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 <i>Bacillus subtilis</i> to nanomolar concentration without any special sample preparation. Among the chemical species tested for potential interferences (other reduced sulfides methionine and cystine, carboxylate, salt (MgCl₂)), 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

Novel Apatite-Based Sorbent for Defluoridation: Synthesis and Sorption Characteristics of Nano-micro-crystalline Hydroxyapatite-Coated-Limestone

Elevated levels of fluoride (F^–) in groundwaters of granitic and basaltic terrains pose a major environmental problem and are affecting millions of people all over the world. Hydroxyapatite (HA) has been shown to be a strong sorbent for F^–; however, low permeability of synthetic HA results in poor sorption efficiency. Here we provide a novel method of synthesizing nano- to micrometer sized HA on the surfaces of granular limestone to improve the sorption efficiency of the HA-based filter. Our experiments with granular limestone (38–63, 125–500 μm) and dissolved PO₄^3– (0.5–5.3 mM) as a function of pH (6–8) and temperature (25–80 °C) indicated rapid formation of nano- to micrometer sized HA crystals on granular limestone with the maximum surface coverage at lower pH and in the presence of multiple additions of aqueous PO₄^3–. The HA crystal morphology varied with the above variables. The sorption kinetics and magnitude of F^– sorption by HA-coated-fine limestone are comparable to those of pure HA, and the F^– levels dropped to below the World Health Organization’s drinking water limit of 79 μM for F^– concentrations commonly encountered in contaminated potable waters, suggesting that these materials could be used as effective filters. Fluorine XANES spectra of synthetic HA reacted with F^– suggest that the mode of sorption is through the formation of fluoridated-HA or fluorapatite at low F^– levels and fluorite at high F^– loadings

Novel Apatite-Based Sorbent for Defluoridation: Synthesis and Sorption Characteristics of Nano-micro-crystalline Hydroxyapatite-Coated-Limestone

Elevated levels of fluoride (F^–) in groundwaters of granitic and basaltic terrains pose a major environmental problem and are affecting millions of people all over the world. Hydroxyapatite (HA) has been shown to be a strong sorbent for F^–; however, low permeability of synthetic HA results in poor sorption efficiency. Here we provide a novel method of synthesizing nano- to micrometer sized HA on the surfaces of granular limestone to improve the sorption efficiency of the HA-based filter. Our experiments with granular limestone (38–63, 125–500 μm) and dissolved PO₄^3– (0.5–5.3 mM) as a function of pH (6–8) and temperature (25–80 °C) indicated rapid formation of nano- to micrometer sized HA crystals on granular limestone with the maximum surface coverage at lower pH and in the presence of multiple additions of aqueous PO₄^3–. The HA crystal morphology varied with the above variables. The sorption kinetics and magnitude of F^– sorption by HA-coated-fine limestone are comparable to those of pure HA, and the F^– levels dropped to below the World Health Organization’s drinking water limit of 79 μM for F^– concentrations commonly encountered in contaminated potable waters, suggesting that these materials could be used as effective filters. Fluorine XANES spectra of synthetic HA reacted with F^– suggest that the mode of sorption is through the formation of fluoridated-HA or fluorapatite at low F^– levels and fluorite at high F^– loadings

Production of Black Carbon-like and Aliphatic Molecules from Terrestrial Dissolved Organic Matter in the Presence of Sunlight and Iron

Photochemical processing of dissolved organic matter (DOM) in natural waters can alter its composition and structure, supply particulate organic matter (POM) to sediments, and deliver modified terrestrial DOM to the ocean. Our studies show that terrestrial DOM exposed to simulated sunlight is altered to produce POM with a markedly different molecular composition enriched with newly formed aliphatic and condensed aromatic molecules. This process is closely tied to the chemistry of iron, which primarily exists as dissolved Fe­(II) and Fe­(III)–organic complexes in initial DOM and photochemically matures to Fe­(III) oxyhydroxides before coprecipitating out with POM. The newly formed condensed aromatic compounds resemble black carbon, which until now was thought to be produced by only combustion. These new molecules contribute a pool of Fe-rich, aliphatic, and black carbon-like organic matter to sediments as the terrestrial DOM is transported through rivers. We estimate that the annual global flux of this photoproduced black carbon, most of which may be preserved in sediments, is nearly equivalent to the estimated flux of dissolved black carbon to the ocean from all other sources

Nanoscale Transforming Mineral Phases in Fresh Nacre

Nacre, or mother-of-pearl, the iridescent inner layer of many mollusk shells, is a biomineral lamellar composite of aragonite (CaCO₃) and organic sheets. Biomineralization frequently occurs via transient amorphous precursor phases, crystallizing into the final stable biomineral. In nacre, despite extensive attempts, amorphous calcium carbonate (ACC) precursors have remained elusive. They were inferred from non-nacre-forming larval shells, or from a residue of amorphous material surrounding mature gastropod nacre tablets, and have only once been observed in bivalve nacre. Here we present the first direct observation of ACC precursors to nacre formation, obtained from the growth front of nacre in gastropod shells from red abalone (<i>Haliotis rufescens</i>), using synchrotron spectromicroscopy. Surprisingly, the abalone nacre data show the same ACC phases that are precursors to calcite (CaCO₃) formation in sea urchin spicules, and not proto-aragonite or poorly crystalline aragonite (pAra), as expected for aragonitic nacre. In contrast, we find pAra in coral

Ubiquitous Presence of Fe(II) in Aquatic Colloids and Its Association with Organic Carbon

Despite being thermodynamically less stable, small ferrous colloids (60 nm to 3 μm in diameter) remain an important component of the biogeochemical cycle at the Earth’s surface, yet their composition and structure and the reasons for their persistence are still poorly understood. Here we use X-ray-based Fe L-edge and carbon K-edge spectromicroscopy to address the speciation and organic–mineral associations of ferrous, ferric, and Fe-poor particles collected from sampling sites in both marine and freshwater environments. We show that Fe­(II)-rich phases are prevalent throughout different aquatic regimes yet exhibit a high degree of chemical heterogeneity. Furthermore, we show that Fe-rich particles show strong associations with organic carbon. The observed association of Fe­(II) particles with carboxamide functional groups suggests a possible microbial role in the preservation of Fe­(II). These finding have significant implications for the behavior of Fe­(II) colloids in oxygenated waters, and their role in different aquatic biogeochemical processes