THE PH EFFECT IN THE DIFFUSION-COEFFICIENT OF HUMIC MATTER - INFLUENCE IN SPECIATION STUDIES USING VOLTAMMETRIC TECHNIQUES

The dynamic aggregation properties of humic matter and the influence o f intramolecular electrostatic interaction lead to an increase of poly dispersity at pH > 5 in two humic samples studied (''Purified Feat Hum ic Acid'', PPHA and ''Laurentian Fulvic acid'', LFA). The smaller enti ties formed at pH increasing above 5 are responsible for the increase of the diffusion coefficient as ''seen'' by voltammetry. The voltammet ric results show that the diffusion coefficient of the humic complex i ncreases from 5 x 10(-12) m(2) s(-1) (pH less than or equal to 5) up t o 2 x 10(-11 )m(2 )s(-1) (pH = 6) for Cd/PPHA and from 6 x 10(-11 )m(2 )s(-1) (pH < or = 5) up to 1.2 x 10(-10) m(2) s(-1 )(pH = 5.5) for Pb /LFA. Systematic shifts in voltammetric results with changing ligand c oncentrations often attributed to adsorption of humic matter on the el ectrode and/or heterogeneity effects, may also be due to incorrect dif fusion coefficient values used in speciation calculations. The aim of this paper is to show that the diffusion coefficient of metal-humic co mplexes is a function of pH. It is necessary to take this into account to obtain more accurate speciation data from voltammetric results and interpret the physicochemical background of the metal humic interacti on. (C) 1998 Elsevier Science B.V.