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Chemical and Environmental Sciences, Univ. of Limerick, Ireland
* Corresponding author (Michael.H.Hayes{at}ul.ie)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONASPECTS OF THE COMPOSITIONS...PROPERTIES OF SOLVENT SYSTEMSSEPARATION OF SOM COMPONENTS...ISOLATION OF SOM COMPONENTS...CONCLUSIONS AND RELEVANCE OF...REFERENCES |
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Abbreviations: DMF, N,N-dimethylformamide • DMSO, dimethylsulfoxide • DOSY, diffusion ordered spectroscopy • EDA, ethylenediamine • EF, electrostatic factor • HA, humic acid • HS, humic substance • IHSS, International Humic Substances Society • NMR, nuclear magnetic resonance • SOM, soil organic matter
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONASPECTS OF THE COMPOSITIONS...PROPERTIES OF SOLVENT SYSTEMSSEPARATION OF SOM COMPONENTS...ISOLATION OF SOM COMPONENTS...CONCLUSIONS AND RELEVANCE OF...REFERENCES |
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Soil organic matter or humus is perhaps the most complex material in nature. Most emphasis has focused on the humic substances (HSs). In the classical definitions, HSs have been regarded as a series of relatively high-molecular-weight dark-colored substances formed by secondary synthesis reactions (Stevenson, 1994), and as a "category of naturally occurring, biogenic, heterogeneous organic substances that can generally be characterized as being yellow to black in color, of high molecular weight, and refractory" (Aiken et al., 1985). In classical definitions, unaltered biological molecular components of plants and animals, though components of humus, would not be considered to be HSs. Thus, unaltered flavonoids, tannins, terpenes, sporopollenins, and large aliphatic molecules such as algaenans, cutans, suberans (Derenne and Largeau, 2001), though components of humus, would not be regarded as HSs. There is increasing emphasis on the presence of charcoal, or char, from the burning of vegetation (Skjemstad et al., 1996; Swift, 2001). Although some charcoal compounds fall within the operational definitions of HSs, based on solubilities data, these cannot be considered to be true HSs.
The operational definitions of SOM fractions based on solubilities were first introduced by Sprengel (1837). Soil scientists define HAs as humus materials that are soluble in aqueous alkaline solutions but precipitate when the pH is adjusted to 1 (water scientists consider the precipitates at pH 2 to be HAs). There are, of course, many nonhumic components (e.g., some proteins) that are precipitated under similar conditions, and so the precipitates at pH 1 might be considered as the HA fraction. Fulvic acids remain in solution after the aqueous alkaline extracts are acidified. These materials are best referred to as the FA fraction. The FA standards of the International Humic Substances Society (IHSS) were processed by passing the acidified solutions on to XAD-8 resin [(poly)methylmethacrylate], which retain the FAs and allow polar components to pass through the resin. The FAs are recovered in 0.1 M NaOH, and H+–exchanged by passing through IR-120 (styrenedivinylbenzene with sulfonic acid functionality, H+–exchanged) resin, followed by freeze drying. Polar, nonhumic components, such as various peptides, saccharides, and some small organic chemicals, are not retained on XAD-8 but may be held by XAD-4 (styrenedivinylbenzene) resin.
To some extent, polar contaminants may also be removed from HAs using XAD-8 resin technology. Häusler and Hayes (1996) removed some of the saccharide and peptide components from HAs in a Sapric Histosol using acidified dimethylsulfoxide [DMSO + 12% (v/v) of 12 M HCl]. The acidified solution was passed on to a XAD-8 resin which retained the HAs. The DMSO was washed through with acidified water, then with distilled water, and the HAs were recovered in 0.1 M NaOH. These were then H+–exchanged by passing through IR-120, and then freeze dried.
Humin is defined as the HSs that are insoluble in aqueous media. There are several biological molecules that would meet the humin solubility criteria but, on the basis of the definitions outlined, do not satisfy the criteria for HSs. Thus, in general, humin is SOM components that are strongly adsorbed to the soil mineral colloids and are not extracted in aqueous acids or bases. In organic soils, or Histosols, the humin would be composed of the components that bear no morphological resemblances to their parent organic structures.
The importance of SOM to agricultural productivity has long been recognized (Hayes and Swift, 1978; Stevenson, 1994; Clapp et al., 2005). More recently, there is emphasis on its importance in terms of global warming effects. Soil management is important for considerations of the roles of soils as sources and sinks of carbon (Lal et al., 1995). Hence there is increased interest to study the chemistry of SOM in relation to its abilities to resist biological transformations.
Significant advances have been made in recent times in understanding aspects of the compositions of SOM fractions. However, the fractions are still largely operationally defined based on solubilities in aqueous basic and acidic media, and such media, as used in conventional extractions, are unlikely to isolate >50% of the SOM components. Thus, there is a need for more complete isolation, more careful fractionation, and more complete descriptions of the fractions to advance our awareness of the compositions of SOM components and of their stabilities and mechanisms of action in the soil environment.
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ASPECTS OF THE COMPOSITIONS OF THE COMPONENTS OF SOIL ORGANIC MATTER |
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TOPABSTRACTINTRODUCTIONASPECTS OF THE COMPOSITIONS...PROPERTIES OF SOLVENT SYSTEMSSEPARATION OF SOM COMPONENTS...ISOLATION OF SOM COMPONENTS...CONCLUSIONS AND RELEVANCE OF...REFERENCES |
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Although Engebretson and von Wandruszka (1997) did not dismiss the concept of macromolecularity, they concluded that the quenching of pyrene fluorescence by Br– ions indicated that humic molecules could spontaneously aggregate through hydrophobic associations. They suggested that pyrene reacted with hydrophobic sites within the humic structures and was protected by the SOM from the Br–. On the basis of the procedures used in earlier work of Piccolo, and the suppression of pyrene fluorescence by Br–, Kenworthy and Hayes (1997) showed that pyrene was best protected by the most hydrophobic HA fractions. Protection was lost when dilute ethanoic acid followed by base was added to the medium and the hydrophobic associations that protected the pyrene were disrupted.
Simpson (2002) and Simpson et al. (2002) used diffusion ordered spectroscopy (DOSY) nuclear magnetic resonance (NMR) for studies of the compositions of HA and FA molecules. The DOSY technique separates structures on the basis of diffusion properties, and the spectrum directly correlates diffusion coefficients to proton chemical shift in a 2-D plot. In a pure aqueous solvent, Simpson (2002) found that all the components of the IHSS Standard peat HA had a single diffusion coefficient, and that would suggest that a single molecular structure, or a strongly associated group of molecules was being observed. However, when a trace amount of ethanoic acid was added, separate diffusivities were observed for lignin-derived, carbohydrate, peptide, and hydrocarbon-derived structures (Fig. 1 ). Ethanoic acid, as used by Piccolo et al. (2001), is known to disaggregate protein structures, and the separate diffusivities of the component molecules, as seen in Fig. 1, can be regarded as evidence that molecular associations, and not covalently linked macromolecular structures, compose HSs. This led Simpson et al. (2002) to depict HAs as associations of altered (oxidized) lignin structures, peptides, saccharides, waxes, long chain fatty acids, etc. At high concentrations, the aggregates displayed diffusivities consistent with those for large proteins (>66 000 Da), but after disaggregation with low concentrations of acids, the diffusivities observed were consistent for molecules in the range of 200 to 2500 Da. The components in the FAs were shown not to aggregate strongly, and tended to be smaller than the HAs.
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PROPERTIES OF SOLVENT SYSTEMS |
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TOPABSTRACTINTRODUCTIONASPECTS OF THE COMPOSITIONS...PROPERTIES OF SOLVENT SYSTEMSSEPARATION OF SOM COMPONENTS...ISOLATION OF SOM COMPONENTS...CONCLUSIONS AND RELEVANCE OF...REFERENCES |
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Aqueous Systems
Water is the major solvent used for the isolation of SOM components. Water, A Comprehensive Treatise, edited by Franks (1975), and Water (Franks, 1983) give details of relevant structures and properties of water. On the basis of these treatises, Hayes (1985) and Clapp et al. (2005) have summarized aspects of water structure that are important for the solvation of SOM components. Theoretical calculations suggest that the most stable form of interaction between water molecules involves the linear H bond. Hydrogen bonding gives considerable structure to liquid water, and liquid water can be considered to be composed of clusters of various sizes in equilibrium with each other, and with free unstructured water molecules. When soluble ions or highly polar compounds are introduced into water, solvation occurs because of associations, through electrostatic effects, between the solvent and the solute. The ordering of the solvent around the solute gives a decrease in entropy, but this is countered by the increased entropy from the breakdown of the water cluster structure. However, when nonpolar molecules enter spaces between water clusters they associate with each other (rather than with the water molecules) in the void spaces. The clusters grow at the expense of free water and the energy and entropy decrease as free water H bonds in the clusters. The associations of nonpolar solutes gives rise to the concept of hydrophobic bonding in the presence of water (Franks, 1975, 1983).
Organic Solvent Systems
Organic molecules will dissolve a solute when the attraction forces between the solvent and solute molecules are similar. The important parameters for consideration of an organic solvent for SOM include the basicity of the system, the ability to make and to break H bonds, the relative permittivity (Kr), and the dipole moment (µ) of the solvent. The boiling point, viscosity, and density are peripheral properties that do not necessarily affect the isolation processes, but these are important in considering the recovery of the solute from the solvent. Relevant values for these properties for water and the organic solvents discussed in this article are given in Table 1.
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) is attributed to Hilderbrand and co-workers (Hildebrand and Scott, 1951, 1962; Hildebrand et al., 1970). Solubility parameters are useful for predicting the solubility of a solute in a solvent. A good solvent for a nonelectrolyte solute will have, for example, a solubility parameter () value close to that of the solute. A mixture of two solvents, one with a value above, and another with a value below that of the solute can provide a better solvent system for the solute than any one of the solvents alone. Hayes (1985) has outlined aspects of the concepts relevant to the present topic.
The one component solubility parameter is appropriate in specific interactions for solutions lacking polarity. To some extent, the one component parameter is replaced by multicomponent solubility parameters which give values for the different interaction forces, such as the dispersion (d) and the polar forces (p), and hydrogen bonding (h). Table 2 gives relevant solubility parameter data for solvents discussed in the present study. A review by Barton (1975) provides the information needed to familiarize the reader with the subject. He pointed out that the data become empirical when multicomponent parameters are used, and thus it is important to use sets of data that are internally consistent. Keller et al. (1971) and Karger et al. (1976) further divided the hydrogen bonding parameter into the proton donor (a), or acid, and the proton acceptor (b), or base parameters. The data for these parameters, listed in Table 2, are from Snyder (1978) and not from the same source as those compiled by Barton (1975) and given in the same Table. Hence, values for h should not be compared directly with those for a and b.
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SEPARATION OF SOM COMPONENTS IN AQUEOUS MEDIA |
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TOPABSTRACTINTRODUCTIONASPECTS OF THE COMPOSITIONS...PROPERTIES OF SOLVENT SYSTEMSSEPARATION OF SOM COMPONENTS...ISOLATION OF SOM COMPONENTS...CONCLUSIONS AND RELEVANCE OF...REFERENCES |
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As mentioned, classical solubility procedures for HAs and FAs are operationally defined and the fractions will contain codissolved or coprecipitated biological molecules that are not regarded as HA and FA. Hayes et al. (1996) and Hayes (1996) used aqueous solvent systems to fractionate SOM components at different pH values. The soil was exhaustively extracted with 0.1 M pyrophosphate (pH 7) until the extracts had negligible color, then exhaustively extracted sequentially with 0.1 M pyrophosphate (pH 10.6), and finally with 0.1 M NaOH + 0.1 M pyrophosphate (pH 12.6). The objective was to isolate organic fractions, and HSs in particular, on the basis of charge density differences. Humic and FAs were isolated by the IHSS procedure (Swift, 1996), and by a modification that used XAD-8 and XAD-4 resins in sequence (Malcolm and MacCarthy, 1992). In the latter procedure, the aqueous extracts were diluted to a concentration <15 ppm, the pH of the solution was adjusted to 2, and then pumped slowly (
40 mL min–1) on to XAD-8 resin in tandem with XAD-4 resin. The components held by the resin were then desalted with distilled water, back eluted in 0.1 M NaOH, H+–exchanged, and freeze dried. Associations of HA molecules can slowly take place in the dilute solutions to give precipitates. The composition of these precipitates were significantly different from those which remained in solution. Also, some insoluble materials may be recovered during the desalting process. These and the precipitates referred to can further be fractionated by dissolving, diluting as mentioned above, adjusting the pH to 2.5, and repeating the resin treatment process. Should any further precipitates be formed, the recovery and dissolution and resin application procedures can be repeated at pH 3. Ethanol extracts (Soxhlet) of the resins following recoveries in the aqueous media gave what are termed the neutral components.
Compositional differences between the samples are emphasized in the cross polarization magic angle spinning 13C NMR spectra in Fig. 2 for the HAs, FAs, and XAD-4 acids isolated from a silty clay grassland soil (Hayes et al., 1996; Hayes, 1996). These spectra show distinct compositional differences between the isolates at the different pH values. Note the progressive decrease in carboxyl contents in all fractions (180 ppm resonance), as would be expected, in the extracts as the pH is raised. The spectra for the HAs (1–3) were obtained for samples processed by the IHSS procedure, and those for HA 4 and the FAs (5–7), and XAD-4 acids (8–10) were for samples isolated by the XAD-8 and XAD-4 resin in tandem process outlined above. Spectrum 11 was isolated using the Soxhlet with ethanol as the solvent.
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Spectrum 3, for the HAs isolated at pH 12.6 is significantly different from spectra 1 and 2, and indicates strong contributions from hydrocarbon structures, and the ratio of the resonances at 120–140 and 140–150 ppm, as well as the resonance at 56 ppm would suggest that aromaticity is largely contributed by partially altered lignin structures. The clear resonance centered around 105 ppm would suggest that much of the 60- to 90-ppm resonance is for carbohydrates.
Spectrum 4 (material that precipitated from the dilute solution at pH 2 for samples isolated by the resin in tandem procedure) is very different from Spectrum 3. Aromaticity is significantly lower, and there is little evidence for lignin content. The evidence for anomeric C is weak; hence the strong resonance at 60 to 90 ppm is from functionalities (alcohol and ether) other than carbohydrate. The rounded shape of the resonance peak at 50 to 60 ppm is more suggestive of amine (peptide) than methoxyl functionalities.
The resonances for the FAs (spectra 5, 6, and 7) isolated at the different pH values are again significantly different from each other, and from the corresponding spectra for the HAs. Note the decreases in carboxylic and aromatic resonances and the increase in lignin-type and carbohydrate signals as the pH values of the extractants increase.
The XAD-4 acids can be considered as components of the FA fraction that were separated from the FAs by the XAD-8 treatment. These are not by definition HSs. Note the very low aromaticity in all these fractions (spectra 8, 9, and 10), and how aromaticity decreases as the pH of the extractant is raised. Aliphatic hydrocarbon resonances are also low, and the shoulder in the 50- to 90-ppm resonance could indicate peptide structures, and the strong anomeric C resonances indicate that carbohydrates are the major contributors to the 60- to 90-ppm resonance.
Spectrum 11 is designated as hydrophilic neutrals, or the fraction removed after the XAD-4 resin had been eluted in 0.1 M NaOH and then extracted in ethanol using a Soxhlet procedure. It is clearly compositionally different from the other fractions isolated. Note the strong aromatic resonance, the anomeric C, and the strong 60- to 90-ppm resonance. The resonance at 50 to 60 ppm would suggest peptide structures.
In the classical definitions, humic components that are not extracted in aqueous base are considered to be humin. The author and his colleagues recently performed exhaustive extractions at pH 7, 10.6, and 12.6, and then subjected the residual materials to repeated extractions with 0.1 M NaOH + 6 M urea. The NMR spectrum of the material in the urea extract from a Mollisol was very similar to that extracted at pH 12.6 (Fig. 3 ). However, the extracts in the basic urea medium were different from those in base in the cases of Irish Brown Podzolic soils. It is likely that the urea extract in the case of the Mollisol was protected from the base by, for example, hydrogen bonding processes or by steric constraints. It would appear that the humin materials (Fig. 4 ) that are not extracted in the base + urea system are very different from the HA (Fig. 3) and the FA components. Note the large resonance at 0 to 40 ppm (Fig. 4), indicating that hydrocarbon structures were major contributors to the composition, and that would explain the nonpolar nature of the material. There is clear evidence for resonance in the 60- to 80-ppm region, and there is evidence for a weak contribution from carboxyl groups.
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ISOLATION OF SOM COMPONENTS IN ORGANIC SOLVENTS |
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TOPABSTRACTINTRODUCTIONASPECTS OF THE COMPOSITIONS...PROPERTIES OF SOLVENT SYSTEMSSEPARATION OF SOM COMPONENTS...ISOLATION OF SOM COMPONENTS...CONCLUSIONS AND RELEVANCE OF...REFERENCES |
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to the keto groups in quinone structures, and the functional groups that are formed resist cleavage by acid washing (see Clapp et al., 2005, p. 33). Hayes (1985) tested the abilities of water and of a range of organic solvents to dissolve the H+–exchanged IHSS Standard HA isolated from a Florida (Belle Glade) Sapric Histosol. The HAs were mixed with solvents (0.2% w/v), some of which are listed in Table 3, and swelling was allowed to take place overnight. After centrifugation, each supernatant solution was diluted with the extractant until an absorbance reading (against the solvent) at 400 nm could be obtained. The absorbance values in Table 1 are the product of the readings obtained and the dilution factors used.
The data in Table 3 show that water is a poor solvent for H+–exchanged HAs. By definition, HAs are insoluble in water, although traces of H+–exchanged HAs can become separated from the hydrogen bonded matrix and will become dissolved. Both water and the H+–exchanged HAs have strong hydrogen bonds, and the intermolecular hydrogen bonding interactions between the water molecules are stronger than similar bonds that might form with the HAs. Hence, sufficient H2O–HA associations will not occur for solvation of the humic molecules to take place. The solvation effects will be different when the acidic functional groups in the HAs become dissociated as the pH is raised, as mentioned above. Similarly, monovalent metal cation-exchanged HAs will dissolve in water because both the cations (and especially Na+ and Cs+ ions with high hydration energies) and the conjugate bases of the acidic functional groups will solvate in water. Compared with the data obtained for dissolution by 0.5 M NaOH, the data in Table 3 show that acetonitrile, ethanol, and pyridine are poor solvents for HAs. On the other hand, formamide (a polar protic solvent), and N,N-dimethylformamide (DMF) and DMSO can be considered to be good solvents. Acetonitrile, DMSO, and DMF are dipolar aprotic solvents defined by Parker (1962) to have relative Kr values >15 and incapable of donating H atoms to form strong hydrogen bonds.
On the basis of the data in Table 1 it is reasonable to predict from the data in Table 3 that organic liquid solvents with electrostatic factor (EF) values >140 and with pKHB values >2 should be good solvents for HSs. The EF is the product of Kr and µ, the dipole moment, and the pKHB is a measure of the ability of the solvent to be an acceptor in hydrogen bonding. Acetonitrile lacks the abilities to make and to break hydrogen bonds (low pKHB value), although its Kr and EF values are in the ranges that would apply for the good solvents. It should be emphasized that swelling in organic solvents is slow, as opposed to that in aqueous base, and several hours of contact between the organic solvent and solute are required for swelling and solvation.
Water, of course, has an abundance of the desirable properties, but as pointed out, its strength of intermolecular hydrogen bonding inhibits its abilities to form the necessary hydrogen bonds with the H+–exchanged HAs. The Kr and EF values for ethanol are well below the ranges for the good solvents for H+–exchanged HSs, and the Kr value for pyridine indicates that it is not a good solvent for H+–HAs, as shown in Table 1. In the absence of water dry HAs do not swell extensively in pyridine. In the presence of water, however, and at pH values >9 (the pKb for pyridine is 8.96), pyridine molecules predominate in solution, and theory suggests that it will solvate functional groups in HSs. As the pH falls there is a logarithmic increase in pyridinium ions, and the organocations formed are held by ion exchange to the conjugate bases of the acidic functional groups of the HAs. Pyridinium humate salts are less dissociated than humate-monovalent cation salts, but significantly more so than humate-divalent cation salts. Hayes (1985) has shown that HAs more readily dissolve in water after prior extraction with pyridine, and that might be attributable in part to the removal of hydrophobic components in the HAs. However, extractions with pyridine are not recommended because pyridine sorbs to humic structures and the associations are not readily reversible.
For the most effective solvation, the self attraction forces in the solute and solvent should be similar, and that would be reflected in similar solubility parameter data. Such data are not available for the components of SOM, and especially for these as they exist in their associations in the soil environment. On the basis of the EF (Table 1) data, formamide is a good solvent for H+–HAs (Table 3). Large values for the various solubility parameter data (Table 2) could suggest that the magnitude of these also would be conducive to solubilization. When the data for formamide are compared with those for ethanol, it would seem that the low value of p is a limiting factor in the ability of ethanol to dissolve HAs to a significant extent. Also, ethanol has an EF value that is well outside the range for good solvents.
The data for acetone (a dipolar aprotic solvent) in Table 2 suggests that it could be a good solvent when subjected to the experimental procedures used for the data in Table 3. However, this was not the case, likely because of the low values for h and b (Table 2), and the values for Kr (20.7) and for pKHB (1.18) are not in the ranges suggested above for good solvents. The strengths of any hydrogen bonds that might form between acetone and the HAs would be less energetic than those between the solute molecules.
Dimethylsulfoxide is recognized as a good solvent for hydrogen bonded systems. However, it is a good solvent for cations but a poor solvent for anions (Martin and Hauthal, 1975). Thus, it is important to have the SOM components H+–exchanged for solvation to take place in DMSO. In metal cation-exchanged systems, the metal cations will dissolve but the anionic conjugated bases will only sparingly dissolve in the solvent. Data to illustrate this were presented by Hayes (1985). Thus, it is important to use DMSO-concentrated acid mixtures to isolate HSs from non-H+–exchanged soils. Data from Hayes (1985) stress the importance of establishing the optimum amounts of concentrated HCl to be added to DMSO for extractions of OM from mineral soils.
Piccolo (1988), and Piccolo et al. (1998) have compared the yields and compositions of extracts from a Sapric Histosol soil using aqueous NaOH, pyrophosphate, and dipolar aprotic solvents (acetone, DMF, and DMSO) aqueous HCl systems. The yields from the dipolar aprotic systems were significantly less than those from the base and pyrophosphate, and the NMR spectra showed significant differences in the compositions of the various isolates (Piccolo et al., 1998). The dipolar aprotic solvents isolated more of the hydrophobic components of the SOM, although the NMR spectra indicated considerable similarities between the spectra of the base and DMSO extracts (Piccolo et al., 1998).
The amounts of water in the dipolar aprotic media strongly influence their abilities to dissolve SOM components. Hayes (1985) has described how water (5% of the total weight) in a urea (10%) DMSO solution was an excellent solvent for H+–HAs and for SOM. However, dissolution was depressed as increasing increments of water were added. The amounts of water in the dipolar aprotic solvent systems used in the work of Piccolo (1988) and of Piccolo et al. (1998) could explain the relatively low yields obtained. The yields from DMSO and the good solvents described by Hayes (1985) were significantly greater than those for solvent systems containing excess water (and no mineral acid additives) as used by Hayes et al. (1975).
Dimethylsulfoxide–H2O interactions are stronger than the associations between water molecules, and there are strong interactions also between DMSO and carboxyl and phenolic hydroxyl groups. In Fig. 5 , a hypothetical representation is given, based on a model by Martin and Hauthal (1975), of the types of associations that can occur when humic and other SOM components are dissolved in DMSO. It shows the links between the sulfoxide functional group and carboxyl and phenolic hydroxyls in the humic structures. Dimethylsulfoxide can be considered to have hydrophobic (represented by the methyl groups) and hydrophilic faces. Thus, the hydrophobic face can associate with hydrophobic components, and that would explain the increased C contents in dipolar aprotic solvent extracts compared with those in a base (Hayes et al., 1975; Piccolo, 1988).
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CONCLUSIONS AND RELEVANCE OF EXTRACTION PROCEDURES FOR APPLICATIONS IN SOIL AND WATER ENVIRONMENTS |
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TOPABSTRACTINTRODUCTIONASPECTS OF THE COMPOSITIONS...PROPERTIES OF SOLVENT SYSTEMSSEPARATION OF SOM COMPONENTS...ISOLATION OF SOM COMPONENTS...CONCLUSIONS AND RELEVANCE OF...REFERENCES |
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Alterations of component molecules during the extraction and separation of SOM fractions can be minimized. For example, negative oxidation during extractions in base can be significantly decreased by H+–exchanging the OM, and using 0.1 M base in an atmosphere of N2. Also, use of aqueous solvents with increasing pH values will minimize oxidation. Another approach is to avoid pH values >10, and use DMSO-acid or DMSO-urea solvent systems after the SOM has been exhaustively extracted at pH 10. The data in Fig. 2 show that the components isolated at increasing but discrete pH values are significantly different. It can be seen from the data in Fig. 3 and 4 that HA and FA can be removed more effectively by using combinations of aqueous base and urea. The residual humin fraction is a major part of SOM strongly sorbed by the mineral colloids, and has so far defied dissolution in aqueous or organic solvents. However, based on the NMR spectrum shown in Fig. 4, the compositions of the hydrophobic components of humin may be relatively simple to resolve by using procedures such as pyrolysis mass spectrometry.
On the basis of the experience of the author, more organic components might have been isolated by Piccolo (1988) and by Piccolo et al. (1998), had less water been used in the solvent systems. However, the NMR spectra of the isolates in the acetone system from three mineral soils (Spaccini et al., 2000) were largely composed of aliphatic hydrocarbon constituents, and were very different from the components isolated in base or pyrophosphate solutions. This indicates that combinations of solvent systems, both aqueous and organic, singly or in combination, are useful for studying SOM. Applying this to isolate the components of SOM that have special growth promoting or inhibition effects on plants could help resolve the debate regarding whether or not dissolved OM components can influence plant growth by hormones (Clapp et al., 2001) or by enhanced uptake of nutrients (especially Fe; Chen and De Nobili, 2004). Isolation and some fractionation of SOM components was essential, for example, to provide the fractions used by Ve et al. (2004a, 2004b), and by Schmidt-Rohr et al. (2004) to determine the availability of N for rice grown in lowland soils.
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ACKNOWLEDGMENTS |
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Received for publication April 4, 2005.
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TOPABSTRACTINTRODUCTIONASPECTS OF THE COMPOSITIONS...PROPERTIES OF SOLVENT SYSTEMSSEPARATION OF SOM COMPONENTS...ISOLATION OF SOM COMPONENTS...CONCLUSIONS AND RELEVANCE OF...REFERENCES |
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), density (
), relative permittitivity (Kr), electrostatic factor (EF), and base parameter (pKHB) values for solvents used for data in 
represents an aromatic functionality.