摘要:

IntroductionHumic substances are the breakdown products of plant and animal matter in environmental matrices, especially soil. The major components of these materials—humic acid (HA), fulvic acid (FA), and humin—are defined on the basis of their solubilities in aqueous solution. HA, the subject of the present study, is soluble in solutions of pH > 2, while FA is soluble at all pHs and humin is entirely insoluble. HA is comprised of a suite of polydisperse carbonaceous polymers with molecular weights ranging to tens of kDa.1Both molecular size and functional group characteristics are variable, depending on the origin of the material and the environmental factors implicated in its formation. Considerable diversity also exists within a single HA sample. Generally, however, HAs have a significant aromatic content (30–60% of C) and an abundance of carboxy (15–25% of C) and hydroxy groups.2Their behavior in aqueous solution has been studied extensively3–6and it has been noted that they have significant surface activity.Based on this observation, the detergent model of HA was developed7–10and has been found to account for much of its behavior in aqueous solution. According to this model, HA polymers aggregate spontaneously in water, forming micelle-like structures comparable to those formed by synthetic surfactants. The term “pseudomicelle” is often used to describe these bodies, especially in dilute solution, since they are composed of various HA polymer fragments and are likely to be more structurally diverse than customary micelles.11,12It is thought that the formation of humic pseudomicelles in aqueous solution can proceed by both intramolecular and intermolecular mechanisms.13–15In the former, a single polymer chain arranges itself in a manner that exposes its hydrophilic parts (e.g., carboxy groups) to the aqueous surroundings, while isolating its hydrophobic portion in the center of the structure. This type of “aggregation” is not characterized by a critical micelle concentration (CMC). It has been shown that the formation of humic pseudomicelles is promoted by the presence of metal ions, especially multivalent ones.16The secondary structure of aqueous HAs can be studied by a variety of methods. Non-intrusive procedures are preferred, since the arrangements of the polymers in water are both changeable and fragile, precluding techniques that interfere with the integrity of the system. Studies of this kind are of interest because of the ubiquitous nature of humic substances in the environment, and their implication in the transport of pollutants through soil.17–21In view of the variations among HAs from different origins, it is important to compare their behavior in aqueous solution, in order to understand how their environmental interactions may differ. The present communication offers a comparison of 6 HAs, studied by 5 different methods.

ISSN:1467-4866    

DOI:10.1039/b100038l

出版商:RSC    

年代:2001

数据来源: RSC