| Abstract
| - Fluorescence correlation spectroscopy was used to studythe disaggregation kinetics of a peat humic acid (PPHA)at several pH. FCS measures diffusion coefficients offluorescent molecules and aggregates, thus allowing forthe determination of disaggregation rates with a temporalresolution of seconds to minutes. Disaggregation wasinitiated by dilution of a peat concentrate consisting of amixture containing 80% large aggregates (averagehydrodynamic radius, rH, of about 300 nm) and freemonomers (average rH of about 1 nm). Upon dilution atdifferent pH values, aggregate size decreased, and theproportion of free monomers in solution increased untilcomplete disaggregation occurred. The mechanism appearedto involve the release of monomers from the surface ofthe aggregates. The pH markedly affected the disaggregationrate. Complete disaggregation took 1 month at pH 3.6,took less than 1 h at pH 5.6, and was extremely rapid inalkaline solutions. The results suggested that at least twoprocesses were operating in parallel with the overallrate being the sum of both processes. At pH higher than4.5, the disaggregation rate increased more than 3 orders ofmagnitude per pH unit increase. For concentrationslower than 30 mg L-1, the equilibrium condition for thePPHA was complete disaggregation even for a pH as lowas 3.6.
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