Treatment of metals-contaminated wastewaters by use of natural zeolites

Abstract

This paper assesses the potential of natural zeolite utilization as a low-cost in exchange material for heavy metals removal. 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 (Pb, Cd, Cu, Zn, Cr, Ni and Co). The effects of relevant parameters such as chemical treatment, metals concentration, pH, and presence of competing ions were examined. The results showed that the received zeolites contained exchangeable K, Ca and Na ions, but exposing them to concentrated NaCl solutions converted them to a homoionic state in the Na form which improved their exchange capacity. Clinoptilolite and chabazite exhibited different selectivity profiles for all metals studied except for Pb for which both zeolites performed exceptionally well. The results also showed that chabazite exchange capacity is superior to that of clinoptilolite mainly due to the higher Al substitution of Si which provides chabazite with a negative framework favourable to higher exchange capability. The pH was found to have an effect on metal removal as it can influence both the character of the exchanging ions and the zeolite itself. Overall, the removal mechanism was controlled by ion exchange and precipitation was proven negligible.