Clay Minerals.

Hexavalent chromium is a water-soluble pollutant, the mobility of which can be controlled by reduction of Cr(VI) to less soluble, environmentally benign Cr(III). Iron(II/III)-bearing clay minerals are widespread potential reductants of Cr(VI), but the kinetics and pathways of Cr(VI) reduction by such clay minerals are poorly understood. We reacted aqueous Cr(VI) with two abiotically reduced clay minerals: an Fe-poor montmorillonite and an Fe-rich nontronite. The effects of ionic strength, pH, total Fe content, and the fraction of reduced structural Fe(II) [Fe(II)/Fe(total)] were examined. The last variable had the largest effect on Cr(VI) reduction kinetics: for both clay minerals, the rate constant of Cr(VI) reduction varies by more than 3 orders of magnitude with Fe(II)/Fe(total) and is described by a linear free energy relationship. Under all conditions examined, Cr and Fe K-edge X-ray absorption near-edge structure spectra show that the main Cr-bearing product is a Cr(III)-hydroxide and that Fe remains in the clay structure after reacting with Cr(VI). This study helps to quantify our understanding of the kinetics of Cr(VI) reduction by Fe(II/III)-bearing clay minerals and may improve predictions of Cr(VI) behavior in subsurface environments.

Estimation of reactive thiol concentrations in dissolved organic matter and bacterial cell membranes in aquatic systems.

Organic thiols are highly reactive ligands and play an important role in the speciation of several metals and organic pollutants in the environment. Although small thiols can be isolated and their concentrations can be estimated using chromatographic and derivatization techniques, estimating concentrations of thiols associated with biomacromolecules and humic substances has been difficult. Here we present a fluorescence-spectroscopy-based method for estimating thiol concentrations in biomacromolecules and cell membranes using one of the soluble bromobimanes, monobromo(trimethylammonio)bimane (qBBr). The fluorescence of this molecule increases significantly when it binds to a thiol. The change in the sample fluorescence due to thiols reacting with qBBr is used to determine thiol concentration in a sample. Using this method, small thiols such as cysteine and glutathione can be detected in clean solutions down to ~50 nM without their separation and prior concentration. Thiols associated with dissolved organic matter (DOM) can be detected down to low micromolar concentration, depending on the DOM background fluorescence. The charge on qBBr prevents its rapid diffusion across cell membranes, so qBBr is ideal for estimating thiol concentration at the cell membrane-water interface. This method was successfully used to determine the thiol concentration on the cell envelope of intact Bacillus subtilis to nanomolar concentration without any special sample preparation. Among the chemical species tested for potential interferences (other reduced sulfides methionine and cystine, carboxylate, salt (MgCl(2))), carboxylates significantly influenced the absolute fluorescence signal of the thiol-qBBr complex. However, this does not affect the detection of thiols in heterogeneous mixtures using the presented method.