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.     

Comparison of the Complexation of Cu and Cd by Humic or Fulvic Acids and by Ligands Observed in Lake Waters

The complexation of Cu and Cd by ligands observed in filtered unfractionated lakewaters is compared to the complexation by humic and fulvic acids. Complexation parameters (conditional stability constants and ligand concentrations) of Suwannee River fulvic acids (FA), purified peat humic acids (HA) and of ligands in lakewater samples have been determined using the same methods (ligand-exchange and CSV (cathodic stripping voltammetry) or ASV (anodic stripping voltammetry)), and the same titration ranges of Cu, Cd and organic carbon concentrations. The performance of the used techniques is first evaluated in FA and HA suspensions, and gives comparable results with the literature values for the same materials, according to published models (5-site model, NICA model) and parameters. Model calculations using the WHAM model for FA and HA (Tipping, 1994) are also presented. The comparison of titrations of FA and HA with Cu and Cd with those of lakewater samples indicates that stronger ligands than FA and HA are present at low concentrations in the lakewaters. Specific strong ligands occur in particular in eutrophic lake waters, whereas in a lake with higher metal concentrations and low biological productivity the ligands more closely match the fulvic acid characteristics.     

Deuterium NMR characterization of noncovalent interactions between monoaromatic compounds and fulvic acids

Deuterium nuclear magnetic resonance spectroscopy (²H-NMR) spin–lattice relaxation (T₁) experiments were used to measure noncovalent interactions between deuterated monoaromatic compounds (phenol-d₅, pyridine-d₅, benzene-d₆) and fulvic acids isolated from the Suwannee River and Big Soda Lake. Noncovalent interactions, in aqueous solution, were examined as a function of monoaromatic hydrocarbon functional groups, fulvic acid concentration and identity, and solution pH. Phenol did not exhibit noncovalent interactions with either fulvic acid at any pH. Pyridine, in a pH range from 3 to 8, interacted with Suwannee River fulvic acid, forming a bond involving the lone pair of electrons on nitrogen. Conversely, no interactions were observed between pyridine and Big Soda Lake fulvic acid; the difference in noncovalent interactions is attributed to the structural and chemical differences of the two fulvic acids. The translational and rotational molecular motion of benzene increased in the presence of both fulvic acids, indicating that in aqueous solution, fulvic acids solubilize benzene rather than forming discrete bonds as with pyridine. The results of this study demonstrate that monoaromatic functional groups, solution pH, and identity and concentration of fulvic acid can influence the type and degree of noncovalent interactions with dissolved organic matter.     

Sedimentary humic acid and fulvic acid as surface active substances

Surface tension of sedimentary fulvic acid (FA) and humic acid (HA) with molecular weight from < 10,000 to > 300,000 was measured at 5°C and 25°C, over a wide range of concentrations (0.114-107.4 g/l) at pH 8. HA was in the form of sodium humate. Surface tension decreases with an increase in HA and FA concentration and both HA and FA were found to be surface active materials with FA exhibiting the lowest surface tension (31 dynes/cm).Plots of surface tension vs. log concentration gave two straight lines with a break at a certain concentration similar to surfactants. From the concentration at the break point, aggregation concentration (AGC) was determined. For HA with molecular weight above 10,000, the AGC decreased with an increase in molecular weight. The more hydrophobic the HA, the greater was the tendency to form aggregates. Surface excess (surface concentration) was determined (2.3 × 10^?10?5.5 × 10^?10mol/cn²) from the slope of the plot of surface tension vs. log concentration for concentrations lower than the AGC. Adsorption of HA into the surface layer increased with increasing molecular weight of HA.     

Effects of pH and Ca competition on complexation of cadmium by fulvic acids and by natural organic ligands from a river and a lake

The technique of competitive ligand-exchange/anodic stripping voltammetry (CLE-SV) was used to investigate effects of pH and Ca concentration on cadmium complexation by fulvic acid (FA), as well as Cd speciation in two different freshwaters, a hardwater Lake Greifen and a softwater River Wyre. Binding of Cd to Suwannee river FA (10 mg/l) was determined at different pH (7–8.5) and in the presence of various concentration of Ca²⁺ (0–2 mmol/l). The results from one-ligand discrete models were compared to simulations by the WHAM VI model. In Lake Greifen, the determined dissolved [Cd²⁺] ranged from 10⁻¹³ to 10⁻¹² mol/l, and the conditional stability constant with natural ligands was log K _CdL about 9.5–10.5 (pH 8.6–8.8) with ligand concentrations of 1.2–7.8 × 10⁻⁶ mol/g C. In the softwater River Wyre, dissolved [Cd²⁺] ranged from 4 × 10⁻¹² to 1 × 10⁻¹¹ mol/l, and the ligands were weaker (log K _CdL 8.9–9.8, pH 8.0) with lower ligand concentrations (0.9–2.3 × 10⁻⁶ mol/g C). The titration curves of FA samples were close to the simulated curves by the WHAM VI model at pH 8.0–8.5, but deviated more from the model at lower pH, indicating that the results determined with CLE-SV for Cd-FA complexation are relevant to the data base in the model. Calculation of the Ca competition for Cd binding by FA showed a competition effect of similar strength as the measured one, but indicated a systematic difference between measured and modeled data at pH 7.5. Using the WHAM model for comparison with FA, the complexation of Cd by the River Wyre ligands was close to that of FA, whereas stronger complexation was observed in the Lake Greifen water. These differences may originate from different ligand composition in the lake and the river.     

Sorption on Humic Acids as a Basis for the Mechanism of Primary Accumulation of Gold and Platinum Group Elements in Black Shales

In order to asses the contribution of sorption by complexation to the concentration of noble metals at early stages of the formation of their deposits in black shales, the sorption of Au(III), Pt(IV), Rd(II), Rh(III), Ru(IV), and Os(IV) ions was studied on ash-free preparations of humic acids (HA) separated from peat of the Tver region and marine sediment samples taken on the Peruvian shelf. Data on the nature and protolytic characteristics of oxygen-containing HA functional groups were obtained. It has been shown that carboxyl groups and phenol oxygroups, which ensure the HA complexation with ions of noble metals, are present in the HA structure. The dissociation constant values for HA carboxyl groups (pK_a) and the distribution function of these groups in their pK_a values have been established. It has been revealed that the pK_a value for both of the HA groups varies within two orders of magnitude: the average value is equal to 6.1 for HA from peat and 7.0 for HA from marine sediments. A fairly high and similar for both of the HA groups sorption capacity with respect to Au(III), Rd(II), Rh(III), Ru(IV), and Os(IV) ions was established in model experiments. It is equal to 320–350 mg g^–1 for Au, 100–110 mg g^–1 for Pd, 11–12 mg g^–1 for Rh, 16–19 mg g^–1 for Ru, and 23 mg g^–1 for Os. The study of platinum(IV) sorption revealed that humic acids from peat and marine sediments do not virtually sorb Pt(IV), and this observation is important for understanding genetic features of the formation of noble metal deposits in black shales. Based on sorption isotherms for Au(III), Pd(II), Rh(III), and Ru(IV), the conditional affinity constant values for HA sorption centers with respect to ions of these metals were calculated by the method of quantitative physicochemical analysis. These values prove that complex compounds forming at the HA surface possess a high strength: the log values for the Au(III)–HA, Pd(II)–HA, Rh(III)–HA, and Ru(IV)–HA compounds are equal to 6.0, 5.0, 3.2, and 3.5, respectively.     

Ligand extraction of rare earth elements from aquifer sediments: Implications for rare earth element complexation with organic matter in natural waters

The ability of organic matter as well as carbonate ions to extract rare earth elements (REEs) from sandy sediments of a Coastal Plain aquifer was investigated for unpurified organic matter from different sources (i.e., Mississippi River natural organic matter, Aldrich humic acid, Nordic aquatic fulvic acid, Suwannee River fulvic acid, and Suwannee River natural organic matter) and for extraction solutions containing weak (i.e., CH₃COO⁻) or strong (i.e., ) ligands. The experimental results indicate that, in the absence of strong REE complexing ligands in solution, the amount of REEs released from the sand is small and the fractionation pattern of the released REEs appears to be controlled by the surface stability constants for REE sorption with Fe(III) oxides/oxyhydroxides. In the presence of strong solution complexing ligands, however, the amount and the fractionation pattern of the released REEs reflect the strength and variation of the stability constants of the dominant aqueous REE species across the REE series. The varying amount of REEs extracted by the different organic matter employed in the experiments indicates that organic matter from different sources has different complexing capacity for REEs. However, the fractionation pattern of REEs extracted by the various organic matter used in our experiments is remarkable consistent, being independent of the source and the concentration of organic matter used, as well as solution pH. Because natural aquifer sand and unpurified organic matter were used in our experiments, our experimental conditions are more broadly similar to natural systems than many previous laboratory experiments of REE-humic complexation that employed purified humic substances. Our results suggest that the REE loading effect on REE-humic complexation is negligible in natural waters as more abundant metal cations (e.g., Fe, Al) out-compete REEs for strong binding sites on organic matter. More specifically, our results indicate that REE complexation with organic matter in natural waters is dominated by REE binding to weak sites on dissolved organic matter, which subsequently leads to a middle REE (MREE: Sm-Ho)-enriched fractionation pattern. The experiments also indicate that carbonate ions may effectively compete with fulvic acid in binding with dissolved REEs, but cannot out compete humic acids for REEs. Therefore, in natural waters where low molecular weight (LMW) dissolved organic carbon (DOC) is the predominant form of DOC (e.g., lower Mississippi River water), REEs occur as “truly” dissolved species by complexing with carbonate ions as well as FA, resulting in heavy REE (HREE: Er-Lu)-enriched shale-normalized fractionation patterns. Whereas, in natural terrestrial waters where REE speciation is dominated by organic complexes with high molecular weight DOC (e.g., “colloidal” HA), only MREE-enriched fractionation patterns will be observed because the more abundant, weak sites preferentially complex MREEs relative to HREEs and light REEs (LREEs: La-Nd).     

1H NMR spectra of humic and fulvic acids and their peracetic oxidation products

¹H NMR spectra of humic (HA) and fulvic (FA) acids and their oxidative degradation products are reported. The HA shows the presence of -(CH₂)_n - CH₃ (n > 6) chemical fragments belonging to n-alkanes and/or n-fatty acids physically adsorbed onto the macromolecule structure. These fragments are absent in the FA fraction. Both humic fractions reveal the presence of similar amounts of aromatic protons which partly undergo exchange phenomena. The importance of this experimental observation is discussed. Oxidative degradation seems to cause partial cleavage of aromatic rings, more pronounced in the FA than in the HA. The degraded FA shows a higher total acidity and a higher phenolic OH content than the degraded HA. Both degraded fractions display some sharp singlet signals at 1.9 and 3.9 ppm arising from protons belonging to repetitive chemical fragments probably formed during the oxidation reaction. Tentative assignments of these signals are given. A general analysis of the HA and FA degraded spectra seems to indicate that the chemical fragments which undergo peracetic oxidation are substantially similar. The extent of oxidation of the two humic fractions is different. The HA degradation products reveal the presence of oligomeric structures, whereas the degraded FA appears less resistant to the oxidizing agent.     

Distribution of As,Cr, Ni,Rb, Ti and Zr between peat and its humic fraction along an undisturbed ombrotrophic bog profile (NW Switzerland)

Ombrotrophic bogs are useful records of the impact of historical human activity on heavy metal contamination. Several studies concerning the trace element record (mainly Pb and Hg) in these particular environments have been carried out in recent years, although the role of humic substances has often not been considered. In particular, of the components of peat organic matter, fulvic acids and low molecular weight compounds are generally responsible for the mobility of trace elements through the profile, while humic acids (HAs) are involved in the formation of more stable organo-mineral complexes. In order to study the parallel distribution of As, Cr, Ni, Rb, Ti and Zr in bulk peat and the corresponding HAs, a peat core (10 × 10 × 81 cm) was collected from Etang de la Gruère (Switzerland) and cut into 27 slices of 3 cm. The samples were freeze-dried and milled very finely, and HAs extracted from each sample. Both peat and HAs were analyzed using an energy-dispersive miniprobe X-ray fluorescence multielement analyser (EMMA-XRF). Of the considered elements, Ni showed a great affinity for the humic acid component, while Cr was concentrated mainly into humic material from the deeper layers. On the other hand, Ti, Zr and Rb seemed to reflect the variation in mineral material both in peat and HA samples, while the As content of both materials reflected the environmental conditions characterizing the bog.     

Phosphorus in marine and non-marine humic substances

The phosphorus content of marine humic acids (HA) is in the range of 0.1–0.2%. The C/P ratios of the HA are 300 to 400. Marine fulvic acids (FA) contain 0.4–0.8% P and have C/P ratios of 80 to 100. High molecular weight organic matter dissolved in pore waters (DOM) contains 0.5% P and has C/P of 90. The data suggest that during the formation sequence: Plankton → DOM → FA → HA → Kerogen, phosphorus is lost, mainly in the FA → HA (and possibly also in the HA → Kerogen) step. Diagenesis of sedimentary humic acids is accompanied by loss of phosphorus (as well as of nitrogen) to form HA with C/P ratios of 1000.Soil humic substances resemble marine humates in P content (0.3%) and soil FA's are about three to fivefold enriched in P relative to HA. C/P ratios are lower in soil HA (ca. 200) as compared with marine HA. Humic acids from diagenetic products such as peat and lignite are highly depleted in P. Rough calculations indicate that humate bound P may account for 20–50% of the organic phosphorus reservoir in sediments. The chemical speciation of this P is unknown, but lack of correlation with ash, Fe, Ca or Al content (in marine humates, at least) indicates that it is organically bound.     

Spectroscopic studies on soil organic fractions—II. IR and 1H-NMR spectra of methylated and unmethylated fulvic acids

The IR and ¹H-NMR spectra of fulvic acids (FA) are discussed. The FA, extracted by traditional methods, were fractionated on the basis of molecular weight (m.w.). Three fractions were obtained labelled FA I (m.w. > ～ 2000), FA II (m.w. > ～ 12000) and FA III (～2000 < m.w. < ～ 12000). Fraction FA II was methylated with CH₃I-Ag₂O. The ¹H-NMR spectra of unmethylated FA fractions show some signals common to all three fractions and some differences in the 3–5 ppm range due to the resonances of OCH₃ and O-CH₂ groups. The proton spectrum of the methylated fraction shows absorption areas for OCH₃ groups of phenols and carboxyls. The results confirm that NMR spectroscopy is a convenient technique which can contribute to defining the chemical structure of humic substances.     

Organic geochemistry and pore water chemistry of sediments from Mangrove Lake,Bermuda

Mangrove Lake, Bermuda, is a small coastal, brackish-water lake that has accumulated 14 m of banded, gelatinous, sapropelic sediments in less than 10⁴ yr. Stratigraphic evidence indicates that Mangrove Lake's sedimentary environment has undergone three major depositional changes (peat, freshwater gel, brackish-water gel) as a result of sea level changes. The deposits were examined geochemically in an effort to delineate sedimentological and diagenetic changes. Gas and pore water studies include measurements of sulfides, ammonia, methane, nitrogen gas, calcium, magnesium, chloride, alkalinity, and pH. Results indicate that sulfate reduction is complete, and some evidence is presented for bacterial denitrification and metal sulfide precipitation. The organic-rich sapropel is predominantly algal in origin, composed mostly of carbohydrates and insoluble macromolecular organic matter called humin with minor amounts of proteins, lipids, and humic acids. Carbohydrates and proteins undergo hydrolysis with depth in the marine sapropel but tend to be preserved in the freshwater sapropel. The humin, which has a predominantly aliphatic structure, increases linearly with depth and composes the greatest fraction of the organic matter. Humic acids are minor components and are more like polysaccharides than typical marine humic acids. Fatty acid distributions reveal that the lipids are of an algal and/or terrestrial plant source. Normal alkanes with a total concentration of 75 ppm exhibit two distribution maxima. One is centered about n-C₂₂ with no odd/even predominance, suggestive of a degraded algal source. The other is centered at n-C₃₁ with a distinct odd/even predominance indicative of a vascular plant origin. Stratigraphic changes in the sediment correlate to observed changes in the gas and pore water chemistry and the organic geochemistry.     

Rapid quantification of humic and fulvic acids by HPLC in natural waters

A simple method based on high-performance size-exclusion chromatography (HPSEC) has been developed for rapid quantification of humic and fulvic acids (HA and FA) in stream waters. A Tsk-gel column was used to separate natural dissolved organic matter (DOM) into two components: peak A and B. In terms of HPSEC chromatograms and fluorescence patterns, peak A and B were similar to the corresponding XAD-extracted HA and FA, respectively. It is suggested that peak A fraction mainly consisted of HA, and peak B fraction FA. The similar separation of HA and FA using HPSEC and a conventional XAD method suggests the consistency of molecular size distribution and physical–chemical properties of DOM. HPSEC offers a simple and rapid method for the quantification of HA and FA instead of tedious extractions of humic substances. Analyses of natural water samples show that the calculation of HA/FA based on UV absorbance was under- or over-estimated, the calibration using the extracted HS allows a more accurate quantification. The fast screening of HA and FA provides useful quantitative and qualitative information that can be used in environmental or monitoring studies.     

Sorption of Ni on Na-rectorite studied by batch and spectroscopy methods

Sorption of Ni²⁺ on Na-rectorite as a function of contact time, temperature, pH and fulvic acid (FA)/humic acid (HA) was studied under ambient conditions. A pseudo-second-order rate equation was used to simulate the kinetic sorption. The removal of Ni²⁺ increased with increasing pH. The presence of FA/HA enhanced the sorption of Ni²⁺ at low pH values, whereas no drastic effect of FA/HA on Ni²⁺ uptake to rectorite was found at high pH values. The diffuse layer model (DLM) fitted the experimental data of Ni²⁺ sorption in the absence and presence of FA/HA very well with the aid of FITEQL 3.2. The Langmuir, Freundlich and Dubinin–Radushkevich (D–R) models were used to simulate the sorption isotherms of Ni²⁺ at different temperatures. The thermodynamic data (ΔH⁰, ΔS⁰, ΔG⁰) were calculated from the temperature dependent sorption isotherms and the results suggested that the sorption process of Ni²⁺ on rectorite was spontaneous and endothermic. The sorption and species of Ni²⁺ on rectorite in the presence and absence of FA/HA was also investigated and characterized by XPS. The spectroscopic analysis indicated no drastic structural changes of Na-rectorite and the sorption of Ni²⁺ mainly occurred on the surface and at the edge position of Na-rectorite.     

[1H] and [13C]NMR studies on the importance of aromatic structures in fulvic and humic acids

To obtain information on the contribution of aromatic fragments to the chemical structure of humic substances, we carried out a study on the [¹H]NMR and [¹³C]NMR spectra of humic and fulvic acids and their oxidative degradation products extracted from an Andosol soil.[¹H]NMR spectra of all organic fractions present considerable adsorption between 7.4 and 8.8 ppm, due to the presence of aromatic protons.The percentages of aromatic protons in respect to the total amount of protons are as follows: FA 20%, HA 19%, degraded FA < 12%, degraded HA 14%. The values indicate that the contribution of aromatic structures to the humic substances is significant, also considering that they are highly substituted.The degraded fractions contain smaller amounts of aromatic protons, because degradation causes the opening of the aromatic rings. Thus results obtained from the degradation do not seem to be reliable for defining the importance of aromatic structures in humic substances.Also the [¹³C]NMR spectra show signals in the aromatic region which derive from unsubstituted carbon atoms, while signals originating from tertiary carbon atoms merge with the noise. We believe that, at present, [¹H]NMR spectroscopy is more suited for studying the role played by aromatic compounds in organic soil fractions.     

Vanadium-fulvic acid chemistry: conformational and binding studies by electron spin probe techniques

Two fractions of soil fulvic acid (FA) were separated by gel filtration chromatography. An observed increase in volume of the heavier fraction (FA I) with increasing pH was attributed to aggregation, intramolecular negative charge repulsions and the rupture of hydrogen bonds, which control molecular conformation. Optical absorption properties and elemental analyses of both fractions were determined. The stability constants and stoichiometries of FA complexes with vanadyl, VO²⁺, at pH 5.0 and ionic strength of 0.04 M were measured by electron paramagnetic resonance (EPR) spectroscopy. EPR spectra of model VO²⁺ complexes with phthalic and salicylic acids, which are the probable functional groups present in FA, are identical to those of the VO²⁺-FA complexes. Aggregation of FA I occurs in the presence of VO²⁺ to form a complex that can be approximated as ‘(VO)₂(FA I)₆’. The average site distance between vanadyl ions in this complex is shown to be greater than 1.2 nm. EPR parameters for FA I suggest binding by carboxylate groups. These parameters are compared with those of other vanadyl complexes with fulvic and humic acids reported by others. Reduction of VO₃^? to VO²⁺ by these materials is discussed.     

Metal distribution and nature of some Cu,Mn and V complexes in humic and fulvic acid fractions of soil organic matter

Organic matter from an arable soil derived from base rich parent material was extracted by alkali and fractionated on the basis of solubility in 0.1 N HCl, hot water and hot 6 N HCl and by selective adsorption on charcoal. The distribution of associated metals was determined and Cu had the largest proportion, 15%, associated with the organic matter. Moderate proportions of the total Al, Co, Ni, and V (3–8%) but only small amounts (?1%) of the Mn, Fe, Ti, Cr, Ba and Sr were extracted from the soil by alkali. The Fe and Ti were concentrated mainly in the humic fraction whereas Mn and V were both found largely in the fulvic acid.Electron paramagnetic resonance spectra of the various fractions were examined and attempts made to relate the spectra to the forms of some of the metals present. In the humic acid fraction Cu was present partly as a copper porphyrin-type complex but in the fulvic acid it was in some other complexed form. VO²⁺ occurred in complexed forms in the fulvic acid which were more covalent than VO²⁺ humic acid complexes, whereas the Mn²⁺ components of the humic and fulvic acids all had a high degree of ionicity.     

Relationships between heavy metals and iron oxides,fulvic acids,particle size fractions in urban roadside soils

Urban roadside soils are the “recipients” of large amounts of heavy metals from a variety of sources including vehicle emissions, coal burning waste and other activities. The behavior of heavy metals in urban roadside soils depends on the occurrence as well as the total amount. Accordingly, knowledge of the interactions between heavy metals and other constituents in the soil is required to judge their environmental impact. In this study, correlations of heavy metal concentrations (Pb, Zn, Cu, Ag, Se, Ni, Cr and Ba) to iron extracted using dithionite–citrate–bicarbonate (DCB) buffer (Fe_DCB), fulvic acids and particle size fractions were examined from the Xuzhou urban roadside soils. Heavy metals except for Cr and fulvic acids had a positive significant correlation with Fe_DCB, indicating these metals and fulvic acids are principally associated with the surfaces of iron oxides of the soils. Significant positive correlations were also found between the contents of fulvic acids and heavy metals, showing these heavy metals (especially for Cu, Ni and Cr) form stable complexes with fulvic acids. Such finding is of importance with regard to the increased mobilization of heavy metals, e.g., into freshwater ecosystems. Ag, Se and Cr are independent of particle size fractions because of their low concentrations of Ag and Se in the studied soils. Pb, Zn, Cu, Ba and Ag are mainly enriched in the finer soil particles (especially <16 μm).     

Bioaccumulation of arsenic,chromium and lead in fish: constraints imposed by sediment geochemistry

Determining metal bioavailability is critical in assessing the necessity to remediate contaminated sediments. In the Halls Brook Holding Area Pond (HBHAP) sapropel, As (3000 mg/kg), and Cr (1400 mg/ kg), are sequestered by amorphous Fe(OH)₃ (Kd_As=560; Kf_Cr=59,0001/kg), while Pb solubility is limited by PbS_(am). Fillet As concentrations in detritivorous and omnivorous fish were similar in the HBHAP (1.19 mg/kg), and the adjacent unimpacted Phillips Pond (1.18 mg/kg). Cr and Pb in both HBHAP and Phillips Pond fish were below analytical detection limits, except for one (0.73 mg/kj Pb), in the HBHAP. The low sediment bioaccumulation factors for As, Cr, and Pb (6.5 × 10⁻⁴, < 1.1 × 10, and 1.8 × 10⁻⁶, respectively in HBHAP) suggest that the sediment acts to sequester metals, rendering them non-bioavailable due to precipitation of solids, and sorption to iron phases.     

Structural environments of carboxyl groups in natural organic molecules from terrestrial systems. Part 2: 2D NMR spectroscopy

Carboxyl groups are abundant in natural organic molecules (NOM) and play a major role in their reactivity. The structural environments of carboxyl groups in IHSS soil and river humic samples were investigated using 2D NMR (heteronuclear and homonuclear correlation) spectroscopy. Based on the ¹H-¹³C heteronuclear multiple-bond correlation (HMBC) spectroscopy results, the carboxyl environments in NOM were categorized as Type I (unsubstituted and alkyl-substituted aliphatic/alicyclic), Type II (functionalized carbon substituted), Type IIIa, b (heteroatom and olefin substituted), and Type IVa, b (5-membered heterocyclic aromatic and 6-membered aromatic). The most intense signal in the HMBC spectra comes from the Type I carboxyl groups, including the ²J_CH and ³J_CH couplings of unsubstituted aliphatic and alicyclic acids, though this spectral region also includes the ³J_CH couplings of Type II and III structures. Type II and III carboxyls have small but detectable ²J_CH correlations in all NOM samples except for the Suwannee River humic acid. Signals from carboxyls bonded to 5-membered aromatic heterocyclic fragments (Type IVa) are observed in the soil HA and Suwannee River FA, while correlations to 6-membered aromatics (Type IVb) are only observed in Suwannee River HA. In general, aromatic carboxylic acids may be present at concentrations lower than previously imagined in these samples. Vibrational spectroscopy results for these NOM samples, described in an accompanying paper [Hay M. B. and Myneni S. C. B. (2007) Structural environments of carboxyl groups in natural organic molecules from terrestrial systems. Part 1: Infrared spectroscopy. Geochim. Cosmochim. Acta (in press)], suggest that Type II and Type III carboxylic acids with α substituents (e.g., -OH, -OR, or -CO₂H) constitute the majority of carboxyl structures in all humic substances examined. Furoic and salicylic acid structures (Type IV) are also feasible fragments, albeit as minor constituents. The vibrational spectroscopy results also suggest that much of the “Type I” signal observed in the HMBC spectrum is due to carboxylic acid esters and possibly α-substituted alicyclic acids.