Application of a membrane model to the sorptive interactions of humic substances.

Abstract

Humic substances, the dark-colored, natural organic polyelectrolytes that are found in practically all soils, sediments, and natural water, strongly interact with both inorganic and organic pollutants. Inorganic cationic species generally undergo complexation reactions with humic substances. The binding of cations, such as cupric ions, by humic substances often markedly reduces their toxicity to aquatic organisms. Some inorganic anionic species, in the presence of metal ions, are sorbed by humic substances. In these instances the metal ions appear to form bridges between the humic substances and the anions. Several different types of interactions take place between organic compounds and humic materials. Hydrophobic organic species partition into either insoluble or soluble humic substances. The insoluble humic substances will remove hydrophobic organic compounds from the aqueous phase, thereby rendering them less mobile. However, soluble humic substances will solubilize hydrophobic organics, increasing their mobility. Other types of interactions between humic substances and organic compounds, such as adsorption and ion exchange, also have been observed. These various interactions between humic substances and pollutants are important in governing their fate and movement in natural water systems, and, for this reason, a detailed understanding of the mechanisms of the interaction is important. A recently developed membrane model of the structure of humic substances is described; this model enables one to better understand the physical-chemical properties of these materials.ImagesFIGURE 2.FIGURE 3.

Pesticide sorption on soils treated with evergreen oak biomass at different humification stages

“…The preferential sorption on soil organic colloids rather than on the mineral fraction reported for most pesticides (Benin et al, 1990) involves different mechanisms (Khan, 1980;Wershaw, 1989), such as physical fixation, van der Waals forces, hydrogen bonding, ionic bonds, complexation through polyvalent metal ions, hydrophobic bonding and charge transfer complexes. The formation of covalent bonds is also possible.…”

“…The formation of covalent bonds is also possible. In fact, it has been reported that some products from the enzymatic transformation of pesticides may polymerize into humus macromolecules (Riggle et al 1987;Bartha, 1980;Wershaw, 1989;Haider et al, 1988 (Martínez-Iñigo, 1991). In the present study, the humified biomass from this plant-starting material is applied to soils with a negligible humus content to prove the effect of organic matter on pesticide fixation processes.…”

“Humification” Process or Formation of Refractory Soil Organic Matter

“…These aggregates develop hydrophobic areas on their inner surface that are able to incorporate lipophilic compounds, whereas hydrophilic areas are located on the outside unable to coat the surface of mineral particles or to take up clay minerals. Such organo-mineral associates substantially contribute to the stabilization of the aggregates of organic molecules (WERSHAW, 1989(WERSHAW, ,1993.…”

The spatio-temporal patterns and ecological significance of free amino acids and humic substances in contrasting oligotrophic and eutrophic freshwater ecosystems

“…Humic substances (HS), or Gelbstoff are a chemically heterogeneous group of amphiphilic, tluorescent, high molecular weight compounds (lo"-lo9 Daltons) consisting of C, H, 0, S and N atoms with a preponderance of hydroxyl, carboxyl and phenolic groups (Cheshire et al, 1967). According to Wershaw (1989) they have membrane or micelle like properties. Although in terms of mass they constitute the major generic group of DOM in freshwaters, they were, until recently, considered to be refractory and relatively unimportant biologically.…”