In a previous study, we studied the ability of native soil bacteria to biotransform a lignite-type LRC, which is generated in Colombian open cast coal mining. Three outstanding bacterial strains in the production of HS from LRC were selected and identified; these isolates were denominated coal solubilizing bacteria CSB. On the other hand, it has been found that the effects on soil properties and bioactivity of HS are related to the structural characteristics; 27 in addition, it has been observed that the HS extracted from LRC through conventional extraction generate plant physiological responses that are lesser than those generated by HS extracted from compost or vermicompost, which have been the object of greater interaction with microorganisms that can influence the structure and properties; 12 other characteristics such as hydrophobicity degree also have been related to bioactivity.
According to the above, the objective of this study was to characterize the supramolecular structures of the HA fraction obtained with CSB activity on LRC HA-CSB , as compared with HA obtained with the traditional alkaline extraction method HA-NaOH as a basis for future studies on the effects in the soil structure, such as plant growth and development, or indirect effect on plant growth, promoting microorganisms.
This as a strategy of soil management in agriculture or in disturbed soils with a low organic carbon content, or in constructed Technosols, in post-coal mining land reclamation activities using LRC as a source of HOM that is susceptible to being released by soil microbiota, as well as applications for coal transformation for other industrial or environmental purposes. Subsequently, the cultures were centrifuged at rpm for 15 min, the pellet of bacterial biomass was discarded, and the supernatant was subjected to a centrifugation procedure three times; the brown dark supernatant was filtered with Whatman paper, 2.
Microbial Conversions of Low Rank Coals
The THE was treated with 0. The 13 C NMR spectroscopy was carried out in the solid state using a Bruker AV spectrometer, equipped with a 4 mm thick probe; the NMR spectra were obtained applying the following parameters: rotational speed of the rotor of Hz, 1 s recycle time; contact time of 0. To correct the dipolar dephasing, a time delay of 0. The development of the NMR spectra was carried out with Mestre-C version ; the Fourier transformation was applied and the base line of each spectrum was manually corrected.
For the interpretation of the 13 C-CPMAS-NMR spectrum, the chemical shift recorded between 0 and mg L -1 was divided into the following regions of resonance: C-alkyl mg L -1 ; methoxy C- and N-C mg L -1 ; C- O -alkyl mg L -1 ; aromatic C mg L -1 ; C-carboxyl and carbonyl mg L -1 , then the integration of the area under the curve in each of the intervals was carried out, thus the quantitative data on the relative percentage of 13 C in each group were obtained.
The gaseous products derived from the thermochemolysis were collected using a He stream of 10 mL min -1 and transferred to two serial containers with mL of chloroform in each one, which were kept in an ice bath. After 30 min of pyrolysis, the contents of the two containers with the chloroform solution were combined and carried to rotoevaporation, then the residue was dissolved in 1 mL of chloroform and transferred to storage in a 1 mL glass vial; each sample was treated three times.
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The identification of the compounds was based on comparison of the mass spectra with the spectra published in the database library of the NIST and real standards. With the data for the presence or absence of each compound identified in each type of HA and LRC, a principal component analysis was carried out. To run the samples, a sodium phosphate buffer NaH 2 PO 4 , 0.
The chromatograms were recorded on a PE-Nelson Series interface and PerkinElmer software to integrate the area under the curve of the recorded peaks. To evaluate the reproducibility of the analysis, each sample was run three times. To calculate the average number of the different molecular weights Mn of the supramolecules of HA, the average molecular weight Mw and polydispersity P , a standard curve was constructed with the recorded values of these three parameters using solutions from a polymer of standards of sodium polystyrene sulfonate with molecular weights of , , , and Da.
The results were subjected to variance analysis and comparison of means with the Dunnett method. The LRC exhibited These results confirmed the characteristics of a lignite-type LRC with high moisture content and volatile materials, and calorific power under kcal kg The ash minerals had values of 4. The results of elemental composition indicated that the alkaline extraction generated a HA that was more oxidized, maybe because of the typical formation of OH— fragments and carboxyl groups after the breakup of the ester and ether bonds of the LRC structure; while in the HA-CSB, there was enrichment in nitrogen and carbon content.
The above findings suggested the occurrence of some degree of structural modification of the HA released from the LRC with CSB activity, but the modifications of the HA structure did not alter the maturity. In the FTIR analysis, some differences between the spectra are observed; the OH band in alcohols and phenols is much more pronounced in HA obtained from Microbacterium sp. A band at cm -1 was seen in the HA samples of A.
Microbial Transformations of Low Rank Coals
Generally, the qualitative differences observed in the FTIR analysis were not relevant with regard to the predominant organic group in the macrostructure of the two types of HA. However, evidence of protein derivatives in the HA-CSB, in line with the results of the elemental composition, indicated that the bacterial activity on the LRC generated HA-enriched N, C and H, but with a lower oxygen content; this result showed CSB activity in the oxygen functional groups, which are the most reactive groups within the HS.
Perhaps this fact is a direct consequence of the microbial metabolism that causes structural changes in HS. It has also been demonstrated the incorporation of N into the molecules of amino sugars and amino acids present in HA obtained with biodegradation of LRC.
The spectra of the HA-CSB had larger peaks in the region corresponding to the carboxylic groups, with a predominant content of aromatic groups, smaller peaks between and 60 mg L -1 corresponding to the region of polysaccharides, alcohols and aminosugars , a peak at about 60 mg L -1 corresponding to amino acids, a small peak corresponding to methoxy groups, and, in the region of the aliphatic groups, larger peaks than the HA-NaOH.
The three HA-CSB had a decreased aromatic carbon content and an increased carbon content in the alkyl groups, corresponding to nitrogen compounds; they also had an increased carbohydrate content and di-oxy alkyl groups and aliphatic compounds, in comparison to the HA-NaOH. These differences suggest that CSB can act metabolically on the original structure of the HA contained in the LRC and partially modify the structure, mainly by acting on the polyaromatic nucleus. Previously, Dong and Yuan 35 found that the HA obtained with alkaline extraction from LRC contained greater amounts of aromatic structures in comparison with HA obtained with Penicillium sp.
The lower aromaticity index for HA-CSB compared with HA-NaOH, together with other described features, provides a more hydrophilic character for the HA-CSB; this feature could determine certain implications in interactions with other soil components; for example, less recalcitrance against biological activity, according to Kleber, 38 a fact that may involve stimulation of microbial activity.
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However, a more hydrophilic characteristic of the HA, given by decreasing the aromatic condensation, can reduce the effect on the conformation of soil aggregates, which are determinant for the presence of HS with a more hydrophobic characteristic, as has been previously described.
Nardi et al. Dobbss et al. The pyrogram of HA-NaOH retained a similar pattern to that of the original LRC, with changes in the composition and magnitude peaks of compounds of lower molecular weight, which were present at the start of the chromatogram, and the aliphatic linear compounds that were high in molecular weight, which were recorded at the end of the chromatogram with a greater variety, but lesser minor amounts. The thermochemolysis of the studied HA released 77 different molecules, identified by characterizing their mass spectra; most of them were identified as methyl esters and esters of natural compounds.
The most abundant compounds were lipid hydrophobic molecules in large aliphatic and alicyclic Lp type plant-derived compounds Bp and units derived from lignin residues Lg , a group of products represented by protein-derived molecules and other nitrogenated compounds Pr and a group represented by polymeric byproducts with a microbial origin Mic Table 3. Based on maceral content and its appearance in a hand specimen, coal is classified into four principal types: clarain , durain , fusain , and vitrain.
Coal may also be classified in grades using subjective terms e. Coal classification. Info Print Cite. Submit Feedback. Thank you for your feedback. Written By: Otto C. See Article History. Start your free trial today for unlimited access to Britannica.
Learn More in these related Britannica articles:. Coal , one of the most important primary fossil fuels, a solid carbon-rich material that is usually brown or black and most often occurs in stratified sedimentary deposits.
Coal is defined as having more than 50 percent by weight or 70 percent by volume carbonaceous matter produced by…. Coke , solid residue remaining after certain types of bituminous coals are heated to a high temperature out of contact with air until substantially all of the volatile constituents have been driven off. The residue is chiefly carbon, with minor amounts of hydrogen, nitrogen, sulfur, and oxygen. Also present in coke…. Peat , fuel consisting of spongy material formed by the partial decomposition of organic matter, primarily plant material, in wetlands such as swamps, muskegs, bogs, fens, and moors.