Standard Suwannee River Fulvic Acid Research Articles

Changes in Fe-redox and Fe-species across the end-Permian ‘Dead Zone’ in the Sydney Basin, Australia (252.10 ± 0.06 Ma): Evidence from X-ray absorption spectroscopy

The end-Permian mass extinction event is traceable across several non-marine basins in Australia. In the Sydney Basin, the lithological succession is characterized by a change from coal seams to mudstones and sandstones, recording a major environmental change following the disappearance of the Permian vegetation. A few millimeter-thick iron-rich ‘rusty’ layer occurs between the uppermost Permian coal seam and the mudstone, a layer that extends laterally across the basin and which has also been documented from coeval successions in Antarctica. This layer is overlain by the <1.5-m-thick Frazer Beach Member, whose basal 10-cm-thick microbreccia bed comprises 99% kaolinite and quartz, and is dated as 252.10 ± 0.06 Ma. The Frazer Beach Member corresponds to the so-called end-Permian ‘Dead Zone’ lacking fossil pollen and leaves. This distinctive member was deposited directly following the extinction of the Permian peat-forming forests.Here we identify, through X-ray absorption spectroscopy, a drastic redox shift across the extinction interval with increasing amount of reduced Fe-species followed by highly oxidized Fe-species, most resembling Fe(III) complexed with organic matter. Values subsequently normalise in younger samples through the ‘Dead Zone’, attaining only slightly higher redox-levels than before the event. The organically complexed Fe-species in the event bed is consistent with the standard Suwannee River fulvic acid, an acid Fe-complex with iron bound to organic matter, whereas the samples above and below the extinction layer yield spectra predominantly resembling magnetite (Fe3O4) mineral phase. We consider that the iron redox fluctuation marking the extinction interval is related to significant environmental changes with accumulation of organic matter following the mass extinction. The highly reduced iron in the extinction layer may relate to methane release from bacterial degradation, or emissions from clathrates. The presence of fulvic acid in the distinct iron-rich extinction layer indicates that an abrupt onset of the process of degradation of plant matter, lipids and calcium hydroxide (CaOH) took place, resulting in this ‘Death layer’. This was followed by millions of years of erosive conditions before new, complex vegetation could establish.

Effects of fulvic acids on the electrochemical reactions and mass transfer properties of organic cation toluidine blue: Results of measurements by the method of rotating ring-disc electrode

This study examined effects of aquatic and soil natural organic matter (NOM) exemplified by standard Suwannee River fulvic acid (SRFA) and Pahokee Peat fulvic acid (PPFA), respectively, on the electrochemical (EC) reactivity and mass transfer properties of the cationic organic probe toluidine blue (TB) that forms complexes with NOM. EC measurements that were carried out using the method of rotating ring-disc electrode (RRDE) showed that for disc potentials below −0.4 V vs. the standard Ag/AgCl reference electrode, TB molecules undergo EC reduction accompanied by the formation of EC-active products that undergo oxidation at the ring electrode. EC reactions of TB in the range of potentials −0.2 to −0.4 V were determined to involve free TB+ cations and TB species adsorbed on the electrode surface. The EC reduction of TB species at the disc potentials < −0.4 V was controlled by the mass transfer of the free TB+ cations and TB/NOM complexes to the electrode surface. Formation of TB/NOM complexes caused the mass transfer-controlled TB currents to undergo a consistent decrease. The observed changes were correlated with the extent of TB/NOM complexation and decreases of the diffusion coefficients of TB/NOM complexes that have higher molecular weights (MW) than the free cations. Properties of the intermediates formed upon the reduction of TB+ cations were also affected by NOM. These results demonstrate that RRDE measurements of EC reactions of TB or possibly other EC active probes allow probing the complexation of EC-active organic species with NOM and mass transfer properties of NOM complexes and ultimately NOM itself.

Aggregation and charge reversal of humic substances in the presence of hydrophobic monovalent counter-ions: Effect of hydrophobicity of humic substances

To investigate the effect of hydrophobicity of humic substances (HSs) on their charging and aggregation, we studied the electrophoretic mobility and aggregation-dispersion of HSs in the presence of hydrophobic monovalent cations, namely, tetraphenylphosponium TPP+. The used HSs were standard Suwannee river fulvic acid (SRFA), Suwannee river humic acid (SRHA), and Leonardite humic acid (LHA) with different contents of aromatic carbons. All of the HSs in the presence of TPP+ showed charger reversal. The charge reversal pH or iso-electric point (IEP) of LHA was higher than that of SRFA and SRHA in every concentrations of TPPCl, demonstrating the strong hydrophobic interaction between HSs, especially LHA, and TPP+. We also found that the formation of large visible aggregates of all the HSs at lower pH in the presence of TPP+. Large HS aggregates were markedly formed for LHA, manifesting the existence of stronger hydrophobic attraction among LHAs with TPP+. The appearance of aggregates of all the HSs with TPP+ was confirmed by microscopic observation and the size determination by dynamic light scattering. The HS aggregates showed fractal structure. The values of fractal dimension Df of HS aggregates were 2–2.2 in quiescent conditions, indicating that the HS aggregates with TPP+ were formed via cluster-cluster aggregation with restructuring. The Df increased to 2.8–2.9 in stirring conditions, implying that the compact aggregates were formed through the continuous aggregate breakage and the regrowth between smaller aggregates and larger aggregates.

Study of iron and aluminum binding to Suwannee River fulvic acid using absorbance and fluorescence spectroscopy: Comparison of data interpretation based on NICA-Donnan and Stockholm humic models

This study examined the evolution of absorbance and fluorescence spectra of standard Suwannee River fulvic acid (SRFA) induced by its interactions with iron and aluminum. The results show that changes of SRFA absorbance are associated with a consistent response of the carboxylic and phenolic functional groups to iron and aluminum forming bonds with these groups, and their deprotonation induced by such binding. The observed changes of SRFA absorbance were quantified via the use of DSlope325–375 parameter that determines the behavior of the slope of logarithms of SRFA absorbance in the range of wavelengths 325–375 nm in the presence of varying concentrations of iron or aluminum. DSlope325–375 values were correlated linearly with the concentration of SRFA-bound iron and aluminum determined using either NICA-Donnan or Stockholm Humic Model (SHM) but the correlation was stronger for the former model (R2 > 0.98). The slopes of these correlations were similar for both iron and aluminum concentrations <10.0 μM and at a wide pH range. Fluorescence of SRFA was responsive to metal binding but it changed less consistently in the presence of the examined metals, especially in the case of aluminum. The combination of these techniques can help explore in more detail manifestations of DOM site specificity at realistically low concentrations of DOM and metal ions.

Effects of NOM properties on copper release from model solid phases

This study examined impacts of concentrations and properties of natural organic matter (NOM) on copper release from characteristic copper solid model phases such as tenorite CuO and malachite Cu2(OH)2CO3. Unaltered Aldrich humic acid (AHA) and standard Suwannee River fulvic acid (SRFA) strongly increased copper release from the model phases but NOM alteration by chlorination or ozonation gradually suppressed or, at higher oxidant doses, eliminated these effects. The nature of NOM changes induced by chlorination and ozonation was examined using differential absorbance spectroscopy (DAS) and high-performance size-exclusion chromatography (HPSEC). The data of these methods show that NOM molecules with higher apparent molecular weight (AMW), higher aromaticities and contributions of protonation-active phenolic and carboxylic groups play a key role in adsorption and colloidal dispersion of the model solids. The data also show that metal release from model phases was well correlated with a number of spectroscopic parameters characterizing NOM properties, notably SUVA254, spectral slopes of NOM absorbance, and differential absorbance at wavelength of 280 nm and 350 nm that is indicative of the contributions of carboxylic and phenolic functional groups. Changes of ζ-potential of the model solid phases were the strongest predictor of the enhancement of copper release especially in the system controlled by malachite. While effects of NOM on the ζ-potential of tenorite and malachite were prominent for unaltered NOM, its oxidation by chlorine and ozone was accompanied by a gradual decrease and ultimately disappearance of its surface activity.

Optical property of iron binding to Suwannee River fulvic acid

In this work, absorbance and fluorescence spectra were used to study iron binding to standard Suwannee River fulvic acid (SRFA). The differential logarithm-transformed absorbance and fluorescence spectra of SRFA induced by iron binding were processed to examine the nature of the observed phenomena and to investigate the contributions of discrete binding sites present in SRFA. Both the Fe-differential log-transformed absorbance and fluorescence were well correlated to the bound iron concentrations predicted based on the Non-ideal Competitive Adsorption (NICA-Donnan) model at iron concentrations below 10.0μM (R2>0.99 for absorbance and R2>0.97 for fluorescence) and over a wide pH range of 3.5–8.0. At pH3.5, both the Fe-differential log-transformed absorbance and fluorescence vs. iron bound spectra exhibited significantly lower slopes than those at pH5.0, 7.0, and 8.0. These results suggest that a different set of complexation-active chromophores and fluorophores are responsible for iron binding at low pH values or that the NICA-Donnan model is limited at low pH. Because phenolic and carboxylic complex sites of different fluorophores respond to iron quenching, the fluorescence data indicate three stages of iron binding to phenolic, carboxylic, and Donnan gels (electrostatic interactions) in SRFA (with R2>0.99 at each stage). The agreement between observations from spectroscopic indices and established metal-binding models shows that the absorbance and fluorescence spectra provide important information about the involvement of metal complexation of specific functional groups typical for fulvic acids.

Dissolved Organic Carbon Reduces Uranium Bioavailability and Toxicity. 2. Uranium[VI] Speciation and Toxicity to Three Tropical Freshwater Organisms

The influence of dissolved organic carbon (DOC) on the toxicity of uranium (U) to three Australian tropical freshwater species, the Northern Trout Gudgeon (Mogurnda mogurnda), green hydra (Hydra viridissima) and unicellular green alga (Chlorella sp.) was assessed. Exposures were conducted in synthetic soft water without DOC and with DOC added in the form of standard Suwannee River Fulvic Acid (SRFA). Organisms were exposed to a range of U concentrations at a range of DOC concentrations (0-20 mg L(-1)). U toxicity was up to 20 times less in water containing 20 mg L(-1) DOC, relative to DOC-free test waters. U toxicity was also assessed using natural water from a tropical Australian billabong containing 10 mg L(-1) DOC. U toxicity was up to ten times less in the billabong water, relative to DOC--free test waters. SRFA was twice as effective at reducing U toxicity as the billabong water at equivalent DOC concentrations. Geochemical speciation modeling confirmed the decreased U toxicity that resulted from both DOC sources was primarily due to a decrease in the free uranyl ion (UO2(2+)) through complexation with DOC. A predictive model is presented for each of the organisms that can be used to predict U toxicity at a given U and DOC concentration.

A fluorescence quenching study of the interaction of Suwannee River fulvic acid with iron oxide nanoparticles

The fluorescence quenching behaviour of a manufactured nanoparticle (NP, iron oxide, 7 nm) on the standard Suwannee River fulvic acid (SRFA) was investigated for the first time. Size, aggregation and fluorescence was examined as a function of NP:SRFA ratio and of pH. Aggregation state varied as both a function of pH and NP:SRFA ratio, with maximum aggregation at near neutral pH values (6–8). SRFA fluorescence quenching increased non-linearly with increasing NP concentrations (>0.22 × 10 −3 M iron nanoparticles), indicating the complex nature of NP:SRFA interactions. Aggregates of iron oxide present at pH 7–8 appeared to have a much larger effect on quenching compared with dispersed NPs or dissolved phase iron. Fluorescence quenching is demonstrated to indicate different mechanisms of NP:SRFA binding with pH.

Fragmentation Studies of Fulvic Acids Using Collision Induced Dissociation Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

The complex natural organic matter standard Suwannee river fulvic acid (SRFA) was analyzed by negative ion mode electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FTICR MS) using on-resonance collision induced dissociation (CID) of single ultrahigh resolved mass peaks in the ICR cell. Molecular formula assignment of precursor masses resulted in exactly one molecular formula for each of the peaks. Analyses of the corresponding fragment spectra and comparison to different standard substances revealed specific neutral losses and fragmentation patterns which result in structures consisting of a high degree of carboxyl- and fewer hydroxyl groups. The comparison of fragmented mass peaks within different pseudohomologous series (CH(2)-series, and CH(4) vs O exchange) suggested structurally based differences between these series. CID FTICR MS allowed isolating single mass peaks in a very complex natural organic matter spectrum. Subsequently, fragmentation gave structural insights into this material. Our results suggest that the structural diversity in complex humic substances is not as high as expected.

Characterization of proton and copper binding properties of natural organic matter from an Australian drinking water source by differential absorbance spectroscopy

This study examined the effects of changes in pH and copper concentration on the absorbance spectra of natural organic matter (NOM) from a reservoir in Western Australia. Differential absorbance spectra generated for this NOM under changing pH and copper concentration conditions revealed features that could be correlated to the activity of distinct types of chromophores. A comparison of results with those generated for experiments with standard Suwannee River fulvic acid highlighted important differences in chemistry between the two samples.

In Situ Examination of the Protonation Behavior of Fulvic Acids Using Differential Absorbance Spectroscopy

The effects of pH changes on the absorbance spectra of fulvic acids, which constitute a major part of natural organic matter (NOM) were examined based primarily on the example of standard Suwannee River fulvic acid (SRFA). Differential absorbance spectra of SRFA generated at varying pH values were determined to be rich in features that are absent in the conventional spectra. Analysis of the differential absorbance revealed the existence of two predominant types of pH-active chromophores which produced features that emerged in the pH ranges associated with carboxylic and phenolic functional groups of fulvic acids. Comparison of the differential absorbance spectra of SRFA and those of the fraction of hydrophobic acids of South Platte River fulvic acid show the existence of differences indicative of fulvic acids's site-specificity. The evolution of the pH-differential spectra of fulvic acids was in accord with predictions made based on the NICA-Donnan theory. These results show that pH-differential absorbance spectroscopy can be used to quantify in situ the protonation and site-specificity of fulvic acids.

The effect of dissolved natural organic matter on the rate of removal of ferrous iron in fresh waters

The ease of removal of iron in water treatment is determined principally by the form of iron present. If iron is complexed to natural organic matter (NOM) and present in dissolved form, it is quite difficult to remove by conventional deep-bed filtration methods while if present as particulate iron oxyhydroxides it is readily removed. A major source of iron in reservoirs is the benthic sediments which, on becoming anoxic, release ferrous iron (Fe(II)) to the water column. This Fe(II) may either bind to NOM and be retained in dissolved form or may form inorganic hydroxyl complexes which oxidize to Fe(III) species which typically precipitate rapidly. In this paper, we report on studies of the kinetics of Fe(II) removal from solution in the presence and absence of the IHSS standard Suwannee River Fulvic Acid (SRFA). Oxidation of inorganic Fe(II) by oxygen is negligible at low pH but addition of organics changes the kinetics of removal of Fe(II) remarkably, reducing the half life of Fe(II) from hours to minutes. Increasing the concentration of SRFA also enhances the degree of Fe(II) removal. Experimental results obtained over a wide range of conditions are successfully described using a kinetic model which accounts for the transformations between Fe(II) and Fe(III) species.

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.

Atomic force microscopy of soil and stream fulvic acids

Atomic force microscopy (AFM) was used to image fulvic acid (FA) deposited from aqueous solution on to the basal-plane surfaces of freshly cleaved muscovite, and allowed to air dry. Two fulvic acid samples were used: a soil fulvic acid (SFA) prepared by NaOH extraction from a muck soil underlying a freshwater fen in the New Jersey Pinelands and the IHSS standard Suwannee River fulvic acid (SRFA). The use of tapping-mode AFM (TMAFM), a relatively new technique which reduces the lateral frictional forces generally associated with contact-mode AFM, allowed excellent images of delicate FA structures to be obtained with minimal sample disturbance. Four main structures were observed on SFA. At low concentrations, sponge-like structures consisting of rings (∼ 15 nm in diameter) appeared, along with small spheres (10–50 nm). At higher concentrations, aggregates of spheres formed branches and chain-like assemblies. At very high surface coverage, perforated sheets were observed. On some samples, all of these structures were apparent, perhaps owing to concentration gradients on drying. SRFA samples were only imagined at higher concentrations. Spheres, aggregated branches, and perforated sheets were apparent. The results agree with previous work by Stevenson and Schnitzer [Soil Sci., 133(1992) 179], who applied TEM to soil FAs freeze-dried on muscovite. However, the TEM images did not detect the smaller spheres and sponge-like structures observed by AFM at low concentrations. The relevance of imaging dried samples remains questionable; hence, it is hoped in the future to use new in situ TMAFM to image FAs sorbed to surfaces in solution. Although TMAFM provided excellent images, a variety of artifacts and potential problems were encountered, as discussed.