DYNAMIC PROPERTIES OF HUMIC MATTER BY DYNAMIC LIGHT-SCATTERING AND VOLTAMMETRY

The diffusion coefficients of humic matter samples (Purified Feat (PP) Moss and Fluka) in solution were obtained by both dynamic light scatt ering (DLS) and voltammetric methods. The diffusion coefficients from voltammetric measurements are greater than those determined by DLS, du e to the different way each technique responds to the size distributio n of polydisperse samples. Large aggregates are present in both PP hum ic acid (HA) and Fluka samples (diameters larger than 30 nm), their si ze depending on the origin and/or preparation procedure of the sample, as well as on the experimental conditions of pH and ionic strength. T he influence of ionic strength on the diffusion coefficient of PPHA wa s analysed. Similar results were obtained by DLS and voltammetry in sa mples with low polydispersity up to ionic strength of 0.1 M. For highe r ionic strengths aggregation may occur, leading to an increase in the apparent molecular size. The influence of pH on the diffusion coeffic ient of PPHA shows one population formed by large aggregates (185 nm d iameter) at pH 5 in the absence of added salt. When pH is lowered belo w 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 adju sted to pH between 3 and 5, present somehow different characteristics from samples directly set to the required pH, which stresses the influ ence of the preparation method on final sample properties.