Removal of dissolved organic matter by anion exchange: effect of dissolved organic matter properties

Ten isolates of aquatic dissolved organic matter (DOM) were evaluated to determine the effect that chemical properties of the DOM, such as charge density, aromaticity, and molecular weight, have on DOM removal by anion exchange. The DOM isolateswere characterized as terrestrial, microbial, or intermediate humic substances or transphilic acids. All anion exchange experiments were conducted using a magnetic ion exchange (MIEX) resin. The charge density of the DOM isolates, determined by direct potentiometric titration, was fundamental to quantifying the stoichiometry of the anion exchange mechanism. The results clearly show that all DOM isolates were removed by anion exchange; however, differences among the DOM isolates did influence their removal by MIEX resin. In particular, MIEX resin had the greatest affinity for DOM with high charge density and the least affinity for DOM with low charge density and low aromaticity. This work illustrates that the chemical characteristics of DOM and solution conditions must be considered when evaluating anion exchange treatment for the removal of DOM.     

Major structural components in freshwater dissolved organic matter

Dissolved organic matter (DOM) contains a complex array of chemical components that are intimately linked to many environmental processes, including the global carbon cycle, and the fate and transport of chemical pollutants. Despite its importance, fundamental aspects, such as the structural components in DOM remain elusive, due in part to the molecular complexity of the material. Here, we utilize multidimensional nuclear magnetic resonance spectroscopy to demonstrate the major structural components in Lake Ontario DOM. These include carboxyl-rich alicyclic molecules (CRAM), heteropolysaccharides, and aromatic compounds, which are consistent with components recently identified in marine dissolved organic matter. In addition, long-range proton-carbon correlations are obtained for DOM, which support the existence of material derived from linear terpenoids (MDLT). It is tentatively suggested that the bulk of freshwater dissolved organic matter is aliphatic in nature, with CRAM derived from cyclic terpenoids, and MDLT derived from linear terpenoids. This is in agreement with previous reports which indicate terpenoids as major precursors of DOM. At this time it is not clear in Lake Ontario whether these precursors are of terrestrial or aquatic origin or whether transformations proceed via biological and/ or photochemical processes.     

Compositional differences between size classes of dissolved organic matter from freshwater and seawater revealed by an HPLC-FTIR system

The molecular complexity of dissolved organic matter (DOM) hinders its characterization. New approaches are thus needed for a better understanding of DOM reactivity and fate in aquatic systems. In this study, high-performance liquid chromatography (HPLC), using size-exclusion separation, was coupled with Fourier transform infrared spectroscopy (FTIR). A solvent-elimination interface was used to deposit DOM fractions onto a germanium disk that were then analyzed by FTIR. Samples included ultrafiltered DOM (UDOM) and fulvic acids from the St. Lawrence Estuary and its tributaries. Results showed significant compositional changes with molecular size and origin, especially in UDOM. Larger fractions of UDOM contained more carbohydrates, amides, aromatics/alkenes and aliphatics, while smaller fractions contained more carboxylate and OH groups. Small marine molecules (500-900 Da) were also enriched in sulfate groups that appeared bound to UDOM. Large marine molecules were the most amide-rich fractions. Fulvic acids were enriched in carboxylate and OH groups, showed little changes in composition, and appeared similar to small terrigenous (riverine) UDOM even in marine water. This work shows that an HPLC-FTIR system is a powerful and complementary tool in the characterization of DOM. The compositional changes observed may explain the reported contrasting reactivity and fate of DOM having different size and origin.     

Binding of pyrene to hydrophobic fractions of dissolved organic matter: effect of polyvalent metal complexation

The effects of polyvalent metal cations on pyrene binding to hydrophobic acid and neutral fractions (HoA and HoN, respectively) of dissolved organic matter (DOM) were elucidated. The DOM was isolated from sewage sludge; pyrene binding was estimated from fluorescence measurements. Isotherms of pyrene binding to both fractions were nonlinear. Pyrene binding was higher for HoN due to the combined effect of greater hydrophobicity, aromaticity and the large molecular size of this fraction relative to HoA. The complexation of HoA with Cu2+, Al3+, and Fe3+ increased the binding of pyrene only when the HoA was equilibrated with polyvalent cations before pyrene was added. The maximal increase in pyrene binding to HoA was 56%, 64%, and 118% when pre-equilibrated with Cu2+, Fe3+, and Al3+, respectively. Pyrene binding to HoN was not affected by the presence of metal cations. HoA complexation with metal cations increased the apparent molecular size of this fraction. We suggest that the presence of metal cations induces the formation of pseudomicelles, which are more efficient in binding pyrene than the low-molecular-weight components. Our results demonstrate that HoA and HoN components can significantly affect the transport of organic contaminants in soils irrigated with treated wastewater or amended with sewage sludge.     

Photosensitized degradation of bisphenol A by dissolved organic matter

The direct and indirect photolysis of bisphenol A (BPA) was investigated using a solar simulator in the absence and presence of dissolved organic matter (DOM). BPA degradation by direct photolysis was significantly slower than its rate in the presence of DOM. In natural waters, the direct photolytic pathway would be even less important due to light screening effects. Surprisingly, differences in the rate of indirect BPA photolysis were relatively small between DOM samples. Two of the DOM samples represented terrestrial (Suwannee River fulvic acid) and autochthonous (Lake Fryxell) geochemical endmembers. The third DOM (Fulton County, Ohio) was derived from a temperate artificial wetland. We were unable to correlate BPA photoreactivity to the structural components of DOM or its extinction coefficient at 280 nm. The addition of methanol, a hydroxyl radical scavenger, to reaction solutions slowed but did not completely quench the indirect photolysis of BPA. This observation suggests that BPA photodegrades via multiple pathways involving other transients formed by the photolysis of DOM. Competitive experiments using 2,4,6-trimethylphenol also reduce the reaction rate of BPA by DOM and implythat other DOM-derived phototransients (e.g., excited triplet state DOM) are involved in the reaction. The reaction rate coefficients reported under solar-simulated irradiance in the presence of DOM are significantly faster than those reported for the microbial degradation of BPA. Thus, in natural surface waters photosensitized transformation of BPA by dissolved organic matter may be as important as biodegradation.     

Ultrafiltration of nonionic surfactants and dissolved organic matter

A brownwater sample with a high content of humic substances (HS) was fractionated by multistage ultrafiltration (mst-UF) into five fractions with nominal molecular weights ranging from >30 to <1 kDa. Fractions were characterized with respect to molecular size distribution and structure. Size exclusion chromatography with online DOC detection revealed that mst-UF yielded fractions with decreasing Mp (molecular weight at peak maximum) and polydispersities from nominally large to small mst-UF fractions. 13C MAS NMR analysis showed that the content of carbohydrate structures decreased from the original sample toward smaller molecular weight (MW) fractions, which in turn contained more carboxylic groups and branched aliphatic structures. Specific UV absorbances (SUVA254) were highest in the >30 kDa fraction and decreased with decreasing MW. To evaluate whether separation mechanisms other than size exclusion were of importance during the fractionation, the behavior of low molecular weight model compounds (MC) with a range of polarities was studied. Recoveries decreased with increasing hydrophobicity of the MC. For selected nonylphenol ethoxylates and 4-nonylphenol the recovery correlated well with the hydrophile-lipophile balance value. The presence of dissolved organic matter (DOM) caused an additional loss of hydrophobic MC, possibly because of sorption of the compounds onto DOM fouling layers. The hydrophilic MC caffeine was recovered almost completely (85-86%) regardless of the DOM content of the model solution. It was concluded that size exclusion was the dominant fractionation mechanism for caffeine, whereas hydrophobic interactions played a major role during the mst-UF fractionation of nonpolar contaminants. For a better understanding of the behavior of polyfunctional molecules such as HS, the effect of other physicochemical properties needs to be investigated in further studies.     

Kinetics of desorption of organic compounds from dissolved organic matter

This study presents a new experimental technique for measuring rates of desorption of organic compounds from dissolved organic matter (DOM) such as humic substances. The method is based on a fast solid-phase extraction of the freely dissolved fraction of a solute when the solution is flushed through a polymer-coated capillary. The extraction interferes with the solute-DOM sorption equilibrium and drives the desorption process. Solutes which remain sorbed to DOM pass through the extraction capillary and can be analyzed afterward. This technique allows a time resolution for the desorption kinetics from subseconds up to minutes. It is applicable to the study of interaction kinetics between a wide variety of hydrophobic solutes and polyelectrolytes. Due to its simplicity it is accessible for many environmental laboratories. The time-resolved in-tube solid-phase microextraction (TR-IT-SPME) was applied to two humic acids and a surfactant as sorbents together with pyrene, phenanthrene and 1,2-dimethylcyclohexane as solutes. The results give evidence for a two-phase desorption kinetics: a fast desorption step with a half-life of less than 1 s and a slow desorption step with a half-life of more than 1 min. For aliphatic solutes, the fast-desorbing fraction largely dominates, whereas for polycyclic aromatic hydrocarbons such as pyrene, the slowly desorbing, stronger-bound fraction is also important.     

Binding of organic ligands with Al(III) in dissolved organic matter from soil: implications for soil organic carbon storage

The binding characteristics of organic ligands with Al(III) in soil dissolved organic matter (DOM) is essential to understand soil organic carbon (SOC) storage. In this study, two-dimensional (2D) FTIR correlation spectroscopy was developed as a novel tool to explore the binding of organic ligands with Al(III) in DOM present in soils as part of a long-term (21-year) fertilization experiment. The results showed that while it is a popular method for characterizing the binding of organic ligands and metals, fluorescence excitation-emission matrix-parallel factor analysis can only characterize the binding characteristics of fluorescent substances (i.e., protein-, humic-, and fulvic-like substances) with Al(III). However, 2D FTIR correlation spectroscopy can characterize the binding characteristics of both fluorescent and nonfluorescent (i.e., polysaccharides, lipids, and lignin) substances with Al(III). Meanwhile, 2D FTIR correlation spectroscopy demonstrated that the sequencing/ordering of organics binding with Al(III) could be modified by the use of long-term fertilization strategies. Furthermore, 2D FTIR correlation spectroscopy revealed that the high SOC content in the chemical plus manure (NPKM) treatment in the long term fertilization experiment can be attributed to the formation of noncrystalline microparticles (i.e., allophane and imogolite). In summary, 2D FTIR correlation spectroscopy is a promising approach for the characterization of metal-organic complexes.     

Determination of the mercury complexation characteristics of dissolved organic matter in natural waters with "reducible Hg" titrations

A new method for the determination of the concentration and conditional stability constant of dissolved organic matter that binds mercury (Hg) has been developed using an in vitro assay of reducible Hg. The technique is a wet chemical analogue to electrochemical approaches now in use for ligand studies of many other trace transition metals in natural waters. Ligand characteristics are obtained from additions of ionic Hg to buffered lake, river water, and seawater and determination of the wet chemically reducible fraction following equilibration of the spike. This approach is robust, as demonstrated by (i) analysis using three reducing agents of varying strengths, (ii) replicate analyses, (iii) comparison to well-characterized complexing species (chloride and EDTA) using a competitive ion-exchange resin, and (iv) kinetic studies. Results indicate that Hg-complexing equivalents are present in the dissolved phase (<0.2 microm) ranging from <1 to >60 nN concentrations and with log conditional stability constants (log K') in the range of 21-24. Only one ligand class was found in the natural waters analyzed. There was indirect evidence for a class of organic ligands that formed reducible complexes with Hg in freshwater. Such ligand characteristics indicate that the vast majority of ionic inorganic Hg dissolved in freshwater and coastal saltwaters is associated with organic complexes. Concentrations, affinities, and kinetics implicate multidentate chelation sites as the principal complexing moieties for Hg and discourage the use of humic carboxylic acids as a proxy for the ligands/functional groups.     

A multifactor exploration of the photobleaching of Suwannee River dissolved organic matter across the freshwater/saltwater interface

A four factor central composite experimental design was applied to explore the photobleaching of Suwannee River dissolved organic matter (SRDOM) at 350 nm as a function of the tetravariate system of [SRDOM], total [Fe(III)], [NO3-], and salinity. The ranges of each factor were setto cover their likely concentrations atthe freshwater/ saltwater interface, to encompass the possible conditions encountered during the transition from the terrestrial to marine environment. Each experiment was carried out using a minimum of 25 different initial conditions, with 3-6 replicates/condition. The resulting data set mapped out the effects of multiple photoactive components on the rate of photobleaching. Under the conditions tested (nominally total [Fe(III)] 0.00-4.00 microM; [NO3-] 0.00-60.00 microM; SRDOM 0.00-30.00 mg/L; salinity 0.00-35.00 ppt, polychromatic illumination, pH 8.2) all samples photobleached at all wavelengths measured, and the absorption at 350 nm bleached the most rapidly. The most important factor for predicting the rate of photobleaching at 350 nm was the initial loading of SRDOM; the effect of all other factors on photobleaching was not significant at the 95% level of confidence. Varied salinity, Fe(III), or added D2O had no effect on the rate of photobleaching, indicating that hydroxyl radical, singlet oxygen, and superoxide did not contribute significantly to the loss of the chromophore at 350 nm. The addition of hydroquinone or thiosulfate inhibited photobleaching, suggesting photobleaching may depend on a weaker oxidant such as the excited-state acceptors (derived from SRDOM directly) produced during photoinitiated charge-transfer processes. The advantages of multifactor experimental techniques for exploring SRDOM photochemistry are discussed.     

Photoformation of low-molecular-weight organic acids from brown water dissolved organic matter

This work describes the effects of simulated solar UV light on the bulk properties of dissolved organic matter (DOM) of bog lake water and on the formation of low-molecular-weight organic acids (LMWOAs). By means of size-exclusion chromatography it was shown that the more hydrophilic moieties of the DOM were preferentially photodegraded while the more hydrophobic ones remained relatively unaffected or were even formed. The combined photochemical-biological degradation proved to be more important than the pure photochemical mineralization. Formic, acetic, pyruvic, oxalic, malonic, and succinic acids were identified as important degradation products. Their contribution to the dissolved organic carbon increased from 0.31% before to 6.4% after 24 h irradiation. About 33% of the bioavailable photoproducts of DOM were comprised of these LMWOAs. The influence of nitrate on the formation of carboxylic acids could not be observed in the investigated system. Kinetic experiments indicated that degradation of LMWOAs occurred simultaneously during irradiation experiments, alpha-oxygen-substituted LMWOAs being more amenable to these processes. Dissolved iron acted as a catalyst of DOM photodegradation and LMWOA photoformation. Copper played an antagonistic role in the irradiation experiments, reducing the formation of formic, acetic, and malonic acids while increasing the formation of oxalic acid.     

Molecular fractionation of dissolved organic matter with metal salts

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.     

Reduction of oxamyl and related pesticides by FeII: influence of organic ligands and natural organic matter

The reduction of oxamyl and related oxime carbamate pesticides (OCPs; methomyl and aldicarb) by FeII is an important pathway for the degradation of these compounds in soil and groundwater. A series of batch kinetic experiments was carried out to assess the effects that selected carboxylate and aminocarboxylate ligands have on these reactions. In the absence of FeII, no OCP reduction by the ligands is observed. In the presence of FeII, the rate of OCP reduction varies by several orders of magnitude and can be described by the expression k(red) = [FeII]sigma(i)k(i)alpha(i) where k(red) is the observed pseudo-first-order rate constant for OCP reduction, [FeII] is the total FeII concentration, alpha(i) is the fraction of each FeII species in solution, and k(i) is the second-order rate constant for OCP reduction by each FeII species. The reactivity of individual FeII species is dependent upon the standard one-electron reduction potential of the corresponding FeIII/FeII redox couple (E(H)o) and the availability of inner-sphere FeII coordination sites for bonding with Lewis base donor groups within the OCP structure. A linear free energy relationship is proposed. Kinetic measurements demonstrate that natural organic matter from the Great Dismal Swamp facilitates OCP reduction by FeII in the same manner as the individual organic ligands. Results from this study improve our understanding of the pathways and rates of pesticide degradation in reducing subsurface environments, especially those rich in organic matter.     

Electron transfer capacities and reaction kinetics of peat dissolved organic matter

Information about electron-transfer reactions of dissolved organic matter (DOM) is lacking. We determined electron acceptor and donor capacities (EAC and EDC) of a peat humic acid and an untreated peat DOM by electrochemical reduction and reduction with metallic Zn and H2S (EAC), and by oxidation with complexed ferric iron (EDC) at pH 6.5. DOC concentrations (10-100 mg L(-1)) and pH values (4.5-8) were varied in selected experiments. EAC reached up to 6.2 mequiv x (g C)(-1) and EDC reached up to 1.52 mequiv-(g C)(-1). EDC decreased with pH and conversion of chelated to colloidal iron, and the electron-transfer capacity (ETC) was controlled by the redox potential Eh of the reactant (ETC = 1.016x Eh - 0.138; R(2) = 0.87; p = 0.05). The kinetics could be adequately described by pseudo first-order rate laws, one or two DOM pools, and time constants ranging from 2.1 x 10(-3) d-1 to 1.9 x 10(-2) d(-1) for the fast pool. Reactions were completed after 24-160 h depending on the redox couple applied. The results indicate that DOM may act as a redox buffer over electrochemical potentials ranging from -0.9 to +1.0 V.     

Strong colloidal and dissolved organic ligands binding copper and zinc in rivers

The speciation or physicochemical form of copper and zinc in freshwater plays an important role in reactivity, bioavailability, and toxicity. Strong metal-binding ligands, which determine speciation, were detected by voltammetric methods, both anodic stripping voltammetry (ASV) and competitive ligand equilibration adsorptive stripping voltammetry (CLE-AdSV); the latter technique can detect nanomolar levels of extremely strong (log K' > 13) ligands. Through careful field site selection and the investigation of ultrafiltration permeate samples, natural organic ligands were measured with limited interferences of colloidal inorganic iron- and aluminum-based trace metal-binding phases. Furthermore, ultrafiltration allowed measurement of colloidal and dissolved ligands independently, and differences of ligand abundance and strength in different size classes are reported. For copper, ultrafilterable (<3 kDa) organic ligand site concentrations (expressed normalized to dissolved organic carbon) were on average 33% of the colloidal level, but ultrafilterable ligand log K' values were 0.5 log units stronger than those of the 0.4 microm filterable concentration. The ultrafilterable copper-binding ligand concentration showed a smaller variation across the rivers (25% rsd) than zinc-binding ligands (90% rsd). For all field sites and size fractions, strong ligand sites greatly exceeded metal concentrations; subsequently, equilibrium speciation modeling predict picomolar levels of free metal. Modeling also indicated that the very strong ligands (detected by CLE-AdSV) predominate, so modeling based solely on ASV data in freshwater may be inadequate. Competition experiments indicated that the very strong ligand sites are metal specific for copper and zinc.     

Binding constants of mercury and dissolved organic matter determined by a modified ion exchange technique

Ion-exchange techniques have been widely used for determining the conditional stability constants (logK) between dissolved organic matter (DOM) and various metal ions in aqueous solution. An exception is mercuric ion, Hg2+, whose exceedingly strong binding with reduced sulfur or thiol-like functional groups in DOM makes the ion exchange reactions difficult. Using a Hg-selective thiol resin, we have developed a modified ion-exchange technique which overcomes this limitation. This technique allows not only the determination of binding constants between Hg2+ and DOM of varying origins, but also the discrimination of complexes with varying coordination numbers [i.e., 1:1 and 1:2 Hg:thiol-ligand (HgL) complexes]. Measured logK values of four selected DOM isolates varied slightly from 21.9 to 23.6 for 1:1 HgL complexes, and from 30.1 to 31.6 for 1:2 HgL(2) complexes. These results suggest similar binding modes that are likely occurring between Hg2+ and key thiolate functional groups in DOM particularly at a relatively low Hg to DOM ratio. Future studies should further elucidate the nature and precise stoichiometries of binding between Hg2+ and DOM at environmentally relevant concentrations.     

Effect of dissolved organic matter on the uptake of trace metals by American oysters

To examine the effects of dissolved organic matter on metal bioavailability, uptake of trace metals (Cd, Co, Hg, Cr, Ag, Zn) by American oysters (Crassostrea virginica) was compared between treatments with different dissolved organic carbon (DOC) concentrations and contrasting low molecular weight (LMW, 1 kDa) and high molecular weight (HMW, 1 kDa-0.2 micron) DOC fractions, using radiotracer techniques and short-term exposure experiments. Uptake rate constants (mL g-1 h-1) of metals, in general, increased with increasing DOC concentrations, with an initial decrease at lower DOC concentrations. Oyster dry weight concentration factors (DCF, mL g-1), determined at the end of exposure experiments (8 h), also increased for Cd, Co, Cr, Ag, and Zn, but decreased for Hg, with increasing DOC concentrations. Changes of metal uptake rate constants and DCF values with DOC concentration suggest that metal uptake pathways by American oysters vary from predominantly uptake (by diffusion of neutral) of free ionic, inorganically complexed, and LMW organic ligand complexed metals at very low DOC concentration to direct ingestion and digestion of HMW or colloidally complexed metals at higher DOC concentrations. Measured partition coefficients (Kc) between dissolved and colloidal phases were comparable between metals, ranging from 10(5.12) to 10(5.75) mL g-1. However, DCF values and uptake rate constants differed considerably between metals, with the highest DCF values and uptake rate constants found for B-type metals, e.g., Ag, Hg, Zn, and Cd, and the lowest ones for several intermediate-type metals (e.g., Co, Cr). Metal types and thus the interaction of metals with organic ligands, such as strong complexation of B-type metals with S-containing organic ligands, may play an important role in the bioavailability and toxicity of metals to aquatic organisms. Differences in metal uptake in contrasting LMW and HMW DOC treatments suggest a generally depressed bioavailability of colloidally complexed metals at low DOC concentration (0.5 ppm) but a generally enhanced uptake at higher DOC concentrations.     

Comparison of the effects of sonolysis and gamma-radiolysis on dissolved organic matter

The effects of 640 kHz sonolysis and 60Co gamma-radiolysis on dissolved organic matter (DOM) were compared through UV/ vis absorption spectrometric, dissolved organic carbon concentration ([DOC]), and potentiometric titration analyses. A reverse-phase chromatographic technique was used to compare changes in the DOM hydrophobicity distribution, and a size exclusion chromatographic technique with inline UV-A absorbance, fluorescence, and [DOC] detectors was used to compare changes in the DOM molecular weight distribution. Whereas upon radiolysis major decreases in absorbance and [DOC] were induced and near-total DOC removal was achieved, upon sonolysis there were major decreases in UV/vis absorbance but only minor decreases in [DOC], and a substantial quantity of hydrophilic nonchromophoric material remained in solution. In radiolysis, hydrophilic and hydrophobic DOM solution components were exposed to equal hydroxyl radical (*OH) concentrations. However, in sonolysis, hydrophobic DOM components were exposed to more elevated *OH concentrations than the hydrophilic components and consequently had enhanced rates of degradation. Sonolysis may be of interest in the design of advanced oxidation processes in which the selective elimination of hydrophobic solution components, such as hydrophobic organic contaminants and hydrophobic DOM domains into which they partition, is desired.     

Accurate quantification of aromaticity and nonprotonated aromatic carbon fraction in natural organic matter by 13C solid-state nuclear magnetic resonance

An improved approach for accurately determining the aromatic carbon fraction (fa) and nonprotonated aromatic carbon fraction (faN) in natural organic matter by solid-state 13C NMR is described. Quantitative peak areas are obtained from direct polarization 13C nuclear magnetic resonance (NMR) under high-speed magic angle spinning (MAS). The problem of overlap between aromatic and alkyl carbon resonances around 90-120 ppm in 13C NMR spectra is solved by a 13C chemical shift anisotropy (CSA) filter technique. After correction for residual spinning sidebands, an accurate value of the aromaticity fa is obtained. To obtain a quantitative faN fraction, dipolar dephasing was adapted for high-speed MAS 13C NMR; the separation of the signals of nonprotonated alkyl and aromatic carbons was achieved by CSA filtering plus dipolar dephasing. The method is demonstrated on a peat humic acid, yielding fa = 45 +/- 2% and faN = (0.64 +/- 0.07) x 45%.