Clay-Humic Complexes and Interactions in Soil: Impact on Sorption and Availability of Introduced Organic Compounds.
Benny K.G. Theng, Landcare Research, Private Bag 11-052, Massey University, Cnr Riddet Road and University Avenue, Palmerston North, New Zealand
The non-living component of soil organic matter (after separation of the macroorganic matter or ‘light fraction') consists of non-humic and humic substances. The former constituents comprise well-defined classes of organic compounds, such as carbohydrates, lipids, and proteins. The bulk of the organic matter in soil is made up of Humic Substances (HS) which, in turn, are conventionally and operationally divided into 3 principal fractions according to their aqueous solubility as a function of pH: (i) fulvic acid, soluble in both acid and alkali; (ii) humic acid, soluble in alkali; and (iii) humin, insoluble in either medium. A large proportion (> 50%) of HS in soil (and sediment) is closely associated with the clay fraction to form a clay-organic complex. Thus, humin is a chemically stable clay-organic complex since it is the material that remains after treatment of the soil/sediment with acid and alkali. Because of their importance to controlling soil pollution, the interactions of organic compounds with HS and clay-humic complexes have received a great deal of attention. However, unlike the situation with organically modified clay minerals (‘organoclays'), the underlying mechanisms are still imperfectly understood. This is partly because HS are chemically complex, structurally heterogeneous, and polydisperse. There is also some uncertainty about the size and shape of HS; that is, whether these substances are polymeric assuming a random-coil conformation in solution, or whether they represent aggregates of relatively small molecules held together by weak dispersive forces forming micelles in solution. Either way, HS by themselves, or in association with clay minerals, are highly reactive toward introduced non-ionic organic compounds. The sorption of hydrophobic organic molecules by Soil Organic Matter (SOM) and its complexes with clay minerals (e.g., humin) has been well documented. Nevertheless, the driving principle behind this process remains controversial. Partitioning of the organic solute between the solution and solid (SOM) phases is widely accepted. This mode of sorption is characterized by linear isotherms, non-competition between solutes, and sorption-desorption reversibility. The uptake of many non-ionic organic molecules, however, is non-linear and competitive, while their desorption shows hysteresis over a wide range of concentrations. These observations have been rationalized in terms of the simultaneous presence in SOM of an expanded and flexible (‘rubbery') region, and one that is condensed and rigid (‘glassy'). Partitioning can apparently take place in both domains, while adsorption (i.e., chemical interactions) together with micropore filling only occur in the condensed domain. Non-linear sorption has alternatively been ascribed to the presence and reactivity of a carbonaceous material with a high surface area, arising from the burning of vegetation and plant residues. More often than not the sorption of organic solutes by SOM (and clay-humic complexes) is not fully reversible because of solute-surface interaction, physical entrapment of solute into SOM, and chemical/microbial transformation. As a result, the mobility and bioavailability of sorbed organic compounds in soil are effectively retarded. Here we assess and summarize the literature that has accumulated over the past two decades on soil organic matter, with particular reference to its interactions with clay minerals, and reactivity toward introduced (anthropogenic) organic compounds. Where appropriate we will indicate how sorption by SOM and clay-humic complexes affects the fate of sorbed organic contaminants, and mention possible areas for future research.