Coal, humic fraction

Jince the time of Berzelius, chemists have proposed structures for the amorphous, black substance known as humic acid. In the past 150 years, much experimental work has appeared on the nature of humic acid, most of it based on classical chemical and microbiological studies. Very little information about the molecular structure of humic add has resulted from these studies however. Some of the problems plaguing investigators in this field have been (a) variation in the source of humic acid, (b) variation in the definition of humic fractions of soil and coal, (c) lack of crystallinity of the samples, (d) uncertainty of molecular weight measurements, (e) variation in extraction techniques, and (f) variation in elemental composition. The little unambiguous information that exists today is based on extensive degradation of the humic acid polymer and represents only a small fraction of the total molecule. 

It is suggested here that the greater insolubility of the humic fraction of coal may be the result, in part, of polymerization by complexing with metals, particularly aluminum and silicon. These elements are suggested because of their presumed greater availability. The minor elements also play a part, however, only in relation to their availability compared with major elements such as aluminum and silicon. 

The conditions of pyrolysis either as low or high temperature carbonization, and the type of coal, determine the composition of Hquids produced, known as tars. Humic coals give greater yields of phenol (qv) [108-95-2] (up to 50%), whereas hydrogen-rich coals give more hydrocarbons (qv). The whole tar and distillation fractions are used as fuels and as sources of phenols, or as an additive ia carbonized briquettes. Pitch can be used as a biader for briquettes, for electrode carbon after coking, or for blending with road asphalt (qv). 

Piccolo, A., Celano, G., and Pietramellara, G. (1993). Effects of fractions of coal-derived humic substances on seed-germination and growth of seedlings (Lactuga sativa and Lycopersicon esculentum). Biol. Fertil. Soils 16(1), 11-15. 

Various separation methods have been used to isolate, fractionate, and characterize humic materials. Originally it was fractionation, based on solubility differences of humic components in diluted alkalis and acids, which laid the ground work for the first classifications of humic substances (HS) in the 19th century (Mulder, 1861 Sprengel, 1837) and provided for operational definition of HS (Kononova, 1966). And now, alkali extraction is the method of choice for isolating HS from solid humus-containing substrates like soil, peat, coal, and so on (Swift, 1996), while hydrophobic resins (e.g., Amberlite XAD resins) are typically used to extract HS dissolved in natural waters (Aiken, 1985). Initial research on HS began with the used simple separation methods to prove, examine, and define characteristics of components of humic matter (Oden, 1919).Today, however, advances in HS research require ever more sophisticated techniques of separation combined with structural analysis (Orlov, 1990 Stevenson, 1994). 

Nine different chlorophenoxyalkanoic acids and esters (Pestanal grade, > 99%, Riedel-De-Haen AG Seelze-Hanover, West Germany) (Table 1) and 10 humic acid (HA) samples of different origin and nature, have been examined in this study. Traditional procedures of extraction, fractionation, and purification were used for the isolation of the humic acids of natural origin (soil, peat, coal, anlaboratory-prepared by oxidative... 

The particle size distribution for the humic acid fraction is depicted in Figure 4. No material sedimented out until the most extreme conditions were applied (40,000 rpm for 24 hr), when some lightening of color at the top of the solution was observed. The sedimented particles had a Stokesian diameter of around 2 nm, which means that a particle size gap of three orders of magnitude exists between these and the next largest particles detected (5 xm). From the experimentally determined coal particle density of 1.43 g/cm, it was calculated that a solid sphere of diameter 2 nm would have a molecular mass of 4000. If the molecules were rod-shaped, even smaller molecular masses would be predicted. Literature values of the molecular mass of regenerated humic acids range between 800 and 20,000, with the values clustering around 1,000 and 10,000 (i5, 16, 17). 

Since the humic acid fraction constitutes 30% of the dry coal mass, about one-third of the coal is in the form of small macromolecules that are bound to the coal structure with bonds weak enough to be disrupted by dilute alkali. 

The most serious problem in the study of humic substances is the lack of reproducibility of analytical results. One would expect soil humates to vary with soil type, aquatic humates to vary with water sources, and coal humates to vary with coal rank. But even within one well-defined source, the elemental composition will vary between samples, depending on extraction and fractionation procedures. There are cases in which the same authors have used the same source and the same extraction procedure and have obtained significantly different elemental analyses. Before any meaningful structural conclusions can be deduced from elemental analysis, a rational definition of humic substances will have to be established (MacCarthy, 1976 Malcolm and MacCarthy, 1979). 

Humins (humus coal) are soluble neither in alkalies nor in acids. This group in humic substances have been very little investigated. They are characterized as an insoluble form of humic acids. Humins are considered as the oldest, and ultimate products of the humification process. They are assumed to be strongly carbonized substances, where the high stability is increased by a strong bond to the mineral fraction. Due to their high chemical stability or even inert character, humins do not participate in the soil formation process and they do not fulfil the function of the true humus. 

Humus acids are a main component extracted by water from organic matter of rocks and deposits, first of all from soil and peat. The content of humic acids reaches in soils 10% by mass, in peats 25-50%, in coals up to 60%. The contents of fulvic acids - up to 15% in peats and 60% in coals. Relative contents of humic and fulvic acids in soils depend on their properties. In tundra and bleached forest soils dominate fulvic and in peats, chestnut and black-earth soils, humic acids. Frequently (but not always), with depth fraction of fulvic acids increases. 

Humic substances Humic acids were obtained from brown coals (Tatabdnya, Hungary, and Steinberg, FRG) by extraction with 0.1 mol dm NaOH. After the usual purification process, the sample from Tatab ya was fractionated with NaCl as described in [32]. The total acidity (method in [32]) and elemental composition of these humic acids are listed in Table 2. Humic acid solution was made by passing Na-humate solution through a column of hydrogen cation-exchange resin. The Na-humate solution used in the experiments was made by neutralization of the humic acid solution with NaOH. 

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