Molecular Structure Of Humic Acids Research Articles

Combined application of non-discriminated conventional pyrolysis and tetramethylammonium hydroxide-induced thermochemolysis for the characterization of the molecular structure of humic acid isolated from polluted sediments from the Ravenna Lagoon

Humic acid (HA) isolated from highly polluted sediment from the Ravenna Lagoon (Italy) was subjected to pyrolysis/tetramethylammonium hydroxide (TMAH)-induced thermochemolysis to reveal the impact of industrial activities on humification. Special effort was made to distinguish between analytes originating from the polymeric humic organic matter network along with sequestered compounds (which cannot be released by solvent extraction), and the solvent-extractable lipid fraction sorbed onto the organic matrix. Exhaustive solvent extraction of the isolated HA proved mandatory to avoid biased results when identifying the origin of the pyrolyzates of untreated samples. Conventional pyrolysis at 750 °C of the “degreased” HA revealed a characteristic polycyclic aromatic hydrocarbon (PAH) pattern, which was significantly different than patterns obtained from the pyrolysis of natural humic acids. TMAH-induced thermochemolysis at 500 °C provided information on carboxylic acids covalently bound inside the HA network via ester bonds. Thermochemolysis of the “degreased” matrix at 750 °C, resulting in the cleavage of C–O and C–C bonds, revealed a significant PAH pattern very similar to that obtained by conventional pyrolysis. The uncommon PAH pattern points to the formation of bound residues as the humification/detoxification pathway. Recalcitrant hopane hydrocarbons showed very similar patterns for both the untreated and exhaustively solvent-extracted samples, which points to hopanes being linked inside the humic network via C–C bonds. The technique of non-discriminating flash pyrolysis used in this study, based on extremely rapid capacitive discharge heating in a Silcosteel capillary, proved to be an excellent method to obtain high-boiling pyrolyzates (PAHs beyond 252 Da molecular weight, C 29–C 31-hopanes).

Chemolytic and solid-state spectroscopic evaluation of organic matter transformation during vermicomposting of sugar industry wastes

The molecular structure of humic acid (HA) extracted was investigated by FT-IR and 13C CP/MAS NMR spectroscopy during the vermicomposting of sugar industry wastes, viz. pressmud, trash and bagasse for 60 days. A rapid decrease in C/N and lignocellulosic (lignin, cellulose, hemicellulose) content was observed in vermicompost during early phase of the process. The FT-IR and 13C CP/MAS NMR spectra of HA indicated a high rate of change in structure with increase in the alkyl C/O-alkyl C ratio during the process. Aromatic structures and carboxyl groups showed an initial increase but decreased after ∼40 days indicating extensive mineralization during final stages of vermicomposting.

Study of the organic matter evolution during municipal solid waste composting aimed at identifying suitable parameters for the evaluation of compost maturity

In this work the composting process of municipal solid wastes was studied in order to characterize the transformations of organic matter, particularly humic acid (HA). A composting process, lasting three months, was monitored by chemical methods; the following parameters were measured: water-soluble carbon concentration (WSC) and humic substances content (humic and fulvic acid (FA)). The effects of humification on the molecular structure of humic acid (HA) were also evaluated by Fourier transform infrared (FT-IR) and 13C NMR spectroscopy. WSC concentration rapidly increased reaching a maximum at day-14 of the composting process and then declined. The humic and fulvic acid content (HA and FA, respectively) slightly increased during the process. The FT-IR and 13C NMR spectra of HA indicate a high rate of change in structure during composting. The groups containing aromatic and carboxylic C increased, while polysaccharides and other aliphatic structures degraded during composting, resulting in HA structures of higher aromaticity. Therefore, spectrometric measurements could provide information significantly correlated to conventional chemical parameters of compost maturity.

Effect of shelterbelt on two kinds of soils on the transformation of organic matter

The changes of the total nitrogen, average yearly concentration of ammonia, nitrate ions, urease activity, total amount of amino acids in humic acids (HA) and the structure of humic acids were monitored in relation to the effect of shelterbelt as biogeochemical barrier located on a mineral and mineral-organic soil. The transformation of different forms of nitrogen in the soil under shelterbelt was strongly connected with the humification process and the molecular structure of humic acids.

Spectroscopic Characterization of Pyrophosphate Incorporation during Extraction of Peat Humic Acids

AbstractExtraction procedures represent a crucial point in the study of the molecular structure of humic substances (HS). Although alkaline extractants such as NaOH plus Na4P2O7 are usually employed for the extraction of the HS from soil and peat, little information concerning the interaction between extractants and humic acids is available. In this study, the results on the molecular structure of humic acids (HA) extracted and separated from a Sphagnum Irish peat sample with an alkaline pyrophosphate solution (0.1 M NaOH + 0.1 M Na4P2O7) are reported. The HA were redissolved in 5 mM NaCl, fractionated in six cut‐off fractions (nominal molecular weight: HA >300, 100–300, 50–100, 20–50, 10–20, and 5–10 kDa) using a tangential ultrafiltration system. All samples were dialyzed against distilled water and then freeze‐dried. Fourier‐transform infrared (FT‐IR) and 31P nuclear magnetic resonance (NMR) spectroscopies were applied to characterize the incorporation of pyrophosphate (PP) in both the unfractionated HA and in the six HA fractions. Spectroscopic analyses showed that it was possible to assign a given pattern of bands to define the structure of phosphate: PP was present only in the unfractionated HA, whereas orthophosphate was present in both the high molecular weight and lower molecular weight fractions. The high concentration of phosphate found in the lower molecular weight HA fractions has been attributed to the presence of metals and the oxygenated groups of HA. The changes in the relative intensity of the bands attributed to oxygenated functional groups observed in the FT‐IR spectra could be assigned to the interaction of phosphate groups with metal cations, which are involved in a coordination complex with the oxygenated groups.

Effects of the extraction temperature on the characteristics of a humic fertilizer obtained from lignite

Solutions of 0·1 m and 0·25 m potassium hydroxide were employed to extract a humic fertilizer from lignite at 20, 50 and 80°C. The effects of the temperature on the yields of humic and fulvic acids and on the characteristics of the former were investigated. Higher extraction temperatures did not significantly enhance yield in either case, and neither were the ratios between humic and fulvic acids significantly affected. Humic acids released by hot 0·25 M KOH solutions held less N-rich material and exhibited a lower content of functional groups than those obtained at 20°C. When 0·1 M KOH was employed at 50 and 80°C, changes in the molecular structure of humic acids were less apparent but there were fewer functional groups than in the humic acids obtained at 20°C. Increasing extraction temperatures were matched by the larger molecular size of humic acids extracted with both KOH solutions.

A tentative approach to the molecular structure of humic acids: the spectral evidence for a derivation of humic acids from plant-borne esters. 3. Pyrolysis-mass spectrometry.

Pyrolysis mass spectrometry of humic acids collected from oak forest litter and humus horizons, of esters extracted from oak leaves and of bark extracts indicated a close relationship between plant esters and humic acids. The data support the hypothesis that humic acids can be derived from simple mixed esters formed in the plants. (Abstract retrieved from CAB Abstracts by CABI’s permission)

A tentative approach to the molecular structure of humic acids: The spectral evidence for a derivation of humic acids from plant-borne esters. 2. Infrared and chemical analyses.

It is suggested that: humic acids are polyesters, originate as monomers in the secondary metabolism of both higher and lower plants, and are formed in an acid environment (including high-moor peat); the formation of humic acids is promoted by desiccation, particularly under alkaline conditions; humic acids exhibit keto-enol tautomerism, can form semi-quinones and therefore stable radicals also, and derive their colour from these radicals. (Abstract retrieved from CAB Abstracts by CABI’s permission)

A tentative approach to the molecular structure of humic acids: the spectral evidence for a derivation of humic acids from plant-borne esters. 1. The electron paramagnetic resonance (EPR) spectra.

It is suggested, from the evidence of EPR spectra, that humic acids are heterogeneous polymers of mixed esters derived from tannins. (Abstract retrieved from CAB Abstracts by CABI’s permission)