B. van Lagen

Amount and composition of clay-associated soil organic matter in a range of kaolinitic and smectitic soils

Abstract In the global carbon cycle, soil organic matter (SOM) is a major source/sink of atmospheric carbon. Clay minerals stabilize part of the SOM through mineral–organic matter binding. Stabilization of organic matter is essential for tropical soils. Since the climatic conditions of the tropics favor decomposition of organic matter, tropical soils would be very poor in organic matter without this stabilization process. This research aims at determining the effect of clay mineralogy on the amount and composition of organic matter that is bound to the mineral surface. We focused on organic matter that is associated with kaolinite and smectite. We characterized kaolinite- and smectite-associated SOM in soils from seven countries, employing 13C NMR spectroscopy and Py-GC/MS. The content of carbon in the total clay-size fraction showed no significant difference between kaolinitic and smectitic soils. This suggests that the total amount of organic carbon in the clay-size fraction is independent of the clay mineralogy. We first extracted the clay fraction with NaOH and thereafter with Na4P2O7. About half of the kaolinite-associated SOM was extractable by NaOH. In the smectitic soils, pyrophosphate extracted more organic carbon than did NaOH. The Py-GC/MS and NMR results indicate that kaolinite-associated SOM is enriched in polysaccharide products, while smectite-associated organic matter contains many aromatic compounds. We suggest that different clay minerals use different binding mechanisms to complex SOM. As a result, the composition of clay-associated organic matter would be influenced by the type of clay that is dominantly present in the soil.

Alkyl-Functionalized Oxide-Free Silicon Nanoparticles: Synthesis and Optical Properties†

Highly monodisperse silicon nanoparticles (1.57 +/- 0.21 nm) are synthesized with a covalently attached alkyl monolayer on a gram scale. Infrared spectroscopy shows that these silicon nanoparticles contain only a few oxygen atoms per nanoparticle. XPS spectra clearly show the presence of unoxidized Si and attached alkyl chains. Owing to the relatively efficient synthesis (yields approximately 100-fold higher than of those previously reported) the molar extinction coefficient epsilon can be measured: epsilon(max) = 1.7 x 10(-4) M(-1)cm(-1), only a factor of 4 lower than that of CdS and CdSe nanoparticles of that size. The quantum yield of emission ranges from 0.12 (C(10)H(21)-capping) to 0.23 (C(16)H(33)-capping). UV/Vis absorption and emission spectroscopy show clear vibrational progressions (974 +/- 14 cm(-1); up to five vibrational bands visible at room temperature), resembling bulk SiC phonons, which support the monodispersity observed by TEM. This was also confirmed by time-resolved fluorescence anisotropy measurements, which display a strictly monoexponential decay that can only be indicative of monodisperse, ball-shaped nanoparticles.

Quantitative aspects of solid-state 13C-NMR spectra of humic substances from soils of volcanic systems

Copyright (c) 1997 Elsevier Science B.V. All rights reserved. Cross-Polarisation Magic Angle Spinning Carbon-13 Nuclear Magnetic Resonance spectroscopy (CPMAS 13 C-NMR) represents one of the most powerful tools to investigate soil organic matter (SOM) mainly because of its inherent capacity to provide a semi-quantitative evaluation of carbon distribution. A critical parameter during acquisition of CPMAS 13 C-NMR spectra is the contact time required to obtain the cross-polarisation between proton and carbon nuclei. The procedure to evaluate the best contact time for the acquisition of a quantitative CPMAS 13 C-NMR spectrum is to perform Variable Contact Time (VCT) experiments. In this work the structural features of a number of purified humic substances from Italian and Costarican volcanic soils were investigated by CPMAS 13 C-NMR spectroscopy after having performed preliminary VCT experiments. The VCT experiments showed that the average contact times vary according to the origin and chemical structure of the humic material. The optimal contact times (OCT) for nine humic samples were between 250 and 800 μs. These values were different from the time of 1000 μs that is commonly applied as the best average contact time for humic materials. Moreover, by comparing the NMR data to those obtained by elemental analysis (C/H ratio), it appeared that the efficiency of the cross-polarisation between protons and carbons, and hence the contact time, is affected not only by the number of protons, but also by their distribution over the molecules. The evaluation of errors in quantitative estimation of the different carbons revealed that the use of OCT generally reduced by half the loss of signals occurring when the average contact time of 1000 μs is used in CPMAS 13 C-NMR spectra of humic substances.

Quantitative differences in evaluating soil humic substances by liquid- and solid-state 13C-NMR spectroscopy

Abstract We compared the quantitative responses of liquid-state (LS) and solid-state (CPMAS) 13 C-NMR spectroscopy of four different soil humic substances. The intensities of signals for the alkyl carbons (0–40 ppm) were significantly larger in CPMAS than in LS spectra. This difference is in agreement with the pseudo-micellar model of the conformational nature of humic substances. By this view, the hydrophobic interactions holding together the heterogeneous molecules of humic micelles inhibit the molecular motions of the alkyl carbons, thereby enhancing the spin-lattice relaxation times and consequently lowering the sensitivity of liquid-state NMR. Conversely, regardless of their position in the humic conformation, a better estimation of the number of alkyl carbons can be obtained by CPMAS-NMR because of the cross-polarization of hydrogen nuclei in CH 2 and CH 3 groups. The intensity of the 40–110 ppm region is also slightly lower in LS than in CPMAS-NMR spectra, despite the hydrophilicity of the oxidized and peptidic carbons resonating in this chemical shift interval. Their molecular motion may also be reduced by either the formation of intra- and inter-molecular hydrogen bondings due to poorly acidic hydroxyl groups of saccharides, or the degree of conformational rigidity that a pseudo-micellar arrangement confers even to hydrophilic domains. The higher content of aromatic carbons (110–160 ppm) found in the LS spectra was attributed partly to the high degree of substitution of the aromatic ring that slows down cross-polarization in CPMAS experiments and partly to the relative overestimation of this region by LS-NMR due to a lack of signal in the aliphatic interval. The slightly lower content of carboxyl carbons estimated in CPMAS spectra as compared to LS spectra was also attributed to slow cross-polarization. This work shows that the combined use of both NMR techniques is profitable in conformational analysis of humic substances and of dissolved organic matter in general.

Increase in stability against thermal oxidation of soil humic substances as a result of self association

The stability to thermal oxidation of soil humic extracts saturated with H, Na, Ca, or Al, was followed after treatment with relatively polar organic compounds, such as methanol, formic acid, and acetic acid. While thermal characteristics of H-humates did not change upon addition of the polar molecules, thermal decomposition (oxidation) of Na-humates was shifted to much higher temperatures (750–830 � C) than control. Substantially less dramatic was the effect on Ca-humates, whereas hardly any alteration was observed when polar organic compounds were added to Alhumates. These results can be explained by considering the forces that hold humic molecules together. Humic components are strongly bound to each other by hydrogen bonding in H-humates and by electrostatic bridges in Ca- and Alhumates. These binding forces were not overcome by the simple addition of polar organic molecules, and their stability remained generally unchanged. In Na-humates, associations of humic molecules are held together only by non-specific hydrophobic interactions. Our results showed that wetting the relatively more flexible Na-humates with organic solvents slightly less polar than water caused a significant increase in thermal stability. Because most polysaccharide-C has largely disappeared at 400 � C, this thermal behaviour can be explained by the rearrangement of largely hydrophobic humic components in methanol, leading to an increase in association energy. The intensity and reversibility of thermal stabilization indicate that association occurred among relatively small molecules rather than among macromolecules. Also Na-saturated humus of bulk samples showed an increase in stability against oxidation upon addition of methanol. These findings suggest that, counter-ions and amphiphilic organic compounds may affect organic matter stability also in natural soils. # 2002 Elsevier Science Ltd. All rights reserved.