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Humic Substances and Nitrogen-Containing Compoundsfrom Low Rank Brown CoalsAyhan Demirbas a , Yakup Kar a & Huseyin Deveci aa Selcuk University, Konya, Turkey

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Energy Sources, Part A, 28:341–351, 2006Copyright © Taylor & Francis Group, LLCISSN: 1556-7036 print/1556-7230 onlineDOI: 10.1080/009083190890111

Humic Substances and Nitrogen-ContainingCompounds from Low Rank Brown Coals

AYHAN DEMIRBASYAKUP KARHUSEYIN DEVECI

Selcuk UniversityKonya, Turkey

Coal is one of the sources of nitrogen-containing compounds (NCCs). Recovery ofNCCs from brown in high yield was carried out from tars of stepwise semicokingof brown coals. Humic acids have been shown to contain many types of nitrogencompounds. Humic acids are thought to be complex aromatic macromolecules withamino acids, amino sugars, peptides, and aliphatic compounds that are involved inthe linkages between the aromatic groups. Humic acids extracted from peats, browncoals, and lignites, are characterized using different techniques. Humic substances(HSs) have several known benefits to agriculture. The properties of humic substancesvary from source to source, because they are heterogeneous mixtures of biochemi-cal degradation products from plant and animal residues, and synthesis activities ofmicroorganisms. HSs have been considered to be a significant floculant in surfacewater filtration plants for the production of drinking water as well as the processingof water. HSs are produced from chemical and biological degradation of plant andanimal residues and from synthetic activities of microorganisms.

Keywords brown coal, humic substances, nitrogen-containing compound, humaterecovery

In recent years, attention has focused on the subject of humus and humates from car-bonaceous materials such as peat, lignite and low rank brown coal. Humus is a com-plex aggregate of brown to dark colored amorphous substances that originated duringthe decomposition of plant and animal residues by microorganisms, under aerobic andanaerobic conditions, usually in soils, composts, peat bogs, and water basins. They aregenerated by the microbiological and chemical degradation and transformation of organicmatter resulting in chemical structures which are more stable than the starting material(Beck et al., 1993). Humic substances (HSs) are ubiquitous natural materials occurring inhuge amounts in soils, sediments and waters as a product of the chemical and biologicaltransformation of animal and plant residues (Janos, 2003). There are many reasons whyhumates work. Their importance in agriculture and soil sciences has been acknowledgedfor over 150 years. There is basic agreement on the benefits of humus, but there is quite

Address correspondence to Prof. Ayhan Demirbas, Selcuk University, Muhendislik MimarlikFaculty, Department of Chemical Engineering, 42031 Konya, Turkey. E-mail: ayhandemirbas@hotmail.com

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a controversy about the benefit of application of applied humate (Ziechmann, 1993).There has been increasing industrial interest in lignite-based humic acids as chemicalsource material used in the fertilizer industry (Yildirim, 2003).

In earlier studies (Demirbas, 2002; Demirbas, 2003), humic acid derivatives (HAD)from low rank brown coals and humic substances from five lignite samples were in-vestigated by gas chromatographic methods. The conversion and product fractionationobtained from supercritical benzene extraction of five Turkish lignite samples have beeninvestigated. The lignite samples differ considerably in mode of occurrence and in theirphysical and chemical properties, thus there is variation in the amount of humic acidsfound in different deposits. Humic substances were extracted from the lignite samplesusing benzene or toluene solvents in supercritical conditions. Phenolic substances suchas humic acid derivatives were characterized with a Gas Chromatography-Mass Spec-trometry (GC-MS) combined system (Demirbas, 2003).

Humic acids have been shown to contain many types of nitrogen compounds. Oneof the major approaches for characterizing the nitrogen in humic substances is by acid orbase hydrolysis (Stevenson, 1982). More recently, chromatographic, spectrophotometric,and x-ray analyses have added much to our knowledge about the organic structural groupspresent in humus (Okudan et al., 1998). Humic acids are thought to be complex aromaticmacromolecules with amino acids, amino sugars, peptides, and aliphatic compoundsinvolved in linkages between the aromatic groups. Humic substances contain small quan-tities of polyphenolic compounds. Their importance in agriculture and soil sciences hasbeen acknowledged for over 150 years. The brown coal or lignites differ considerably inmode of occurrence and in their physical and chemical properties, thus the variation inthe amount of humic acids found in different deposits. Separation methods are widelyused to isolate humic substances (HSs), to fractionate them before further investigation,and to obtain information about their structure and properties (Janos, 2003). To separateand identify the phenols from humic acids of the lignites, GC Hewlett-Packard 5790 andMS-VG 70-250-SE were used (Demirbas, 2002).

Since humic substances originate from the chemical and biological degradation ofplant and animal residues and from metabolic activities of microorganisms, they mightbe expected to show hormonal character (Young and Chen, 1997).

Coal is one of the sources of nitrogen-containing compounds (NCCs). NCCs arevaluable raw materials for preparing a wide set of pesticides, pharmaceuticals, dyes, heat-resistant fibers, special-purpose rubbers, ion-exchange resins, flotation agents for rare andrare-earth metals, surfactants, and vitamins. Recovery of NCCs from brown in high yieldwas carried out from tars of stepwise semi-coking of brown coals (Platonov et al., 2001).Table 1 shows the yields of organic bases and nitrogen-containing compounds fromtars of stepwise semicoking of lignite. Humic acid and humin contain 2–6% nitrogen,whereas the nitrogen content of fulvic acid ranges from <1 to 3% (Schnitzer and Preston,1986). Table 2 shows carbon, hydrogen, and nitrogen contents of humic acid samplesfrom compost, peat, and soil samples by different methods (Simpson et al., 2003). Theelemental compositions of humic substances and several plant material are given inTable 3 (Okudan et al., 1998). The proximate analysis of humic acid (dry basis) is givenin Table 4 (Okudan and Kara, 2001).

Status of Humic Materials

Figure 1 shows the status of humic materials in soil, peat, lignite, leonardite, and bio-waste. Organic matter in the soils and sediments exists in three different forms or states.

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Table 1Yields of organic bases and nitrogen-containing compounds from tars of

stepwise semicoking of lignite

Group of compound Yield (wt% based on whole fraction)

Pyridine 0.50–2.45Picolines 6.82–11.65Lutidines 8.50–13.95Ethylpyridines 0.90–1.65C3-Pyridines 6.65–9.45>C4-Pyridines 5.00–8.52Quinolines 1.95–8.45Isoquinolines 0.95–4.45Benzoquinolines 0.65–5.55Aromatic amines 1.95–2.50Indoles 2.45–6.25Carbazoles 0.65–2.85Polycyclic nitrogen-containing compounds 0.95–5.62Aliphatic amines 0.85–1.90Hydrogenated quinolines 8.90–13.45Hydrogenated benzopyrroles 9.30–15.35

Source: Platonov et al., 2001.

Table 2Carbon, hydrogen, and nitrogen contents of humic acid

samples from different sources by different methods (wt%)

Sample C H N

Compost humic acids 50.6–65.4 4.8–6.5 2.9–7.5Peat humic acids 55.2–68.7 3.7–6.5 2.5–9.4Soil humic acids 34.1–60.3 3.7–6.5 2.5–9.4

Source: Modified from Simpson et al., 2003.

Table 3Elemental compositions of humic substances and

several plant materials (wt% daf)

Substance C H O N

Fulvic acids 44–49 3.5–5.0 44–49 2.0–4.0Humic acids 52–62 3.0–5.5 30–33 3.5–5.0Proteins 50–55 6.5–7.3 19–24 15.0–19.0Lignin 62–69 5.0–6.5 26–33 —

Source: Okudan et al., 1998.

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Table 4Proximate analysis of humic acid (wt% dry basis)

Volatile material Fixed carbon Ash Total

42.7 44.9 12.4 100.0

Source: Okudan and Kara, 2001.

They are: (a) living plant and animal matter, (b) dead plant and animal matter, and(c) decomposed plant and animal matter or humus. The term “humus” dates back tothe time of the Romans, when it was frequently used to designate the soil as a whole.Wallerius first defined “humus” in 1761 in terms of decomposed organic matter.

Humus is the product of the decay of organic matter, and it contains both humic andnonhumic material. Nonhumic matter is relatively undecomposed organic matter, and isrelatively insoluble in water. Humic matter is completely decomposed organic matter,and is readily soluble either in acids or in bases. Humic acids may be separated from

Figure 1. Forms of dead organic matter in soil, peat, lignite, leonardite, biowaste.

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Humic Compounds from Lignites 345

humic matter by alkaline extraction. Humic acid is the acid radical found in humic matterwhich is soluble in alkali but insoluble in acid, methyl ethyl ketone, and methyl alcohol.Humates are the salts of humic acids. Fulvic acid is the acid radical found in humicmatter which is soluble in alkali, acid, and methyl ethyl.

Humic acid is primarily found in manure, peat, lignite coal, and leonardite. Leonarditeis a soft brown coal-like deposit usually found in conjunction with deposits of lignite.Highly oxidized lignite was called leonardite after the geologist, Dr. A. G. Leonard, whodiscovered it. Leonardite has been oxidized by nature, resulting in a highly active humicacid at reasonable cost. Humic acid contents of leonardite are about 85%. Humic acidextracted from leonardite is an excellent balance of effectiveness and low cost.

Commonly, HSs are operationally subdivided according to their solubility into hu-mic acids (HAs) and fulvic acids (FAs). HAs comprise high-molecular-mass organicsubstances that are soluble in alkaline media (e.g., in 0.1 mol/l NaOH) and insolublein acidic media (at pH 1–2), whereas FAs comprise moderate-molecular-mass organicsubstances of nonspecific composition that are soluble at all pH values. The portion oforganic matter present in soils and sediments that is insoluble at any pH value is calledhumin. Strongly alkaline extraction agents, typically aqueous solutions of NaOH (Okudanet al., 1998; Demirbas, 2002; Demirbas, 2003), are used for the isolation of HSs fromsoils and sediments, as well as from coals and peat.

Humic acids are thought to be complex aromatic macromolecules with amino acids,amino sugars, peptides, and aliphatic compounds that are involved in the linkages be-tween the aromatic groups. The hypothetical structure for humic acid contains free andbound phenolic OH groups, quinone structures, nitrogen and oxygen as bridge units, andcarboxylic acid (COOH) groups variously placed in aromatic rings. Humic substancesare comprised of three operationally defined fractions: water soluble fulvic acids, acid-precipitated humic acids, and water insoluble humans.

Humic substances appear to exist as polyfunctional macromolecules and constantlychange their structural conformations due to interactions involving carboxylic acid andphenolic and alcoholic hydroxyl functional groups (Okudan et al., 1998; Okudan andKara, 2001).

Coal represents a number of humus types in an advanced state of decomposition,produced from various plant residues at different periods during prehistoric times andlater stratified and compressed by superimposed layers of mineral matter (Stevenson,1982). Humic fertilizers can be recovered from brown coal. Humic substances work formany reasons, depending on soil and environmental conditions. The consensus is thatthey work well in low organic matter soils. In low amounts they do not perform in soilsof high organic content, and at high rates they seem to tie up soil nutrients.

The procedure used to isolate humic acids from young brown coal has previouslybeen described in detail (Hänninen et al., 1986). Brown coal was air dried, ground, passedthrough a 2-mm sieve and sequentially solvent extracted with water (24 h) and chloroform(24 h). A 10 g portion of washed, solvent-extracted peat was extracted 10 times with100 ml portions of fresh 0.1 M NaOH. The extracts were combined and centrifuged for2 h at 14,000 rpm. The supernatant was acidified with 6 M HCl to pH 2. Humic acidswere isolated by centrifugation (4500 rpm, 5 min), redissolved in minimal amounts of0.1 N NaOH, and purified by dialysis against distilled water. Purified humic acids weresubsequently freeze-dried, weighed, placed in mini bags, and stored in the dark.

Ash content determination and functional group analyses were carried out accord-ing to Hänninen (1987). More recently, chromatographic, spectrophotometric, and x-rayanalyses have added much to our knowledge about the organic structural groups presentin humus (Okudan and Kara, 2001).

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In water they have colloidal properties and the ability to form water-soluble andwater-insoluble complexes with metal ions and hydrous oxides and to interact with clayminerals and organic pollutants (Beck et al., 1993; Christman and Gjessing, 1983). Manyinvestigations have been done to study the processes of complexation, aggregation andsorption of humic substances (Perdeu and Gjessing, 1990; Eggins et al., 1997).

Upgrading Studies on Lignites from Different Resources

Coal researchers may conduct lignite (brown coal) upgrading investigations on 14 broadpathways: combustion, cofiring with natural gas or biomass, desulfurization and oxy-desulfurization, pyrolysis and hydropyrolysis, liquefaction and hydroliquefaction, extrac-tion and supercriticaluid extraction, gasification, oxidation, briquetting, sructure identi-fication, cogeneration, hazardous metal ion adsorption, activated coal production, andutilization as fertilizer sources.

For coal to remain competitive with other sources of energy in the industrializedcountries of the world, continuing technological improvements in all aspects of coalextraction are necessary. A number of studies have been done to study the desulfurizationand oxydesulfurization of the lignites (Kara and Ceylan, 1998; Boncukcuoglu et al., 1994;Demirbas, 1999; Yaman and Kucukbayrak, 1997; Ceylan and Kucuk, 2004).

There has been considerable interest in the supercritical fluid extraction (SFE) offossil fuels for the production of liquid fuels and chemicals from lignites and coals. It isbelieved that coal is composed of condensed aromatic units that have various degrees ofsubstitution on the periphery and are linked by methylenic, etheric, and hydroaromaticbridges. The cleavage of these bridges plays an important role in the thermal processes,such as high temperature and pressure extraction, and the liquefaction of coal (Pehlivanand Olcay, 1994). Considerable work has been done to study the supercritical tolueneextraction of lignites (Olcay et al., 1979; Olcay and Togrul, 1979; Togrul and Olcay,1978).

There are several studies on the oxidation of coals (Azik et al., 1993, 1994; Hepbasli,2004). The reactivity of a coal in direct liquefaction processes is highly dependent onthe processing conditions employed, i.e., temperature, pressure, residence time, and theamounts and kinds of hydrogen donor and catalyst available (Artok et al., 1992a, 1992b,1993, 1994; Ceylan and Bredenberg, 1982; Bredenberg and Ceylan, 1983).

Low-quality lignite was mixed with a high-quality bituminous coal, and optimumbriquetting conditions were investigated for blends with and without binder materials(Gurbuz-Beker, 2000; Gurbuz-Beker and Kucukbayrak, 1996). Coal-derived humic acidwas used as binder for coal briquetting (Yildirim and Ozbayoglu, 2002).

Proximate and ultimate analyses and microscopic investigations, thermal reactivities,coal structures, combustion characteristics, and extract qualities were carried out for coaland diesel fuel samples (Piskin et al., 1992; Yavuz and Kucukbayrak, 1998; Okudan et al.,1998; Kara and Mirzaoglu, 1998; Durusoy, 1999; Gullu et al., 2001). Lately, adsorptionof hazardous metal ions on humic substances of various origins was extensively studied(Martyniuk and Wieckowska, 2003).

The conversion of Elbistan (in Turkey) lignite into nitrogen-rich, alkali-soluble humicacid (ammonium nitrohumate) and its use as a coal binder were investigated (Yildirimand Ozbayoglu, 1997). In that study, the optimum conditions for nitric acid oxidationand ammoniation were determined. The optimum HNO3 concentration was determinedas 4.87 wt%. As a result, a humic acid solution containing 10.75 wt% nitrogen wasobtained with 88.2 wt% recovery from Elbistan lignite.

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Humic Compounds from Lignites 347

Isolation of Humic Acid Derivatives and Extraction of Phenolic Compounds

Ten grams of the humic acid derivatives (HAD) obtained from lignite samples are ex-tracted four times with 250 ml of a 5% heated solution of NaOH at 343 K. Then 200 mlof the caustic solution is extracted twice with 25 ml of pentane to remove hydrocarbons.After addition of a 15% HCl solution to adjust the pH of the solution to 6, a 5% solutionof NaHCO3 is added until the pH of the solution is 8.3, and any precipitate is completelydissolved. The clear solution is extracted four times with per 50 ml of diethyl ether, andethereal layer containing the phenols is washed with distilled water and dried overnightwith MgSO4. This phenolic fraction is known as Phenolic Fraction-A (PF-A) (Phenolcontents in PF-A extracted at pH = 8.3). Table 5 shows the phenolic compounds inisolated PF-A from HAD at pH = 8.3 from the lignite samples (Demirbas, 2002).

After the diethyl ether extraction at pH 8.3, to extract the residue phenols in theaqueous layer, the pH solution is adjusted from 8.3 to 7.5 with the 5% solution of HCl.After adjusting the pH of the aqueous solution, the residue phenols are extracted fourtimes with per 75 ml of the diethyl ether as mentioned above. The solution is washedwith water and dried overnight with MgSO4. This phenolic fraction is known as PhenolicFraction-B (PF-B) (Phenol contents in PF-B extracted at pH = 7.5). After removal of theMgSO4 and diethyl ether, the extracts (PF-A and PF-B) were concentrated in a rotaryevaporator to approximately 5 ml. Table 6 shows the phenolic compounds in isolatedPF-B from HAD at pH = 7.5 from the lignite samples (Demirbas, 2002).

Trimethylsilyl (TMS) derivatives of PF-A and PF-B fractions are separated andcharacterized by a GC-MS combined system.

Humic acids extracted from peats, brown coals, and lignites, were characterized usingdifferent techniques (Francioso et al., 2003). Humic acid removal from coal was carriedout with activated carbon from rice husk by chemical activation using phosphoric acid(Daifullah et al., 2004).

Table 5Phenolic compounds in isolated PF-A from HAD at

pH = 8.3 from the lignite samples

Phenolic compound %

α-Naphtol 20.5–27.43,5-Dimethyl phenol 13.0–15.74-Hydroxy-3-metoxyphenyl pyrivic acid metoxim 11.1–12.3o-Cresol 8.9–9.82-Isopropyl-5-methyl phenol 6.1–7.52-Hydroxy benzamide 5.6–6.34-Hydroxy benzaldehyde 5.3–5.7Dimethyl phenol 4.3–4.73-Hydroxy-4-metoxybenzoic acid 3.8–4.12,4-Dimethyl phenol 3.5–3.7β-Naphtol 2.3–2.73-Hydroxy propiophenone 2.1–2.4Simple phenol 1.3–1.7Unidentifed 1.9–4.1

Source: Demirbas, 2002.

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Table 6Phenolic compounds in isolated PF-B from HAD at

pH = 7.5 from the lignite samples

Phenolic compound %

2-Hydroxy benzamide 69.1–73.22-Hydroxy benzoic acid 9.1–10.32-Hydroxymethylester, benzoic acid 8.2–9.42-Hydroxy, N-acetyl benzamide 4.2–5.64-Hydroxy phenylpropanol 1.2–1.74-Hydroxy benzene acetic acid 0.8–1.04-(5, 6, 7, 8-Tetrahydro-1,3-dioxolo, 0.7–0.9

4,5-G isoquinolin) phenol3-Hydroxy benzaldehyde 0.5–0.82-Amino-4-nitro phenol 0.2–0.43-Hydroxy-4-metoxybenzoic acid 0.2–0.3Unidentified 0.8–1.2

Source: Demirbas, 2002.

Humic acid is a mixture of organic acids resulting from the controlled oxidationof carbonaceous matter. The structure is believed to be that of condensed cyclic rings,mostly aromatic in nature with carboxylic acid groups (−COOH), hydroxy-substitutedbenzenecarboxylic acids, phenolic (−OH) and carbonyl (−C=O) groups and aliphaticdicarboxylic acids such as succinic, oxalic, and possibly monocarboxylic acids (Yildirimand Ozbayoglu, 1997; Okudan et al., 1998).

HAs isolated from coal samples differ from one another according to the grade ofcoalification and conditions under which they were formed (Kurkova et al., 2004).

Utilization of Humic Substances for Agricultural Purposes

Humic substances have several known benefits to agriculture. Scientists have agreed onthe following benefits from the soil humus formation:

• Improves soil physical properties (increases water holding capacity, increases aer-ation of soil, improves soil workability, helps resist drought, improves seedbed,makes soil more friable or crumbly and reduces soil erosion).

• Holds exchangeable plant nutrients.• Improves moisture conditions.• Affects the release of plant nutrients through slow decomposition by organisms,

especially nitrogen release.• Improves trace element nutrition through chelation.• Has a growth-promoting effect.• Has a high base exchange capacity, which is an important basis for soil fertility

concepts.

Soil formation is closely linked with the action of diverse forms of organic sub-stances on the parent rock. The pioneers in this process (biogeochemical) are apparentlymicroorganisms, whose participation in the natural circulation of iron, sulfur, calcium,

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Humic Compounds from Lignites 349

silica, phosphorus, and other elements has been shown by many investigators. In theproduction of a fertile soil, organic substances play a direct part as they are the sourcesof plant nutrients which are liberated in available forms during mineralization.

At the present time most soil scientists hold a more moderate view and at leastrecognize that humus influences soil fertility through its effect on the water-holdingcapacity of the soil.

Humic acids are colloids and behave somewhat like clays, even though the nomen-clature suggests that they are acids and form true salts. The humates of monovalent alkalimetals are soluble in water, but the humates of multivalent metals are insoluble. Apartfrom their effect on the solubility of the materials and their absorption by clays, thedifferent cations have little effect on the humic molecules.

High oxygen-containing low-rank coals (peat, young lignites, and leonardite) areusually used for the production of humic acids which are in the form of alkali-solublehumate salts. Nitrogen-rich coal humic acids (ammonium nitrohumates) have good valueas a fertilizer and act as growth stimulators, improve plant resistance under unfavor-able conditions, accelerate ripening, act on biochemical processes favorably during plantgrowth. Humic acid has adsorption and cation-exchange properties, so it can be used forthe treatment of water (Yildirim and Ozbayoglu, 1997).

Conclusion

Humic substances (HSs) are a major class of naturally occurring organic colloidal parti-cles which not only demonstrate colloidal phenomena by themselves but display a rangeof important colloidal interaction in the presence of other substances. Specifically, notonly do they interact with other naturally occurring soil components, such as clays andmetals ions but all interact with man-made materials such as herbicides and pesticidesused in agriculture. Despite the fact that the characterization in terms of molecular massand chemical composition of HSs has presented and still continues to present majorproblems, due primarily to the heterogeneity of humics and their tendency to form col-loidal aggregates, considerable progress has been made towards an understanding of theirbehavior in both the laboratory and in the environment.

The properties of HSs vary from source to source, because they are heterogeneousmixtures of biochemical degradation products from plant and animal residues, and syn-thesis activities of microorganisms. Humic substances (HSs) have been considered to bea significant floculant in surface water filtration plants for the production of drinkingwater as well as the processing of water. HSs are produced from chemical and biologicaldegradation of plant and animal residues and from synthetic activities of microorganisms.

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