Cadmium adsorption on hydrous aluminium (III) oxide: effect of adsorbed polyelectrolyte

Abstract

Freshly precipitated hydrous oxides of Al (flocs) are important sorbents of heavy metal ions and are used in water treatment processes. The addition of chelating agents or polyelectrolytes can increase the efficiency of heavy metal removal by flocs. The polymer polyethylenimine is commonly used as a flocculant aid in water treatment and has been modified with phthalic acid. Adsorption studies have shown that the addition of millimolar concentrations of polyelectrolyte increases the removal of Cd ions from a 29.7 μM aqueous solution by aluminium flocs from ca. 40% to up to 75%. Experiments in which the pH was varied demonstrated that the observed enhancement of Cd adsorption is caused by a shift of the pH edge to lower pH values in the presence of polyelectrolyte. Maximum polyelectrolyte adsorption occurs between pH 6 and pH 7, corresponding to the point of enhanced Cd adsorption in the presence of polyelectrolyte. The enhancement of Cd adsorption observed below pH 7 is therefore likely to be due to the formation of a ternary surface complex. At pH values above pH 8 the polyelectrolyte forms a stable complex with Cd and Al ions in solution causing floc dissolution and decreased Cd adsorption.