Substance humic

Humic substances represent a broad spectrum of different compounds. In the formation of humic substances saccharides, pectins, lignin, proteins, fats, waxes, resins, tannins, etc. participate. During humification, biochemical and chemical processes take place. Of these, the polymerization and condensation reactions of the products of the degradation of primary organic matter into products which are the result of the humification process are important. 

Humic substances are high-molecular cyclic compounds representing a complex of organic substances as products of condensation of phenol-type aromatic substances with amino acids and proteins. The amount of humic substances in soil ranges from 0 to 5%, and in peat from 40 to 55%. 

Humic substances are classified according to physico-chemical and chemical properties into 

The classification is based on differing solubility in acid and alkaline media as well as in alcohol. There is no general agreement over the classification into individual groups. 

Generally, humic substances are defined as condensed polymers of aromatic and aliphatic compounds produced by decomposition of plant and animal residues and by microbial synthesis. They are amorphous, dark-colored, and hydrophilic, with a wide range in molecular weight from a few hundreds to several thousands. Furthermore, humic substances contain a large number of nonidentical functional groups, with different pKa values, and are partitioned into three main fractions based upon their solubility behavior (Fig. 3.16) (Evangelou, 1995b)  

Humic acid—soluble in dilute alkali, but precipitates in acid solution 

Fulvic acid—soluble both in alkali and acid solutions 

Functional groups of humic substances are fractioned into three clusters of different dissociation constants  

FIGURE 5.14 Three major groups of humic substances. 

A variety of functional groups are present in humic substances  

Carboxyl Phenolic OH Alcoholic OH Quinone and ketone Amino Sulfhydryl 

Major elements in humic and fulvic acids are oxygen and carbon. The carbon content of fulvic acid ranges from 41 to 51% as compared to 54-59% for humic acid. In contrast, the oxygen content of fulvic acids is higher (40-50%) compared to 33-38% for humic acids (Table 5.9). These data demonstrate that distinct differences exist between fulvic and humic acids. 

FIGURE 5.15 (a) Generalized structure of humic substances showing various functional groups, (b) Sche- 

A similar overview but focussing on the interactions between humic substances and heavy metals was found by Munari [75] and Gotz [76]. The two main arguments can be used as an example to reveal the complexity of the theme  

Suffet and MacCarthy [22] give the most comprehensive overview about appearance, chemistry and properties of humic substances. They discuss influences on waste, on the drinking water preparation, on several interactions with heavy metals, on detergents and organic pollutants, reactions with light (UV-radiation) and ion exchange effects. 

Humic substances deriving from aquatic and terrestrial environments are not identical in their composition and structure but very similar which is mentioned by Suffet and MacCarthy [22] as well as by Chen and Inbar [63]. Such complex chemical interactions with ecological consequences may therefore appear in all ecosystems. 

Compost contains humic substances in very high concentrations. Their appearance should be given more attention since they are one of the few anal) ically measurable parameters and are additionally an indicator for compost maturity, as claimed by Chen and Inbar [63]. 

Other groups include, for example, hymatomelanic acid, the alcohol-soluble fraction (Senesi and Loffredo 1998). Clearly, these fractions are loosely defined and can include many different compounds. Usually, HSs are obtained from the soil through alkaline extraction, which separates FA and HA but not humin, albeit other procedures have also been proposed (Swift 1996). The structure and properties of HSs are considered in Chapter 10. 

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In some systems, such as lake and river waters, the suspended inorganic particles may be coated by biological polymers, termed humic substances, which prevent flocculation by either steric or electrostatic mechanisms. These can also interact with added inorganic salts (31) that can neutralize charged functional groups on these polymers. 

In the area of municipal and iadustrial wastewater treatment, the principal environmental issue is the toxicity of residual flocculating agents ia the effluent. Laboratory studies have shown that cationic polymers are toxic to fish because of the iateraction of these polymers with giU. membranes. Nonionic and anionic polymers show no toxicity (82,83). Other studies have shown that ia natural systems the suspended inorganic matter and humic substances substantially reduce the toxicity of added cationic polymer, and the polymers have been used successfully ia fish hatcheries (84—86). Based on these results, the EPA has added a protocol for testing these polymers for toxicity toward fish ia the presence of humic acids (87). The addition of anionic polymers to effluent streams containing cationic polymers to reduce their toxicity has been mentioned ia the patent Hterature (83). 

In the removal of contaminating ions such as (PO or Fe " a precipitate such as Ca2(P0 2 Fe(OH)2, after oxidizing ferrous ion to ferric, is formed and the soHd is removed. The addition of surfactants is usually not essential (nor desirable) since most waters contain natural surfactants that would render the soflds sufficiently hydrophobic for flotation to occur. Such surfactants derive from the degradation of organic matter, and humic substances abundantly available in nature (30). 

G. A. Aiken and co-workers. Humic Substances in Soil, Sediment and Water,]o m Wiley Sons, Inc., New York, 1985. 

Hydrodechlorination is a common reaction of chlorinated pesticides such as atrazine (eq. 15), alachlor, and metolachlor (2) (eq. 16). These reactions are catalyzed primarily by transition metals or by soil surfaces (clays or humic substances). 

The development of methods of analysis of tria2ines and thek hydroxy metabohtes in humic soil samples with combined chromatographic and ms techniques has been described (78). A two-way approach was used for separating interfering humic substances and for performing stmctural elucidation of the herbicide traces. Humic samples were extracted by supercritical fluid extraction and analy2ed by both hplc/particle beam ms and a new ms/ms method. The new ms /ms unit was of the tandem sector field-time-of-flight/ms type. 

N. Masque R. M. Marce and E. Boirull, Chemical removal of humic substances interfering with the on-line solid-phase exti action-liquid chi omatographic determination of polar water pollutants , Chromatographia 4 231-236 (1998). 

F. J. Stevenson, Humus Chemistry, Genesis, Composition, Reactions, Wiley-Interscience, New York (1982). M. Schnitzer and S. U. Khan, Humic Substances in the Environment., Marcel Dekker, Inc., New York (1972). 

Humic substances in sediments and soils have commonly been, defined as heteropolycondensates of decomposing plant and animal detritus 46. For lack of a better structural definition, these macromolecular substances have been divided into three categories fulvic acids and humic acid and humin. Fulvic acids and humic acids are soluble in dilute alkaline solutions, whereas humin is insoluble. 

According to Hatcher and co-authors47 the CP/MAS NMR technique opens up new means of distinguishing between various structural features of aquatic and ter-restric humic materials of rather old origin. They found, for instance that the aliphatic carbons of the humic substances in Holocene sediments, are dominant components suggesting an input of lipid-like materials. 

The problems involved in the study of humic substances are, as expected, also encountered in the case of fossil fuels. Most C-13 CP/MAS spectra of solid fossil fuels (coals, oil shales) do not exhibit a high level of spectral resolution50,51). They consist essentially of two broad bands — one in the aromatic/olefinic region from about 170 ppm to 95 ppm and one in the aliphatic region from about 90 to —5 ppm relative to TMS. On the other hand, lignite, an imperfectly formed coal, shows a considerable amount of fine structure. 

Biogeochemical compartment. The biogeochemical compartment consists of the O, A, E, and B horizons of soils, in which substantial organic matter can be an integral component of the soil. The load of the water leaving the soil from the biogeochemical compartment consists mainly of soluble compounds because the matrix of the soil acts as a filter that retains particulate matter. In some cases, however, clay and particulate humic substances are also car-... 

Stuermer, D. H. and Payne, J. R. (1976). Investigation of seawater and terrestrial humic substances with carbon-13 and proton nuclear magnetic resonance. Geochim. Cosmochim. Acta 40,1109-1114. 

Bone protein was extracted following the method described by Sealy (1986). Bone chips were demineralized in a weak HCl solution, then soaked in 0.1 M NaOH to remove base-soluble humic substances. Remaining material, which is mainly collagen, but includes non-collagenous proteins, was... 

Adhikari M, Das PK, Das K. 1986. Studies on adsorption and desorption of methyl parathion on humic substances. J Indian Chem Soc 63 1027-1029. 

Perdue EM, NL Wolfe (1982) Modification of pollutant hydrolysis kinetics in the presence of humic substances. Environ Sci Technol 16 847-852. 

Zepp RG, GL Baugham, PA Scholtzhauer (1981a) Comparison of photochemical behaviour of various humic substances in water. I. Sunlight induced reactions of aquatic pollutants photosensitized by humic substances. Chemo sphere 10 109-117. 

Zepp RG, PF Schlotzhauer, RM Sink (1985) Photosensitized transformations involving energy transfer in natural waters role of humic substances. Environ Sci Technol 19 74-81. 

Coates JD, KA Cole, R Chakraborty, SM O Connor, LA Achenbach (2002) Diversity and ubiquity of bacteria capable of utilizing humic substances as electron donors for anaerobic respiration. Appl Environ Microbiol 68 2445-2452. 

Hatcher PG, JM Bortiatynski, RD Minard, J Dec, J-M Bollag (1993) Use of high-resolution NMR to examine the enzymatic covalent binding of C-labeled 2,4-dichlorophenol to humic substances. Environ Sci Technol 21 2098-2103. 

NMR has been used in conjunction with NMR in studies on reactions of [ N] hydroxylamine with fulvic and humic substances (Thom et al. 1992). 

Scott DT, DM McKnight, EL Blunt-Harris, SE Kolesar, DR Lovley (1998) Quinone moieties act as electron acceptors in the reduction of humic substances by humics-reducing microorganisms. Environ Sci Tech-nol 32 2984-2989. 

Thorn KA, JB Arterburn, MA Mikita (1992) N and C NMR investigation of hydroxylamine-derivatized humic substances. Environ Sci Technol 26 107-116. 

Thorn KA, PJ Pettigrew, WS Goldenberg, EJ Weber (1996) Covalent binding of aniline to humic substances. 2. N NMR studies of nucleophilic addition reactions. Environ Sci Technol 30 1764-1775. 

Mierle G, Ingram R. 1991. The role of humic substances in the mobilization of mercury from watersheds. Water Air Soil Pollut 56 349-357. 

Curtis PJ. 1998. Climatic and hydrologic control of DOM concentration and quality in lakes. In Hessen DO, Tranvik LJ, editors. Aquatic humic substances ecology and biogeochemistry. Berlin, Germany Springer-Verlag, p. 93-105. 

Currently available CRMs for Cr(lll)/(VI) species There exist a lyophilized water certified for Cr(III)/Cr(VI) and a binder-free glass fiber filter loaded with welding dust certified for Cr(VI) and total Cr (Vercoutere et al. 1998 Christensen et al. 1999) issued by the BCR. They consist of a set of specimens for single use. There is a need for more CRMs, such as a Cr(VI) in industrial effluents and in river water containing, e.g. humic substances. 

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