Extraction Kinetics and Molecular Size Fractionation of Humic Substances From Two Brazilian Soils

Abstract

No presente trabalho foi estudada sistematicamente a extração de substâncias húmicas (SH) de um Oxisol e de um Mollisol do Sul do Brazil, empregando-se soluções de NaOH 0.1 e 0.5 mol L In the present study, the extraction behaviour of humic substances (HS) from an Oxisol and a Mollisol from South Brazil, by using 0.1 and 0.5 mol L -1 NaOH and 0.15 mol L -1 neutral pyrophosphate solutions, respectively, was systematically studied. The kinetics and efficiency of HS extraction were evaluated by means of UV/Vis spectroscopy. The isolated humic acids (HA) and fulvic acids (FA) were size-classified by multistage ultrafiltration (six fractions) in the molecular weight range of 1 to 100 kDa. The obtained data show that the HS extraction yield depended not only on the extractant, but also on the soil type. Within 3 h approximately 90% of the soluble HS could be extracted following complex extraction kinetics by both methods and none or little structural modification was verified as observed from their stable extinction ratio E350/E550. In the Mollisol the pyrophosphate extraction was more effective, suggesting that a great part of HS occurred as macromolecules bonded to clay minerals and aggregated between themselves through cationic bridges. In the Oxisol a higher HS yield was verified with the alkaline method, presumably due to HS fixation onto the oxide surface by H-bonds and/or surface complexation reactions. In general, HS extracted by the pyrophosphate procedure showed higher molecular weights than those extracted by NaOH.

Humic Substances and Chemical Properties of an Acrisol Amended with Vermicomposted Vegetal and Animal Residues

“…The FTIR spectra of HA and FA fractions exhibited a similar shape ( Figure 2) and are in accordance with the FTIR spectra of HS extracted from Brazilian soils, either under natural conditions or subjected to agricultural management (eventually receiving external organic amendments), as reported by authors applying HS extraction and purification procedures similar to that used in the present work (Dick et al, 1999;Dias et al, 2009;Moraes et al, 2011;Santana et al, 2011;Coelho et al, 2013).…”

Sepiolite and sepiolite-bound humic acid interactions in alkaline media and the mechanism of the formation of sepiolite-humic acid complexes

“…In contrast, there was not a correlation between UV 254 and the amount of organic carbon released from sepiolite depending on increased extraction time. This was confirmed by the subtraction E 450 -E 700 which expresses the relative amount of extracted HA (Dick and Burbab, 1999), as a function of time, indicating that HA extraction reached its maximum within the first 20 min and then no significant change was observed thereafter ( Table 2).…”

Comparative study of humic acids of the mound of a wood-feeding termite and of the litter directly below in the Amazon river delta

“…These molecules could be the intermediate compounds of cellulose degradation as described by Kononova [39]. Dick and Burba [32] have studied humic and fulvic acids extracted from two Brazalian soils and reported that fulvic acids were mainly found in the smaller fractions in contrast to the corresponding humic acids whose main fractions were found between 50 and 100 kDa and >100 kDa. This result can confirm the fulvic character of mound material and the humic character of litter.…”

“…The E4/E6 ratio varied only slightly, regardless of the origin of the organic matter. Dick and Burba [32] reported for humic substances that the absorbance ratio E4/E6 is a traditional parameter to estimate their humification degree and/or their molecular size. This ratio was slightly higher for the mound humic acids, indicating that the molecules were less aromatic with carbon mainly located in side chains [33] and a higher level of oxygen.…”