Monitoring heavy metals in seawater by their electrochemically induced deposition as hydroxides

A novel method for monitoring heavy metals in seawater is presented. The method is based on the electrochemical codeposition of metal hydroxides driven by the change of ph at the surface of a gold electrode. Altering the ph is achieved by applying a negative potential or current that reduces the water and thereby increases the concentration of hydroxyl ions. This in turn causes metals to coprecipitate with mg(oh)₂. The continuous deposition enriches the precipitate with the metals (as compared with their concentration in the aqueous phase) and allows their determination by icp-ms upon dissolving the deposit. A set of experiments in which seawater was spiked with 1 - 10 ppm of cu, cr, co, zn and pb, was conducted. It was observed that metals were accumulated in the precipitate as a function of time and their concentration in seawater. The precipitates were analyzed by sem, eds and xps indicating that the metal hydroxides formed a separate phase from mg(oh)₂ and even water electroreducible metals, e.g., cu²⁺, preferentially precipitated as hydroxides. Distribution constants correlating the concentrations of the metals in the deposited salts to their concentrations in seawater were calculated. These calculations imply that the mechanism governing the precipitation of the metal hydroxides by the electrochemically induced process is likely to be kinetically and mass-transport driven rather than thermodynamically controlled.