Aquatic Humic Solutes Research Articles

NMR Spectroscopy Study of Freshwater Humic Material in Light of Supramolecular Assembly

The structural similarity-dissimilarity of several humic-type derivatives, separated from a strongly colored freshwater sample by different sorbing solid techniques, tangential-flow ultrafiltration (UF), and large-scale preparative high-performance size-exclusion chromatography (HPSEC), were in detail studied with one-dimensional liquid 1H and solid-state 13C nuclear magnetic resonance (NMR) spectroscopy, especially in light of the native humic-type dissolved organic matter (DOM-HM). The results support the applicability of functional cross-linked poly(vinylpyrrolidone) (PVP) or diethylaminoethyl-cellulose (DEAE) sorbents in concentrating representative integrated wholes of aquatic humic-type material along with a conventional nonionic XAD-8/DAX-8 (polymethyl methacrylates) technique. Apart from the fact that the acidification of the original humic water before a separation procedure seems not to be so destructive to the original structural composition of the DOM-HM as expected, the refinement of aquatic humic solutes, independent of the selected sorbing solid technique, will cause structural changes in the separated humic complexes in comparison with the situation predominating in the original starting material. Tangential-flow ultrafiltration (UF) proved an overpowering reliability to concentrate the aquatic DOM-HM. Most fundamental is the combined outcome of different HPSEC experiments and determined structural functionalities which indicate that almost all original DOM-HM solutes are aggregated mixtures consisting of structurally similar associations possessing various molecular size ranges, which can be separated from their integrated whole as nearly homogeneous and uniform species. This finding permits a reasonable starting point to go on working with more advanced multidimensional NMR techniques in resolving the uncertainty about supramolecular assembly of dissolved humic material. The tested conformity between the obtained molecular NMR descriptors and the corresponding previously collected FT-IR parameters was acceptable thus speaking for the fact that the less sensitive FT-IR spectroscopy can also provide valuable information on the structural and functional properties of heterogeneous humic-type mixtures.

Comparative study for separation of aquatic humic-type organic constituents by DAX-8, PVP and DEAE sorbing solids and tangential ultrafiltration: elemental composition, size-exclusion chromatography, UV–vis and FT-IR

Aquatic humic-type solutes were separated in parallel from the same fresh water source by four different procedures: non-ionic polymethyl methacrylate (DAX-8) and functional cross-linked polyvinylpyrrolidone (PVP) resins, functional diethylaminoethyl cellulose (DEAE) and tangential ultrafiltration completed with a weakly basic anion exchange resin (IRA-67). The similarity–dissimilarity between the quantities and qualities of the different humic samples is discussed, especially in the light of the original dissolved organic matter (DOM). During the past two decades, a significant progress has occurred in the aquatic humic research due to the so-called hydrophobic–hydrophilic properties possessed by certain non-ionic sorbing solids. As a result of many coincidences, it may be justifiable to examine critically the prevailing isolation techniques of aquatic humic solutes and to try to update their complicated definitions. For that reason, it is reasonable to summarize the leading principles of different isolation techniques in Section 1 of this article. The results of the present study strongly support the applicability of the PVP resin, alone or completed in sequence with a suitable non-ionic sorbing solid, for isolation of aquatic humic-type solutes from both quantitative and qualitative points of view. In certain cases, the DEAE cellulose gives a useful alternative for conventional sorbing solids in the isolation of the bulk of aquatic humic solutes. The base-catalyzed ester hydrolysis of the HM during the chromatographic isolation of the DOM seems to be relative minor.

Sorption of aquatic humic matter by DAX-8 and XAD-8 resins: Comparative study using pyrolysis gas chromatography

Aquatic humic solutes were separated in parallel by the non-ionic macroporous DAX-8 and XAD-8 resins from four different fresh water sources. The sorptive power of the DAX-8 resin was systematically somewhat greater compared to that of the XAD-8 resin. The DAX-8 resin seems to have more precise column characteristics compared with the XAD-8 resin. According to the tetramethylammonium hydroxide (TMAH) procedure followed up with pyrolysis gas chromatography–mass spectrometry (Py-GC–MS) the content and quality of aliphatic carbons were the most systematic and powerful discriminating factors between the humic extracts obtained by these two resins. Generally speaking, the DAX-8 and XAD-8 resins seem to isolate humic solute bulks almost equally, although the content of aliphatics is slightly greater for the former, producing mixtures with similar structural compositions for general purposes. The environmental impact was also visible on the quality of the structural fine chemistry of the different humic isolates obtained both by the DAX-8 and XAD-8 resins.

Characterization, differentiation and classification of aquatic humic matter separated with different sorbents: synchronous scanning fluorescence spectroscopy

Aquatic humic solutes were separated by the non-ionic macroporous XAD-8 and DAX-8 resins and a weakly basic DEAE cellulose anion exchanger from seven different fresh water sources. Synchronous fluorescence spectroscopy was applied for characterization, differentiation and classification of the different humic-solute aggregates. Fluorescence properties verified that humic-solute fractions isolated parallelly with the non-ionic XAD-8 and DAX-8 resins resembled very closely each other speaking strongly for their structural similarities. DAX-8 resin separated ca. 19% more aquatic humic matter than did the analogous XAD-8 resin. It was possible to tentatively differentiate the untreated water samples according to their fluorescent materials. Several distinct classes of chromophores were detected in both DOM and isolated humic fractions by the synchronous technique: λ ex/ λ em 280/298, 330/348, 355/373, 400/418, 427/445, 460/478, 492/510 and 516/534 nm.

Comparisons of sorption of aquatic humic matter by DAX-8 and XAD-8 resins from solid-state 13C NMR spectroscopy's point of view

Aquatic humic solutes were separated in parallel by the non-ionic macroporous DAX-8 and XAD-8 resins from four different fresh water sources. On average, the sorptive power of the DAX-8 resin does not differ systematically from that of the XAD-8 resin. The DAX-8 resin seems to have more precise column characteristics compared with the XAD-8 resin. There was no significant difference between the major elemental compositions of the parallel humic-solute bulks obtained by these two resins. According to the 13C NMR spectroscopy the content and quality of aliphatic carbons, especially those representing terminal methyl groups or methylene carbons, were the most systematic and powerful discriminating factors between the humic extracts obtained by these two resins. Generally speaking the DAX-8 and XAD-8 resins seem to isolate humic-solute bulks almost equally, although the content of aliphatics is slightly greater for the former, producing mixtures with similar structural compositions for general purposes. The structural composition and quantity of the humic-solute mixture isolable with a weakly basic DEAE-cellulose anion exchange resin differs partially from any humic fraction obtained by non-ionic sorbing solids. The environmental impact was also visible on the quality of the structural fine-chemistry of the different humic isolates obtained both by the DAX-8 and XAD-8 resins.

The Effects of a Strong Disaggregating Agent on Sec-Page of Aquatic and Soil Humic Matter

Size-exclusion chromatographic (SEC) fractionation and electrophoretic separation of aquatic humic matter samples from a Finnish lake using Sephadex G-75 with 7 M urea solution as eluent and 10% polyacrylamide gel (PAGE) with urea and sodium dodecyl sulphate solution (SDS), respectively, were performed and compared to similar analyses performed on a Russian chernozem soil humic acid sample and Nordic reference fulvic and humic acid samples. The integrated whole of aquatic humic solutes and soil humic acids were found to exhibit similar SEC-PAGE behaviours. Humic matter was not excessively disaggregated by the 7 M urea and hence SEC-PAGE can with confidence be applied as a coarse, initial fractionation procedure or for certain predeterminations of the structural composition.

Binding of pyrene on lake aquatic humic matter: the role of structural descriptors

Partition coefficients for the binding affinities of pyrene to 28 different humic materials were determined by the solubility enhancement method. The humic materials used in the present study were isolated by different methods from different environmental sources, and characterized by various methods including spectroscopic and pyrolysis (Py) approaches. Several structural descriptors were derived for testing the binding affinity of pyrene. A significant correlation was observed, supporting the earlier literature results, between the pyrene binding coefficient and the molecular weights, molar absorptivities at 280 nm, and aromaticity of the aquatic humic solutes. Also other structural descriptors, e.g. nitrogen and especially aliphatic moieties, have their own importance to the binding affinity of pyrene. The results confirm the fact that it is not meaningful to treat aquatic and terrestrial humic material as an integrated whole. The results also disclosed that the isolation and fractionation procedures of aquatic humic probes will strengthen, as a result of breaking hydrophobic bondings of aggregates between different humic solutes, the binding affinities of pyrene on different humic isolates compared with the original situation predominating in the water sample. One has to be very careful in the light of the forthcoming predictions what kind of humic probe has been adopted for representing aquatic humic matter (HM). A statistical approach for estimation of the most probable carbon distribution in a humus sample proved quite workable for the prediction of binding affinity of pyrene on aquatic humic materials.

Characterization of lake-aquatic humic matter isolated with two different sorbing solid techniques: pyrolysis electron impact mass spectrometry

The thermal degradation of different kinds of aquatic humic-matter fractions by means of pyrolysis (Py) in combination with mass spectrometric (MS) analyses using low energy (14 eV) electron impact (EI) ionization is reported. According to the data obtained it is difficult to divide aquatic organic solutes into real humic and non-humic matters based solely on qualitatively examination of thermal degradation products. On the contrary, the quantities of thermal degradation products were strongly correlated with the various humic-matter fractions isolated/fractionated by means of different operations. The close similarities between the principal building blocks of various organic solute–fractions isolated indicates that the formation of aquatic humic solutes based on certain universal one–way precursor–product relationship is an oversimplification of a complex process. Accordingly, isolation/fractionation procedures being purely connected with strongly acidic treatments are somewhat man-made for modelling aquatic acidic humic solutes.

Multi-method characterization of lake aquatic humic matter isolated with two different sorbing solids

Aquatic humic solutes were isolated from lake water by two different methods: non-ionic sorbing solid (XAD technique) and weakly basic anion exchanger (DEAE cellulose). The humic solutes removed by the DEAE procedure from the fresh water sample at natural acidity consist of practically equivalent quantities of organic acids (ca. 80% of the dissolved organic carbon (DOC)) to the total amount of the various organic acids (so-called `hydrophobic' acids and `neutrals' and `hydrophilic' acids) obtained by the XAD technique at preadjusted acidity (pH 2) with different adsorbents. According to the 13 C and 1 H NMR spectra, the structural composition of acidic humic solutes obtained by the DEAE isolation procedure appears to be a combination of certain `hydrophobic' and `hydrophilic' acidic solutes obtained by the multi-stage XAD procedure. For setting up the structural composition of an average humic solute molecule, it is important to realize the significant (mostly also predominating) role of alkenyl, cycloalkyl, ester or amide groups as structural subunits in the complicated web of aggregated solutes. Although, the strongly acidic treatments connected to the isolation and fractionation of organic solutes by the XAD technique are somewhat coincidental, especially from the structural chemistry point of view, structural compositions of the different aquatic humic solute fractions derived by the XAD technique seem to be, to a certain degree, real products, not just accidental ones, of the isolation method. With a computational method developed, specially for the complex mixture of humic solutes, it was possible to estimate the most probable carbon distributions to an average humus molecule.

Multimethod characterization of lake aquatic humic matter isolated with sorbing solid and tangential membrane filtration

According to this work, it is evident that structural alterations take place in the organic solutes defined as humic substances, in addition to the removal of certain “impurities”, during specific isolation of aquatic humic solutes at preadjusted acidity by non-ionic sorbing solids (e.g. XAD-resins). The acidic partitioning of the humic solutes at strongly acidic conditions into humic- (HA) and fulvic-acid (FA) fractions does not lead to a chemically unambiguous result, especially in the case of humic solutes with smaller molecular sizes (most significant fraction of humic solutes). The combined molecular formula (C,H,O) recalculated from distinct HA- and FA-entities, after acid precipitation of the smaller size humic solutes, was ca. 50% greater than that obtained for their corresponding unseparated HA↔FA mixtures. Also, the combined relative unsaturation (mainly due to aromatics and in particular cycloalkyl groups) recalculated from distinct HA- and FA-entities after acid precipitation of the smaller size humic solutes was ca. 10% greater than that obtained for their corresponding unseparated mixture. The combined degree of substitution of aromatic rings recalculated from distinct HA- and FA-entities after acid precipitation was, on an average, 40% greater than that estimated for their total unseparated mixture, and even ca. 90% greater in the case of smaller size humic solutes. The combination of the distinct HA- and FA-subfractions with different molecular sizes generates a molecular formula close to that obtained for the corresponding individual HA- and FA-solutes, respectively (relative differences of C, H and O: 5±2%). Also, the composition, based on the NMR spectra of different types of carbons and hydrogens, was particularly similar ( r=0.98 and 0.82, respectively) for the computationally combined distinct HA- and FA-subfractions with different molecular sizes and for the corresponding individual HA- and FA-solutes, respectively. Isolation/fractionation of aquatic humic solutes with the XAD technique appears to occur with nearly the same mechanism independent of the molecular size or of the degree of polydispersity of the solutes. Therefore the isolated humic solutes (especially the unseparated mixture of HA and FA) must play a role as certain definite entities in the dissolved organic matter (DOM) and they cannot be merely accidental products of the isolation processes, though they do not seem to be fully in accord with the original DOM. With a computational method, the most probable carbon distribution was generated for a humus sample.

Estimation of origin of lignin in humic dom by CuO-oxidation

Ligneous components of aquatic humic solutes were analysed by cupric oxide (CuO) oxidation. Vanillyl, syringyl, and cinnamyl phenols were applied as tracers to indicate the contribution of lignin to the aquatic humic matter. The lake aquatic humic solutes were isolated from the dissolved organic matter (DOM) by the XAD technique at pH 2 and by the DEAF cellulose at natural acidity. The characteristic lignin-derived phenols classified the humic-solute samples into clusters reflecting the surrounding area of the aquatic environments. No, or at most only traces of, ligneous phenols were found from the effluent of the XAD-8 column. The aquatic humic matter isolated by the DEAF cellulose produced practically the same amount of ligneous phenols than the combined amount obtained from the humic- and fulvic-acid fractions isolated by the XAD technique from the same water sample.

Isolation and characterization of natural organic matter from lake water: Two different adsorption chromatographic methods

Aquatic humic solutes were isolated from lake water by two different methods: non-ionic sorbing solid (XAD technique) and weakly basic anion exchanger (DEAE cellulose). The DEAE procedure removed from the fresh water sample at natural acidity 80% of the DOC as humic solutes. This was practically equal to the total amount of the three organic acids obtained directly by the XAD-8 resin at preadjusted acidity (pH 2) and of the acidic solutes obtained by a specific anion exchanger in connection with the XAD-8 column. The acidic gross character of the humic solutes obtained by the DEAE procedure was similar to the combined character of the four distinct acidic fractions obtained at pH 2 by different adsorbents. The structural composition of humic solutes obtained by the DEAE isolation procedure appears to be a given combination of the various hydrophobic and hydrophilic acidic solutes obtained at pH 2 by the XAD technique. It is likely the acidic treatments connected to the isolation and fractionation of organic solutes by the XAD technique can be to some extent coincidental, especially from the structural chemistry point of view. However, aquatic humic solutes (especially so-called fulvic and humic acids) modelled by the XAD technique are in some degree real, rather than accidental products of the isolation method.

Molecular size distribution and spectroscopic properties of aquatic humic substances

The number- and weight-averaged molecular weights of different isolated humic fractions and unfractionated organic matter from natural water samples were measured using high-performance size-exclusion chromatography (HPSEC). The chromatograms obtained, with sodium acetate as the mobile phase on a TSK G3000SW high-speed column, provided illustrative information about the nature of the dissolved organic matter that was not available with phosphate buffer eluents. The calibration of the column system merely with protein standards generated misleading information about the molecular weight and size distribution of humic solutes. Instead, the present results support the utilization of polystyrene-sulphonates (PSS) either alone or together with certain other model compounds for the calibration of HPSEC in studying molecular weights of humic solutes. The data obtained with the HPSEC system indicate that the average molecular weight of humic solutes is distinctly lower than is generally assumed. Besides, aquatic humic solutes also contain some very large-sized though minor constituents, leading to a fairly polydisperse mixture. Number-averaged molecular weights obtained using the HPSEC system were 4–5-fold greater than those from vapour-pressure osmometry. Bulk spectroscopic properties such as molar absorptivity at 280 nm and the quotient, E 2 E 3 , correlated strongly with the total aromaticity and averaged molecular weights of all the humic solutes. This observation suggests that bulk spectroscopic properties can be applied, as a first approximation, for estimating the size of humic solutes and their aromaticity in natural surface waters.