Yu-Ping Chin

Molecular weight, polydispersity, and spectroscopic properties of aquatic humic substances

The number- and weight-averaged molecular weights of a number of aquatic fulvic acids, a commercial humic acid, and unfractionated organic matter from four natural water samples were measured by high-pressure size exclusion chromatography (HPSEC). Molecular weights determined in this manner compared favorably with those values reported in the literature. Both recent literature values and our data indicate that these substances are smaller and less polydisperse than previously believed. Moreover, the molecular weights of the organic matter from three of the four natural water samples compared favorably to the fulvic acid samples extracted from similar environments

FRACTIONATION OF AQUATIC NATURAL ORGANIC MATTER UPON SORPTION TO GOETHITE AND KAOLINITE

Natural organic matter (NOM) consists of a complex mixture of organic molecules; previous studies have suggested that preferential sorption of higher molecular weight, more hydrophobic, and more aromatic components may lead to fractionation of the NOM pool upon passage through porous media. Our work expands upon previous studies by quantifying the change in solution-phase weight average molecular weight (Mw) upon sorption of bulk (rather than isolated) surface water NOM from the Suwannee River (SR) and the Great Dismal Swamp (GDS) to goethite and kaolinite at different sorption densities and at pH 4, 22°C. High pressure size exclusion chromatography (HPSEC) was used to quantify changes in Mw upon sorption, and molar absorptivities at λ=280 nm were used to approximate changes in solution NOM aromaticity. Two SR water samples were used, with Mw=2320 and 2200 Da; a single GDS sample was used, with Mw=1890 Da. The SR NOM was slightly more hydrophobic and aromatic. These differences were reflected in greater sorption of SR NOM than GDS NOM. Both surface water NOMs showed a much greater affinity for goethite than for kaolinite. HPSEC analysis of the NOM remaining in solution after 24 h reaction time with goethite revealed that the largest changes in solution phase Mws (decreases by 900–1700 Da) occurred at relatively low equilibrium sorbate concentrations (approximately 5–20 mg C l−1); the decrease in solution Mw suggested that reactive surface sites were occupied disproportionately by large and intermediate size NOM moieties. At higher equilibrium NOM concentrations (>20 mg C l−1), as percent adsorption decreased, Mw in solution was similar to original samples. A smaller decrease in solution NOM Mw (300–500 Da at 10–20 mg C l−1 ∼100 Da at >20 mg) also occurred upon sorption to kaolinite. Overall, our results showed that factors (as related to NOM composition, clay mineral surface properties, and position along the sorption isotherm) which promote a higher percent sorption lead to the most pronounced decreases in solution Mw.

The role of fulvic acid composition in the photosensitized degradation of aquatic contaminants

Abstract.Dissolved organic matter (DOM) chemical composition varies depending upon the source of its precursor materials. Results show that the indirect photodegradation rate coefficients for the compounds sulfadimethoxine (SDM) and triclocarban (TCC) differ depending on the source of fulvic acids used as DOM surrogates. For sulfadimethoxine, little to no enhanced photolysis occurred for the terrestrially derived Suwannee River fulvic acid, but the rate coefficients for triclocarban increased 28%. In contrast, a large photo-enhancement (48% for SDM and 45% for TCC) occurred in the presence of Pony Lake and Old Woman Creek fulvic acids, both of which are derived from predominantly autochthonous material. Conversely, the iron-promoted photodegradation of alachlor occurred more quickly in the presence of Suwannee River fulvic acid than another autochthonous fulvic acid isolated from Lake Fryxell, Antarctica. This pathway is dominated by reaction through hydroxyl radicals generated by the photolysis of the fulvic acids. Taken together, these data provide evidence that autochthonous fulvic acids are more reactive than allochthonous fulvic acids in promoting pathways involving triplet dissolved organic matter intermediates, whereas the latter are more reactive than the former in promoting degradation by some reactive oxygen species (ROS). Thus, more systematic studies are needed to determine the full extent of the linkage between fulvic acid composition and its ability to promote indirect photolytic processes and how the presence of other DOM fractions may affect these reactions.

Photochemical Fate of Sulfadimethoxine in Aquaculture Waters

Sulfadimethoxine (SDM) is an antibiotic often used in combination with ormetoprim to prevent the spread of disease in freshwater aquaculture. It is known to undergo photochemical degradation in natural sunlit surface waters, but the role of dissolved organic matter (DOM) in this process is poorly understood. Our results show that water from a eutrophic catfish pond at the Mississippi State University Delta Research and Extension Center facility in Stoneville, MS facilitates the rapid phototransformation of SDM. In contrast, water from a nearby stream (Deer Creek) whose DOM is derived from allochthonous precursors does not enhance SDM photodegradation. We attribute these disparate results to DOM composition, whereby dissolved organic matter originating from highly eutrophic water bodies is a better SDM photosensitizer. Experiments conducted concurrently using respective autochthonous (Pony Lake, Antarctica) and allochthonous (Suwannee River) derived fulvic acids corroborate these findings. Scavenging experiments and experiments conducted anoxically show that the main indirect photodegradation pathway occurs by triplet excited-state DOM oxidation. Finally, transformation products assayed by mass spectrometry reveal the same major SDM photoproducts in the presence and absence of dissolved organic matter.

Effect of detector wavelength on the determination of the molecular weight of humic substances by high-pressure size exclusion chromatography

High-pressure size exclusion chromatography (HPSEC) has proven to be an effective method for determining the molecular weights (MW) of humic substances (HS) from a variety of aquatic and terrestrial environments. HPSEC systems often use a variable wavelength UV-vis detector, which detects the analytes based upon their chromophoric composition. HS contain a range of moieties with chromophores having unique molar absorptivities (for a given wavelength), and the calculated MW may be dependent upon the wavelength chosen for the analysis. As a consequence, the choice of wavelength becomes an important parameter for the reliable determination of MW by HPSEC. The effect of UV-vis detector wavelength on the determination of the MW distribution of selected humic and fulvic acids by HPSEC is examined in this paper. For the HS examined, both the number (Mn) and weight average (Mw) MW increased with increasing wavelength. The relative increase in MW was most pronounced for Lake Fryxell fulvic acid, with a 63 and 21% increase in Mn and Mw, respectively, between 220 and 380 nm. The increases observed for Suwannee River humic and fulvic acids were less pronounced. Mn was more sensitive to changes in detector wavelength than Mw, and as a result the target HS appeared to be less polydisperse at higher wavelengths. Within the range of wavelengths commonly used for the determination of MW of HS by HPSEC (i.e., 220-280 nm), the magnitude of the increases in MW was not significant compared to variability in MW that results from changes to other operational parameters in HPSEC.

Characterizing the properties of dissolved organic matter isolated by XAD and C-18 solid phase extraction and ultrafiltration

Abstract.The properties of aquatic dissolved organic matter (DOM) isolated by solid phase extraction (SPE) C-18 cartridges, ultrafiltration, and XAD chromatography are compared. Samples taken from the Suwannee River, Georgia, USA and McDonalds Branch in the Pine Barrens, New Jersey, USA were chosen to represent waters where DOM originates from predominantly terrestrially-derived (allochthonous) precursors. Pony Lake, Antarctica represented an exclusively algal/microbially-derived (autochthonous) DOM. Fluorescence, UV absorption, 13C NMR spectroscopy, and high-pressure size exclusion chromatography (HPSEC) were employed to discern differences and similarities between the DOM isolated by these three methods. Only subtle differences between isolation methods were observed for the terrestrially derived DOM samples when assayed by light and fluorescence spectroscopy. Conversely, the Pony Lake DOM isolates exhibit greater variability when analyzed by these methods. 13C-NMR analyses showed structural differences between the methods for all samples. HPSEC analysis also revealed differences with the C-18 isolates exhibiting the highest molecular weights. Thus, it appears that each method isolates sufficiently different fractions of DOM that can only be delineated when a consortium of analytical methods are used to assay the samples. Nonetheless real differences between autochthonous and allochthonous derived DOM were observed with the algal-derived samples exhibiting high fluorescence ratios and lower aromaticity relative to the terrestrially derived materials. These results demonstrate that caution must be exercised when interpreting DOM reactivity data that rely upon the use of specific fractions.

Direct and indirect photolysis of polycyclic aromatic hydrocarbons in nitrate-rich surface waters

The photolysis of three polycyclic aromatic hydrocarbons (PAHs)-pyrene, phenanthrene, and naphthalene-were studied in waters taken from creosote-contaminated sites in Gary (IN, USA) and Wilmington (NC, USA). Direct photolysis of all PAHs was observed under simulated solar radiation, with pyrene degrading at a faster rate than either phenanthrene or naphthalene. Phenanthrene degradation, when compared to its direct photolysis rate, increased in Gary water but decreased in Wilmington water. Analysis of the waters for dissolved organic carbon (DOC) and nitrate revealed higher levels of DOC in the Wilmington sample (9.29 mg/L) compared with the Gary sample (6.73 mg/L), as well as significantly less nitrate (0.046 mM vs 0.205 mM for the Gary sample). The slightly lower rate of phenanthrene degradation observed for the Wilmington sample, corrected for light attenuation effects, is statistically the same as that in the direct photolysis experiments. Therefore, we attribute the lower rate of degradation in the presence of Wilmington water to light screening by DOC, but we believe the faster reaction rate observed for the Gary water results from hydroxyl radical (OH*) chemistry generated by nitrate photolysis. Indeed, degradation of the target compound increased when nitrate (at 0.2 and 0.4 mM) was added to the Wilmington sample, further corroborating this conclusion. Overall photoreaction rates decreased for the lower-molecular-weight PAHs, because the fastest naphthalene photolytic rate was roughly two orders of magnitude slower than that of pyrene.

Photosensitized degradation of caffeine: role of fulvic acids and nitrate.

The photolysis of caffeine was studied in solutions of fulvic acid isolated from Suwannee River, GA (SRFA) and Old Woman Creek Natural Estuarine Research Reserve, OH (OWCFA) with different chemical amendments (nitrate and iron). Caffeine degrades slowly by direct photolysis (>170 h in artificial sunlight), but we observed enhanced photodegradation in waters containing the fulvic acids. At higher initial concentrations (10 μM) the indirect photolysis of caffeine occurs predominantly through reaction with the hydroxyl radical (OHⁱ) generated by irradiated fulvic acids. Both rate constant estimates based upon measured OHⁱ steady-state concentrations and quenching studies using isopropanol corroborate the importance of this pathway. Further, OHⁱ generated by irradiated nitrate at concentrations present in wastewater effluent plays an important role as a photosensitizer even in the presence of fulvic acids, while the photo-Fenton pathway does not at neutral or higher pH. At lower initial concentrations (0.1 μM) caffeine photolysis reactions proceed even more quickly in fulvic acid solutions and are influenced by both short- and long-lived reactive species. Studies conducted under suboxic conditions suggest that an oxygen dependent long-lived radical e.g., peroxyl radicals plays an important role in the degradation of caffeine at lower initial concentration.

Sonochemical reactions of dissolved organic matter

Property changes of Aldrich and Pahokee peat dissolved organic matter (DOM) at different ultrasonic frequencies and energy densities were systematically investigated. Exposure of DOM to ultrasound resulted in decreases in TOC, Color465, specific UV absorbance (SUVA), aromaticity and molecular weight, while DOM acidity increased. Compared to 20 kHz ultrasound, greater sonochemical transformation of DOM occurred at 354 kHz and at higher energy density, due to greater ·OH radical production. The changes to DOM properties suggest that ultrasound may significantly affect DOM-pollutant interactions (e.g.facilitate desorption of hydrophobic organics from DOM or promote complexation between metallic cations and DOM).

An Improved Screening Tool for Predicting Volatilization of Pesticides Applied to Soils

Pesticide volatilization and vapor drift can have adverse effects on nontarget, sensitive ecosystems and human health. Four approaches for pesticide volatilization screening based on Ficks Law were investigated. In each approach, vapor pressures or environmentally relevant partition coefficients were used to describe pesticide behavior in an agricultural field system and to predict 24-h cumulative percentage volatilization (CPV(24h)) losses. The multiphase partitioning approach based on soil-air (K(soil-air)) and water-air (K(water-air)) partition coefficients was found to most accurately model literature-reported pesticide volatilization losses from soils. Results for this approach are displayed on chemical space diagrams for sets of hypothetical K(soil-air) and K(water-air) combinations under different temperature, relative humidity, and soil organic carbon conditions. The CPV(24h) increased with increasing temperature and relative humidity and with decreasing soil organic carbon content. Pesticides and the conditions under which the greatest volatilization losses exist were easily identified using this visual screening technique.