Performance of natural zeolites for the treatment of mixed metal-contaminated effluents

Two natural zeolites, clinoptilolite and chabazite, have been evaluated with respect to their selectivity and removal performance for the treatment of effluents contaminated with mixed heavy metals, namely Pb, Cd, Cu, Zn, Cr, Ni and Co. Parameters such as metal concentration, pH and presence of competing ions were examined and removal performance was determined in terms of the zeolites, ion-exchange capacity measured at room temperature. The study showed that at metal concentrations ranging from 1 mgl(-1) to 10mgl(-1) the zeolites exhibited an optimum removal efficiency at metals concentration of 10 mgl(-1). Clinoptilolite and chabazite exhibited different selectivity profiles for all metals studied except for Pb for which both zeolites performed exceptionally well. The selectivity sequences for clinoptilolite and chabazite are summarized as follows: chabazite (Ph>Cd>Zn>Co>Cu>Ni>Cr); and clinoptilolite (Pb>Cu>Cd>Zn>Cr>Co>Ni). The study also showed that the chabazite exchange capacity is superior to that of clinoptilolite which is mainly due to the higher AI substitution of Si which provides chabazite with a negative framework favourable to higher exchange capability. Solution pH was found to have an effect on metal removal as pH can influence both the character of the exchanging ions and the zeolite itself. The metal removal mechanism was demonstrated to be controlled by ion exchange and precipitation was negligible. The results also showed that Ca was a major competing cation for ion exchange for both clinoptilolite and chabazite when concentrations exceeded 1000 mgl(-1). Overall, chabazite and clinoptilolite removal efficiency was not affected by the presence of more than one heavy metal in solution which demonstrates their potential application in the treatment of effluents contaminated with mixed heavy metals. (C) 1997 ISWA.