Electrochemical recognition of selective mercury adsorption on minerals

Mercury species are highly toxic contaminants of the environment, and their dissemination in aquatic media is governed by sorption processes on both organic (including biological) and inorganic particles. An electrochemical method is provided for the direct monitoring of the selective binding of inorganic mercury(II) to the surface hydroxyl groups of some minerals (silica, goethite, pyrite) by using carbon paste electrodes modified by these mineral particles. After accumulation from aqueous solutions at open circuit, anodic stripping voltammetry was performed with the electrode immersed in a detection cell containing typically 0.2 M HNO3 in order to desorb and measure the amount of previousl- y adsorbed mercury. Several experimental parameters were optimized with a silica-modified electrode to ensure quantitative and reproducible results, including the electrode composition, the accumulation time, and the voltammetric detection mode. Mercury adsorption was studied as a function of pH and chloride concentration. It was found that only the soluble mercury(II) species with hydroxyl moieties (HgOH+, Hg(OH)2, HgOHCl) were able to adsorb on these minerals by reacting with their surface hydroxyl groups. Moreover, in a rather low pH region (4-7), this reaction was extremely selective for mercury over other soluble metal species. The electrochemical monitoring, though mainly controlled by kinetics, gave significant insight in the adsorption processes as far as equilibrium states were rapidly reached.