Aggregation of aquatic humic substances

Humic substances (HS) were isolated from Penwhirn Reservoir (PR) and Esthwaite Water (EW) and their removal from solution by centrifugation was studied as a function of pH, humic concentration and molecular weight, and CaCl₂ concentration. Large amounts (up to 50%) of PR HS could be removed but only small amounts (? 3%) of EW HS. At pH ? 5 removal of PR HS by Ca²⁺ can be explained satisfactorily in terms of decreases in humic solubilities induced by complexation with the cation. However, removal induced by protonation of the PR HS is unusual in that it decreases with increasing humic concentration.The results suggest that PR HS comprise a range of molecules differing in solubility, with the high-molecular-weight (40,000) components being the least soluble. The EW HS consist of molecules of weight-average molecular weight 5000 and resemble similarly sized PR HS in that they remain unaggregated in solution even when highly complexed with Ca²⁺.     

Molecular size of aquatic humic substances

Aquatic humic substances, which account for 30 to 50% of the organic carbon in water, are a principal component of aquatic organic matter. The molecular size of aquatic humic substances, determined by small-angle X-ray scattering, varies from 4.7 to 33 Å in their radius of gyration, corresponding to a molecular weight range of 500 to greater than 10,000. The aquatic fulvic acid fraction contains substances with molecular weights ranging from 500 to 2000 and is monodisperse, whereas the aquatic humic acid fraction contains substances with molecular weights ranging from 1000 to greater than 10,000 and is generally polydisperse.     

The usefulness of UV–visible and fluorescence spectroscopies to study the chemical nature of humic substances from soils and composts

The aim of this work is to study the suitability of the use of fluorescence and UV–visible spectroscopies to evaluate the humification degree of different organic alkaline extracts obtained from diverse materials (humic substances, composted and non-composted materials). Our results indicate that the complementary utilization of the fluorescence index proposed by Milori et al. [Milori, D., Martin-Neto, L., Bayer, C., Mielniczuk, J., Vagnato, V., 2002. Humification degree of soil humic acids determined by fluorescence spectroscopy. Soil Science, 167, 739–749] (the total area of the emission spectra collected over a range of 460–650 nm using an excitation wavelength of 465 nm), the ratio between the electron-transfer band and benzenoid band of benzene UV light absorption, and the molar absorption coefficients at 600 nm and 280 nm proved capable of discriminating between extracts obtained from non-humified, partially humified and very humified materials.Likewise, our results suggest that humification is related to increments in the aromatic character of the molecules, which may be reflected in the presence of more aromatic rings and/or more condensed polyaromatic structures, and the degree of substitution in aromatic rings with polar functional groups, as well as to the possible increase in the conjugation degree in unsaturated aliphatic chains.     

The adsorption of aquatic humic substances by iron oxides

The interactions of humic substances from Esthwaite Water with hydrous iron oxides (α-FeOOH, α-Fe₂O₃, amorphous Fe-gel) have been examined by measuring adsorption isotherms and by microelectrophoresis. In Na⁺-Cl^?-HCO₃^?at I = 0.002 M (medium I) the extent of adsorption decreases with increasing pH. The results are consistent with a mechanism involving ligand exchange of humic anionic groups with H₂O and OH^?of surface Fe-OH₂⁺and Fe-OH groups respectively, with an increasing degree of protonation of the adsorbed humics as the adsorption density increases at constant pH.At pH 7 in a medium containing Mg²⁺, Ca²⁺ and SO₄^2?, at their Esthwaite Water concentrations and at I= 0.002 M (medium II) the adsorption capacity of goethite (α-FeOOH) is approximately twice that in medium I. Electrophoresis experiments show that the extra capacity is associated with coadsorption of Mg²⁺ and/or Ca²⁺ ions.When the iron oxides are added to samples of Esthwaite Water itself they become negatively charged and plots of electrophoretic mobility against pH for the natural water are identical to those in medium II plus humics.     

The adsorption of aquatic humic substances by two oxides of manganese

A study was made of the adsorption of humic substances (HS) by Mn₃O₄ and by oxide B, a preparation with the β-MnOOH diffraction pattern but having a manganese oxidation state of 3.4. The interactions follow trends found for other oxides. Thus in 0.01 mol dm^?3 NaCl adsorption decreases with increasing pH, while it is enhanced by Ca²⁺. The HS adsorb more strongly to the oxide with the higher zero point of charge (Mn₃O₄), while the effect of Ca²⁺ is greater for oxide B.Microelectrophoretic measurements show that the oxide particles take on the electrokinetic characteristics of the adsorbed HS. However it was found that the magnitude of the mobility depends on the underlying oxide surface and on the source of the HS. The electrokinetic properties of the two oxides dispersed in surface water samples of Esthwaite Water, Cumbria, England, can be accounted for by the adsorbed HS together with coadsorbed Ca²⁺.     

Comparison of the structures of humic substances from aquatic and terrestrial sources by pyrolysis gas chromatography-mass spectrometry

Four humic extracts isolated from terrestrial, mahne, plankton and freshwater sources have been investigated by pyrolysis-gas chromatography-mass spectrometry. The results show that substantial quantities of phenols (some not derived from lignin), pyrroles and nitrites (derived from proteins, nucleic acids or porphyrins) are decomposition products. Small amounts of furans (derived from carbohydrates) and unsaturated ketones (derived from polycarboxylic aliphatic acids) are also present. The results illustrate how parent material, transport factors and biological activity affect the composition of humic extracts.     

Methylation patterns of aquatic humic substances determined by13C NMR spectroscopy

¹³C NMR spectroscopy is used to examine the hydroxyl group functionality of a series of humic and fulvic acids from different aquatic environments. Samples first are methylated with¹³C-labeled diazomethane. The NMR spectra of the diazomethylated samples allow one to distinguish between methyl esters of car☐ylic acids, methyl ethers of phenolic hydroxyls, and methyl ethers of phenolic hydroxyls adjacent to two substituents. Samples are then permethylated with¹³C-labeled methyl iodide/NaH.¹³C NMR spectra of permethylated samples show that a significant fraction of the hydroxyl groups is not methylated with diazomethane alone. In these spectra methyl ethers of carbohydrate and aliphatic hydroxyls overlap with methyl ethers of phenolic hydroxyls. Side reactions of the methyltion procedure including carbon methylation in the CH₃I/NaH procedure, are also examined. Humic and fulvic acids from bog, swamp, groundwater, and lake waters showssome differences in their distribution of hydroxyl groups, mainly in the concentrations of phenolic hydroxyls, which may be attributed to their different biogeochemical origins.     

Determining the seasonality of groundwater recharge using water isotopes: a case study from the upper North Han River basin,Korea

The stable isotopes of oxygen and hydrogen were used to determine the seasonal contributions of precipitation to groundwater recharge at a forested catchment area in the upper North Han River basin, Korea. A comparison of the stable isotopic signatures of groundwater and precipitation indicates that the precipitations which occurred during both the dry and rainy seasons are the important source of groundwater recharge in this region. A stable isotopic signature shown in the stream waters at the upstream reaches is similar to that of groundwaters, indicating that stream waters are mostly fed by groundwater discharge. Reservoir waters in the downstream flood control dams have lower deuterium excess values or d-values compared with those of the upstream waters, indicating a secondary evaporative enrichment. These results can provide a basis for the effective management of groundwater and stream water resources in the North Han River basin.     

Unique properties of humic substances from sapropel

Doklady Earth Sciences - Sapropel from inland Russian water reservoirs is becoming a popular raw material for medicinal purposes, production of sorbents, organomineral fertilizers, and food...     

Interactions of mercury with different molecular weight fractions of humic substances in aquatic systems

Interactions of mercury (Hg) with different molecular weight fractions of humic substances (HS) play an important role in controlling distribution, diffusion, speciation, and bioavailability of Hg in natural systems. This study suggests that Hg prefers to associate with higher molecular weight fractions of HS and this association particularly predominates at low pH and high ionic strengths of the medium. The concentrations of aggregated HS (with higher molecular weight) become high at lower pH (acidic condition) and high ionic strength. Molecular weight of HS gradually decreases with the increasing pH (basic condition) and low ionic strength of the medium. The disaggregation property of HS which involves the release of monomers from the surface of the aggregates produces HSs of different intermediate molecular weight with different Hg complexing capacity. Distribution of Hg in different molecular weight fractions of HS is dependent on aggregational and disaggregational properties of HS in aquatic medium. Association of Hg with high molecular weight fraction of HS may alter distribution and bioavailability of Hg in a system as the bioreactivity of organic matter decrease along a continuum of size in aquatic medium.     

Chemical coagulation of humic substances: A comparison of natural aquatic versus soil-extracted materials

Aquatic humic substances can be removed by chemical coagulation during water treatment. Hydrolyzing metals such as Al(III) and Fe(III), as well as commercially available cationic polyelectrolytes, can effectively remove humic substances prior to chlorination, thus reducing the formation of trihalomethanes. Synthetic waters, produced by adding soil-extracted fulvic acids to distilled water containing a clay source and various salts, have some merit in approximating the behavior of aquatic humic substances during chemical coagulation; however, caution must be exercised in the interpretation and applicability of the results.     

Chemical weathering of carbonates and silicates in the Han River basin,South Korea

A detailed investigation of the fluvial geochemistry of the Han River system allows to estimate the rates of chemical weathering and the consumption of CO₂. The Han River drains approximately 26,000 km² and is the largest river system in South Korea in terms of both water discharge and total river length. It consists of two major tributaries: the North Han River (NHR) and the South Han River (SHR). Distinct differences in basin lithology (silicate vs. carbonate) between the NHR and SHR provide a good natural laboratory in which to examine weathering processes and the influence of basin geology on water quality. The concentrations of major elements and the Sr isotopic compositions were obtained from 58 samples collected in both summer and winter along the Han River system in both 2000 and 2006. The concentrations of dissolved loads differed considerably between the NHR and SHR; compared with the SHR, the NHR had much lower total dissolved solids (TDS), Sr, and major ion concentrations but a higher Si concentration and ⁸⁷Sr/⁸⁶Sr ratio. A forward model showed that the dissolved loads in the NHR came primarily from silicate weathering (55 ± 11%), with a relatively small portion from carbonates (30 ± 14%), whereas the main contribution to the dissolved loads in the SHR was carbonate weathering (82 ± 3%), with only 11 ± 4% from silicates. These results are consistent with the different lithologies of the two drainage basins: silicate rocks in the NHR versus carbonate rocks in the SHR. Sulfuric acid derived from sulfide dissolution in coal-containing sedimentary strata has played an important role in carbonate weathering in the SHR basin, unlike in the NHR basin. The silicate weathering rate (SWR) was similar between the NHR and SHR basins, but the rate of CO₂ consumption in the SHR basin was lower than in the NHR basin due to an important role of sulfuric acid derived from pyrite oxidation.     

Quantitative monitoring of aquatic humic substances in Lake Biwa,Japan, using the DAX-8 batch method based on carbon concentrations

Aquatic humic substances (AHS) have been intensively studied because they have a number of significant biogeochemical functions in water chemistry. In order to evaluate such functions, which are regulated by their quantity and quality, a quantitative understanding of AHS is necessary. This study shows the seasonal and vertical changes in the AHS concentration in Lake Biwa, the largest monomictic lake in Japan, over 4 years. The concentration of dissolved organic matter carbon (DOM-C) and AHS carbon (AHS-C) ranged from 0.85–1.6 mg C/l and 0.32–0.71 mg C/l, respectively. The percentages of AHS-C in the DOM-C ranged from 32% to 65%. At all depths, the DOM-C decreased and the AHS-C slightly increased or remained at nearly the same level, suggesting that the quality of the water may have changed mainly due to changes in the composition of the organic matter. Although the AHS-C to DOM-C ratio fluctuated and had no seasonal tendency, the average AHS-C ratio increased during the 4 year monitoring period. Because AHS can influence the entire food web starting with phytoplankton, unraveling the mechanism by which they accumulate remains an important goal.     

Calculation of molecular weights of humic substances from colligative data: Application to aquatic humus and its molecular size fractions

A rigorous mathematical expression for the dependence of colligative properties on acid dissociation of water soluble humic substances is presented. New data for number average molecular weights of a river derived humic material and its gel permeation Chromatographic fractions are compared with $\text{M}\text{?}{}_{\mathrm{n}}$ values obtained by a reevaluation of previously published experimental observations on soil and water fulvic acids. The results reveal a remarkable similarity of fulvic acids from widely different sources with respect to number-average molecular weight.     

Comparative characterization of humic substances from the open ocean, estuarine water and fresh water

Humic substances were isolated from ocean, estuarine water and fresh water using a two column array of XAD-8 and XAD-4 resins in series. The extracted fulvic acids and XAD-4 fraction from different origins were characterized using UV–vis., molecular fluorescence, Fourier transform infrared (FTIR) spectroscopy and cross polarization magic angle spinning (CPMAS)-¹³C nuclear magnetic resonance (NMR) spectroscopy. The isolation procedure allowed us to obtain the necessary amount of sample for characterization, even in the case of open ocean water, which has a very low amount of dissolved organic carbon (DOC). Humic substances from the open ocean showed the lowest chromophore and fluorophore contents and showed relatively greater fluorescence at lower wavelengths than those from fresh water. FTIR and ¹³C NMR spectra highlighted the idea that humic substances from a marine environment have a more branched aliphatic structure and less aromatic structure than those highly influenced by terrestrial sources. The spectra also suggest that the open ocean humic substances have a higher content of olefinic carbons than aromatic- or alkyl-substituted carbons.     

Uranium isotopes as a tracer of sources of dissolved solutes in the Han River,South Korea

The uranium (U) content and ²³⁴U/²³⁸U activity ratio were determined for water samples collected from Korea's Han River in spring, summer, and winter 2006 to provide data that might constrain the origin of U isotope fractionation in river water and the link between U isotope systematics in river waters and the lithological nature of the corresponding bedrock. The large difference in the major dissolved loads between the two major branches of the Han River, the North Han River (NHR) and South Han River (SHR), is reflected in the contrasting U content and ²³⁴U/²³⁸U activity ratio between the tributaries: low U content (0.08–0.75 nM; average, 0.34 nM) and small ²³⁴U/²³⁸U activity ratio (1.03–1.22; average, 1.09) in the NHR; and high U content (0.65–1.98 nM; average, 1.44 nM) and large ²³⁴U/²³⁸U activity ratio (1.05–1.45; average, 1.24) in the SHR. The large spatial differences in U content and ²³⁴U/²³⁸U activity ratio are closely related to both lithological differences between the two tributaries and groundwater input. The low U content and small ²³⁴U/²³⁸U activity ratio in the NHR arise mainly from a combination of surface and meteoric weathering of the dominant silicate rocks in this branch and congruent dissolution of already weathered (secular equilibrium) materials. In contrast, the high U content and large ²³⁴U/²³⁸U activity ratio in the SHR are ascribed to the dissolution of carbonates and black shales along with significant inputs of deep groundwater.     

Long-living free radicals study of stepwise pyrolyzed melanoidins and humic substances

ESR measurements of stepwise-pyrolyzed melanoidins and humic substances (at various temperatures, mesh size, and pH values) furnished the following information: the melanoidin structure stabilizes the long-living free radicals in a manner similar to humic substances; the g and Ng values of melanoidins are similar to those of the humic substances, the cleavage of CC and CX (X = heteroatom) bonds increases the Ng value. Thermogravimetric curves, weight loss by stepwise pyrolysis, and ¹³C-CP/MAS NMR were found to be in good correlation with ESR data regarding the structural features of melanoidins and humic substances.