摘要:

IntroductionHumic substances are a ubiquitous depository of carbon in soil and natural waters. As the decay products of the total biota in the environment, they are highly refractory—mean residence times in soil vary fromca.800 to >3000 years.1The humic substances found in soils, oceans, and deep lakes are generally autochthonous (produced within the system), while those in streams and shallow lakes are more often allochthonous (produced outside the system). Flowing surface and ground water are responsible for their distribution throughout the hydro- and lithosphere. They are formed through aerobic and anaerobic decomposition of (mostly) plant detritus, as well as secondary microbial synthesis. Differences in origin, age, and genesis lead to a high degree of chemical and morphological complexity that makes the compounds difficult to characterize. They generally have a high aromatic content, estimated (from13C NMR spectra) to range from 20 to 60% of the carbon present.2Heteroatomic functionalities include phenols and other alcohols, ketones/quinones, aldehydes, carboxylic acids, amino- and nitro-groups, and sulfur containing entities such as mercaptans, sulfates, and sulfonates.The diversity among humic substances makes their classification difficult. It is customary to divide them into three operationally defined classes, broadly based on their solubility characteristics. Thushuminis the fraction that is insoluble in both acid and base,humic acid(HA) is the fraction soluble at pH 2,3andfulvic acid(FA) is soluble at all pH values. Since the focus of this review is on dissolved humic substances, humin will not be considered here; emphasis will be given to HA. The relative quantities of the three fractions present in soil depend on the type of soil considered: in a typical mollisol the distribution is roughly equal, while humin prevails in vertisol, and FA in spodosol B.3Humic deposits such as those found in association with leonardite (vide infra) consists almost entirely of HA. In aqueous media, FA is, for obvious reasons, generally more plentiful.The differences in solubility between FA and HA are necessarily based on chemical differences. While the two classes of compounds share many structural features, including an abundance of carboxy, hydroxy, phenolic, and ketonic groups, FA has a lower molecular weight, a higher functional group density, and higher acidity than HA. Molecular weights for FA are in the 500–2000 Da range, while they extend from 2.0 to 1300 kDa for HA. The oxygen content is reported as 32.8–38.3% for HA, and 39.7–49.8%4for FA.

ISSN:1467-4866    

DOI:10.1039/b001869o

出版商:RSC    

年代:2000

数据来源: RSC