Removal of heavy metals by electrocoagulation from hydrogenocarbonate-containing waters: Compared cases of divalent iron and zinc cations

Divalent iron and zinc cations can be removed from water by electrocoagulation (EC) with aluminium electrodes in a discontinuous system: the effect of hydrocarbonate HCO3- ion often present in liquid waste and in groundwater on the EC process has been investigated. For the two ions, the presence of hydrocarbonate strongly limits the pH variations by its buffering properties and reduces the rates of Al dissolution by corrosion. Removal of the two cations was then shown to require longer treatment times and larger amounts of dissolved aluminium. Whereas the local pH gradients near the electrode surface with HCO3- free water was previously shown to allow local formation of stable Zn and Fe hydroxides, which actively contribute to their elimination, the presence of hydrocarbonate nearly suppresses this positive phenomenon, resulting in far less efficient EC treatment. Whereas removal of zinc cations from carbonated water can be considered as their simple adsorption on the Al flocs, Fe2+ ions are oxidized to Fe(OH)(3) by air oxidation after their adsorption. Use of an overall adsorption model allowed quantitative comparison of the EC treatments, with very different adsorption parameters for the two metal studied.