Super-faradaic charge yields for aluminium dissolution in neutral aqueous solutions

• Super-faradaic dissolution of Al electrodes in electrocoagulation processes studied.
• Importance of chloride ions and humic acid on super-faradaic Al charge yields.
• Al oxidation in parallel with proton/water reduction giving net current densities.
• Role of localised evolution of hydrogen in Al dissolution pits studied.

Dissolution of elemental aluminium or iron in aqueous solutions is the essential reaction in electrocoagulation and other processes. These and several other such metals have been reported as exhibiting dissolution rates that are higher than expected from the current/charge passed, assuming electron stoichiometries of three (Al) or two (Fe). In batch dissolution experiments with aluminium electrodes, the addition of 30 g m−3 humic acid to a neutral solution of 0.5 mol m−3 Na2SO4 and 8.8 g m−3 NaCl led to an unexpected increase in the charge yield from 1.0 to ca. 1.5, i.e., the measured concentration of aluminium species in solution was 50% higher than expected from Faraday's law. This phenomenon may be explained when net currents are used for the determination of charge yields, whereas partial dissolution currents should be used. The latter may be significantly greater than the former when a concurrent reduction reaction, such as hydrogen evolution, occurs at the dissolving electrode.