Solvent Systems for the Isolation of Organic Components from Soils

Abstract

The classical methods for the isolation of soil organic matter (SOM) components use aqueous base or neutral salt solutions, and combinations of aqueous base and pyrophosphate. Organic solvents have been rarely used, largely because of difficulties in recovering solutes. This review provides the relevant chemistry of aqueous and organic chemicals relevant to extracting and fractionating the components of SOM that are bound and are not bound by the soil mineral surfaces. Uses of aqueous media to separate the SOM components on the basis of charge density differences are described. Combinations of aqueous base and urea enhance the isolations of the SOM components that have a high degree of polarity [humic and fulvic acids (HAs and FAs), polysaccharides, peptides]. However, the nature of the associations between the solute molecules has prevented the isolation of any purified SOM component. Properties are listed of organic solvents that have potential for the isolation of SOM components, and novel procedures are described for the recovery of the SOM components dissolved in organic solvents. The procedures include the uses of resins with varying degrees of polarity and the recovery of the SOM components in aqueous media. No satisfactory solvent system has been found that can isolate all of the humin materials sorbed by the inorganic colloids. However, procedures are outlined that can extract much of the material classified as humin in the classical definitions. Because the polar components of the SOM can now be removed, it is likely that the compositions of the nonpolar humins strongly held by the mineral colloids will be resolved using procedures such as pyrolysis–mass spectrometry.