Humic Acid Components Research Articles

Photodegradation of the antibiotics nitroimidazoles in aqueous solution by ultraviolet radiation

The objective of this study was to analyze the efficacy of ultraviolet (UV) radiation in the direct photodegradation of nitroimidazoles. For this purpose, i) a kinetic study was performed, determining the quantum yield of the process; and ii) the influence of the different operational variables was analyzed (initial concentration of antibiotic, pH, presence of natural organic matter compounds, and chemical composition of water), and the time course of total organic carbon (TOC) concentration and toxicity during nitroimidazole photodegradation was studied. The very low quantum yields obtained for the four nitroimidazoles are responsible for the low efficacy of the quantum process during direct photon absorption in nitroimidazole phototransformation. The R 254 values obtained show that the dose habitually used for water disinfection is not sufficient to remove this type of pharmaceutical; therefore, higher doses of UV irradiation or longer exposure times are required for their removal. The time course of TOC and toxicity during direct photodegradation (in both ultrapure and real water) shows that oxidation by-products are not oxidized to CO 2 to the desired extent, generating oxidation by-products that are more toxic than the initial product. The concentration of nitroimidazoles has a major effect on their photodegradation rate. The study of the influence of pH on the values of parameters ɛ (molar absorption coefficient) and k′ E (photodegradation rate constant) showed no general trend in the behavior of nitroimidazoles as a function of the solution pH. The components of natural organic matter, gallic acid (GAL), tannic acid (TAN) and humic acid (HUM), may act as promoters and/or inhibitors of OH· radicals via photoproduction of H 2O 2. The effect of GAL on the metronidazole (MNZ) degradation rate markedly differed from that of TAN or HUM, with a higher rate at low GAL concentrations. Differences in MNZ degradation rate among waters with different chemical composition are not very marked, although the rate is slightly lower in wastewaters, mainly due to the UV radiation filter effect of this type of water.

Photochemical Effect of Humic Acid Components Separated Using Molecular Imprinting Method Applying Porphyrin-like Substances as Templates in Aqueous Solution

To elucidate the relationship between photochemical functions with the structure of humic acids (HA), we developed a molecular imprinting method to separate the substances with given structure and investigated their photochemical behavior in aqueous solution. The substances with porphyrin-like core structure, such as chlorophyll or heme, were employed as template substances for preparing molecular imprinting polymers (MIP). The polymers were used to separate the substances with porphyrin-like structure from HA. Photochemical experiments were conducted to evaluate effects of the separated HA fractions on the photodegradation of coexisting organic pollutant. The results showed that all fractions bound by MIP accelerated photochemical degradation of coexisting 2,4-dichlorophenoxyacetic acid (2,4-D) under simulated sunlight (lambda>290 nm) irradiation, indicating that HA with porphyrin-like structure possesses better photoactivity than ones without the structure. The photochemical degradation of 2,4-D was enhanced when Fe(III), the ubiquitous element in natural aquatic systems, was added owing to the formation of Fe(III) complex with the HA. Electron paramagnetic resonance (EPR) spectra indicated that OH* and 1O2 radicals were generated in the solutions of HA fractions bound by MIP under simulated sunlight irradiation, implying that 2,4-D degradation could be related to oxidation reactions caused by reactive oxygen species (ROS).

Selective digestion of the proteinaceous component of humic substances by the geophagous earthworms Metaphire guillelmi and Amynthas corrugatus

Abstract Humic substances play a key role in the global carbon cycling and the sequestration of micropollutants in soil. The transformation of these substances by earthworms, the dominant soil macroinvertebrates of many terrestrial ecosystems, and the mechanisms involved are still obscure. We prepared two chemically identical humic model compounds that were specifically 14 C-labeled either in the aromatic or the proteinaceous component, and added them to soil incubated with the geophagous earthworm species Metaphire guillelmi (anecic) and Amynthas corrugatus (endogeic). In the absence of the earthworms, both the aromatic and the proteinaceous components were mineralized at similarly low rates (5−8% after 9 days of incubation). In the presence of the earthworms, mineralization rate of the proteinaceous component was strongly stimulated (2-fold by M. guillelmi and 1.4-fold by A. corrugatus ). The mineralization rate of the aromatic component was (slightly) stimulated (1.2-fold; P A. corrugatus . In all cases, the stimulated mineralization was accompanied by a transformation of radiolabeled humic acids to fulvic acids within the earthworm guts and by an incorporation of radiolabel into the earthworm tissues. Digestion of the proteinaceous component of humic acids by the earthworms was corroborated also by a decrease of extractable humic acids in fresh cast and a stimulated mineralization of soil nitrogen; in the case of M. guillelmi , the fresh cast contained sixfold more NH 4 + than the non-ingested soil. Our study provides direct evidence for the selective digestion of humic components by earthworms. Considering the ubiquity of geophagous earthworms and their large biomass, the alteration of the chemical structure of humic substances by the earthworms through their selective digestion of peptidic components may have significant impacts on the stability of humic substances and the bioavailability of micropollutants in soil.

Differences of stabilized organic carbon fractions and microbiological activity along Mediterranean Vertisols and Alfisols profiles

This study examined the chemical and structural properties of humic substances and microbiological activity in order to verify differences in carbon dynamics along soil profiles in two Vertisols ( Typic Haploxerert and Xeric Epiaquert) and two Alfisols ( Mollic Haploxeralf and Ultic Haploxeralf) developed under Mediterranean climate in Italy. Humification parameters, thermal methods, including differential scanning calorimetry (DSC) and thermogravimetry (TG), together with Fourier transform infrared (FT-IR) and fluorescence spectroscopies were used to characterize humic acids (HA). Microbiological activity of soils was assessed by basal respiration, metabolic quotient ( qCO 2) and C mic:C org ratio. FT-IR spectra and thermal analysis DSC/TG of HA extracted from the upper horizons showed a higher aliphatic character, whereas HA extracted from the lower horizons had a higher content of aromatic structures and polysaccharides. Moreover, the fluorescence index of the HA (HIX flu) showed a higher degree of aromatic polycondensation in the subsoils. The C mic:C org ratio was negatively correlated with the HIX flu of HA ( P < 0.05), while qCO 2 was positively correlated with the labile components of HA (Exo1/(Exo2+Exo3)) ( P < 0.05). These results suggest reduced C availability in deep horizons where HA structures resulted in complex and thermally more stable molecules. Following discriminant function analysis all the chemical and microbiological properties with the exception of labile thermal fraction of HA and soil pH largely varied between B and the upper horizons. In deep horizons of both Vertisols and Alfisols the microbial biomass was nutritionally stressed with a low efficiency in C turnover. No correlation was found between specific soil processes, such as argilliturbation or clay illuviation and humic substances properties or soil microbial biomass and activity.

Capillary electrophoresis profiles and fluorophore components of humic acids in Nebraska corn and Philippine rice soils

As humic substances represent relatively high molecular mass polyelectrolytes containing aromatic, aliphatic and heterocyclic subunits, capillary electrophoresis (CE) has become an attractive method for “finger-print” characterization of humic acids. In addition, fluorescence excitation-emission matrix (EEM) spectroscopy is also a highly sensitive analytical method, providing information on flurophore properties of humic acids. However, basic information on the CE profile patterns and fluorophore components of humic acid fractions from different agricultural soils is rare. Consequently, we sequentially extracted the mobile humic acid (MHA) and recalcitrant Ca humate (CaHA) fractions from Nebraska corn soils and Philippine rice soils under different fertilization schemes, and acquired their CE profiles and fluorophore component composition. We observed greater differences in the CE profiles between soils than between management practices on the same soil, reflecting the distinct “finger-print” CE features of these soil humic fractions. Modeling analysis of EEM data indicated that four fluorophore components were present in these humic fractions. Whereas three have previously been reported in literature, we propose the features of the unknown component as being relevant to agricultural humic fractions. UV irradiation seemed to impact the Nebraska soils by disaggregating the humic fractions into their structural subunits, while impacting the Philippine soils by altering the functional groups of humic fractions. This research provides insight into the CE profiles and fluorophore features of humic acid fractions from different agricultural soils.

Abiotic association of PAEs with humic substances and its influence on the fate of PAEs in landfill leachate

An equilibrium dialysis combined with a 14C-labeling method was used to study the abiotic association of phthalic acid esters (PAEs) with dissolved humic substances (HS) and its influence on the fate of PAEs in landfill leachate. Elemental analysis and FTIR spectral analysis were carried out on the humic acid (HA) and fulvic acid (FA) components of HS to examine the relationship between the structure of HS and the intensity of the association of the PAEs with HS ( K A). The results show that the association intensity of HS with PAEs depends greatly on both the properties of the PAEs and the source of HS. The HS have a strong association reaction with dibutyl phthalic acid, which might explain the higher concentration of dibutyl phthalic acid detected in landfill leachate. The pH value strongly affects the K A of HS, and it decreases radically when the pH increase from 3.0 to 9.0. The non-specific hydrophobic interaction between HS and PAEs plays a more important role in the abiotic association of HS with PAEs. Elemental analysis and FTIR spectra suggest that the high K A values are related to the high aromatic content and larger molecular weight of HS.

Sorption of Sulfonamide Antimicrobial Agents to Humic Acid–Clay Complexes

The interaction of sulfonamide antimicrobial agents with smectite clay minerals and humic acid (HA)-clay complexes was investigated in batch experiments to assess the influence of adsorbed humic acid on sulfonamide sorption. Soil HA-clay complexes were produced at HA:clay ratios of 1:5, 1:50, and 1:100 (w/w). Vibrational and electronic spectroscopy indicated the preferential adsorption of polar and aliphatic components of HA to smectite surfaces, a phenomenon most readily discerned at the two lower HA:clay ratios (1:50 and 1:100). Humic acid adsorption to smectite clay minerals enhanced sulfamethazine [2-(p-aminobenzenesulfonamido)-4,6-dimethyl pyrimidine] sorption, especially at the highest HA:clay ratio (1:5). Sulfamethazine sorption to adsorbed HA generally increased with the abundance of carboxyl (and possibly other O- and N-containing) moieties and aliphatic carbon content. Both the smectite surface and the adsorbed humic acids contributed to sulfamethazine sorption-desorption hysteresis; hysteresis in sulfamethazine sorption to the adsorbed humic acids was equal to or larger than for adsorption to the smectites. Competitive sorption with a structurally similar sulfonamide antimicrobial was apparent only at high competitor concentrations. This study indicates that sorption to organic carbon and clay mineral surfaces as well as the hysteretic sorption behavior warrant consideration in predicting the transport of these antimicrobial agents in soils and subsurface environments.

Structural Organization of Humic Acid in the Solid State

Studies of the structural organization of humic acid in solution have suggested that it is composed of three fractions, two humic acid-like and one lipid-like, that have distinct roles in the process of its self-organization. The present work extends the study of humic acid's structural organization to the solid state by directly comparing its organization with a physical mixture composed of its three fractions using differential scanning calorimetry. Comparative measurements of the specific heat capacity as a function of temperature reveal differences between the original humic acid and the mixture with the same chemical composition. These differences provide direct evidence that humic acid's solid state structure is more than just a mixture of components and is determined by specific interactions between its components. This study indicates that humic acid in the solid state has a hierarchical or "structure within a structure" architecture. The lower-level structure is determined by the self-assembly of amphiphilic components of humic acid with lipids into a nanostructured composite material. A higher-level structure is formed by the association of this composite material with the nonamphiphilic components of humic acid.

Characterization of humic substances in landfill leachate and impact on the hydraulic conductivity of geosynthetic clay liners

A detailed characterization was performed on the humic substances present in landfill leachate derived from the older (10-year) and younger (6-month) municipal landfill cells at a site in Inchion, Korea. The characterization focused on the humic and fulvic acid components of the leachate, relying on information gleaned from the UV/visible spectroscopy, molecular weight distribution, and Fourier transform infrared spectroscopy. The effect of the leachates, and specific components of the leachates, on the hydraulic conductivity of a geosynthetic clay liner (GCL), was evaluated. The humic acid extracted from the older leachate was composed primarily of high molecular weight and aromatic compounds, which is typical for humic acids. However, the humic acid extracted from the younger leachate showed characteristics more similar with fulvic acids, indicating that the younger humic acid was at the initial stage of humification. The hydraulic conductivity of the GCLs to the humic and fulvic acids of the older and younger leachate was similar to those permeated with the distilled deionized water (DI). However, the hydraulic conductivity of the samples tested with the raw leachate was more than 200 times the DI value. This fact suggests that cations present in leachate, rather than humic substances, are the key factor in the increase of the permeability.

Molecular characteristics of humic acids extracted from compost at increasing maturity stages

The molecular composition of humic acids (HA) extracted from compost at increasing maturity stages was determined by off-line TMAH-thermochemolysis-GC-MS, in combination with solid-state nuclear magnetic resonance (NMR) and infrared (IR) spectroscopies. While spectroscopy measurements followed the bulk changes, thermochemolysis provided a detailed molecular variation of HA composition. Both thermochemolysis and spectroscopy indicated that polysaccharides, alkyl, cyclic, and aromatic compounds were the predominant components of HA, the stable fraction of compost. NMR dipolar dephasing (DD) experiments confirmed that HA extracts contained lignin in lower amount than its oxidized degradation products. The progressive compost maturity was reflected in HA extracts by a decrease of carbohydrate content and a selective preservation of hydrophobic alkyl molecules, such as medium- and long-chain fatty acids, aliphatic alcohols, linear hydrocarbons, and plant polyester derivatives, like long-chain alkyl dicarboxylic acids, and ω-hydroxyacids. Spectroscopy results showed a concomitant entrapment in HA of biolabile compounds, such as peptidic moieties. The wide range of identified lipid components and plant biomarkers may represent useful tools to trace origin, quality, and transformation of amended compost in soil ecosystems.

Quinone–hydroquinone complexes as model components of humic acids: Theoretical studies of their structure, stability and Visible–UV spectra

Humic substances present a geochemically and environmentally important, yet poorly characterized, component of dissolved organic matter. In the past they have generally been described as macromolecular polymers containing many different functional groups. Recently, it has been suggested, partly on the basis of new experimental data, that such materials are rather supramolecular in nature, consisting of smaller molecular units held together by noncovalent forces such as van der Waals forces and H-bonds. A perplexing difficulty in characterizing humic acids has always been that data expected to be informative, such as their Visible–UV spectra, were sadly lacking in structure. This has usually been explained using models in which ensembles of molecules are present characterized by either long-range charge–charge interactions or random short-range donor–acceptor interactions. Structural components resembling hydroquinone:quinone donor–acceptor complexes have been postulated to explain the near-IR and visible spectra of humic acids (Del Vecchio R., and Blough N.V. (2004) On the origin of the optical properties of humic substances. Environ. Sci. 38, 3885–3891). We have calculated structures, energetics and Visible–UV spectra for several different quinone and hydroquinone monomers and for donor–acceptor complexes formed between hydroquinone, H 2Q, the donor, and quinone, Q, the acceptor. Most of the Visible–UV spectral calculations are carried out using time-dependent density functional theory. For the monomers the calculated energies are in good agreement with experiment. We confirm that the absorption spectra of the D:A complexes have maxima at much lower energy than their monomeric components. These absorption energies are influenced by substituents on the aromatic rings, but are also sensitive functions of the distances between the aromatic rings. The importance of D:A complexes in generating a spectrum like that of natural humic acids is consistent with the model of Del Vecchio and Blough (2004). However, our calculations of reaction free energies indicate that the donor–acceptor interactions do not arise from free D:A complexes, but rather from forced or random close approach of D and A rings.

Behavior of two different constituents of natural organic matter in the removal of sodium dodecylbenzenesulfonate by O 3 and O 3-based advanced oxidation processes

The objective of this study was to analyze the role played by two components of natural organic matter (NOM), gallic acid (GAL) and humic acid (HUM), in the removal of the surfactant sodium dodecylbenzenesulfonate (SDBS) from waters by O 3-based oxidation processes, i.e., O 3/H 2O 2, O 3/granular activated carbon (GAC), and O 3/powdered activated carbon (PAC). It was found that the presence of low concentrations of these compounds (1 mg/L) during SDBS ozonation increases both the ozone decomposition rate and the rate of SDBS removal from the medium. Because of the low reactivity of SDBS with ozone, these effects are mainly due to an increase in the transformation rate of ozone into HO • radicals. Results obtained demonstrate that the presence of GAL and HUM during SDBS ozonation increases the concentration of O −• 2 radicals in the medium, confirming that GAL and HUM act as initiating agents of ozone transformation into HO •. It was also found that this effect was smaller with a larger molecular size of the acid. Presence of GAL and HUM during SDBS removal by O 3/H 2O 2, O 3/GAC, and O 3/PAC systems also increases the SDBS degradation rate, confirming the role of these compounds as initiators of ozone transformation into HO • radicals.

Multiple polar components in poorly-humified humic acids stabilizing free radicals: Carboxyl and nitrogen-containing carbons

To reveal the structural factors associated with free radicals in humic substances (HSs) with lower degrees of humification that are comparable to Type Rp, correlations between free radical content and chemical properties of 10 humic acids (HAs) and 2 fulvic acids (FAs) from different sources were investigated. Free radical contents of the HSs were analyzed using electron spin resonance spectroscopy (ESR) and ranged from 7.0 × 10 15–5.7 × 10 17spins g −1 carbon. The relative abundances for aromatic and/or olefin, alkyl, and carboxylic carbon species in the HSs were determined using solid state cross polarization magic angle spinning (CPMAS) 13C nuclear magnetic resonance spectroscopy (NMR). Aromaticity of the HSs estimated from 13C NMR varied slightly (30–40%) and no correlation was observed between aromaticity and free radical content. In contrast, a positive exponential relationship was observed between relative abundance of carboxyl groups and free radical content in HAs, while a negative exponential relationship was observed between relative abundance of alkyl carbon and free radical content. Two FAs were outliers in the correlations, probably due to the different chemical properties between HAs and FAs. Analysis of structural fragments using pyrolysis–gas chromatography–mass spectrometry (GC/MS) showed that additional chemical properties were also positively correlated with free radical content in HAs: two or more acidic functional groups-substituted phenolic compounds, short chain dicarboxylic acids, hydroxyacetic acid, nitrogen-containing compounds and heterocyclic carbonyl compounds. Possible semiquinone precursors, such as methoxyphenols, were not related to free radical content. These results show that free radicals are stabilized by combinations of multiple polar components such as carboxyl and nitrogen-containing carbons within HA structures during the early stages of humification.

Spectroscopic properties of humic acids isolated from the rhizosphere and bulk soil compartments and fractionated by size-exclusion chromatography

The humic acids (HAs) isolated by conventional procedure from rhizosphere (r) and bulk (c) soils were analyzed by means of chemical and physico-chemical techniques. Two different crops were selected, tomato (T) and artichoke (A), and each HA was fractionated by size-exclusion chromatography (SEC) into three fractions with increasing molecular size, respectively, Fraction I (FrI) < Fraction II (FrII) < Fraction III (FrIII). Elemental analysis data indicated greater N and S contents in the rhizosphere T-HAs, with respect to rhizosphere A-HA, which suggests the occurrence in the former ones of a large amount of organic nitrogen- and sulfur-containing compounds that are released by the rhizodeposition processes. Further, the three HA fractions from the bulk soils of the two series showed a gradual increase of C, H, and N contents, and a decrease of O and S contents and C/N and C/H ratios. These results suggested that the lowest molecular size fractions are richer in oxygenated functional groups, whereas the higher molecular size fractions are richer in N-containing groups and structural C- and H-containing units. The three HA fractions from the rhizosphere soils of the two series showed a gradual decrease in C content, and an increase of H, N, and O contents, which suggests the possible incorporation into soil HAs of a multitude of C-containing compounds of low molecular size released by plant roots. The FT-IR data, in general, suggested that the contents of carboxylic, phenolic and N-containing groups and polysaccharide-like components in HAs from rhizosphere soils are larger than those of HAs from the corresponding bulk soils. Further, the FrI fraction consisted mainly of simple structural units, likely quinonic and phenolic units with a prevalent aromatic character, whereas the FrII and, especially, FrIII fractions featured a mixed aliphatic/aromatic nature and a greater molecular complexity. The extent of these differences appeared to depend on the plant species and age, and is mainly due to the partial incorporation into rhizosphere HAs of typical root exudate components, such as amino acids, amides, aliphatic and aromatic acids of low molecular size, polysaccharides and sugars, fatty acids and sterols, and enzymes.

Exploring the high‐mass components of humic acid by laser desorption ionization mass spectrometry

Leonardite and Elliot soil humic acids have been analyzed by laser desorption ionization mass spectrometry (LDI MS) in the m/z 4000-200,000 range. Positive ion mass spectra for each humic acid obtained under optimum conditions showed a broad high-mass distribution between m/z 20,000 and 80,000. The dependence of the mass distribution on instrumental parameters and solution conditions was used to investigate the nature of the high-mass peaks from humic acid spectra. Our data suggests that macromolecular ions and humic acid aggregates have the same probability of occurrence while cluster ion formation has a low probability of occurrence.

Removal of humic acid using TiO 2 photocatalytic process – Fractionation and molecular weight characterisation studies

The photocatalytic removal of humic acid (HA) using TiO 2 under UVA irradiation was examined by monitoring changes in the UV 254 absorbance, dissolved organic carbon (DOC) concentration, apparent molecular weight distribution, and trihalomethane formation potentials (THMFPs) over treatment time. A resin fractionation technique in which the samples were fractionated into four components: very hydrophobic acids (VHA), slightly hydrophobic acids, hydrophilic charged (CHA) and hydrophilic neutral (NEU) was also employed to elucidate the changes in the chemical nature of the HA components during treatment. The UVA/TiO 2 process was found to be effective in removing more than 80% DOC and 90% UV 254 absorbance. The THMFPs of samples were decreased to below 20 μg l −1 after treatments, which demonstrate the potential to meet increasingly stringent regulatory level of trihalomethanes in water. Resin fractionation analysis showed that the VHA fraction was decreased considerably as a result of photocatalytic treatments, forming CHA intermediates which were further degraded with increased irradiation time. The NEU fraction, which comprised of non-UV-absorbing low molecular weight compounds, was found to be the most persistent component.

Distribution of As, Cr, Ni, Rb, Ti and Zr between peat and its humic fraction along an undisturbed ombrotrophic bog profile (NW Switzerland)

Ombrotrophic bogs are useful records of the impact of historical human activity on heavy metal contamination. Several studies concerning the trace element record (mainly Pb and Hg) in these particular environments have been carried out in recent years, although the role of humic substances has often not been considered. In particular, of the components of peat organic matter, fulvic acids and low molecular weight compounds are generally responsible for the mobility of trace elements through the profile, while humic acids (HAs) are involved in the formation of more stable organo-mineral complexes. In order to study the parallel distribution of As, Cr, Ni, Rb, Ti and Zr in bulk peat and the corresponding HAs, a peat core (10 × 10 × 81 cm) was collected from Etang de la Gruère (Switzerland) and cut into 27 slices of 3 cm. The samples were freeze-dried and milled very finely, and HAs extracted from each sample. Both peat and HAs were analyzed using an energy-dispersive miniprobe X-ray fluorescence multielement analyser (EMMA-XRF). Of the considered elements, Ni showed a great affinity for the humic acid component, while Cr was concentrated mainly into humic material from the deeper layers. On the other hand, Ti, Zr and Rb seemed to reflect the variation in mineral material both in peat and HA samples, while the As content of both materials reflected the environmental conditions characterizing the bog.

Oxidation of humic acids from an agricultural soil and a lignite deposit: Analysis of lipophilic and hydrophilic products

The composition of humic acids (HAs) isolated from an agricultural soil and a lignite deposit was examined via H 2O 2 and RuO 4 oxidation. The oxidation digests were separated into lipophilic and hydrophilic components. Information with regard to the source, degree of humification and preservation of easily degradable constituents of the HAs was obtained and results were compared with those obtained earlier for base hydrolysates of solvent-extracted fractions. H 2O 2 oxidation of both HAs afforded lipophilic fractions containing high molecular weight compounds. The composition of the base hydrolysates of the lipophilic fractions strongly differed with the origin of the HA. The lipophilic components of the soil HA derived mainly from the higher plant polyesters cutin and suberin. The lipophilic components of the lignite HA predominantly comprised long chain alkanoic acids and alkanols. The patterns for the hydrophilic components released upon H 2O 2 oxidation were found to be identical irrespective of the origin of the HA. The hydrophilic fractions comprised aliphatic (poly)carboxylic acids related to carbohydrate moieties and benzene polycarboxylic acids. The relative abundance of benzene polycarboxylic acids increased with the degree of humification. For both HAs, RuO 4 oxidation resulted in a lipophilic fraction containing low molecular weight products identical to those found in the base hydrolysate of the lipophilic fraction released upon H 2O 2 oxidation. The hydrophilic components released upon RuO 4 oxidation were independent of the HA origin and consisted mainly of monosaccharides and disubstituted aromatic compounds. In agreement with the greater aromaticity of lignite HA, the aromatic compound/carbohydrate ratio was higher for lignite HA than soil HA. The results show that the fused aromatic structures had a small size and that carbohydrates could escape degradation during the humification process.

31P NMR peak width in humate–phosphate complexes

The mobility of inorganic phosphate (P) attached to solid humic acid (HA) and fulvic acid (FA) via a metal “anchor” was investigated by 31P NMR spectroscopy. The peak width of the 31P resonance was monitored as an indicator of the degree of attachment of the element to the humic matrix. The concept was demonstrated by contrasting peak widths of thoroughly dry M–HA–P complexes with those that had been allowed to absorb different amounts of moisture. It was shown that the presence of moisture, which enhances the mobility of P, results in a significant reduction of chemical shift peak width. The work was extended to comparisons between dry systems with and without metal anchors; systems with anchors consisting of different metals; systems comprising different humates and fulvates; and systems with different size fractions of a humate. It was shown that both the type of humate/fulvate, and the metal anchor used lead to different degrees of mobility within the humic matrix. It was also found that the effect of metal addition on 31P peak width is greater with fulvates and smaller HA fractions than with the larger HA components.

Analysis of polynuclear aromatic and aliphatic components in soil humic acids using ruthenium tetroxide oxidation

SummaryAlthough condensed aromatic components are considered to be one of the major structural units of soil humic acids (HAs) and to be responsible for the dark colour of HAs, their amount and composition remain largely unknown. In ruthenium tetroxide oxidation (RTO), condensed aromatic components are detectable as their degradation products, mainly benzenepolycarboxylic acids (BPCAs). We applied this technique to soil HAs with various degrees of humification (darkening). The yields of water‐ and dichloromethane‐soluble products from HAs upon RTO after methylation ranged from 210 to 430 mg g−1 and 10–40 mg g−1, respectively. Eight kinds of BPCAs with two to six carboxyl groups, and seven kinds of BPCAs with additional side chains (tentative assignment) were obtained as methylated counterparts. The yield of each BPCA and the sum of the yields of BPCAs (12–85 mg g−1 HAs) increased with increasing degree of humification and aromatic C content. The compositions of BPCAs indicated that the degree of condensation was greater in the HAs with greater degrees of humification. The sum of the yields of aliphatic compounds ranged from 0.1 to 6.5 mg g−1, and decreased with increasing degree of humification. The C12 to C30 monocarboxylic acid methyl esters accounted for &gt; 56% of the aliphatic compounds assigned, which may be present mainly as end alkyl groups in the HA molecules. We also obtained the methylated counterparts of C14 to C24 dicarboxylic acids; these were possibly derived from polymethylene bridges between adjacent aromatic rings.