Adsorption and coprecipitation of heavy metals from ammoniacal solutions using hydrous metal oxides

The importance of metal ion hydrolysis to the processes of adsorption and coprecipitation is investigated. The metal ions studied were Cr(III), Zn(II) and Ni(II) and the substrates used were amorphous hydrous iron(III) oxide (HFO) and amorphous hydrous chromium(III) oxide (HCO). Adsorption and coprecipitation experiments were performed using an ammoniacal background electrolyte, where the ammonia present could form ammine complexes with the metal ions, and hence significantly suppress hydrolysis. It is suggested that this suppression of hydrolysis inhibits adsorption, both through competitive equilibria and also through diminished capability for hydrogen bonding in the presence of ammoniacal ligands. The removal profiles obtained for Zn(II) adsorption and coprecipitation on HFO and HCO were modelled using a modified version of the James-Healy model for metal ion adsorption. (C) 1997 Elsevier Science B.V.