Extraction kinetics and molecular size fractionation of humic substances from two Brazilian soils

In the present study, the extraction behaviour of humic substances (HS) fro m 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 sy stematically studied. The kinetics and efficiency of HS extraction were eva luated by means of UV/Vis spectroscopy. The isolated humic acids (HA) and f ulvic acids (FA) were size-classified by multistage ultrafiltration (six fr actions) in the molecular weight range of 1 to 100 kDa. The obtained data s how that the HS extraction yield depended not only on the extractant, but a lso 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 stab le extinction ratio E-350/E-550. In the Mollisol the pyrophosphate extracti on was more effective, suggesting that a great part of HS occurred as macro molecules bonded to clay minerals and aggregated between themselves through cationic bridges. In the Oxisol a higher HS yield was verified with the al kaline method, presumably due to HS fixation onto the oxide surface by II-b onds and/or surface complexation reactions. In general, HS extracted by the pyrophosphate procedure showed higher molecular weights than those extract ed by NaOH.