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

• Autor(a) principal: Dick,Deborah Pinheiro
• Data de Publicação: 1999
• Outros Autores: Burba,Peter
• Tipo de documento: Artigo
• Idioma: eng
• Título da fonte: Journal of the Brazilian Chemical Society (Online)
• Texto Completo: http://old.scielo.br/scielo.php?script=sci_arttext&pid=S0103-50531999000200013
• topic: humic substances; oxisol; mollisol; molecular-weight fractionation; extraction kinetics

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.