Abstract
The diffusion coefficients of humic matter samples (Purified Peat (PP) Moss and Fluka) in solution were obtained by both dynamic light scattering (DLS) and voltammetric methods. The diffusion coefficients from voltammetric measurements are greater than those determined by DLS, due to the different way each technique responds to the size distribution of polydisperse samples. Large aggregates are present in both PP humic acid (HA) and Fluka samples (diameters larger than 30 nm), their size depending on the origin and/or preparation procedure of the sample, as well as on the experimental conditions of pH and ionic strength. The influence of ionic strength on the diffusion coefficient of PPHA was analysed. Similar results were obtained by DLS and voltammetry in samples with low polydispersity up to ionic strength of 0.1 M. For higher ionic strengths aggregation may occur, leading to an increase in the apparent molecular size. The influence of pH on the diffusion coefficient of PPHA shows one population formed by large aggregates (185 nm diameter) at pH 5 in the absence of added salt. When pH is lowered below 2.5, coagulation occurs quickly leading to the precipitation of PPHA. Some irreversible disagreggation (120 nm diameter) is promoted by pH increase up to 10. PPHA samples, initially set to pH 10 and then adjusted to pH between 3 and 5, present somehow different characteristics from samples directly set to the required pH, which stresses the influence of the preparation method on final sample properties.
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