Soil: acidification 352 humic substances

The first attempt to isolate humic substances from soil appears to have been made by Achard (1786), who extracted peat with alkali and obtained a dark, amorphous precipitate upon acidification. This alkali-soluble, acid-insoluble material became known by a number of names, of which the term humic acid survived. 

Humus, which has lost all the visible features of the organic residues from which it formed, is subdivided into amorphous brown-colored polymers according to the scheme depicted in Figure 2.21. The polymers, termed humic substances, can be further separated (operationally, if not intrinsically) into humic acid, fulvic acid, and humin, according to their solubility in strong acid and base. Aqueous sodium hydroxide extracts humic and fulvic acid from soils, leaving the humin unextracted. Acidification of this dark-colored extract then causes the humic acid to precipitate, while the fulvic acid remains soluble. 

Both Pu(VI) and Pu(V) are relatively strong oxidants and are reduced by organic compounds (17). Reduction of Pu(VI) was observed when Pu(VI) was equilibrated with citric acid, humic substances extracted from soil, and acidic polysaccharides (23). This reduction of Pu (VI) by humic substances is illustrated in Table IV. After 13 days, 43% of the Pu(IV) and 84% of the Pu(VI) remained in solution. Of the Pu remaining in solution in the Pu(IV) experiment, only 32% would extract into TTA-xylene [i.e., ionic Pu(IV)] upon acidification of an aliquot of the Pu(IV)-bicarbonate solution. The remainder of the unextractable, soluble Pu was found to be mostly Pu polymer as well as a small amount of Pu(VI) formed through dis-porportionation of Pu(IV). When Pu(VI) was the initial... 

Based on the results of zinc and cadmium uptake by the T. caerulescens in contaminated soils, Knight et al. (1997) observed the mobilization of metals along with a significant increase in pH in the rhizosphere, which facilitated the dissolution of humic substances in the soil matrix. They also suggested that acidification of the soil solution was not the reason for the increase in zinc availability (Knight et al., 1997). A similar conclusion was also reached in a study performed by McGrath et al. (1997). 

A number of studies have been made on the occurrence of N in the various humic substances and the distribution between them. In many studies soil organic matter is separated into humic (HA) and fulvic (FA) acids by the traditional technique of alkaline extraction followed by acidification. 

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