Humic acid adsorption on the Au(111) and an polycrystalline electrode surface

Our aim was to investigate the adsorption of humic acid (HA) on the Au(111) and Au polycrystalline electrode, as well as the adsorption competition be tween PIA and chloride anion. The investigations are relevant to the situat ion encountered in estuaries, where HA interacts with increasing concentrat ions of inorganic salts from seawater. The adsorption of HA was investigate d using cyclic voltammetry, differential capacitance measurements, chronoco ulometry and electrochemical quartz crystal microbalance measurements. The investigations were performed in NaClO4 and NaCl solutions at pH 5 and 2. H A adsorbs on the Au(111) surface practically in the whole double layer pote ntial region (the potential region in which the faradaic current is negligi ble), from about -0.6 V(SCE) through all the measured positive potentials. Gibbs surface excess of HA increases with positive potentials. In the prese nce of NaCl two opposite effects are encountered: Na+ ions neutralize negat ive charges on HA and thus enhance its adsorption, while Cl- compete with H A for the adsorption sites on Au(111). Ratio of HA to NaCl concentration, e lectrode potential, and structural transitions of the Au(111) surface dicta te which of the two effects shall predominate.