ADSORPTION OF HUMIC-ACID ON A MERCURY AQUEOUS-SOLUTION INTERFACE

The adsorption on a mercury/water interface of a sample of humic acids from an Irish peat has been analyzed by a.c. voltammetry, using a han ging mercury drop electrode in an unstirred and/or stirred medium, at different pH values (pH = 2.5, 5 and 8) and two potential values (E = -0.5 and -0.9V), in order to have a better insight into the kinetics a nd equilibrium adsorption which may drastically affect the voltammetri c signal in trace metal speciation of natural waters. The results at p H 2.5 present a slower kinetic of adsorption but a higher coverage deg ree at the equilibrium when compared with pH 5 and/or 8 due to the cha rge of the molecules. At -0.5 V, close to the potential of zero charge , the kinetics of adsorption is discussed in terms of a diffusion mech anism, but at -0.9 V the experimental curves clearly show that the glo bal adsorption process cannot be explained by a pure diffusion mechani sm based on Frumkin isotherm.