Humic acids high molecular weight

Many fluids of natural origin contain detectable quantities of high molecular weight organic anions, such as those from humic, fulvic, and tannic acids, which can be carried to and deposited on AX membranes. Such deposits can behave as thin films partially selective to cations (6). The iaterfaces between such films and the undedyiag AX membranes then act as very thin stagnant depletion compartments and the AX membranes may exhibit polarization at current densities that are much lower than would be expected for new membranes ia the abseace of such anioas. 

About half of the dissolved organic carbon may appear in humic or fulvic acids. These are high-molecular weight organic compounds of a composition which is somewhat uncertain. They contain aromatic hydroxyl and carboxyl groups which have the ability to bind to metal ions. Rivers and estuaries typically contain 10 mg/liter of acid with an exchange capacity of 5-10 mmol/g, mainly due to carboxylic... 

Since lignins are polymers of phenolics and are major plant constituents with resistance to microbial decomposition, they are the primary source of phenolic units for humic acid synthesis (178, 179). Once transformed, these humic acids become further resistant to microbial attack and can become bound to soils (180) form interactions with other high molecular weight phenolic compounds (ex. lignins, fulvic acids) and with clays (181) and influence the biodegradation of other organic substrates in soils (182, 183). 

The enhancement of kerosene dissolution occurs even at low humic acid content in the aqueous solution. In view of the fact that humic substances are relatively high molecular weight species containing nonpolar organic moieties, Chiou et al. (1986) assumed that a partition-like interaction between a solute of very low solubility in aqueous solution and a microscopic organic environment of dissolved humic molecules can explain solute solubility enhancement. 

Humic acid is composed of aromatic, aliphatic and carbohydrate carbon compounds. An average humic acid s elemental composition is 55.1% C, 5.0% H, 3.5% N, 35.6% O, and 1.8% S (Rice and MacCarthy, 1991). Its molecular weight distribution is typically broad, and it is a relatively high-molecular-weight material relative to the fulvic acid isolated from the same soil or sediment. It s predominantly functionalized by carboxylic acid and phenolic groups. At least some components of humic acid are surface-active, and these components have been shown to form micelles in concentrated, alkaline aqueous solutions (Piret et al., 1960 Visser, 1964 Wershaw et al., 1969 Tschapek and Wasowski, 1976 Chen et al., 1978 Rochus and Sipos, 1978 Hayano et al., 1982 Hayase and Tsubota, 1984 Guetzloff and Rice, 1994). 

Operationally, it is common to define HS in terms of the methods used to extract or isolate them from soils, sediments, and natural waters. The classic soil extraction procedure yields three main fractions humic acid [also defined as high-molecular-weight (HMW) or high-molecular-size (HMS) fraction], fulvic acid (FA) [also defined as low-molecular-weight (LMW) or low-molecular-size (LMS) fraction], and humin. These fractions are defined in terms of their solubility in aqueous media as a function of pH or in terms of their extractability from soils or sediments as a function of the pH of the extracting medium. Humic acid is the fraction of HS that is not soluble in water under acidic conditions, but becomes soluble (or extractable) at higher pH values. Fulvic acid is the fraction that is soluble in aqueous media at all pH values. Humin represents the fraction that is not soluble in an aqueous medium (or is not extractable with an aqueous medium) at any pH value. Actually, humin consists of an aggregate of humic and nonhumic materials (Rice and Mac-... 

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