66 research outputs found

    Molecular Fractionation of Dissolved Organic Matter with Metal Salts

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    Coagulation of dissolved organic matter (DOM) by hydrolyzing metals is an important environmental process with particular relevance, e.g., for the cycling of organic matter in metal-rich aquatic systems or the flocculation of organic matter in wastewater treatment plants. Often, a nonremovable fraction of DOM remains in solution even at low DOM/metal ratios. Because coagulation by metals results from interactions with functional groups, we hypothesize that noncoagulating fractions have a distinct molecular composition. To test the hypothesis, we analyzed peat-derived dissolved organic matter remaining in solution after mixing with salts of Ca, Al, and Fe using 15 T Electrospray Ionization Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry (ESI-FT-ICR-MS). Addition of metals resulted in a net removal of DOM. Also a reduction of molecular diversity was observed, as the number of peaks from the ESI-FT-ICR-MS spectra decreased. At DOM/metal ratios of ∼9 Ca did not show any preference for distinct molecular fractions, while Fe and Al removed preferentially the most oxidized compounds (O/C ratio >0.4) of the peat leachate. Lowering DOM/metal ratios to ∼1 resulted in further removal of less oxidized as well as more aromatic compounds (“black carbon”). Molecular composition in the residual solution after coagulation was more saturated, less polar, and less oxidized compared to the original peat leachate and exhibited a surprising similarity with DOM of marine origin. By identifying more than 9200 molecular formulas we can show that structural properties (saturation and aromaticity) and oxygen content of individual DOM molecules play an important role in coagulation with metals. We conclude that polyvalent cations not only alter the net mobility but also the very molecular composition of DOM in aquatic environments

    The Molecular Composition of Dissolved Organic Matter in Forest Soils as a Function of pH and Temperature

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    <div><p>We examined the molecular composition of forest soil water during three different seasons at three different sites, using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR-MS). We examined oxic soils and tested the hypothesis that pH and season correlate with the molecular composition of dissolved organic matter (DOM). We used molecular formulae and their relative intensity from ESI-FT-ICR-MS for statistical analysis. Applying unconstrained and constrained ordination methods, we observed that pH, dissolved organic carbon (DOC) concentration and season were the main factors correlating with DOM molecular composition. This result is consistent with a previous study where pH was a main driver of the molecular differences between DOM from oxic rivers and anoxic bog systems in the Yenisei River catchment. At a higher pH, the molecular formulae had a lower degree of unsaturation and oxygenation, lower molecular size and a higher abundance of nitrogen-containing compounds. These characteristics suggest a higher abundance of tannin connected to lower pH that possibly inhibited biological decomposition. Higher biological activity at a higher pH might also be related to the higher abundance of nitrogen-containing compounds. Comparing the seasons, we observed a decrease in unsaturation, molecular diversity and the number of nitrogen-containing compounds in the course of the year from March to November. Temperature possibly inhibited biological degradation during winter, which could cause the accumulation of a more diverse compound spectrum until the temperature increased again. Our findings suggest that the molecular composition of DOM in soil pore waters is dynamic and a function of ecosystem activity, pH and temperature.</p></div

    H/C and O/C data for the different groups of molecular formulae exhibited clear trends with pH and DOC concentration.

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    <p>The data for each group of formulae are summarized as centroid data in the van Krevelen diagram. The number of formulae per group is represented by the scaled size of the symbols and given in brackets in the legend.</p

    Boxplots representing the characteristics of the different groups of molecular formulae.

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    <p>The data are standardized to the maximum value of each parameter. Pairs with equal medians are marked *.</p

    Ordination plots from PCA, based on all detected molecular formulae and their normalized FT-ICR-MS signal intensities.

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    <p>DOC concentration and pH were not used for PCA, but plotted as supplementary variables. Variability explained: PC1, 32.9%; PC2, 15.7%; PC3, 12.5%. (a) Plot of first and second axes, (b) plot of first and third axes.</p

    Overview of abundance of molecular formulae per month and main site.

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    <p>(a) Average abundance of formulae, (b) average relative abundance of nitrogen-containing compounds. Error bars indicate standard deviation between individual sampling spots or replicate measurements.</p

    Median, quartiles Q1 (25<sup>th</sup> percentile) and Q3 (75<sup>th</sup> percentile) and minimum and maximum values of various molecular parameters.

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    <p>Data are shown for those molecular formulae that correlated negatively and positively with pH in the soil pore waters.</p><p>Median, quartiles Q1 (25<sup>th</sup> percentile) and Q3 (75<sup>th</sup> percentile) and minimum and maximum values of various molecular parameters.</p

    Species diversity diagram based on RDA with molecular formulae as responsive variables and pH, DOC concentration and length of growing season as explanatory variables.

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    <p>Variability explained: Axis 1, 30%; Axis 2, 8.6%. DOC concentration, pH and length of growing season together explain 42% of the variability in formulae. The greater the circle diameter, the greater the number of molecular formulae per measurement (range of formulae per measurement: 1062 to 1334).</p

    Land Use Controls on the Spatial Variability of Dissolved Black Carbon in a Subtropical Watershed

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    Rivers export roughly 250 Pg of dissolved organic carbon (DOC) to coastal oceans. DOC exported from rivers can be a reflection of watershed dynamics, and changes in land use can lead to shifts in the molecular composition and reactivity of riverine DOC. About 10% of DOC exported from rivers is dissolved black carbon (DBC), a collection of polycondensed aromatic compounds derived from the incomplete combustion of biomass and fossil fuels. While DOC and DBC export are generally coupled, the effects of watershed land use on DBC quality are not well understood. In this study, DBC samples were collected throughout the Altamaha River watershed in Georgia, USA. DBC was characterized using the benzenepoly­(carboxylic acid) method and Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). DBC had a more polycondensed character in areas of the watershed with less anthropogenic disturbance. Furthermore, FTICR-MS revealed that DBC became enriched with a lower molecular weight, heteroatomic signature in response to higher anthropogenic activity. As global land cover continues to change, this study demonstrates on a localized scale that watershed land use can influence the export and composition of DBC, which may have further implications for global carbon and nutrient cycling