Soil organic matter beyond molecular structure Part I: Macromolecular and supramolecular characteristics

Abstract

This contribution reviews and discusses structural aspects of soil organic matter (SOM) and humic substances (HS) with special respect to the macromolecular and the supramolecular view. It can be concluded that (1) dissolved humic acids behave as supramolecular associations of relatively small molecules with an enormous flexibility of reaction of environmental conditions, (2) multivalent cations may increase the apparent molecular weight by the formation of coordinative crosslinks in dissolved and undissolved natural organic matter (NOM), (3) sorption nonlinearity in solid humic acids and SOM may be due to polymer properties of NOM, (4) sorbates affect sorbent characteristics of SOM, and (5) hysteresis and conditioning effects in SOM can up to now best be explained with the polymer analogy. A distinct polydispersivity of SOM over a wide range of molecular masses is to be assumed. The supramolecular and the macromolecular models were derived from humic acids with different composition and on the basis of different sample states. Although the supramolecular model has not explicitely been shown for unfractionated DOM, the combination of all discussed studies suggests supramolecular as well as macromolecular characteristics of NOM. Neither macromolecules nor small molecules can be fully excluded in solid and dissolved SOM. Microregions with different properties provide different types of sorption sites. SOM is suggested to be regarded as amorphous material. This point of view is not restricted to high molecular masses and may supplement our understanding of SOM by the model of physical aging.

Soil humic acid aggregation by dynamic light scattering and laser Doppler electrophoresis

“…Hence, the HS structure has been discussed adopting different viewpoints including molecular conformation, molecular aggregation, macromolecularity, and supramolecular characteristics (Schaumann, 2006). The traditional view, based on the concept of macromolecularity, considers HS to be coiled conformations of long-chain molecules, which may be slightly cross-linked (Stevenson, 1994).…”

Metal Cation Complexation with Natural Organic Matter in Aqueous Solutions: Molecular Dynamics Simulations and Potentials of Mean Force

“…[6][7][8][9][10][11] The formation of NOM coatings is also a significant factor controlling the stability of viruses in aquatic systems 12 and the performance characteristics of inorganic and polymeric nanofiltration membranes. 13,14 Common cations such as Na + , Mg 2+ , and Ca 2+ are thought to play an important role in the supramolecular aggregation of NOM molecules into colloidal particles, 2,3,[15][16][17][18][19][20] the formation of NOM surface coatings, [10][11][12][13] and the aggregation of mineral particles. 21,22 These background cations also successfully compete for binding sites with toxic metals, thus effecting their mobility in the environment.…”

Fate and Determination of Triazine Herbicides in Soil

“…Humic substances (HS) are heterogeneous and still poorly characterized macromolecules or supramolecular associations (Schaumann, 2006). A number of mechanisms have been proposed for the interaction of atrazine and HS: partitioning resulting from hydrophobic interactions (Lima et al, 2010;Prosen & Zupančič-Kralj, 2000), hydrogen bonding (Prosen & Zupančič-Kralj, 2000), electron transfer, charge transfer (Mudhoo & Garg, 2011).…”

“…Desorption hysteresis has been observed for longer contact times (Drori et al, 2005;Prosen & Zupančič-Kralj, 2000). The currently accepted model explaining the effect of contact time, nonlinear sorption kinetics, desorption hysteresis and conditioning effect of sorbate on sorbent affinity is the dual-mode sorption process of sorbate in the interchangeable rubbery and glassy state of polymerous SOM material (Schaumann, 2006).…”