Humic Acid Adsorption on the Au(111) and Au 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 between PIA and chloride anion. The investigations are relevant to the situation encountered in estuaries, where HA interacts with increasing concentrations of inorganic salts from seawater. The adsorption of HA was investigated using cyclic voltammetry, differential capacitance measurements, chronocoulometry and electrochemical quartz crystal microbalance measurements. The investigations were performed in NaClO(4) and NaCl solutions at pH 5 and 2. HA adsorbs on the Au(111) surface practically in the whole double layer potential region (the potential region in which the faradaic current is negligible), from about -0.6 V(SCE) through all the measured positive potentials. Gibbs surface excess of HA increases with positive potentials. In the presence of NaCl two opposite effects are encountered: Na(+) ions neutralize negative charges on HA and thus enhance its adsorption, while Cl(-) compete with HA for the adsorption sites on Au(111). Ratio of HA to NaCl concentration, electrode potential, and structural transitions of the Au(111) surface dictate which of the two effects shall predominate.