Chemical composition of aquatic dissolved organic matter in five boreal forest catchments sampled in spring and fall seasons

The chemical composition and carbon isotope signature of aquatic dissolved organic matter (DOM) in five boreal forest catchments in Scandinavia were investigated. The DOM was isolated during spring and fall seasons using a reverse osmosis technique. The DOM samples were analyzed by elemental analysis, FT-IR, solid-state CP-MAS 13C-NMR, and C-1s NEXAFS spectroscopy. In addition, the relative abundance of carbon isotopes (12C, 13C, 14C) in the samples was measured. There were no significant differences in the chemical composition or carbon isotope signature of the DOM sampled in spring and fall seasons. Also, differences in DOM composition between the five catchments were minor. Compared to reference peat fulvic and humic acids, all DOM samples were richer in O-alkyl carbon and contained less aromatic and phenolic carbon, as shown by FT-IR, 13C-NMR, and C-1s NEXAFS spectroscopy. The DOM was clearly enriched in 14C relative to the NBS oxalic acid standard of 1950, indicating that the aquatic DOM contained considerable amounts of organic carbon younger than about 50years. The weight-based C:N ratios of 31 ± 6 and the $$\delta^{13}\hbox{C}$$ values of -29\pm2\permille indicate that the isolated DOM is of terrestrial rather than aquatic origin. We conclude that young, hydrophilic carbon compounds of terrestrial origin are predominant in the samples investigated, and that the composition of the aquatic DOM in the studied boreal forest catchments is rather stable during low to intermediate flow condition

Interactions of ferrous iron with clay mineral surfaces during sorption and subsequent oxidation

In submerged soils and sediments, clay minerals are often exposed to anoxic waters containing ferrous iron (Fe2+). Here, we investigated the sorption of Fe2+ onto a synthetic montmorillonite (Syn-1) low in structural Fe (<0.05 mmol Fe per kg) under anoxic conditions and the effects of subsequent oxidation. Samples were prepared at two Fe-loadings (0.05 and 0.5 mol Fe added per kg clay) and equilibrated for 1 and 30 days under anoxic conditions (O2 < 0.1 ppm), followed by exposure to ambient air. Iron solid-phase speciation and mineral identity was analysed by 57Fe Mössbauer spectroscopy and synchrotron X-ray absorption spectroscopy (XAS). Mössbauer analyses showed that Fe(ii) was partially oxidized (14-100% of total added Fe2+) upon sorption to Syn-1 under anoxic conditions. XAS results revealed that the added Fe2+ mainly formed precipitates (layered Fe minerals, Fe(iii)-bearing clay minerals, ferrihydrite, and lepidocrocite) in different quantities depending on the Fe-loading. Exposing the suspensions to ambient air resulted in rapid and complete oxidation of sorbed Fe(ii) and the formation of Fe(iii)-phases (Fe(iii)-bearing clay minerals, ferrihydrite, and lepidocrocite), demonstrating that the clay minerals were unable to protect ferrous Fe from oxidation, even when equilibrated 30 days under anoxic conditions prior to oxidation. Our findings clarify the role of clay minerals in the formation and stability of Fe-bearing solid phases during redox cycles in periodically anoxic environments.ISSN:2050-7887ISSN:2050-789

Isolation and purification of Cu-free methanobactin from Methylosinus trichosporium OB3b

<p>Abstract</p> <p>Background</p> <p>The isolation of highly pure copper-free methanobactin is a prerequisite for the investigation of the biogeochemical functions of this chalkophore molecule produced by methane oxidizing bacteria. Here, we report a purification method for methanobactin from <it>Methylosinus trichosporium </it>OB3b cultures based on reversed-phase HPLC fractionation used in combination with a previously reported resin extraction. HPLC eluent fractions of the resin extracted product were collected and characterized with UV-vis, FT-IR, and C-1s NEXAFS spectroscopy, as well as with elemental analysis and ESI-MS.</p> <p>Results</p> <p>The results showed that numerous compounds other than methanobactin were present in the isolate obtained with resin extraction. Molar C/N ratios, mass spectrometry measurements, and UV-vis spectra indicated that methanobactin was only present in one of the HPLC fractions. On a mass basis, methanobactin carbon contributed only 32% to the total organic carbon isolated with resin extraction. Our spectroscopic results implied that besides methanobactin, the organic compounds in the resin extract comprised breakdown products of methanobactin as well as polysaccharide-like substances.</p> <p>Conclusion</p> <p>Our results demonstrate that a purification step is indispensable in addition to resin extraction in order to obtain pure methanobactin. The proposed HPLC purification procedure is suitable for semi-preparative work and provides copper-free methanobactin.</p

Relating Ion Binding by Fulvic and Humic Acids to Chemical Composition and Molecular Size. 1. Proton Binding

Proton binding by a soil fulvic acid, humic acid, and a set of size fractions of the humic acid was studied as a function of pH and ionic strength by potentiometric titrations. The negative charge of the humic substances resulting from deprotonation of acidic functional groups generally increased with increasing pH and increasing ionic strength. At any given pH and ionic strength, the fulvic acid fraction exhibited much higher negative charge than the humic acid fraction. For the size-fractionated humic acids, negative charge decreased steadily with increasing apparent molecular weight, as determined by size exclusion chromatography. Observed differences in proton binding by the various humic substances corresponded well to differences in functional group composition, which has been extensively characterized in a previous study using a combination of analytical techniques. The proton binding behavior of the humic substances was described very well by the consistent NICA-Donnan model. However, when all adjustable model parameters were determined using a least-squares minimization technique without introducing parameter constraints, the values of some parameters turned out physically and chemically unreasonable. Therefore, we propose to derive some model parameters from chemical characterization results obtained by size exclusion chromatography and solid-state 13C NMR spectroscopy. Using this approach, we obtained excellent descriptions of all titration data, and the model parameter values were more consistent and chemically reasonable. Our results demonstrate that characterization results of humic substances can be used in NICA-Donnan modeling to reduce the number of free fitting parameters without arbitrary constraints and, thereby, obtaining a more reliable database for environmental modeling

The Effect of Aeration on Mn(II) Sorbed to Clay Minerals and Its Impact on Cd Retention

Manganese is a redox-sensitive element in soils and sediments that plays an important role in the retention of trace elements. Under anoxic conditions, clay minerals were shown to increase Cd retention by favoring the precipitation of Mn(II) phases. In this study, we investigated the influence of aeration on anoxically formed Mn solid phases and its impact on Cd retention in the presence of two clay minerals with low Fe contents, a natural kaolinite (KGa-1b) and a synthetic montmorillonite (Syn-1). Ca-saturated KGa-1b and Syn-1 were pre-equilibrated with Mn2+ and Cd2+ under anoxic conditions for 1 or 30 days and subsequently exposed to air for 1 or 30 days. The analysis with synchrotron X-ray absorption spectroscopy (XAS) revealed that extended anoxic pre-equilibration (30 days) partially prevented the oxidation of sorbed Mn(II) (MnSiO3 and Mn(II)Al-LDH). Extended exposure to ambient air and short anoxic pre-equilibration favored the formation of feitknechtite (β-MnOOH) and birnessite (Î-MnO2). Aeration resulted in a decrease of pH and a net release of Cd2+ into the solution, indicating that Cd re-sorption by Mn(III/IV)-phases was insufficient to compensate for the release of Cd2+ due to dissolution of Mn(II)-containing phases and the decrease in pH. Our results demonstrate the significance of clay minerals in the (trans)formation of Mn-containing phases and their impact on trace metal retention in environments undergoing fluctuating redox conditions. ©ISSN:0013-936XISSN:1520-585

Effects of natural organic matter (NOM), metal-to-sulfide ratio and Mn2+ on cadmium sulfide nanoparticle growth and colloidal stability

Redox-dynamic environments such as river floodplains and wetlands have been identified as sources of natural metal sulfide nanoparticles (MS NPs). However, little information is available on how their growth and colloidal stability are affected by the concentrations of metals and sulfide in solution, the presence of natural organic matter (NOM), and the possible incorporation of other metal cations such as Fe2+ or Mn2+. Here, we performed experiments on the formation of CdS nanoparticles (CdS NPs) in anoxic solutions with varying Cd (50, 100, 500 μmol L−1) and sulfide (50, 100, 1000 μmol L−1) concentrations in the absence and presence of Suwannee River fulvic acid (SRFA, 0, 5, 50 mg C per L). Additionally, we studied the influence of different metal-to-sulfide ratios and varying Mn2+ (0, 0.1, 0.5, 1 mmol L−1) concentrations on CdS aggregation using dynamic light scattering (DLS), transmission electron microscopy (TEM), and electrophoretic mobility measurements. The incorporation of Mn into the crystal lattice of CdS over 8 weeks was investigated with X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). Our results show that after 24 hours, small CdS primary particles with median diameters of a few nanometers ([d with combining tilde]TEM = 2–14 nm) formed large aggregates ([d with combining tilde]TEM = 167 nm) and that increasing SRFA concentrations progressively constrained the size of these aggregates (down to 19 nm) irrespective of the initial reactant stoichiometry. When NOM was absent or at low concentration, higher metal-to-sulfide ratios (≥1) and Mn2+ concentrations (≥0.5 mmol L−1) led to reduced colloidal stability of the suspensions. We found that in suspensions containing Mn2+, 10–30% of the Cd atoms in the crystal lattice were substituted by Mn during the formation of CdS, which was prevented by NOM.ISSN:2051-8153ISSN:2051-816

Arsenic contamination of the Ogosta River floodplain and its environmental impacts

ISSN:1029-7006ISSN:1607-796