Structure humic substances

Humic earth results from the decomposition of organic matter, particularly cellulose from dead plants. The organic phase contains humic acids, allomelanins with complex phenolic macro-molecular structure. Humic substances fall between plants and geological materials (coals and petroleum). Degraded organic matter when released into the environment will form relatively soluble fulvic acids which will condense to form humic acids and then humins with lower solubilties. Humic earths are transitional between the low grade coal lignite (. v.) and peaty soils. 

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. 

This chapter focuses on the effects of humic substances present at the rhizo-sphere on plant growth and nutrient uptake. The main structural features of humic substances, their nutritional function, and the capacity to interact with plant metabolism are also presented. 

A. Piccolo, S. Nardi, and G. Concheri, Structural characteristics of humic substances as related to nitrate uptake and growth regulation in plant systems. Soil Biol. Bio-chem. 24 313 (1992). 

Malcolm RE, Vaughan D (1979) Effects of humic acid fraction on invertase activities in plant tissues. Soil Biol Biochem 11 65-72 Malcolm RL (1989) Application of solid-state 13C NMR spectroscopy to geochemical studies of humic substances. In Hayes MHB, MacCarthy P, Malcolm RL, Swift RS (eds) Humic substances II. In search of structure. Wiley, Chichester, UK, pp 339-372... 

Almendros G, Gudalix G, Gonzalez-Vila F, Martin F (1998) Distribution of structural units in humic substances as revealed by multi-step selective degradations and C-13 NMR of successive residues. Soil Biol Biochem 30 755-765... 

Kopinke F, Georgi A, Mackenzie K (2001) Sorption of pyrene to dissolved humic substances and related model polymers. 1. Structure-property correlation. Envion Sci Technol 35 2536-2542... 

Kulikova N, Perminova I (2002) Binding of atrazine to humic substances from soil, peat, and coal related to their structure. Environ Sci Technol 36 3720-3724... 

Perminova I, Grechishcheva N, Peterosyan V (1999) Relationships between structure and binding affinity of humic substances for polycyclic aromatic hydrocarbons relevance of molecular descriptors. Environ Sci Technol 33 3781-3787... 

Field JA, Cervantes FJ (2005) Microbial redox reactions mediated by humus and structurally related quinones. In Perminova IV, Hatfield K, Hertkom N (eds) Use of humic substances to remediate polluted environments from theory to practice, vol 52. Springer, Dordrecht, pp 343-352... 

Dissolved humic substances (DHS) are the main constituents of the dissolved organic carbon (DOC) pool in surface waters (freshwaters and marine waters), groundwaters, and soil porewaters and commonly impart a yellowish-brown color to the water system. Despite the different origins responsible for the main structural characteristics of DHS, they all constitute refractory products of chemical and biological degradation and condensation reactions from plant or animal residues and play a crucial role in many biogeochemical processes. 

Senesi N, Steelink C (1989) In Hayes MHB, MacCarthy P, Malcolm RL, Swift RS (eds) Humic substances In search of structure. Wiley, NewYork, p 46... 

Averett RC, Leenheer JA, McKnight DM, Thorn KA (1987) Humic substances in the Suwannee river, Georgia interactions, properties, and proposed structures, Open-File Report 87-557, US Geological Survey, Denver, CO... 

The basic structure of humic substances involves a backbone composed of alkyl or aromatic units crosslinked mainly by oxygen and nitrogen groups. Major functional groups attached to the backbone are carboxylic acids, phenolic hydroxyls, alcoholic hydroxyls, ketones, and quinones. The molecular structure is variable as it is dependent on the collection of DOM available in seawater to undergo the various polymerization, condensation, and oxidation reactions and reaction conditions involved in humification, as well as the ambient physicochemical reaction conditions, such as temperature and light availability. 

Recall from Chapter 23.2.4 that humic substances are isolated from seawater by adsorption on a hydrophobic resin followed by elution using solvents of varying pH. The desorbed compounds are fractionated into two classes, humic acids fulvic acids based on their solubility behavior. A model structure for a humic acid is illustrated in Figure 23.10a in which fragments of biomolecules, such as sugars, oligosaccharides. 

Labile and refractory DOM undergo abiotic photochemical reactions in the photic zone, especially in the sea surfece microlayer where physical processes concentrate DOM into thin films. Some of these reactions appear to be important in the formation of refractory DOM and others in its degradation. For example, DOM exuded by diatoms during plankton blooms has been observed to be transformed into humic substances within days of release into surfece seawater. Laboratory experiments conducted in seawater have demonstrated that photolysis of labile LMW DOM promotes the chemical reactions involved in humification and produces chemical structures foimd in marine humic substances. 

Humic substances High-molecular-weight organic compounds that are variable in composiUon, have complex structures, and are relaUvely inert. They comprise a large fracUon of the DOM. Found in sods, sediment, fresh, and seawater. 

These types of models, while incomplete, are steps toward the formulation of composite models, which depend on future availability of compositional data. Moreover, these structural models are an important aid in understanding the interactions between anthropogenic chemicals and terrestrial organic matter. However, due to the heterogeneity of humic substances in the environment, provision of an exact, general structure does not seem feasible. 

A reevaluation of molecular structure of humic substances based on data obtained primarily from nuclear magnetic resonance spectroscopy, X-ray absorption near-edge structure spectroscopy, electrospray ionization-mass spectrometry, and pyrolysis studies was presented by Sutton and Sposito (2005). The authors consider that humic substances are collections of diverse, relatively low molecular mass components forming dynamic associations stabilized by hydrophobic interactions and hydrogen bonds. These associations are capable of organizing into micellar structures in suitable aqueous environments. Humic components display contrasting molecular motional behavior and may be spatially segregated on a scale of nanometers. Within this new structural context, these components comprise any molecules... 

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