Model Dissolved Organic Matter Research Articles

Fluorescence excitation-emission matrix regional integration to quantify spectra for dissolved organic matter.

Excitation-emission matrix (EEM) fluorescence spectroscopy has been widely used to characterize dissolved organic matter (DOM) in water and soil. However, interpreting the > 10,000 wavelength-dependent fluorescence intensity data points represented in EEMs has posed a significant challenge. Fluorescence regional integration, a quantitative technique that integrates the volume beneath an EEM, was developed to analyze EEMs. EEMs were delineated into five excitation-emission regions based on fluorescence of model compounds, DOM fractions, and marine waters or freshwaters. Volumetric integration under the EEM within each region, normalized to the projected excitation-emission area within that region and dissolved organic carbon concentration, resulted in a normalized region-specific EEM volume (phi(i,n)). Solid-state carbon nuclear magnetic resonance (13C NMR), Fourier transform infrared (FTIR) analysis, ultraviolet-visible absorption spectra, and EEMs were obtained for standard Suwannee River fulvic acid and 15 hydrophobic or hydrophilic acid, neutral, and base DOM fractions plus nonfractionated DOM from wastewater effluents and rivers in the southwestern United States. DOM fractions fluoresced in one or more EEM regions. The highest cumulative EEM volume (phi(T,n) = sigma phi(i,n)) was observed for hydrophobic neutral DOM fractions, followed by lower phi(T,n) values for hydrophobic acid, base, and hydrophilic acid DOM fractions, respectively. An extracted wastewater biomass DOM sample contained aromatic protein- and humic-like material and was characteristic of bacterial-soluble microbial products. Aromatic carbon and the presence of specific aromatic compounds (as indicated by solid-state 13C NMR and FTIR data) resulted in EEMs that aided in differentiating wastewater effluent DOM from drinking water DOM.

Partitioning of polycyclic aromatic hydrocarbons to dissolved organic matter from different soils

Dissolved organic matter (DOM) in the soil solution or groundwater can have a considerable effect on the dissipation of hydrophobic organic pollutants. We used the fluorescence quenching technique as well as the reversed-phase separation technique to determine the partition coefficient of 3- to 5-ring PAH to dissolved organic matter from agricultural soils and forest floor materials. Several techniques (centrifugation, pressure filtration, and percolation) are compared for obtaining DOM solution most representative of the DOM in the natural soil solution to use for the sorption experiments. Centrifugation was shown to be the most suitable procedure to obtain DOM solutions in large quantities with a composition most similar to the field soil solution. PAH binding to DOM from mineral soil was found to be considerably lower compared to DOM from acid forest floor materials. These differences can partly be explained by compositional differences with respect to hydrophobic and hydrophilic components of DOM. Binding capacity of DOM was only slightly affected by changes in pH and electrical conductivity of the soil solution. The partition coefficients for DOM from soils are rather different to the data given for model DOM solutions, such as humic acids, fulvic acids, or Aldrich humic acid. To obtain realistic estimates for the effect of DOM on the mobility of hydrophobic organic chemicals in soils, it is essential to use DOM solutions representative of the dissolved organic matter in the soil solution.

Monitoring the Biological and Physical Reactivity of Dextran Carbohydrates in Seawater Incubations Using Flow Field-Flow Fractionation

In order to better understand the factors that modulate the size and reactivity of high molecular weight organic matter dissolved in seawater, fluorescently labeled dextrans were used as the model compounds whose molecular weight distributions were monitored by flow field-flow fractionation (flow FFF or F1FFF) during incubations in seawater matrices. Two fluorescein isothiocyanate (FITC) labeled dextrans (145k and 2M Da) were incubated in whole seawater (natural microbial population and natural dissolved organic matter (DOM) present), 0.02 μm filtered seawater (all microorganisms removed, but natural DOM largely unaltered), and UV-oxidized seawater (no microorganisms or natural DOM present). Flow FFF fractograms of the two FITC-dextrans incubated in UV-oxidized seawater showed no changes, signifying that the dextrans did not undergo any alteration or aggregation. The dextrans incubated in filtered seawater with natural DOM present resulted in fractograms that are shifted to higher retention times, consistent with aggregation of the dextran and natural DOM. In the whole seawater incubations, the complex changes in the fractograms over time indicated that the dextrans underwent both aggregation with natural DOM and degradation by heterotrophic microorganisms. The latter was confirmed by microscopic examination of the collected fractions after selectively staining the microorganisms with the fluorescent dye 4,6-diamidino-2-phenylindole (DAPI). The FITC dextrans were observed to be “attached” to the bacteria's outer cell membrane where they are subsequently hydrolyzed into small low molecular weight fragments by extracellular enzymes. Some of these degraded dextrans are assimilated by the bacteria and the rest (μ70%) are released into the seawater. Approximately half of the released degraded dextrans are of high enough molecular weight to be retained by flow FFF at the conditions employed in these studies. The data presented in this paper illustrate that, when used as both a qualitative and semipreparative tool, flow FFF can provide information on the relationships between natural and model DOM and microorganisms that would be difficult or impossible to obtain using other methods.

Kinetics of copper complexation with dissolved organic matter using stopped‐flow fluorescence technique

This study provided direct observations on reaction kinetics of Cu and dissolved organic matter (DOM). Stopped‐flow method and fluorescence technique were used to measure Cu(II)‐DOM complexation over time. Suwannee River reference fulvic acid and water soluble soil organic matter were used as model DOM. Experimental conditions of pH 6, 5 × 10‐5 M Cu(II), and 5 mg C/L of DOM were used and the results were to simulate the relevant natural conditions. Both organic ligands reacted fairly rapidly with Cu(II). Reaction half‐life of DOM was in the millisecond range. Two types of metal binding sites of organic ligands were identified. Fulvic acid yield forward rate constants: 1 x 106 M‐1 s‐1 and 5.15 × 103 M‐1 s‐1, based on second‐order reaction rate model. Water soluble soil organic matter gave forward rate constants: 1.34 × 105 M‐1 s‐1 and 2.2 × 103 M‐1 s‐1. From comparison of kinetic natures, the fast‐reacting ligands were comparable to nitrilotriacetic acid (NTA) and slow‐reacting ligands were more like salcy...

Ground water transport of hydrophobic organic compounds in the presence of dissolved organic matter

AbstractThe effects of dissolved organic matter (DOM) on the transport of hydrophobic organic compounds in soil columns were investigated. Three compounds (naphthalene, phenanthrene and DDT) that spanned three orders of magnitude in water solubility were used. Instead of humic matter, molecularly well‐defined DOM represented by Triton X‐100, a nonionic industrial detergent, and bovine serum albumin protein were used. In batch isotherm studies, the sorption of naphthalene to both model DOMs appeared to be hydrophobic in nature and quantitatively similar to the binding to humic materials. Equations were derived to model the enhanced transport of organic compounds by DOM based on octanol/water partition coefficients, Kow. For a specific soil and DOM level, it was shown theoretically and experimentally that all organic compounds with Kow values above a specific value should move at the same rate in ground water. In some situations, DOM can increase the movement of highly hydrophobic compounds, such as DDT, by a factor of a thousand or more. This enhanced transport in the presence of DOM can either be a problem, as with ground water contaminant spreading, or a benefit, as with contaminated aquifer cleanup.