International Humic Substances Society Research Articles

Triazine retention by nanofiltration in the presence of organic matter: The role of humic substance characteristics

The influence of different types of humic substances (HS) is studied, in the presence or absence of calcium ions, on the retention of two triazines, atrazine and prometryn. To correlate the retention of triazines with the characteristic chemical parameters of the humic substances, three standard humic materials from the International Humic Substances Society (IHSS) as well as a NOM surrogate, tannic acid, are employed. Differences are observed, in the triazine retention behaviour, between organic-free triazine solutions and those of mixed HS–triazines. The results suggest that physicochemical interactions alone, between triazines and humic substances, cannot fully account for the observed triazine retention by nanofiltration. The HS acidity (total, carboxylic and phenolic) exhibits a weak correlation with triazine rejection. However, the conformation of humic species appears to play a more significant role. The presence of calcium ions improves both HS–triazine association and membrane rejection of the HS–triazine complexes. In support of the membrane filtration results, gel permeation chromatography shows that the hydrophobic interactions can explain the different triazine absorbing capacities of the HS used. In general, the results illustrate the complexity of the adsorption and retention mechanisms of triazines in the presence of humic substances, and confirm the important role of membrane fouling on process efficiency.

Compositional differences between soil humic acids extracted by various methods as evidenced by photosensitizing and electrophoretic properties

Humic acids (HAs) were isolated from Elliott soil provided by the International Humic Substances Society (1BS102M) by three commonly used methods: (i) 0.1 M NaOH (EI-HA), (ii) neutral 0.1 M Na 4P 2O 7 (L-HA) and subsequent 0.1 M NaOH (S-HA), and (iii) 0.1 M NaOH + 0.1 M Na 4P 2O 7 (NP-HA). The objective was to evaluate the impact of these extractants on the photosensitizing properties of the isolated HAs. HAs were analyzed for their elemental composition, functional acid groups content, absorption and emission properties, electrophoretic characteristics and ability to produce singlet oxygen using furfuryl alcohol (FFA) as a scavenger. L-HA was slightly more aromatic and oxygenated than the other HAs and contained a higher portion of long-wavelength fluorophores and macromolecules showing low molecular size (MS) and high electrophoretic mobility. L-HA also gave a rate of FFA photooxygenation between 1.25- and 1.6-fold higher than the other HAs. This suggests that the free humic macromolecules ionized at pH 7 and/or weakly bounded on mineral surfaces via cation bridges are of relatively low MS and contribute significantly to the photosensitizing and long-wavelength emitting properties. Differences among the other HAs were more subtle, but the parallel evolution of the reactivity and electrophoretic characteristics was observed. Photochemical and electrophoretic measurements seem to be sensitive indicators to evaluate differences among the extraction procedures of HAs.

N-15 NMR spectra of naturally abundant nitrogen in soil and aquatic natural organic matter samples of the International Humic Substances Society

The naturally abundant nitrogen in soil and aquatic NOM samples from the International Humic Substances Society has been characterized by solid state CP/MAS 15N NMR. Soil samples include humic and fulvic acids from the Elliot soil, Minnesota Waskish peat and Florida Pahokee peat, as well as the Summit Hill soil humic acid and the Leonardite humic acid. Aquatic samples include Suwannee River humic, fulvic and reverse osmosis isolates, Nordic humic and fulvic acids and Pony Lake fulvic acid. Additionally, Nordic and Suwannee River XAD-4 acids and Suwannee River hydrophobic neutral fractions were analyzed. Similar to literature reports, amide/aminoquinone nitrogens comprised the major peaks in the solid state spectra of the soil humic and fulvic acids, along with heterocyclic and amino sugar/terminal amino acid nitrogens. Spectra of aquatic samples, including the XAD-4 acids, contain resolved heterocyclic nitrogen peaks in addition to the amide nitrogens. The spectrum of the nitrogen enriched, microbially derived Pony Lake, Antarctica fulvic acid, appeared to contain resonances in the region of pyrazine, imine and/or pyridine nitrogens, which have not been observed previously in soil or aquatic humic substances by 15N NMR. Liquid state 15N NMR experiments were also recorded on the Elliot soil humic acid and Pony Lake fulvic acid, both to examine the feasibility of the techniques, and to determine whether improvements in resolution over the solid state could be realized. For both samples, polarization transfer (DEPT) and indirect detection ( 1H– 15N gHSQC) spectra revealed greater resolution among nitrogens directly bonded to protons. The amide/aminoquinone nitrogens could also be observed by direct detection experiments.

Potentiometric acidity determination in humic substances influenced by different analytical procedures

Carboxylic (CA), phenolic (PhA) and total (TA) acidity contents of five International Humic Substances Society (IHSS) standards and four model compounds were determined via the potentiometric titration method. Titration curves were scrutinized both by the BEST7 algorithm and the modified Henderson-Hasselbalch model (MHHM). In the case of IHSS samples, the fitting data depended on the analytical procedure undertaken. Whilst high CA and low PhA were usually recorded using the MHHM, the opposite trend was observed employing the BEST7 algorithm. In contrast, in the case of model compounds the acidity values matched well with theoretical data regardless of the procedure. In order to better understand the reasons for such discrepancies changes in the titrations procedure (e.g.: fast or slow) were also considered. General data strongly suggest that acidity determination of humic substances (HS) by potentiometric methods is extremely dependent on both the choice of mathematical model to fit experimental data points as well as the experimental conditions employed.

Quantifying Interactions between Singlet Oxygen and Aquatic Fulvic Acids

Singlet oxygen (1O2) is a reactive oxygen species produced by dissolved organic matter (DOM) in sunlit waters. While the production of 1O2 by DOM has been studied, little is known on interactions between 1O2 and DOM. The central objective of this work was to quantify the rate constants of reaction and quenching of 1O2 with Suwannee River and Pony Lake fulvic acids, the terrestrial and microbial end-member reference aquatic humic substances of the International Humic Substance Society. Fulvic acids were reacted with 1O2 generated through visible light irradiation of Rose Bengal. Uptake of 1O2 by the fulvic acids was followed through changes in dissolved oxygen concentrations via membrane inlet mass spectrometry (MIMS). Results from multiple diagnostic tests for 1O2-processes in solution suggested that 64-70% of the observed uptake of oxygen by the fulvic acid solutions was due to reaction with 1O2; the remaining O2 uptake was likely due to non-1O2 processes initiated by the excited-state sensitizer. The rate constants of reaction (krxn) and physical quenching (kphys) with 1O2 were determined to be 2.6 x 10(5) M-C(-1) s(-1) and 2.7 x 10(5) M-C(1-) s(-1) (krxn) and 1.5 x 10(5) M-C(1-) s(-1) and 1.3 x 10(6) M-C(1-) s(-1) (kphys) for Suwannee River and Pony Lake fulvic acids, respectively. Results from this study demonstrated that1O2 reacts with microbially and terrestrially derived DOM at rate constants comparable to phenols, naphthols, or aromatic amines, on a per carbon basis.

Quantification of refractory organic substances in freshwaters: further insight into the response of the voltammetric method

A recently published method for quantifying refractory organic matter (often referred to as humic substances) in freshwaters was applied to a wide range of International Humic Substance Society (IHSS) humic compounds in order to (i) gain a better understanding of the mechanism of the voltammetric response which is the basis of the analytical method and (ii) provide guidance on choosing the optimal standard to be used. At the same time, the sensitivity of the technique has been increased by switching from the pulse mode initially proposed to the square-wave mode. The results obtained show that (i) differences in adsorption onto the electrode rather than differences in complexation strength are responsible for the differences in the intensity of the signal obtained for the different humic compounds, (ii) carboxylate, N- and S-containing groups do not play a role in the voltammetric signal.

Capillary Electrophoresis and Fluorescence Excitation‐Emission Matrix Spectroscopy for Characterization of Humic Substances

Capillary electrophoresis (CE) and fluorescence spectroscopy have been used in natural organic matter (NOM) studies. In this study, we characterized five fulvic acids (FAs), six humic acids (HAs), and two unprocessed NOM samples obtained from the International Humic Substances Society (IHSS) using these two analytical methods. The electropherograms of all samples revealed three peak features. The first and third peaks were sharp. The second peak had a broad, hump‐shaped feature. The pattern and shapes of these peaks were different among the FA, HA, and unprocessed NOM samples. Excitation‐emission matrix (EEM) fluorescence spectroscopic analysis revealed that each of the 13 investigated samples contained four components. However, the relative amounts of the four components varied with sample origin. Autoclaving these samples for 1 h (heat decomposition) produced additional CE peaks and changed portions of the four fluorophore components, indicating that both methods can be used to investigate the dynamics of NOM decomposition. Although four fluorophore components were present in each of the three CE fractions, their relative abundances varied among the three CE fractions. Specifically, Fraction 1 and 2 were rich in Component 1 and 4, but sparse in Component 2, compared with their original samples. Fraction 2 also contained less Component 3. The distribution of the four components in Fraction 3 was similar to that of the original samples. The mutual relevance of data collected from each of the two methods provided novel insight into the correlation of complex NOM fluorescence spectra to specific NOM fractions.

Compositional relationships between organic matter in a grassland soil and its drainage waters

SummaryWe present a novel study of the compositional relationships between soil organic components extractable in aqueous extractants and those in sub‐soil drainage and surface runoff waters from the soil. The surface soil (0–20 cm) of a stagnogley in long‐term grassland was sequentially and exhaustively extracted in aqueous media at pH values of 7, 10.6 and 12.6. Extracts from the soils and their runoff and drainage waters were processed by the procedures of the International Humic Substances Society (IHSS), and fractionated into humic, fulvic, and XAD‐4 acids. Elemental, δ13C, δ15N, sugar, amino acids, and solid state CPMAS 13C NMR analyses were used to identify similarities and differences between the fractions from the different extracts. There were few differences between the compositions of drainage water samples taken 1 year apart, and these had compositional features similar to those from the more highly oxidized fractions isolated from the soil at pH 7. There were significant differences between the humic components from the drainage waters and isolated from the soil at pH 7 and those of the humic fractions isolated at the higher pH values whose compositions are more clearly related to origins in plants. The compositions of the surface runoff waters indicate origins in transformed plant and animal manures on the soil surface, whereas those of the deep drainage waters originate in more extensively transformed materials, including products of microbial metabolism. The resin technique used in the fractionation allowed the isolation of novel humic acid fractions from the soil extracts, in particular at pH 7 and 12.6. These fractions clearly originated in microbial sources, were rich in saccharides and amino acids (peptides), and low in lignin‐derived components.

Thermogravimetric characteristics of water-soluble organic matter from atmospheric aerosols collected in a rural–coastal area

The influence of chemical composition on the thermal degradation behaviour of water-soluble organic compounds (WSOC) from fine atmospheric aerosols collected over a period of 12 months was investigated by using thermogravimetric (TG) analysis. The obtained results were further compared to those of Suwannee River Humic Acid (SRHA) and Fulvic Acid (SRFA) standards, which were obtained from the International Humic Substances Society (IHSS). Prior to the TG analyses, the WSOC samples were separated into hydrophobic acids and hydrophilic acids fractions by using a XAD-8/XAD-4 isolation procedure. The data reported in this study were illustrative of the very complex nature of aerosol WSOC hydrophobic acids samples (three to six peaks of thermal degradation) in comparison to that of SRHA and SRFA standards (two degradation peaks). A pseudo-first order mathematical approach was applied to estimate the apparent activation energy of the different multi-step degradation regions. It was found that the degradation processes of the SRHA and SRFA standards occurred with lower apparent activation energies than those obtained for the aerosol WSOC hydrophobic acids samples, although these latter samples appear to exhibit the most thermally labile structural component. The WSOC hydrophobic acids showed a trend towards high apparent activation energies at high temperatures, thus suggesting the decomposition of very stable structural units with relatively strong bond energies above ca. 460 °C. The thermal profiles presented in this study are unique in showing the annual evolution of the thermal-oxidative properties of the fine aerosol WSOC hydrophobic acids samples, and in providing important supplementary information on the structural stability of the bulk aerosol WSOC.

Removal of natural organic material by a magnetic ion exchange resin

The aim of this research was to elucidate the interactions among dissolved organic carbon (DOC), inorganic anions and a magnetic ion exchange (MIEX) resin. Model waters containing well characterised natural organic matter (NOM) extracts in addition to DOC-free model waters containing sulphate and bicarbonate were used in all experiments. The NOM extracts used in this work included two reference fulvic acids supplied by the International Humic Substances Society (IHSS): Suwannee River Fulvic Acid (SRFA) and Pony Lake Fulvic Acid (PLFA). These reference fulvic acids possess unique chemical characteristics that are representative of their distinct end-member environments. The charge density of the NOM extracts, obtained by potentiometric titration, was used to perform charge balance analyses for ion exchange reactions involving DOC, sulphate, bicarbonate and chloride. For SRFA, the results clearly show the stoichiometric exchange of DOC and bicarbonate for chloride. This work provides an improved understanding of the interactions between NOM, MIEX resin and the background solution matrix, and should result in more effective applications of ion exchange treatment for the removal of DOC during drinking water treatment.

Frequency-Domain Fluorescence Lifetime Measurements via Frequency Segmentation and Recombination as Applied to Pyrene with Dissolved Humic Materials

In this study, the association behavior of pyrene with different dissolved humic materials (DHM) was investigated utilizing the recently developed segmented frequency-domain fluorescence lifetime method. The humic materials involved in this study consisted of three commercially available International Humic Substances Society standards (Suwannee River fulvic acid reference, SRFAR, Leonardite humic acid standard, LHAS, and Florida peat humic acid standard, FPHAS), the peat derived Amherst humic acid (AHA), and a chemically bleached Amherst humic acid (BAHA). It was found that the three commercial humic materials displayed three lifetime components, while both Amherst samples displayed only two lifetime components. In addition, it was found that the chemical bleaching procedure preferentially removed red wavelength emitting fluorophores from AHA. In regards to pyrene association with the DHM, different behavior was found for all commercially available humics, while AHA and BAHA, which displayed strikingly similar behavior in terms of fluorescence lifetimes. It was also found that there was an enhancement of pyrene's measured lifetime (combined with a decrease in pyrene emission) in the presence of FPHAS. The implications of this long lifetime are discussed in terms of (1) quenching mechanism and (2) use of the fluorescence quenching method used to determine the binding of compounds to DHM.

Interaction between manufactured gold nanoparticles and naturally occurring organic macromolecules

The increasing exploitation of nanomaterials into many consumer and other products is raising concerns as these nanomaterials are likely to be released into the environment. Due to our lack of knowledge about the environmental chemistry, transport and ecotoxicology of nanomaterials, it is of paramount importance to study how natural aquatic colloids can interact with manufactured gold nanoparticles as these interactions will determine their environmental fate and behaviour. In this context, our work aims to quantify the effect of naturally occurring riverine macromolecules – International Humic Substances Society (IHSS) Suwannee River Humic Acid Standard (SRHA) – on citrate- and acrylate-stabilized gold nanoparticles. The influence of SRHA on the stability of the gold colloids was studied as a function of pH by UV–visible absorption spectroscopy, dynamic light scattering (DLS) and transmission electron microscopy (TEM). At high ionic strengths (0.1 M), extensive and rapid aggregation occurred, while more subtle effects were observed at lower ionic strength values. Evidence was found that SRHA enhances particle stability at extreme pH values (ionic strength < 0.01 M) by substituting and/or over-coating the original stabilizer on the gold nanoparticle surface, thus affecting surface charge and chemistry. These findings have important implications for the fate and behaviour of nanoparticles in the environment and their ecotoxicity.

ISOLATION AND CHARACTERIZATION OF HUMIC ACIDS IN IDAHO SLICKSPOT SOILS

Naturally occurring humic substances in soils hold promise as a factor related to restoring native plants. We have characterized the humic acids (HA) from inside and outside slickspots of two sites in the Juniper Butte Range soils on the Snake River Plain in Southwest Idaho. These HA may promote the establishment of native slickspot peppergrass (Lepidium papilliferum) seedlings. The HA were extracted from the silt, vesicular, and clay layers using a modified International Humic Substances Society method, and isolated by membrane filtration, precipitation, dialysis, and freeze-drying. Elemental analyses showed minor differences between HA; the inside surface silt samples had higher C and N values than the outside or the deeper samples. Differences were also noted between sample sites and between the samples for carbohydrates, amino acids, and acidity functionalities. Fourier transform infrared and fluorescence, and especially cross-polarization magic angle spinning 13C-nuclear magnetic resonance spectroscopies, provided compositional information and highlighted differences between the samples.

Radiation sensitivity of natural organic matter: Clay mineral association effects in the Callovo-Oxfordian argillite

Clay-rich low-organic carbon formations (e.g., Callovo-Oxfordian argillite in France and Opalinus Clay in Switzerland) are considered as host rocks for radioactive waste disposal. The clay-organic carbon has a strong impact on the chemical stability of the clays. For this reason, the nature of the clay-organic carbon, the release of hydrophilic organic compounds, namely, humic (HA) and fulvic acids (FA) and the radiation sensitivity of the undisturbed host rock organics was investigated. The clay sample originates from Oxfordian argillite (447 m depth, borehole EST 104). HA and FA were extracted following the standard International Humic Substance Society (IHSS) isolation procedure. Synchrotron based (C-, K-, Ca-, O- and Fe-edge XANES) scanning transmission X-ray microscopy (STXM) and FT-IR microspectroscopy was used to identify under high spatial resolution the distribution of clay-organic matter with different functionality using principal component and cluster analysis. The results show that in this old (Jurassic) geological formation, small parts of the organic inventory (1–5%) keeps the structure/functionality and can be mobilized as hydrophilic humic substance type material (HA and FA). Target spectra analysis shows best correlation for isolated humic acids with organics found in smectite-rich regions, whereas the extractable FA has better spectral similarities with the illite mixed layer minerals (MLM) regions. After radiation of 1.7 GGy under helium atmosphere the same rock sample area was investigated for radiation damage. Radiation damage in the smectite and illite–MLM associated organic matter is comparably low with 20–30% total oxygen mass loss and 13–18% total carbon mass loss. A critical dose d c of 2.5 GGy and a optical density after infinite radiation (OD ∝) of 54% was calculated under room temperature conditions. C(1s) XANES show a clear increase in C C bonds especially in the illite–MLM associated organics. This results suggests a combination of the formation of C C bond due to crosslinking via polymerization and mass loss due to bond breaking (scissioning) in the main chain or in side groups of the organic macromolecules upon irradiation.

Analytical constraints on acidic functional groups in humic substances

Four standard fulvic acids and two standard humic acids from the International Humic Substances Society (IHSS) were analyzed to quantify the in situ generation of new acidity during direct titrations, as observed by hysteresis between tandem forward and reverse titration curves. The resulting concentrations of carboxyl and phenolic groups were then corroborated by analytical constraints imposed by elemental data and 13C NMR data. Collectively, carboxyl groups in the IHSS samples, as determined by rapid forward direct titrations, consistently accounted for 69% (±6) of the maximum possible carboxyl-like structures that were estimated by 13C NMR spectra. The concentrations of carboxyl groups in the samples increased for two separate reverse titrations initiated at 30 min and 24 h after the completion of the forward titrations, representing 73% (±7) and 84% (±8) of the maximum carboxyl-like groups, respectively. Conversely, the concentrations of phenolic groups were invariant between forward and reverse titrations. Elemental constraints coupled with titration data predicted that the IHSS samples have, on average, one acidic hydroxyl group per every two phenyl rings, as constrained by 13C NMR estimates of aromatic carbon.

Synthesis and utilization of Mg/Al hydrotalcite for removing dissolved humic acid

It has been synthesized Mg/Al layered double hydroxide anionic clay (Mg/Al hydrotalcite) through direct precipitation by adding 0.5 M NaOH solution into a mixed solution containing Mg(NO 3) 2 and Al(NO 3) 3 with molar ratio of 0.1:0.05 until the medium acidity reached pH 10.1. The synthesized Mg/Al hydrotalcite was then utilized to remove dissolved humic acid in aqueous medium. The humic acid was isolated from peat soil taken in Gambut District, South Kalimantan, Indonesia using the recommended procedure of IHSS (International Humic Substances Society). The removal of humic acid was mostly occurred through simple sorption process without accompanied by significant intercalation. The sorption was optimum at pH 9.0, with the first order rate constant, capacity and energy of sorption were 5.50 × 10 −3 min −1, 0.12 mmol g −1 (69 mg g −1), and 28.32 kJ mol −1, respectively.

Alternating current anodic stripping voltammetry in the study of cadmium complexation by a reference Suwannee river fulvic acid: a model case with strong electrode adsorption and weak binding

The possibilities of anodic stripping voltammetry (ASV) using an alternating current (AC) scan in the stripping step have been checked through the study of the complexation of cadmium by Suwannee river fulvic acid (SRFA), a reference fulvic acid from the International Humic Substances Society. Because of the strong electrode adsorption of SRFA, AC mode appears to be a good approach to the study when proper selection of the phase angle is made. The goodness of AC mode in ASV has been demonstrated, and the complexation constant of 3.71 +/- 0.04 determined is in good agreement with the value of the constant obtained by the reference technique of reverse pulse polarography. Some particularities of SRFA have been observed, among them its homofunctional and strongly heterogeneous behaviour in cadmium complexation and the impossibility of avoiding electrode adsorption problems in ASV measurements at very low metal concentrations. Figure DP anodic stripping and AC anodic stripping voltammograms at -12 degrees and -65 degrees during the titration of a 10(-7) mol L(-1) Cd(II) solution with SRFA at pH 7.5 in 0.05 L(-1) Tris.

The complementary use of 1H NMR, 13C NMR, FTIR and size exclusion chromatography to investigate the principal structural changes associated with composting of organic materials with diverse origin

The aim of this work is to study the structural changes involved in humification processes. Total humic extracts (THE) obtained from five composted materials of diverse origin (solid wastes of wineries, solid mill olive wastes, domestic wastes, ovine manures plus straw, and a mixture of animal manures), and their corresponding initial raw fresh organic mixtures were studied using 13C nuclear magnetic resonance (NMR) using the cross-polarization magic angle spinning technique (CPMAS), 1H NMR, Fourier transform infrared spectroscopy (FTIR) and high pressure size exclusion chromatography (HPSEC). One group of three humic acids extracted from soils, and a second group consisting of two reference humic acids and two reference fulvic acids (1S104H, 1R103H, 1R101F and 1R107F) obtained from the International Humic Substances Society were also characterized using these techniques, in order to compare the features of reference humic and fulvic acids with those of composted materials. Likewise, the results were compared with those obtained in previous studies, in which UV–Visible and fluorescence spectroscopies were employed to characterize the humification degree of the molecular systems. The results obtained by 13C CPMAS NMR, 1H NMR and FTIR indicate that, in general, humification seems to be associated with an increase in the aromatic character of the systems, with the presence of phenol groups as principal substituents and a reduction in oxygen containing functional groups, principally carboxylic or carbonylic groups, as well as the development of molecular fractions with larger size. These results also support the suitability of UV–Visible and fluorescence spectroscopies in the assessment of the humification course of humic extracts in composting processes.

Fine fractionation and purification of the fulvic acid fraction using adsorption and precipitation procedures

A new scheme for fine fractionating and purifying a crude fulvic acid fraction was proposed. The fractionation procedure includes an adsorption process of the fulvic acid fraction on a hydrophobic resin (e.g. XAD-8 and DAX-8) and a sequence of successive elution processes. This procedure yields a non-adsorbed fraction (FA-1), a 0.1 mol L−1 HCl eluted fraction (FA-2), a distilled water eluted fraction (FA-3) and a 0.1 mol L−1 NaOH eluted fraction (FA-4). To remove salts and water, the fractionated organic molecules were co-precipitated with Al hydroxides at pH 5 and collected by centrifugation. Recoveries of carbon from the crude fulvic acid fraction of an Andosol were 42, 20, 19 and 14% for FA-1, FA-2, FA-3 and FA-4, respectively, and 96% of carbon was recovered in total. Optical properties and 13C nuclear magnetic resonance spectra indicated that the chemical properties of FA-1 were similar to those of FA-2 and were hydrophilic, while the chemical properties of FA-3 were very close to those of FA-4 and were hydrophobic (estimated hydrophobicity: humic acid >> FA-4 ≥ FA-3 >> FA-2 ≥ FA-1). A fulvic acid that can be isolated using the International Humic Substances Society method would be, in practice, a mixture of hydrophilic fulvic acid (FA-2) and hydrophobic fulvic acid (FA-3 + FA-4). The method developed in the present study will provide more detailed, quantitative and complete information of the fulvic acid fraction of soils.

The freshwater dissolved organic matter fluorescence–total organic carbon relationship

AbstractThe fluorescent properties of dissolved organic matter (DOM) enable comparisons of humic‐like (H‐L) and fulvic‐like (F‐L) fluorescence intensities with dissolved organic carbon (DOC) in aquatic systems. The fluorescence‐DOC relationship differed in gradient, i.e. the fluorescence per gram of carbon, and in the strength of the correlation coefficient. We compare the fluorescence intensity of the F‐L and H‐L fractions and DOC of freshwater DOM in north Shropshire, England, featuring a river, wetland, spring, pond and sewage DOM sources. Correlations between fluorescence and DOC varied between sample sites. Wetland water samples for the F‐L peak gave the best correlation, r = 0·756; the lowest correlation was from final treated sewage effluent, r = 0·167. The relationship between fluorescence and DOC of commercially available International Humic Substances Society standards were also examined and they generally showed a lower fluorescence per gram of carbon for the F‐L peak than the natural samples, whereas peat wetland DOM gave a greater fluorescence per gram of carbon than river DOM. Here, we propose the strength of the fluorescence–DOC correlation to be a useful tool when discriminating sources of DOM in fresh water. Copyright © 2007 John Wiley &amp; Sons, Ltd.